17 October 2011
Concentrated photovoltaic (CPV) solar technology is a cost effective way to generate clean, renewable commercial solar power on a scale large enough to be used by electric utilities. CPV is a “disruptive innovation” in the solar industry with the potential to displace traditional thin-film and silicon-based PV and Concentrated Solar Power (CSP) technologies in utility-scale projects in sunny and dry climates. Because CPV does not require water in operation, uses land better, and produces more energy per acre than any other solar technology, CPV is the best choice in areas where there is an abundance of sunshine, water is a concern and land use is an issue.
CPV does not require water in operation, uses land better, and produces more energy per acre than any other solar technology.
Incorporating inexpensive optics into the design of CPV systems can significantly reduce the amount of expensive semiconductor material needed to produce each watt of electricity. Successful optical design for high-concentration photovoltaics needs to strike a balance between a number of optical system parameters, including cost, efficiency, manufacturability, tracking error sensitivity, thermal load, and durability.
The Amonix CPV system, for example, uses efficient and cost-effective acrylic Fresnel lenses to collect sunlight. Originally designed to concentrate the light emitted by lighthouse lamps, Fresnel lenses are thinner, larger, and flatter than conventional lenses. The Fresnel lens, combined with a proprietary secondary optical element, concentrates the sunlight to more than 500 times its usual intensity on to III-V multijunction solar cells. III-V multijunction solar cells are manufactured by layering semiconductor materials that have different band gaps. Sunlight enters the top layer, which has the largest band gap and continues to penetrate the solar cell until it reaches the layer that has a smaller band gap than its photon’s energy, at which point it is absorbed. By matching the band gap to the photon energy in this way, multijunction solar cells are more efficient than single junction solar cells, because less of the photon’s energy is lost to heat during the absorption. Current commercial III-V multijunction solar cells are capable of converting sunlight into electricity at around 40% efficiency.
Using the right optics with advanced III-V multijunction solar cells leads to unprecedented power generation.
Though III-V multijunction cells have a long heritage in rigorous space applications, their relatively recent introduction into CPV systems leaves considerable headroom for improvement. In the last year, optimization of the optics design has led to more than a 10% increase in the system’s rated power. To continue these upward trends in power and energy, engineers are exploring trade-offs in concentration, alternative optical designs, materials and thermal management.
Here are some key trade-off elements to consider in achieving good optic design:
A high-concentration design with good tolerance to alignment errors leads to higher efficiency and energy generation. But pushing the higher concentration envelope eventually involves trade-offs in efficiency.
Higher concentration brings cost savings from lower part count and can, up to a point, deliver increased cell efficiency. However, going to higher concentration increases the thermal load on a solar cell, which reduces the efficiency. Consequently, the optimum concentration is a balance between the detriments of lower operating efficiency and reduced durability that can arise from higher temperatures.
III-V multijunction solar cells can achieve high fill factors with uniform power flux distributions. Unfortunately the flux distribution generated by a Fresnel lens is non-uniform. Adequate Fresnel lens design with improved flux distribution can mitigate the losses caused by series resistance in the solar cell and achieve relatively higher fill factors at higher concentration.
Without cleaning, the performance of solar panels slowly falls over time. For CPV, the average height of the optics above ground level is an important variable in mitigating the soiling of the optical surface. An on-going soiling study in Las Vegas has quantified the impact of soiling using acrylic samples. For the wavelength ranges of interest for III-V multijunction cells, the average soiling rate was only 2% over a three month period. This was consistent with the decrease in power observed on adjacent CPV generators. Measurement of the soiling samples after they have been cleaned provides insight into the rate of damage to the acrylic due to sources such as UV and wind-blown particulates.
The impact of optics
Optic design has a huge impact on the operating performance of CPV systems. Using the right optics with advanced III-V multijunction solar cells leads to unprecedented power generation and a drastic reduction in the amount of semiconductor material required. Achieving the most efficient optics requires the engineer to strike the right balance in cost, materials and operating parameters.
About the Author
Adi Nayak is an Optics Engineer with Amonix.
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