Performance analysis of Cascade solar heat pump for simultaneous heating and cooling agricultural product

Abstract

The performance investigation of water to water cascade heat pump for simultaneous cooling and heating applications was presented in this study. The refrigerant cycles of cascade system were R245fa and R134a at high temperature cycle (HTC) and low temperature cycle (LTC), respectively. Two thermal storage tanks at water volume of 300 and 350 L and two internal heat exchangers were integrated with the heat pump system to investigate the system cycle performance. The heat pump system was tested at chilled storage temperature of 5, 7 and 12 °C and hot storage temperature was varied from 60 °C to 80 °. At a given chilled storage temperature, the result showed that the total heating and cooling capacities decreased and the consumed compressor powers increased with the rising of hot storage temperature since the pressure ratio was growing with the condensing temperature. Besides, the degrees of superheat of high and low temperature cycles were also increased with the condensing temperature of R245fa. The degree of superheat of LTC was closely to the R245fa liquid temperature resulting in high discharge superheat at the outlet of LTC compressor which reduced the heat transfer rate at the cascade heat exchanger. As a result, the heat capacity at the condenser was decreased though of that at the evaporator was almost constant. Moreover, the intermediate temperature was investigated to obtain the maximum total COPs. At evaporating temperature of 1 °C and condensing temperature of 85 °C, the optimum intermediate temperature was 43 °C at the total COPs of 3.02. The highest total COPs was obtained at chilled storage temperature of 12 °C at all various hot storage temperatures due to the highest total heat rate and lowest total compressor powers compared to that at chilled storage temperature of 5 and 7 °C. At hot storage temperature of 80 °C, the total COPs were 3.06, 2.91 and 2.85 at chilled storage temperature of 12, 7 and 5 °C, respectively. Moreover, the heat pump model was validated with the experiment data and the result showed well agreement within the deviation less than 7%. A case study of simultaneous drying and cooling longan was conducted to investigate the energy consumption and system performance. The fresh longan at the capacity of 1000 kg and 1540 kg were dried and cooled in the drying and refrigeration room, respectively. The products were dried at drying of 70 °C and cooled at 10 °C for the same period of 60 h. The fresh longan was dried from initial moisture content of 400% (dry basic) to final moisture content of 20% (dry basic). Besides, the cascade heat pump system was compared to the existing unit and the payback period of the heat pump system was 1.77 year. Additionally, the solar photovoltaic (PV) was sized with the cascade heat pump system. The power model (Pm) of the solar PV was validated with experimental data at the variation less than 10%. At the grid electricity price of 4 Baht/kWh, the shortest payback period of the cascade solar heat pump was 5.46 years at 32 modules of 320 W polycrystalline solar photovoltaic. A quick method to predict a cycle performance of a standard single stage heat pump cycle was proposed from figure of merit (FOM) as function of Jacob number and operating temperature. Then, the correlations of COP-FOM were given and the performance of low and high temperature cycle of cascade heat pump could also be found. In addition, the data obtained by FOM model were compared with the experimental data and the deviation was less than 10% which was well agreement of the model.