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18 Oct 2010
More powerful focusing optics doubles concentration ratio from 500x to 1000x for commercial rooftop installations, reducing the cost of solar electricity.
California-based Soliant Energy has added to the energy-generating potential of its rooftop solar systems by using higher-efficiency, triple-junction cells.
The Soliant panels are based on concentrated photovoltaics (CPV), using an optical focusing element to image a tiny spot of sunlight onto each of eight cells within each solar module. Eight modules make up the overall panel, meaning that 64 cells feature in the panel altogether.
By adapting the optical setup, which is based on a Fresnel lens, sunlight is now concentrated by a factor of 1000 in the company’s latest SE-1000X products - launched at the annual Solar Power International show held in Los Angeles last week. That is enough to deliver a peak output power of 504 W per 2.6 m2 panel – a substantial increase from the 335 W maximum that its previous generation, which concentrate sunlight by a factor of 500 and comprise six, rather than eight, modules.
As well as using 16 more cells than the 500x version, the 1000x panel features more efficient cells, says Soliant. The overall result is a module efficiency of 25.3 per cent (under a direct normal irradiation of 900 W/m2), up from 22.5 per cent in the previous generation, according to the company’s data sheets.
Efficiency vs cost trade-off
Because the modules use very high efficiency triple-junction cells grown epitaxial on a germanium substrate, they are able to convert more sunlight into electricity much more effectively than a conventional rooftop panel based on silicon. These advanced cells are much more expensive than silicon - although far less semiconuctor material is required - meaning that the CPV approach is only economically viable in very sunny locations.
The need for precision dual-axis tracking (to keep the sunlight focused on precisely the center of the triple-junction cell) means that CPV has also traditionally been thought of as a utility-scale technology, suitable for power generation at solar “farms” but not such a good option for rooftops.
Soliant is challenging that theory, and says it has a number of 10 kW systems installed around the world. It recently connected a 10 kW rooftop system on the rooftop of customer MBK Tape Solutions in Los Angeles to the grid, and Soliant CEO Terry Bailey sees the city as an ideal market for the technology:
“The city of Los Angeles has millions of square feet of unused commercial rooftop space,” Bailey said. “Our concentrated solar panel solutions are the ideal way to turn those rooftops into energy-producing power plants. We believe this is just the beginning in what will be many similar installations across Los Angeles and beyond.”
Thinking big, the company suggests that there are some five million flat roofs across the US that might be suitable for the technology, representing an opportunity for the roof owners to collectively generate more than 500 GW of power – assuming that it is a sunny day across the entire country. But Soliant’s technology is specifically designed to maximize the energy yield from rooftop space, and the switch to increased concentration should make CPV technology a more viable option for many of those roofs.
“Our business is turning empty, unused flat commercial rooftops into the most energy-dense and lowest energy cost solar plants possible,” Bailey added. “The only way to do that is through high-concentration solar panels.”
While many remain skeptical about the ability of CPV to meet its technological promise in the real world, where cloudy skies, wind and slightly misaligned trackers could all conspire against it, Bailey’s hopes will be aided by the continued advances in triple-junction cell efficiency: Spire Semiconductor recently set a new world best of 42.3 per cent, and tells optics.org that it will come close to that level of performance in volume production by the end of this year. Another key trend in CPV’s favor is the increasing number of triple-junction cell manufacturers – both Spire and JDSU have joined the growing list of commercial suppliers in recent months, and so the cost of the cells ought to fall accordingly.
Soliant also stresses that its closed-loop tracker system is designed to tolerate significant misalignment while still producing maximum output power - unlike typical ground-mounted trackers.
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