We investigate the optical performance of dichroic filters used in solar spectrum-splitting applications. Photovoltaic (PV) systems utilizing spectrum splitting have higher theoretical conversion efficiency than single-bandgap PV modules. Dichroic filters have been used in several spectrum-splitting optical system designs with success. However, dichroic filters only achieve ideal performance under collimated incident light. With an incident angle constraint the optical concentration ratio is limited. A high-concentration ratio helps to achieve high-conversion efficiency and control cost by reducing the PV cell area. In a dual-junction spectrum-splitting PV configuration with a gallium arsenide (GaAs) PV cell and a 2.1-eV bandgap PV cell, the experimental dichroic filter can provide 86.3% of the ideal designed performance. The filter nonideal performance under focused incident light is simulated with ZEMAX. System efficiency under different F-number and filter refractive index is simulated for dual-junction and three-junction systems to show the performance of dichroic filters. We have found that for a dual-bandgap spectrum-splitting system there is a 0.32% system efficiency gain associated with a filter refractive index increased from 1.5 to 1.95. An efficiency gain of 0.41% is associated with an aperture size reduction from F2.0 to F3.0. In a three-junction configuration, simulation shows that a 0.57% system efficiency gain is possible when the filter refractive index is increased from 1.5 to 1.95. An efficiency gain of 0.63% is associated with an aperture size reduction from F2.0 to F3.0.
In grating-over-lens spectrum splitting designs, a planar transmission grating is placed at the entrance of a plano-convex lens. Part of the incident solar spectrum is diffracted at 15-30° from normal incidence to the lens. The diffracted spectral range comes to a focus at an off-axis point and the undiffracted spectrum comes to a focus on the optical axis of the lens. Since the diffracted wave is planar and off-axis, the off-axis focal points suffer from aberrations that increase system loss. Field curvature, chromatic and spherical aberrations are compensated using defocusing and a curved focal plane (approximated with each photovoltaic receiver). Coma is corrected by modifying the off-axis wavefront used in constructing the hologram. In this paper, we analyze the use of non-planar transmission gratings recorded using a conjugate object beam to modify the off-axis wavefront. Diverging sources are used as conjugate object and reference beams. The spherical waves are incident at the lens and the grating is recorded at the entrance aperture of the solar concentrator. The on-axis source is adjusted to produce an on-axis planar wavefront at the hologram plane. The off-axis source is approximated to a diffraction limited spot producing a non-planar off-axis wavefront on the hologram plane. Illumination with a planar AM1.5 spectrum reproduces an off-axis diffraction-limited spot on the focal plane. This paper presents ray trace and coupled wave theory simulations used to quantify the reduction in losses achieved with aberration correction.
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.
In this paper a method to characterize the anisotropy of diffuse illumination incident on photovoltaic systems is presented. PV systems are designed based on standard conditions in which only consider direct and isotropic diffuse illumination. Anisotropic illumination can cause the PV system output to step outside of the design specifications. A baffled multi-detector sensor system is described having a discrete set of azimuthal and declination angle combinations in order to constantly sample the irradiance and the incidence angle of the diffuse illumination in all zenith directions. The sensor was deployed in the Tucson Electric Power Solar Test Yard alongside with commercially available PV systems that are independently monitored. Constant and transient sources of anisotropic diffuse illumination, such as surface reflection and cloud edge effects respectively, are measured and modeled with ray tracing software. Results of the method are described for characterizing diffuse illumination at the TEP Solar Test Yard. Understanding the anisotropic diffuse illumination can potentially allow to more accurately predict PV system or to optimize energy harvesting of systems with non-standard mounting conditions as well as building integrated photovoltaic applications.
In this work, a concentrating photovoltaic (CPV) design methodology is proposed which aims to maximize system
efficiency for a given irradiance condition. In this technique, the acceptance angle of the system is radiometrically
matched to the angular spread of the site’s average irradiance conditions using a simple geometric ratio. The optical
efficiency of CPV systems from flat-plate to high-concentration is plotted at all irradiance conditions. Concentrator
systems are measured outdoors in various irradiance conditions to test the methodology. This modeling technique is valuable at the design stage to determine the ideal level of concentration for a CPV module. It requires only two inputs: the acceptance angle profile of the system and the site’s average direct and diffuse irradiance fractions. Acceptance angle can be determined by raytracing or testing a fabricated prototype in the lab with a solar simulator. The average irradiance conditions can be found in the Typical Metrological Year (TMY3) database. Additionally, the information gained from this technique can be used to determine tracking tolerance, quantify power loss during an isolated weather event, and do more sophisticated analysis such as I-V curve simulation.
