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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - HTEPV (Novel hybrid thermoelectric photovoltaic devices: modeling, development, and characterization)

Teaser

This project aims at the practical realization, the study, and the commercial evaluation of optimized hybrid thermoelectric – photovoltaic (HTEPV) devices for the efficient harvesting of solar energy. It is in fact well known that common photovoltaic (PV) cells have limited...

Summary

This project aims at the practical realization, the study, and the commercial evaluation of optimized hybrid thermoelectric – photovoltaic (HTEPV) devices for the efficient harvesting of solar energy. It is in fact well known that common photovoltaic (PV) cells have limited efficiencies, since most of the incoming power is lost as heat. Thermoelectric generators (TEGs), which convert heat into electricity, may be used to recover these losses, enhancing the effectiveness of solar cells.
More efficient PV systems can lead to a lower cost of solar harvesters, and to a wider availability and diffusion of this kind of renewable energy in the European community. This can in turn help to meet the important goal of lowering the use of fossil fuel, and reduce the emission of CO2 in the atmosphere, which has dramatic effects on climate and air quality.
The overall objective of this action is therefore the practical development of at least two kind of HTEPV prototypes, achieving performances higher by more than 25% than the PV cell alone. This innovative kind of harvesters are realized with special solar cells (not sensitive to temperature increases), a TEG part with optimized design, and a proper encapsulation to prevent heat losses.

Work performed

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.

Final results

As described in the “work performed” section the first part of the project was focused on the study and the realization of an optimized TEG to be implemented within the final HTEPV system.
The theoretical model for the description and the prediction of the system behavior was novel and it has been object of a scientific publication and several presentation at international meetings. The analysis of the various system part and the model to compute their behavior were therefore beyond the state of the art.

In addition some parts of the model were implemented to generate additional original results which were published in two other scientific publications and reported at international conferences.
The first publication focused on the determination and the description of heat losses in solar cells. This is a fundamental part of the project, since it can be used to determine the amount of recoverable heat available for the thermoelectric part (and in turn predict its efficiency). In addition the description of the characteristics of the available losses showed the direction in which the design of the HTEPV device has to point in order to achieve the highest efficiency increase. In particular this analysis showed that heat losses in solar cells are equally distributed over the whole solar spectrum, indicating an advantage of thermally coupled HTEPV systems, over spectrum splitting approaches (see attached figure for a scheme of the two systems). This study put an end to the debate on which should be the strategy to follow for a successful hybridization of TEG and PV system.
The second publication focused on the possibility of electrically hybridize the TEG and PV part in order to achieve a fully hybridized system (thermally and electrically). Actually most of the publications appearing in literature regarding the hybridization of solar cells with thermoelectrics, suppose only thermally hybridized system, in which the TEG and PV part are electrically separated. Only few works considered electrically hybridized systems, with results confined to very specific cases. A comprehensive investigation of the interplay between the PV and TEG resistances, and the optimal harvester working temperature was needed in order to evaluate the actual applicability of such kind of hybridization. The model mentioned above served as a tool to study and analyze this scenario, showing the very small window of suitability for electrically hybridized HTEPV system. This study showed therefore that the a fully hybridized HTEPV system is possible, but unlikely applicable, and was well beyond he state of the art.

Furthermore both the studies reported above, one focusing on the characterization and the evaluation of several kind of solar cells, and the second related to the novel kind of encapsulation, generated new interesting data and some important results. Both studies will be published soon in scientific papers and presented at international meetings.

Finally, so far the project has neither relevant socio-economic impact or wider societal implications. Such impacts are expected to happen once the hybrid device prototypes will be completed, and their economical/commercial evaluation will be performed. This is expected to happen at the end of the project.

Website & more info

More info: https://sites.google.com/site/brunosonzelorenzi/.