## 1 Introduction
### Experiments on the Improvement of the Vapor Compression Cycle  
The vapor compression cycle has thermodynamic losses in comparison to the ideal reverse Carnot cycle. The compression of a single-phase gas and isenthalpic expansion are related to these losses [PERSON] et al. (2018). The first set of losses results in high refrigerant temperatures, high compression work, and significant heat release in the condenser. When considering the vapour compression cycle for refrigeration purposes, these losses are seen as a negative factor. However, when the cycle is used for heating, some of these drawbacks can be turned into advantages, converting energy losses into useful energy. Additionally, in cold climate zones where air-source heat pumps operate, a high compression ratio is necessary, as it determines the outdoor temperature at which the system can function. The second set of losses leads to significant throttling losses and low refrigeration capacity. In examining the advancements in vapour compression cycle technologies aimed at reducing thermodynamic losses and improving efficiency/effectiveness, three directions of cycle improvement are distinguished: sub-cooling cycles, expansion loss recovery cycles, and multi-stage cycles. Research into sub-cooling cycles focuses on the internal heat exchanger of the suction line [PERSON] et al. (2018); [PERSON] et al. (2018), the thermoelectric cooler [PERSON] et al. (2018); [PERSON] et al. (2018), and the mechanical secondary cooler [PERSON] et al. (2018); [PERSON] et al. (2018). Expansion loss recovery cycles mainly concentrate on expanders (essentially, turbine use) [PERSON] et al. (2018); [PERSON] et al. (2018) and ejectors [PERSON] et al. (2018); [PERSON] et al. (2018) used in vapour compression cycles. Multi-stage cycle studies include vapour or liquid-phase refrigerant injection cycles [PERSON] et al. (2018), two-phase refrigerant injection cycles [PERSON] et al. (2018), and saturation cycles [PERSON] et al. (2018). Furthermore, the heat pump cycle is examined from the perspective of the discharge temperature of the refrigerant. In one study [PERSON] et al. (2018), three cycle improvements were modelled to reduce the compressor discharge temperature. The impact of two-phase refrigerant pumping, liquid-phase injection, and two-phase injection on the compressor discharge temperature was analyzed at fixed condensation and evaporation temperatures. The research shows that all three methods have the potential to reduce the discharge temperature and thus increase the adiabatic efficiency of the compression process. However, two-phase injection outperforms liquid injection and two-phase suction for both heating and cooling efficiency by 4.8% and 11.8%, respectively. Some researchers aim to improve the efficiency of the vapor compression cycle not through individual cycle measures (directly in contact with the refrigerant) but by altering the parameters of the secondary fluid. In their work [PERSON] et al. (2018), the authors presented a heat pump operating in a refrigeration mode with air, pre-cooled by an indirect water vapour cooling condenser, passing through it. Thermodynamically, this solution reduces the refrigerant gas condensation temperature, consequently decreasing the work performed by the compressor. In hot and dry climates, this method can reduce cooling energy consumption by about 20%, but it requires a significant amount of water for the water vapour cooling condenser to function. Efficiency is sought through a combination of maximizing load selection with modulation and on/off control systems [41], where the use of storage tanks reduces the number of compressor on/off cycles. An extractable air heat pump or extractable air heat recovery system is essentially an air-to-air heat pump.