To reduce carbon emission and achieve carbon neutrality, deep geothermal energy has been widely extracted for building heating purpose. In recent years, deep borehole heat exchanger (DBHE) heating system has gained more attention, especially in densely populated urban areas in Weihe Basin, northern China. The long-term performance and the economic feasibility are essential for the system application. In this work, the DBHE model implemented in OpenGeoSys software is verified against an analytical solution and a comprehensive economic analysis approach is further proposed. Then the short-term thermal performance tests are conducted to obtain the tentative heat extraction capacity for long-term simulation. The long-term simulations are further performed with the heat pump unit under the adjusted tentative heat extraction rate imposed on the DBHE. Finally, a comprehensive economic analysis is applied to the DBHE heating system over 15 heating seasons. Results show that the minimum coefficient of performance value of the heat pump is 4.74 over the operation of 15 heating seasons. With the increase of depth for the DBHE, the total electricity consumption of heat pumps and circulation pumps has a prominent promotion. With the comprehensive approach of economic analysis, the depth of 2,600 m has the lowest levelized cost of total heating amount, which is the best system design for the application in Weihe Basin. The present results are specific to the conditions in Weihe Basin, but the proposed economic analysis approach is generic.

This study proposed a combined solar-assisted heat pump (SAHP) system that could operate in the serial mode or parallel mode. For this proposed system, a stable year-round operation could be achieved without the assistance of electric heating or low-temperature heat pump. By analyzing the heat balance equations, a correlation of the combined SAHP system for the two modes switched was obtained, which provided a theoretical basis for the optimal operation of this system. In addition, the performance of the proposed system applied in a university bathroom in Xi’an district was investigated using TRNSYS. The results illustrated that compared to the serial and parallel systems, the proposed system exhibited a good performance on energy efficiency. The annual average coefficient of performance (COP) of the proposed system was 5.7, obviously higher than those of the serial system and the parallel system, which were 3.3 and 4.3, respectively. Therefore, the results in this study could provide the theoretical guidance and reference for practical engineering design.

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.