mist cooling coupled with air conditioning to …...sale in the guava production chain in rajasthan,...

Mist Cooling Coupled with Air Conditioning to Provide Low Cost/Low Energy

Guava Cold Storage for Indian Producer Companies

Abdullah AlSharhan

Sydney Lance

Henry Magun

Kathleen Miller

Chase Milligan

Ufuoma Ovienmhada

Moses Swai

Alexandra Warner

ME 170B

Mechanical Engineering Design: Integrating Context with Engineering

Spring Quarter 2018

Final Report

June 11, 2018

ECOGuavo! Final Report | June 11, 2018 1

Abstract

The goal of this project is to reduce the amount of food loss between harvesting and consumer

sale in the guava production chain in Rajasthan, India. When stored at 7°C, guava can resist

spoilage for up to 3 weeks, however, at ambient conditions of up to 30°C, this shelf life is

reduced to just 2-3 days [1]. The lack of a cold chain for rural guava farmers in India results in

significant produce losses between farm and market, leading to decreased crop yields and

economic hardship for farmers. Additionally, the inability to safely store produce prohibits

farmers from avoiding market lows, waiting until market highs, and aggregating export-quality

fruit for foreign markets.

We have developed a low-cost, environmentally friendly, and easily deployable system for the

storage of guavas at temperatures between 10°C and 15°C. Our system utilizes a combination of

an active cooling system, comprised of an air conditioning unit supplemented with a CoolBotTM,

and a passive cooling system, comprised of a series of misting nozzles spraying onto a heat

exchanger. These technologies are deployed within a trailer outfitted with insulation rated with

an R-value of 26.2.


Acknowledgements

We would like to thank:

● Our teaching team Shoshanah Cohen, Isaac McQuillen, Rachel Reed, and Jeff Wood for

their guidance on how to tackle engineering problems throughout the design process.

● Our project sponsors Dr. Michael Machala and Andrey Poletayev of Stanford University,

and Rajesh Sharma, Nitin Sharma, Rupendra Sharma, and Vikram Rajput of the Reliance

Foundation for their critical insights into our users’ needs and their unwavering support

throughout our project and our travel to India.

● The Stanford Mechanical Engineering faculty who advised our analysis throughout this

project including Professor John Eaton, Professor Arun Majumdar, Professor Hai Wang,

Dr. Lester Su, and Dr. Scott Crawford.

● John Bergher and Freddy Remolina from Store It Cold for supplying us with a CoolBot

module and John Lerch from Axiom Energy for meeting with us to discuss Axiom’s

approach to energy storage for grocery store refrigeration.

● The Stanford Mechanical Engineering Department, the Stanford Precourt Institute for

Energy, the Haas Center for Public Service, and the Fankuchen Innovation Fund for

funding our project, including support for our prototype equipment and materials and our

incredibly impactful travel to India to meet our users.

● All our other kind supporters throughout the project, including Patrick Archie of the

O’Donohue Family Stanford Educational Farm and Angelos Deltsidis, post-harvest food

specialist from UC Davis.


Table of Contents

Abstract 1

Acknowledgements 2

Table of Contents 3

List of Figures 6

List of Tables 7

1. Introduction 8

2. Background 100

2.1 User Research 11

2.2 Technical Requirements 111

3. Ethical Considerations 133

3.1 Human Safety 143

3.2 Clean Energy and Sustainability 144

3.3 Product-Context Fit 155

4. Preliminary Cooling Research 18

4.1 Active Cooling 19

4.1.1 Air Conditioner 190

4.1.2 Active Cooling - Air Conditioner Coupled with CoolBot 19

4.2 Passive Cooling - Structure and Insulation Design 201

4.3 Evaporative Cooling 222

4.3.1 Direct Evaporative Cooling—Matki Pots 234

4.3.2 Direct Evaporative Cooling—Zero Energy Cooling Chamber 245

4.3.3 Direct Evaporative Cooling—Swamp Cooling 256

4.4 Mist Cooling 266

5. Preliminary Cooling Experiments 27

5.2 Swamp Cooling 299

5.3 Mist Cooling 30

6. Preliminary Structure Design Research 311

6.1 Retrofitting an Existing Building 312

6.2 Prefabricated Insulated Structure 322

6.3 Retrofitting a Prefabricated Storage Structure 322


6.4 Insulation Materials 333

7. Thermal Model 334

7.1 Overall Energy Use 344

8. Proposed Cold Storage System 389

8.1 Structure and Insulation 389

8.2 Cooling 41

8.2.1 System Description 423

8.2.2 Methods 444

8.2.3 Analysis and Discussion 46

8.3 Energy Source and Storage 49

9. FMEA Summary 53

10. Conclusion 58

11. Future Work 59

12. References 61

13. Appendices 64

Appendix A: Team Members, Roles and Responsibilities 64

Appendix B: Gantt Chart 66

B-1 ECO Guava Winter Quarter 66

B-2 GuavO! Winter Quarter 67

B-3 ECOGuavO! Spring Quarter 67

Appendix C: FMEA 68

Appendix D: Matki Pot Test 736

Appendix E: Trailer BOM 75

Appendix F: Summary of Expenses and Budget 79

Appendix G: User and Technical Requirements 846

Appendix H: User Manual for Producer Company 91

Appendix I: O’Donohue Farm Trailer Guide 105

Appendix J: Existing Cooling Products Considered 124

List of Figures

Figure 1: Maps of India and the state of Rajasthan. 11

Figure 2: Photo of Part Farmers Producer Company Limited 12


Figure 3: Photos from observed conditions in trip to India 15

Figure 4: Cooling Energy Spectrum 19

Figure 5: Igloo geometry 22

Figure 6: Psychrometric Chart 23

Figure 7: Matki Pot 25

Figure 8: Zero Energy Cooling Chamber 26

Figure 9: Typical swamp cooling setup with warmer, drier air flowing being pulled by

a fan through a cooling pad moistened by a water distribution system

27

Figure 10: A typical mist cooling setup. 28

Figure 11: Data from swamp cooling experiment 31

Figure 12: Diagram of mist cooling prototype 31

Figure 13: Data from mist cooling experiment 32

Figure 14: Produce temperature as a function of time. 38

Figure 15: Power decreases as chamber temperature temperature increases. 38

Figure 16: Sensitivity analysis shows heat leak as a function of temperature

differential and R

39

Figure 17: A before and after photo of the retrofitted cold-storage trailer 40

Figure 18: Insulation & AC test 41

Figure 19: Insulation & reheating test 42

Figure 20: Labeled photo of the back of the trailer showing the air conditioner, water

inlet, air inlet and energy monitor.

43

Figure 21: Labeled photo of the inside of the trailer showing the CoolBot, heat

exchanger and drain.

44

Figure 22: Labeled close up photo of the heat exchanger highlighting the sealant,

nozzles and HVAC tape.

45

Figure 23: Photo of our testing set up inside the trailer 47

Figure 24: Misting system experiment results 49


Figure 25: Air conditioner and Coolbot experiment results: 50

Figure 26: Air conditioner + Coolbot and misting system experiment results:

Temperature vs Time.

51

Figure 27: Rate of apple temperature increase after turning off A/C. 54

Figure 28: FMEA - Door gasket 57

Figure 29: FMEA - Drain 58

Figure 30: FMEA - Nozzles 59

Figure 31: FMEA - Misting joints 59

Figure 32: Gaps in Insulation 60


List of Tables

Table 1: High priority engineering requirements 13

Table 2: Air and wet bulb temperature data from Jaipur, Rajasthan to calculate the

maximum temperature drop that can be achieved with evaporative cooling

24

Table 3: Air, wet bulb and groundwater temperature data from Jaipur, Rajasthan to

calculate the maximum temperature drop that can be achieved with evaporative cooling

29

Table 4: Insulating materials with associated price per square foot and R-value. 34

Table 5: Specifications of our cold-storage trailer prototype and our future structure in

India.

41

Table 6: Description of large-scale cooling system tests. 47

Table 7: Air conditioner + Coolbot and misting system experiment results:

Temperature vs Time.

52

Table 8: Costs and deployability of alternative energy production and storage methods. 53

Table 9: Cost comparison of trailer building materials and translation to India 55


1. Introduction

In India, roughly 327 million metric tons of food is wasted every year [1]. This can be largely

attributed to an underdeveloped supply chain that lacks the means to extend the shelf life of

produce. Post-harvest losses—produce that spoils before reaching the market—range from 15%-

50% of a farmer’s total harvest, representing a significant economic loss. Furthermore, only

10%-11% of produce grown in India is preserved in cold storage throughout its supply chain [2].

For guava farmers in India, this problem is particularly troublesome. Currently, picked guavas

have a 2-3 day shelf life in India under their current storage and transportation methods. When

placed in a cold storage or chemical preservation environment, the shelf life of guava can be

extended for up to 2-3 weeks shelf life [3]. Field interviews in India revealed that farmers show

risk-averse behavior as sellers in the market, opting to sell their produce as soon as possible for

fear of spoilage, unpredictable fluctuations in market price, or financial stress. Furthermore,

farmers take on all operational costs of sorting, grading, packaging, and delivering their produce

to the market. These deliveries often occur over long distances and unfavorable weather

conditions that degrade the quality of the produce by over ripening or bruising. Farmers therefore

face operational challenges and a limited window of opportunity to ensure that their harvest is

sold.

The rise of Producer Companies (PC)—cooperative social enterprises that aggregate produce

from multiple local farmers for higher bargaining power and reduced costs—have helped

mitigate these risks and reduce the scope of challenges faced by farmers. Many guava farms are

located up to 600 km from their local markets; produce must therefore be trucked from farming

cooperatives to market. Guavas are typically transported in two-ton trucks that run once per day.

When farmers have aggregated more guavas than can fit into a standard truck, the extra produce

is kept at the cooperative in outdoor conditions, risking spoilage and reduced yield.

The objective of our project is to provide PCs with a prototype low-cost cold storage unit capable

of storing 2 tons of guava to reduce produce losses and maximize profit for PCs and their

members. With a cold storage system, farmers can store excess produce, wait for favorable

market conditions, and aggregate higher quality guavas for export to more distant markets. These

opportunities allow for a reduction in produce loss, an increase in sales price, and the potential


for high-paying international customers, providing farmers with operational flexibility and risk

mitigation to increase overall farm-to-market yield, thus increasing farmer profits and improving

their economic status.

Our goal for this project is to develop a cold storage system for guava farmers that is:

● Low cost such that the installation and operational costs do not surpass the income gain that

the farmers can realize from using the system.

● Environmentally friendly and maximizes use of a green/renewable energy solution.

● Easily deployable to the farmers considering their location, remoteness, technical

capabilities, and available materials.

We have worked closely with the Reliance Foundation, the philanthropic arm of Reliance

Industries, and the Parth Farmers PC Ltd., located in the state of Rajasthan, to understand and

scope the overarching goals of our collaboration, study regional challenges in the produce supply

chain, and most importantly, ensure a product-market fit, more accurately a product-context fit,

for successful deployment of our final product. This project has the potential to have a significant

impact on the guava production chain in Rajasthan, the production chains of produce in other

regions, and food and energy losses in the wider agriculture ecosystem in resource-challenged

communities.


2. Background

The initial project background was provided in the form of a Project Brief prepared by Dr.

Michael Machala. The Project Brief provided an overview of the impact of the lack of affordable

cold storage facilities on guava farmers in Sawai Madhopur, Rajasthan (Figures 1a and 1b). In

addition to the Project Brief, we conducted email interviews and had WhatsApp conversations

throughout the 20 weeks of the course to assess our users and their needs. Three members of our

team visited Rajasthan from March 22 - April 1, giving us the opportunity to gain additional

valuable information about our users’ specific requirements. We used this information to create

and refine a comprehensive list of user and technical requirements, to define and create a

mitigation plan for potential ethical and environmental consequences, and to create a Failure

Modes and Effects Analysis.

Figure 1a (left): Map of India highlighting the state of Rajasthan [4]; Figure 1b (right): Map of

the state of Rajasthan highlighting Sawai Madhopur [5].


2.1 User Research

We designed our cooling system based on the needs of Parth Farmers Producer Company Ltd.

(PFPCL), a producer company located in Sawai Madhopur, Rajasthan, India, seen in Figure 2.

Producer companies (PCs) are collectives of small farmers who pool their resources to gain

benefits they could not achieve individually [6]. PFPCL is a collective of 500 rural guava

farmers near Sawai Madhopur, Rajasthan who together operate approximately 400 Hectares (Ha)

of guava orchards. PFPCL offers its constituents focused training, capacity building events, and

higher prices for their guavas at market as a result of the PFPCL’s improved leverage in

negotiations.

Figure 2: (From left to right) Four members of Parth Farmer’s Producer Company Ltd., one

member of the Reliance Foundation and three members of our team in Sawai Madhopur, Rajasthan.

2.2 Technical Requirements

We designed our cold storage unit to help farmers aggregate their guavas over 2-3 days to fill

larger trucks. Large 2-10 metric ton trucks are more cost efficient than the 0.5 metric ton lorries

the farmers currently use and can travel longer distances (up to 600 km) to reach far away

markets where guavas sell for higher prices. Based on background research, initial interviews


with members of PFPCL and the Reliance Foundation, and in-person discussions during our trip

to India, we identified six high priority engineering requirements that are critical to our system’s

success, seein in Table 1.1

Table 1: High priority engineering requirements

Engineering Requirement Justification

Have an internal volume of 4 m3 PFPCL has asked for a minimum of 2 tons worth of storage space

for guavas. Each ton of guavas uses approximately 2 m3

Maintain temperature of 10-15°C PFPCL expressed the need to store guavas for 1-4 days in order to

aggregate a sufficient quantity to fill a two ton truck. Storing

guavas at 10-15°C delays ripening for 11 days [7].

Operate at different settings based on

produce stored inside

Temperature control is necessary to maintain produce quality,

slow the ripening processes, and avoid sogginess or water loss.

Farmers should be able to store tomatoes, green chilies and onions

in addition to guavas to make use of system during the off season

for guavas.

Have an up front cost less than $3,000 Upfront material and installation cost to the user must be less than

$3,000 for the entire unit. This value is based on PFPCL’s stated

cost requirement for the system.

Have operational cost less than

$2.96/ton/day*

This value is based on PFPCL’s estimate of what they currently

pay to rent cold storage space at the markets of Jaipur, Rajasthan.

Keep guavas cool during electrical

blackouts of up to four hours.

Grid data taken from a village outside of Sawai Madhopur,

Rajasthan shows that villages have electricity for approximately

20 hours a day and regain electricity within four hours after most

blackouts.2

1 For more detailed information about our medium and low priority engineering requirements, please refer to tables

H-1, H-2 and H-3 in the Appendix.

2 For more detailed information on grid availability, please refer to section 3.4.

*Based on a conversion factor of 1 INR = .014823 USD


3. Ethical Considerations

Deploying a low cost, low energy cold storage system for guavas requires an understanding of

the multiple ethical dimensions of our product’s features, both upstream (when exploring

manufacturing methods and costs) and downstream (when exploring operating costs and

operating carbon footprint). Taking into consideration that our end users are producer companies

comprised of low-income farmers, the ethical implications of our product should be assessed

based on three primary criterions. The first is in how our storage unit fulfills its intended goal of

raising farmers’ livelihoods and economic status, the second is in how the features of our product

align themselves with the sustainability and clean energy goals of the course, and the third is to

ensure product-context fit in all facets of our design. Our ethics analysis is informed by the key

tenets of ethics in engineering design:

● Do not cause harm or create an unreasonable risk of harm to others

● Prevent harm and any unreasonable risk of harm

● Try to alert and inform about the risk of harm

3.1 Human Safety

First and foremost, we must consider human safety in all aspects of our design. This includes

both preventing harm to human health from system malfunctions and ensuring the financial well-

being of our users by providing a reliable product. The reliability of the product, both structurally

and cooling efficacy, carries its own ethical considerations.

While many of the evaporative cooling systems we found in our preliminary research use water

as an evaporant, certain fluids, such as ammonia, have the potential to increase the efficiency of

evaporative cooling systems. Before comparing the thermal properties of ammonia and water, we

compared the health risks: Ammonia gas is fatal if inhaled and burns exposed tissue. We were

not confident that our users would properly maintain the system; throughout our visit to India we

observed imprecise technology solutions including exposed wires and precarious construction

scaffolding, seen below in Figure 3a and Figure 3b. We chose to eliminate ammonia gas as a

potential solution as the added efficiency would not be worth the risk to human health. While

ammonia would have offered similar advantages to our misting system, our solution uses water


instead for the same reasons. We made similar precautions in choosing and assembling our

insulation material, the CoolBot, and our AC unit into our final product.

