Main results

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. In particular 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 is needed to support and guide the experiments during all the main phases of the action. In particular it helps finding 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 and study several TEGs systems, optimized to be implemented in the HTEPV devices. In particular, this part of the action was aimed to find/realize the right thermoelectric material and assembling it to form the TEG systems to be implemented within the final device. 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 (when available on the market) 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, which is commercially available. Wafers of this material were then bought, cut and characterized with a dedicated setup. After, implementing the model described above, and the previous solar cells characterization, the optimal design of the TEG part was calculated (for the given kind of solar cell), and the final HTEPV efficiency calculated. Finally the thermoelectric material was cut in the optimal size and some TEG devices were built and characterized.

3. Development and characterization of a proper encapsulation to minimize the thermal exchange between the ambient and the top surface of the HTEPV device. In this phase a preliminary study on a possible structure for the optimized encapsulation was done and several depositions and characterizations of the chosen material were performed. The study showed interesting results pointing in the expected direction.

In the second part of the project the work was manly dedicated to the development of the two hybrid prototypes, to their characterization, and economical evaluation. Part of the efforts were also focused to the realization of the encapsulation minimizing the thermal exchange between the ambient and the HTEPV device. In particular the following points were addressed:

1. Perovskite solar cells were acquired from collaborators, and their efficiency was characterized as a function of temperature and optical concentration. The solar cells were also optically characterized, defining their reflectance and emittance. The results of these characterizations 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. The efficiency gain was then found as function of temperature and optical concentration. 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 per watt 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.