EARTH AIR TUNNEL HEAT EXCHANGER (EATHE)

Submitted by

Amandeep Singh

Vikas Mahala

Ashok Dhayal

CONTENTS Introduction Passive cooling Earth Air Tunnel Principle Factors affecting thermal conductivity Applications of EAT Design guidelines Classification Advantage and limitations Potential issues Conclusion References

INTRODUCTION

Energy Saving: One of the most important global challenges

Energy Efficiency:

Supply Side: Higher

Efficiency power plants,

renewable sources of energy, Smart Grids, etc.

Demand Side: Energy efficient,

Building Envelopes (direct systems),

Earth Air Tunnels(indirect systems), etc.

PASSIVE COOLING

• Passive cooling systems are least expensive means of cooling a home which maximizes the efficiency of the building envelope without any use of mechanical devices.

•It rely on natural heat-sinks to remove heat from the building. They derive cooling directly from evaporation, convection, and radiation without using any intermediate electrical devices.

•All passive cooling strategies rely on daily changes in temperature and relative humidity.

•The applicability of each system depends on the climatic conditions.

•These design strategies reduce heat gains to internal spaces.

- Natural Ventilation- Shading- Wind Towers- Courtyard Effect

- Earth Air Tunnels- Evaporative Cooling- Passive Down Draught Cooling- Roof Sprays

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EARTH AIR TUNNEL

• The Earth Air Tunnel (EAT) systems utilizes the heat-storing capacity of earth.

• The fact that the year round temperature four meter below the surface remains almost constant

throughout the year. That makes it potentially useful in providing buildings with air-

conditioning.

• It depends on the ambient temperature of the location, the EAT system can be used to provide

both cooling during the summer and heating during winter.

• The tunnels would be especially useful for large buildings with ample surrounding ground.

• The EAT system can not be cost effective for small individual residential buildings.

• The ground temperature remains constant and air if pumped in appropriate amount that allows

sufficient contact time for the heat transfer to the medium attains the same temperature as the

ground temperature.

EARTH-AIR TUNNEL: PRINCIPLE

Underground heat exchanger

Also called:

Earth-Air Heat Exchangers

Air-to-soil Heat Exchangers

Earth Canals

PRINCIPLE DIAGRAM

EARTH-AIR TUNNEL: PRINCIPLE

Earth acts a source or sink High thermal Inertia of

soil results in air

temperature fluctuations

being dampened deeper

in the ground Utilizes Solar Energy

accumulated in the soil Cooling/Heating takes

place due to a temperature

difference between

the soil and the air

FACTORS AFFECTING THERMAL CONDUCTIVITYSOIL: Moisture content

Most not able impact on thermal conductivity

Thermal conductivity increases with moisture to a certain point (critical moisture content)

Dry density of soil

As dry density increase thermal conductivity increase Mineral Composition

Soils with higher mineral content have higher conductivity

Soils with higher organic content have lower conductivity Soil Texture

Coarse textured, angular grained soil has higher thermal conductivity Vegetation

Vegetation acts as an insulating agent moderating the affect of temperature [2]

APPLICATIONS OF EAT’S

EAT’s can be used in a vast variety of buildings:

Commercial Buildings: Offices, showrooms, cinema halls etc.

Residential buildings

University Campuses

Hospitals

Greenhouses

Livestock houses

DESIGN GUIDELINES

IMPORTANT DESIGN PARAMETERS:

The design parameters that impact the performance of the EAT are:• Tube Depth• Tube Length• Tube Diameter• Air velocity• Air Flow rate• Tube Material• Tube arrangement

Open-loop system Closed-loop system

• Efficiency• Coefficient of Performance (COP)

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TUBE DEPTH

Ground temperature defined by: External Climate Soil Composition Thermal Properties of soil Water Content

Ground temperature fluctuates in time, but amplitude of fluctuation diminishes with depth.

Burying pipes/tubes as deep as possible would be ideal.

A balance between going deeper and reduction in temperature needs to be drawn.

Generally ~4m below the earth’s surface dampens the oscillations significantly.

TUBE LENGTH Heat Transfer depends on surface area. Surface area of a pipe:

Diameter Length

So increased length would mean increased heat transfer and hence higher efficiency. After a certain length, no significant heat transfer occurs, hence optimize length. Increased length also results in increased pressure drop and hence increases fan energy. So economic and design factors need to be balanced to find best performance at lowest cost.

TUBE DIAMETER

Heat Transfer depends on surface area. Surface area of a pipe:

Diameter Length

Smaller diameter gives better thermal performance. Smaller diameter results in larger pressure drop increasing fan

energy requirement. Increased diameter results in reduction in air speed and heat

transfer. So economic and design factors need to be balanced to find best

performance at lowest cost. Optimum determined by actual cost of tube and excavation cost.

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AIR VELOCITY As the velocity of air increases the exit

temp decreases

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AIR FLOW RATE

For a given tube diameter, increase in airflow rate results in:

Increase in total heat transfer Increase in outlet temperature

High flow rates desirable for closed systems

For open systems airflow rate must be selected by considering:

Outlet temperature Total cooling or heating capacity

TUBE MATERIAL

The main considerations in selecting tube material are: Cost Strength Corrosion Resistance Durability

Tube material has little influence on performance.

