## 4 Simulation Results
### Comparison of the Different Working Pairs
Interestingly, when the waste heat source temperature is increased to 50 \({}^{\circ}\)C, different working pairs can be considered suitable for the application, as represented in Figure 7 and Table 4, guaranteeing the possibility of providing up to 20 K of GTL, meaning 70 \({}^{\circ}\)C available, which can be used for feeding the DHN at higher operating temperature. As highlighted in Figure 7a and Table 4, all the investigated working pairs can be operated up to 10 \({}^{\circ}\)C of ambient temperature to achieve a GTL of 10 K, while some specific ones can also exploit higher ambient temperature, i.e., 20 \({}^{\circ}\)C. In particular, this is the case for the two composite sorbents using water as a refrigerant and the MWCNT-LiCl with  
Figure 6: Thermal COP (**a**) and specific energy (**b**) obtained by the different investigated working pairs, exploiting 40 \({}^{\circ}\)C as waste heat source, with a GTL of 10 K, as a function of the ambient temperature. methanol. Furthermore, the pure silica gel Siogel can also be operated up to 20 \({}^{\circ}\)C of ambient temperature, even if very limited COP and specific energy is expected. The MWCNT-LiCl/methanol working pair shows both GTLs of 10 K and 20 K, to be able to achieve COP almost constant at 0.5, with a decrease down to 0.35 at 20 \({}^{\circ}\)C ambient temperature (Figure 7a). The expected specific energy looks even more attractive, always being above 1000 kJ/kg when the ambient temperature is below or equal to 10 \({}^{\circ}\)C, both for 10 K and 20 K GTL (Table 4). This is mainly due to the feature of the composite sorbent which, under these operating conditions, is almost always able to exchange a constant amount of methanol, thus guaranteeing a flat evolution of these performance parameters, according to the equilibrium data reported in [28]. Other working pairs that show appreciable performance are the ones based on ammonia as a working fluid, especially when 10 K of GTL is requested. Nevertheless, both activated carbons using ammonia never overcome 380 kJ/kg as specific energy, thus making them less attractive than the MWCNT-LiCl/methanol working pair. As pointed out before, the two investigated composite sorbents using water as working fluid can also achieve appreciable performance under these conditions, and the SGST-LiCl especially shows quite high COP even at high ambient temperature and specific energy slightly lower that the MWCNT-LiCl/methanol, thus also making this working pair a valuable option. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Specific Energy [k]/kg/COP [-] @ 50 \({}^{\circ}\)C Waste Heat**} \\ \hline
**Working Pair** & **GTL [K]** & **T\({}_{\text{amb}}\)** = **-10 \({}^{\circ}\)C** & **T\({}_{\text{amb}}\)** = 0 \({}^{\circ}\)C** & **T\({}_{\text{amb}}\)** = 5 \({}^{\circ}\)C** & **T\({}_{\text{amb}}\)** = 10 \({}^{\circ}\)C** & **T\({}_{\text{amb}}\)** = 20 \({}^{\circ}\)C** \\ \hline  & 10 & — & — & 225.18/ & 149.60/ & 3.28/ \\ SG & & & & 0.49 & 0.48 & 0.13 \\ Siogel/Water & & & & 76.42/ & 16.51/ & — \\ \cline{2-6}  & 20 & — & — & 0.45 & 0.30 & — \\ \hline  & 10 & — & — & 77.06/ & 45.39/ & — \\ \cline{2-6}  & 20 & — & — & — & — & — \\ \hline  & 10 & — & — & 1066.69/ & 736.71/ & 35.17/ \\ MWCNT- & & & & 0.50 & 0.49 & 0.28 \\ MiCl/Water & & & & 426.16/ & 96.97/ & — \\  & & & & 0.48 & 0.39 & — \\  & 30 & — & — & 97.28/ & — \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Specific energy evaluated for each working pair when the waste heat temperature is available at 50 \({}^{\circ}\)C, considering a GTL of 10 K and 20 K for all the investigated ambient temperatures. Figure 7: Thermal COP obtained by the different investigated working pairs, exploiting 50 \({}^{\circ}\)C as waste heat source, with a GTL of 10 K (**a**) and 20 K (**b**), as a function of the ambient temperature.