Solar Thermal Gears Up for a Comeback

A few years ago, when a polysilicon shortage suddenly drove up the price of photovoltaic panels, solar thermal was all the rage.

Start-ups were emerging every week, introducing new super-concentrating mirror technologies, special reflective films and other innovations.

Companies began announcing plans for utility-scale solar thermal plants anywhere there was sun in the United States. Solar thermal, also called concentrating solar power (CSP), not only had a cost advantage over photovoltaics, it offered one thing PV never could: storage, and thus stability.

So where did all the solar thermal go?

While there have been a few highly publicized bouts between large-scale solar thermal proponents and conservation groups concerned about the land required to build such plants, the real issue comes down to simple economics. Back when there was private capital available to fund projects like giant solar plants in the desert, the technology was still new and relatively untested. Now, just as the technology has matured, private capital has dried up with the recession.

Federal stimulus money has provided some grants and loan guarantees, but by all accounts the government just can’t afford to be the only funder of large-scale solar thermal plants. Moreover, the silicon glut is long gone, and PV is now the better option for utilities looking to get renewable energy into their portfolios cheaply and quickly.

Elsewhere in the world, CSP is still the technology of choice for large-scale solar, according to Jayesh Goyal, North American sales director for French utility Areva, which recently acquired Silicon Valley solar thermal start-up Ausra Solar.

The key for the U.S. market is to bring down the cost of the equipment, its installation and its operation and maintenance.

"For awhile there was a lot of development down the path of very customized solutions—lots of complicated lenses and materials," said Sumeet Jain, a principal with CMEA Capital, a longtime investor in solar technology. "That means more expense—and maybe higher performance—but definitely at a lot of expense."

Now companies are leveraging more off-the-shelf components, Jain said. “Solar thermal projects, for example, might go with a standard boiler or opt for flat mirrors instead of custom, curved glass.”

Such choices make it easier to get financing, because companies are using tested, proven components, Jain added. It also makes it easier to partner with manufacturers to get better deals on parts and drive down the overall cost of a project.

A number of solar thermal companies are working on the cost problem, each finding new ways to make the economics more attractive to American utilities.

There are three primary CSP designs on the market today: solar towers, parabolic troughs and linear-Fresnel systems — and proponents of each have a rivalry similar to that between PV and thin film. Engineers can wax poetic for hours on the differences between the three, but the fact is that all CSP systems work in essentially the same way: Reflective surfaces with tracking systems are used to concentrate heat from the sun into a receiver filled with a heat-conducting fluid. It is then transferred to an engine that converts the heat to electricity.

In parabolic trough systems, each trough has its own receiver, while linear-Fresnel systems feature several rows of mirrors that point to a single receiver. In tower systems, thousands of tracking mirrors in a field capture and reflect sunlight to a central receiver atop a tower. Each technology has been touted as the most efficient, stable, cost-effective choice in the solar thermal repertoire. So far, linear-Fresnel — the technology used by Ausra Solar — has been dominating the market.