Daniel Chemisana

Characterization of volume holographic optical elements recorded in Bayfol HX photopolymer for solar photovoltaic applications.

Volume Holographic Optical Elements (HOEs) present interesting characteristics for photovoltaic applications as they can select spectrum for concentrating the target bandwidth and avoiding non-desired wavelengths, which can cause the decrease of the performance on the cell, for instance by overheating it. Volume HOEs have been recorded on Bayfol HX photopolymer to test the suitability of this material for solar concentrating photovoltaic systems. The HOEs were recorded at 532 nm and provided a dynamic range, reaching close to 100% efficiency at 800 nm. The diffracted spectrum had a FWHM of 230 nm when illuminating at Bragg angle. These characteristics prove HOEs recorded on Bayfol HX photopolymer are suitable for concentrating solar light onto photovoltaic cells sensitive to that wavelength range.

Broadband behavior of transmission volume holographic optical elements for solar concentration

A ray tracing algorithm is developed to analyze the energy performance of transmission and phase volume holographic lenses that operate with broadband illumination. The agreement between the experimental data and the theoretical treatment has been tested. The model has been applied to analyze the optimum recording geometry for solar concentration applications.

New CPV Systems With Static Reflectors

New designs of low concentrating photovoltaics have been studied, where static reflectors and moving absorbers that track the concentrated solar rays, aim to cost effective solar devices. These systems are based on the concept that strip absorbers can use the beam radiation and convert it to electricity, while the diffuse radiation is absorbed by flat absorbers and converted into heat. The work on these designs follow the research on the linear Fresnel lenses, which are combined with PV or PV/T absorbers and can be used not only for the conversion of solar radiation into electricity and heat but also to control the illumination and the temperature of interior building spaces. Design aspects and optical results give a figure of the optical performance of these new CPV collectors, which are based on the converged solar radiation distribution profiles for East‐West orientation of parabolic trough reflectors. The concentration ratio depends on the geometry of the parabola axis and the higher values correspond...

Full modeling and experimental validation of cylindrical holographic lenses recorded in Bayfol HX photopolymer and partly operating in the transition regime for solar concentration

Concentrating photovoltaics for building integration can be successfully carried out with Holographic Optical Elements (HOEs) because of their behavior analogous to refractive optical elements and their tuning ability to the spectral range that the photovoltaic (PV) cell is sensitive to. That way, concentration of spectral ranges that would cause overheating of the cell is avoided. Volume HOEs are usually chosen because they provide high efficiencies. However, their chromatic selectivity is also very high, and only a small part of the desired spectral range reaches the PV cell. A novel approach is theoretically and experimentally explored to overcome this problem: the use of HOEs operating in the transition regime, which yield lower chromatic selectivity while keeping rather high efficiencies. A model that considers the recording materials response, by determining the index modulation reached for each spatial frequency and exposure dosage, has been developed. It has been validated with experimental measurements of three cylindrical holographic lenses with different spatial frequency ranges recorded in Bayfol HX photopolymer. Simulations of systems comprising two lenses and a mono-c Si PV cell are carried out with the standard AM 1.5D solar spectrum. Promising results are obtained when using the system with lower spatial frequencies lenses: a total current intensity equal to 3.72 times the one that would be reached without the concentrator.

Performance of a dielectric PVT concentrator for building-façade integration

Concentrating photovoltaic-thermal (CPVT) systems, which can be integrated on buildings façades and use low-accuracy trackers and standard cells, have the potential to produce cost-effective electricity and heat. In this paper, a refractive cylindrical CPVT module with cells directly immersed in deionized water (DIW) or isopropyl alcohol (IPA) is designed, fabricated and experimentally tested. The interfaces between the cylinder and the fluids cavity have been optimized to maximize optical efficiency and irradiance uniformity, obtaining better results for a geometric concentration of 10x and IPA. The system achieves an optical efficiency of 81%, an acceptance angle of 1.07° and a non-uniformity coefficient of 0.13.

Energy analysis of holographic lenses for solar concentration

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.

Design and characterization of refractive secondary optical elements for a point-focus Fresnel lens-based high CPV system

Point-focus Fresnel lens-based High Concentrator Photovoltaic (HCPV) systems are usually equipped with refractive secondary optical elements (SOE) in order to improve their performance. Two basic SOE designs are optically modeled and simulated in this work: Domed-Kaleidoscope (D-K) with breaking-symmetry top and SILO (SIngle-Lens-Optical element). Wavelength-dependent optical material properties like refractive index and absorption coefficient, as well as the spectral response of a typical triple-junction (TJ) solar cell, are included in the ray tracing simulations. Moreover, using a CPV Solar Simulator “Helios 3198”, both HCPV units are experimentally characterized. The acceptance angle characteristics of both HCPV units, obtained through optical simulations and through indoor characterization, are compared. The acceptance angle characteristic is better for the HCPV unit with the D-K SOE both in simulations and in experimental measurements, showing concordance between simulation and experiment. However, ...

Simple models for complex devices

Summary form only given. The search for higher efficiencies in photovoltaics has led us to develop increasingly complex solar cell architectures that rely on increasingly complex physical processes. The desire to overcome the Shockley-Queisser efficiency limit has caused us to consider stacks of junctions with cascading bandgaps, quantum dots and impurities for the creation of intermediate bands, amongst others. The need to improve in-coupling and absorption in thinner layers requires the development of surface textures and nanoparticles working either in the geometric- or wave-optical regimes, or a combination of both. Quantum wells have been employed to tune bandgaps or to induce angle-selective luminescent emission. More recently, photonic methods have been used to control the black-body emission of solar cells and thus influence their operating temperature. Understanding these processes, requires a detailed knowledge of the paths incident photons take in the device, complex generation and recombination mechanisms (sometimes involving multiple sequential transitions), carrier drift and diffusion, and the paths of emitted photons (either luminescent or incandescent). This sounds like a job for a supercomputer. However, with a few carefully made assumptions, accurate, predictive and insightful models can be developed that allow calculations to be performed on a laptop computer. We will show how these models can help to develop nano-photonic space solar cells that are more radiation hard; to better understand quantum-dot intermediate-band solar cells and improve them using light trapping; and to understand and control black-body emission in silicon solar cells.

Specially designed solar cells for hybrid photovoltaic-thermal generators

The performance of hybrid photovoltaic-thermal systems can be improved using PV cells that are specially designed to generate both electricity and useful heat with maximum efficiency. Present systems, however, use standard PV cells that are only optimized for electrical performance. In this work, we have developed two cell-level components that will improve the thermal efficiency of PV-T collectors, with minimal loss of electrical efficiency. These are a spectrally-selective low-emissivity coating to reduce radiative thermal losses, and a nanotextured rear reflector to improve absorption of the near-infrared part of the solar spectrum for heat generation.

Building-Integration of High-Concentration Photovoltaic Systems

The chapter addresses building-integration (BI) issues from the basic concepts to the most specific concerns related to high-concentration photovoltaic (HCPV) systems. The reader is introduced to the topic by learning the main aspects of the true BI of PV systems. Although CPVs were developed to generate electricity at a decreased price compared with nonconcentrating systems, their BI can provide additional advantages related to a range of building energy needs and functions. Characteristic case studies of building-integrated concentrating systems are presented and discussed to show how different types of optical arrangements are designed to be architecturally integrated. BI HCPV systems require two-axis tracking arrangements, which poses a range of challenges in addition to those general for low-concentration or nonconcentrating BI solar systems. Two examples of truly BI HCPV, from the very few that can be found at present, are presented.