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Concentrated thinking targets photovoltaics

02 Jun 2009

GreenVolts of California is building a 2 MW power plant to demonstrate the commercial viability of concentrator photovoltaics technology. Richard Stevenson reports on the optical design and engineering challenges that will need to be addressed along the way.

Concentrator photovoltaics (CPV) stand at a crossroads. The supply chain for building these solar-energy systems is falling into place, but the technology must demonstrate that it is capable of delivering cost-competitive, reliable power on a large scale if it is to achieve commercial success.

Proof of the viability of this technology – which concentrates the sunlight by factors of several hundred onto multi-junction cells – could come from the performance data of a handful of kilowatt-scale plants under construction. This includes a 2 MW plant for Pacific Gas and Electric Company (PG&E), which is being built by GreenVolts of San Francisco, CA, on an 8 acre site at Tracy, about 50 miles east of the Bay area.

"It's a full-on commercial power plant with a 20 year power purchase agreement with one of the nation's largest utility companies," asserted Bob Cart, GreenVolts' executive chairman. "It's an opportunity to get real field data at a considerable scale, and to understand installation and permitting, electrical wiring and losses, equipment and maintenance, and all of the other issues that go into building a power plant."

When this facility is up and running it will give a welcome insight into the true cost of electrical generation with CPV. Comparisons of projected and actual performance will also be possible, and the facility should eventually help to answer the most pertinent question – can CPV plants deliver acceptable performance levels for 20 years or more?

Setting sail

Cart formed GreenVolts in 2005, following an 18-month sailing trip from San Francisco to New Zealand and back. During this voyage he mulled over ideas for a new business venture and concluded that renewable energy, and solar in particular, presented the greatest opportunity. Large solar panels were installed on his yacht, but they produced little power and were very expensive. He wondered how solar energy could ever generate electricity at a cost comparable to existing technologies.

"I started researching different possibilities, studying patents and doing analysis, and then I spent another year in the garage developing and tinkering." Cart then entered and won a business plan competition and a grant application with the Californian Energy Commission. Raising $45 m (€33.2 m) in two funding rounds followed along with signing the power plant contract with PG&E in summer 2007.

The current focus is the construction of one of the largest CPV plants in the world, which was scheduled for completion this year. However, the deadline has been pushed back to June 2010 due to the deterioration of financial markets. "In a capital-constrained economy it makes sense to slow the deployment, deploy enough to test and get data, and take advantage of a couple of opportunities for innovation that we had planned for the next generation."

Cart's original plan was to rapidly deploy an earlier generation of the technology while raising more cash. This approach may appear reckless, but it makes a lot of sense, according to him. That is because the photovoltaic industry has seen a 20% fall in module costs for every doubling of capacity produced.

Optical challenges

GreenVolts' CPV system has two distinguishing features: it uses parabolic reflectors rather than mirrors to focus sunlight and it has a novel tracker system that circumvents the need for a concrete base.

Lenses are popular because they can be manufactured from a single part that concentrates the light and protects the module from the environment. However, there are some concerns about the reliability of plastic lenses, says Cart, and silicones mounted on glass can be very expensive.

High losses are another issue for lenses. Those deployed in CPV are not conventional, solid lenses, which are too expensive, but far thinner Fresnel designs made from a set of concentric annular sections known as Fresnel zones. Scattering occurs at the intersection between zones, leading to optical losses. "As you increase the concentration ratio, you either have to have more grooves or make the mater-ial thicker," said Cart. "If you make the material thicker you have more absorption and greater cost, and if you increase the number of grooves, you have more facets and more losses."

Parabolic mirrors are simpler and deliver a higher quality of the light to the cell. "There is no practical limit to the concentration," said Cart. "We're already at 625× in our first generation and now we are testing at concentrations well beyond that." Higher concentrations will make thermal management more challenging, but it is the obvious route to driving down generation costs because the multi- junction chip currently accounts for one-fifth of the cost of the entire system.

Maintaining a high tracking accuracy is another major hurdle to increasing concentration. Most systems use expensive steel and concrete structures to support an array of modules off the ground, but this subjects the small gearboxes used to position the optics to very high loads, according to Cart. "We are using a big, wide ground base. This gives us the support of the ground with no concrete." The GreenVolts design also places less demand on the gearing system, which consumes less power as a result.

Partnership matters

Interest in CPV has rocketed over the last few years, and the two leading multi-junction cell manufacturers, Emcore and Spectrolab, are likely to face competition from other chip makers. Cart has some advice for new players looking to enter the market: "I think there is a concept that there will be a large number of CPV companies and they all want the same thing, a standard little chip. That is fundamentally wrong." Ultimately, he doesn't expect the number of CPV companies to be any higher than the number of cell companies, and points out that each optical system will require a different approach.

Successful chip makers will be the ones that partner closely with a small number of systems companies, says Cart, to develop products and manufacturing processes that meet their needs. It is no surprise, therefore, that he urges the chip companies to talk to their potential customers: "Find the requirements and dig in deep; don't make the assumption that they understand the requirements."

Heeding his advice will hopefully allow CPV to take the correct turn at the crossroads and ultimately become a competitive source of solar energy.

• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.

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