Material Optimization for Concentrated Solar Photovoltaic and Thermal Co-generation

We conducted an analytic study of concentrated solar photovoltaic and hot water co-generation based on various solar cell technologies and micro channel heat sinks. By co-optimizing the electricity generation and heat transport in the system, one can minimize the cost of the key materials and compare different tradeoffs as a function of concentration ratio or other parameters. Concentrated solar Photovoltaic (PV) based on multi junction cells can yield around 35-40% efficiency. They are suitable for high photon energy flux and they are already available in the market. However, due to high heat fluxes at large concentrations, such as 100-1000 Suns, heat sinks could be costly in terms of material mass, space, energy for pumping fluid, and system complexity. In addition, since the efficiency of solar cells decreases as the ambient temperature increases, there is a tradeoff between electricity and hot water cogeneration. Similar to our previous analysis of thermoelectric (TE) and hot water co-generation, PV/solar thermal system is also optimized. The results are compared with thermoelectric systems as a function of the concentration ratio. The solar concentrated co-generation system using either PV or TE for direct electricity generation collects more than 80% of solar energy when it is optimized. We calculate the overall cost minima as a function of concentration ratio. Although there are some differences between PV and TE, the optimum concentration ratio for the system is in the range of 100-300 Suns for both. INTRODUCTION There is an emergent need for renewable and zero emission energy sources. The Sun is the basis of almost all renewable energies on earth. There are extensive activities in the area of solar photovoltaic (PV) electricity generation as well as in solar thermal hot water systems. The goals is to decrease the production cost ($/W) and increase the system reliability and lifetime. Conventional fossil fuel based power plants can achieve high efficiencies using co-generation of electricity and hot water. However conventional power plants do not scale well for individual houses or small buildings. There is no doubt that photovoltaic and solar thermal are both well developed and commercialized. However, there is still a big challenge in harvesting energy in specific areas, which turns in to the harvesting energy density. This discussion is most relevant for energy supplies in residential housing. Both electricity and hot water are essential in everyday life applications. We previously reported the energy efficiency of an optimized solar concentrated thermoelectric and hot water system [1]. At the optimum concentration, around 100x, we could harvest 10% of the solar energy as electricity and 70% of the solar energy as heat, for a total of 80%. The thermoelectric device can be designed to match the temperature range with proper choices of temperature dependent materials [2]. This was the advantage we considered in the previous work. However, the conversion efficiency is moderate [3] in comparison to the concentrated solar photovoltaic, for which multi-junction cells are typically used, and efficiency is around 35-40% [4]. Photovoltaic materials are typically temperature dependent and it was reported that the conversion efficiency from the Proceedings of the ASME 2011 Pacific Rim Technical Conference & Exposition on Packaging and Integration of Electronic and Photonic Systems InterPACK2011 July 6-8, 2011, Portland, Oregon, USA