A design is presented for a planar spectrum-splitting photovoltaic (PV) module using Holographic Optical Elements (HOEs). A repeating array of HOEs diffracts portions of the solar spectrum onto different PV materials arranged in alternating strips. Several combinations of candidate PV materials are explored, and theoretical power conversion efficiency is quantified and compared for each case. The holograms are recorded in dichromated gelatin (DCG) film, an inexpensive material which is easily encapsulated directly into the panel. If desired, the holograms can focus the light to achieve concentration. The side-by-side split spectrum layout has advantages compared to a stacked tandem cell approach: since the cells are electrically isolated, current matching constraints are eliminated. Combinations of dissimilar types of cells are also possible: including crystalline, thin film, and organic PV cells. Configurations which yield significant efficiency gain using relatively inexpensive PV materials are of particular interest. A method used to optimize HOE design to work with a different candidate cells and different package aspect ratios is developed and presented. (Aspect ratio is width of the cell strips vs. the thickness of the panel) The relationship between aspect ratio and HOE performance properties is demonstrated. These properties include diffraction efficiency, spectral selectivity, tracking alignment sensitivity, and uniformity of cell illumination.
In this paper we investigate the use of holographic filters in solar spectrum splitting applications. Photovoltaic (PV)
systems utilizing spectrum splitting have higher theoretical conversion efficiency than single bandgap cell modules.
Dichroic band-rejection filters have been used for spectrum splitting applications with some success however these
filters are limited to spectral control at fixed reflection angles. Reflection holographic filters are fabricated by recording
interference pattern of two coherent beams at arbitrary construction angles. This feature can be used to control the angles over which spectral selectivity is obtained. In addition focusing wavefronts can also be used to increase functionality in the filter. Holograms fabricated in dichromated gelatin (DCG) have the benefit of light weight, low scattering and absorption losses. In addition, reflection holograms recorded in the Lippmann configuration have been shown to produce strong chirping as a result of wet processing. Chirping broadens the filter rejection bandwidth both spectrally and angularly. It can be tuned to achieve spectral bandwidth suitable for spectrum splitting applications. We explore different DCG film fabrication and processing parameters to improve the optical performance of the filter. The diffraction efficiency bandwidth and scattering losses are optimized by changing the exposure energy, isopropanol dehydration bath temperature and hardening bath duration. A holographic spectrum-splitting PV module is proposed with Gallium Arsenide (GaAs) and silicon (Si) PV cells with efficiency of 25.1% and 19.7% respectively. The calculated conversion efficiency with a prototype hologram is 27.94% which is 93.94% compared to the ideal spectrum-splitting efficiency of 29.74%.
A comparison of static and single-axis tracking holographic planar concentrator systems is made. Tracking is used as a
design parameter that can provide additional degrees of freedom in the spectrum and uniformity of the beam
illuminating the photovoltaic cell surface. These parameters impact the energy yield of the system. An overview of
these factors and an estimate of the cost differences for the two systems will be presented.
A design methodology for low-concentration ratio holographic solar concentrators with one-axis tracking is investigated. This methodology maximizes the energy collected by cascaded holographic gratings and reduces diffracted beam cross talk between gratings. Several types of transmission gratings, optimized to work with single-axis tracking systems, are used in a cascaded configuration to concentrate a large fraction of the useable solar spectrum on the surface of photovoltaic cells. A model is developed that determines the energy yield of the holographic planar concentrator (HPC). Good agreement is found between simulation and measurement of a prototype system. Simulation of an optimized HPC design shows that 80% optical efficiency at 2X geometric concentration ratio is possible. The acceptance angle in the nontracking direction is ±65 deg, and a ±16-deg tracking tolerance is sufficient to maintain 80% of the maximum optical efficiency. Simulation also shows that the single-axis tracking HPC system has a 43.8% increase in energy yield compared to a nontracking holographic solar concentrator.
We present the results of combining copper indium gallium (di)selenide (CIGS) photovoltaic cells with holographic planar concentrating film over a broad range of illumination levels. The film, originally designed for silicon bifacial solar applications worked well with the CIGS cells. The Voc, cell efficiency and fill factor reached full operating values at lower light levels; with a significant boost in performance being recorded. The holographic regions of the concentrator act as extended heat transfer surfaces, allowing the CIGS cells to operate at lower operational temperatures than they normally would in a traditional PV application.
In this presentation we evaluate the energy collection efficiency and energy yield of different holographic planar
concentrator designs. The holographic planar concentrator replaces expensive photovoltaic cell material with
holographic collectors that cost approximately 1% of the photovoltaic material. An analysis is performed using a
combination of raytracing and coupled wave theory. Other loss factors such as Fresnel reflection and polarization are
also incorporated. The performance of single gratings is optimized to maximize the spectral and angular bandwidth that
matches the spectral responsivity of different photovoltaic devices. Multiple grating collectors are also modeled to
maximize energy collection over the course of a year accommodating the movement of the sun. The results show that
approximately half of the light illuminating the hologram can directly be collected by diffraction and directed to the
photovoltaic cell. A test system is evaluated and the experimental results compare well with the analysis.
Holographic elements have several unique features that make them attractive for solar collector and concentrator
systems. These properties include the ability to diffract light at large deflection angles, Bragg selectivity, grating
multiplexing, and angle-wavelength matching. In this presentation we review how these properties can be applied to
solar collection and concentrator systems. An algorithm is presented for analyzing the energy collection properties of
holographic concentrators in specific geometries and is applied to a planar collection format. Holographic elements are
shown to have advantages for low concentration ratio solar concentrator systems.