## 2 Design of the System
### Thermal and Electrical Design  
The thermal design of the collector had some particular features since the heat exchange/cooling system aimed to have a double function: (1) offering PV-cell cooling and (2) acting as a thermal collector (collecting the heat removed). In addition, the coolant acted as an optical element in conjunction with the PMMA cylindrical casing, focusing incident rays onto the PV cells. To this end, the thermal fluid needed to have the following:  
1. High light transmission so as to optimise electrical output (considering the effective spectrum of the PV cells). 2. High absorption in the spectral range where the PV cells do not generate electricity in order to achieve effective heat dissipation, optimising thermal production. 3. The necessary characteristics to act as a coolant: 1. High specific heat and thermal conductivity to maximise thermal exchange. 2. Low coefficient of expansion to prevent the increase in pressure in the hydraulic circuit when temperature increases. 3. Appropriate range of operating temperatures, taking into account melting and boiling points as well as real operating conditions. 4. High density and low viscosity so that the system can maximise heat-removal capacity and attain low pressure losses in pumping. 5. The materials should be available and not expensive. On the basis of a previous study that analysed several liquids for directly immersed PVT concentrators [20], two liquids with characteristics that meet the requirements previously described are the following: DIW and IPA. In Figure 2, the spectral-transmittance curves of both liquids are illustrated. It can be noticed that these liquids have high transmittance of photons in the frequency range in which silicon-based PV cells operate and absorb all the photons for wavelengths longer than 1400 nm. It should also be noted that these liquids present two drawbacks: (1) DIW freezing-temperature limits can be easily exceeded (this problem can be solved by mixing DIW and IPA), and (2) IPA degrades some polymeric compounds, and this means that it was necessary to select IPA-resistant materials for module fabrication. \begin{table}
\begin{tabular}{c c c c} \hline  & \(\mathbf{C_{\text{g}}}\) (\(\times\)) & \(\mathbf{\eta_{\text{o}}}\) (\%) & \(\mathbf{\alpha}\)=\(\frac{C_{\text{max}}-C_{\text{min}}}{C_{\text{max}}+C_{\text{min}}}\) (-) \\ \hline DIW & 12 & 76.14 & 0.05 \\ IPA & 12 & 80.18 & 0.22 \\ \hline \end{tabular}
\end{table} 
Table 1: Optical characteristics of the system. Figure 2: Spectral response (SR) of the utilised crystalline silicon (c-Si) cells. Transmittance of DIW, IPA and AM1.5D solar spectrum [41]. The thermal design was related to the previous optical design. The resulting liquid/cavity geometries (one for the DIW configuration and the other for the IPA one) were those optimising the optical requirements. On the basis of these cavity geometries, computational fluid dynamics (CFD) analysis was carried out in Comsol Multiphysics to determine the following: (1) the optimum geometry/position of the inlet/outlet ducts and (2) the velocity that minimised pressure losses while enhancing thermal exchange with the cells (previous study: [37]). Figure 3 shows the plots of representative temperature contours for both configurations. Longitudinal and cross-sectional views are presented. It can be observed that due to the shape of the IPA cavity, the PV-cell area was a bit warmer than in the case of DIW. On the other hand, the greater proportion of PMMA solid body was more efficient in isolating the system from the exterior. It can be noted that the inlet was located at the bottom part of the cavity and the outlet was located on the top, in agreement with the flow lines/path traced by the liquid. The flow lines were obtained by means of multiphysics simulations, combining fluid mechanics and energy equations. In terms of the electrical production, according to the design, every module was formed by 6 cells connected in series (length: 12 cm; width: 0.5 cm each one).