Research And Application Of Solar Assisted Heat Pump In-store Drying

In-store drying is a grain drying method adopted by grain depots widely. The conventional natural ventilation in-store drying has the disadvantages of long drying time, high power consumption, bad drying uniformity and weather-dependent. To reduce energy consumption and improve the drying effect, it's necessary to develop new in-store drying process. This paper focuses on a novel in-store drying system which is composed of solar air collector, heat pump and grain stirrer. The drying process aims for accomplishing the drying process in energy saving, short time and high efficiency. Related theoretical design methods are developed and experimental measurement on the demonstrated system is conducted for the evaluation of the new drying process.In terms of ventilation parameters design, the mathematical model on heat and mass transfer in grain drying are developed. Three deep bed drying models are summarized and the calculation analysis is conducted to analyze their applicability. Based on the mathematical model, the features of deep bed drying are analyzed. Besides, a calculation method on grain drying with the application of CFD software is developed. The simulation results based on typical meteorological parameters of Kunming shows that the drying time will be less than 10 days for the moisture content reduction of 2% when the inlet air has 8℃temperature rise relative to ambient air.For the ventilation system design, the outside ventilation system and inside air distribution system are designed and analyzed. 96m2 solar air collectors and an air source heat pump with rated power of 22kW were manufactured and installed in Kunming depot. Combined with the actual situation of experimental granary, parallel connection scheme of the outside equipments and aboveground ventilation cage of inside distributor are determined. Resistance calculation is done to select the supply fans.To evaluate the performance of heating equipments and the effect of drying process, experimental measurements on solar assisted heat pump equipments and practical drying process are conducted. Results show that thermal efficiency of solar air collector can reach 63% under designed condition and the maximum heating capacity can reach 62.8 kW. Average COP of heat pump can reach 5.7 under experimental condition and the average heating capacity is 78kW. The experimental results show the advantages of solar assisted heat pump in-store drying process are obvious. The power consumption per ton grain for the moisture content reduction by 1% is 1.24 kWh/(1%·t) which indicates the lower power consumption level. The drying rate is faster and it takes 42h for the moisture content decreasing from 12.9% to 12.5%. The drying uniformity is improved and maximum moisture content difference decreases from 4.7% to 2.8%. Besides, the grain quality is maintained well.