Selection would be determined by other factors like ease of installation, corrosion resistance etc.

Spacing between tubes should enough so that tubes are thermally independent to maximize benefits.

TUBE ARRANGEMENT EAT can be used in either:

Closed loop system Open loop system

Open Loop system: Outdoor air is drawn into tubes and delivered to AHUs or directly to the inside of the building Provides ventilation while hopefully cooling or heating the building interior Improves IAQ

Closed Loop system: Interior air circulates through EATs Increases efficiency Reduces problem with humidity condensing inside tubes.

Hybrid System: EATHE system is coupled to another heating/cooling system, which

may be an air conditioner , evaporative cooling system or solar air heater

TUBE ARRANGEMENT

EAT can be used in either: One-tube system Parallel tubes system

One tube system may

not be appropriate to meet

air conditioning requirements

of a building, resulting

in the tube being too large

Parallel tubes system More pragmatic design option Reduce pressure drop Raise thermal performance

CLASSIFICATIONClassification of EATHE system

According to layout of pipe in ground According to mode of arrangement

There are four different types according to layout of pipe in the ground

Horizontal/ straight Loop Vertical Looped Slinky/ spiral Looped Pond/Helical Looped

CONTD….

EAT EFFICIENCY

Calculating benefits from EAT is difficult due to: Soil Temperatures Conductivity

Performance of EAT can be calculated as:

where;

To = Inlet Air Temperature

To (L) = Outlet Air Temperature

Ts = Undisturbed ground temperature

CO-EFFICIENT OF PERFORMANCE(COP)

COP based on:

Amount of heating or cooling done by EAT (Heat Flux) Amount of power required to move the air through the EAT

Q= Heat Flux

W= Power

COP decreases as system is operated COP can be integrated into system control strategies When COP down to a certain point, EAT should be shut down and

conventional system should take over

ADVANTAGE AND LIMITATIONS

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ADVANTAGE ETHE based systems cause no toxic emission and therefore, are

not detrimental to environment.

Ground Source Heat Pumps (GSHPs) do use some refrigerant but

much less than the conventional systems.

ETHE based systems for cooling do not need water - a feature

valuable in arid areas like Kutch. It is this feature that motivated

our work on ETHE development.

ETHEs have long life and require only low maintenance

Low operating cost.

LIMITATIONS

Require large space to make setup.

Give a limited cooling effect.

Initial cost high.

POTENTIAL ISSUES

MOISTURE ACCUMULATION AND IAQ PROBLEM

ISSUE

• Condensation inside the tubes has been observed

• Condensation occurs if temp. in the tube is lower that dew point temp.

• Condensation occurs in systems with low airflow and high ambient dew point temperature

• Removal of moisture from the cooled air is always an issue and system may be used with a regular air conditioner or a desiccant

• Water in tubes also results in growth of mould or mildew leading to IAQ issues

SOLUTIONS• Good construction and

drainage• Tubes are tilted to prevent

water from standing in the tubes

• In the service pit at the lowest point water can be captured and pumped

• Water tight tubes can be used to prevent ground water from entering into the system

CONCLUSIONS

CONCLUSIONS

EATs are based on the following principles

Using earth as a source or sink Uses Soil Thermal inertia Depends on the Thermal Conductivity of Soil

Various Factors affect the performance of EAT which need to be optimized to maximize performance.

Integrate the EAT into the building systems to maximize performance and maximize energy savings.

REFRENCES1. A passive solar system for thermal comfort conditioning of buildings in composite climates†,1 p.

RAMAN, SANJAY MANDE and V. V. N. KISHORE received 19 august 1998; revised version accepted 13 october 2000

2. Earth air heat exchanger in parallel connection manojkumardubey1, dr. J.L.Bhagoria2, dr. Atullanjewar M.Tech student1 MANIT bhopal professor mech deptt. , MANIT bhopal asst. Professor mech deptt, MANIT bhopal(figures)

3. Jalaluddin, Miyara A, Thermal performance investigation of several types of vertical ground heat exchangers with different operation mode, Applied Thermal Engineering 33-34 (2012) 167–74.

4. Performance analysis of earth–pipe–air heat exchanger for winter heating vikas bansal *, rohit misra, ghanshyam das agrawal, jyotirmay mathur

5. Performance analysis of earth–pipe–air heat exchanger for summer cooling vikas bansal *, rohit misra, ghanshyam das agrawal, jyotirmay mathur

6. Performance evaluation and economic analysis of integrated earth–air–tunnel heat exchanger–evaporative cooling system vikas bansal , rohit misra, ghanshyam das agrawal, jyotirmay ∗mathur

7. Thermal performance investigation of hybrid earth air tunnel heat exchanger rohit misraa, vikas bansala, ghanshyam das agarwala, jyotirmay mathura, , tarun aserib∗

8. ANALYTICAL MODEL FOR HEAT TRANSFER IN ANUNDERGROUND AIR TUNNEL MONCEF KRARTI and JAN F. KREIDER (received 27 october 1994; received for publication 11 july 1995)

THANK YOU