Why are photovoltaic panels more popular than solar thermalcollectors on homes? One big reason is easy storage. New technology maychange that.
When it comes to energy storage, solar panels have it easy.
Homeowners with PV panels on their roofs effectively store power byshuttling electricity generated in the daytime onto the grid, said JaneDavidson, a professor at the University of Minnesota and the directorof the Solar Energy Laboratory there, during a presentation at theFifth Germany California Solar Day taking place at PG&Eheadquarters in San Francisco today.
It’s not so easy in solar thermal. Concentrated solar thermal plantsin 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 hoursafter the sun goes down.
But in homes it is not so easy. Although roughly 75 percent of thehomes and commercial buildings in the U.S. could potentially derivesome of their power from solar systems, most homes aren’t located inthe center of the desert and thus don’t get the kind of solar radiationa CSP plant will.
To make solar thermal economical, many of these buildings will needseasonal storage. "There is a mismatch," she said. "They need systemsso that we can store it in the summer for use in the winter."
Which brings us to the headline. For long-term storage, storingenergy in chemical bonds – the secret sauce behind fuel cells – may bethe answer. Theoretically, heat generated in the summer could be usedto 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 producehydrogen for fuel cells and zinc, which can be used to releaseelectrons in other reactions. Some researchers have proposed storingheat through a zinc-to-zinc oxide reaction.
For more near-term storage, phase change materials – materials likezeolites and desiccants that move relatively easily from solid toliquid or liquid to gas states – could be used.
And for really near-term storage, says Werner Koldehoff, a boardmember of the German Solar Industry Association, households could usethe ultimate phase change material: water. Water could be turned intoice (through a solar-driven chiller) and changed into water.
In Germany, energy storage for some residential thermal systems isaccomplished through storing liquids heated by the sun in pipes in theearth.