Sustainable Energy

Hybrid Solar Power Works Even When It’s Not Sunny

The government’s energy research agency is spending $30 million to demonstrate cheap solar power that’s available day and night.

Solar power is getting cheaper, but its intermittency limits how much utilities can depend on it.

Combining the strengths of two different solar technologies could yield “hybrid solar power” that works even at night or when it’s cloudy.

The Advanced Research Projects Agency–Energy is devoting $30 million to several demonstration projects that will attempt to combine photovoltaics with solar thermal. Early-stage work being conducted by researchers around the U.S. hints at how the combined technology might work.

The effort seeks to solve an important problem. ARPA-E estimates that the intermittency of solar electricity could limit it to providing just 5 percent of total energy in the United States. “You end up having so much solar at times that you can’t use it, that you end up having to dump it,” says Cheryl Martin, ARPA-E’s deputy director.

Currently, storing electricity from solar panels is either prohibitively expensive or, in some areas, unfeasible. Solar thermal power, which concentrates sunlight to heat water and make steam for turbines, can store energy by keeping heat in insulated containers. But overall, solar thermal power is twice as expensive as power from solar panels.

According to ARPA-E, there are several ways the two types of solar power might be combined.

Some solar power systems involve concentrating sunlight on tiny, super-efficient solar cells. As they’re currently configured, the heat from the concentrated sunlight is quickly extracted and allowed to dissipate into the atmosphere. If it could be collected instead, it could be stored and used to generate electricity later. The challenge is that this approach would require operating solar cells at much higher temperatures than is normal, and this can damage them. Researchers are looking at ways to make solar cells more resistant to high temperatures.

Another possibility is to split up the solar spectrum. Solar cells are very good at converting certain wavelengths of light into electricity—but not others. It may be possible to redirect wavelengths that can’t be used efficiently, and to use these to heat up water and produce steam.

Yet another approach is being developed by Todd Otanicar, a professor of mechanical engineering at the University of Tulsa. He uses nanoparticles suspended in a translucent fluid to absorb certain wavelengths but allow others to pass through to a solar cell. As the nanoparticles absorb sunlight, they heat up, and the fluid can be used to generate steam.

The idea of using both heat and electricity from solar panels isn’t new. Some companies run tubes along the back of conventional solar panels and then pump water through the tubes. The waste heat from the solar panel heats up the water, replacing water heaters. But these systems do not work very efficiently.

ARPA-E is also considering funding novel energy storage technologies that use both heat and electricity. Adding heat to electrolysis, for example, might improve the economics of splitting water to produce hydrogen. The hydrogen could then be run through a fuel cell to generate electricity. Heat could also aid other electrochemical reactions, such as those that can be used to make liquid fuels for vehicles.

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