During the past few years there has been a significant interest in spectrum splitting systems to increase the overall efficiency of photovoltaic solar energy systems. However, methods for comparing the performance of spectrum splitting systems and the effects of optical spectral filter design on system performance are not well developed. This paper addresses these two areas. The system conversion efficiency is examined in detail and the role of optical spectral filters with respect to the efficiency is developed. A new metric termed the Improvement over Best Bandgap is defined which expresses the efficiency gain of the spectrum splitting system with respect to a similar system that contains the highest constituent single bandgap photovoltaic cell. This parameter indicates the benefit of using the more complex spectrum splitting system with respect to a single bandgap photovoltaic system. Metrics are also provided to assess the performance of experimental spectral filters in different spectrum splitting configurations. The paper concludes by using the methodology to evaluate spectrum splitting systems with different filter configurations and indicates the overall efficiency improvement that is possible with ideal and experimental designs.

No abstract

The optical efficiency of a holographic spectrum-splitting optical system with transmission holographic lenses is investigated. Spectrum-splitting is a promising approach to improve the efficiency of photovoltaic (PV) systems. By removing the lattice-matching constraints, it is possible to utilize low-cost thin-film PV materials and fabrication techniques. Transmission holograms are fabricated with the recording of the interference patterns of two or more coherent beams. It is also possible to use converging construction wavefronts to record holographic gratings that are matched to the focusing beam from the primary concentrator optics. Experimental holograms are fabricated in dichromated gelatin, and high diffraction efficiency is obtained. A single holographic lens is used to divide a broad spectrum into two types of PV cells. The position and orientation of the PV cells are chosen to match the dispersion properties of the holographic lens. The optical transfer efficiency of the holographic lens is measured to be ∼90% at the peak with fast transitions between the high diffraction efficiency and the high transmission spectral regions. With a GaAs solar cell and a 2.1-eV bandgap solar cell, the system efficiency is 31.0% under one-sun which is improved by 11.9% over the best single PV cell. The achievable system efficiency with the prototype filter is 96% compared to that of the ideal system.

SUMMARYA holographic module is designed to split light into two spectral bands for hybrid solar energy conversion. Incoming light is either transmitted to a large subsystem receiver or diffracted through an aperture in this receiver toward a second subsystem receiver. The holographic element is simulated using rigorous diffraction and ray-tracing methods. Two applications of the design are described and simulated. A photovoltaic/thermal system with 93% optical efficiency and adjustable subsystem power output ratio is designed to address solar intermittency and provide energy storage. A photovoltaic system added to an alga biofuel operation significantly increases energy output while maintaining 92% of the original algae yield. The energy return on investment of this photovoltaic/biofuel system is 2.4× greater than that of the biofuel system alone, leading to economically viable operation. Modifications to the standard holographic lens provide additional increases in spectrum-splitting capability, optical efficiency, and energy conversion efficiency. The diffraction-through-aperture concept is demonstrated as a successful approach to spectrum splitting for hybrid solar applications.

Two types of solar concentrators for use with standard silicon photovoltaic cells are compared. The first is a spectral shifting luminescent concentrator that absorbs light in one spectral band and re-emits light at longer wavelengths where the absorption of standard silicon photovoltaic cells is more efficient. The second type is a holographic planar concentrator that selects the most useful bands of the solar spectrum and concentrates them onto the surface of the photovoltaic cell. Both types of concentrators take advantage of total internal reflected light, do not require tracking, and can operate with both direct and diffuse sunlight. The holographic planar concentrator provides a simpler and more cost effective solution with existing materials and construction methods.