## 2 Research in Sustainable Energy Systems at DTG: Main Results and Discussion
### Energy and Economic Analysis of Direct and Indirect Evaporative Cooling  
Direct evaporative cooling (DEC) and indirect evaporative cooling (IEC) use the evaporative cooling potential by means of a suitable control logic, varying the external air flow in the Air Handling Unit (AHU) from minimum (G\({}_{\text{vent}}\), that is, ventilation air flow) to maximum value (G\({}_{\text{I}}\), that is, inlet air flow) (Figures 10 and 11). Figure 9: (**a**) Thermal efficiency of PVTWIN, COGEN, and MSS collectors, (**b**) HYBRIS collector (**c**) and THYBRIS collector. Figure 11: _Cont._  
Figure 10: DEC: (**a**) Subdivision of the zones on the psychometric chart; (**b**) AHU scheme, input and output data. A simulation worksheet was developed, based on the hourly Typical Meteorological Years of some European resorts [56] and verified by experimental measurements [57]. The psychrometric chart was divided into seven parts, considering the outside air enthalpy \(\mathrm{h_{E}}\) and the humidity ratio \(\mathrm{x_{E}}\), fixing the indoor (\(\mathrm{h_{A}}\)), the minimum outdoor (\(\mathrm{h_{E,min}}\)) and the inlet air enthalpy \(\mathrm{h_{I}}\) values, and the inlet air humidity ratio (\(\mathrm{x_{I}}\)) (Table 2). For each zone, the simulation worksheet determined the external air flow \(\mathrm{G_{E}}\) that maximizes the effect of free cooling, subdivided into sensible and latent components. Different AHU configurations for air conditioning (cooling) of a room were considered to evaluate the possible energy and economic advantages using such techniques:  
* Heat Recovery (HR): Sensible heat recovery, typically by a cross-flow heat exchanger, is always useful when the external air enthalpy \(\mathrm{h_{E}}\) is greater than the ambient enthalpy \(\mathrm{h_{A}}\); in contrast, the heat exchanger must be bypassed so as to use the outdoor air (eventually mixed with the recirculated air) for free cooling (Figure 12a);
* Direct Evaporative Cooling (DEC): External air can help achieve the cooling load by directly humidifying it by an adiabatic saturator when the enthalpy of the outside air \(\mathrm{h_{E}}\) is lower than that of the inside air \(\mathrm{h_{A}}\), and the humidity ratio (\(\mathrm{x_{E}}\)) is suitably lower than that of the inside air (\(\mathrm{x_{A}}\)) in order to cover internal latent load (Figure 12b);
* Indirect Evaporative Cooling (Single Stage) (IEC_SS): The idea is to place an adiabatic saturator in the inside air flow \(\mathrm{G_{O}}\) to humidity it before being exhausted; thus, its temperature decreases, allowing to cool the outside air flow \(\mathrm{G_{E}}\). This is useful to improve the performance of the sensible heat exchanger in the cooling season, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Zone** & **Enthalpy** & **Humidity** & **External Air Flow \(\mathrm{G_{E}}\)** & **Description** \\ \hline D1 & \(\mathrm{h_{E}>h_{A}}\) & & \(\mathrm{G_{Vent}}\) & No energy saving zone \\ D2 & \(\mathrm{h_{I}<h_{E}<h_{A}}\) & \(\mathrm{x_{E}>x_{I}}\) & \(\mathrm{G_{I}}\) & Possibility of energy saving with dehumidification \\ D3 & \(\mathrm{h_{E}<h_{I}}\) & & \(\mathrm{G_{I}}^{\mathrm{T}}(\mathrm{h_{A}-h_{I}})/(\mathrm{h_{A}-h_{E}})\) & Possibility of energy saving with dehumidification \\ U1 & \(\mathrm{h_{E}>h_{A}}\) & & \(\mathrm{G_{Vent}}\) & No energy savings and with dehumidification \\ U2 & \(\mathrm{h_{I}<h_{E}<h_{A}}\) & \(\mathrm{G_{I}}\) & Partial free cooling and humidification \\ U3 & \(\mathrm{h_{E,min}<h_{E}<h_{I}}\) & \(\mathrm{x_{E}<x_{I}}\) & \(\mathrm{G_{I}}^{\mathrm{T}}(\mathrm{h_{A}-h_{I}})/(\mathrm{h_{A}-h_{E}})\) & Total free cooling and humidification \\ U4 & \(\mathrm{h_{E}<h_{E,min}}\) & & \(\mathrm{G_{Vent}}\) & Heating and humidification \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Description of the zone subdivision on the psychrometric chart and the outside air flow \(\mathrm{G_{E}}\) that maximizes the free cooling. Figure 11: IEC: (**a**) Subdivision of the zones on the psychrometric chart; (**b**) AHU scheme, input and output data.