SOLAR DRYERS INTEGRATED WITH PHASE CHANGE MATERIALS BASED THERMAL ENERGY STORAGE

What are Phase Change Materials

Materials that can store large amount of heat energy in the form of latent heat. Latent heat is typically 100 times higher than sensible heat capacity

Less material is required to store the same amount of heat. Reduces the need for high contact area or a high thermal conductivity

These materials Release/absorb heat at constant temperature.

Heat Absorption / Release process is repeatable over a substantial number of cycles (~ 3000 or more).

What are Phase Change Materials

PCMs are chemicals in the form of solid and liquids which have the ability to store thermal energy for heating /cooling application at a constant temperature. PLUSS manufactures both organic & inorganics PCMs with a wide temperature range of -36 ⁰ C to 89 ⁰C

Temperature Regulator Heating/cooling

PCM

Solar Dryer Schematic

Exhaust air

Absorber Sheet

Glass Sheet

• Drying is an essential process in the preservation of agricultural products. • The dried produce fetches significantly higher prices than fresh produce

while maintaining the nutritional value. • Most food products, especially fruits and vegetables require hot air in the

temperature range of 40-55 OC for safe drying. • Solar drying provides much higher quality dried produce as compared to

open sun drying and is cheaper and environment friendly as compared to fuel based drying.

• Day time only operation of solar drying has been a limiting factor in wider industrial adoption.

Common dried food articles: • Spices & Herbs: Turmeric, Coriander, Moringa, wheatgrass etc. • Fruits & Vegetables: Apricots, Grapes, Tomatoes, Onions etc. • Sea Food: Fish, Prawns, Seaweed • Tea leaves & coffee beans • Pulses

Food Drying

To create a properly optimized system to deliver heat for solar drying at a constant temperature throughout a 24 hour day. Advantages: 1. Improves system productivity due to 24/7 operation as compared to day

time only operation.

2. Increases the efficiency of the solar dryer due to controlled temperature and continuous operation.

3. The consistency in air temperature and continuous operation beyond the day time significantly improves the quality of the dried product providing better nutritional value, appearance, aroma & taste.

1. Shortens the duration of drying which leads to reduced food wastage & increased farm productivity.

Why Thermal Storage?

Collector Efficiency

The efficiency of a solar collector is defined as the heat transferred to the air divided by the solar irradiance η = useful energy/solar energy mathematically m = Air mass flow rate, Cp = Specific heat of air, T= air temperature I= solar radiation, Ap = Surface area of the absorber plate

Solar dryers with storage would achieve 55-60% efficiency as compared to 45-50% for conventional solar dryers • Reduced radiation & convective losses due to controlled temp. • Storage of excess heat during the peak day time. • The constant temperature throughout a 24 h day avoids startup

effects in the morning as well as energy loss when the cycle top temperature drops at night.

• Absorber sheet with selective coating having high absorbance & low emissivity.

• Allows heat to be transferred directly to and from the absorber plate to the storage medium.

• Eliminates the need for a separate apparatus and control systems for external storage as well as the associated heat losses.

• The low heat capacity working fluid is not used to transfer heat to the storage.

• Using a built-in storage system also eliminates the need for a secondary working fluid loop and the inherent increase in complexity and cost.

• PCM is placed below the absorber plate in direct contact with the absorber

Collector Design

Heat Transfer

1. The transmittance of the glass is 91% 2. The Absorber plate has a black chrome selective coating that has

high absorption of 96% and very low emissivity of 13%. 3. This energy absorbed is distributed to:

• Useful energy gain that heats the air during the sunshine hours and useful energy that melts the PCM.

• Radiation losses, thermal losses through the glass cover, and thermal losses through the glass wool insulation

PCM Specifications

PCM – Organic PCM with melting point @55 degrees C Latent Heat: 255 KJ/kg Specific Heat: 2.75 KJ/Kg/K Thermal Conductivity: 1.12 W/m/K (Solid); 0.24 W/m/K (liquid) Density: 850 kg/Cum (Solid); 800 kg/Cum (Liquid) Too enhance the poor conductivity of the PCM the tank has been embedded an Aluminum mesh at 5% by volume which ensures that it takes only 10 mins for the heat absorbed to reach the PCM at the bottom of the tank

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40 kg PCM is required per Sqm Of collector area to achieve 100% melting

% P C M M E L T E D

Time in hrs

CFD Analysis: Heat Transfer Coefficient

Flow rate: 60 kg/hr havg: 5.4 W/m2/K

Flow rate : 36 kg/hr havg: 4.0 W/m2/K

CFD Analysis - Outlet Temperature

Flow rate : 60 kg/hr Mass flow averaged Temperature at the outlet: 52° C

Flow rate : 36 kg/hr Mass flow averaged Temperature at the outlet: 51° C

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T_final

Ambient temperature

Temperature Profile

T E M P

Time in Days

Temperature range of 51-52 degrees as compared to conventional Dryers that would provide a range from 30-75 degrees

PLUSS’ innovative solar dryer helps in: • Providing higher remuneration to farmers • Creating entrepreneurship opportunities for rural women • Controlling food inflation by reducing food wastage • Preserving the environment by avoiding fossil fuels

Benefits to Society