Combining solar PV & thermal could be the way

The Advanced Research Projects Agency–Energy in the US is devoting $30 million to several demonstration projects that will attempt to combine photovoltaics with solar thermal. The effort seeks to solve the important problem of intermittency of solar electricity.

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.

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.