Figure 3a (left): Photo of a light switch with exposed electrical wiring as observed in a farmer’s

house in Sawai Madhopur, Rajasthan; Figure 3b (right): Photo of precarious slanted scaffolding

held together with knotted rope as observed in Jaipur, Rajasthan.

We considered multiple factors related to physical labor and hygiene. Our design is intended to

be easy to use and does not require unreasonable bending and lifting. The mist generated by the

misting module does not touch the fruit inside the cooling chamber, limiting the risk of

contamination through unclean water. In addition, our build instructions include a maintenance

plan that specifies periodic deep cleaning inside the storage unit. The cooling chamber is

relatively easy to build and uses lightweight insulation that is easy to cut and install.

3.2 Clean Energy and Sustainability

Second, we must consider regional and global sustainability in the form of resource use and

carbon dioxide emissions. As climate change continues to occur, we must help the developing

world find “clean” opportunities to improve their quality of life. One design consideration in

particular—how to meet our system’s energy requirements—presents a significant challenge

both technically and ethically.

We considered two main sources of electricity to power our system: grid and solar. Tapping into

the grid would provide inexpensive electricity, however power availability is intermittent—

which would make our system unreliable—and electricity generation is predominantly fueled by


coal, which is a heavy emitter of carbon dioxide. Solar panels would provide clean, “free”

electricity once installed; however, the upfront cost of solar panels and the necessary batteries

are too high and prohibitive to the farmers for which we are designing.

Our solution is to reduce the system’s cooling load by supplementing an active cooling system

with passive cooling. Reducing the cooling load would reduce the necessary electrical input to

power the storage unit, thus reducing the operating carbon footprint. The implementation of our

mist cooling system in our final product is ethically advantageous for that reason.

3.3 Product-Context Fit

Successful deployment of our final product requires a strong understanding of ground conditions

in India, including available resources, operational and logistical difficulties, and how our end-

users plan on integrating our cold storage unit into their business model. On our trip to India, we

spent the majority of our time there interviewing farmers about their cold storage needs and the

feasibility of potential solutions.

Can our system be used for other produce?

After visiting India and meeting with our users, we concluded that for our solution to be effective

for the entire producer company, it must be able to preserve produce other than guavas. A

cooling solution that is viable for other produce will enable shared costs across farmers and

allow the system to be used throughout the year, rather than during guava season only. Our

system was designed with this consideration in mind; the storage capacity and temperature

controls will work for other types of produce and all farmers can share the investment, lowering

the cost for each.

Is it ethical to produce a unit that relies on expensive services and/or replacement parts to

maintain it?

We decided to implement the most basic system possible, using locally available low-cost

materials and forgoing extra functionality that adds cost without corresponding economic

benefit. Furthermore, our experiences have shown that there is a technical gap between what is

feasible on a technical/engineering level and what can be successfully deployed and used by our

end-users. To avoid the possibility of harm, it is our ethical responsibility to adhere to the latter


while simultaneously ensuring that PCs are adequately informed of the storage unit’s functional

and practical limitations.

Does our design require changes in consumption of resources?

Rural villages in India face higher limits on resources We examined the use of water and

electricity, recognizing that there are tradeoffs in the use of resources used in the villages. By

having a product that demands one resource over another, how are we changing their

consumption behavior? Is that an ethical request to make? Speaking to the farmers in India, we

saw that there was a general

Is our design safe for people using the system?

We considered multiple factors related to physical labor and hygiene. Our design is intended to

be easy to use and does not require unreasonable bending and lifting. The mist generated by the

misting module does not touch the fruit inside the cooling chamber, limiting the risk of

contamination through unclean water and fungal infection. In addition, our build instructions

include a maintenance plan that specifies periodic deep cleaning of the chamber. The cooling

chamber is relatively easy to build and uses lightweight insulation that is easy to cut and install.

Will our solution cause the loss of jobs/livelihood?

The ethical problem of job loss is inherently linked to the introduction of new technologies into

society. While it is an important factor to consider, ultimately it did not impact our design with

respect to delivering a product that allows producer companies to cut out private contractors. The

problem that we are solving is one directed towards the farmers, whose solution is dependent on

eliminating their need to pay private contractors.

Can our system be deployed beyond its initial implementation?

The trip to India provided valuable insight regarding the transportation capabilities for the

delivery of materials to build the system. We realized that our system must be capable of being

disassembled and reassembled, and that each component must be able to fit on a standard 3-4 ton

truck. This had a significant impact on our design, as it has driven us to focus on self-standing

structures that have detachable walls. While our prototype uses an existing trailer as the base

chamber, the design can be easily translated to a structure with detachable walls. If the producer

company opts to use a similar trailer as a solution, it will be easily transportable.


Are our materials recyclable?

A further ethical consideration that we took into account is the disposability/recyclability of the

insulation and other components used. We will need to specify the amount of time materials in

our system can be used for, and how to properly dispose of the components involved. To further

reduce waste our system produces, we will also need to improve the recyclability of the water in

our system.


4. Preliminary Cooling Research

We conducted preliminary research on existing cooling technology to inform the design of our

cooling system. We met with faculty from Stanford University’s Mechanical Engineering

Department including Professor John Eaton, Professor Arun Majumdar, Professor Hai Wong, Dr.

Lester Su and Dr. Scott Crawford, as well as representatives from cooling companies including

Store It Cold and Axiom Energy to learn about small and large scale methods for cooling with

varying levels of required energy input. Passive cooling solutions such as evaporative cooling

and underground storage require zero energy input, while active cooling systems such as air

conditioning units require energy input to operate. Figure 4 below organizes the cooling

concepts we explored by amount of energy input required. We then evaluated each cooling

method in the context of our user and technical requirements, our project budget, and expected

ease of implementation Cooling methods that showed initial potential against these criteria were

further considered via rapid experimentation. For a detailed list of cooling products considered,

see Table K-1 in Appendix K.

Figure 4: Cooling methods organized based on amount of required energy input.


4.1 Active Cooling

We considered standard home air conditioners as an active cooling mechanism for our system

because they are relatively inexpensive compared to traditional refrigeration systems and are

readily available throughout India.

4.1.1 Air Conditioner

We first considered a simple solution of a traditional air conditioner operating in an insulated

storage chamber. Given the size of the required chamber, we evaluated the use of a window air

conditioning unit designed to cool a space of up to 4 m3.

Most window air conditioning units have a recommended minimum temperature of 20°C/68°F,

well above the 10-15°C required for effective guava storage [8]. In addition, most are

electronically limited so that they cannot go below 15.5°C and the ability to utilize the additional

cooling power drops drastically as this level is approached due to a lack of of supplemental fans

and extra surface area built into units intended to be used for cold storage. We therefore

concluded that an air conditioner would not be a suitable solution on its own.

4.1.2 Active Cooling - Air Conditioner Coupled with CoolBot

We next considered forced continuous cooling via a CoolBot that works in conjunction with a

standard 18K BTU window air conditioning unit. The AC unit was sized based on the expected

volume of our cooling unit (4m3) and the sensible heat load associated with cooling 2 tons of

guava from ambient temperature conditions. The CoolBot is a temperature controller that allows

bypass of the electrical limitations of standard units so that a system can reach temperatures that

are typically only achievable with refrigerators. The unit uses multiple temperature sensors, a

heating element, and a microcontroller that directs the air conditioner’s compressor to operate in

a such a way as to cool to 1°C without stalling its operation.

When the CoolBot begins to lose efficiency when the air conditioner is close to 0°C, it is

programmed to shut off the compressor, thus minimizing wasted energy.

The main drawback of this solution is that it requires an active energy input that must come from

the grid or a renewable source. Based off of data obtained for the grid availability at the location


of PFPCL in Rajasthan [9], we calculate that the air conditioner/CoolBot combination must

operate in an environment that can hold the temperature below our upper technical requirement

of 15°C for up to four hours.

4.2 Passive Cooling - Structure and Insulation Design

We investigated several different types of structures and insulating materials that could enhance

the passive cooling of our cold-storage unit. Choosing a structure and insulating material that

increases the thermal resistance, R-value, of the storage unit decreases the amount of heat

entering the cold-storage unit. A high R-value also decreases the energy required to maintain the

storage unit temperature within the required temperature range of 10-15°C. Furthermore, a

higher R-value increases the response time of the inside storage unit temperature to ambient

temperature. Therefore, a storage unit with a higher R-value can withstand longer electrical grid

failures before the temperature inside the storage unit exceeds our maximum temperature of

15°C.

4.2.1 Igloo Structure

The first structure that we considered was that of an igloo. Igloos take advantage of natural

convection to create a thermal gradient inside of the igloo structure that could be used for storing

produce at various temperatures. Shown in Figure 5 below, colder air enters the bottom of the

igloo and warmer air exits from the top.

Figure 5: Sketch of potential igloo geometry for cold storage chamber.

Additionally, natural materials such as dirt, straw, and cow manure could be used to build the

structure, reducing building costs. However, we were concerned that building materials could

present sanitary issues if cow manure were in contact with fruit. Furthermore, we needed a

storage unit that could be quickly disassembled and transported to other harvesting locations,

whereas an igloo structure could not be moved.


4.2.2 Underground Structure

Next, we considered a conventional box-shaped structure and examined different methods for

increasing the insulative properties of the structure. One technique would be to place the

structure underground. Utilizing the ground as a natural insulator could increase the R-value of

the storage space without adding significant building material costs. Underground units also take

advantage of the ground’s naturally cooler temperature than air. We considered building a cellar,

or using an already existing basement and placing our storage unit inside.

However, discussions with the PFPCL during our trip to India made it clear that an underground

storage space would not be feasible. The PFPCL did not want to require workers to carry heavy

crates of produce up and down stairs. Additionally, our partners thought that flooding from

monsoon rains could threaten a structure located underground. We also learned that the up-front

cost for building a basement would be well-beyond our budget. Furthermore, the equipment

needed to dig a large hole would be difficult to transport to the rural villages where our storage

unit would be used.

4.3 Evaporative Cooling

The evaporation of water can be used to cool a space by removing thermal energy from air as the

water changes phase from liquid to gas. Water’s high enthalpy of vaporization makes

evaporative cooling an effective cooling solution [10]. Evaporative cooling is limited by the wet

bulb temperature3 of the air as well as the temperature of the water being evaporated. The wet

bulb air temperature can be taken from a psychrometric chart as seen in Figure 4. Psychrometric

analysis uses the variables of relative humidity and dry bulb temperature4 to calculate the

theoretical wet bulb temperature of a system. We used Figure 4 to determine the wet bulb

temperatures for Rajasthan during guava season (November - January), which are summarized in

Table 2.

3 Wet bulb temperature refers to the temperature of air at 100% relative humidity. Increases in humidity typically

result in a lower temperature. Thus, in an evaporative system, the wet bulb temperature represents the lowest

theoretical temperature that can be achieved through perfect evaporation and 100% efficiency

4 Dry bulb temperature refers to the temperature of air measured by a thermometer freely exposed to the air, but

shielded from radiation and moisture


Figure 6: Psychrometric chart in standard units. The X-axis is dry bulb (DB) temperature. The Y-

axis is humidity ratio. The red lines are lines of constant relative humidity. If the DB temperature

and relative humidity (RH) are known, one can use the diagonal blue lines to calculate the resulting

wet bulb (WB) temperature. An example of this calculation was performed with a DB temperature

of 30°C and RH 47%. We found a WB temperature of approximately 22°C for these inputs.

We can calculate the actual air temperature exiting an evaporative system if the efficiency, 𝜀, of

our system is known from eq. (1) where i refers to the inlet conditions, db refers to dry bulb

temperature and wb refers to wet bulb temperature [11].

𝜀 = (𝑇

𝑖,𝑑𝑏 − 𝑇

𝑒,𝑑𝑏) ÷ (𝑇𝑖,𝑑𝑏 − 𝑇𝑖,𝑤𝑏) (1)

In order to evaluate the potential for evaporative cooling as a solution, we performed these

analyses using the environmental conditions in Rajasthan where PFPCL is located. The result of

these calculations based on climate data for Jaipur, Rajasthan5 are shown in Table 2 as the

theoretical ΔT assuming an efficiency of 𝜀 = 0.8; standard evaporative mediums, such as those

made of high density cellulose, operate with an efficiency between 80% - 90% [12].

Table 2: Air, wet bulb and groundwater temperature data from Jaipur, Rajasthan to calculate the maximum

temperature drop that can be achieved with evaporative cooling. Weather data was obtained from timeanddate.com.

5 Jaipur, Rajasthan is a city 150 km from Sawai Madhopur with available historical weather data.


Month

Avg. Daily High Air

Temp (°C) RH (%)

Wet Bulb

Temp (°C)

Theoretical ΔT

(°C)

November 30.0 47.0 21.8 7.0

December 26.0 51.0 19.0 6.0

January 23.0 56.0 17.1 5.0

February 27.0 46.0 19.1 6.0

Average 26.5 50.0 19.3 6.0

These initial calculations indicated that evaporative cooling could prove to be an excellent

passive cooling solution. Evaporative cooling systems can be separated into two categories:

direct evaporative cooling (DEC) and indirect evaporative cooling (IEC). DECs introduce

moisture into the inlet air stream, cooling the air stream adiabatically [13]. IECs lower the inlet

ambient air temperature at a constant absolute humidity level via sensible cooling6 [14].

4.3.1 Direct Evaporative Cooling—Matki Pots

Matki pots are traditional Indian clay pots used to cool drinking water, seen in Figure 5. Matki

pots use direct evaporation to cool the water held inside. As water percolates through the porous

clay walls of the pot, heat energy is drawn out of the water reservoir through increasing entropy.

We considered Matki pots as a way to supplement a cooling system by pre-cooling the water

used for evaporative cooling. To test this we filled a Matki pot with water entering at 23℃ to

match the outdoor air temperature at the time of the test, and monitored the water temperature in

comparison to the ambient air temperature; however, we found the cooling process to be very

slow and only able to provide a temperature drop of 3-5℃ below the surrounding air

temperature.7 We ultimately chose not to incorporate Matki pots into our final cooling solution to

reduce the complexity of our design and the human involvement required for its operation.

6 The sensible cooling of air is the process in which only the sensible heat of the air is removed so as to reduce its

temperature; there is no change in the moisture content of the air. During the sensible cooling process, the dry bulb 7 See Appendix E for test plans and results.


Figure 7: Matki Pot that can be used to lower water temperature

4.3.2 Direct Evaporative Cooling—Zero Energy Cooling Chamber

Zero Energy Cooling Chambers (ZECC), see in Figure 8, are a form of direct evaporative

cooling used around the world to cool produce in an inexpensive and sustainable way [15].

ZECCs store roughly 200 kg of produce and consists of a shaded double-walled box with an

internal space to store produce and a gap between the inner and outer walls containing a sand-

water mixture. Walls are made of a porous material to allow water to percolate from the sand-

water mixture outside of the chamber. As the water evaporates through the walls, the inner

chamber is cooled due to the heat energy of the air being converted to latent heat of the

evaporated water [15]. The unit is kept in the shade to reduce the amount of water evaporated by

the sun’s energy rather than the produce inside the chamber. Depending on the weather

conditions where these units are located, ZECCs have been shown to decrease and maintain the

temperature inside the cooling chamber up to 10-15℃ below the ambient air temperature [15].

We considered this method further in Section 5.1.


Figure 8: Typical ZECC with double-layered brick and sand walls, shaded from the sun by a straw

roof.

4.3.3 Direct Evaporative Cooling—Swamp Cooling

Swamp cooling (see Figure 9), is a form of Direct Evaporative Cooling that uses a fan to pass air

through a membrane saturated with water in order to cool the ambient air. As air passes through

the membrane, the water in the membrane evaporates and the resulting cool air is then circulated

throughout the surrounding environment. Although swamp cooling requires more energy to run

than a completely passive evaporative cooling system, it requires far less energy than cooling

options such as air conditioning and refrigeration [13]. DEC systems have been implemented in

temperate and dry climates similar to the climate in the region in Rajasthan where PFPCL is

located and have shown the ability to produce a temperature drop of 10℃ [13]. As a result, we

proceeded with preliminary experimentation in Section 5.2.


Figure 9: A typical swamp cooling setup with warmer, drier air flowing being pulled by a fan

through a cooling pad moistened by a water distribution system [10].

4.4 Mist Cooling

Mist cooling is similar to evaporative cooling, but with several important advantages. It involves

forcing water through a tiny nozzle which produces ultra fine droplets that rapidly evaporate as

seen on Figure 10. The evaporation absorbs heat and cools the surrounding air. In mist cooling,

water exiting the nozzle is at a lower temperature than the water at the intake. This is due to mass

and energy conservation principles. Since water does not accumulate in the water pipe, the mass

flow rate through the nozzle has to be the same as the mass flow rate at the inlet as described by

equation (2) below.

