Catching the Sun


February 2013

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Archimedes, the Greek engineer from the third century B.C., defended his home of Syracuse from a besieging Roman fleet by focusing sunlight reflected off highly polished shields onto the ships, setting them afire. As a historical matter, it's probably a myth, but it neatly illustrates the power of solar energy. When concentrated to a high degree, sunlight generates intense heat.

Capturing that heat and putting it to work has captivated minds of engineers since Archimedes's time, if not before. But it has taken advances in materials science and control technologies to bring this ancient dream to the cusp of practical power production.

Photovoltaic cells, which turn light into electricity, are the most common means of capturing solar energy. Concentrating solar power systems are more complex: mirrors—sometimes 100,000 or more—controlled by sophisticated tracking systems reflect and concentrate sunlight, which is then converted to heat to generate electricity via steam turbines or heat engines. CSP systems range in size from large, utility-scale operations to smaller units that power individual buildings or plants.

Parabolic troughs bring sunlight to focus along a line. Although they produce lower temperatures than solar towers do, linear concentrators can be easier to build and operate. Image: DOE/NREL 00113/Warren Gretz

The potential for CSP is enormous. More solar energy reaches the earth in one hour than the combined worldwide consumption of energy by human activities in one year. A recent study by the International Energy Agency's SolarPACES group, in conjunction with the European Solar Thermal Electricity Association and Greenpeace International, suggested that concentrating solar power systems could provide up to 25 percent of the world's electricity needs by 2050.

In spite of the promise of CSP, the challenge is making solar thermal energy cost-competitive with fossil fuels and other alternative energy sources. This means increasing the efficiency of CSP technologies, as well as making them more affordable.

"The primary challenge that is driving all the development work on this technology is how to reduce the installation and operating costs to the point where the generated electricity is cost-competitive with other conventional forms of electricity generation," said Scott R. Hunter, senior research scientist at the Oak Ridge National Laboratory in Tennessee. "This is driving the design of lower-cost mirror materials, thermal storage, scaling to larger mirrors and facilities, and higher operating temperatures."

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by Mark Crawford, ASME.org