Solar Thermal Power May Make Sun-Powered Grid a Reality

• solar power
• alternative energy
• electricity
• breakthrough awards

Modularity has other benefits, too. Since each 25-kw SunCatcher has its own Stirling engine producing electricity, there’s no single point of failure. “If something goes wrong with one dish, it doesn’t matter,” Osborn says. In contrast, the thousands of mirrors in a parabolic trough plant all feed a central turbine, so when the turbine is down for maintenance, power production stops. The SunCatcher design also shortens the wait for power during construction: Electricity will flow once the first 40 are built—a “solar group” that can churn out 1 Mw.

The breakthrough efficiency of the dish results from focusing the sun’s rays on a single spot instead of on a long pipe, which allows temperatures to reach 1450 F, compared to 750 F for parabolic troughs. In addition, the Stirling engine has a relatively flat effi­ciency curve: It produces close to maximum output even when the sun is obscured or low in the sky. So while the record 1-hour effi­ciency achieved earlier this year was 31.25 percent, the SunCatcher’s full-year, sunrise-to-­sunset efficiency is still a respectable 24 to 25 percent, roughly double that of parabolic trough systems.

Another twist on CST designs confronts the challenge that dogs every solar power scheme: “When the sun sets, that’s it for the day,” as Tisdale puts it. “But in Arizona in midsummer, it’s hot as hades, so people have their a/c cranked until 9 or 10 in the evening.” A hot liquid can be stored more efficiently than electricity; the analogy used by one industry executive is that a $5 thermos can hold as much energy in the form of heat as a $150 laptop battery can store electrochemically. Two 50-Mw plants that should begin operations by the end of this year in Spain will operate on this principle, using what amounts to a giant thermos filled with molten salt.

In the U.S., a thermal storage facility is scheduled for completion in Gila Bend, Ariz., in 2011. The 280-Mw Solana plant, being built by Spanish company Abengoa Solar, will use a parabolic trough design, but will incorporate a thermal storage tank that can keep the plant running for 6 hours with no sun. “We could design a plant that runs 24 hours a day,” says Fred Morse, an adviser for Abengoa who was formerly the Department of Energy’s solar czar, “but that would make no economic sense.” Instead, the plant is designed to cover Arizona’s peak energy-use periods, when power is most expensive.

A Matter of Scale

The enormous scale of the Abengoa and Stirling Energy plants provides an answer to skeptics who doubt whether a few rooftop panels here and there can ever play a meaningful role in the world’s energy portfolio. But size also creates its own set of problems. For one thing, the power has to be transmitted to where it’s needed, and the empty deserts best suited for sprawling CST plants tend to be in the middle of nowhere. The site of Stirling Energy’s future plant for the San Diego market currently has enough transmission capacity for 300 Mw, or 12,000 dishes. The remaining 24,000 dishes will be built only if San Diego Gas & Electric is able to complete a proposed 150-mile transmission line between the plant and the city.

Water use is another issue. CST plants with steam turbines can require hundreds of millions of gallons of water to cool their con­densers—a challenge in regions where water is already at a premium. In this respect, Stirling Energy’s hydrogen­-based system has a significant advantage, since it only uses water to rinse the mirrors every few weeks. Osborn estimates that the San Diego plant, when producing power for 500,000 households, would use the same amount of water as 33 average homes.

Utility-scale solar power also requires enormous capital, which keeps it out of reach of people in the developing world, where such solutions are desperately needed. That’s a challenge RawSolar, an MIT spinoff, is trying to meet with a dish that is just 12 ft. wide, and simple and cheap enough to make for stand-alone operation. The nonprofit Solar Turbine Group, another MIT spinoff, built an even more bare-bones mini-CST system in Lesotho last summer, using spare car parts for the heat engine.

The most natural fit for small-scale solar, though, is the good old photovoltaic cell. It takes in sunlight and spits out electricity with no moving parts, requires no water and can be situated wherever electricity is needed, to avoid transmission losses. PV panels can generate useful amounts of electricity even in the weaker sunlight of northern states where big CST plants aren’t practical. Also, they’re ideal for homeowners, since they are simple to install and maintain—in fact, integrated building materials like PV roof tiles will make new homes even easier to connect.

Comment Title
Your Name
Email Address
Website	make public
Comment
Please enter the characters shown below:
	Make sure your comment is relevant to the topic discussed. Comments not relevant to the topic will be deleted. Neither you nor Popular Mechanics has the ability to make your e-mail address public. However, we ask that you submit your e-mail address to us just in case we need to contact you. Thank you for your understanding--The Editors.