When it comes to energy storage, solar panels have it easy.

Homeowners with PV panels on their roofs effectively store power by shuttling electricity generated in the daytime onto the grid, said Jane Davidson, a professor at the University of Minnesota and the director of the Solar Energy Laboratory there, during a presentation at the Fifth Germany California Solar Day taking place at PG&E headquarters in San Francisco today.

It’s not so easy in solar thermal. Concentrated solar thermal plants in the desert store heat from the sun in large tanks of molten salt. That can be used to create steam to run a generator for a few hours after the sun goes down.

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But in homes it is not so easy. Although roughly 75 percent of the homes and commercial buildings in the U.S. could potentially derive some of their power from solar systems, most homes aren’t located in the center of the desert and thus don’t get the kind of solar radiation a CSP plant will.

To make solar thermal economical, many of these buildings will need seasonal storage. “There is a mismatch,” she said. “They need systems so that we can store it in the summer for use in the winter.”

Which brings us to the headline. For long-term storage, storing energy in chemical bonds – the secret sauce behind fuel cells – may be the answer. Theoretically, heat generated in the summer could be used to generate a reaction, which could then be unwound later in the year.

Researchers at the Paul Scherrer Institut, for instance are looking at ways to take heat from the sun, zinc, oxygen and a dash of carbon to create zinc oxide and carbon monoxide. Zinc oxide could then be unwound in further reactions to produce hydrogen for fuel cells and zinc, which can be used to release electrons in other reactions. Some researchers have proposed storing heat through a zinc-to-zinc oxide reaction.

For more near-term storage, phase change materials – materials like zeolites and desiccants that move relatively easily from solid to liquid or liquid to gas states – could be used.

And for really near-term storage, says Werner Koldehoff, a board member of the German Solar Industry Association, households could use the ultimate phase change material: water. Water could be turned into ice (through a solar-driven chiller) and changed into water.

In Germany, energy storage for some residential thermal systems is accomplished through storing liquids heated by the sun in pipes in the earth.