How inefficient are solar cells at the moment?

We posed this question Niraj Lal from the University of Cambridge...

Niraj - A typical diesel generator would be about 40% efficient. What that means is heat energy to electrical energy is about 40% efficient or maybe 45% for a really good one. Just recently, they cracked 40% efficiency with solar cells. So the efficiencies are about the same but for converting heat energy right now, your diesel engine generator or heating up a hot bucket of steam is more efficient.

But what about CO2 efficiency?

Niraj - A lot of cells are made from silicon and that's a pretty energy intensive thing to make. I think for a standard silicon solar cell panel, it's about 4 years that it takes of making energy in a really nice sunny place to just pay back all the energy that it took to make the solar panel in the first place. But after 4 years, it starts giving you back in carbon terms and the life of the silicon solar panel is about 25 years plus more. Chris - When you say 40%, what sort of light are they absorbing? Are they just taking the light we can see?

Niraj - Yes. A solar cell has a band gap, so it has a particular wavelength and it'll absorb light of all energy higher than that particular wavelength. So, definitely, we can't see some of the light from the sun. Some of it is infrared and in the UV, but a solar cell will absorb at a particular wavelength and absorb everything that we see, plus all the UV ahead of it and plus all the x-rays and gamma rays.

Chris - So could you make one that will work in the dark because it will just use heat? Because that's infrared, it's a form of light. We just can't see it.

Niraj - I think technically, yes, but the efficiencies that you get wouldn't be worth it today.