Modeling and Experimental Validation of a New Hybrid Photovoltaic Thermal Collector.

Photovoltaics and solar thermal collectors are often competing technologies used to harness the energy of the sun.  Photovoltaic (PV) systems convert solar energy directly into an electric current whereas solar thermal collectors harness the heat of solar radiation to generate electricity in a process similar to the one used in conventional power plants.  Both systems have advantages and drawbacks.  Photovoltaics generate lower conversion rates under direct sunlight and experience a decrease in efficiency when the temperature of the solar cell increases.  Current technology also does not allow electricity to be stored for later use.  Solar thermal collectors reach relatively high temperatures under a clear sky which allows them to reach higher conversion rates.  Thermal energy can be efficiently stored in the form of fluid storage tanks or molten salts.  However, solar thermal collectors generate little power under diffused light conditions while photovoltaics can continue operating under such weather.  The weaknesses and advantages of both systems complement each other and there have been attempts to integrate the two systems into a hybrid cell.  Touafek et al. (2011) have attempted to create precisely such a hybrid photovoltaic thermal collector (PVT collector).  The PVT collector consists of a photovoltaic layer placed adjacent to a thermal collector unit.  As such, excess heat that decreases the efficiency of the photovoltaic cell will be transferred to the solar thermal unit, which can generate electricity with the heat.  Touafek et al. present the results of their modeling and experimentation on a prototype hybrid PVT collector.—Alan Hu
Touafek, K., Haddadi M., Malek A. 2011. IEEE Transactions on Energy Conversion 26, 176-183.

            Touafek at the Unit of Applied Research in Renewable Energy, Haddadi at the Algerian National Polytechnic University, and Malek at Renewable Energy Development Center at Algeria constructed a prototype PVT collector and created a theoretical framework used to calculate various indicators.  The PVT collector prototype consists of two main components: the first is the photovoltaic section of the system and the second is the thermal collector.  The photovoltaic section of the system consists of three layers in the following order: tempered glass, a PV cell, and Tedlar (used as a backsheet to protect the PV system).  Immediately adjacent to the Tedlar layer is thermal collector which essentially is a system of pipes that pumps cooling fluid.  Any excess heat generated by the PV system is absorbed and transferred by the pipes and coolant. 
Toaufek et al. develop a numerical model that attempts to take into account all sources of heat that may affect the PVT collector, including heat from ambient air, from the ground, from the sky, from solar radiation absorbed by the glass, and from other sources.  Also included in the model is the transfer of heat between components of the PVT collector.  For example, heat transferred from the glass to the PV cell through conduction is considered in the calculations.  Toaufek et al. are able to isolate the temperature of the solar cell and the fluid through the model.  The two figures are important as the goal of the PVT collector is to minimize the temperature of the PV cell and to maximize the temperature of the fluid.
The researchers use the temperature of the solar cell and the temperature of the fluid to judge the optimal thickness of the pipe material.  Thicknesses of 0.01, 0.02, 0.03, 0.04, and 0.05 m were tested and the thickness that maximized fluid temperature and minimized solar cell temperature.  Toaufek et al. continue to present calculations from the model and from experimentation to validate the accuracy of their model.  The paper claims a total efficiency of 80%, though it is unclear if this refers to EQE or thermal efficiency.  Further research and experimentation is recommended in the conclusion.

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