𝜌𝑉𝑖𝑛𝑙𝑒𝑡𝐴𝑖𝑛𝑙𝑒𝑡 = 𝑛 ∗ 𝜌𝑉𝑛𝑜𝑧𝑧𝑙𝑒𝐴𝑛𝑜𝑧𝑧𝑙𝑒 (2)

The area of the nozzle 𝐴𝑛𝑜𝑧𝑧𝑙𝑒is very small compared to the area of the inlet 𝐴𝑖𝑛𝑙𝑒𝑡 . Assuming

that the fluid is incompressible, 𝜌 remains constant. Therefore, from mass conservation 𝑉𝑛𝑜𝑧𝑧𝑙𝑒is

greater than 𝑉𝑛𝑜𝑧𝑧𝑙𝑒 by a factor of 𝐴𝑖𝑛𝑙𝑒𝑡

𝑛∗𝐴𝑛𝑜𝑧𝑧𝑙𝑒 where 𝑛is the number of nozzles. The increase in

velocity comes increases the kinetic energy since 𝐾𝐸 = 1/2 ∗ 𝑚𝑉2. Energy conservation states

that energy cannot be created from nothing but can be transferred from one form to another.

Therefore, this increase in KE comes from the decrease in thermal energy of the water. The

decrease in thermal energy lowers the temperature of water at the nozzle exit. Since evaporative


cooling is limited by the temperature of the fluid evaporating, the lower temperature theoretically

allows more cooling through mist cooling. To further examine mist cooling, we developed a

small prototype as explained in Section 5.3.

Figure 10: A typical mist cooling setup. Water from a hose is sped up and evaporated

through the nozzles.

5. Preliminary Cooling Experiments

From our initial research, Zero Energy Cooling Chambers (ZECC), swamp cooling and mist

cooling were the most promising low-energy solutions; we then ran a series of experiments to

understand which of these methods would be best to implement for our final product.

From a consultation with Professor Hai Wang, we learned that our previous understanding of

evaporative cooling has an additional performance limitation other than the wet bulb

temperature, which we previously thought was the primary relevant factor. Theoretically, the wet

bulb temperature is the lowest temperature that can be achieved by means of evaporative

cooling. However, in reality, the temperature of the water reservoir used is an additional

limitation of the cooling capacity of these systems. If the temperature of the available water is

higher than the wet bulb temperature, the maximum cooling potential of the evaporative system

is limited to the difference between the ambient air temperature and the water temperature; this is

shown in Table 3 as ‘Actual ΔT’. We found that the groundwater available in Rajasthan is always

hotter than the wet bulb temperature of the air for months of the guava season. Table 3 below

represents the temperature drop when accounting for groundwater temperature.

Table 3: Updated maximum temperature drop to account for groundwater temperature limitation. Air, wet bulb and

groundwater temperature data from Jaipur, Rajasthan to calculate the maximum temperature drop that can be

achieved with evaporative cooling. Weather data was obtained from timeanddate.com


Month

Avg. Daily

High Air

Temp (°C) RH (%)

Wet Bulb

Temp (°C)

Groundwater

Temp (°C)8

Theoretical

ΔT (°C)

Actual ΔT

(°C)

November 30.0 47 21.8 26 7 4.0

December 26.0 51 19.0 26 6 0.0

January 23.0 56 17.1 26 5 -3.0

February 27.0 46 19.1 26 6 1.0

Average 26.5 50 19.3 26 6 0.5

To test the impact of this additional variable against ZECC, we performed calculations on the

temperature drop possible when accounting for groundwater temperature in the following

section. To test the impact of this additional variable against evaporative cooling, we prepared an

experiment in a controlled environment and ran tests with various water reservoir temperatures

below or above the calculated wet bulb temperature of our control environment.

5.1 Zero Energy Cooling Chambers

As mentioned earlier in Section 4.3.2, ZECCs based on psychrometric analysis can decrease and

maintain the temperature inside the cooling chamber up to 10-15℃. However, when groundwater

data is accounted for in the calculations, we predict that these units would only have a maximum

cooling ability of only 4℃ and an average cooling ability of only 0.5℃ during the Sawai

Madhopur guava season from November to January. While ZECCs are an excellent solution to

the challenge of green energy, this level of cooling did not seem like a worthwhile pursuit to

meet our technical requirements of cooling guava to a range of 10-15℃.

8 Groundwater temp shown is the average temperature taken from 13 sites throughout the year. While data for

specific months was not available, the minimum temperature logged was 25°C and the maximum temperature

logged was 27°C so the temperature does not vary much throughout the year.


5.2 Swamp Cooling

The setup of the swamp cooling experiment was as follows. We used a space heater to heat a

room to 30.0℃. In this experiment, we were not able to achieve steady state at 30.0℃ because of

the room ventilation so the actual ambient temperatures were between 27-28℃ for the duration

of our test as seen in Figure 11 below. We also used a sous-vide to maintain the water reservoir

temperature at approximately 21.2℃ which is higher than the calculated room wet bulb

temperature of 20.0℃. We used an insulated duct to contain our chamber and a simple

evaporative medium from Home Depot. We measured temperature inside of the closed duct and

the ambient temperature of the room using wireless sensors. Based on the theoretical calculations

of the maximum ΔT explained in Section 4.3, we expected a temperature differential of 10℃

below the ambient temperature. However, based on the principle presented in Table 3 that states

that the actual maximum ΔT is limited to the differential between the reservoir and the wet bulb

temperature, we would expect to see a ΔT of just 1.2℃. The experimental data presented in

Figure 9 confirms this constraint of the water reservoir temperature. Over the span of 2 hours,

we saw a temperature differential of approximately 1.0℃. As a result, we deemed DEC to be

insufficient for our purposes as hypothesized in Table 3.


Figure 11: Data from swamp cooling experiment in semi-controlled environment. Results show a

temperature differential of about 1.0℃

5.3 Mist Cooling

The setup of the swamp cooling experiment was as follows. The water reservoir used was from a

backyard hose at pressure 50 psi and 17℃. Water is drawn through nozzles then sprayed into an

enclosed upper chamber where air is introduced via a fan. The mist is sprayed onto an aluminum

sheet which serves as a heat exchanger. The insulated lower chamber is the volume that

represents the storage container where guavas would hypothetically be placed. Ambient

temperature and the lower chamber temperature were recorded. A diagram of the prototype can

be seen in Figure 12.

Figure 12: Diagram of mist cooling prototype

The results from our experiment are shown in Figure 13. Over the span of one hour, a

temperature differential of 2.5°C is attained. In addition, while this is not shown on the graph,

relative humidity remained constant at 96%. This factor was important because it shows that mist

cooling does not rely purely on adiabatic heat transfer via introduction of moisture into the air in

the same way that DEC swamp cooling does. We hypothesized that with an increase in water

reservoir pressure we may see a larger change in kinetic energy and greater ΔT. These results

seemed promising to us so we selected mist cooling as a semi-passive solution to prototype

further.


Figure 13: Temperature in the lower chamber of misting prototype vs time. Results from misting experiment show a

temperature differential of 2.5°C

6. Preliminary Structure Design Research

Through the course of our research and user interviews, we considered several different cold-

storage structures and insulation materials. Our primary goal was to find a low-cost structure that

could be easily assembled without requiring significant technical knowledge. As for insulation,

we aimed to find rigid insulation that inexpensive, easy to handle, water resistant, locally

available, and has a high R-value.

6.1 Retrofitting an Existing Building

One structural concept that members of the PFPCL conceived was to retrofit an existing building

in a village with insulation and a cooling system to become a cold-storage unit. We had not

considered using an existing building before travelling to India, given that repurposing an

existing building could displace residents and space used for storing goods already. Although

retrofitting an existing building would avoid most construction costs, we realized during our trip

that the variability between building layouts was too significant for us to design a practical

solution at Stanford that we could easily transfer to India. Members of the PFPCL also wanted

the option to use the room for other purposes during non-harvest times, which could require the

insulation and cooling system to be removed and then reinstalled with each harvest season. We

considered reinstalling insulation and a cooling system every year too costly and labor-intensive

to be a feasible option.


6.2 Prefabricated Insulated Structure

We also considered purchasing a prefabricated insulated structure from a manufacturer in India.

Buying a prefabricated structure would avoid the costs associated with constructing a new

building, and many prefabricated structures can be quickly assembled or disassembled without

requiring significant technical skills. However, we found that the manufacturing costs for a

prefabricated insulated structure exceeded our budget of $3000.

6.3 Retrofitting a Prefabricated Storage Structure

We chose to purchase a prefabricated storage structure, and retrofit the structure with insulation

and a cooling system. Prefabricated storage structures at our required storage capacity of 4m3 are

easily found for under $500 both in India and the United States. Purchasing a storage unit

without insulation also allows us to choose an R-value that balances up-front material costs with

operating costs. In addition, prefabricated storage units can often be disassembled, allowing the

users

6.4 Insulation Materials

While researching suitable structure designs for the cold-storage unit, we also considered

different types of insulation materials. Some of the design considerations we took into account

while researching insulation were price per square foot, R-value, water resistance, structural

integrity, and ease of handling. Table 4 below lists various insulating materials that we

considered along with the materials’ price per square foot and associated thermal resistance, R-

value.

Table 4: Insulating materials with associated price per square foot and R-value [16].

Type of Insulation Description Price/sq. ft. R-value/in

Polyurethane Spray/rigid

Foam

$2.50 6.50

Polyisocyanurate Spray/rigid

foam

$1.07 6.00


Expanded Polystyrene

(EPS)

Rigid Foam $0.15 3.85

Extruded Polystyrene

(Styrofoam)

Rigid Foam $0.70 5.00

Expanded polystyrene (EPS), known in India as Thermocol©, is the cheapest insulation per

square foot. One drawback of EPS insulation is that it breaks apart easily during handling and

cutting. Insulating a storage unit at a large scale with EPS panels would be the cost-effective, but

would also likely result in the most waste and maintenance since the insulation panels degrade

easily.

Polyisocyanurate and extruded polystyrene foams are both commonly used to insulate buildings,

have good structural rigidity, water resistance, and are easy to handle [16]. Both panels are also

easy to cut into custom sizes with a saw or knife, which will be required when insulating a

storage unit. Choosing between polyisocyanurate and extruded polystyrene insulation will

require comparing local cost and availability of each insulating material in India.

7. Thermal Model

In order to determine the energy requirements of our system, we created a thermal model to

estimate the various sources of heat into and out of the system. By creating variables in the

model that represent different design parameters such as system size, insulation R-value, mass of

produce, initial temperature of produce, ambient temperature, and system internal temperature,

we were able to individually vary each one to aid in determining the final product specifications.

Our model utilized Newton’s cooling equation, and assumed 2-D heat transfer into and out of the

system through the walls. Through this modeling, we determined the size of air conditioner

required for our final system to be 10,000 BTUs and settled on insulation with an R-value of

26.2.


7.1 Energy and Power Requirements to Cool Guavas

We devised a thermal model in order to analyze the theoretical required cooling capacity of our

cooling methods. Our aim was to use the model results to determine the minimum cooling

requirements that our desired methods needed to meet. First we wanted to know the energy

requirements of the system so we could determine the power required to cool the system. The

maximum heat energy flowing into the system at any point was modeled as a combination of

sensible heat in the mass of guavas, all 2 tonnes introduced to the system at once, and the heat

leaking into the system as due to the different in temperature between the surroundings and the

inside of the cooling chamber. This is shown in equation (3)

𝐸𝑐𝑜𝑜𝑙𝑖𝑛𝑔 = 𝐸𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒 + 𝐸𝑠𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔 (3)

In equation (3) above, 𝐸𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒represents the thermal input from the mass of guavas. Its initial

temperature is assumed to be at25°𝐶. It needs be cooled to a desired final temperature of

15°𝐶.This final temperature is the steady-state temperature that our system should be able to cool

guavas to as defined in our technical requirements on Section 2.2. 𝐸𝑠𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔represents the

thermal energy leaking in because the surroundings are at a higher temperature that the cooling

chamber. For a given surface area, this heat influx is a product of the temperature difference

between the cooling chamber and the outside, and the insulation capacity of the walls. The initial

temperature of the surroundings used in our model was set to 30°𝐶. The interior temperature was

varied at 7°𝐶,10°𝐶and14°𝐶.The insulation was set to an R-value of 25 because this is the

common R value of refrigeration equipment.

From this setup, the energy required to remove the sensible heat energy 𝐸𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒was calculated

from equation (4)

𝐸𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒 = 𝑚 ∗ 𝐶𝑝 ∗ 𝛥𝑇 (4)

From the equation above, 𝑚is the mass of the guavas, 𝐶𝑝is the specific heat capacity of the

guavas and 𝛥𝑇 is the temperature difference between the initial temperature of the guavas and

the desired final temperature of the guavas. In our case 𝛥𝑇 = 10°𝐶.


In order to determine the heat influx from the surrounding 𝐸𝑠𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔 ,we used the thermal

conductivity equation (5) to find the rate of heat transfer 𝑄 between the cold chamber and

warmer surroundings.

𝑄 = (𝐴𝑠𝑢𝑟𝑓𝑎𝑐𝑒/𝑅 ) ∗ 𝛥𝑇 (5)

In this case, 𝛥𝑇is the difference in temperature between the surroundings and the cooler

chamber. This value depends on the temperature at which we maintain the cooling chamber.

From the heat transfer rate, 𝐸𝑠𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔becomes

𝐸𝑠𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔 = 𝑄 ∗ 𝑡 (6)

where 𝑡 is the time that elapses. Since we are focused on the energy required to cool the guavas

from their initial temperature to the desired temperature, 𝑡 is the same as the time it takes to

achieve this. Using Newton’s law of cooling and lumped capacitance assumption for a single

guava, the cooling curve of the guava could be defined as

𝑇 − 𝑇𝑖𝑛𝑓 = (𝑇𝑖 − 𝑇𝑖𝑛𝑓) ∗ 𝑒𝜏𝑡 (7)

Equation (7) shows that we could find the time it takes to cool down a guava from a given initial

temperature 𝑇𝑖to a desired temperature 𝑇when it is exposed to an ambient temperature 𝑇𝑖𝑛𝑓.The

missing parameter was the time constant 𝜏which we decided to find empirically find.

To find 𝜏performed cooling experiments with a single guava, a crate of guavas, and a crate of

apples in a refrigerator maintained at a temperature of 7°𝐶.We used apples because they have

similar thermal characteristics as guavas but are cheaper and readily available at our location.

During the experiments we took measurements of time and temperature. For the crate

experiments, the temperature was measured at two points, an interior fruit and a surface fruit.

From the data we performed an exponential fit on each run and used it to determine the time

constant 𝜏for each case. This can be seen on Figure 14. To stay on the conservative side, we


characterized the time constant of the guavas as the smallest time constant from all the

experiments to get 𝜏 = −0.003355 𝑝𝑒𝑟 𝑚𝑖𝑛.On Figure 14, this is the time constant from the

exponential fit of the surface apple data.

Figure 14: Produce temperature as a function of time. We see that there is minimal difference between the interior

and surface cooling rates.

From the above results, we could figure out the total energy 𝐸𝑐𝑜𝑜𝑙𝑖𝑛𝑔needed to cool the guavas at

different cooler chambers. Knowing the time 𝑡 we could figure out the power required by our

cooling system as shown on Figure 15.


Figure 15: Power decreases as chamber temperature temperature increases.

This means maintaining the cooling chamber temperature at 7°𝐶takes the shortest time to cool

the guavas to the desired temperature but also requires the most cooling power at 5.7 kW.

Maintaining the cooling chamber at 14°𝐶takes the longest time to cool the guavas to the desired

temperature but requires the least cooling power at 2.2 kW.

From the model above we performed a sensitivity analysis to determine the power required to

maintain the guavas given 1m/s of insulation space. This gave us a sense of how to scale the

power required to maintain the temperature in the cooler for a given thermal resistance value.

During guava harvesting months, the optimal temperature for storage between the interior

temperature of the cold storage unit and the exterior surroundings yields a ΔT =15C. At R = 25,

the average heat flux into our system is 3 W/m^2. Using that value and total surface area of our

cold storage unit, we find that that is an optimal value to operate below the maximum operating

costs that our user is willing to incur.


Figure 16: Sensitivity analysis shows heat leak per m2 as a function of temperature differential and R

8. Proposed Cold Storage System

Assessing our thermal model, technical requirements, and the ethical dimensions of our proposed

final product was necessary before beginning the design and actual build of the cold storage

system. Based on those assessments, our proposed cold storage system combines an active

cooling element involving a CoolBot and AC unit for active cooling, a misting system to provide

passive cooling, and a low-cost insulated structure to serve as the cold storage unit.

