Abstract
The use of volume and phase holographic elements in the design of photovoltaic solar concentrators has become very popular as an alternative solution to refractive systems, due to their high efficiency, low cost and possibilities of building integration. Angular and chromatic selectivity of volume holograms can affect their behavior as solar concentrators. In holographic lenses, angular and chromatic selectivity varies along the lens plane. Besides, considering that the holographic materials are not sensitive to the wavelengths for which the solar cells are most efficient, the reconstruction wavelength is usually different from the recording one. As a consequence, not all points of the lens work at Bragg condition for a defined incident direction or wavelength. A software tool that calculates the direction and efficiency of solar rays at the output of a volume holographic element has been developed in this study. It allows the analysis of the total energy that reaches the solar cell, taking into account the sun movement, the solar spectrum and the sensitivity of the solar cell. The dependence of the recording wavelength on the collected energy is studied with this software. As the recording angle is different along a holographic lens, some zones of the lens could not act as a volume hologram. The efficiency at the transition zones between volume and thin behavior in lenses recorded in Bayfol HX is experimentally analyzed in order to decide if the energy of generated higher diffraction orders has to be included in the simulation.
Highlights
Holographic Optical Elements (HOEs) are a versatile technology that has multiple applications
HOEs can be designed so that their efficiency reaches high values for a particular wavelength range, which in solar concentration applications needs to take into account the incident solar spectrum and the kind of receiver the solar radiation will be directed to
It is necessary to calculate the concentration coefficient defined in equation (5) as the ratio of irradiances, and the ratio of current intensities generated by the PV cell with and without the holographic concentrator, in order to take into account the spectral response of the cell
Summary
Holographic Optical Elements (HOEs) are a versatile technology that has multiple applications. HOEs can be designed so that their efficiency reaches high values for a particular wavelength range, which in solar concentration applications needs to take into account the incident solar spectrum and the kind of receiver the solar radiation will be directed to. Bayfol HX, a photopolymeric recording material developed by Covestro AG8, offers very interesting advantages, such as the lack of requirement of a wet processing after the recording; a simple photocuring process is needed instead It can provide the dynamic range necessary for recording HOEs that are highly efficient at 800 nm[9]. Holographic cylindrical lenses have been studied in certain systems designed to concentrate solar irradiance onto one or more photovoltaic cells. Gordon et al.’s design[10] is based on a system formed by two different holographic cylindrical lenses, each diffracting more efficiently a spectral range towards a different PV cell, so a broader part of the incident spectrum can be used.
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