## 4 Conclusions  
A novel double skin transpired solar collector was analyzed via CFD simulations aiming to determine the thermal behaviour and performance of such a system, thus providing guiding values and a technical point of view for engineers, architects, and constructors when designing such transpired solar collectors. The numerical grid used for the numerical simulation was verified in a previous article on this issue in terms of a mesh independency test. Additionally, validation of the numerical results with experimental measurements was performed to trust the conclusions of the numerical results. Three important parameters less studied in the literature were addressed in this study, namely: the influence of a separation plate introduced in a transpired solar collector turning it into a double skin transpired solar collector, the influence of air layer thickness on the performance of the collector, and the influence of the used absorber materials for the separation plate material. The numerical simulation studies have shown that a separation plate placed inside the solar air collector will not negatively impact the thermal performance of the solar air collector, but quite the opposite--results demonstrate that the DSTSC has slightly better thermal performances than TSC in the studied range of airflow. Additionally, higher heat exchanger efficiency for the DSTSC at every imposed airflow rate can be noticed compared with the TSC. All this shows the potential of the DSTSC prototype for future use integrating latent heat thermal storage in the back-pass. The influence of the DSTSC thickness was also investigated. Results have shown that if the DSTSC is too thin, it is susceptible to overheating in the upper part of the absorber plate. The efficiency gradually increases when increasing the solar collector thickness until it reaches a value of 20 cm. For a thickness of 30 cm, DSTSC reached close values with a 20 cm case. When varying the thickness from 30 cm to 10 cm we were able to notice a variation of about 4% regarding the solar collector efficiency for an airflow of 500 m\({}^{3}\)/h. Figure 20: Distribution of the temperature on the perforated absorber plate of the HDPE (**left**) and Aluminum (**right**) with an air flow of 500 m\({}^{3}\)/h. The numerical study was performed for steady-state conditions, not considering the wind influence, as the time-step is too small for the current computational capabilities. Additionally, the simulation period was limited to a typical summer day, but further investigation should be performed to overcome these limitations. Regarding the influence of the used absorber materials for the separation plate material, the results indicate that low-conductivity material does not have homogeneous thermal behaviour when comparing to high-conductivity material; however, for Transpired Solar Air Collectors, the differences regarding thermal performances are not significant. Thus, low-conductivity materials are conceivable for TSCs, especially since their price remains lower. These results cast new light on alternative materials for solar air collectors, especially since recycled materials have particular interest in buildings construction. The balance between quality and price is interesting for low-conductivity materials, and this allows this type of collector to be low-cost and low-tech, which is very interesting for new low-carbon policies and for the development of these technologies in developing countries looking for reliable and cheap products. However, this led us to ask ourselves several open questions about the durability of such materials for this type of technology. First, plastic-based materials can be harmful to air quality and health [22; 23]. Indeed, they can release fine plastic particles or even release harmful particles when they heat up.