Evaporative Cooling Technology
Most of the under developed countries and rural areas do not have sufficient – and in some cases even minimal access to electricity. But preservation of food being an integral part of human life sustenance, exploring alternatives to commercial refrigerators has become necessary...
Food preservation is an important issue in the desert zone of Rajasthan. The heat and dust take their toll on precious crops. Most of the fruits and vegetables are rotten within three days. For increasing their shelf life, it is necessary to preserve them with the help of refrigeration. Conventional Refrigerators require electricity to run. Most of the under developed countries and rural areas does not have sufficient and in some cases even minimal access to electricity. But preservation of food being an integral part of human life sustenance, exploring alternatives to commercial refrigerators has become necessary. These refrigerators need to be inexpensive, easily available and should be environmentally friendly. Refrigeration by evaporative cooling is found to be successful in this regard. The working principle behind evaporative cooling is described hereafter.
Figure 1: Principle of Evaporative Cooling Refrigerator...
The evaporative cooling system design which is popularly used consists of two clay pots. The first clay pot is a large clay pot inside which the second smaller clay pot can be placed. The food that is to be preserved is placed inside the smaller inner pot. The space in between the two pots is filled with sand. The sand acts as a medium to retain water required for evaporative cooling. The water poured will fill the voids present in the sand. The outer clay pot being porous, allows water to permeate through it by hydraulic conductivity. The water thus arriving on the outer surface of the outer pot will be exposed to surrounding air. The latent heat of evaporation energy required is observed from the inner pot and thus water, by evaporating cools the inner pot area and brings about refrigeration of stored food.
Figure 2: Development of Evaporative cooling system (Grameen Fridge) at Village Swarupsar, District Sikar (Rajasthan)...
Parameters affecting evaporative refrigeration
A. Ambient temperature: The ambient temperature plays an important role in evaporative cooling. The higher the temperature higher will be the rate of evaporation, ensuring better cooling of the storage space.
B. Ambient relative humidity: Relative humidity is the measure of water content already present in air. When the relative humidity of air surrounding the refrigerator is less, more water can be evaporated and hence cooling will be better. Theoretically it is found that with decrease in ambient relative humidity for a given temperature, the temperature of the storage space decreases.
C. Geometry and surface area of refrigerator: The amount of evaporation taking place is directly proportional to area available for evaporation. Hence increase in the area for evaporation around the storage space enhances the cooling process.
D. Thermal conductivity of material used to hold water: The material used to hold water between the pots should have as low a thermal conductivity as possible to obtain lesser temperature in the storage space for a given ambient temperature and relative humidity. Theoretical as well as experimental studies have also confirmed that the material used should be of low thermal conductivity.
Figure 3: Development of Evaporative cooling system (Grameen Fridge) at Village Kohari, District Sikar (Rajasthan)...
E. Thermal conductivity of material exposed to evaporation: The material in contact with the ambient air should have lower thermal conductivity as well. But theoretical results have shown that though the temperature of storage space decreases with decrease in thermal conductivity, the effect is minimal.
F. Hydraulic conductivity of material: Hydraulic Conductivity of porous material in contact with ambient air should be higher, but not too high to allow seepage. Theoretical analysis shows that the effect of hydraulic conductivity is also minimal on the cold temperature achieved at storage space.
G. Movement of air: The movement of air enhances the evaporation process and hence the cooling. Experimental results show that the temperature of storage space decreases with increase in wind speed for a given temperature and humidity.
AUTHORS CREDIT & PHOTOGRAPH
Er Kapil Samar
cum Project Manager
and Training Centre,