## 3 Results and Discussion
### PAR Isocontours
Considering the threshold of 4 MJ m\({}^{-2}\) d\({}^{-1}\) and the results presented in Table 9, it can be seen that only the case of the OPV/PE combination that results in normal \(\tau_{\text{n,PAR}}\) of 60% can satisfy this threshold for the period between May to September. The rest of the OPV/PE combinations result in a daily PAR radiation integral lower than this threshold all year round. Another important finding presented in Table 9 is that although the daily average greenhouse transmission to PAR simulated for the case of the PE-covered greenhouse without integrated OVPs (87%) was close to the normal PAR transmission value given for this case (89%). The simulated values estimated for the other cases were significantly lower than the normal PAR transmission of each \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Day** & **PAR Energy at Plant Level [M] m\({}^{-2}\)d\({}^{-1}\)] and Greenhouse Transmission Coefficient to PAR** & **C\({}_{\text{n,PAR}}\) = 30\%** & **\(\tau_{\text{n,PAR}}\) = 45\%** & **\(\tau_{\text{n,PAR}}\) = 60\%** & **\(\tau_{\text{n,PAR}}\) = 89\%** \\ \hline
6 February & 0.84 (0.19) & 1.25 (0.28) & 2.13 (0.46) & 4.48 (0.88) \\
21 March & 1.08 (0.18) & 1.64 (0.27) & 2.33 (0.38) & 4.92 (0.81) \\
6 May & 1.84 (0.18) & 2.66 (0.27) & 3.79 (0.39) & 8.25 (0.81) \\
21 June & 2.20 (0.19) & 3.28 (0.29) & 4.59 (0.40) & 9.38 (0.82) \\
6 August & 2.09 (0.20) & 3.09 (0.29) & 4.29 (0.41) & 8.64 (0.82) \\
21 September & 1.67 (0.20) & 2.45 (0.30) & 3.40 (0.42) & 6.79 (0.83) \\
6 November & 0.71 (0.20) & 1.05 (0.30) & 1.46 (0.42) & 2.96 (0.85) \\
21 December & 0.50 (0.18) & 0.82 (0.30) & 1.14 (0.42) & 2.34 (0.87) \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Daily cumulative PAR solar energy at plant level and daily average greenhouse transmission to PAR (in parentheses) for the different OPV/PE combinations studied. case. More specifically, the daily average greenhouse transmission to PAR simulated for the cases of combinations OPV/cover that correspond to normal transmittance in the PAR spectrum equal to 30%, 45%, and 60% were 19%, 29%, and 41%, respectively. The respective difference between the normal PAR transmission and the simulated daily average greenhouse transmission to PAR for the cases of OPV/cover combinations with normal PAR transmittance of 30%, 45%, 60%, and 89% were 36%, 35%, 31%, and 6%, respectively. This nonlinearity is due, on the one hand, to the logarithmic relation that links transmissivity to the extinction coefficient and to the light path length [20] and, on the other hand, to the fact that the average equivalent transmissivity is the result of reflections within the interior of the greenhouse. The above difference was lower during the period from September to December for almost all the OPVs/PE combinations studied. This is in agreement with the measurements presented by [8] who showed that the lowest difference was found in December, and it increased during the rest of the year until reaching the maximum in June and July. In practice, the overall greenhouse transmissivity is lower than the normal transmission value declared for the cover material due to factors such as shading of the structural elements of the roof, dust, and the angle of incidence of the sunrays, which is always different from the perpendicular rays used by manufacturers to test the transmissivity of their materials [31; 32]. In addition, the results presented in [8], which studied the propagation of solar radiation in a greenhouse with south-oriented photovoltaic roofs, showed that the solar radiation measured below the photovoltaic roof consists only of diffuse radiation.