Solar advocates beef up solar thermal efforts

A National Renewal Energy Laboratory "irradiance" map, which shows the available sunlight around the world, suggests that the United States is the Saudi Arabia of solar energy, with twice the irradiance of Europe. It's a picture worth a thousand words to solar activists looking to make a convincing case for the emerging energy source.

Harnessing global sun power "is just an engineering effort," said Werner Koldehoff of the German Solar Industry Association.

Koldehoff's group and other European solar enthusiasts have come to America to make the case for solar thermal technology, an alternative to photovoltaics (PV) that attempts to harness the efficient phase change from water to steam. For cost and technology reasons, solar thermal is emerging as the preferred alternative energy technology in the race to replace fossil fuels with sustainable energy sources, many experts agree.

Solar benefits
Along with cost per watt—eventually cost per kilowatt—solar thermal's biggest selling point is its ability to store energy and deliver electricity to consumers during periods of peak power demand. Experts at a recent solar energy conference said "concentrating" solar thermal power could allow utilities and other emerging operators to store steam energy for up to six hours. Super-heated steam is used to drive turbines that generate electricity.

For residential, business and other lower-temperature applications, solar thermal could be used to heat water as well as for space heating. Koldehoff said the approach could also be harnessed for an emerging application he calls "solar-assisted cooling." Air conditioning requires roughly 4.5x the energy as heating. The largest amount of solar energy is available in the late afternoon during peak demand for air conditioning. Hence, advocates say, solar thermal power offers the least-expensive source of electricity when demand is highest.

Koldehoff said pilot solar-cooling projects are already under way in sunny Spain, and the technique could also be used for applications such as operating power-hungry desalinization plants. "The real future application in the next five years is [solar] cooling, and we need it badly, because we can't afford [the soaring cost of cooling] anymore," he said.

Concentrating, or sun-tracking, photovoltaics and solar thermal power collectors such as parabolic troughs follow the sun across the sky at one of more axis points, focusing sunlight by as much as 1,500-fold in high-end systems to improve the efficiency of solar panels.

Power concentration
Experts note that solar thermal's so-called "dispatchability" means stored power could reliably generate electricity that could then be sold to utilities during load peaks on electric grids, usually after 5 p.m. This "load-shifting" approach makes solar thermal power far more valuable for plant operators than, say, photovoltaic energy that must be used immediately.

"Thermal energy storage is the killer app of concentrating solar power technology," Andrew McMahan, VP for technology and projects at SkyFuel, told a packed solar technology conference last month held in conjunction with Semicon West. Solar thermal collector technologies like parabolic troughs have a good track record after more than 20 years of use, McMahan added. "The technology has steadily improved and is being demanded by utilities" when negotiating power supply agreements with solar operators.

Industry analysts like Jim Hines, Gartner Inc. research director for semiconductors and solar, agree that solar thermal appears best suited to large power projects aimed at supplying electricity to utilities. Other technologies, such as flat-plate photovoltaics and concentrating PV systems, work best in residential and commercial applications, Hines said. Photovoltaic "cost projections are encouraging, but future demand will depend on external factors" like solar thermal becoming the technology of choice.

Among the large solar thermal projects discussed at the solar confab were several "power tower" projects that use concentrating solar collectors to refocus sunlight on "solar boilers." For example, solar developer Brightsource Energy is building a 400MW solar thermal complex in California's Mojave Desert, a prime location for a number of planned solar thermal projects. Along with other industry executives, Brightsource CEO John Woolard noted that the primary challenge for solar thermal is efficiently transmitting power from remote desert locations to cities.

"Solar has become an important part of our resource mix," said Hal LaFlash, director of resource planning for Pacific Gas & Electric. "The big challenge is transmission [because] the highest resource potential is remote from population centers."

'Fossil-assisted solar'
Still, experts agreed that for large alternative-energy projects, solar thermal appears to be the best approach. According to estimates compiled by the Prometheus Institute for Sustainable Development, solar thermal power- generating costs could drop from about $4.25/W in 2008 to $2.5/W by 2020.

Solar thermal "is an extremely cost-effective technology compared with other [solar] technologies," though costs may not drop as fast as competing technologies like flat-plate or concentrating photovoltaics, said Travis Bradford, founder of the Prometheus Institute.

Nevertheless, solar thermal's "load-shifting" capability allows producers to store electricity, and then sell it during periods of peak demand. The predictability of solar thermal power along with technology innovations could help drive down start-up costs as the solar power infrastructure is built. Proponents add that the amount of energy needed to build and deploy solar thermal technologies is recovered in less than a year, more than twice as fast as comparable photovoltaic systems.

For now, advocates envision an energy future where solar energy supplements current fossil fuels. But as a sustainable energy infrastructure is built and solar technologies become more reliable and affordable, solar boosters like the German activist Werner Koldehoff are talking about a future in which dwindling fossil fuels are used to back up abundant solar power.

The earth's energy future hinges on "fossil-assisted solar," said Koldehoff. "We have a responsibility to future generations to make this happen."

- George Leopold
EE Times