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With the development of the economy and society, energy problems have become a great concern. The heat pump-coupled thermal energy storage (TES) system is a potential form of building heating, which can improve the stability of the grid and promote the consumption of renewable energy. Phase change materials (PCMs) are widely used in the field of building heating, but there are still some problems such as unsatisfactory melting points, low thermal conductivity, phase separation, and supercooling, which limit the application of PCMs in heat pump heating systems. Therefore, it is very important to improve PCMs by a performance improvement method. This work first summarizes the classification, advantages and disadvantages of PCMs, and introduces the connection between PCMs and heat pumps. Then, a detailed summary of PCMs applied in heat pump heating systems is presented, and a comprehensive review of the performance improvement methods for PCMs, which include additives, encapsulation, and eutectic compounds, is discussed. Finally, the existing problems, solutions, and future research directions are proposed. The emphasis of the research is to clarify the influence of PCMs on heat pump performance and the effect of different performance improvement methods on PCMs, and to illustrate the future development direction for PCMs in heat pump heating technologies, including the matching of heat pumps and PCMs, multi-standard decision methods and advanced control strategies.

In the process of development and utilization of a large-scale borehole heat exchanger (BHE) array system, the phenomenon of load shifting within BHE array can be observed. In this paper, OpenGeoSys software coupled with TESPy toolkit is used to establish a comprehensive numerical model of BHE system (without depicting the heat pump part), and the behaviors of load shifting between BHEs with different design parameters are studied. The results show that the outlet temperature of single BHE and BHE array is generally rising, and the soil temperature around the BHE has accumulated unbalanced heat. The soil temperature near the BHEs array fluctuates more obviously than the single BHE system, and the distribution is uneven. At the end of the 15th year, the soil temperature near the center BHE increased by 2 °C compared with the initial soil temperature, which was more favorable in winter, but was not conducive to the performance improvement in summer. Further analysis by changing the inter-borehole spacing shows that with the increase of the inter-borehole spacing, the load shifting behaviors are gradually weakened, and the maximum shifted load of the central BHE is linear with the change of the inter-borehole spacing. After changing the layout methods, we observe that the more intensive the layout is, the more load shifting behavior is, and the unbalanced rate of soil temperature distribution around the linear layout is lower than other layouts. With the increase in the number of BHEs, the load shifting behaviors are further enhanced. By analyzing the proportion of shifted load amount relative to the average value, it is found that the system will take a longer time to reach heat balance with the increase of BHEs’ number. A shutdown of part of BHEs for a certain period of time will help to improve the long-term operational efficiency of the large-scale shallow ground source heat pump (GSHP) system.

To contribute to the goal of carbon neutralization, the closed-loop borehole heat exchanger system is widely applied to use geothermal energy for building cooling and heating. In this work, a new type of medium-shallow borehole heat exchanger (MSBHE) is proposed, which is coaxial type and has a depth range between 200 m to 500 m. To investigate the long-term performance of MSBHE in the area with unbalanced cooling and heating load of buildings and the sustainable load imbalance ratio under different design parameters, a comprehensive numerical model is established. The results show that the drilling depth significantly influences the sustainable load imbalance ratio of MSBHE. As the drilling depth is increased from 200 m to 500 m, the load imbalance ratio of the MSBHE increases from 20.76% to 60.29%. In contrast, the load imbalance ratio is always kept at the same level with different inlet velocities and operation modes. Furthermore, in a 9-MSBHE array system, the heat exchanger located in the middle of the array has the lowest load imbalance ratio of 48.97%, which is 15.98% lower than the borehole in the edge location. This is caused by the significant influence of the shifted-load phenomenon among MSBHEs in an array system. The findings of the work imply that this newly proposed MSBHE can sustain a notable load imbalance ratio, which is particularly applicable to the areas with a strong imbalance of annual building load.

When a Borehole Heat Exchanger (BHE) array is coupled with heat pump to provide cooling and heating to the buildings, thermal interaction between BHEs may occur in the subsurface. In the long term, imbalanced seasonal thermal load may lead to low or high temperature zones accumulating in the centre of the array. In this study, numerical models are configured according to a real BHE array project in Leicester, UK, and verified against monitoring data. Based on this reference model, a series of numerical experiments are conducted to investigate the response of circulation fluid temperature to different settings of imbalanced thermal load. It is found that over long-term operation, the sub array with a larger number of installed BHEs is shifting its thermal load towards the other branch with less BHEs installed. Within each sub array, the heat injection rate on the central BHEs is gradually shifted towards those located at the edge. A linear correlation is also found between the working fluid temperature increment and the amount of the accumulated heat injected into the subsurface.

Application of the medium-deep borehole heat exchanger (MDBHE) coupled ground source heat pump (GSHP) system on building heating has been gradually accepted by public and its sustainability becomes a key concern. In this work, a long-term transient heat transfer model for the MDBHE coupled GSHP system is proposed to investigate the effects of various influencing factors on the thermal extraction and energy efficiency of this system. Sensitive analysis highlights the significant effects of the specific heat transfer rate, rock-soil thermal conductivity, geothermal gradient, pipe depth and fluid velocity on the annual decline of the system performance during its long-term operation. Furthermore, the operation optimization for this system should be conducted based on its performance in quasi-steady state. An optimal fluid velocity exists to acquire the maximum specific heat transfer rate of MDBHE under a constant Seasonal Performance Factor of Heat Source (SPF2). In order to guarantee an efficient thermal extraction, the comprehensive effects of both specific heat transfer rate and fluid velocity on the energy efficiency should be taken into account. For the benchmark MDBHE, the operation condition is optimized with the specific heat transfer rate of 142 W/m, and the fluid velocity of 0.7 m/s; the annual SPF2 is always greater than or equal to 4.0 during the long-term operation, which ensures a sustainable and efficient operation.