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September 2008

It’s All Done With Mirrors

Parabolic trough systems consist of a linear, parabolic-shaped reflector that focuses the sun’s energy on a receiver pipe, heating a transfer fluid flowing through the pipe; the transfer fluid then generates superheated steam which is fed to a turbine and electric generator to produce electricity. Source: Hong Kong Engineer Online.

The rise of concentrating solar power

by Bruce Wright

When you hear about solar power, you may think of the solar panels cropping up on everything from homes to digital calculators — devices that yield relatively small amounts of power for immediate use.

But a lesser-known form of solar power can do far more. Concentrating solar power, or CSP, can produce electricity on an industrial scale — enough to power a city, or to make a significant daily contribution to the power grid.

CSP technology is not new. In 1982, for instance, a U.S. Department of Energy (DOE)-sponsored CSP demonstration project began generating about 10 megawatts of power in California’s Mojave Desert. That initiative produced power for six years.

But today’s energy prices are bringing new attention to this highly promising power source.

Coming to a Boil

While the photovoltaic (PV) cells in solar panels turn sunlight directly into electricity, CSP converts sunlight into heat by using mirrors to focus the sun’s rays on vessels filled with a heat-transfer fluid, typically water, oil or molten salt. The heat absorbed by this fluid then is used to create steam, which powers a turbine to generate electricity.

Several different types of technology can be used to generate CSP. Parabolic trough systems concentrate solar energy with fields of long, curved mirrors; a pipe running down the center of the mirrored trough contains the heat-absorbing liquid. Dish engines use a mirror resembling a satellite dish to heat the fluid, while power towers use a field of mirrors to focus the sun’s rays on a large tower containing a receiver filled with molten salt.

CSP is used almost exclusively for relatively large power production, rather than to power a single building or installation. CSP’s land needs are large, but so is its power potential. DOE estimates that CSP installations covering about 9 percent of Nevada’s land area could supply enough electricity to power the entire nation.

Solar Dish/Engine System - The sun’s energy is concentrated on a receiver and generator located at the focal point of the parabolically shaped dish. Sources: Florida A&M University and Florida State University.

Time-Shifting the Sun

CSP has one drawback compared to PV systems: its need for intense, direct sunlight. “[CSP] relies on direct solar radiation,” says Steven Wiese of Austin-based Clean Energy Associates. “Flat-plate photovoltaics can use any kind of sunlight, whether it’s direct from the sun or diffused — scattered by clouds or pollution or even bounced up off the ground.”

But that limitation is countered by a huge advantage. PV systems make electricity only when they are struck by sunlight. Fluids heated by CSP installations stay hot for hours after the sun goes down, allowing them to continue making steam and producing power.

“[Electricity usage] in the summer tends to peak between about 4 p.m. and 8 p.m. — that’s when electricity is most expensive,” says Wiese. “Solar radiation, and thus PV power, tends to peak at about 1 p.m., and by 4 p.m. it’s trailing off. So if I can build in three or four hours of storage, I can be collecting that energy at 1 p.m. but not releasing it until those peak hours of 4 to 8 p.m. I can actually shift the distribution of that energy to the hours in which it’s most needed and most valuable.”

Light is Where You Find It

Solar Power Towers Step-by-step:   1. Sun heats salt in the receiver.  2. The salt is stored in hot storage tank.  3. Hot salt is pumped through the steam generator.   4. Steam drives turbine/generator to produce electricity.  5. Salt returns to cold storage tank. Sources: Florida A&M University and Florida State University.

CSP’s demand for intense sunlight makes the Southwest the most promising area for its development in the U.S. “The very best regions in the country are Nevada, Arizona, New Mexico, the Mojave Desert in California and the far-western fringe of Texas,” says Wiese.

While Texas is a generally sunny place, “CSP makes a lot more sense in places like West Texas, where you don’t have [as much] humidity in the cloud cover,” Wiese says. “The difference from one end of the state to the other, in terms of direct-beam solar radiation, is pretty large, because of humidity from the gulf.

“We get plenty of sunshine in Austin and Houston, but it tends to be more scattered,” he says. “The further West you go, the better off you’re going to be, for any [CSP] technology. We’ve got the best solar resources in far West Texas that you can find.”

Markets and Hurdles

Today, the U.S. and Spain are world leaders in the development of CSP. Spain has encouraged the growth of solar power by establishing a so-called feed-in tariff that guarantees the nation’s utilities will buy the energy at a fixed price for a fixed period of time, a factor that has ensured the industry’s stable growth.

Most solar installations in Spain have been photovoltaic, but the country’s first commercial-scale CSP plant came online in 2007, an 11-megawatt facility located near Seville. Since then, activity in the field has exploded. A recent report in the industry publication Renewable Energy World indicated that Spanish CSP projects capable of producing 800 megawatts of energy are operating, under construction or planned.

In the U.S., CSP systems currently produce about 419 megawatts of power in all, but many more installations are planned. The Mojave Desert is home to the world’s largest CSP facility. There, nine power plants collectively called Solar Energy Generating Systems cover about 1,600 acres and produce about 600 million kilowatt-hours of electricity each year, enough to power more than 100,000 homes.

Texas does not yet have a major CSP installation, but San Antonio’s utility, CPS Energy, recently released a request for proposals to build up to 100 megawatts of solar electricity generation for the city, a bid that should attract CSP-related offers.

One major hurdle facing CSP in Texas also affects the development of wind energy — a lack of adequate power transmission facilities to link far West Texas with the power grids that serve most Texans. Recent legislation intended to benefit wind energy, however, may come to the aid of solar power as well, by extending new transmission lines to these distant areas.

According to Wiese, the new transmission lines may well do double duty.

In West Texas, wind and solar resources in general [reach their peak] at different times,” he says. “Let’s say you’ve got a billion-dollar transmission line that goes out there to a bunch of wind farms. Those wind farms are generally producing power in the evening hours. If you put a solar plant out there, you can better use that same transmission capacity. You may not need another transmission line, because you’re using the line at a different time of day. You’ve increased the utilization of that line. That can help to justify its cost.” FN

The Solúcar Platform near Seville, Spain – Photo courtesy of Abengoa Bioenergy
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