This chapter focuses on several advanced applications of implementing various RES in buildings and DHSs. An energy and environmental analysis of an existing ground-coupled heat pump (GCHP) system with a single U-tube borehole heat exchanger (BHE) installed in an experimental laboratory located in Timisoara, Romania, is performed. Exploratory measurements are used to determine the GCHP's performance at various operating modes. A variable flow rate control method for the circulation pump is proposed to enhance system efficiency and reduce energy consumption during heating and cooling operations. Thus, energy optimisation equipment for the system utilising quantitative adjustment with a buffer tank (BT) and a variable-speed circulation pump is implemented. A comparative analysis of primary efficiency parameters (e.g., coefficient of performance (COP), CO2 emission ( \(M_{{CO_{2} }}\) )) achieved over one month of operation, utilising the two control strategies—the standard and optimised adjustment of the water pump speed in various operating modes—is presented. In addition, a numerical simulation model for heat and cold consumption in heating and air conditioning modes, utilising TRNSYS software, is developed. At last, the simulation acquired is investigated and compared with test estimations. The thermal protection influence of a residential building on the energy and environmental efficiency of the air-source heat pump (ASHP) floor heating system is explored through in-situ investigations and numerical simulations in two test cases: (1) conventional ASHP and (2) hybrid ASHP-PV. The energy and environmental performances (COP, \(M_{{CO_{2} }}\) ) of the heating system interconnected to the conventional ASHP are evaluated by in-situ monitoring during two months of heating mode operation. Additionally, the main performance parameters of the heating system supplied by the ASHP, combined with a PV array, are investigated through numerical simulation using Polysun software for 8760 h, both in heating mode and in heating and domestic hot water (DHW) mode. The complex efficiency indicator (seasonal performance factor), SPFASHP-PV, is used to estimate the energy efficiency of the ASHP-PV system. Finally, a comparative financial analysis of the two heating systems is achieved. The performances (COP and CO2 emission) of various radiant heating and cooling systems (floor, wall, ceiling, combined floor-ceiling) of low- and very low-temperature and the conventional heating system with medium-temperature water radiators, connected to a closed-loop GCHP in double U-tube BHE configuration, are also experimentally investigated together with indoor thermal comfort. Some numerical simulations of the useful thermal energy and the system COP in heating and cooling modes are performed with TRNSYS software for a duration of 8760 h and then analysed and compared with the experimental tests to validate the simulation models. Restoring the thermal imbalance of the soil caused by conventional GCHP systems through seasonal and continuous regeneration using a geothermal-solar installation with a vertical reversible GCHP, combined with PV and PV/T panels, in a heating-dominated climate is also addressed. Thus, the fundamental efficiency parameters of a GCHP system in single and double U-tube BHE configurations with regeneration during the summer season, achieved by injecting and storing heat into the BHE integrated with the heating circuit of an experimental office, are experimentally obtained. Additionally, the results of a comparative theoretical study carried out with the Polysun numerical simulation software are presented regarding the performances of the hybrid GCHP-PV/T system with continuous regeneration throughout the year by PV/T array and of a conventional GCHP, integrated in the heat supply and DHW production system for both the experimental office and a single-family building. A detailed comparative transient heat transfer analysis is conducted between single and double U-tube BHEs with two independent circuits. In addition, the possibility of storing industrial waste heat in the ground using the BHE and GCHP and then utilising it to heat living spaces or produce hot water for domestic use, offering, in specific scenarios, the efficiency of the storage, is discussed. A comparative economic, energy, and environmental impact study is conducted for the heating and cooling of an existing multifunctional building using various primary energy sources, which justifies the opportunity for the heating and cooling solution with closed-loop GCHPs featuring vertical collectors. A case study for Timisoara, Romania, is included regarding the integration of RES in the form of a solar-assisted water-source heat pump (SAWSHP) system composed of water-source heat pumps (WSHPs) and ST and PV panels into a 3GDH system. This heating system is converted into a low-temperature 4GDH system, which supplies the water radiators serving as heating terminal units for consumers and produces the DHW they require. Installed PV panels generate electrical energy that is utilised to balance the electricity consumption of WSHP systems. Additionally, it demonstrates the potential for recovering excessive hydraulic energy from the heat transmission network, which is dissipated by pressure-reducing valves (PRVs), by utilising micro-hydropower technology with pumps operating as micro-hydro turbines (PATs). The results of an experimental study on the thermal wastewater potential energy conversion from a surface-water heat pump (SWHP) into electricity by connecting it to a Pelton microturbine coupled to a synchronous electric generator (SG) are also included. Based on the experimental curves, analytical equations are determined for the calculation of the recoverable hydraulic power at the outlet of the evaporator and the inlet of the Pelton microturbine, the load operating characteristic of the SG, and the efficiency of the hydraulic energy conversion. Finally, a literature review on several applications of implementing various RES in buildings and DHSs is included.