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Solar drying is the one of the effective means for preserving grains and pulses. It is a simpler, cleaner and safer method of drying. However, most available solar dryers are inefficient due to non-availability of solar energy during the periods of low insolation which eventually leads to moisture re-absorption with pronounced effect particularly at night, causing growth on dried grains. To solve this problem, an energy collector and storage chamber was developed and connected to a developed solar dryer to serve as a heat source at night.
No load test was carried out at night when the dryer was subjected to four different conditions: valve opened and surface of the collector covered; valve opened and surface of the collector uncovered; valve closed and surface of the collector covered; and valve closed and surface of the collector uncovered. Load test was conducted by drying five different grains; maize, soyabean, groundnut, cowpea and sorghum during daytime using the dryer and applying the best combination of energy supplied and surface of the collector at night.
The result showed that the highest temperature difference of 35ºC and 33ºC were attained in the flat plate collector and drying chamber respectively, when the valve was opened and the surface of the collectors were covered. The load test revealed that there was a slight reduction in the moisture content of grains between midnight and 5 am. The highest moisture reduction (4%) was recorded during drying of sorghum while the least moisture reduction (1%) was recorded when drying cowpea at night. In conclusion, for continuous solar drying of grains at night using stored energy, the valve controlling the heat storage chamber should be opened and the surface of the collector should be covered to obtain optimum temperature within the drying chamber. Conserved heat energy from the energy storage chamber ensures continuous drying of smaller grains (sorghum) and at least prevents moisture re-absorption in larger grains (cowpea and maize).