Energy efficiency of a new heat pump system for drying grain

An experimental closed-loop heat pump grain drying system was designed and evaluated for energy performance while drying shelled corn and grain sorghum. The system was designed based on an estimated heat pump coefficient of performance (COP) of 4.0, an air-to-air exchanger efficiency of 80%, and a fan efficiency of 50%. The objective of this research was to develop and evaluate a closed-loop heat-pump system for drying grain that is more efficient than conventional gas-fired dryers. Four drying tests using three corn lots with initial moisture contents of 25%, 20%, and 18% w.b. and one grain sorghum lot with initial moisture content of 25% w.b. were conducted during the 1998 fall harvest season. Temperature of the heated air was controlled in the range of 43 to 48°C with airflow rates of 0.26 and 0.25 m3/h per dry mass of grain for corn and grain sorghum, respectively. The average amount of input energy (all electric) required to remove 1 kg of water from the corn lots was 1186 kJ based on grain lot weight differences before and after drying. The best drying performance observed in a 4-6 h period was 942 kJ/kg of water removed. The average drying capacity was 230 kg/h based on an equivalent moisture reduction from 25% to 15.5% w.b. The drying performance for drying grain sorghum was lower (1516 kJ/kg of water) than that for drying corn. The average thermal energy transfer rate of the air-to-air heat exchanger was 11.47 kW, which represented about 28% of the total (source-sink) thermal energy transfer rate of 39.38 kW. The average efficiency of the air-to-air heat exchanger was about 34%. The average COP for the heat pump was 6.4 including the air-to-air thermal transfer and 4.7 excluding the air-to-air thermal transfer.