Conventional silicon Photovoltaic (PV) modules often have a significant surface obstruction that reduces the collection of incident solar illumination and energy conversion efficiency. In this paper, light management methods that combine low cost holographic optical elements and diffusers into conventional PV modules are evaluated to capture unused illumination. It is found that by using reflection volume holograms (RVHs) with 300 nm spectral bandwidth in combination with a diffuser on a PV module with 12% of its surface area not covered with active PV cell regions that an improvement in power collection efficiency of 9.36% is possible.
The most expensive electrical energy occurs during early morning and late afternoon time periods. This poses a problem for fixed latitude mounted photovoltaic (PV) systems since the sun is low in the sky. One potential solution is to use vertically mounted bifacial PV modules to increase the East-West collection area and solar energy production during high energy usage time periods. However, vertically mounted PV modules have reduced conversion efficiency during mid-day time periods. In this paper the use of a horizontally mounted collector with holographic elements is examined as a way of increasing the energy yield of vertically mounted bifacial PV (VMBP) modules during mid-day time periods. The design of a holographic `cap’ collector is evaluated that considers dimensional constraints, holographic diffraction efficiency characteristics, and system solar collection efficiency properties. The irradiance illuminating the vertical mount is modeled with and without the cap. The design process also includes the optimization of separation between rows of vertically mounted modules and the use of directional diffusers in the proximity of the modules to maximize system energy yield.
The technique of designing, optimizing, and fabricating broadband volume transmission holograms using dichromate gelatin (DCG) is summarized for solar spectrum-splitting applications. The spectrum-splitting photovoltaic (PV) system uses a series of single-bandgap PV cells that have different spectral conversion efficiency properties to more fully utilize the solar spectrum. In such a system, one or more high-performance optical filters are usually required to split the solar spectrum and efficiently send them to the corresponding PV cells. An ideal spectral filter should have a rectangular shape with sharp transition wavelengths. A methodology of designing and modeling a transmission DCG hologram using coupled wave analysis for different PV bandgap combinations is described. To achieve a broad diffraction bandwidth and sharp cutoff wavelength, a cascaded structure of multiple thick holograms is described. A search algorithm is then developed to optimize both single- and two-layer cascaded holographic spectrum-splitting elements for the best bandgap combinations of two- and three-junction spectrum-splitting photovoltaic (SSPV) systems illuminated under the AM1.5 solar spectrum. The power conversion efficiencies of the optimized systems are found to be 42.56% and 48.41%, respectively, using the detailed balance method, and show an improvement compared with a tandem multijunction system. A fabrication method for cascaded DCG holographic filters is also described and used to prototype the optimized filter for the three-junction SSPV system.
In this work, the technique of designing and optimizing broadband volume transmission holograms using dichromate gelatin (DCG) is summarized for solar spectrum-splitting application. Spectrum splitting photovoltaic system uses a series of single bandgap PV cells that have different spectral conversion efficiency properties to more fully utilize the solar spectrum. In such a system, one or more high performance optical filters are usually required to split the solar spectrum and efficiently send them to the corresponding PV cells. An ideal spectral filter should have a rectangular shape with sharp transition wavelengths. DCG is a near ideal holographic material for solar applications as it can achieve high refractive index modulation, low absorption and scattering properties and long-term stability to solar exposure after sealing. In this research, a methodology of designing and modeling a transmission DCG hologram using coupled wave analysis for different PV bandgap combinations is described. To achieve a broad diffraction bandwidth and sharp cut-off wavelength, a cascaded structure of multiple thick holograms is described. A search algorithm is also developed to optimize both single and two-layer cascaded holographic spectrum splitters for the best bandgap combinations of two- and three-junction SSPV systems illuminated under the AM1.5 solar spectrum. The power conversion efficiencies of the optimized systems under the AM1.5 solar spectrum are then calculated using the detailed balance method, and shows an improvement compared with tandem structure.
