III-Vs for terrestrial solar: the time is now

These are heady times for photovoltaics. With renewables and the potential impact of traditional energy sources on the future climate now firmly entrenched in both the public psyche and in policymaking, solar energy is big news. From Spain to South Korea, photovoltaics is booming.

Whether the economics of solar power will ultimately stack up remains to be seen. The German government, the biggest backer of the technology, cannot continue to subsidize the industry through its so-called "feed-in tariff" scheme forever. For developing countries, such subsidies are out of the question.

So there is a need to come up with photovoltaic technologies that can generate electricity at genuinely competitive rates. This is what the proponents of concentrating photovoltaics (CPV) claim is possible - in the right conditions, and with very-high-efficiency III-V cells.

Relocated to Milan s vast trade fair center for the first time, the 22nd European Photovoltaic Solar Energy Conference featured 520 exhibitors - up from 350 the year before - and some 3000 delegates. The question for the compounds sector was this: where exactly do these more exotic materials fit into the rapidly growing scene; and just how important a market could it become?

Well, on the trade show floor, you could be forgiven for thinking that compound semiconductors were a non-starter. For most people in this industry, photovoltaics clearly means silicon. But there was an odd paradox in Milan - while silicon dominated the exhibition, in the conference talks III-Vs and concentrator systems based around high-performance III-V chips enjoyed a very high profile.

This much was evident right from the opening plenary session, which featured both Richard King from Spectrolab and Matthias Meusel from the key European supplier Azur Space, who detailed the many recent breakthroughs made in III-V multijunction solar cells.

Italy s pledge
The Milan location also coincided neatly with the Italian government s pledge to imitate near neighbors Germany and Spain in backing the photovoltaics sector. 3 GW of deployments are planned for the next decade. Of more interest to the III-V world will be the fact that some 500 MW of that total has been earmarked for CPV systems.

Whether or not those promises will come to fruition, and whether or not they will involve III-V cells, remains to be seen. But Emcore - the only III-V multi-junction cell maker to host a booth at the Milan exhibition - certainly senses a new opportunity.

David Danzilio, the general manager of Emcore s solar business unit, reckons that "now is the time" for multijunction technology to make a real impact in terrestrial systems. With the price of polysilicon still high, but with a capacity expansion to change that situation in the pipeline, there may well be a limited window of opportunity for concentrator solar to prove itself in the wider industry.

Not everybody is a great proponent of concentrating systems, of course. Applied Materials, which signaled the extent of its ambitions in photovoltaics by rolling out virtually its entire senior management team in Milan, is sticking to what it knows best - silicon.

Charlie Gay is the general manager of Applied Materials solar business. He says that the big downside of CPV is that, to be an economic solution, it demands clear blue skies.

Not only that, contends Gay, the huge heat loads generated by 500- or 1000-sun CPV make for severe reliability challenges. "Wires [carrying the PV current] have to be huge, and there's a lot of expansion and contraction. Metals change shape, and this has reliability implications," said Gay.

He s right, of course. Only certain geographies have a suitable climate and sky for CPV to be a serious option. Gay joked, albeit with a serious undertone, about "Albuquerque, New Mexico, and a few plateaus in the Andes" as the only places on Earth where CPV deployment makes sense - even though some countries around the Mediterranean Sea are either already deploying the technology or seriously looking at it.

Where the views of Gay and some in the CPV community really diverge is in their assessment of the range of locations where CPV can be successfully deployed. Spain is a key location, a battleground where CPV can win. In the south of the country, the local company Isofoton is one of three CPV system suppliers involved in the government s Castilla La Mancha project, a 3 MW CPV installation scheduled to begin operating next year.

Having been involved in building solar systems since 1981, with a range of different technologies, Isofoton is in a good position to assess the relative merits of CPV. Vicente Diaz, the manager of Isofoton s CPV business unit, said in Milan: "The community is more sensitive to CPV now. Its moment has come."

As long ago as the 1970s, there was a lot of hope around GaAs-based CPV, Diaz told compoundsemiconductor.net. In the post-Oil Crisis era, somewhat analogous to today's economic climate, Sandia Labs in the US had developed a 10 kW system. But, as oil prices dropped, so did the interest of energy providers and consumers alike, and today there are only a handful of CPV systems in the field.

Perfect storm
What s changed now is that oil prices are not expected to drop by as much as they did in the early 1980s. Add to that the concerns over energy security and the use of hydrocarbon fuels, plus an unprecedented appetite for renewable sources, and you have what some have described as a "perfect storm" scenario behind solar.

