Generating electricity from the sun's heat

Virginia Lacy is a consultant with the Energy and Resources Team at Rocky Mountain Institute.

For most of us, the words "solar power" bring a few images to mind: an array of flat, iridescent panels perched on a roof, quietly generating electricity from the sun's rays; or, if you've been following the latest technological developments, a thin, flexible film attached to a backpack for charging a laptop computer.

Both of these are examples of different types of solar photovoltaic (PV) technology. Whether it's in the news or in the stock market, solar PV has been a shining star among renewable energy technologies.

But what about other forms of solar power? Until recently, solar thermal electric power has not received the same level of attention as solar PV. But the advantages of this technology and the recent surge of activity in this space may soon change that trend.

In contrast to solar PV, which coverts sunlight directly into electricity, solar thermal technologies harness the sun's energy as heat. This heat can be used directly -- to heat water for a hot shower, for example -- or indirectly to generate electricity by creating steam to power a turbine. The latter is commonly referred to as "solar thermal electric power" or "concentrating solar power."

Although there are a variety of solar thermal electric technologies, they function under the same basic principle.

Remember when you were a kid and you used a magnifying glass to concentrate sun light on an object to set it on fire? Solar thermal electric plants apply the same principle on a grand scale.

Most often by using mirrors, these facilities focus the sun's rays on a focal point, which is designed to absorb a high degree of heat. This concentrated heat is used to create steam, which is used to turn a turbine and generate electricity -- just like in a conventional power plant.

In fact, one way to think about solar thermal electric power is that it simply replaces a fossil fuel, such as coal, with the sun to power a conventional electricity plant.

In addition to being a clean, renewable source of energy, solar thermal electric power has several advantages.

First, solar thermal electric plants can be designed to store excess energy to be used when it is needed, such as periods of peak energy use or when there is no sun. This is a key advantage over other renewable technologies.

One of the principle criticisms of wind and solar PV, for instance, is their variability. With energy storage, some solar thermal power designs are projected to be able to generate a steady stream of power for up to 20 hours in a given day.

Second, solar thermal electric power is technologically straightforward. Sure, the design and configuration of a large-scale project can be quite impressive, but the principles behind the technology and the basic materials used to construct it -- often steel, glass, and concrete -- are common and readily available.

The lack of technological complexity lowers the cost of the solar thermal electric plant, which in turn, lowers the cost of the power that it produces. Although estimates vary considerably, utility-scale solar thermal electric power can be generated at 10 cents to 15 cents per kilowatt-hour.

Not only is this comparatively less expensive than the price of unsubsidized solar PV (30 cents per kilowatt-hour), it's also competitive with the cost that many utilities pay for peak power. The National Renewable Energy Lab expects costs to drop even further over the next decade.

As with any solar technology, the availability of the sun is a critical component to the system's operation. Although the ability to store energy as thermal energy helps to fill in for short-term solar shortages, regular doses of daily sun are still necessary to continually generate power.

Solar thermal power plants also require a sizable amount of land area -- between 5,000 to 15,000 square meters (or 1 to 3 American football fields) per megawatt. To be fair, the land required for a utility-scale solar thermal plant goes to producing the solar "fuel."

Comparing these land requirements to those of a coal plant, for example, would have to include the coal mine and transportation. In addition, the best sites for solar thermal plants typically are so dry and hot that there are not a lot of competing uses. The more pressing issue may be accessing water needed for cooling in these hot, dry areas.

Currently, the United States is home to almost all the installed solar thermal capacity in the world, including the largest solar power plant of any kind ever installed, the Solar Energy Generating Systems (SEG) in the Mojave Desert. Constructed in the 1980s, SEG is a collection of nine installations, which can generate 354 megawatts, the equivalent of a mid-sized coal plant.

Over the last few years, there has been a burst of activity in developing and installing solar thermal electric plants in the United States and Europe, especially Spain. Two new plants were recently completed in the U.S., including the 64-megawatt Solar One plant outside Las Vegas. California utility PG&E plans to buy 1,000 megawatts of solar thermal power within the next five years. The utility has already signed deals for over 650 megawatts.

The next expected addition to the world's solar thermal electric power capacity is the 15-megawatt Solar Tres plant in sun-soaked southern Spain.

Even if these solar arrays won't be powering your home anytime soon, here are some resources you can consult to find out more about the technology:

• National Renewable Energy Laboratory
• U.S. DOE Efficiency and Renewable Energy