Investigation On Key Technology Of A Solar Assisted Heat Pump In-Bin Grain Drying System

Grain depot is an important base to ensure food storage safety. For every year, the grain in the depot should be alternated, especially in the harvest season. Due to climatic conditions, food sources and other factors, some part of grain with high water content may be stored. To guarantee the safe storage of grain, in bin drying operation is needed. There have been two commonly used methods, namely, grain drying unit and ventilation. Grain drying unit, usually spends a lot of fossil fuels, and the operation cost is high and the workload is heavy. Ventilation drying is limited by a long drying cycle, and is strongly dependent on weather conditions, etc. so there is an urgent requirement for development of new type of grain drying technology.In this paper, a solar assisted heat pump in-bin grain drying unit has been proposed and studied. This technology is advantageous in energy saving, short drying time and uniform drying distribution, etc. the main work of this paper is summarized below:(1) Based on the climate and solar radiation conditions for a typical grain depot, a solar assisted heat pump in-bin drying process, which involves several operation modes, such as drying by heating , drying through dehumidification, drying by ventilation, etc., has been designed and analyzed.(2) Using topology network methods, a mathematical model, which can calculated the fluid flow and flow resistance of a solar air collector array, has been developed. The distribution for air flow and resistance in a solar collector array, and arrangement of the solar collectors are also optimized.(3) Three other mathematical models including solar air heating system, heat pump system, as well as grain drying unit within the depot, are also developed. Based on the models, the performance of the system, the operation process and the matching method of the components for the system as a whole has been analyzed, and optimized.(4) A solar assisted heat pump in-bin grain drying system for a grain depot, which is a typical large depot (length 39m×high 9m×width 24m), the capacity is 3000t~5000t, has been built. The experimental results show that the system works well, heat pump's COPH is up to 5.0, the whole system's COPE is 3.33~3.69, the whole system's COPCH is 5.88~6.86 (only system power consumption is considered), solar heating contribution SF is 23%~36%.(5) It is found that the solar assisted heat pump in-bin grain drying technology is a safe, energy-saving and efficient drying technology, and can resolve the issue of uniformity of grain drying by using one kind of machine-grain butler. The cost for drying is 9.3$/t, much lower than the grain dryer(36.5 $/t).