## 2 Materials and Methods
### Materials
#### 2.5.4 Other Materials Properties  
The thermal properties of the OPV are not taken into account in the simulation of the cover, since are not expected to modify the thermal properties of the cover. The bare cover is a polyethylene film (PE) with density \(\rho\) = 950 kg m\({}^{-3}\), thermal conductivity k = 0.38 W m\({}^{-1}\) K\({}^{-1}\), and specific heat capacity C\({}_{\text{P}}\) = 1900 J kg\({}^{-1}\) K\({}^{-1}\). The thermal and optical properties of the other materials present in the greenhouse are summarized in Table 6.  
The optical properties values marked with an asterisk (*) correspond to the four wavelength spectrum ranges presented in the previous tables. The porosity of the tomato crop was 40%. Side walls have the same thermal properties with the cover, but they are considered opaque. The optical properties  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Property** & **Ground** & **Tomato Solid Part** & **Tomato Fluid Part** \\  & & & [24,25] & **Air** \\ \hline Absorption coef. & 10,000/10,000/ & 1.23 (for thickness & \\ as [1/m] * & 10,000/10,000 & 0.5 m) & 3.075/3.075/7.5 & 0.000582/0.000582/0.0015/0.0015 \\ Refractive index, n * & 1.92/1.82/2.03/2.03 & 2.77 & 2.69/2.69/1.58/1.58 & 1.009 \\ Density, \(\rho\) [kg m\({}^{-3}\)] & 1300 & 700 & Boussinesq (1.225) & Boussinesq (1.225) \\ Thermal conductivity, & & & \\ k\({}_{\text{c}}\) [W m\({}^{-1}\) K\({}^{-1}\)] & 1 & 0.173 & 0.0242 & 0.0242 \\ Emissivity & 0.92 & 0.5 & 0.00343 & 0.9 \\ Scattering coefficient & \(-\)10 & 0.46 & \(1.7894\times 10^{-5}\) & 0.15 \\ Specific heat capacity, & & & \\ Cpef [J kg\({}^{-1}\) K\({}^{-1}\)] & 800 & 2130 & 1006 & \(1.7894\times 10^{-5}\) \\ \hline \hline \end{tabular}
\end{table}
Table 6: Thermal and optical properties of the rest greenhouse materials considered in the simulations.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Spectrum** & \(\rho_{\text{t/c}}\)**—Reflectance [\%]** & \(\tau_{\text{t/c}}\)**—Transmittance [\%]** & \(\alpha_{\text{t/c}}\)**—Absortance [\%]** \\ \hline UV (100–400 nm) & 13.2 & 0.2 & 86.6 \\ PAR (400–700 nm) & 16.8 & 17.5 & 65.7 \\ NIR (700–1400 nm) & 41.0 & 27.4 & 31.6 \\ IR (1400–100,000 nm) & 14.5 & 60.1 & 25.4 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Optical properties for the combination of structure I/polyethylene (PE) cover.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Spectrum** & \(\rho_{\text{t/c}}\)**—Reflectance [\%]** & \(\tau_{\text{t/c}}\)**—Transmittance [\%]** & \(\alpha_{\text{t/c}}\)**—Absortance [\%]** \\ \hline UV (100–400 nm) & 13.4 & 1.9 & 84.6 \\ PAR (400–700 nm) & 24.6 & 62.2 & 13.3 \\ NIR (700–1400 nm) & 40.4 & 48.3 & 11.2 \\ IR (1400–100,000 nm) & 52.4 & 28.8 & 18.8 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Optical properties for the combination of structure II/PE cover.  
of air inside the greenhouse have been calculated [26; 27; 28; 29] for a mixture of air and humidity corresponding to air temperature of 25 \({}^{\circ}\)C and relative humidity HR of 65%.  
According to the Boussinesq approximation, the density is considered constant in all the solved equations except from the buoyancy term in the momentum equation [15]. In the current simulation, the value 1.225 [kg m\({}^{-3}\)] corresponds to the density reference value used in Boussinesq approximation.