Simultaneous optimization of working fluid and temperature matching for heat pump assisted geothermal cascade heating system

The heat pump assisted geothermal cascade heating system (GCHS) has a promising prospect in space heating. In this paper, considering thermodynamic, economic and environmental performance, the optimal working fluid from four hydrocarbon candidates and temperature-matching for the GCHS are determined simultaneously based on multi-objective optimization and grey relational analysis. The results show that according to the optimal compromise solutions, the GCHS with R600 has the highest thermodynamic performance (COPtot) of 22.58 and the least annual carbon emission (ACE) of 2.80 × 103 ton while R290 has the largest net present value (NPV) about 45.83 × 106 CNY. Additionally, all temperature variables considered generally tend to reach their extreme value except for the temperature lift of middle circulation water in the secondary plate heat exchanger. The optimal working fluid based on comprehensive performance is considered to be R600 with the largest grey relational degree of 0.752. Besides, the global sensitivity analysis on the GCHS with R600 indicates that the pinch point temperature difference of condenser is the most important influential parameter for COPtot with total Sobol index about 0.918 and the geothermal tail water temperature plays a significant role for both NPV and ACE with total Sobol index up to 0.806 and 0.992, respectively.