# A Long-Term Dynamic Analysis of Heat Pumps Coupled to Ground Heated by Solar Collectors
## 1 Introduction  
In Europe, natural gas is widely used for winter heating, and, nowadays, many greenhouse gas emissions still come from burning fossil fuels [1]. For this reason, the European directives have obliged the European States to develop alternative renewable energy sources, such as solar, wind, and geothermal, to reduce the carbon dioxide emissions in the atmosphere [2, 3]. Heat pumps and solar thermal systems realize the objective to reduce fossil fuels and to achieve decarbonization goals, including net zero greenhouse gas emissions at 2050 [4]. Heat pumps are particularly helpful for winter heating in temperate and mild climates and, therefore, could be particularly effective in Italy. A general analysis of the Italian energy system is reported in [5] and focuses on the possible energy, economic, and environmental effects of the use of individual heat pumps for winter space heating. Another aspect driving the decarbonization goals in the building sector is related to the envelope. For instance, different researchers investigated the use of glass facades [6, 7, 8]. Concerning the heat generation system, in the literature, many papers have investigated the evaluation of the seasonal coefficient of performance (SCOP) of heat pumps, both geothermal [9, 10, 11, 12] and air-source, also considering, in the latter case, the effect of the defrosting cycle [13]. Regarding the integration of heat pumps with solar panels, the so-called solar-assisted heat pump systems, the most used system considers a heat pump connected to photovoltaic panels [14, 15]. These systems suffer from a decreasein electric conversion efficiency, mainly at the latitudes of Southern Europe, when the temperature in the solar collectors rises. Moreover, in [16], the authors demonstrate that climate changes have influenced PV energy production in the past period and will continue to cause a reduction in energy production. In fewer papers, attention is paid to hybrid solar-geothermal heat pumps, although solar thermal panels and geothermal heat pumps have a high potential to reach the objective of the Paris Agreement [17]. As stated by [PERSON] and [PERSON] [18], thermal storage facilities ensure a heat reservoir for optimally tackling the dynamic characteristics of district heating systems. [PERSON] et al. [19] simulated a solar district heating system for a period of 5 years when serving a district composed of six typical single-family houses under the climatic conditions of Naples (central Italy). The analysis showed that the use of a seasonal borehole TES connected to a solar DH system allowed for a primary energy reduction of 6% and a reduction in carbon dioxide emission of more than 4%. Another work [20] demonstrated that seasonal storage combined with a solar plant could decrease energy consumption by about 26%. An experimental study [21] of a ground-coupled heat pump used in a 180 m\({}^{2}\) private residence, combined with thermal solar collectors, showed that the energy injected into the ground was 34% of the heat extracted from the ground by using a heat pump with a COP of 3.75 after 11 months of operation. In a recent work [22], the authors investigated, by means of dynamic simulation over a 15-year period, a solar-assisted, ground-coupled heat pump system that met the domestic hot water demand of 960 students. The simulation outcomes regarding the underground heat balance were compared with a standalone heat pump. The results showed that solar energy storage in the ground could lessen the thermal imbalance caused by continuous heat extraction and correspondingly could improve the system's performance. In addition, when the entire system was employed in regions where solar energy resources were consistent, the average temperature of the soil may increase year by year instead of decreasing as when a stand-alone heat pump is used. Thermal energy storage presents many advantages with respect to other storage systems, when compared with batteries in particular. [PERSON] et al.