8.1 Cold-Storage System Structure and Insulation

The design goal for the structure and insulation of our proposed cold-storage unit was to create a

durable, easy-to-assemble unit with a R-value of 25-30 that could hold up to two tons of guava

and maintain a temperature of 15C or lower during electrical grid failures of up to four hours.

We aimed to keep the cost of our initial prototype under $3000 to meet the requirement of the

PFPCL.


For our prototype structure, we chose to retrofit a storage trailer lent to us by the Stanford

O’Donohue Family Farm. A picture of the storage trailer before and after construction can be

seen in Figure 17 below.

Figure 17: A before and after photo of the retrofitted cold-storage trailer. The air conditioner unit can be seen

protruding from the front of the trailer in the right-hand photo.

The completed cold-storage trailer took approximately 4 ½ weeks to construct, and required

approximately 225 hours of labor divided among three team members dedicated to building the

trailer. We used a 10K BTU air conditioner and powered the AC through the electrical grid. For

insulation, we used two layers of R-13.1 Rmax© polyisocyanurate insulation on the walls and

ceiling, and one layer of the same insulation on the floor and large drop down door on the back

of the trailer. A complete description of our building process can be found Appendix I, and our

bill of materials is in Appendix F. The specifications of our completed cold-storage trailer can be

seen in Table 5 below. We have listed our desired specifications for our storage unit in India for

comparison.

Table 5: Specifications of our cold-storage trailer prototype and our future structure in India.

Specification Cold-Storage Trailer Future Prototype in India

Volume 7.86 m3 8.00 m3

Produce Capacity 670 kg 2000 kg

Insulation R-Value 20 25-30

Air Conditioner Rating 3.40 kW 2.93 kW


Although the volume of our cold-storage trailer was close to the volume of our ideal storage size

for 2000 kg of guava, the weight rating on the suspension of the trailer limited the storage

capacity to 670 kg.

8.1.1 Time to Cool the Cold-Storage Trailer

After completing construction of the cold-storage trailer, we conducted a test to determine the

time required to cool the inside of the trailer from 25°C to the required minimum temperature of

10°C. The results of the experiment can be seen in Figure 18 below.

Figure 18: The plot shows the temperature outside and inside the cold-storage trailer during a test in which the

trailer was cooled from 25°C to 10°C using the AC unit with no produce in the trailer. The inside trailer temperature

decreased below 15°C in approximately 0.3 hours and reached 10°C in 1 hour.

The temperature inside of the trailer decreased from 25°C to 15°C in approximately 0.3 hours,

and reached 10°C after 1 hour of cooling. During the test, the AC unit consumed 0.57 kWh of

energy. The cyclic heating and cooling beginning after 0.4 hours of test time is a result of the AC

unit turning on and off.

8.1.2 Time to Reheat the Cold-Storage Trailer


After determining the amount of time and energy required to reduce the temperature of the trailer

from 25°C to 10°C, we turned the AC unit off and let the trailer warm back up. We sought to

determine how long the temperature inside the storage unit stayed below our maximum operating

temperature of 15°C. In Figure 19 below, the temperatures inside and outside of the trailer

shown as a function of time.

Figure 19: The plot shows the temperature inside and outside of the cold-storage trailer while the trailer is reheating

from a starting temperature of 10°C to a final temperature of approximately 16°C. The temperature inside the trailer

required approximately 1.75 hours to exceed the maximum operating temperature of 15°C when there was no

produce stored in the trailer.

The temperature inside the trailer required approximately 1.75 hours to exceed our maximum

cold-storage temperature of 15°C when no produce was stored inside the trailer. Over the entire

test time of approximately two hours, the inside of the trailer reached a temperature of

approximately 16°C.

8.2 Cooling

To meet our system’s cooling requirements with the lowest amount of energy input possible, our

cooling design couples a semi-passive mist cooler with an active air conditioning unit and a

CoolBot. Based on our preliminary cooling experiments described in Section 5, mist cooling was


the most promising low energy solution. Based on our research of active cooling methods

described in Section 4.1, the air conditioning unit-puls-CoolBot combination was the most

promising active energy solution.

8.2.1 System Description

Our active cooling system consists of an air conditioner, shown in Figure 20 below and a

CoolBot, shown in Figure 21. Both the air conditioner and the CoolBot require electrical input

which is satisfied by the grid.

Figure 20: Labeled photo of the back of the trailer showing the air conditioner, water inlet, air inlet

and energy monitor.


Figure 21: Labeled photo of the inside of the trailer showing the CoolBot, heat exchanger and

drain.

The body of our misting system is made of four aluminum air ducts that are held together with

hose clamps and HVAC tape and hung from the ceiling of the cooling unit with steel chains, as

seen in Figure 20 and Figure 21 below. The aluminum air ducts allow heat to be transferred out

of the chamber to the water inside the ducts which is then removed from the system. To prevent

water from pooling, the ducts are hung at a slight incline. Since the air ducts are not meant to

hold water, all leaky areas are sealed with silicone caulk as shown in Figure 21.


Figure 22: Labeled close up photo of the heat exchanger highlighting the sealant, nozzles and

HVAC tape.

Our misting system consists of 16 nozzles (two are highlighted in Figure 22) which are fed by a

garden hose that supplies a pressure of 45-50 psi. The water input to the system is shown above

in Figure 20. Air is pushed through the misting system by a fan that requires an electrical input

that is satisfied by the grid. The air inlet to the system is shown above in Figure 20.

8.2.2 Methods

We ran four cooling tests, described in Table 6 below, each with 100 kg of apples inside the

trailer to determine the cooling ability of each system with the additional thermal load from

produce. As mentioned previously in Section 7, we chose to use apples rather than guavas

because they have similar thermal properties and are significantly less expensive at our local

supermarkets.

Table 6: Description of large-scale cooling system tests.

Experiment Objective Length of

Test*

Misting system on with

100kg apples.

1. Identify cooling potential for misting system.

2. Identify power requirements of misting system.

2.5 hours

Air conditioner and CoolBot

on with 100kg apples.

1. Identify time required to cool apples.

2. Identify power requirements of air conditioner and

CoolBot.

24 hours

Misting system, air

conditioner and CoolBot on

with 100kg apples.

1. Compare power use with test for air conditioner

alone.

2. Compare rate of apple cooling with test for air

conditioner alone.

4 hours

Apples starting at 10º C, No 1. Identify time for apples to heat to upper bound of 5 hours


cooling** optimal temperature range

2. Determine the need for an off-grid cooling solution

*Tests involving the misting system are shorter because we chose not to run the misting system without supervision.

We ran into issues with the heat exchanger and nozzles leaking during our construction process and wanted to

ensure we could stop the test early if the trailer flooded.

**Data displayed and discussed in section 8.3.

A photo of our experimental set up is shown in Figure 23 below. For each test, the temperature

inside trailer and of the apples started at 25°C to simulate the conditions during guava season in

India. When the outside air conditions were below 25°C, we used a space heater to heat the

trailer to 25°C. Throughout each test we tracked temperature and relative humidity inside of and

outside of the trailer, as well as the internal temperature of three apples: one at the bottom of the

pile, one midway through the pile and one at the top of the pile. We used a power meter to track

energy use throughout each test; however, the power meter was only able to track cumulative

energy use. To get an approximate power throughout each test we divided the total energy use

throughout the test by the length of the test. This approximation does not account for the fact that

the air conditioner likely uses more power to drop the temperature initially than it does to

maintain the temperature in the trailer.


Figure 23: Photo of our testing set up inside the trailer with 100 kg of apples, three temperature

probes inside apples (one apple at the bottom of the pile, one in the middle and one on the surface)

and two temperature and relative humidity sensors to measure the air conditions inside and outside

of the trailer.

8.2.3 Analysis and Discussion

The first test as described on Table 6 above was with the misting system alone. The results of

this test can be seen below in Figure 24. Both the inside of the trailer and the apples cooled at a

similar slow rate. The overall temperature drop of the apples was 2°𝐶 over the 2.5 hours that the

test was conducted. The energy meter showed that the fan consumed 0.9 kWh of energy. This

corresponded to 0.36 kW of power as the fan was running non-stop for the whole 2.5 hour

duration. The small temperature drop showed that the misting system was insufficient on its own

as the primary cooling method.

Figure 24: Misting system experiment results: Temperature vs Time. The average drop in temperature of the apples

is 2°𝐶for a duration of 2.5 hours. The results show that the misting system is not sufficient on its own as a primary

cooling method.

Our second test was with the air conditioner and Coolbot. The Coolbot was set to maintain the

inside of the trailer at10°𝐶.This can be observed as the wavy temperature fluctuation on the plot

since the Coolbot works by turning the air conditioner off when the room gets to the desired

temperature and turning it on when the room drops below the desired temperature. The results


are shown on Figure 25. As expected, the exterior apple cooled much faster than the interior

apple. Although the test was run for 24 hours, we were interested in the duration it took the

exterior apple to get down to15°𝐶, which was 8 hours. Over this period, the energy meter

showed that the air conditioner consumed 2 kWh of energy. This corresponded to 0.25 kW of

power draw. Although this is not the most accurate way to convert the energy consumed to the

power required ss explained in section 8.2.2, it gave us an average metric to compare it to the

misting system. The results show that the air conditioner and Coolbot draw less power that the

misting system but with better performance.

Figure 25: Air conditioner and Coolbot experiment results: Temperature vs Time. It took 8 hours to cool the

exterior apple down to15°𝐶when the inside of the trailer was maintained at10°𝐶. The results show that the power

draw of this setup is less than that of the misting system alone.

Given that our misting system experiment showed a cooling potential albeit small, we wanted to

test if it would reduce the energy consumed by the air conditioner and Coolbot. To test this, we

ran the two cooling methods simultaneously and the results are seen on Figure 26. We ran the

test for 4 hours only because we could not leave the misting system running without supervision.

4 hours were not enough to cool the apples down to our desired temperature of 15°𝐶but they

were enough to give us enough data to compare the performance of the air conditioner and

Coolbot alone against when they were supplemented with the misting system.


Figure 26: Air conditioner + Coolbot and misting system experiment results: Temperature vs Time. The experiment

duration was 4 hours because we could not leave the misting system running without supervision.

In order to compare the two cooling systems, we truncated the data from the air conditioner and

Coolbot experiment down to the first 4 hours. The results of the comparison are shown on Table

7 below. The apples cool down faster over the 4 hour period when the air conditioner is

supplemented with the misting system than when the air conditioner is running alone. However,

the combined method draws more power. This is because the fan stays on for the 4 hour duration

as opposed to the AC which swaps back and forth between being on and off.

Table 7: Air conditioner + Coolbot and misting system experiment results: Temperature vs Time. The experiment

duration was 4 hours because we could not leave the misting system running without supervision.


Experiment AC AC + Mist

Avg ΔT of apples

over 4 hours 2.4°C 4.6°C

Avg. Power 0.25kW 0.49kW

The take away from these experiments is that swamp cooling is not sufficient as the primary

cooling system on its own. Nonetheless, it has the potential to supplement the air conditioner

especially in a case where the power draw from the fan is optimized to meet the cooling needs.

8.3 Energy Source and Storage

The primary source of energy used to cool our system is electrical power drawn from the local

grid in Rajasthan, India. Our system operates on the commercial grid,which provides electricity

at a cost of $0.14 per kilowatt-hour, and is accessible 94.2% of the time according to data

collected between May 13th, 2018 and May 20th, 2018. During this week, the longest blackout

lasted for 100 minutes, and most lasted only 10-30 minutes. Despite the short nature of these

blackouts, due to the relatively unpredictable nature of the grid’s “down times,” in conjunction

with the variation of grid reliability from region to region, a purely grid reliant system would be

insufficient to provide the necessary level of reliability to protect the farmers’ crops from system

failure.

Our goal was to design a cooling system that both minimized operational costs, and

simultaneously did not fully rely upon grid-electricity. In order to accomplish this goal, we

investigated various alternative methods of energy production and storage, and assessed the

deployability and costs of each. Table 8 shows this research, and demonstrates that no alternative

energy source currently falls into the budget set by our partners in India. For this reason, we

investigated the ability of our insulation to maintain produce (apples used as substitute) within

the desired temperature range with cooling sources turned off. Figure 27 shows the rate of

temperature increase of 100 kg apples, measured from the interior, midway between the interior


and exterior, and from the exterior of the stack of apples. From a starting temperature of 10º C,

the exterior apples, which heated up the fastest, took 4.37 hours to heat to 15º C, the upper bound

of our optimal temperature range. Using this data, and that from our electrical grid sensor in

India, we found that our system had a factor of safety of 2.5, comparing the longest blackout to

the length of time that all of the produce stayed within the optimal range without a cooling

source. Having performed this experiment using only a fraction of the fruit that would typically

be housed in the system, we hypothesize that this factor of safety would increase, as a larger

quantity of fruit would provide a larger thermal mass, and thus slow the heating process.

Table 8: Costs and deployability of alternative energy production and storage methods.

Energy Storage Method Upfront Costs* Operating Costs* Deployability*

Ice $0 $23/day Very Difficult

Batteries + Solar $3000 $0 Support Infrastructure

In Place

Fuel $350 $17/day Support Infrastructure

In Place

Gravitational Potential

(of pumped water)

Estimated Cost Too

High To Build

Water Tower

Cost of routine water

pumping Very Difficult

*For a system with a 2-ton storage capacity

*Red text indicates a prohibiting factor


Figure 27: Rate of apple temperature increase after turning off A/C.

8.4 Cost

Building the trailer at Stanford not only demonstrated the feasibility of our prototype, but

also the kinds of materials that we would need to build a structure of this kind and their

subsequent prices. From our insulated trailer unit that is volumetrically capable of housing two

tons of guavas, however, structurally incapable of holding said weight, we can scale up the

amount of materials required using the ratios of different design parameters between the

prototype and the final solution.

The table below shows a breakdown of our most significant costs and a comparison of

the total costs that each unit would be. The table does not include the cost of tools that were

purchased or additional construction materials. The unit would be significantly cheaper in India,

as evidenced by our examination of material prices in the United States and in India, and is

subsequently well within the upfront budget outlined by the producer company. For a more

comprehensive cost breakdown of building materials at Stanford and in India, please reference

Appendices F and I.

Table 9: Cost comparison of trailer building materials and translation to India


Part of trailer Cost at Stanford Cost in India

Structure $4,399

(Donated to project) $380

Insulation $443 $498

AC $369 $420

CoolBot $349

(Donated to project) $349

Misting System $200 $75

Total (main components of

system)

$5,760

(We spent $1,012) $1722

Working off of a budget outlined by our partners in India of $2.96/ton of storage/day, we

determined

From our thermal model, we determined that were our system to house the entire 2 tons of

produce, as it would in India, our air conditioner would have to be upgraded to a 12,000 BTU

model. Using the EER value of 12 advertised by the manufacturer of our air conditioner, we

determined the amount of power required to run it to be roughly 24 kWh/day.

EER = 12

12,000 / 12 = 1 kW = Required Power For Air Conditioner

1 kW * 24 hr = 24 kWh / day


Using the cost of electricity per kWh as reported by our partners in India of $.14/kWh, we

determined the estimated cost to operate a 12,000 BTU air conditioner to be $3.36/day. For 2

tons and a desired operational cost of $2.96/ton/day, the total daily operational cost of $3.36 is

well below this requirement of $5.92. This allows for significant leeway with regards to energy

requirements, and introduces a safety factor for power inefficiencies.

9. FMEA Summary

Looking at our overall design and structure, there are several key subsystems of our design that

require additional risk mitigation. To summarize, there are three overarching modes of failure

that are critical to the system’s performance: thermal leaks in our structure that could reduce the

efficiency of our cooling unit, water leaks from our misting system that could degrade the

produce stored inside, and the assembly/disassembly process of our structure. A comprehensive

Failure Modes and Effects Analysis on our cold storage unit can be found in Appendix D.

9.1 Door Gasket

The function of the door gasket is to seal the interior of the storage unit by sealing the gap

between the door and the wall. This prevents environmental damage due to dust or rain and

insulates the interior. The door gasket could fail due to clogging or damage from use. Mitigating

this requires regular maintenance and cleaning.


Figure 28: A door gasket protects the interior of the trailer when the door is closed.

9.2 Drain

Installing a floor drain is necessary for regular upkeep and hygiene inside the trailer. To prevent

thermal leaks, we covered the drain with insulation when the drain is not in use.


Figure 29: Floor Drain from misting system used in trailer covered in HVAC tape.

9.3 Misting System Nozzles

Our misting system uses highly pressurized flow for passive cooling inside our storage unit.

Water leaving the nozzles produces a strong opposite force that could detach the nozzle from the

heat exchanger and flood the interior of the storage unit. Unless properly fastened onto the heat

exchanger, we risk damaging the produce stored inside. Mitigating this requires using sealant to

prevent potential leaks from the nozzles and the use of permanent adhesive or mechanical

fasteners to lock it in place.


