## Materials and Methods
### Monthly Energy Analysis  
Some first results are reported here on a monthly basis (Figures 11-15), referring to the A3 case only (40 m\({}^{2}\) evacuated tube--300 m ground probes). Solar energy was used to provide hot water for heating (green in Figure 11) and sanitary uses (red) during the whole year, and to regenerate the ground mainly in the summer months (light blue). While the latter quota increased proportionally to the increasing of the solar field area, the former increased less than proportionally due to the very small heating demand in summer. The quota of solar energy directed to the HP evaporator was negligible. Only A5 presented a meaningful value. This was due to the lower thermal efficiency of the flat plate collectors in winter conditions with respect to the evacuated tube, so it was more frequent that outlet solar circuit temperatures were lower than the hot tank or DHW tank temperatures. Figure 12 reports the absorption HP/chiller energy balance; the limited use of solar energy as heat source of the HP is confirmed. The highest efficiency (1.83) was reported during the mid seasons (April and October), whereas during cooling season the gas utilization efficiency (GUE) was between 0.74 and 0.79. Figures 13 and 14 report the energy balance of the DHW tank and hot tank, respectively. Solar energy was assured an appreciable contribution to DHW demand, even if the hot tank contribution prevail \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Alternative**} & \multirow{2}{*}{\begin{tabular}{c} **Type of** \\ **Collector** \\ \end{tabular} } & \multicolumn{2}{c}{**Solar Field Area**} & \multirow{2}{*}{\begin{tabular}{c} **DHW** \\ **Tank (L)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Hot** \\ **Tank (L)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Cold** \\ **Tank (L)** \\ \end{tabular} } \\ \hline A1 & - & 0 & 1500 & 1500 & 1000 \\ A2 & Evacuated tube & 20–400 & 1500 & 2000 & 1000 \\ A3 & Evacuated tube & 40–300 & 3000 & 3000 & 1000 \\ A4 & Evacuated tube & 60–200 & 3000 & 5000 & 1000 \\ A5 & Flat plate & 50–300 & 3000 & 5000 & 1000 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Different alternatives simulated with solar field, ground boreholes, and storage tank dimensions (all considering sensible (water) storage). Figure 11: Solar energy balance for the A3 alternative. The contribution of electric auxiliaries was quite limited, and it was present mainly during the summer or mid-season. This was because in winter the absorption machine operated as HP, and it was controlled by the hot tank set point temperature. In this way, the DHW tank was also satisfied mainly by solar energy and the hot tank. Differently, during the summer or mid-season the absorption machine operated as chiller, controlled by the cold tank set point temperature. Then heat recovered by the condenser was available from the absorption machine only when it is on. The hot tank was quite fully satisfied by solar energy (light blue in Figure 14) during the summer months. Figure 12: Absorption HP/chiller energy balance for the A3 alternative. Figure 13: Energy balance for the DHW tank for the A3 alternative. Figure 15 reports the percentage covering of different heat generators (solar collectors, absorption HP, and electric auxiliaries), and the primary energy ratio (PER) for the A3 alternative. PER is defined as the ratio between useful energy produced by the plant (energy from the three tanks to the main collectors of Figure 8) and the total no-renewable requested primary energy. The latter includes natural gas (NG) supplied to the absorption machine, and electricity to the auxiliaries of the hot tank and DHW tank. According to Italian standard DM 26/06/2015, such electricity consumptions are converted to primary energy by the no-renewable primary factor \(\mathrm{f_{p,nren}}=1.95\). During winter months, PER was always greater than 1.94, and that was the nominal value of the HP GUE. The heat pump covered the largest fraction of heating and DHW demand.