## 3 Results and Discussion
### Heat Pump Cycles in a p-h Diagram  
The previous part of the results showed how increasing the system volume affects the output and the performance of the plant. This subsection shows the characteristic changes in the heat pump operation cycle in a _p-h_ diagram when the plant is operated in the above-mentioned fashion. The graphs below are based on the experimental results. Figure 15 shows a typical case of compressor control, with constant expansion valve capacity at 70%, the compressor speed varying between 1800 and 2220 (intermediate cycles of 1900 and 2100 are not shown), and the heat pump loop volume of V0. It can be seen that the variation of the compressor speed has a small effect on the differences between the superheating and sub-cooling temperatures of the cycle, which remain similar. In addition, increasing the compressor speed increased the condensation temperature and decreased the evaporation temperature. This trend comes into light because increasing the compressor speed results in a higher rate of refrigerant flow through the compressor, which also results in a higher flow rate through the expansion valve, and a lower pressure is generated behind the expansion valve than at speed. It can therefore be argued that the change in the compressor speed has a direct influence on the operating mode of the evaporator. Similar trends can be seen in other throttle cases as well. Figure 16 shows a case where the heat pump is controlled by an expansion valve, its flow rate varying between 30% and 70%, with a fixed compressor speed and a fixed volume of the HP loop at V0. The change of the expansion valve flow rate had a negligible effect on the condensing temperature (the difference between the limiting cases is 2.75 \({}^{\circ}\)C), but a more significant effect on the evaporating temperature of 6.83 \({}^{\circ}\)C between the valve flow rate cases of 30% and 70%. In addition, controlling the heat pump with the expansion valve was able to affect the superheating and sub-cooling temperatures of the cycle. Reducing the valve permeability from 70% to 30% resulted in an increase in the differences between the superheating and sub-cooling temperatures of 19.33 \({}^{\circ}\)C and 6.87 \({}^{\circ}\)C, respectively. Therefore,  
Figure 15: The operation cycles of the HP in a _p-h_ diagram (compressor speeds: 1800 and 2220 rpm; expansion valve capacity: 70%; volume: V0). the position of the expansion valve has a significantly greater impact on the operating mode of the evaporator than the case of changing the compressor mode. Changing the volume of the heat pump circuit had the most significant effect on the heat pump operation cycle (Figure 17). Increasing the volume from V0 to V1 resulted in a virtually uniform \"lowering\" of the cycle in the graph. In this case, the condensation and evaporation isotherms were reduced by 8.30 \({}^{\circ}\)C and 8.22 \({}^{\circ}\)C, respectively, while the temperatures upstream and downstream of the compressor were similar. As the system volume was further increased to V2 and V3, the condenser and evaporator lines moved steadily in the direction of the pressure drop. However, the cycles did not reach/pass into the two-phase region, which means that no refrigerant condensation process occurred in the condenser and the whole cycle was in the superheated vapour region. The high temperature and pressure vapour leaving the compressor was cooled but not condensed in the condenser. In these cases (V2, V3), the enthalpy differences between the condenser and the evaporator were reduced by several times compared to the cases V0 and V1. From the _p-h_ diagrams, it can be seen that changing the volume of the system extends the operating range compared to standard measures. An increase in volume by 17% improves the controllability, but an increase in volume by more than 26% is detrimental to the performance as no evaporation or condensation are achieved.