Sunday, December 18, 2011

Solar is fusion power too!

In my day job I research fusion energy, so I'll probably say a lot about the energy we use and where it comes from.  And I believe energy is probably the most important issue facing our world this century, because every societal problem reduces to an energy problem*.  Well, a lot of them do, anyway, the ones that wars get fought over.  I'll talk more about that later.

For now I just want to point out a really fascinating bit of work coming from the University of Texas at Austin.  A researcher named Xiaoyang Zhu has discovered a way to double the efficiency of solar cells made of a plastic semiconductor called pentacene.  Their test batch has an efficiency of 40% in normal sunlight.

(If you head over to that link, you can read a bunch of mumbo jumbo quantum jargon about how they did it which will leave you pleasantly confused, if you're into that sort of thing.  Science reporters:  please don't tell people about dark quantum "shadow states" and leave it at that.  You are just confusing people.)

Basically: when an electron absorbs a particle of light, it is too energetic to be captured.  It's a 90 MPH fastball coming at you; and you're not a major-league catcher, you're just you.  So they found a way to take all that energy and split it into two, slower electrons that can be captured more easily, kind of like if the baseball hit another baseball in midair so they both came at you with a more reasonable speed - and if you look closely, you can see just where the metaphor breaks down.

So goody! yay solar cells.  Why's it important?

Well, the discovery is exciting in its own right because of two things.  Firstly, 2x improvement is a lot in a field that used to fight for percentage points.  Secondly, the fact that these cells are plastic instead of typical crystal semiconductors like Silicon (or more expensive stuff like Gallium Arsenide) means they should be much easier and cheaper to manufacture.  I.e. this could be that greatest of things, the heart's desire of any creative person: A Breakthrough.

But this is just the latest in an ongoing trend of breakthroughs where solar cells get cheaper and cheaper in terms of energy/dollar.  And we (as usual) have the benefit of living in interesting times: a recent study has showed that solar photovoltaics are now at grid parity with conventional electricity sources in some sunny places.

This is kind of a surprise: most sources prior to this study were telling us solar was years away from grid parity**.  But the field's been growing exponentially, and exponentials are powerful stuff - very confusing to analysts (I guess Excel only does polynomial fits***).

I could (and probably will) write a whole nother bit on the numbers and problems of solar electricity, but for now just keep the following numbers in mind:  To power the entire US with solar continuously, you would need ~ 27000 square kilometers of solar panels operating at 30% efficiency.  (By the way, that's ALL US power use, not just electricity: that includes natural gas, oil, bio-fuels, wood, peat, horsepower, candlepower, and candypower.)  That's about the area of Massachusetts.  That's the area of one half of the Mojave desert in southern California.

A sense of scale is probably the biggest gift that a physicist can give you.  Happy Chanukah!

All the forces in the world are not so powerful as an idea whose time has come.
-Victor Hugo

*level up +1 Hyperbole!
** "grid parity" just means that the average cost of electricity from solar is equal to the cost from other sources (coal/nuclear/hydro/...) when you include the full lifetime cost (installation, maintenance, legal fees for relocating the protected bird species that decides to nest there, ...)
*** This line is really funny, by the way, in case you didn't notice.


  1. Obviously, one needs to click on all your links to get the whole, complete, unadulterated picture. Love it!
    Also love that you still love quotes.

  2. That works out to about 1000 sq ft per person, or not quite the surface area of all housing units combined. Throw in the commercial properties, and who knows!

  3. Another interesting thought is that the US interstate system has a total length of 75,440 km (Wikipedia), and the total highway system has ~ 270,000 km (Wolfram Alpha). You could get a good start on solar electricity by bordering the highways with solar panels. Easy to access, easy to service; hard to legislate & pay for. Or you could build solar roadways: (