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 ⁰ C PCM Temperature Regulator Heating/cooling
Solar Dryer Schematic Exhaust air Glass Sheet Absorber Sheet
Food Drying • 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 O C 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
Why Thermal Storage? 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.
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, C p = Specific heat of air, T= air temperature I= solar radiation, A p = 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.
Collector Design • PCM is placed below the absorber plate in direct contact with the absorber • 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.
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 120 % P 100 40 kg PCM is required per Sqm C M 80 Of collector area to achieve M 60 100% melting E 40 L T 20 E D 0 0 10 20 30 40 50 60 Time in hrs
CFD Analysis: Heat Transfer Coefficient Flow rate: 60 kg/hr h avg : 5.4 W/m 2 /K Flow rate : 36 kg/hr h avg : 4.0 W/m 2 /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
Temperature Profile 60 50 40 T E 30 M P 20 T_final Ambient temperature 10 0 0 0.5 1 1.5 2 2.5 Time in Days Temperature range of 51-52 degrees as compared to conventional Dryers that would provide a range from 30-75 degrees
Benefits to Society 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
Recommend
More recommend
