Thermoeconomic optimization and comparison of the simple single-stage transcritical carbon dioxide vapor compression cycle with different subcooling methods for district heating and cooling

Abstract

The transcritical carbon dioxide vapor compression cycle is commonly used for heating and cooling applications because of the high discharge temperature and large evaporation heat. Whereas, high return water temperature is usually required by the heating demands, which significantly degrades the system performances. Nowadays, various kinds of subcooling methods have been proposed for the transcritical carbon dioxide vapor compression cycle, but a systematic comparative study based on the trade-off between the thermodynamic efficiency and economic cost is needed to analyze the applicability of these methods for large-scale district heating and cooling applications. In this work, the hot water temperature increases from 323.15 K to 343.15 K for district heating, and chilled water temperature reduces from 285.15 K to 280.15 K for district cooling. The simple single-stage transcritical carbon dioxide vapor compression cycles using subcooling methods with and without expansion work recovery, including internal heat exchanger, dedicated mechanical subcooling, cooling tower and dry cooler, are respectively modeled from the energetic, exergetic and economic perspectives. Based on the dual-objective Jaya algorithm and Technique for Order of Preference by Similarity to Ideal Solution decision-making method, the parametric study is conducted with optimized system performances to analyze the effects of different ambient conditions, namely the winter mode with ambient temperature from 248.15 K to 278.15 K and the summer mode with ambient temperature from 288.15 K to 318.15 K, and the relative humidity from 30% to 90%. Afterwards, the optimal subcooling methods under different ambient temperatures are selected based on the overall optimization and comparative study. The effects of different ambient temperatures on the system performances vary with different subcooling methods without expansion work recovery, and the dry cooler is the optimal subcooling method at the ambient temperatures from 248.15 K to 268.15 K, while the cooling tower is suggested under ambient temperatures higher than 278.15 K in spite of the performance degradation with high relative humidity. Insignificant differences of system performances using different subcooling methods can be observed with expansion work recovery, and the internal heat exchanger is selected as the optimal subcooling method for winter-mode operations, while the cooling tower is also suggested as the suitable subcooling method in the summer mode.