Solar Windows Generate Questions

As a general rule, I’m optimistic about the long-term potential of clean technologies. But once in awhile it pays to curb your enthusiasm and ask some skeptical questions.

I decided to do that the other day after reading the umpteenth jazzy news story about “solar windows,” a great-sounding concept to turn windows into mini-generators of electricity by substituting clear solar cells for glass.

At first it sounds like a win-win. A moment’s reflection suggests it might be too good to be commercially viable. Here’s why, in a nutshell.

The more energy you extract from photons (bundles of light energy) for electricity, the less there is left over for illumination. The most efficient solar cell will take all the available light energy and pass nothing through. So you can be transparent and inefficient, or opaque and efficient, but not (as far as I know) as transparent as a window and efficient enough to be competitive as a photovoltaic cell.

In fact, if you think about it for more than a second, wouldn’t it make more sense to make the opaque sides of buildings out of photovoltaics, rather than windows? (As we’ll see, even that approach has issues.)

This issue came to mind recently when scientists at Brookhaven and Los Alamos National Laboratories announced an apparently significant step toward making power-generating windows.

Turns out they fabricated a new “thin film” material made of organic polymers (chains of molecules like those in plastics) and hollow carbon molecules called fullerenes. Their material forms into tiny hexagons, with clear centers that let light through. The edges of the hexagons absorb light and emit electrons.

The press release quoted Mircea Cotlet, a physical chemist at Brookhaven National Laboratory, as saying, “Imagine a house with windows made of this kind of material, which, combined with a solar roof, would cut its electricity costs significantly. This is pretty exciting.”

The press release’s talk of nanotechnology, honeycomb structures, and time-resolved confocal fluorescence microscopy was intimidating, but I called up Dr. Cotlet to raise my doubts.

Cotlet acknowledged that the team doesn’t have a working device yet or any measures of efficiency of converting light to electricity. For other organic solar cells, he noted, “efficiency is about 6 percent, which is quite low compared to silicon.” (Many silicon cells achieve more than 20 percent efficiency.)

His lab’s new material is likely to be even less efficient than other organic cells, he conceded, precisely because it is transparent. And, needless to say, it’s way to early to say anything about the cost of producing working cells.

Journalists who write up these press releases need to ask a few other basic questions. Even if the cells were as efficient as silicon, and cheaper to boot, how much would it cost to install them? Retrofitting existing windows would cost a fortune. Even with new construction, wiring one window to another along the entire face of a building could be an expensive task.

In addition, many roofs have unobstructed views of the sun. Many walls, on the other hand, are blocked by other walls. When the sun reaches its peak at mid-day, light hits vertical walls at an oblique angle. In short, walls are lousy places to put solar cells.

I asked the CEO of a company that is working to commercialize solar windows some of these questions. (Since it’s publicly traded, I won’t name it.)

For all of his optimistic press releases, he didn’t yet have figures for the efficiency or cost of his prototype cells, either.

And while admitting that installing cells on windows would be costly, he suggested that the cell could be installed inside the building to harvest energy from fluorescent lights. Hmmm, anyone else have a problem with that?

Many great inventors have proven skeptics greater than me wrong, and the world is a better place for it. Here’s hoping that my doubts are indeed misplaced.

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