Periodic Reporting for period 2 - HTEPV (Novel hybrid thermoelectric photovoltaic devices: modeling, development, and characterization)
Okres sprawozdawczy: 2019-07-01 do 2020-06-30
This project aimed 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 was the practical development of at least two kind of HTEPV prototypes, achieving performances higher by more than 25% than the PV cell alone.
The project was successfully implemented and the goal was met with perovskites, and amorphous silicon solar cells. The thermoelectric part of the system was developed with commercially available bismuth telluride but with an optimized and novel device geometry. Efficiency gains were found to be higher in the case of perovskite solar cells, and in the case of optical concentration (the use of lens or parabolic mirrors to enhance the solar input power). The economical evaluation showed that for mid-low optical concentration (5 – 10) hybrid systems based on perovskite solar cells can be cost-effective and potentially competitive in the actual photovoltaic market.
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 was the practical development of at least two kind of HTEPV prototypes, achieving performances higher by more than 25% than the PV cell alone.
The project was successfully implemented and the goal was met with perovskites, and amorphous silicon solar cells. The thermoelectric part of the system was developed with commercially available bismuth telluride but with an optimized and novel device geometry. Efficiency gains were found to be higher in the case of perovskite solar cells, and in the case of optical concentration (the use of lens or parabolic mirrors to enhance the solar input power). The economical evaluation showed that for mid-low optical concentration (5 – 10) hybrid systems based on perovskite solar cells can be cost-effective and potentially competitive in the actual photovoltaic market.
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.
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.
The theoretical model for the description and the prediction of the system behavior is novel and it has been object of a scientific publication and several presentation at international meetings. The analysis is beyond the state of the art, with an impact expected to be mainly at academic and research level.
Some parts of the model were implemented to generate additional original results published in two other scientific publications and reported at international conferences. The first focused on the determination and the description of heat losses in solar cells and the second focused on the possibility of electrically hybridize the TEG and PV part in order to achieve a fully hybridized system (thermally and electrically). Both studies were well beyond the state of the art, with impact expected to be mainly at academic and research level.
The novel kind of encapsulation based on heat mirrors is innovative and beyond state of the art. This part of the project was evaluated patentable and an IP protection procedure is currently under way. The interesting fact about this part of the project is that it is applicable not only to hybrid thermoelectric – photovoltaic systems but to all solar – thermal applications. In this perspective the impact of novel encapsulation is expected to be relevant at industrial level.
Also the thermoelectric hybridization of perovskites solar cell, and its economical evaluation is innovative. The technological novelties consist mainly in some advances at the system layout. These advances could stimulate new products in the TEG industry.
In terms of socio-economic impact or wider societal implications, the project has not direct impact on citizens. Such impacts are expected to happen once new kind products, stimulated by the project results (such as the novel encapsulation, the hybrids with perovskite, or even a smaller cost for renewable energy) will be made available to European citizens.
Some parts of the model were implemented to generate additional original results published in two other scientific publications and reported at international conferences. The first focused on the determination and the description of heat losses in solar cells and the second focused on the possibility of electrically hybridize the TEG and PV part in order to achieve a fully hybridized system (thermally and electrically). Both studies were well beyond the state of the art, with impact expected to be mainly at academic and research level.
The novel kind of encapsulation based on heat mirrors is innovative and beyond state of the art. This part of the project was evaluated patentable and an IP protection procedure is currently under way. The interesting fact about this part of the project is that it is applicable not only to hybrid thermoelectric – photovoltaic systems but to all solar – thermal applications. In this perspective the impact of novel encapsulation is expected to be relevant at industrial level.
Also the thermoelectric hybridization of perovskites solar cell, and its economical evaluation is innovative. The technological novelties consist mainly in some advances at the system layout. These advances could stimulate new products in the TEG industry.
In terms of socio-economic impact or wider societal implications, the project has not direct impact on citizens. Such impacts are expected to happen once new kind products, stimulated by the project results (such as the novel encapsulation, the hybrids with perovskite, or even a smaller cost for renewable energy) will be made available to European citizens.