## 3 Results and Discussion  
### Estimation of Quality  
While estimating the state of the refrigerant at the compressor suction by the EEV opening control in a heat pump system, it is assumed that the capacity of the water and refrigerant side is equal, and that the refrigerant, through the expansion valve, is under a constant enthalpy process without experiencing a throttling loss, implying that the expansion process results in no heat loss. For the validation of the energy balance between the water and refrigerant side, the mass flow rate of the refrigerant was calculated by the water capacity and the enthalpy difference between the evaporator inlet and outlet. The error was confirmed between the calculated mass flow rate and the measured mass flow rate at the condenser outlet. As shown in Figure 2, the results were in good agreement, within \(\pm 5\%\) for the entire experimental range. The quality (\(x\)) at the compressor suction was calculated using Equations (1)-(5), as follows:  
\[\dot{Q}_{\text{w,evap.}}=\dot{Q}_{\text{r,evap.}} \tag{1}\]  
\[\dot{Q}_{\text{w,evap.}}=\dot{m}_{\text{w}}\cdot c_{\text{p}}\cdot\big{(}EWT_{ \text{evap.}}-LWT_{\text{evap.}}\big{)} \tag{2}\]  
\[\dot{Q}_{\text{r,evap.}}=\dot{m}_{\text{r}}\cdot\big{(}h_{\text{o,evap.}}-h_{ \text{in,evap.}}\big{)} \tag{3}\]  
\[h_{\text{o,evap.}}=\frac{\dot{Q}_{\text{w,evap.}}}{\dot{m}_{\text{r}}}+h_{ \text{in,evap.}} \tag{4}\]  
\[x=\frac{h_{\text{o,evap.}}-h_{\text{l,sat.}}}{h_{\text{g,sat.}}-h_{\text{l, sat.}}} \tag{5}\]  
where \(h_{\text{o,evap.}}\) is the local specific enthalpy of the fluid (single phase or two-phase) at the compressor inlet, and \(h_{\text{l,sat.}}\) and \(h_{\text{g,sat.}}\) are the saturated liquid and vapor enthalpies, respectively. Thus, \(0<x<1\) (\(h_{\text{l,sat.}}<h_{\text{o,evap.}}<h_{\text{g,sat.}}\)) is in the two-phase compression region (called wet compression), whereas \(x>1\) (\(h_{\text{o,evap.}}>h_{\text{g,sat.}}\)) is in the superheated compression region (called dry compression).  
Figure 3 shows the quality as a function of the EEV opening at a different entering water temperature of the outdoor heat exchanger (EWT of OD HEX). With an increase in the EEV opening, the quality decreased for all EWT of OD HEX, until it remained nearly constant at larger openings, depending on the EWT of OD HEX. The increase in the EEV opening results in an increase in the refrigerant flow rate, leading to a larger capacity of the refrigerant side than of the water side. Thus, because of the increase in the liquid droplets, the state of the compressor inlet varied to two-phase mixtures, with a quality lower than one. The slope of quality decreased gradually, owing to the decrease in the pressure difference between the high and low pressure side, by increasing the EEV opening. The quality remained constant after a specific EEV opening, namely the EEV openings of 35%, 30%, and 25% for the EWT of OD HEX at 10 \({}^{\circ}\)C, 5 \({}^{\circ}\)C, and 0 \({}^{\circ}\)C, respectively. Generally, if the system works at a constant superheat, it has to decrease the mass flow rate to meet the required superheat, as the evaporator capacity decreases with a decrease in the EWT of OD HEX. Thus, the quality decreased with a decrease in the EWT of OD HEX, for the same EEV opening (see Figure 3).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Uncertainty** & **Full Scale** \\ \hline Pressure transducer & \(\pm 0.3\%\) & 50 bar \\ Temperature (T-type thermo-couples) at refrigerant side & \(\pm 0.5\) \({}^{\circ}\)C & \(-200\)–\(400\) \({}^{\circ}\)C \\ Temperature (Pt-100 \(\Omega\)) at water side & \(\pm 0.1\) \({}^{\circ}\)C & \(-100\)–\(100\) \({}^{\circ}\)C \\ Mass flow meter & \(\pm 0.25\%\) & 10 kg-min\({}^{-1}\) \\ Turbine flow meter & \(\pm 0.5\%\) & 2.2 m\({}^{3}\cdot\)h\({}^{-1}\) \\ Power meter & \(\pm 0.1\%\) & 12 kW \\ \hline \hline \end{tabular}
\end{table}
Table 3: Specification of the sensors.