• Energy Research

A new practical method for thermal response test (TRT) is proposed herein to estimate the groundwater velocity and effective thermal conductivity of geological zones. The relaxation time of temperature (RTT) is applied to determine the depths of the zones. The RTT is the moment when the temperature in the borehole recovers to a certain level compared with that when the heating is stopped. The heat exchange rates of the zones are calculated from the vertical temperature profile measured by the optical-fiber distributed temperature sensors located in the supply and return sides of a U-tube. Finally, the temperature increments at the end time of the TRT are calculated according to the groundwater velocities and the effective thermal conductivity using the moving line source theory applied to the calculated heat exchange rates. These results are compared with the average temperature increment data measured from each zone, and the best-fitting value yields the groundwater velocities for each zone. Results show that the groundwater velocities for each zone are 2750, 58, and 0 m/y, whereas the effective thermal conductivities are 2.4, 2.4, and 2.1 W/(m∙K), respectively. The proposed methodology is evaluated by comparing it with the realistic long-term operation data of a ground source heat pump (GSHP) system in Kazuno City, Japan. The temperature error between the calculated results and measured data is 6.4% for two years. Therefore, the proposed methodology is effective for estimating the long-term performance analysis of GSHP systems.

Ground source heat pump (GSHP) systems are being applied in various buildings to achieve carbon neutrality and zero energy building. Large-scale residential buildings have seen limited use of GSHP systems due to the absence of design guidelines and established operation methods. While design guides for general buildings are available from associations or institutes, there is a scarcity of designs and operation guides that consider dynamic simulations with occupant conditions and detailed load conditions. Generally, it is crucial for the economic analysis to optimally design suitable system capacity and accurately predict the performance. Therefore, in this study, the dynamic energy simulation model was constructed by considering the condition of the occupants and system operation and quantitively analyzed economic feasibility and system performance. Furthermore, the sensitive analysis based on the practical range of operation was conducted for comparison with conventional systems, district heating (DH) and electric heat pump (EHP) system. The heat pump's performance improved by up to 12.6% based on GSHP system design factors. Although the initial investment cost of the GSHP system was 51.5–84.7% higher than DH and EHP systems, its annual operating cost was 20.8–33.1% lower. The ground source heat exchanger had the most significant impact on performance, while the thermal storage tank's capacity had the largest impact on annual operating costs, aligning with previous research findings. In terms of carbon emissions, the annual CO2 emissions of the GSHP system were 49.1% lower than those of the existing system. The study highlights that GSHP systems can achieve high performance, economic efficiency, and low CO2 emissions based on the design approach. Additionally, the study provides valuable insights into GSHP system design, offers quantitative feasibility data, and compares them to traditional heating and cooling systems, taking practical cost considerations into account.

Abstract The purpose of this study was to determine the life cycle cost (LCC) of a ground source heat pump (GSHP) system based on the results of a multilayer thermal response test (TRT). Previous studies by our research team suggested the possibility of significantly reducing the total borehole length, based on the results of the multilayer TRT to identify the partial groundwater flow. In the present study, it was validated that the borehole heat exchangers reduced by the results of the partial groundwater flow could operate with a sustainable performance of the GSHP system over an extended period. LCCs of the GSHP system for 30 years were analyzed in three areas with different climatic and geological conditions. Based on these results, the proposed method demonstrates a reasonable cost reduction advantage compared with conventional TRT analysis. The multilayer TRT analysis was validated using root mean square error and temperature error, obtained from temperature comparison between the numerical simulation results and the TRT data. The required total borehole length was achieved when the performance of the GSHP system attained the target performance for heating, after 30 years of operation. The GSHP system, designed by considering the partial groundwater flow, can reduce 30–40% of the total borehole length and 15–20% of the LCC, compared with the system designed based on conventional TRT analysis.

Thermoelectric generators (TEGs) harness temperature differences to produce electricity and hold promise for diverse industrial applications. However, their limited conversion efficiency casts doubt on their role in achieving energy independence. This study introduces an advanced technique that exploits temperature gradients in water pipes, utilizing supplementary TEGs to augment power generation. This method maintains a stable temperature gradient for TEG operation. Additionally, TEG power output can be efficiently modulated via flow control. In the feasibility evaluation for residential settings, the temperature fluctuations across each system unit were analyzed. In the active system, the chosen sites for TEG integration were units equipped to manage heat transfer using working fluids. The inlet and outlet temperatures were calculated for photovoltaic-thermal (PVT) systems, ground heat exchangers (GHEs), and heat storage tanks (HSTs). The electricity produced by the TEGs was benchmarked against their conversion efficiency, zT. The results indicated that the TEGs yielded 10.95 kWh of electricity when systematically implemented in each unit. To realize a zero-energy building, an area of 64.5 m2 per unit is necessitated for TEG deployment, given a zT value of 1.