Multi-objective optimization and performance evaluation of a cooling tower assisted ground source heat pump system under different intermittent control strategies

Ground source heat pump (GHSP) system is widely utilized currently due to its advantages in economy, environment, and energy aspects. However, different building load demands during heating and cooling seasons cause the difference in heat intakes, leading to the ground thermal imbalance, and will further impact the operating performances of the system. Cooling tower (CT) is employed as an assisting method to alleviate the above problems in cooling-dominated areas. Various CT control strategies have been applied and studied nowadays, including fixed running time (FRT) control strategy, fixed entering fluid temperature (FET) control strategy, and fixed temperature difference (FTD) control strategy. In this study, multi-objective optimization (MOO) method is utilized and an optimizing framework is proposed based on NSGA-II. Concerning the economy, environment, and energy performances of cooling tower assisted ground source heat pump (CT-GSHP) system, operating period cost (OPC), operating period environmental performance (OPEP), and total energy consumption (TEC) are used as objective functions. COMSOL Multiphysics is utilized to establish the numerical model and demonstrate the performances of the system during a 10-year period. One representative set of optimized solutions illustrates that FET method has the best OPC with the value of 43.01 x 106 CNY, FTD method gets the best OPEP value of 2674.44 tCO2, and TEC of FRT is the lowest with the value of 2295.24 MWh. What's more, FRT, FET, and FTD methods keep the coefficient of performance (COP) and energy efficiency ratio (EER) at relatively stable ranges, and decrease the imbalance of ground temperature by 72.64 %, 75.03 %, and 76.98 %, respectively. Results of this research can offer references to the operation and enhance the comprehensive performances of CT-GSHP systems.