Investors and utilities intent on building solar power plants are increasingly turning to solar thermal power, a comparatively low-tech alternative to photovoltaic panels that convert sunlight directly into electricity. This month, in the latest in a string of recent deals, Spanish solar-plant developer Abengoa Solar and Phoenix-based utility Arizona Public Service announced a 280-megawatt solar thermal project in Arizona. By contrast, the world's largest installations of photovoltaics generate only 20 megawatts of power.

In a solar thermal plant, mirrors concentrate sunlight onto some type of fluid that is used, in turn, to boil water for a steam turbine. Over the past year, developers of solar thermal technology such as Abengoa, Ausra, and Solel Solar Systems have picked up tens of millions of dollars in financing and power contracts from major utilities such as Pacific Gas and Electric and Florida Power and Light. By 2013, projects in development in just the United States and Spain promise to add just under 6,000 megawatts of solar thermal power generation to the barely 100 megawatts installed worldwide last year, says Cambridge, MA, consultancy Emerging Energy Research.

The appeal of solar thermal power is twofold. It is relatively low cost at a large scale: an economic analysis released last month by Severin Borenstein, director of the University of California's Energy Institute, notes that solar thermal power will become cost competitive with other forms of power generation decades before photovoltaics will, even if greenhouse-gas emissions are not taxed aggressively.

Solar thermal developers also say that their power is more valuable than that provided by wind, currently the fastest-growing form of renewable energy. According to the U.S. Department of Energy, wind power costs about 8 cents per kilowatt, while solar thermal power costs 13 to 17 cents. But power from wind farms fluctuates with every gust and lull; solar thermal plants, on the other hand, capture solar energy as heat, which is much easier to store than electricity. Utilities can dispatch this stored solar energy when they need it--whether or not the sun happens to be shining. "That's going to be worth a lot of money," says Terry Murphy, president and chief executive officer of SolarReserve, a Santa Monica, CA, developer of solar thermal technology. "People are coming to realize that power shifting and 'dispatchability' are key to the utility's requirements to try to balance their system."

In fact, the capacity to store energy is critical to the economics of the solar thermal plant. Without storage, a solar thermal plant would need a turbine large enough to handle peak steam production, when the sun is brightest, but which would otherwise be underutilized. Stored heat means that a plant can use a smaller, cheaper steam turbine that can be kept running steadily for more hours of the day. While adding storage would substantially increase the cost of the energy produced by a photovoltaic array or wind farm, it actually reduces the cost per kilowatt of the energy produced by solar thermal plants.

The amount of storage included in a plant--expressed as the number of hours that it can keep the turbine running full tilt--will vary according to capital costs and the needs of a given utility. "There is an optimal point that could be three hours of storage or six hours of storage, where the cents per kilowatt- hour is the lowest," says Fred Morse, senior advisor for U.S. operations with Abengoa Solar. Morse says that the company's 280-megawatt plant in Arizona, set to begin operation by 2011, will have six hours of storage, while other recent projects promise seven to eight.