KEYWORDS: Energy conversion efficiency, Holography, Solar energy, Volume holography, Photovoltaics, Solar concentrators, Solar cells, Gallium arsenide, Holograms, Diffraction
In this paper a prototype spectrum-splitting photovoltaic system based on volume holographic lenses (VHL) is designed, fabricated and tested. In spectrum-splitting systems, incident sunlight is divided in spectral bands for optimal conversion by a set of single-junction PV cells that are laterally separated. The VHL spectrumsplitting system in this paper has a form factor similar to conventional silicon PV modules but with higher efficiencies (>30%). Unlike many other spectrum-splitting systems that have been proposed in the past, the system in this work converts both direct and diffuse sunlight while using inexpensive 1-axis tracking systems. The VHL system uses holographic lenses that focus light at a transition wavelength to the boundary between two PV cells. Longer wavelength light is dispersed to the narrow bandgap cell and shorter wavelength light to the wide bandgap cell. A prototype system is designed with silicon and GaAs PV cells. The holographic lenses are fabricated in Covestro Bayfol HX photopolymer by ‘stitching’ together lens segments through sequential masked exposures. The PV cells and holographic lenses were characterized and the data was used in a raytrace simulation and predicts an improvement in total power output of 15.2% compared to a non-spectrum-splitting reference. A laboratory measurement yielded an improvement in power output of 8.5%.
Spectrum-splitting is a multijunction photovoltaic technology that can effectively improve the conversion efficiency and reduce the cost of photovoltaic systems. Microscale PV design integrates a group of microconcentrating photovoltaic (CPV) systems into an array. It retains the benefits of CPV and obtains other benefits such as a compact form, improved heat rejection capacity, and more versatile PV cell interconnect configurations. We describe the design and performance of a two-junction holographic spectrum-splitting micro-CPV system that uses GaAs wide bandgap and silicon narrow bandgap PV cells. The performance of the system is simulated with a nonsequential raytracing model and compared to the performance of the highest efficiency PV cell used in the micro-CPV array. The results show that the proposed system reaches the conversion efficiency of 31.98% with a quantum concentration ratio of 14.41× on the GaAs cell and 0.75× on the silicon cell when illuminated with the direct AM1.5 spectrum. This system obtains an improvement over the best bandgap PV cell of 20.05%, and has an acceptance angle of ±6 deg allowing for tolerant tracking.
In this work a spectrum splitting micro-scale concentrating PV system is evaluated to increase the conversion efficiency of flat panel PV systems. In this approach, the dispersed spectrum splitting concentration systems is scaled down to a small size and structured in an array. The spectrum splitting configuration allows the use of separate single bandgap PV cells that increase spectral overlap with the incident solar spectrum. This results in an overall increase in the spectral conversion efficiency of the resulting system. In addition other benefits of the micro-scale PV system are retained such reduced PV cell material requirements, more versatile interconnect configurations, and lower heat rejection requirements that can lead to a lower cost system. The system proposed in this work consists of two cascaded off-axis holograms in combination with a micro lens array, and three types of PV cells. An aspherical lens design is made to minimize the dispersion so that higher concentration ratios can be achieved for a three-junction system. An analysis methodology is also developed to determine the optical efficiency of the resulting system, the characteristics of the dispersed spectrum, and the overall system conversion efficiency for a combination of three types of PV cells.
In this study the impact of outdoor temperature variations and solar illumination exposure on spectral filter material and holographic optical elements is examined. Although holographic components have been shown to be useful for solar spectrum splitting designs, relatively little quantitative data exist to demonstrate the extent to which these materials can withstand outdoor conditions. As researchers seek to investigate practical spectrum splitting designs, the environmental stability of holographic materials should be considered as an important factor. In the experiment presented, two holographic materials, Covestro Bayfol HX photopolymer and dichromated gelatin, and 3M reflective polymer filter materials are exposed to outdoor conditions for a period of several months. The environmental effect on absorption, spectral and angular bandwidth, peak efficiency, and Bragg matching conditions for the holograms are examined. Spectral bandwidth and transmittance of the 3M reflective filter material are also monitored. Holographic gratings are recorded, measured, and mounted on glass substrates and then sealed with a glass cover plate. The test samples are then mounted on a photovoltaic panel to simulate realistic temperature conditions and placed at an outdoor test facility in Tucson, Arizona. A duplicate set of holograms and 3M filter material is stored as a control group and periodically compared over the test period.
This paper presents a segmented parabolic concentrator employing holographic spectral filters that provide focusing and spectral bandwidth separation capability to the system. Strips of low band gap silicon photovoltaic (PV) cells are formed into a parabolic surface as shown by Holman et. al. [1]. The surface of the PV segments is covered with holographic elements formed in dichromated gelatin. The holographic elements are designed to transmit longer wavelengths to silicon cells, and to reflect short wavelength light towards a secondary collector where high-bandgap PV cells are mounted. The system can be optimized for different combinations of diffuse and direct solar illumination conditions for particular geographical locations by controlling the concentration ratio and filtering properties of the holographic elements. In addition, the reflectivity of the back contact of the silicon cells is used to increase the optical path length and light trapping. This potentially allows the use of thin film silicon for the low bandgap PV cell material. The optical design combines the focusing properties of the parabolic concentrator and the holographic element to control the concentration ratio and uniformity of the spectral distribution at the high bandgap cell location. The presentation concludes with a comparison of different spectrum splitting holographic filter materials for this application.
