Alberto Riverola

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.

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.