# A Sustainable Polygeneration System for a Residential Building
## 4 Discussion  
The first consequence of this configurational analysis is the appropriate selection of the number of PVs or PVTs in the solar field to meet downstream demands. Although PVT technology has a higher overall performance than PV since it also produces heat simultaneously, it could not be the best solution. Suppose such heat serves a demand that can be generated with an electrical technology of lower equivalent consumption and lower investment cost. In that case, it does not justify the purely thermal integration for a polygeneration scheme, including desalinated water. This case is clearly shown in our comparative analysis A-D, when HP and RO are used to produce cooling and water with \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Items** & **Conventional** & **A** & **B** & **C** & **D** \\ \hline PVs & – & 45,738.7 & 69,697.0 & 58,806.8 & 45,738.7 \\ PVTs & – & 51,342.6 & 18,915.7 & 37,831.4 & 43,235.9 \\ Aerotherms (AE) & – & 23,081.3 & 8503.6 & 17,007.3 & 19,436.9 \\ Inverters & – & 6822.4 & 10,396.1 & 8771.7 & 6822.4 \\ Water tanks & – & 2425.0 & 18,430.0 & 26,190.0 & 32,980.0 \\ Water pumps & – & 653.3 & 284.8 & 904.5 & 938.0 \\ Heat pump & – & 14,480.0 & 12,670.0 & – & – \\ SEAC & – & – & – & 6742.1 & 8989.5 \\ MED & – & 57,571.4 & – & – & 57,571.4 \\ RO & – & – & 33,828.6 & 33,828.6 & – \\ Piping & – & 1493.7 & 550.3 & 1100.6 & 1257.8 \\ Wiring & – & 3328.6 & 5072.1 & 4279.6 & 3328.6 \\ Expansion vessels & – & 627.6 & 231.2 & 462.5 & 528.5 \\ Foundations (solar field) & – & 23,650.2 & 22,439.7 & 23,947.3 & 21,791.8 \\ Biomass boiler & – & 40,380.0 & 13,460.0 & 40,380.0 & 40,380.0 \\ Total polygeneration & – & 293,419.8 & 214,479.1 & 260,252.3 & 282,999.5 \\ \% materials & & 94.55 & 93.44 & 92.87 & 93.35 \\ \% transport & & 0.98 & 1.75 & 1.69 & 1.39 \\ \% end life & & 4.47 & 4.81 & 5.44 & 5.26 \\ Electricity (from the grid) & – & 162,000 & 159,000 & 152,000 & 155,000 \\ \hline Natural gas boiler & 67,455 & – & – & – & – \\ LV Electricity, Spanish grid & 723,650 & – & – & – & – \\ Heat, from DH network & 942,510 & – & – & – & – \\ De-ionized water by RO & 1,803,830 & – & – & – & – \\ \hline kgCO\({}_{\rm 2 eq}\)/m\({}^{3}\) & 1.670 & 0.701 & 0.322 & 0.326 & 0.666 \\ kgCO\({}_{\rm 2 eq}\)/m\({}^{3}\)DHW & 6.314 & 0.219 & 0.488 & 0.667 & 0.201 \\ kgCO\({}_{\rm 2 eq}\)/kWh\({}_{\rm H}\) & 0.188 & 0.006 & 0.013 & 0.018 & 0.005 \\ kgCO\({}_{\rm 2 eq}\)/kWh\({}_{\rm C}\) & 0.537 & 0.026 & 0.023 & 0.036 & 0.017 \\ kgCO\({}_{\rm 2 eq}\)/kWh\({}_{\rm E}\) & 0.477 & 0.067 & 0.060 & 0.061 & 0.068 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Total emissions (in kgCO\({}_{\rm 2 eq}\)) of the polygeneration scheme. an electrical rather than thermal technology. This is especially the case when using MED to cover water demand, as it overstretches the required solar field too much compared with having RO as an alternative. The subject of SEAC is more compatible with the higher solar input in summer, and its effect concerning an HP is lower than in the case of desalination. In any case, it is important to note that this four-option analysis arises from the selection of desalination and cooling technologies. Therefore, this optional analysis is meaningless outside the scope of an area with water scarcity and similar building cooling and heating demands. At this point, it is worth remembering that this analysis focused on polygeneration with equipment that produces thermal energy, particularly those equipped with the possibility of integrating TEG devices within them to provide free electricity generation. Finally, it should be remarked that the TEG devices integrated with the PVTs and the BB do not represent a substantial additional advantage in electricity generation. They only contribute a mere 1.5% to the generation obtained with the PVs and PVTs, and they are a dispensable option in any proposed configuration.