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Right now, photovoltaic devices are the cheapest, most efficient way to harvest the energy in sunlight. The problem is that this energy ends up in the form of electricity, which we have difficulty storing in a cost-effective manner. An alternative approach, solar thermal energy, converts solar energy to heat and can use that heat to continue generating power for several hours after the Sun goes down. But that’s not enough to make solar an around-the-clock energy source.

Researchers are apparently working on a third option, one that could potentially store energy indefinitely. It goes by the name of “solar thermal fuel,” but it’s not a fuel in the traditional sense. Rather than breaking apart the fuel molecule through combustion, solar thermal fuels release heat by rearranging bonds within a molecule, leaving all the atoms in place. As a result, they can be recycled repeatedly—in the example that introduced me to solar thermal fuels, a research team ran theirs through more than 2,000 cycles with no loss in performance. . . .

http://arstechnica.com/science/2014/04/new-solar-thermal-fuel-has-energy-density-of-lead-batteries/