## 1 Introduction  
Space heating plays an important role and accounts for nearly 21% of energy consumption in buildings [1]. The heating demand and energy consumption of space heating are expected to increase in the future, challenging the clean-development tasks globally. For energy conservation, as well as sustainable development, electrically driven heat pump systems have been studied and applied for space heating during recent decades [2]. The key factor of this technology is to obtain the high-temperature and stable heat source, which directly determines the energy performance of this system [3]. Among various kinds of heat pump systems, the ground-coupled heat pump systems (GCHPs), which extract heat from the geothermal energy, can perform efficiently and stably [4]. Thus, it has been widely applied and studied worldwide [5]. Some researchers have conducted field tests to examine the practical energy performance of GCHPs. [PERSON] [6] conducted an experiment to analyze the energy performance of GCHPs for space heating in Turkey. Results showed that the annual Coefficient of Performance (COP) of heat pumps reached 3.42. [PERSON] [7] measured the energy performance of 10 GCHPs, and 11 air source heat pumps (ASHPs) applied for space heating in residential buildings in China. Results showed that the COP of GCHPs varied from 2.32 to 5.15 with an average value of 3.72, while the COP of ASHPs varied from 1.70 to 3.74 with an average value of 2.59. Consequently, the GCHPs were more energy-saving and economically preferable than direct electric resistance heating, gas heating, coal fired heating and ASHPs [6]. However, with numerous studies carried out to examine the practical energy performance of GCHPs, thermal imbalance of the ground and the occupation of huge spaces [8] were identified the two typical issues limiting the application of GCHPs. In order to eliminate thermal imbalance and improve the economic effect of GCHPs, the combinations of other energy systems have been put forward. Some researchers [9; 10] analyzed the combination of GCHPs with solar energy, and indicated that the solar energy could compensate the thermal imbalance of ground and improve the energy performance of the GCHPs. You [11] proposed a heat compensation unit with thermosiphon (HCUT) and also applied it for domestic water systems [12]; thus, the HCUT can effectively eliminate the thermal imbalance of ground, as well as decrease the payback period. However, the initial investment and space occupation will increase. A straightforward method to solve these issues and produce space heating is to utilize the deeper geothermal energy, which uses enclosed deep borehole heat exchangers (DBHEs) with a depth more than 2000 m to extract heat from the medium depth geothermal energy with temperature around 70 to 90 \({}^{\circ}\)C (MD GHPs) [7]. Benefiting from high-temperature heat source, the medium-depth geothermal heat pump systems (MD-GHPs) could perform better than conventional GCHPs, which is named shallow-depth geothermal heat pump systems (SD-GHPs) in this paper. This method was first introduced and carried out in America and Europe, where the outlet water temperature of DBHEs could reach nearly 98 \({}^{\circ}\)C in Hawaii [13] and 40 \({}^{\circ}\)C in Switzerland [14]. Besides, this method was successfully used as a heat source in a university in Germany [15]. This method was also applied for space heating with heat pump systems in China. [PERSON] [16] conducted a field test on the energy performance of MD-GHPs for five projects. Results showed that the outlet water temperature from DBHEs with a depth of 2500 m could reach 34.7 \({}^{\circ}\)C with heat extraction of 273 kW per DBHE. Besides, the COP of the heat pump reached 5.70, which was much higher than SD-GHPs. Benefiting from the higher energy performance, the operation cost of MD-GHPs is nearly 51.9% lower than that of ASHPs, 25.4% lower than that of SD-GHPs and 36.4% lower than that of Gas boilers.