Efficiency / Cost

Sam172 · Post by **Sam172** » 15 Sep 2005, 18:08

I know the ordinary PV cells aren't particularly efficient, at about 10-20% tops.

What I was wondering about however, was how efficient, and how cost effective other forms of electricity generation via solar power are.

E.g. Concentrating solar power plants, especially heliostat ones.

Are there any readily available sources of already pooled information on this?

isenhand · Post by **isenhand** » 16 Sep 2005, 07:09

What I would like to know is why are solar cells as efficient as they are? I assume there is some underlying physical reason? Any physicists here be kind enough to explain it? Is there anyway to improve the efficiency of solar cells?

Post by **clv101** » 16 Sep 2005, 07:31

I don't think the actually efficiency (how much of the total energy landing per unit area is captured as useful electricity) is very important at all. The sun is free, 1% would be just as good as 10% if the cost of the panels (both energetic and economic) was one tenth. The only important variables are captured energy per ? and captured energy per unit manufacturing energy.

isenhand · Post by **isenhand** » 16 Sep 2005, 07:48

If they were more efficient then they would occupy a smaller area for the same output. Would that not make the cost go down? Also, if they were more efficient would they not also be more useful for not so sunny climates?

GD · Post by GD » 16 Sep 2005, 11:05

Both very good points Chris and isenhand.

isenhand wrote:What I would like to know is why are solar cells as efficient as they are? I assume there is some underlying physical reason? Any physicists here be kind enough to explain it? Is there anyway to improve the efficiency of solar cells?

It?s about the ability of the photodiode to capture photons, which is dependant on the materials used in creating the structure itself. (The wikipedia article is pretty good, and has a nice diagram depicting the photovoltaic effect.)

I read in an electronics weekly about a year ago that there?s research going on to enable them to capture the sun?s full spectrum, but I think this has been something of a holy grail for some time.

(It?s been a while since I?ve looked at this type of theory, my brain?s been frazzled for the last five years staring at dodgy PCB?s and PC monitors.

Mike will be able to elaborate once he?s on his MSc!)

isenhand · Post by **isenhand** » 16 Sep 2005, 11:24

Silly me, should have looked there first

. Thanks. Well, that explains that.

Now, all we need to do is invest in research in to solar cells rather than build new roads!

fishertrop · Post by **fishertrop** » 16 Sep 2005, 13:15

clv101 wrote:I don't think the actually efficiency (how much of the total energy landing per unit area is captured as useful electricity) is very important at all. The sun is free, 1% would be just as good as 10% if the cost of the panels (both energetic and economic) was one tenth. The only important variables are captured energy per ? and captured energy per unit manufacturing energy.

I would say the only cost that matters is the resource/energy to make them.

Sure it would be nicer and more convientient if they were small, but it wouldnt really matter if they needed to be HUGE so long as they were (resource-) cost-effective to produce.

Post by **mikepepler** » 16 Sep 2005, 13:18

GD wrote: I read in an electronics weekly about a year ago that there?s research going on to enable them to capture the sun?s full spectrum, but I think this has been something of a holy grail for some time.

Mike will be able to elaborate once he?s on his MSc!)

I hope so!

You're certainly right about the full-spectrum thing. Current panels don't capture the full spectrum, but then again, plants don't either - if they did the leaves would be black!

An interesting bit of info from a lecture I went to on this subject a few months ago is that plants' efficiency at converting solar energy to something they can use is only around 2% on average. Sugar cane was the most efficient I think, at 4-5%. So, we're already beating nature on efficiency, but nature has the advantage that plants can build themselves, while we're stuck with using energy and chemical-guzzling factories...

isenhand · Post by **isenhand** » 19 Sep 2005, 07:57

fishertrop wrote: Sure it would be nicer and more convientient if they were small, but it wouldnt really matter if they needed to be HUGE so long as they were (resource-) cost-effective to produce.

In general I agree, but there is some size considerations. No point in having something that is cost-efficient but too large to install anywhere. However, I don?t think that?s a problem to worry about.

BTW, I remember something that you can improve the efficiency by using mirrors to reflex light on the cells as well.

Also, is it possible to convert heat to electricity?

Hmm ? one day I will have to do something about my poor chemistry knowledge!

Bandidoz · Post by **Bandidoz** » 19 Sep 2005, 12:04

isenhand wrote:Also, is it possible to convert heat to electricity?

Yes - it's how most electricity generation works - heat water into steam which then drives turbines.

isenhand · Post by **isenhand** » 19 Sep 2005, 12:13

I was thinking more directly. I have this vary vague memory of seeing some material that converted heat to electricity. The demo I remember was on a guy put a bit in a fire and ran a TV off of it. This was a long time ago so I could have remember this completely wrong.

Post by **mikepepler** » 19 Sep 2005, 13:18

You're thinking of a therocouple. I remember seeing the same thing on TV, but I can't remember how it worked, other than the fact that it was a thermocouple of some sort. An easier way might be to use a Stirling engine to convert the heat in mechanical energy, and drive a generator with that. There are some big solar Stirling engines, using a mirrored dish to collect the heat.

isenhand · Post by **isenhand** » 19 Sep 2005, 13:37

Could be. I remember it as a semiconductor and giving of a bit more than a few uV (but I could have remember all this wrong anyway). However, the thing I was wondering about was if it would be possible (if not already done) to use such a device to increase the output of a solar cell by converting some of the waste heat to electricity as well?

isenhand · Post by **isenhand** » 19 Sep 2005, 13:58

These Stirling engines are interesting. Never heard of them before. Now, I wonder, what would the cost / efficiency of a solar power Stirling engine be for generating electricity?

Post by **mikepepler** » 19 Sep 2005, 15:45

Check out this Google image search for some cool pictures of big solar stirling engines:
http://images.google.com/images?svnum=1 ... tnG=Search

Here's some model ones (inlcuding one which will run off the heat of your palm!):
http://www.stirlingengine.com/

Better still, check this site out for how to make one from tin cans and bits of wood:
http://www.boydhouse.com/stirling/

I want to do this one day, but never seem to find the time....