Energy: Think of solar power, and you probably think of photovoltaic panels. But there is another way to make electricity from sunlight, which arguably has even brighter prospects.

In the past few months BrightSource Energy, based in California, has signed the world’s two largest deals to build new solar-power capacity. 

The company will soon begin constructing the first in a series of 14 solar-power plants that will collectively supply more than 2.6 gigawatts (GW) of electricity—enough to serve about 1.8m homes. But to accomplish this feat BrightSource will not use photovoltaic cells, which generate electricity directly from sunlight and currently constitute the most common form of solar power. Instead, the company specialises in “concentrating solar-thermal technology” in which mirrors concentrate sunlight to produce heat. That heat is then used to create steam, which in turn drives a turbine to generate electricity.

Solar-thermal power stations have several advantages over solar-photovoltaic projects. They are typically built on a much larger scale, and historically their costs have been much lower. Compared with other renewable sources of energy, they are probably best able to match a utility’s electrical load, says Nathaniel Bullard of New Energy Finance, a research firm. They work best when it is hottest and demand is greatest. And the heat they generate can be stored, so the output of a solar-thermal plant does not fluctuate as wildly as that of a photovoltaic system. Moreover, since they use a turbine to generate electricity from heat, most solar-thermal plants can be easily and inexpensively supplemented with natural-gas boilers, enabling them to perform as reliably as a fossil-fuel power plant.

Besides these benefits, the main drivers for the growth of the solar-thermal industry are moves to limit carbon-dioxide emissions and requirements to increase the proportion of electricity produced from renewable sources. According to New Energy Finance, about 12GW of concentrating solar-thermal power capacity is being planned worldwide—a vast amount, given that only about 500 megawatts (MW) of such capacity has been built to date. To maximise the energy that can be collected from the sun, solar-power facilities are being constructed in regions that enjoy daily uninterrupted sunshine for much of the year. According to Mark Mehos of America’s National Renewable Energy Laboratory, solar-thermal power could in theory generate 11,000GW in America’s south-west. That is about ten times America’s entire existing power-generation capacity.

Simple techniques for concentrating sunlight to generate heat date back thousands of years. In China and ancient Greece, people focused the sun’s rays with mirrors or glass to light fires. In times of war, the same approach is said to have been used to set enemy ships ablaze. By the early 20th century several scientists had built simple machines that could run on concentrated heat from the sun.

Now, as the solar-thermal industry is experiencing a revival, parabolic-trough projects are garnering much of today’s investment money because of their proven track record. To improve the economics still further, SkyFuel, a firm based in New Mexico, is replacing curved glass mirrors, which are expensive to make, with a thin, reflective low-cost film. And other competing solar-thermal technologies that were developed in parallel with trough-based systems, but never commercialised, are also ready to be deployed.

Among them is an approach that BrightSource uses, in which a field of small, flat mirrors called “heliostats” redirect and concentrate sunlight onto a central receiver at the top of a tower. The tower contains a fluid, typically water, which boils and the resulting steam is then transferred to a nearby “power block”, where it spins a conventional turbine. The advantage of this “power tower” approach is that it can produce steam at a temperature of 550°C and can thus achieve a higher thermal-to-electric efficiency than trough-based systems, says John Woolard, the chief executive of BrightSource. In addition, he says, power-tower systems suffer from fewer pumping losses than trough-based designs. The first commercial power-tower began operating in Spain in 2007.

Another advance that makes solar-thermal power more economically and technologically viable than in the past is the ability to use a large number of smaller and less expensive mirrors, steered by computer systems, to ensure more accurate and automatic tracking and redirection of sunlight than was ever possible before. Bill Gross, the chief executive of eSolar, a developer of “power tower” technology based in Pasadena, California, says his firm is using software to turn thousands of flat mirrors and shape them into a continuously evolving parabola around the tower.

Both power-tower and parabolic-trough systems can store thermal energy in the form of hot, molten salt. It is then possible to generate steam, and thus electricity, even when the sun is not shining. Solar-thermal plants without storage can operate about 30% of the year; but with storage that number could climb to 70% or higher. Unfortunately storage is expensive, and is only economical when regulators provide incentives. In Spain, for example, producers of solar-thermal power receive a guaranteed feed-in tariff. That makes it particularly appealing for Spanish plants to have storage capabilities, to maximise their ability to sell electricity to utilities. In America the main incentives for solar-thermal projects are a 30% investment-tax credit or an equivalent cash grant. As a result, American plants have to be built more cheaply in order to make a profit, and thus typically do not include storage.

Source: TheEconomist.com