Diaz says that Isofoton has mapped out a five-year plan to roll out CPV technology and, although the company will not reveal precise details of the extent of that roll-out, it is clear that the Spanish company has major deployments in mind.

Also involved in the Castilla La Mancha project is SolFocus, a relative newcomer from the US that has just attracted $52 million in a second round of venture finance. Nancy Hartsoch, the Californian company s VP of marketing, has a radically different view to Gay regarding the abundance of CPV-compatible locations. She believes that as much as one-third of total photovoltaic energy could eventually be produced via CPV. "We ve found that it can be economical as far north as San Francisco," said Hartsoch. Nevertheless, SolFocus will concentrate its efforts on southern Europe, South America and North Africa for the mean time.

It s a critical time in SolFocus evolution right now. Around half of that $52 million will be spent on its new Madrid-headquartered European subsidiary. Meanwhile, the firm has just moved into a new facility in Mountain View, while its manufacturing plant in India begins volume production next month.

That ramp-up will primarily be to support the Castilla La Mancha project in the first instance, with other CPV installations set to drive "controlled demand" in the first three quarters of next year, before a volume ramp scheduled for the final quarter of 2008.

Whereas many cite the huge heat loads encountered in CPV as a key technical problem, Hartsoch spins that argument on its head by suggesting that the efficiency of silicon panels drops off by as much as 30 percent at high temperatures. She adds that keeping the supply of solar energy in line with the projected increase in global demand will become a problem without CPV, because of the huge amount of silicon that will be required.

Concentrix Solar, a 2005 off-shoot of Germany s Fraunhofer Institute for Solar Energy Systems (ISE), is the third CPV system partner involved in Castilla La Mancha. Its managing director, Hansjoerg Lerchenmueller, says that the company is currently building a 500 kW, 90-array system, and he is no doubt that III-V cells are key to the future business. Unlike SolFocus, which uses a pair of mirrors to focus sunlight, the Concentrix systems feature Fresnel lenses that produce a 2 mm concentrated spot in the III-V cell.

Having also recently deployed small demonstrator systems in Cyprus, Egypt and Belgium (at a site owned by germanium wafer supplier Umicore), Concentrix is currently upgrading its manufacturing facility to support a capacity ramp. That expansion, which will give it an annual system capacity of 25 MW, should be completed by the middle of 2008.

2008 - key year
Next year really does look like it will be a critical one for CPV. Emcore s recent $24 million deal with the Australian firm Green and Gold Energy (see related story) illustrates that the demand for III-V cells is really starting to tick up. Much now rests on the real-world performance of CPV systems - are they sufficiently reliable? Can they cope with huge thermal loads, high winds, bad weather, and still generate the power they are designed to at a competitive cost?

In the conference sessions held in Milan, there were some initial studies on reliability. David Faiman from Israel s Ben Gurion University of the Negev has been testing a dense array of GaAs chips using the largest solar dish in the world. Called "PETAL", this dish can concentrate the sun s irradiance by a factor of 6000 if required. Faiman reduced this focusing power to a maximum of only 1000, and admitted that he still did not really believe that the cells would be able to cope with the intense heat and light.

But, after steadily ramping up the concentrating power by unveiling more and more of PETAL s mirrors and extensive tests, he surprised even himself by finding "no signs of degradation" to the GaAs chips and module. Plenty more studies like this will be required, and in the course of the next 12-15 months we should see whether the technology is sufficiently robust.

So the key messages from Milan were these: CPV systems based on III-V cells are up against some very tough competition, especially with as major a player as Applied Materials, with the huge influence and resources at their disposal, backing rival technologies. Nevertheless, CPV is a compelling technology for those places in the world that enjoy clear blue skies. We re talking about the Mediterranean; the south-west US; parts of Australia; and, yes, possibly the odd Andean plateau.

Early-stage deployments are in progress and the technology seems, so far at least, to be sufficiently robust enough to deliver on its promises. But the next stage is critical - CPV must prove that it can deal with everything that is thrown at it.

And if the likes of Isofoton, Emcore, SolFocus and Concentrix can do that, then perhaps the sunnier spots of the world could yield a global demand for some 300-400 MW of power delivered by III-V cells. A mere snip of our total energy requirements, perhaps, but nonetheless a solid market opportunity for compound semiconductors.