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Solar power tower and surrounding heliostats at the Gemasolar concentrating solar power plant in Spain. The 20-megawatt plant is the first such plant to use molten salt technology for thermal storage. (Greg Glatzmaier/NREL)
Web Feature | Environment

Suncatchers: A Solar Tech Rundown

Pictured above: Solar power tower and surrounding heliostats at the Gemasolar concentrating solar power plant in Spain. The 20-megawatt plant is the first such plant to use molten salt technology for thermal storage. (Greg Glatzmaier/National Renewable Energy Laboratory)

Somewhere in the Mojave Desert right now towers are being erected that will stand like shining tributes to the sun. Row upon row of mirrors will genuflect below, taking in the sun's light, each to its measure, and focusing it back on a point atop their corresponding tower. The resulting heat will make the tower glow like a blinding beacon, or a steel-and-glass version of the Eye of Sauron, for Tolkien lovers.

This scene is not set to appear in a new science fiction film, either, but instead may become increasingly familiar as a form of solar energy production. The Ivanpah Solar Electric Generating System in southeastern California will rely on a new form of solar, known rather prosaically as the power tower. The system relies on giant mirrors called heliostats to reflect and concentrate the sun's rays, thereby heating liquid contained in the towers. In this case, that liquid is water. The steam created as a result powers a turbine in the same way burning fossil fuels would in a traditional plant.

The power tower is but one of several emerging solar energy technologies that have received a boost from federal funding. With growing concern about global climate change and a national economic interest in moving away from fossil fuels, renewable energies like solar have become increasingly attractive. After all, that giant nuclear explosion in the sky we call the sun is the ultimate power source. If we captured every bit of solar energy that reaches Earth in just one hour, we could power every house, every factory, every cell phone, computer and car in America, for an entire year. That's according to a report by the U.S. Department of Energy.

Of course, we can't capture all of the sun's radiation. And real-world obstacles such as inclement weather and limitations on storage capacity and transmission exist. But the United States does boast nearly 60,000 square miles of viable sun-collecting land in the Southwest, according to the energy department. Put enough solar-power facilities there and we could theoretically generate four times the amount of electricity currently produced in the country annually.

It sounds like a small enough footprint for what we get, but there are real concerns over the environmental impact of bulldozing or fencing off so much of our delicate desert landscape. For more on that, see our story "The Great Green Rush."

Regardless of the challenges or the controversy facing solar, its portion in our overall energy cocktail only seems set to grow (it currently accounts for less than a tenth of the electricity generated in state).

Solar power technology breaks down into two main categories: concentrating solar power and photovoltaic. Concentrating solar power, or CSP, focuses the sun's light on a receiver, generating enough heat to create steam and power a turbine. The power tower falls into this category. Photovoltaics, or PV, employ the glossy, obsidian solar cells probably familiar to most to capture the sun's radiation and convert it directly to electricity. One kind of PV cells power your calculator and maybe your watch. It's the same technology you may have already installed on your roof.

Here's a quick peek at the key solar power technologies on the market today.

Photovoltaic
Photovoltaic panels on the roof of the National Renewable Energy Laboratory's newest green energy building, the Research Support Facility. (Dennis Schroeder/NREL)


Photovoltaic systems directly convert sunlight into electricity and are generally found on buildings, though there are also power plants employing whole fields of PV cells. Photovoltaics account for a little more than half the solar-powered electricity generated in California.

Compact Linear Fresnel Reflectors (CSP)
One example of a concentrating solar power technology is AREVA Solar's Compact Linear Fresnel Reflector technology at the company's Kimberlina Solar Thermal Power Plant in Bakersfield, California. (AREVA Solar)


Another category of solar power is solar thermal, or concentrating solar power, which turns the sun's energy into heat in order to generate electricity. In linear concentrator systems, mirrors reflect the sunlight onto a receiver tube that contains a liquid. Compact linear fresnel reflectors focus multiple mirrors onto one tube to heat the liquid and generate steam, a setup that allows them to track the sun more easily.

Parabolic trough (CSP)
Elevated photo of SkyFuels parabolic trough system, which uses a lightweight film as its reflector material. (SkyFuel, Inc.)


Another type of linear concentrator, the parabolic trough system has a receiver tube for each reflector. Currently, all of the CSP plants in California use this system. Ivanpah will be the first to rely on another type of solar thermal power in the state.

Stirling Solar Dish Engine (CSP)
Dish-engine solar system. The solar dish generates electricity by focusing the sun's rays onto a receiver, which transmits the heat energy to a Stirling engine, which in turn drives a generator to make electricity. (Sandia National Laboratory)


The dish/engine system again uses the sun's energy to generate heat and steam, but in this case that steam turns the pistons on a mechanical engine. The engine then powers a generator, which produces electricity.

Solar Power Tower (CSP)
Stretched membrane heliostats with silvered polymer reflectors will be used as demonstration units at the Solar Two central receiver in Daggett, CA. (Sandia National Laboratory)


The Ivanpah plant is an example of a power tower. In this type of CSP, reflectors concentrate sunlight onto a receiver atop a central tower. Some power towers hold water, which is turned into steam. But some designs incorporate molten nitrate salt, because it is better at transferring — and storing — heat. That means the system can continue to operate in cloudy weather and even at night.

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