Figure 30: Nozzles need to be securely fastened into heat exchanger to avoid flooding the trailer.

9.4 Misting System Joints

Since our current heat exchanger uses air ducts instead of water ducts, the joints of the system

were not designed to seal water. To avoid leaks onto the produce and the interior of the trailer,

the joints were sealed using caulk and tape.

Figure 31: Misting System joints were sealed using caulk and tape.

9.5 Insulation

The function of the insulation is to prevent the entry of heat from the environment into the

interior of the cold storage unit. There are three primary failure modes that could reduce the

thermal resistance of the insulation: water, gaps between pieces of insulation, and damage to the

insulation. Water, because it is a good conductor, reduces the R-value of our insulation because it

settles into the insulation’s air gaps. Wet insulation therefore reduces the efficiency of our

cooling system because it makes the insulation more conductive to heat coming from the

surroundings.

The door gasket, drain, and misting system were all designed to mitigate produce loss and

protect the insulation from getting wet. However, there is a risk of insulation getting wet due to

water entering the insulation through gaps between insulation panels. To ensure that the gaps


between insulation panels are sufficiently waterproof and thermally sealed, sealant and HVAC

tape was used to cover the gaps.

Figure 32: Gaps in insulation were filled with spray insulation, then taped over using metal HVAC tape.

Physical damage to the insulation, whether by puncturing or reducing the thickness of the panel,

will also reduce the thermal resistance. Proper care and assembly instructions have been

communicated in the trailer build instructions attached in Appendix I.


10. Conclusion

Throughout the past two quarters, we have researched and prototyped a wide array of

cooling methods representing both passive processes, such as evaporative cooling, and active

processes, such as air conditioning. Our final prototype is comprised of an air conditioner linked

to a CoolBot device, along with a misting apparatus built into a series of aluminum ducts. This

solution meets our user and technical requirements outlined in section 1 of this paper, and will be

donated to the O’Donohue Family Stanford Educational Farm for use as a flower and produce

cold storage system. While our misting system required significant amount of electrical energy in

order to power the fan, and thus, was not “passive”, we are optimistic that this method of cooling

has potential as a supplemental cooling process to reduce the energy needs of a primary cooling

source such as an air conditioner.

As a component of our partnership with the Reliance Foundation and the Parth Farmers’

Producer Company Ltd., we have generated a set of building instructions to be used by producer

companies in the Rajasthan region of India for the construction of low-cost, low energy cold

storage systems for produce.

Figure 33: Snapshots of our journey through our exploration, prototyping, and building over the past six months.


11. Future Work

Though our misting system shows promise, we have yet to fully develop and parametrize our

understanding of the underlying physical principles that govern it. There are a number of

experiments and variables that should be explored. First, we would like to further explore the

performance of the pump at different pressures to optimize cooling performance. Second, we

would conduct more experiments with different heat exchanger geometries to improve the

performance and reliability of the misting system. Third, we would run a test of the misting

system without the fan to run a cost-benefit analysis of the added power consumption compared

to temperature drop acquired.

In regards to our current body of work, our next steps would be to gather user feedback on the

proposed cold-storage unit and the build guide. We would like to further assess feasibility of

deployment such as obstacles in translating our technical specifications to India and the product-

context fit, run field tests using a structure built using the build guide, and use design-thinking

techniques to gather user feedback regarding performance and ease of assembly. One

performance metric we can use to assess success of the build guide would be to measure the

experimental R-value of a structure in India compared to the one built on O’Donohue Family

Farm.

Moving forward, we will continue to integrate user feedback, collaborate with the Reliance

Foundation in India, and explore further deployment opportunities.


12. References

[1] 2014, Institution of Mechanical Engineers (IME) in the UK

[2] Qazi, Moin. “India's Failed Food System.” Asianage, The Asian Age, 27 Dec. 2017,

www.asianage.com/india/all-india/271217/indias-failed-food-system.html.

[3] Mahajan, B.V.c & Sharma, Sajeev & Dhall, Rajinder Kumar. (2009). Optimization of

storage temperature for maintaining quality of guava. Journal of Food Science and

Technology. 46. 604-605.

[4] “Large Color Map of India.” MapsofIndia.com, 11 June 2018,

www.mapsofindia.com/maps/india/large-color.html.

[5] “Rajasthan District Map.” MapsofIndia.com, 11 June 2018,

https://store.mapsofindia.com/digital-maps/country-maps-1-2-3/india/districts/rajasthan-district-

map.

[6] ME Capstone India Cold Storage Briefing Document (2018).

[7] Tiwari, S. (2006). Chilling injury as an indicator of critical temperature for cold storage of

guava (Psidium guajava L.) cv. Allahabad Safeda. InderScience Publishers,1(2). Retrieved from

http://www.inderscience.com/info/inarticle.php?artid=11659

[8] “How Much Can Your Air Conditioner Cool: Realistic Temperature Differences Between

Inside and Outside Air.” WM Henderson Inc., WM Henderson Inc., 13 Apr. 2017.

[9] Collected "2018-05-13 22:09:27" - 2018-05-20 20:31:33 from inside a home in Sawai

Modhopur via Michael Machala

[10] Petrucci, Ralph H., William S. Harwood, F. G. Herring, and Jeffry D. Madura. General

Chemistry: Principles & Modern Applications. 9th ed. Upper Saddle River, NJ: Pearson Prentice

Hall, 2007. 474.

[11] Abdel-Fadeel, Waleed & A. Hassanein, Soubhi. (2012). Calculations of the outlet air

conditions in the direct evaporative cooler. Journal of engineering sciences, Assiut university.

Vol. 40. pp. 1351-1358.

[12] “Evaporative Cooler.” Wikipedia, Wikimedia Foundation, 10 June 2018,

en.wikipedia.org/wiki/Evaporative_cooler.

http://www.mapsofindia.com/maps/india/large-color.html

https://store.mapsofindia.com/digital-maps/country-maps-1-2-3/india/districts/rajasthan-district-map

https://store.mapsofindia.com/digital-maps/country-maps-1-2-3/india/districts/rajasthan-district-map


[13] Datta, S, and P N Sahgal. “Design and Operating Characteristics of Evaporative Cooling

Systems.” Egyptian Journal of Medical Human Genetics, Elsevier, 26 Feb. 2003,

www.sciencedirect.com/science/article/pii/0140700787900533.

[14] Heidarine, Ghassem. “Experimental Investigation of Two-Stage Indirect/Direct Evaporative

Cooling System in Various Climatic Conditions.” Egyptian Journal of Medical Human Genetics,

Elsevier, 6 Mar. 2009, www.sciencedirect.com/science/article/pii/S0360132309000511.

[15] Basediya, A. L., Samuel, D. V., & Beera, V. (2011). Evaporative cooling system for storage

of fruits and vegetables - a review. Journal of Food Science and Technology,50(3), 429-442.

doi:10.1007/s13197-011-0311-6 Available:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3602570/

http://www.sciencedirect.com/science/article/pii/0140700787900533

http://www.sciencedirect.com/science/article/pii/S0360132309000511

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3602570/


13. Appendices

Appendix A: Team Members, Roles and Responsibilities

Abdullah AlSharhan, Thermal Modeling, Financial Officer

Abdullah is an undergraduate senior studying Mechanical

Engineering. Born and raised in Kuwait, Abdullah is passionate

about using engineering skills for impact and change.

Sydney Lance, Financial Officer

Sydney is a Biomechanical Engineering major from Alamo,

California. She has a strong passion for sustainability and design in

relation to engineering.

Henry Magun, Thermal Modeling

Henry is an undergraduate senior and coterminal master’s student

studying Mechanical Engineering. He grew up in Westchester, NY,

and has a strong passion for design and manufacturing, with a

specific love for woodworking.

Kathleen Miller, Structure and Insulation

Kathleen is an undergraduate senior studying Mechanical

Engineering. She is passionate about mechanical design and cultural

compatibility.


Chase Milligan, Structure and Insulation Team Lead

Chase is an undergraduate senior studying Mechanical Engineering.

He is interested in mechanical design, manufacturing, and

sustainable development in underserved communities.

Ufuoma Ovienmhada, Cooling Prototyping, Notetaker

Ufuoma is an undergraduate senior studying Mechanical

Engineering.

Moses Swai, Thermal Modeling

Moses is a senior and coterminal master’s student from Tanzania. He

is studying mechanical engineering and is interested in design,

mechatronics and automobiles with a deep passion for soccer.

Alexandra Warner, Project Manager

Alexandra is a senior and coterminal master’s student studying

mechanical engineering. She has strong interests in mechanical

design, healthy living and the environment. In her free time you can

find her outside biking, hiking or walking her dog.


Appendix B: Gantt Chart

B-1 ECO Guava Winter Quarter


B-2 GuavO! Winter Quarter

Task # Task Name Duration Week 5 Week 6 Week 7 Week 8 Week 9

Week

10

Finals

week

1

Build and evaluate first

prototype

1.1 Research cooling solutions 2 days

1.2 Choose 3 design concepts 1 day

steps 1.3 - 1.11 will be iterated over

for each design concept

1.3

Craft experiment to test

design concept 1 day

1.4

Identify experiment metrics

of success 1 day

1.5 Design testing process 1 day

1.6

Make theoretical

predictions 2 days

1.7 Make sketches 2 days

1.8 Develop and source BOM 5 days

1.9 Build prototype 2 days

1.10 Collect Data

depends

on 1.4

1.11

Analyze Data against

metrics of success 1 day

2

Create Week 7 Design

Review 4 days

Delegate Sections of

Design Review 2 days

Consult with Professors

about our Results 5 days

Build Coolant Prototype 1 day

Collect and Analyze Data 1 day

Identify succesful

prototype using data

analysis 1 day

Build Succesful Prototype 1 day

Prepare Final Design

Review 1 week

B-3 ECOGuavO! Spring Quarter

Week 11 12 13 14 15 16 17 18 19 20


Thermal Model

Meet with Hai Wong

Single Guava Test

Crate of Guava Test

Crate of Apple Test

Active Cooling Solution

Cost Benefit Analysis to Choose an AC

Order AC Unit

Install AC Unit

Swamp Cooling

Revamp Swamp Cooler

Swamp Cooler Test

Matki Pot Test

Mist Cooling

Small Scale Test

Plan Large Scale Mister

Purchase Materials for Large Scale Mister

Build and Install Large Scale Mister

Structure and Insulation

Insulation Research

Plan Trailer Retrofit

Purchase Materials for Trailer Retrofit

Trailer Retrofit

Flower Cooler Guide

Installation Guide for India

Testing

R-value Insulation Test

AC no Apples

Mist no Apples

AC with Apples

Mist with Apples

Mist + AC with Apples

Off-grid Solution

Cost Benefit Analysis

Deliverables

Design Review 4

Design Review 5

Final Presentations

Final Report

Appendix C: FMEA

1. Door Gasket

Functions Potential Failure Mode(s) Potential Causes


dFM

EA

line

item Component

Item /

Function

Potential

Failure

Mode(s)

Potential

Effect(s)

of Failure

S

e

v

e

r

it

y

Potential

Cause(s)/

Mechanism(s) of

Failure

P

r

o

b

a

b

il

it

y

Current Design

Controls

D

e

t

R

P

N Crit

d1 Door Gasket

Seals space

between

door and

cold

storage unit

Gasket

separates from

door

Thermal

leak created 4

Adhesive between

door and gasket

cannot withstand

extreme

temperatures,

dust, rain 3

Users inspect

gasket monthly 2 24 12

d2 Door Gasket

Seals space

between

door and

cold

storage unit

Gasket

separates from

door

Rainwater

enters

storage unit 4

Adhesive between

door and gasket

cannot withstand

extreme

temperatures,

dust, rain 3

Users inspect

gasket monthly 2 24

d3 Door Gasket

Seals space

between

door and

cold

storage unit

Gasket

material

degrades over

time

Thermal

leak created 4

Repeated thermal

expansion of

gasket causes

tears in the

material 4

Users inspect


d4 Door Gasket

Seals space

between

door and

cold

storage unit

Gasket clogs

or is worn

down due to

weather

elements

Airtight seal

around door

breaks,

creating

thermal leak

and hygiene

concerns 4

Gasket clogs or

malfunctions due

to exposure due to

weather elements 4

Users inspect



2. Drain


dFM

EA

line

item

Compone

nt

Item /

Function

Potential

Failure

Mode(s)

Potential

Effect(s)

of Failure Sev

Potential

Cause(s)/

Mechanis

m(s) of

Failure

Pr

ob

Current Design

Controls

D

e

t

R

P

N

Cri

t

d1 Drain

Drains

water out

of the

system

Allowing

dust, smog,

pollution, and

heat into the

system

Damage and

degradation to

fruit/produce

while it is in

storage 4

Drain

allows

direct

passage of

dust,

smog, and

pollution

into the

system 2

Add drain cover

and reduce

surface area of

holes 2 16 8

d2 Drain

Drains

water out

of the

system

Improper

assembly of

drain into

ground or

storage unit

Damage and

degradation to

fruit/produce

while it is in

storage 2

Unintende

d leaks

around

the drain

but not

into the

drain 3

Farmer and

builder

experience 2 12 6

d3

Drain

Cover

Blocks

Drain and

insulates

system

Drain Cover

is a thermal

leak due to

conductive

material

choice

We lose thermal

efficiency in our

system 1

Material

choice of

drain

cover 1

Use insulative

material or

design drain

ditch to that its

covered with

insulation 2 2 1

d4

Drain

Cover

Blocks

Drain and

insulates

system

Drain Cover

is a thermal

leak because

it allows air

into the

system

We lose thermal

efficiency in our

system 2

Drain

allows

external

air into

the

system 3

Liberal

Manufacturing

tolerances 2 12 6

3. Misting System


dFME

A line

item

Componen

t

Item /

Function

Potential

Failure

Mode(s)

Potential

Effect(s)

of Failure Sev

Potential

Cause(s)/

Mechanism(s)

of Failure

P

r

o

b

Current

Design

Controls Det RPN Crit


d1

Misting

Nozzles

Cools

water

Detaches

from

connecto

rs

Floods

trailer 10

Not fastened

well-enough 5

Tape and

mechanical

fasteners 2 100 50

d2

Misting

Joints

Cools

and

directs

water

down the

drain

Leaking

water

inside the

produce

Slowly

floods

trailer 7

Joints are not

properly

sealed 5

Sealant,

caulk, and

tape 2 70 35

4. Insulation Panels


dFM

EA

line

item

Componen

t Item / Function

Potential

Failure

Mode(s)

Potential Effect(s)

of Failure

S

ev

Potential Cause(s)/

Mechanism(s) of

Failure

P

r

o

b

Current Design

Controls

D

e

t RPN

Cri

t

d1

Insulation

Panels

Reduce heat

transfer from

ambient air to

inside cold

storage unit

Cracks in

protective

plastic

panel

covers

Because heat leaks

into the system,

more power is

required to cool

unit to desired

temperature 4

Outer plastic walls of

panels damaged

during handling 3

Users inspect panel

covers for cracks 3 36 12

d2

Insulation

Panels

Reduce heat

transfer from

ambient air to

inside cold

storage unit

Cracks in

protective

plastic

panel

covers

Produce lost

because

temperature inside

unit becomes too

high 4

Outer plastic walls of

panels damaged

during handling 3

Users inspect panel

covers for cracks 3 36 12

d3

Insulation

Panels

Reduce heat

transfer from

ambient air to

inside cold

storage unit

Extreme

temperatur

es

More power

required to cool

system to desired

temperature,

increasing

operating cost 4

R-value not high

enough due to too

small of insulation

thickness or wrong

material choice 4

Users monitor

ambient

temperatures 3 48 16

d4

Insulation

Panels

Reduce heat

transfer from

ambient air to

inside cold

storage unit

Extreme

temperatur

es

Produce lost

because

temperature inside

unit is too high 4

R-value not high

enough due to too

small of insulation

thickness or wrong

material choice 4

Users monitor

ambient

temperatures 3 48 16

d5

Insulation

Panels

Reduce heat

transfer from

ambient air to

inside cold

storage unit

Power goes

off for

longer than

expected

time

Produce lost

because

temperature inside

unit becomes too

high after several

hours 4

R-value of insulation

is not sufficient 5

Users switch to

thermal/battery

storage when grid

power unavailable 4 80 20


d6

Insulation

Panels

Serve as walls

and roof of

storage unit

Users

assemble/d

isassemble

structure

improperly

Storage unit cannot

be assembled again

without structural

instability

1

0

Mechanisms that

interlock panels break

during assembly,

handling,

transportation 3

Users inspect

locking mechanism

before assembly 2 60 30

d7

Insulation

Panels

Reduce heat

transfer from

ambient air to

inside cold

storage unit

Panel

joints

sealed

improperly

during

assembly

More power

required to cool

system to desired

temperature,

increasing

operating cost.

