## 2 Research in Sustainable Energy Systems at DTG: Main Results and Discussion
### Energy Performance of PV/T Modules: Experimental and Theoretical Analysis  
A PV/T test rig was placed on the roof of a building of the DTG to investigate the energy performance of different types of PV/T water-cooled plane collectors (Figure 7).  
The collectors tested are described in Table 1 and Figure 8.  
Experimental analysis of the first four PV/T liquid cooled collectors mentioned above conducted by the authors during the 2006-2009 period highlighted that, with glazed collectors, high values of water mass flow rate are necessary during periods of high global solar radiation (\(700\div 800\) W m\({}^{-2}\)) [50; 51; 52] in order to limit the negative effect of high temperature on electric efficiency. In contrast, a lower mass flow rate was useful to avoid high penalization in the thermal energy production of unglazed collectors. During periods with low global solar radiation (\(350\)-\(400\) W m\({}^{-2}\)), low mass flow rate values were effective for both glazed and unglazed technologies. Therefore, glazed collectors are advisable in resorts with low annual global solar radiation; instead, unglazed collectors are more useful with high annual radiation.  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & **COGEN** & **PVTWIN** & **MSS** & **HYBRIS** & **THYBRIS** \\ \hline Glass & Glazed & Glazed & Unglazed & Unglazed & Unglazed \\ PV cells & Single-crystalline Si & Multi-crystalline Si & Multi-crystalline Si & Single-crystalline Si & Amorphous Si \\ Gross area (m\({}^{2}\)) & 1.2 & 2.54 & 2.7 & 1.27 & 2.09 \\ Nominal power (\(W_{\rm p}\)) & 135 & 295 & 300 & 170 & 52 \\ Electrical efficiency & 11.2\% & 11.6\% & 11.5\% & 13.3\% & 10\% \\  & Roll-bond type made & & & Roll-bond type made & \\  & of aluminium suitably & & Plate-and-tube type & Plate-and-tube type & of aluminium suitably & \\ Absorber & glued to the & & made of copper & made of aluminium & glued to the & \\  & Tedlar film of the & & & Tedlar film of the & \\  & PV laminate & & & PV laminate & \\ Manufacturer & By the authors & PVTWINS & Millennium Electric & & \\  & & & (The Netherlands) & T.O.U. (Israel) & SY.T.EN. (Italy) & By the authors \\ \hline \end{tabular}
\end{table}
Table 1: Main technical characteristics of the PV/T module tested at DTG.  
Figure 7: (**A**) Possible configurations of the water-based PV/T module; (**B**) the test rig at DTG (University of Padova, Vicenza).  
The authors developed the THYBRIS collector in order to balance two different requirements: Minimizing dangerous effects of stagnation, typical of glazed modules, without penalizing too much thermal production. This was realized by installing the photovoltaic layer over the glazing (instead of behind): The main idea was not to use a simple glass but a semitransparent PV. The effect was an indirect cooling of the photovoltaic layer (by the fluid and the ambient air), whereas the absorber behaved like a thermal collector. As a drawback, data from experimental measurements highlighted a lower thermal efficiency than conventional PV/T (glazed and unglazed) (Figure 9). Furthermore, a low value of electrical efficiency with a low temperature coefficient was an intrinsic characteristic of the thin film technology, and thus the electrical efficiency decreased less than crystalline PV when increasing the external air temperature.  
Figure 8: COGEN: Front (**a**) and section view (**b**); (**c**) PVTWIN: Back-side view; (**d**) MSS: Schematic and back-side view; \(\xi\) THYBRIS module.  
As a main consequence of such experimental work, more recently the authors developed simulation models in Trnsys(r) to evaluate the energy performance of heating and cooling PV/T systems coupled to both electric [53; 54] and thermal absorption heat pump/chillers [55]. In fact, the use of the suitable type of PV/T (glazed or unglazed) in dual source heat pump systems can offer a useful technical solution for new and refurished nearly zero-energy buildings (NZEB), allowing very high energy performance and economic viability.