Micro-scale PV technology combines the high conversion efficiency of concentrated photovoltaics (CPV) with the low costs and the simple form of flat panel PV. Some of the benefits of micro-scale PV include: reduced semiconductor material usage; improved heat rejection capacity; and more versatile PV cell interconnect configurations. Spectrumsplitting is also a beneficial technique to increase the efficiency and reduce the cost of photovoltaic systems. It spatially separates the incident solar spectrum into spectral components and directs them to PV cells with matching bandgaps. This approach avoids the current and lattice matching problems that exist in tandem multi-junction systems. In this paper, we applied the ideas of spectrum-splitting in a micro-scale PV system, and demonstrated a holographic micro-scale spectrum-splitting photovoltaic system. This system consists of a volume transmission hologram in combination with a micro-lens array. An analysis methodology was developed to design the system and determine the performance of the resulting system. The spatial characteristics of the dispersed spectrum, the overall system conversion efficiency, and the improvement over best bandgap will be discussed.
Concentrating and spectrum splitting photovoltaic (PV) modules have a limited acceptance angle and thus suffer from optical loss under off-axis illumination. This loss manifests itself as a substantial reduction in energy yield in locations where a significant portion of insulation is diffuse. In this work, a spectrum splitting PV system is designed to efficiently collect and convert light in a range of illumination conditions. The system uses a holographic lens to concentrate shortwavelength light onto a smaller, more expensive indium gallium phosphide (InGaP) PV cell. The high efficiency PV cell near the axis is surrounded with silicon (Si), a less expensive material that collects a broader portion of the solar spectrum. Under direct illumination, the device achieves increased conversion efficiency from spectrum splitting. Under diffuse illumination, the device collects light with efficiency comparable to a flat-panel Si module. Design of the holographic lens is discussed. Optical efficiency and power output of the module under a range of illumination conditions from direct to diffuse are simulated with non-sequential raytracing software. Using direct and diffuse Typical Metrological Year (TMY3) irradiance measurements, annual energy yield of the module is calculated for several installation sites. Energy yield of the spectrum splitting module is compared to that of a full flat-panel Si reference module.
KEYWORDS: Solar cells, Solar energy, Photovoltaics, Convection, Thermal modeling, Systems modeling, Clouds, Aluminum, Solar thermal energy, Data modeling
In this paper we introduce an approach to damping intermittency in photovoltaic (PV) system output due to fluctuations in solar illumination generated by use of a hybrid PV-thermal electric (TE) generation system. We describe the necessary constrains of the PV-TE system based on its thermodynamic characteristics. The basis for the approach is that the thermal time constant for the TE device is much longer than that of a PV cell. When used in combination with an optimized thermal storage device short periods of intermittency (several minutes) in PV output due to passing clouds can be compensated. A comparison of different spectrum splitting systems to efficiently utilize the incident solar spectrum between the PV and TE converters are also examined. The time-dependent behavior of a hybrid PV-TE converter with a thermal storage element is computed with SMARTS modeled irradiance data and compared to real weather and irradiation conditions for Tucson, Arizona.
KEYWORDS: Solar cells, Dispersion, Receivers, Photovoltaics, Energy efficiency, Optical filters, Gallium arsenide, Optical components, Optical design, Solar energy
There has been a significant interest in spectrum splitting techniques to increase the overall efficiency of photovoltaic solar energy systems. In spectrum splitting, an optical system is used to spectrally separate the incident sunlight. Although systems with different methods and geometries have been proposed, they can generally be classified as either dispersive or nondispersive. Nondispersive systems are based on reflective spectral filters that have minimum optical losses due to dispersion. Dispersive systems use optical elements that spatially separate light as a function of wavelength. This class of spectrum system typically operates in transmission and is shown to have an inherent optical loss. The dispersive effects of transmission type filters are evaluated using a cross-correlation analysis. The results of the analysis are then used to evaluate different spectrum splitting geometries and to determine parameters that minimize their dispersion losses and optimize optical designs.
