Even though the exergy approach gives a more complete picture of energy flows, the energy balance analysis remains the more common method of energy systems evaluation. However, some investigations have been conducted by researchers in the exergy-based evaluation of heat pumps performance. Cakir et al. [
2] performed an experimental study using an air-to-water heat pump and investigated the relationship between the exergy performance of the heat pump and its components. The authors also analyzed the effects of working conditions of the components on the system performance. It turned out that the influence of the components on the exergy performance varies with the heat source temperature. Li et al. [
3] performed an exergy analysis of both ground source heat pump and air source heat pump in the cooling mode. The authors showed that the ground source heat pump consumes less exergy than the air source heat pump. The reason is that the ground source heat pump drains “cool exergy” from the ground, while no “cool exergy” is available in the surrounding air. The second reason is a smaller temperature difference between cooling water and heat source for the ground source heat pump. Esen et al. [
4] studied the energetic and exergetic performance of a ground source heat pump as a function of the horizontal ground heat exchanger depth for heating purpose. The results confirmed that the energy and exergy efficiencies of the heat pump rise with an increase in the heat source temperature for the heating season. Moreover, it was confirmed that an increase in the surrounding environment temperature causes a decrease in both efficiencies. The conclusion drawn by Esen et al. was confirmed by Verda et al. [
5]. They compared the exergy performance of ground source heat pumps differing in depths of the horizontal ground heat exchanger. The authors showed that the deeper installation allows an increase of exergy output. Habtamu et al. [
6] performed a more detailed exergy analysis of a ground source heat pump coupled with vertical ground loop pipes for heating application. The authors took into consideration the impact of the ground depth, the brine mass flow rate, types of refrigerants, the heat delivery temperature and the reference state of the surrounding environment. In the study by Abbasi et al. [
7] regarding exergy and sustainability analysis of a ground source heat pump that is coupled with a photovoltaic system for cooling and heating application, the authors evaluated the exergy destruction rate and sustainability indices of different components of the system. They considered the influence of monthly thermal load variations on the system performance. They concluded that the system caused significant exergy losses, especially in the heating season. The exergy destruction occurred mostly in PV panels, condenser, and evaporator. Hu et al. [
8] presented energy and exergy analysis of a ground source heat pump installed in a public building under five different control strategies. Parameters, such as exergy efficiency, exergy loss, COP and energy consumption, were calculated. The proposed control strategies were compared with the actual control system—manual operation. It was concluded that the best control strategy is variable flow control by variable speed pumps. Menberg et al. [
9] developed a thermodynamic model of a hybrid ground source heat pump system with a supplementary boiler for heating and cooling. The study showed that for heating and cooling different system components attribute most to the overall exergy loss in the system. The authors concluded that improvement measures, such as changes in the operational settings and an upgrade of the building envelope, have a more significant impact on heating performance than on cooling performance. Moreover, decreasing thermal loads is a more effective improvement measure than changes in the operational settings of energy supply systems. Ozgener et al. [
10] developed an energetic and exergetic model of two ground source heat pump systems—a solar assisted vertical ground source heat pump and a horizontal ground source heat pump. The authors based their investigation on experimental data. The solar assisted vertical ground source heat pump achieved higher values of COP and exergy efficiency. Cho [
11] performed a comparative study on the performance and exergy efficiency of a solar hybrid heat pump using refrigerants R22 and R744. The author ran experimental tests on a sunny and cloudy day. Then he determined the COP values and the exergy losses. The exergy efficiency of the heat pump using the refrigerant R22 was higher than that of the R744. Bi et al. [
12] performed a comprehensive exergy analysis of a ground-source heat pump system for building heating and cooling. The authors derived the analytical formulae of various exergy indices. It was shown that the indices should be used integratively. The largest exergy losses occurred in the compressor. The lowest exergy efficiency and thermodynamic perfect degree were achieved in the ground heat exchanger. Moreover, it was concluded that the exergy loss of the system for the heating mode is larger than for the cooling mode. Hepbasli [
13] performed an exergy analysis of a solar assisted domestic hot water tank integrated with a ground source heat pump for a residence. The author derived exergy relations for each system component and the whole system. Both experimental and assumed values were used for the calculations. The exergy efficiency of the whole system was 44.06% at the reference state of 19 °C and 101.325 kPa. The greatest exergy destruction occurred in the condenser. Saloux et al. [
14] proposed an exergy modeling approach for heat pumps that do not require a knowledge of refrigerant thermodynamic parameters. The energy and exergy balance is based on energy terms and energy quality factors. A mathematical method for refrigerant operating temperatures calculation is proposed. The authors presented a graphical exergy representation that allows localizing exergy losses sources. The model was applied to water-to-water and air-to-air heat pumps. The results were compared with the reference ones that were calculated using the classical thermodynamic cycle method. Stanek et al. [
15] performed an exergetic and thermo-ecological assessment of a heat pump powered by electricity generated from renewable sources. The authors pointed out that the ecological efficiency of heat pumps is dependent on their performance and on the energy mix used for the electricity production. The efficiency can be improved by renewable energy sources application. The authors evaluated a heat pump system using thermo-ecological cost (TEC) indices. The analyzed system consisted of an electricity-driven heat pump, and PV panels or wind turbines that acted as the priority electricity source.
The aforementioned studies are examples of the exergy analysis application. However, the literature survey showed that the energy analysis is still the more popular method of energy systems evaluation. It is used both by researchers and policymakers. Unfortunately, energy analysis is a limited tool to analyze vapor compression heat pumps operation. It is due to the fact that the energy balance equation considers the quantity of various energy forms without distinguishing their quality. That is why the exergy approach based on the first and the second law of thermodynamics has to be used to evaluate heat pump operation. Application of the exergy analysis may contribute to a more rational consumption of energy sources due to a reduction of high-quality non-renewable energy sources consumption. The exergy model of a heat pump and a heat supply system equipped with heat pumps are presented below.