I wonder if they’re talking about the photo-isomerization of azobenzene from cis- to trans- and vice versa. I remember learning about that in Organic Chem 40 years ago. I can’t see the phenomenon as much of a storage for photo energy, but maybe that’s where the density issue comes in.
IronJack and Lepton were close to the point. Perhaps I can clarify. The “density” that needs to be the focus is the solar flux density - how much energy can be collected and or stored on a given patch of material on a perfect sunny day at solar noon. The magic number is considerably less than 1000 watts/square meter, closer to 500 watts on a clear day, depending upon water vapor absorption in the atmosphere, and the sun’s elevation.
When looking at the numbers, especially the efficiency of photoelectric or thermal storage, solar energy conversion to electricity for base load, or even bulk generation has never made economic sense, but is certainly useful for political purposes. Next to “its for the children” and “widows and orphans”, its been worth billions of dollars to cronies of politicians, and thus billions to politicians who can never be faulted for ignorance is promoting “safe”, “clean”, “free” solar generated electricity.
This author actually once, as an assignment by one of the world’s authorities on solar conversion materials science, did a survey of government land that might be available for construction of solar farms, and learned that even if we had no alternative, we haven’t sufficient land, ignoring endangered plants and animals, to dedicate billions of square meters (a typical 1000 megawatt nuclear plant equivalent would require about 200 million square meters of surface in cloudless desert, not counting storage; remember, the sun’s useful production is during about 6 of twenty four hours).
The typical photovoltaic array one sees on a roof in the West, where the people are often well educated and ignorant, a 12’ x 12’ array, wouldn’t power an old-fashioned incandescent light bulb, but would charge batteries - slowly. Figure an optimistic 10% conversion efficiency on sunny days with low humidity. That suggests about 10 watts per square meter. It probably isn’t half of that, and I’m assuming perfect lossless storage, in case someone really thinks they need a lamp on at night.
Solar energy along highways, for emergency communication, to power intermittent instruments, in space where relatively large arrays can be flimsy and there is no water vapor, has applications. For low quality applications, like preheating water for homes in desert climates and for swimming pools, solar energy has economic applications. But the people who put together the Solyndra project deployed the scam always understanding how to make money from the inevitable bankruptcy.
Even people in Silicon Valley who don’t have a background in solar engineering, but know the semiconductor business spotted the scam immediately, since no one can afford to manufacture low-tech semiconductors in California, let alone in Silicon Valley. Remember, the investors in Solyndra walked away with hundreds of millions of our dollars after the bankruptcy. The two big factories employed thousands of union contractors and tens of millions of dollars of equipment was purchased and installed, never to be used. (This was about the time Toyota couldn’t be threatened into floating the NUMMI plant in Fremont any longer). Solyndra was a success to Obama/Holdren/Chiu cronies, who donated tens of millions to Obama’s campaigns. Solar energy is similar to carbon taxes, a ploy to explain stealing money from a naïve public, whose money they take by force of a now well-armed Internal Revenue Service.