Performance enhancement of a two-stage vapour compression heat pump with solution circuits by eliminating the rectifier
Abstract Performances of four versions of a two-stage vapour compression heat pump with solution circuits (VCHSC) are compared. The VCHSC represents a cascade system that requires only one compressor. Performance curves for the best cycle, the modified version with a bleed line and a desuperheater are obtained and studied in detail. The four cycles are compared for the same total UA value (product of overall heat transfer coefficient and area) including all heat exchangers while pumping heat from an average desorber temperature of −2.5°C to an average absorber temperature of 105°C. Simulation results show that the cycle with a bleed line and a desuperheater has 40 to 50% higher cooling COPs and 30 to 40% higher cooling loads as compared to the cycle with a rectifier. Both the rectifier and the bleed line cycles show an increase of up 20% in the cooling COP and capacity by adding a desuperheater. Performance curves are obtained by simulating the best cycle at various weak solution concentrations in the low temperature loop and different solution pump flow rates. The parameters studied are the cooling COP, the solution heat exchanger effectiveness, the pressure ratio, the solution temperature glides in the absorbers and desorbers, the low temperature desorber load and the distribution of the UA value. Cooling COPs up to 1.05 and pressure as low as 7.05 are obtained for the modified VCHSC. The cooling COPs are twice as much and the pressure ratios are two thirds less of what would be found in a conventional single-stage vapour compression system. Changing the weak solution concentration in the low temperature loop from 40 to 80 wt% ammonia increased the cooling load by four times. The results indicate that the two-stage VCHSC with a bleed line and a desuperheater can work at temperatures above 100°C and achieve temperature lifts of more than 100 K with well known and environmentally safe refrigerants.