Produce lost

because

temperature inside

unit is too high 4

Improper assembly,

loose tolerances

between panels 4

Users inspect seals

between panels for

gaps 3 48 16

d8

Insulation

Panels

Reduce heat

transfer from

ambient air to

inside cold

storage unit

Panel

joints

sealed

improperly

during

assembly

Produce lost

because

temperature inside

unit is too high 4

Improper assembly,

loose tolerances

between panels 4

Users inspect seals

between panels for

gaps 3 48 16

5. Temperature Regulators


dFM

EA

line

item

Compone

nt

Item /

Function

Potential

Failure

Mode(s)

Potential Effect(s)

of Failure

Se

v

Potential

Cause(s)/

Mechanis

m(s) of

Failure

P

r

o

b

Current

Design

Controls Det RPN Crit

d1

Temperat

ure

Regulator

Measure

Temperat

ure

Sensor

Failure

Active cooling turned on

for too long, or

prematurely turned off.

Temperature is no longer

measured or improperly

controlled. 5

Soldering

failure 3

Include lost

connection

indicator light 3 45 15

d2

Temperat

ure

Regulator

Measure

Temperat

ure

Sensor

Failure


for too long, or




controlled. 5

Wire

failure 2

Include lost

connection


d3

Temperat

ure

Regulator

Measure

Temperat

ure

Sensor

Failure


for too long, or




controlled. 5

Water

damage 5

Seal housing

with caulk 4 100 25


d4

Temperat

ure

Regulator

Turn

Cooling

On/Off

Switch

Failure


for too long, or




controlled. 5

Soldering

failure 3

Include lost

connection


d5

Temperat

ure

Regulator

Turn

Cooling

On/Off

Sensor

Failure


for too long, or




controlled. 5

Faulty

sensor 4 Factory QA 3 60 20

d6

Air

Condition

er Cooling

Coolant

Leak

Cooling failure, food

contamination 10

Manufact

uring 1

Sealed Air

Conditioner

housing and

frequent

maintenance 2 20 10

d7

Temperat

ure

Regulator Cooling

Sensor

Damage

Exposure to weather

elements such as dust,

rain, wind, and/or high

humidity 10

Extreme

weather

conditions 1

Proper

placement of

sensor 2 20 10

d8

Temperat

ure

Regulator

Turn

Cooling

On/Off

Sensor

miscalibra

tion

Sensor malfunctions over

lifetime 5

Power

outages 2

Frequent

recalibration of

sensors 2 20 10

Appendix D: Matki Pot Test

We tracked the temperature of water inside a matki pot and the outdoor air temperature for five

hours and observed a maximum temperature difference of 3.9°C.


Appendix E: Trailer BOM

Purchase

Date Item

Unit

Cost Quantity Total Cost Source

4/29/18

Insulation Panels--Rmax

Thermasheath-3 2 in. x 4 ft. x

8 ft. R-13.1 Polyisocyanurate

Rigid Foam Insulation Board $31.67 14 $443.38 Home Depot

4/29/18

Liquid Nails Heavy Duty

Adhesive $2.57 4 $10.28 Home Depot

4/29/18 10 oz Dripless Caulk Gun $6.57 1 $6.57 Home Depot

4/29/18

HDX N95 Nonvalve

Respirator 3Pk $5.47 1 $5.47 Home Depot

4/29/18

Husky Quick Release Utility

Knife $7.98 2 $15.96 Home Depot

4/29/18

16 oz. Big Gap Filler

Insulating Foam Sealant

Quick Stop Straw $5.25 3 $15.75 Home Depot

4/29/18

16 oz. Gaps and Cracks

Insulating Foam Sealant with

Quick Stop Straw $4.25 3 $12.75 Home Depot

4/29/18

1.89 in. x 50 yd. 322 Multi-

Purpose HVAC Foil Tape $8.27 5 $41.35 Home Depot

4/29/18

Sales Tax from 4/29/18 Home

Depot Trip $54.90 Home Depot

Universal Light-Duty Air

Conditioner Support $30.97 1 $30.97 Home Depot

HDX Blue Nitrile Gloves 50

Pk. $7.98 1 $7.98 Home Depot

https://www.homedepot.com/p/Thermasheath-Rmax-Thermasheath-3-2-in-x-4-ft-x-8-ft-R-13-1-Polyisocyanurate-Rigid-Foam-Insulation-Board-613010/100573703




https://www.homedepot.com/p/Liquid-Nails-10-oz-Extreme-Heavy-Duty-Adhesive-LN-907/205089262


https://www.homedepot.com/p/HDX-10-oz-Dripless-Caulk-Gun-HD109D/203788459

https://www.homedepot.com/p/HDX-N95-Disposable-Respirator-Small-Blister-3-Pack-H950S/205360109


https://www.homedepot.com/p/HDX-Retractable-Utility-Knife-60037/202051990


https://www.homedepot.com/p/GREAT-STUFF-16-oz-Big-Gap-Filler-Insulating-Foam-Sealant-Quick-Stop-Straw-99053938/207050533?MERCH=REC-_-PIPHorizontal2_rr-_-204352574-_-207050533-_-N



https://www.homedepot.com/p/GREAT-STUFF-16-oz-Gaps-and-Cracks-Insulating-Foam-Sealant-with-Quick-Stop-Straw-99053937/206977048



https://www.homedepot.com/p/Nashua-Tape-1-89-in-x-50-yd-322-Multi-Purpose-HVAC-Foil-Tape-1207792/100030120


https://www.homedepot.com/p/AC-Safe-Universal-Light-Duty-Air-Conditioner-Support-AC-080/100091666


https://www.homedepot.com/p/HDX-Disposable-Nitrile-Gloves-50-Count-00290-50LBD14/202196087



A/C Pan Tablets 6 Pk $3.48 1 $3.48 Home Depot

1-1/2" White Weatherization

Foam Seal $4.96 3 $14.88 Home Depot



5/7/18

LG 10,000 BTU Window AC

Smart Wi-Fi w/ 3 YR

Protection Plan $369.00 1 $369.00 Home Depot

5/7/18



5/3/18 8 Pc. Paint Tray Set $11.51 1 $11.51 Home Depot

5/3/18

Ryobi 18V 1.3AH Battery &

Charger $49.97 1 $49.97 Home Depot

5/3/18

2 YR Warranty for Ryobi

Battery & Charger $7.00 1 $7.00 Home Depot

5/3/18 Ryobi 18V Jig Saw $59.97 1 $59.97 Home Depot

5/3/18

2 YR Warranty for Ryobi

18V Jig Saw $12.00 1 $12.00 Home Depot

5/3/18 3-Pack Paint Roller Cover $9.17 1 $9.17 Home Depot

5/3/18

#8 x 3" Self-Drilling Drywall

Screws $6.28 3 $18.84 Home Depot

5/3/18

#8 x 2" Exterior Phillips Head

Screws (1lb) $8.47 1 $8.47 Home Depot

5/3/18

#10 x 3-1/2" Exterior Phillips

head Screws (1lb) $8.47 1 $8.47 Home Depot

5/3/18 Scotchblue Painter's Tape $6.91 2 $13.82 Home Depot

5/3/18



https://www.homedepot.com/p/AC-Safe-Air-Conditioner-Pan-Tablets-6-Pack-AC-912/100665383

https://www.homedepot.com/p/AC-Safe-Weather-Seal-AC-401/100664956?keyword=Weatherization+foam+seal&semanticToken=22040+++%3E++++st%3A%7Bweatherization+foam+seal%7D%3Ast++cn%3A%7B0%3A0%7D++weatherization+foam+seal+%7Brest%7D+


https://www.homedepot.com/p/LG-Electronics-10-000-BTU-Window-Smart-Wi-Fi-Air-Conditioner-with-Remote-ENERGY-STAR-LW1017ERSM/300422896



https://www.homedepot.com/p/8-Piece-Roller-Tray-Set-RS-701-SP/100064287

https://www.homedepot.com/p/Ryobi-18-Volt-ONE-Lithium-Ion-Battery-Pack-1-3Ah-and-Dual-Chemistry-IntelliPort-Charger-Upgrade-Kit-P128/203466924


https://www.homedepot.com/p/Ryobi-18-Volt-ONE-Orbital-Jig-Saw-Tool-Only-P523/204824014

https://www.homedepot.com/p/9-in-x-3-8-in-High-Density-Polyester-Roller-Cover-3-Pack-RS-1433/100090787

https://www.homedepot.com/p/Everbilt-8-x-2-3-8-in-Self-Drilling-Drywall-Screw-1-lb-Box-128-Pack-116014/205142912


https://www.homedepot.com/p/Grip-Rite-8-x-2-in-Phillips-Bugle-Head-Coarse-Thread-Sharp-Point-Polymer-Coated-Exterior-Screws-1-lb-Pack-PTN2S1/100197689


https://www.homedepot.com/p/Grip-Rite-10-x-3-1-2-in-Philips-Bugle-Head-Coarse-Thread-Sharp-Point-Polymer-Coated-Exterior-Screw-PTN312S1/100161193


https://www.homedepot.com/p/3M-1-88-in-x-60-yds-Original-Multi-Use-Painter-s-Tape-2090-48N/100550611

https://www.homedepot.com/p/Liquid-Nails-10-oz-Heavy-Duty-Construction-Adhesive-LN-903/100176209

https://www.homedepot.com/p/Liquid-Nails-10-oz-Heavy-Duty-Construction-Adhesive-LN-903/100176209


5/3/18

Bosch 3"x24TPI T-Shank Jig

Saw Blade $6.98 1 $6.98 Home Depot

5/3/18 Behr Premium Solid Stain $37.98 1 $37.98 Home Depot

5/3/18 Paintcare Fee $0.75 1 $0.75 Home Depot

5/8/18



5/8/18 2'x4' Lumber $3.30 2 $6.60 Home Depot

5/8/18 4'x8'x3/4" CDX Plywood $34.38 4 $137.52 Home Depot

5/8/18 CA Lumber Fee $1.42 Home Depot

5/8/18



5/8/18 Plastic Cap Roof Nails $7.99 1 $7.99

Orchard Supply

Hardware

5/8/18

Sales Tax from 5/8/18

Orchard Supply Hardware $0.72

Orchard Supply

Hardware

Chase Trips to Home Depot $3.41 6 $20.46

14.2 mi/trip; $3.85/gal

gas on 5/8/18; truck gets

16 mi/gal

5/19/18

Milwaukee Bi-Metal Hole

Saw 13-Piece Kit $69.97 1 $69.97 Home Depot

5/20/18

Vinyl Foam/Aluminum Duct

Insulation $19.71 1 $19.71 Home Depot

5/21/18

Milwaukee 4-1/2" Bi-Metal

Hole saw $34.97 0 $0.00 Home Depot

5/22/18

Nashua Exterior Weather Foil

Tape $22.87 1 $22.87 Home Depot

5/23/18 ABS-PVC Transition Cement $5.40 1 $5.40 Home Depot

5/24/18

ABS General Purpose Drain

Strain $6.13 1 $6.13 Home Depot

5/25/18 3"x2' ABS Pipe $6.94 1 $6.94 Home Depot

https://www.homedepot.com/p/Bosch-3-in-24-Tooth-Per-Inch-Steel-T-Shank-Jig-Saw-Blade-Set-for-Cutting-Metal-5-Pack-T118A/100531668


https://www.homedepot.com/p/BEHR-Premium-1-gal-SC-533-Cedar-Naturaltone-Solid-Color-Waterproofing-Stain-Sealer-5053301/202263942

https://www.homedepot.com/p/2-in-x-4-in-x-92-1-4-in-Prime-Green-Douglas-Fir-Stud-603503/100030742

https://www.homedepot.com/p/23-32-in-x-4-ft-x-8-ft-Fir-Sheathing-Plywood-Actual-0-688-in-x-48-in-x-96-in-439614/100034683

http://www.osh.com/Osh-Categories/Tools-%26-Hardware/Building-Materials/Fasteners/Nails/Nails%2C-Packaged/Grip-Rite-Round-Plastic-Cap-Roofing-Nail%2C-%2312-x-7-8-Inch%2C-Steel%2C-112-Count-lb/p/1867613

https://www.homedepot.com/p/Milwaukee-Hole-Dozer-General-Purpose-Bi-Metal-Hole-Saw-Set-13-Piece-49-22-4025/202327772


https://www.homedepot.com/p/Milwaukee-4-1-2-in-Hole-Dozer-Hole-Saw-49-56-9649/202327764


https://www.homedepot.com/p/Nashua-Tape-2-83-in-x-50-yd-330X-Extreme-Weather-HVAC-Foil-Tape-1207801/100507541

https://www.homedepot.com/p/Nashua-Tape-2-83-in-x-50-yd-330X-Extreme-Weather-HVAC-Foil-Tape-1207801/100507541


5/17/18

18-Volt ONE+ 1/2in.

Hammer Drill(Tool Only) $69.00 0 $0.00 Home Depot

5/17/18

#10 x 3-1/2" Phillips Bugle-

Head Coarse Thread Sharp

Point Polymer Coated

Exterior Screw $8.47 3 $25.41 Home Depot

5/17/18

6in. x 25 ft. Insulated Flexible

Duct R6 Silver Jacket $28.84 1 $28.84 Home Depot

5/22/18

Plymetal Teks 12x2-3/4, 40

pcs $8.48 0 $0.00 Home Depot

5/22/18

Sales Tax 5/22/18 Home

Depot Trip $0.78 1 $0.78 Home Depot

5/26/18

HDX Blue Nitrile Disp Glove

100 PK $14.98 1 $14.98 Home Depot

5/26/18

2"x4'x8' Poly Iso RMatte

R13.1 Insulation $31.67 6 $190.02 Home Depot

5/26/18

1.89"x50yd 322 Aluminum

Foil Tape $8.27 3 $24.81 Home Depot

5/26/18



5/26/18

Sales Tax 5/26/18 Home

Depot Trip $21.97 1 $21.97 Home Depot

Total Cost $2,895.38

https://www.homedepot.com/p/Ryobi-18-Volt-ONE-1-2-in-Hammer-Drill-Tool-Only-P214/206666886

https://www.homedepot.com/p/Ryobi-18-Volt-ONE-1-2-in-Hammer-Drill-Tool-Only-P214/206666886

https://www.homedepot.com/p/Grip-Rite-10-x-3-1-2-in-Philips-Bugle-Head-Coarse-Thread-Sharp-Point-Polymer-Coated-Exterior-Screw-PTN312S1/100161193?keyword=3-1%2F2%22+phillips+exterior+screws&semanticToken=21040+++%3E++++st%3A%7B3-1%2F2%22+phillips+exterior+screws%7D%3Ast++cn%3A%7B3%3A1%7D++phillips+%7Bbrand%7D+3-1%2F2%22+exterior+screw+%7Brest%7D+




https://www.homedepot.com/p/Master-Flow-6-in-x-25-ft-Insulated-Flexible-Duct-R6-Silver-Jacket-F6IFD6X300/100396935

https://www.homedepot.com/p/Master-Flow-6-in-x-25-ft-Insulated-Flexible-Duct-R6-Silver-Jacket-F6IFD6X300/100396935


Appendix F: Summary of Expenses and Budget

The largest aspect of our budget went to building our final prototype, the insulated trailer with

the misting system.

Winter Quarter Team Purchases:


Total: $688.31

Spring quarter Team purchases:


Total: $3,395.75

Total for 2 quarters: $4,084.06

Appendix G: User and Technical Requirements

Table G-1: Performance-related engineering requirements

Req. Priority Engineering

Requirements

Justification

ER1-1 HIGH Have internal volume 4

m3.

PFPCL has asked for a minimum of 2 tons worth of storage

space for guavas. Each half ton of guavas takes up roughly 1

m3 which translates to an internal volume of 4 m3.

ER2-1 HIGH Maintain a temperature

of 10-15 °C.

PFPCL expressed the need to store guavas for 1-4 days in

order to aggregate a sufficient quantity to fill a five ton

truck. Storing guavas at 10 °C -15 °C delays ripening for 11

days [6].

ER2-2 MEDIUM Internal temperature of

storage unit cannot drop

Guavas quality deteriorates when stored at temperatures

below 6 °C. This includes surface pitting, brown streaks,


below 6 °C. bland taste. [1]

ER3-1 MEDIUM Maintains internal space

at 90-95% relative

humidity.

Optimal storage humidity for guavas [1]

ER4-1 HIGH Cooling unit can operate

at different settings

based on produce stored

inside.