Spectrum-splitting is a beneficial technique to increase the efficiency and reduce the cost of photovoltaic (PV) systems. This method divides the incident solar spectrum into spectral components that are spatially separated and directed to PV cells with matching spectral responsivity characteristics. This approach eliminates problems associated with current and lattice matching that must be maintained in tandem multi-junction systems. In this paper, a two-junction holographic spectrum-splitting photovoltaic system is demonstrated with a folded PV geometry. The system is designed to use both direct and diffuse solar irradiation. It consists of holographic elements, a wedge-shaped optical guide, and PV substrates with back reflectors. The holographic elements and back reflectors spatially separate the incident solar spectrum and project spectral components onto matching PV cell types. In addition, the wedge-shaped optical guide traps diffuse illumination inside the system to increase absorption. In this paper, the wedge spectrum splitting system is analyzed using tabulated data for InGaP2/GaAs cells with direct illumination combined with experimental data for reflection volume holograms. A system efficiency of 31.42% is obtained with experimental reflection hologram data. This efficiency is a 21.42% improvement over a similar system that uses one PV cell with the highest efficiency (GaAs). Simulation results show large acceptance angle for both in-plane and out-of plane directions. Simulation of the output power of the system with different configurations at different times of the year are also presented.
Shockley and Queisser have shown that systems based on single junction PV cells are limited to a system efficiency of 33%. This restriction results from the mismatch between the photon energy of the incident sunlight and the inability of a single junction device to optimally convert the broad incident spectrum. One approach to overcome this difficulty is to incorporate multiple PV cells with different bandgaps that are optimized to convert different parts of the incident spectrum to electrical power. Spectrum splitting configurations distribute incident photons onto several single bandgap PV cells that are spatially separated. Although, systems with different methods and geometries have been proposed, optical systems relying on reflective filters have not been compared to transmissive ones. Since reflection-type films are primarily based on the interference of reflected waves from optical interfaces, systems based on these filters do not have dispersion losses. Dispersive spectrum splitting systems rely on optical elements that use diffraction or refraction for spectral separation. The dispersion from a single broad band optical element can be used for spectral separation. The geometrical relationship between focusing power, the degree of dispersion, the system aperture, and the PV cell aperture and position can be used to tailor the spectral shape of the incident spectrum into each of the PV cells comprising the system. In this paper, the effects of dispersion introduced by transmission type filters are presented compared to reflective filters.
Thermodynamic principles limit the conversion efficiency of a single bandgap organic photovoltaic (OPV) cell to 33%1 . In order to increase efficiency, multiple OPV devices can be combined to cover a larger spectral range of the incident solar spectrum. The most common way of doing this is to mount multiple bandgap cells in tandem or series. However, stacked multijunction systems have limitations, such as current-matching constraints and optical quality of the OPV layer. A separated arrangement with spectrum splitting is a promising alternative to the stacked tandem approach. In this paper, two organic photovoltaic cells with complementary EQE curves are integrated into a holographic spectrum splitting module. The highest possible conversion efficiency of this two-cell combination is quantified assuming an ideal spectral filter as a reference. A spectrum splitting module is built, consisting of a reflective hologram oriented at an angle to split the incident beam into two spectral bands. The holographic beamsplitting system is assembled and studied under a solar simulator. Power output and conversion efficiency of the holographic spectrum splitting system is evaluated in terms of Improvement over Best Bandgap (IoBB) of the two-cell combination. The combined system has a measured improvement over its best single cell of 12.30% under a solar simulator lamp and a predicted improvement of 16.39% under sunlight.
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.
KEYWORDS: Photovoltaics, Solar cells, Renewable energy, Atmospheric modeling, Solar energy, Sun, HVAC controls, Control systems, Light sources and illumination, Fluctuations and noise
A hybrid photovoltaic/illumination rooftop module is proposed for greenhouse environmental control. The design counterbalances seasonal irradiance and temperature extremes while simultaneously generating renewable energy. Oneaxis tracking is used to increase photovoltaic collection. The tracking system also provides a seasonal variation of shading and ventilation to control temperature in the greenhouse. A faceted illumination structure is added to the module, which modifies irradiance by changing the solar angle. Irradiance in the greenhouse is simulated throughout the year for a particular site latitude and facet angle. Adjustments in the design can be made to customize the rooftop for a specific site location, illumination range, and irradiance uniformity.
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