In the first period of the project the main efforts were successfully focused on the study and the realization of an optimized TEG to be implemented within the final HTEPV system. The following points were addressed:
1) Develop a theoretical model to properly describe and predict the behavior of the PV, TEG, and HTEPV systems. This model to find the ideal characteristics of the solar cell, the thermoelectric material, and the TEG design to be implemented. It can also help to calculate the efficiency of overall HTEPV device and predict its behavior versus time under real operating conditions.
2) Develop a TEG, optimized to be implemented in the HTEPV devices. Since the optimization of the TEG part had to be done on the characteristics of the solar cell implemented, several kind of solar cells were bought or acquired from research groups around the world. Then a setup for the characterization of these solar cells was built and their relevant physical properties were characterized. On the basis of this study, the best choice for the thermoelectric material were found to be bismuth telluride. Wafers of this material were then bought, cut and characterized with a dedicated setup.
In the second part of the project the work was manly dedicated to the development of the two hybrid prototypes, and to the realization of an encapsulation minimizing the thermal exchange between the ambient and the HTEPV device. The following points were addressed:
1) Perovskite solar cells were acquired from collaborators and characterized. The results of this characterization were then put in the model developed in the first part of the action, returning the prediction of the achievable efficiency gain, the optimal working temperature, and the optimal design of the TEG part. Optimized TEG devices were the developed from commercial bismuth telluride wafers. Namely, legs with optimal aspect ratio and section were cut, and then soldered on copper plates, used as electrodes. Finally the perovskite solar cell was attach on top of the TEG device using thermal conductive paste, and the formed hybrid device was characterized under solar simulator. Same procedure was repeated in order to develop a second kind of hybrid with commercial amorphous silicon solar cells.
2) Once the performances of the hybrids were characterized, they were inputted in a model to predict the system economic feasibility. In particular the cost per watt ($/W), and its ratio with the solar cell cost $/W were found. The model was intentionally made to be general in order to be adaptable to different hybrid cases, and materials. The results showed that hybrids based on perovskites solar cells, working at mid-low optical concentrations, can be economically feasible (their $/W value can compete with the actual PV market). The case of hybrids with amorphous silicon was evaluated less feasible, mainly because to the smaller starting efficiency of this kind of solar cells.
3) The encapsulation minimizing the thermal exchange between the ambient and the HTEPV device is necessary in order to guarantee optimal efficiency gains. In this terms it is fundamental to limit the radiative heat exchange between the device top surface (the solar cell top surface) and the ambient. In this project this was done engineering and developing a glass enclosure cover with a thin film heat mirror. Heat mirrors are systems transparent to sun light but highly reflective for the infrared. This characteristic enable a sort of greenhouse effect that reflects radiative heat exchange (normally with wavelength in the mid infrared) back to the system, minimizing heat losses. In this project an optimized heat mirror was developed using a novel array of thin film materials, currently under patenting.
