## 3 Results and Discussion  
### PAR Isocontours  
The PAR isocontours in the simulation domain are presented in Figure 3a-f, for the 21 June for normal transmittance at PAR spectrum, \(\tau_{\text{n,PAR}}\) = 30% for 6 different hours of the day. It should be noted that in all the presented results the solar time is considered. In addition, Figure 4a-f, Figure 5a-f, and Figure 6a-f present the PAR for the same date for normal transmittance at the PAR spectrum equal to 45%, 60%, and 89% (bare PE cover), respectively. Figure 4: PAR isocontours for 21 June and \(\tau_{\text{n,PAR}}\) =45%. Figure 3: Photosynthetically active radiation (PAR) isocontours for 21 June and \(\tau_{\text{n,PAR}}\) = 30%. Figure 7: Average PAR at plant level during the solar day for 8 indicative days of the year. (i) \(\tau_{\text{n,PAR}}=30\%\) (green line continuous), (ii) \(\tau_{\text{n,PAR}}=45\%\) (red dashed line), (iii) \(\tau_{\text{n,PAR}}=60\%\) (brown dot-dashed line), (iv) \(\tau_{\text{n,PAR}}=89\%\)—bare cover (black dotted line), and (v) PAR incident to horizontal surface in open field (blue continuous line with blue circles). It can be seen that for the region of Thessaly in Central Greece (the region of the case studies considered) the outside incident PAR had a maximum value of about 125 W m\({}^{-2}\) during 21 December (date with the lowest radiation level among the dates studied) and of about 340 W m\({}^{-2}\) during 21 June (date with the highest radiation level among the dates studied). The corresponding values for the case of the greenhouse covered with a bare PE film are 100 W m\({}^{-2}\) and 270 W m\({}^{-2}\), respectively. The daily PAR solar energy at plant level for the examined days and for the examined cover/PV combinations is given in Table 9. The values given in parenthesis correspond to the daily average greenhouse transmission to PAR. The most commonly grown species in greenhouses are vegetables with medium thermal requirements (tomato, pepper, cucumber, melon, watermelon, marrow, green bean, eggplant); the aim is to extend the growing calendars beyond the conventional open-air cultivation season, and thus increase profitability. Tomato cultivation requires high solar radiation levels, but there are other crops that do not need so much sunlight for proper growth. PAR light may not be considered as a limiting growth factor for a tomato crop at levels greater than 200 W m\({}^{-2}\). Of course, optimal light levels are difficult to define since the crop productivity is also related to many other parameters like air temperature, relative humidity, and CO\({}_{2}\) concentration [29]. Considering the above threshold for solar radiation and taking into account the results presented in Figure 7, it can be seen that the PAR levels inside the PE-covered greenhouse without integrated OVPs were higher than the above radiation level for a long part of the day from April to September, while for the other months studied, the radiation levels were lower than 200 Wm\({}^{-2}\). In addition, in all the case studies with different OPV/PE combinations, the PAR levels estimated are lower than 200 W m\({}^{-2}\). Nevertheless, since it is not only the intensity of radiation but also the duration of the lighting period that plays a significant role, the solar radiation integral value will have to be taken into account. Furthermore, the fraction of diffuse to total light plays a significant role, since higher diffuse ratios result in higher radiation use efficiency. The minimum levels of the daily total solar radiation integral requirements of the above species are estimated at around 8.5 MJ m\({}^{-2}\) d\({}^{-1}\) (which is equivalent to about 4.05 MJ m\({}^{-2}\) d\({}^{-1}\) of PAR) [30]). The reduction observed to the mean daily PAR radiation integral (Table 9) due to the presence of the OVPs for the cases with normal PAR transmittance of 30%, 45%, and 60% in relation to the bare PE was 77%, 66%, and 52%, respectively.