SAN FRANCISCO — A National Renewal Energy Laboratory "irradiance" map showing available sunlight around the world suggests 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 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.

A Sandia National Laboratories engineer sets up a calibration device for a parabolic trough used to concentrate solar energy.

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 here 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 4.5 times 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.