Thursday, October 22, 2009

Photovoltaic cell color

A silly controversy has arisen concerning the color of photovoltaic solar panels. Apparently a new book "SuperFreakonomics" contains the following (quoted here):

As an example he points to solar power. “The problem with solar cells is that they’re black, because they are designed to absorb light from the sun. But only about 12 percent gets turned into electricity, and the rest is reradiated as heat — which contributes to global warming.

Yglesias (among others) jumps on this statement (here and here) posting a photo of solar panels that appear to be dark blue and claiming:

... Still, it is worth dwelling a moment on the fact that their critique of photovoltaic literally rests on the idea that PV cells are black whereas in reality they’re usually blue:

First their critique depends on solar cells being dark (low albedo) not black. Perhaps they should have said "dark" instead of "black" but this is not a serious error. Especially since, Yglesias to the contrary, many cells are black. It is hard to judge color from photos because the cells are often reflecting the blue sky. See for instance this photo where the cells appear to be blue except at the upper right where they appear to be brown because they are reflecting brown branches instead of blue sky.

Anyway the critique is because solar cells are dark they may absorb more sunlight than whatever background they are replacing thus heating the earth. This can occur whether the cells are black or dark blue in appearance. However this effect is not important. Coal power plants also generate considerable waste heat as their thermal efficiency is only about 33%. It is doubtful that solar cells produce more excess heating per unit of electricity generated than the waste heat from coal power plants. And in any case the global warming concern with coal power plants is not their waste heat (which is not significant globally compared to solar heating) but the much greater long term heating effect from the CO2 emitted when coal is burned.

Does this mean photovoltaic solar cells make sense? Not really, they are inferior to wind turbines which also don't emit CO2 and are much cheaper .


  1. Yeah, the blue/black thing is an example of something that was both trivially wrong and fundamentally misguided. It doesn't really matter whether the cells are black or just dark in terms of immediate thermal absorption, so if immediate thermal absorption were important to global warming, quibbling about the difference between blue and black would have been nonsensical.

    But given that, as you say, the waste heat from either coal plants or solar cells is pretty insignificant, so the whole point the Freakonomics guys were making would have been pointless and misleading even if technically accurate, it's pretty funny that it wasn't even technically accurate.

  2. I doubt the statement that dark color contributes to global warming as it absorb light from the sun. Why?

    OK. Either white solar cell or black solar cell, the sun shines the similar amount of energy onto them. It doesn't matter whether white dissipates heat or black absorb. The amount of energy is already blessed to the earth. Both color would dissipate the heat eventually, only that white does it faster than black. During the night, they are all cooled.
    I see no side effect from harnessing the sunlight. It is the carbon from the coal burnt that prohibit heat from escaping from the earth, which causes global warming. Not black color.

  3. Stephen, the energy in sunlight that is immediately reflected back to space does not contribute to warming the earth. Overall the earth reflects about 30% of incoming solar radiation. Changing this number does affect the average temperature of the earth. So if solar panels are less reflective than whatever they replace this will slightly warm the earth. However as I said in the post the effect is insignificant compared to the warming from the CO2 emitted when fossil fuels are used to generate the same amount of electricity.

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