Temperature control is necessary to maintain produce

quality, slow ripening processes, and avoid sogginess or

water loss. Farmers should be able to store tomatoes, green

chilies and onions in addition to guavas to make use of

system during the off season for guavas.

ER5-1 LOW System has a form of

ventilation or absorption

for ethylene to keep

interior ethylene levels

below 100 ppm.

Buildup of ethylene released by the fruit causes it to ripen

faster. Preventing this ethylene buildup can keep guavas

fresh longer. 100 ppm studied ethylene levels determined by

Reyes and Paull, 1995 [6].

ER6-1 HIGH Have an upfront cost

less than $3000.

Upfront material and installation cost to the user must be

less than $3000 for the entire unit [3]. This value is based on

PFPCL’s stated price requirement for the system.

ER7-1 HIGH Designed for easy

assembly and

disassembly

Assembly process can reduce cost and provide added

operational flexibility to farmers.

ER8-1 LOW Insulation and cooling

unit should be chosen

from local brands.

In order to localize economic benefits and ensure that all

components of our system can be repaired or replaced using

local resources. This will also ensure lower shipping and

overhead costs.

ER9-1 MED Add features for easy

water drainage out of

storage unit, such as an

open door or slightly

inclined ground.

In order to make cleaning the system easy. Cleaning will

likely be done with large amount of water.

Table G-2: Operational engineering requirements


Req. Priority

Engineering

Requirements Justification

ER10-1 HIGH Function at daytime

temperatures of 25°C

to 30°C ∓ 5 °C.

Daytime temperatures in Rajasthan, India during guava

harvesting season (November - February) [4] typically range

between 25°C and 30°C [7].

ER10-2 MED Function at maximum

daytime temperature of

50°C.

Our system should function throughout the entire year so

that farmers can use it to store other produce during the off-

season for guavas.

ER11-1 HIGH Have an operational

cost less than

$2.96/ton/day

This value is based on PFPCL’s stated price requirement for

the system and includes power consumption and

maintenance.

ER12-1 HIGH Not require constant

grid electricity.

Grid electricity is only available for 12-16 hours each day in

Sawai Madhopur [3]. Specific times of electricity

availability can be unpredictable. Our system should either

use solar energy or utilize passive processes that eliminate

the need for any constant grid-based electrical source.

ER13-1 HIGH System will be

watertight.

System should be resistant to rain and storms.

ER13-2 HIGH Have all components

secured to the ground or

another part of the

system.

System should be resistant to wind and storms.

ER14-1 HIGH Expensive parts are

secured using a lock or

able to be moved

indoors when not in use.

PFPCL should feel confident that no part of their investment

will be stolen. Any cost of stolen components that need to be

replaced would cut away from PFPCL’s profits.

Table G-3: Reliability-related engineering requirements

Req. Priority Engineering

Requirements

Justification


ER15-1 HIGH Use components (AC

units, solar panels,

pumps, and building

materials) that are

standard locally.

This would reduce upfront shipping costs and also ensure

PFPCL would be able to obtain necessary parts quickly

when the system breaks to minimize the time it spends out

of service (and maximize PFPCL’s profits). This will also

localize the economic benefit our system brings to the

community.

ER15-2 HIGH Uses components (such

as pumps) that are

familiar to local repair

workers.

Using local components will ensure nearby repair workers

will be familiar with common maintenance procedures.

ER16-1 HIGH Components of system

should have a minimum

lifetime of 3-5 years.

This value is based on the Reliance Foundation’s estimate

of PFPCL’s needs for the system to ensure PFPCL sees a

return on its investment.


Appendix H: User Manual for Producer Company

ECOGuavo! Insulated Cooling Structure: Building in India

1. Background and Introduction

For the past two quarters, the members of ECOGuavo! have focused on developing a cheap

and energy efficient cold storage system for guavas and other produce for rural farmers in India.

We developed a prototype cold storage solution at the O’Donohue Family Stanford Educational

Farm by retrofitting an aluminum trailer that the farm provided for us. Although we ran

successful tests at the farm, we never had the opportunity to build and scale a prototype in

India. Therefore, we created this build guide with tips and tricks that we learned from our

prototyping experience for those who are going to continue the project in India and hopefully

build a functioning unit there. In this build guide we did not include construction of the misting

system that we tested with, as our data was promising yet not conclusive. During our prototype

construction, we drew heavily upon the insulating guidelines from Store It Cold [1].

2. Proposed Bill of Materials

These materials are for a structure that could contain two tons of guava. For any larger amount

of guava, the quantity of materials should be scaled up following the scaling up suggestions in

section_. We determined the amount of materials needed based on what we bought for the

trailer at the farm which is very similar in size to what this proposed unit would be. All item price

information was taken from indiamart.com and the cost of these items is close to what it would

be in India.

2.1 Building Materials

We recommend that the housing unit of this system should be a waterproof steel shed because

it is a relatively robust, cheap, simply-shaped structure that would be easy to insulate, is

prefabricated, and is already something intended for storage. However, theoretically, any

structure that is the right could fit the appropriate amount of guavas and is relatively robust

could work.

Figure 1: Raw Building Materials

Item Use Quantity Cost

Prefabricated Steel

Garden/Storage

Shed (waterproof;

swinging, non-sliding

doors; flat or

Structure to house cooling system and produce

1 $380


pyramidal roof)

Foil-covered Insulation Panels (R ~13)

To cover the walls and insulate the structure

112 (590mm x 590mm panels)

$498

Liquid Adhesive To stick the panels to the walls and to each other when layering them

18 bottles (1.4 kg) $0.44

HVAC Foil Tape To put over any area that is showing and is not waterproof

9 $33

Spray foam sealant To fill any empty space between insulation panels and spaces that should be insulated

8 $10

Screws (long enough to get through the first layer of insulation)

To screw the wood into the floor and the insulation into the walls of the outer structure. Also to help build any bracket for the Air conditioner

30-40 $4.50

Plywood To put on top of the insulated floor of the structure. Only if the structure is above ground.

12 sq. meters $140*

Waterproof primer To waterproof any wood pieces being used (floor plywood, wooden bracket)

2 cans, 256 oz $60*

Air conditioner Main cooling aspect of the system

1 $420

Air conditioner support *not on IndiaMart

To support a window air conditioner

1 $28

CoolBot To give the air conditioner more cooling power

1 $349


2”x4” wood beam For building additional air conditioner support if needed

1 $5*

Door gasket To prevent thermal leaks from the door

1 $8*

*may not be a necessary purchase TOTAL: $1,795.92

2.2 Tools

Figure 2: Additional tools used in Construction

Item Use Cost (USD)

Jigsaw To cut through the metal of the outer structure, cut wood

$163

Caulk gun To put on the liquid adhesive $1

Boxcutter To cut the insulation $1.50

Tape Measure Measuring $0.75

Power Drill To drill in screws $85

Step Ladder To reach roof of insulated structure/put on roof insulation

$70

TOTAL: $321.25

COMBINED MATERIALS AND TOOLS: $2,117.17

3. Building Instructions

3.1 Scaling and Preliminary Calculations:

3.1.1 Scaling Shed Size:

To get the most accurate size of shed that should be purchased, one must first determine how

many tons of guava will need to be stored in this structure at one time. Assume 18 kg fit into 1

standard crate. Based on the volume of the standard crate and how much guava will be needed,

one can calculate the rough dimensions of how large the shed structure should be. One also

must take into account how much space the insulation will take up on every wall. A standard

crate has dimensions of 0.542 x 0.360 x 0.290 m [2]. Assuming a comfortable height to work

with is 8 crates high, and that this was the desired height of the stacks, the height of the


structure, would need to be about 2.2 meters high at the lowest point of the roof. One should

make a rough map of the desired organization of the stacks of guavas and determine how long

and wide the walkways between the stacks should be.

Example Calculation:

The following calculation is for approximately two tons of guavas.

Number of crates: 2 tons = ~1800 kg/18 kg per crate = ~100 crates

If the desired height of the stacks of crates is 8 crates high:

100/8 = 12-13 stacks of crates

Desired width of the walkway: .6 m

Desired space between crates: 100 mm

Space that insulation will take up: 200 mm on every side

AC unit takes up about .6 x .3m in the interior of the unit

Figure 3: Size of an average crate

Taking into account all of these measurements, the rough volume of this unit would need to be

about 8-9 m3.


Figure 4: Sample Layout of the cooling unit. AC unit is in blue. Not to scale.

Based on the calculation above, one also could gain a very rough estimate of the size that they

would need for the unit by simply multiplying the ratio of the number of guavas compared with 2

tons by the volume of the 2 ton unit.

Example:

If a 5 ton unit is needed

5 𝑡𝑜𝑛𝑠

2 𝑡𝑜𝑛𝑠= 2.5

2.5 x 8 m3 = 20 m3

Figure 5: Rough Sizings of Prefabricated Storage Structure/Shed

Tons of Guava Approximate Shed Size

2 8-9 m3

5 20-22.5 m3

10 40-45 m3

20 80-90 m3

3.1.2 Scaling Air Conditioner:


If the shed is larger than 28 sq. meters, use a mini-split Air Conditioner. Ensure that the air

conditioner used is compatible with the CoolBot using the CoolBot air conditioner knowledge

center [4]. The air conditioner should correspond to the size of the structure, with 220 BTU/hr

needed for each sq. meter. Based on the volume of the shed, one can multiply the volume of

the structure by the 220 BTU/hr to properly size the air conditioner by the BTU/hr rating. When

sizing an air conditioner, one must keep in mind that in areas larger 28 sq. meters, one should

consider using a split rather than a window AC, as if the space was any larger it would not be as

effective [5].

After one calculates the air conditioner specifications necessary for the space, they can also

gain an understanding of how much the air conditioner will cost.

Figure 6: Upfront and Operational Costs of Air Conditioner Sizes

BTU/Hr Upfront Cost (USD) Op Cost (USD/day) If running for 24 hrs/day at $0.14/kWh

8000 239 2.24

12000 369 3.36

15000 445 4.20

18000 559 5.04

25000 600 7.00

3.1.3 Scaling Building Materials:

- Adhesive: Roughly 10z per sq. meter

- Enough insulation boards to cover the entire inside of the shed with a combined R-value

of 26. If the insulation boards have an insulation value of about 13, use two layers of

insulation boards, etc. An R value above 25 is common for refrigerators [2] and is likely

closer to the lower bound of what would make a good insulated unit.

- For the remainder of the materials, one could get a good sense of the amount of

materials needed by multiplying the ratio of the 2 ton unit materials above by the amount

of tons in the desired storage unit.

Example Calculation:

For foam sealant - used 8 bottles on a 2 ton unit


For a 5 ton unit

5 tons/2 tons = 2.5

2.5 x 8 bottles = 20 bottles of foam sealant for a 5 ton system

3.2 Building Instructions:

3.2.1 Assembly

1. Ensure that the metal storage shed/unit that is being used as the housing unit for this system

is waterproof and does not have sliding doors and will be placed somewhere with a power

source. If it is not waterproof, it should be primed with a waterproofing material to prevent

rusting. If the unit has sliding doors, this will prevent the ability to put insulation on the doors and

create a thermal leak.

2. Before beginning anything construction, one should cut any wood that will be used to size.

Cut the 2x4 boards to create a bracket to stabilize the air conditioner on the inside of the shed if

necessary (insert example bracket picture). Then cut enough plywood to cover all of the floors,

leaving little to no extra space on the floor.

Figure 7: Priming the wood

3. Prime all of the wood that will be used throughout the building process, putting on at least two

coats. This is to ensure that no moisture ruins the wood over time. This is also a very time-

intensive process so this should be done as soon as possible.

4. Next, the insulation should all be cut to size using a box cutter or saw, if necessary, in order

to perfectly fit over all of the walls, floor, and roof. If using multiple layers of insulated board,


take into account how the insulation will stick out and cut accordingly if necessary. For example,

to open a swinging door, the second layer of insulation would likely need to be cut to fit when

the door closes, some insulation will need to be cut off of this second layer so that the door can

swing open. Cut the hole in the insulation for the air conditioner later, after the AC panels have

been installed.

5. A space for the air conditioner should then be cut into the side of the shed/storage unit using

the jigsaw. One should leave about 30 mm of extra space on each side of this hole to provide

ample room to put in the air conditioner without difficulty. The bracket for the air conditioner

should also be sized at this time. If necessary, determine the need to build an additional wooden

bracket for the inside of the unit to further support the air conditioner on the inside.

Figures 8,9,& 10 : Air conditioner hole and supports.


6. The insulation should then be glued on using the adhesive. The adhesive should be put on

the walls of the unit using the caulk gun and then the insulation should be stuck on top of it and

held in place, if necessary, as the adhesive dries.

6a. If the structure is slightly above ground, floor insulation is necessary, but if the

structure sits on the ground, it is optional to insulate it. The floor insulation should be put

on last when putting on this first layer, as walking on the floor insulation would damage

it. Primed plywood will need to be drilled on top of this insulation before people walk on it

to prevent damage to this floor insulation. One could choose to out on one or two layers

of floor insulation. If only putting on one layer of floor insulation, using the screws and

hand drill (or a hammer and nails), secure the primed plywood on top of the floor

insulation, ensuring that it covers the entirety of the floor.

Figure 11 : Insulation after it has been screwed in, with sealant between the cracks

7. After the entirety of the adhesive on the first layer of insulation has dried, use the hand drill

and screws to further secure the first layer of insulation to the walls. Ensure that the screws go

through the side of the walls. If the screws do not seem secure, it might be helpful to put a nylon

washer [3] around the screws to ensure a more even force distribution of the screws on the

insulation.

8. Fill any cracks between this insulation and the walls with spray foam sealant. If necessary,

trim the foam sealant after it has dried to make the layer completely flat.

9. After all of the insulation panels have been screwed into the wall and the sealant has dried,

use adhesive once again to stick any additional layers of insulation to the first layer. These do

not need screws.


Figure 12: Using the adhesive. Second layer of insulation [4]

9. Repeat step 8 for every additional layer of insulation.

10. Cut the insulation in the area of the air conditioner unit hole from the outside of the unit.

Then put in the air conditioner housing unit. For any space between the insulation and air

conditioner, use spray foam sealant to seal the cracks, ensuring that no cracks exist between

the outside of the unit and the inside.

Figure 13: Air Conditioner Housing Unit installation [4].

11. For any space where the inner layer of the insulation or the foam sealant shows (between

any two pieces of insulation, on top of the insulated layers, the sides of the door, etc.), or the

outer foil layer of the has ripped, use foil tape to cover it thoroughly.


12. We recommend installing a door gasket to prevent thermal leaks, although there was no

gasket in our prototype. Simply install the gasket using the same adhesive that was used to out

on the insulation layers.

13. Connect the CoolBot to the air conditioning unit based on the instructions on the CoolBot

website - https://www.storeitcold.com/.

3.2.2 Disassembly

If the insulated unit that is being used is made of panels and is able to be disassembled, it could

be beneficial to ensure that the insulation panels that are being used line up along the lines of

the panels of the structure itself. This would allow the insulated unit to be disassembled and

reassembled if need be, without damaging the insulation. Any rip in the foil layer makes it

possible for water to seep in and ruin the insulation, therefore, when ripping layers of insulation

off of the wall, it will likely tear the insulation.

4. Power Supply

To power the AC unit and CoolBot we recommend using grid electricity if it is readily available,

as this is the easiest option. However, using solar power and a battery would also be sufficient

and would have a smaller carbon footprint, yet higher initial cost.

5. Conclusion

This building guide is by no means the only way to create a low-cost insulated chamber in India.

