# A Sustainable Polygeneration System for a Residential Building
## 2 Materials and Methods
### Energy Analysis  
The energy KPI selected is the primary energy saving ratio (PESR). It compares the energy performance of the proposed system (PS) through a comparison with a suitable reference system (RS). The PS solely considers the demand unattended by electricity from the grid, because, in all cases, the demand for air conditioning and water is served at 100%. The RS supposes all demands are produced by conventional technologies based on fossil fuels and appropriate conversion efficiencies (see Table 6 for details). The PES and its ratio (PESR) are calculated as shown in Equations (1) and (2):  
\[PES=PE_{RS}-PE_{PS} \tag{1}\]  
\[PESR=\frac{PES}{PE_{RS}} \tag{2}\]  
\[PE_{RS}=\frac{E_{D}}{\eta_{E}}+\frac{Q_{SH}}{\eta_{Q}}+\frac{Q_{DHW}}{\eta_{Q}} +\frac{Q_{CL}}{COP_{R}\cdot\eta_{E}}+\frac{W_{D}}{SEC_{R}\cdot\eta_{E}} \tag{3}\]  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Component** & **Type** & **Parameter** & **Value** & **Unit** \\ \hline Demand controller & 106 & Heating setpoint & Var. & \({}^{\circ}\)C \\ \hline \multirow{6}{*}{Heat Pump (HP)} & \multirow{6}{*}{927} & Cooling capacity & Var. & kW\({}_{\text{cl}}\) \\  & & Inlet source & 15 & \({}^{\circ}\)C \\  & & temperature & 12 & \({}^{\circ}\)C \\  & & Return temperature & Var. & kg/h \\  & & Load flowrate & Var. & kg/h \\  & & Source flowrate & & kg/h \\ \hline \multirow{6}{*}{Single-effect LiBr absorption chiller (SEAC)} & \multirow{6}{*}{107} & Cooling capacity & Var. & kW\({}_{\text{cl}}\) \\  & & Return temperature & 12 & \({}^{\circ}\)C \\  & & Load flowrate & Var. & kg/h \\  & & Cooling flowrate & Var. & kg/h \\  & & Hot Water & Var. & kg/h \\  & & Cooling temperature & 20 & \({}^{\circ}\)C \\  & & Hot Water & Var. & \({}^{\circ}\)C \\  & & temperature & 70–85 & \({}^{\circ}\)C \\  & & Operating range & & \\ \hline \multirow{6}{*}{Cooling water tank (CWT)} & \multirow{6}{*}{156} & Capacity & Var. & m\({}^{3}\) \\  & & Number of nodes & 10 & - \\  & & Overall loss & 0.35 & W/m\({}^{2}\cdot\)K \\ \hline \multirow{6}{*}{Multi-effect distillation unit (MED)} & \multirow{6}{*}{–} & Desalting capacity & 1309 & L/h \\  & & Recovery factor & 20.65 & \% \\ \cline{1-1}  & & Source flowrate & Var. & L/h \\ \cline{1-1}  & & Source temperature & Var. & \({}^{\circ}\)C \\ \cline{1-1}  & & Operating range & 60–82 & \({}^{\circ}\)C \\ \hline \multirow{6}{*}{Reverse osmosis unit (RO)} & \multirow{6}{*}{–} & Desalting capacity & 3216 & L/h \\ \cline{1-1}  & & Recovery factor & 45 & \% \\ \cline{1-1}  & & Reject factor & 99.31 & \% \\ \hline \multirow{6}{*}{Freshwater tank (FWT)} & \multirow{6}{*}{39} & Capacity & 50 & m\({}^{3}\) \\ \cline{1-1}  & & Minimum volume & 5 & m\({}^{3}\) \\ \cline{1-1}  & & Level (off-desalting) & 35 & m\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 5: _Cont._\[PE_{PS}=\frac{E_{FG}}{\eta_{E}} \tag{4}\]  
where PES is the primary energy saving, and PE\({}_{\text{RS}}\) and PE\({}_{\text{PS}}\) are, respectively, the energy consumption in the reference and proposed system. E\({}_{\text{D}}\) is the electricity demand of the building, Q\({}_{\text{SH}}\), Q\({}_{\text{DHW}}\), and Q\({}_{\text{CL}}\) are the building's space heating, domestic hot water, and cooling demands, and W\({}_{\text{D}}\) is its freshwater demand. Regarding the technical parameters, \(\eta_{\text{IE}}\) is the electric efficiency of the Spanish grid system, and \(\eta_{\text{Q}}\) is the thermal efficiency of the auxiliary boiler. For the reference system, COP\({}_{\text{R}}\) is the coefficient of performance of the cooling system, and SEC\({}_{\text{R}}\) is the specific energy consumption of the desalting system. E\({}_{\text{FG}}\) is the electricity demand of the building taken from the grid.