Ground-source heat pumps (GSHPs) are widely used in heating-dominated regions because air-source heat pumps have extreme performance degradation under cold climate conditions. Although GSHPs use the ground as a heat source with a constant and higher temperature than the air temperature, the performance of GSHPs also can be degraded when GSHPs are used most for heating operation, which decreases the ground temperature gradually over the time. To solve the ground thermal imbalance and decrease in the ground temperature, solar-assisted ground-source heat pumps (SGSHPs) are proposed and investigated comprehensively. Most of studies focus on improving the ground thermal conditions by injecting solar heat into the ground, which is called serial configuration. However, when solar heat is used through a parallel configuration, the performance of SGSHPs can be improved further. Since study on a parallel configuration in SGSHPs is still lacking, this study investigates the parallel configuration in SGSHPs through experiments, simulation, and optimization.
The performance characteristics of the HGSHPs with the serial and parallel configurations are investigated through experiments by varying the temperature and flow rate of the high-temperature heat source. The HGSHP with the serial configuration has two operation modes: serial and forced recovery modes, whereas the HGSHP with the parallel configuration has two operation modes: parallel and natural recovery modes according to the flow rate and temperature of the high-temperature heat source. The heating performance and efficiency of the parallel configuration are higher than those of the serial configuration owing to the direct heat transfer between the refrigerant and working fluid of the high-temperature heat source. In addition, the temperature difference limit where the serial and parallel modes should be switched to natural and forced recovery modes is investigated and covered.
From the experimental results, TRNSYS simulation is conducted to investigate the long-term performance of the SGSHPs with the serial and parallel configurations in heating-dominated regions. With various solar collector area and borehole depth, the operating characteristics and energy consumption of the SGSHPs with the serial and parallel configurations are investigated over 20 years. The operating characteristics of the SGSHPs with the serial and parallel configurations vary according to the climate conditions and heating load. The improvement in the ground temperature of the SGSHPs with both configurations is investigated and compared to the ground temperature of the GSHP. In addition, from the solar heat, the energy savings of the SGSHPs with the serial and parallel configurations are investigated and compared to the GSHP according to various system size and regions. Accordingly, the benefits of applying the serial and parallel configurations in SGSHPs in the regions over the GSHP can be found in this study.
Finally, optimization is performed to evaluate the maximum benefits of the serial and parallel configurations in the SGSHPs compared to the GSHP in the regions. Refrigerants, solar collector area, and borehole depth are optimized to minimize the life cycle cost (LCC) and life cycle climate performance (LCCP) of SGSHPs with serial and parallel configurations. For this, TRNSYS simulation is conducted according to the various design parameters, and articial neural network (ANN) is used to predict the LCC and LCCP of the SGSHPs according to the design parameters. Finally, optimal refrigerant and system size are found using multi-objective genetic algorithm (MOGA). The optimal results of the SGSHPs with the serial and parallel configurations are compared to the GSHP.
This study brings powerful knowledge and insights about the effects and benefits of applying a parallel configuration to SGSHPs in heating-dominated regions over a serial configuration. In addition, the optimization results can bring meaningful strategy for designing SGSHPs with a parallel configuration in heating-dominated regions to minimize the LCC and LCCP considering comprehensive aspects such as climate conditions, heating load, electricity cost, electrical CO₂ emissions, and refrigerant regulation.