However, based on the results of our testing and our experience insulating a trailer, this is the

most cost-effective, simple solution for an insulated cold storage unit.

https://www.storeitcold.com/


6. Appendices

Appendix A - Bibliography

[1] “CoolBot.” Store it Cold. Retrieved from: https://www.storeitcold.com/build-it/

[2] “Plastic Crates.” IndiaMart. Retrieved from: https://dir.indiamart.com/impcat/plastic-

crates.html

[3] “Nylon Washers” IndiaMart. Retrieved from: https://www.indiamart.com/proddetail/nylon-

washers-1670516448.html

[4] “Build it.” Store it Cold. Retrieved from: https://www.storeitcold.com/build-it/ac-selection/

[5] “Air Conditioning Chart.”The Spruce. Retrieved from: https://www.thespruce.com/air-

conditioning-chart-1152654

A.1. Links to Item Costs

Building Materials

Item Cost Link

Prefabricated Steel Garden/Storage Shed

(waterproof; swinging, non-sliding doors; flat

or pyramidal roof)

https://www.indiamart.com/sanjivani-auto-

industries/industrial-sheds.html#garden-shed

Foil-covered Insulation Panels (R ~13) https://www.indiamart.com/proddetail/acoustic

-thermal-insulation-board-7728375830.html

Liquid Adhesive https://www.indiamart.com/proddetail/constru

ction-adhesive-19054731633.html

HVAC Foil Tape https://www.indiamart.com/proddetail/alumini

um-foil-tape-2880940091.html

Spray foam sealant https://www.indiamart.com/proddetail/z2206-

360-puf-3x-self-expanding-sealant-

4405967388.html

Screws (long enough to get through the first layer of insulation)

https://www.indiamart.com/proddetail/pan-

head-wood-screws-12614951373.html

Plywood https://www.indiamart.com/proddetail/commer

cial-plywood-4698399273.html

Waterproof primer https://www.indiamart.com/proddetail/hydro-

https://www.storeitcold.com/build-it/

https://dir.indiamart.com/impcat/plastic-crates.html

https://dir.indiamart.com/impcat/plastic-crates.html

https://www.indiamart.com/proddetail/nylon-washers-1670516448.html

https://www.indiamart.com/proddetail/nylon-washers-1670516448.html

https://www.storeitcold.com/build-it/ac-selection/

https://www.thespruce.com/air-conditioning-chart-1152654

https://www.thespruce.com/air-conditioning-chart-1152654

https://www.indiamart.com/sanjivani-auto-industries/industrial-sheds.html#garden-shed

https://www.indiamart.com/sanjivani-auto-industries/industrial-sheds.html#garden-shed

https://www.indiamart.com/proddetail/acoustic-thermal-insulation-board-7728375830.html

https://www.indiamart.com/proddetail/acoustic-thermal-insulation-board-7728375830.html

https://www.indiamart.com/proddetail/construction-adhesive-19054731633.html

https://www.indiamart.com/proddetail/construction-adhesive-19054731633.html

https://www.indiamart.com/proddetail/aluminium-foil-tape-2880940091.html

https://www.indiamart.com/proddetail/aluminium-foil-tape-2880940091.html

https://www.indiamart.com/proddetail/z2206-360-puf-3x-self-expanding-sealant-4405967388.html



https://www.indiamart.com/proddetail/pan-head-wood-screws-12614951373.html

https://www.indiamart.com/proddetail/pan-head-wood-screws-12614951373.html

https://www.indiamart.com/proddetail/commercial-plywood-4698399273.html

https://www.indiamart.com/proddetail/commercial-plywood-4698399273.html

https://www.indiamart.com/proddetail/hydro-prime-waterproofing-insulating-primer-9215121748.html


prime-waterproofing-insulating-primer-

9215121748.html

Air conditioner https://www.indiamart.com/proddetail/lg-

window-air-conditioner-19257891512.html

Air conditioner Support Bracket https://dir.indiamart.com/search.mp?ss=air+c

onditioner+bracket&src=as-

popular%3Akwd%3Dair+conditioner+bracket

CoolBot https://www.storeitcold.com/

2”x4” wood beam https://www.indiamart.com/proddetail/timber-

beam-10923455130.html

Door gasket https://www.indiamart.com/proddetail/door-

gasket-11577670197.html

Tools

Item Cost Link

Jigsaw https://www.indiamart.com/proddetail/bosch-

jigsaws-2147976388.html

Caulk gun https://www.indiamart.com/proddetail/caulking

-guns-4051694888.html

Boxcutter https://www.indiamart.com/proddetail/box-

cutter-14001828533.html

Tape Measure https://www.indiamart.com/proddetail/level-

measuring-tape-18955701633.html

Power Drill https://www.indiamart.com/proddetail/rotary-

hammer-ud-4-1-year-free-repair-

18895431962.html

Step Ladder https://www.indiamart.com/proddetail/ss-

ladder-11322109012.html



https://www.indiamart.com/proddetail/lg-window-air-conditioner-19257891512.html

https://www.indiamart.com/proddetail/lg-window-air-conditioner-19257891512.html

https://dir.indiamart.com/search.mp?ss=air+conditioner+bracket&src=as-popular%3Akwd%3Dair+conditioner+bracket



https://www.storeitcold.com/

https://www.indiamart.com/proddetail/timber-beam-10923455130.html

https://www.indiamart.com/proddetail/timber-beam-10923455130.html

https://www.indiamart.com/proddetail/door-gasket-11577670197.html

https://www.indiamart.com/proddetail/door-gasket-11577670197.html

https://www.indiamart.com/proddetail/bosch-jigsaws-2147976388.html

https://www.indiamart.com/proddetail/bosch-jigsaws-2147976388.html

https://www.indiamart.com/proddetail/caulking-guns-4051694888.html

https://www.indiamart.com/proddetail/caulking-guns-4051694888.html

https://www.indiamart.com/proddetail/box-cutter-14001828533.html

https://www.indiamart.com/proddetail/box-cutter-14001828533.html

https://www.indiamart.com/proddetail/level-measuring-tape-18955701633.html

https://www.indiamart.com/proddetail/level-measuring-tape-18955701633.html

https://www.indiamart.com/proddetail/rotary-hammer-ud-4-1-year-free-repair-18895431962.html



https://www.indiamart.com/proddetail/ss-ladder-11322109012.html

https://www.indiamart.com/proddetail/ss-ladder-11322109012.html


Appendix I: O’Donohue Farm Trailer Guide

Insulated Trailer Build Guide

EcoGuavo!

2018

Basic Trailer Construction


Introduction

This build guide was created by students of ME170B: Mechanical Engineering Design:

Integrating Context with Engineering. We transformed an existing trailer owned by the

O'Donohue Farm at Stanford University into an insulated structure capable of being cooled by

both passive and active cooling mechanisms in an effort to prototype our class project. The

following document is an detailed overview of the steps taken to turn the basic trailer into an

insulated and air conditioned trailer. The details of constructing the passive misting system are

not included in this guide as they are not relevant for use beyond our project, however we do

include information about permanent changes to the trailer that resulted from our misting

prototype and how they have been repaired.

Materials

Item Quantity

Insulation Panels--Rmax Thermasheath-3 2 in. x 4 ft. x 8 ft. R-13.1 Polyisocyanurate Rigid Foam

Insulation Board 14

Liquid Nails Heavy Duty Adhesive 19

16 oz. Big Gap Filler Insulating Foam Sealant Quick Stop Straw 3

16 oz. Gaps and Cracks Insulating Foam Sealant with Quick Stop Straw 3

1.89 in. x 50 yd. 322 Multi-Purpose HVAC Foil Tape 5

Universal Light-Duty Air Conditioner Support 1

HDX Blue Nitrile Gloves 50 Pk. 1

A/C Pan Tablets 6 Pk 1

1-1/2" White Weatherization Foam Seal 3

LG 10,000 BTU Window AC Smart Wi-Fi w/ 3 YR Protection Plan 1






















8 Pc. Paint Tray Set 1

3-Pack Paint Roller Cover 1

#8 x 3" Self-Drilling Drywall Screws 3

#8 x 2" Exterior Phillips Head Screws (1lb) 1

#10 x 3-1/2" Exterior Phillips head Screws (1lb) 1

Scotchblue Painter's Tape 2

Behr Premium Solid Stain 1

2'x4' Lumber 2

4'x8'x3/4" CDX Plywood 4

Plastic Cap Roof Nails 1

Trailer 1

Tools

Item Quantity

10 oz Dripless Caulk Gun 1

HDX N95 Nonvalve Respirator 3Pk 1

Husky Quick Release Utility Knife 2

Ryobi 18V 1.3AH Battery & Charger 1

Ryobi 18V Jig Saw 1

Bosch 3"x24TPI T-Shank Jig Saw Blade 1

Milwaukee Bi-Metal Hole Saw 13-Piece Kit 1

Milwaukee 4-1/2" Bi-Metal Hole saw 1





















https://www.trailersplus.com/California/Gilroy/6-Wide-Cargo-Trailers/trailer/4RAVS1212JK066838

https://www.trailersplus.com/California/Gilroy/6-Wide-Cargo-Trailers/trailer/4RAVS1212JK066838


















Demolition and preparation

We started by removing all wooden panels on the inner sides of the trailer. This was done to

reduce the overall weight and prevent water damage. All other wood on the floor, drop down

door, swing door, rounded front wall and ceiling, was left in place for structural purposes.

Trailer before construction.

Trailer after non structural wood was removed.

Painting


We coated all exposed wooden surfaces with primer to protect the wood from water damage.

Chase priming floor boards.

Trailer after all wooden surfaces were coated with primer.

Air conditioning opening


We mounted the air conditioner to the front of the trailer. We positioned the air conditioner in the

center of the wall and low enough to account for the thickness of the ceiling insulation. We made

the hole of the in the trailer approximately 1” larger on either side of the air conditioner to allow

room for cords and tubing to run through the opening in addition to the air conditioner itself. We

cut the hole using a jigsaw.

Front of the trailer with air conditioner hole.

We also constructed a wall bracket for the air conditioner to rest on within the opening. We

mounted wooden supports to the trailer’s metal frame so that the air conditioner housing could

be fastened directly to the bracket. We also purchased an air conditioner bracket for the air

conditioning unit to rest on outside the trailer. We cut a notch in the hole in the trailer so the air

conditioner sits flush against the hole, and is still supported by the bracket. It is important that

when in use the AC unit is tilted backwards so that water that condenses during operation pools

outside the trailer.


Partially completed wooden bracket installed to support the air conditioner.

Purchased bracket installed to support the portion of the AC unit outside the trailer.

Floor

We left the original wooden floor in place and installed one layer of insulation on top of it. We

used rigid foam insulation with an R value of 13.1. We cut the insulation so that it covered the

floor, applied a heavy duty construction adhesive to the wooden floor and placed the insulation


on the wood floor to bond. We filled large gaps with spray foam and taped all insulation joints

with HVAC tape to seal thermal leaks. We then cut three quarter inch treated exterior plywood to

cover the insulation and act as the new floor. We coated this wood with the same primer used on

the structural wood. We then applied heavy duty construction adhesive to the insulation and

placed the plywood on top of the insulation to bond. We used screws spaced 12 inches apart to

fasten the top plywood through the foam insulation to the lower plywood.

Insulation adhered on top of existing wooden floor.


Seams in floor insulation were filled with spray foam before being sealed with metal HVAC tape.

Top layer of wooden flooring was secured using screws.

Wall insulation


We used two layers of 2” thick insulation for the walls, each with an R value of 13.1. We

measured and cut the insulation to cover the walls, going as high as the flat portion of the walls

(indicated in the photo below). We applied heavy duty construction adhesive to the studs and

adhered the insulation to the walls. We then used self-drilling screws with large plastic washers

to fasten the insulation to the trailer studs. The plastic washers act to distribute the force to a

greater area as to prevent the screw from further puncturing the insulation. We installed the

screws 14” apart. We then used spray foam to fill all gaps in the insulation, and taped all joints

with metal HVAC tape. To secure the second layer of insulation we applied heavy duty

construction adhesive to the first layer of insulation and held the second layer of insulation in

place until the layers had bonded.

Self-drilling screws with large plastic washers.


Screws were installed 12” apart and spray foam was used to fill large gaps.

Adhesive was used to secure the second layer of insulation to the first layer.


Ceiling insulation

We used two layers of 2” Rmax insulation for the ceiling, each with an R value of 13.1. Because

the trailer’s ceiling is rounded we couldn’t install the insulation directly to the metal roofing, so

we decided to rest it on top of the wall insulation. We held the top layer of insulation in place,

applied heavy duty construction adhesive to the exposed surface, and installed the second layer

of insulation beneath the top layer. We used wooden supports to brace the insulation in place

until the adhesive cured. We used spray foam to fill all gaps, and metal HVAC tap to seal the

insulation joints.

The top layer of insulation was held in place and adhesive was applied to its exposed surface. The lower layer of

insulation was inserted beneath the top layer.


Wooden supports were used to maintain the ceiling’s shape while the adhesive dried.

Interior finishing

We taped the edges of all exposed insulation, floor/side, side/side, and side/ceiling joints with

metal HVAC tape.

Air conditioner and Coolbot installation

We installed the air conditioner to the precut opening in the trailer. The air conditioner frame has

installation holes, which we used to fasten the frame to the wooden bracket previously installed.

We installed a metal grate to the exterior of the AC unit to prevent rocks and debris from


damaging the air inlet mesh. We then fastened the Coolbot to the insulation near the AC unit

using screws and connected it to the AC unit according to the Coolbot instructions.

Metal grate that prevents debris from damaging inlet mesh to the AC unit.

Specific ME170 Modifications

We made a few modifications to the trailer that are specific to our course project. They are

outlined below.

Drain hole

To accommodate our misting system which used water, we included a small hole in the floor for

a tube to drain out excess water. To do this we cut a 2” hole in the floor. For long term use of the

trailer without a misting system, the hole will be filled with spray foam insulation, fit with a

piece of wood, and we will use metallic HVAC tape over the joints to seal thermal leaks.


The floor drain sealed with HVAC tape. It will be sealed with spray insulation for long term use.

Ceiling chains

To hang our misting system from the ceiling we installed chains. The chains wrap around the

ceiling struts of the trailer and drop through both layers of ceiling insulation and attach to the

misting system. To do this we cut 4” holes in the ceiling insulation which we then removed,

pulled the chains through the opening, threaded the chains through a smaller hole inside the 4”

hole, and replaced the insulation to its original place. The Insulation holes are kept in place by

metallic HVAC tape. For long term use of the trailer without a misting system, the 4” holes will

be filled with their original insulation and heavy duty construction adhesive will be used to keep

them in place. Spray foam will be used to fill the smaller inner hole where the chains were

threaded, and lastly, metal HVAC tape can be used to seal all thermal leaks.


The chains are threaded through a small hole in a larger hole removed from the ceiling. Adhesive and spray foam

will be used to install the holes permanently.

Additional Notes

How to use the trailer

To use the air conditioned trailer, first plug in the air conditioning unit and the Coolbot. Press the

check mark on the cool bot – the number on the left will display the set temperature in degrees C

(should be blinking). Use the arrows to increase or decrease the temperature. Press the check

mark again to set the temperature. Then press the power button the AC to turn it on, press mode

on the AC so it turns from energy saver to cool, and press the fan speed button until the display

shows F3. Refer to the Coolbot instruction manual for more information about Coolbot settings.

Important Notes

The AC unit needs to be tilted down (such that the portion outside the trailer is lower than the

portion inside the trailer) during use to prevent water from the AC unit pooling inside the trailer.

To do this we recommend using the trailer jack next to the hitch to change the angle of the trailer

floor. We recommend using the same method of adjusting the trailer jack to tilt the trailer such

that the front of the trailer (AC side) is higher than the back side for cleaning purposes - so that

water can be used on the floor and drained out the back. However, we do not recommend that

water is used on the walls as it is not sealed against liquids. If this is a desired method of

cleaning the unit, explore vinyl siding as a solution.

As a safety consideration, there is no handle on the interior of the trailer. Before closing the

trailer, be sure that no one is still inside.

Reference


We would like to thank and acknowledge the Coolbot Trailer Construction Guide

as it is was our foundation for our trailer construction and build guide.

Trailer Construction Guide DIY Trailer Walk In Cooler. 2017, Trailer Construction Guide DIY Trailer

Walk In Cooler. https://cdn2.hubspot.net/hubfs/2434330/DIY%20Trailer%20Guide.pdf

Appendix K: Existing Cooling Products Considered

We evaluated several existing cooling products, ranging from accessories for home air

conditioning units to systems intended for use in large commercial environments. These

technologies are summarized in Table X.

Table X: Existing Cooling Products

Company Product Features

Store It Cold CoolBotTM A small accessory that attaches to a traditional air

conditioning unit and stimulates the unit with heat,

leading to temperature drops well below the typical

https://cdn2.hubspot.net/hubfs/2434330/DIY%20Trailer%20Guide.pdf




target room temperature.

Axiom Energy Refrigeration Battery A sophisticated platform for large supermarkets that

transforms large refrigeration systems into intelligent,

long-duration batteries through thermal storage. It

requires ice on site storage and is too large and

expensive as a potential solution for PFPCL.

ecoZen ecoFrost An on-farm solar powered cold storage unit similar in

size to a shipping container. Effective at low energy

cooling, but too large, heavy, and expensive as a

potential solution for PFPCL.

Evapoler Window Ducting Air Cooler

Most Evapoler applications are swamp cooling system for large areas (i.e., large office or warehouse buildings)

through a two-stage process. While Evapoler’s window

cooling system is a potential supplemental system, we

can expect temperature drop of only about 7°C.

InfiCold ColdVault A thermal battery that uses water as a medium to

capture and store energy. When energy is available, ice

is produced and energy is stored for later release. Initial

applications are focused on milk and produce storage. In

its current state, the ColdVault exceeds our users’

budget

mist cooling coupled with air conditioning to …...sale in the guava production chain in rajasthan,...

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