Quick question about solar cell for the DU scientists...

[Javaman]

How come current solar designs only capture a small percentage of energy?

or to put it another way...

why is their efficiency so low?

[izquierdista]

1. Quick answer

To capture solar energy, the photon energy has to pop an electron in the solar cell material out of the ground state up into an excited state. Some of the photons are too weak to give the electrons a boost; some that are get reflected right off the surface of the solar cell; some photons hit the solid lattice instead and just give it a good shake. Only if they are just the right energy, hitting at just the right angle, knocking a bound electron in just the right way can they send the electron down the line and turn it into current.

[Kolesar]

2. I enjoyed that...eom

[Javaman]

3. Okay, me being completely ignorant (at a very scary level I might add)

about photons and electrons, could there be a theoretical way to "contain" the electrons for a bit so as to "knock" off an electron?

I am clearly aware that my ass does my talking in these matters but I figured, I might as well ask. LOL

[End Of The Road]

4. I was wondering sort of the same thing, but too scared to ask

I was thinking of the so-called "solar tubes" that you can buy as sort of skylights for your home. They're lined with highly reflective material... the light bounces around in there from your roof, through your attic, into your room. So I was wondering if there was anyway to "contain" the photons that way with solar panels.

Sorry to be such a stoopid doofus.

[tinrobot]

5. You can get more photons with a concentrator

A lens or a mirror can place more photons on a solar cell, thus giving more output.

But this heats up the solar cell, which causes other problems. Many people are working on solving these problems, and IBM just had an announcement this week about ways to cool down the cells.

[End Of The Road]

6. Thanks!

[izquierdista]

7. Containing electrons?

You can do that by charging up a battery. The solar panel makes photovoltaic current that charges a battery until the current is needed later.

Electricity is really a quantity to be used as it is produced. That is why getting it from natural sources instead of from a generator where you can control the fuel flow is problematical. The sun doesn't shine and the wind doesn't blow when it is convenient or needed. That's why solar and wind are good for base loads, but you would always like to have some other generator where you can adjust the output tied into the system.

[NNadir]

8. You have - inadvertantly I'm sure - got the "base load" issue backwards.

Wind and solar are not useful for baseloads. The best of them have 25% capacity utilization. Even gas plants - which are usually designed for meeting peak loads except in places like California - have higher capacity utilization than wind or solar. Of course, as you correctly state, wind and solar are not avaiable on demand, which is why the need for spinning reserve cuts into their utility.

Spinning reserve is defined generally as the amount of current that can be provided to the grid in less than 30 minutes. Most power grids maintain spinning reserves at about 10% of demand.

Recently, in Germany, reserve requirements maintained because of wind power were mismanaged, plunging much of the country into hours of darkness.

[izquierdista]

9. Grid independent

I was thinking of an isolated system where you had a solar panel and a wind generator to keep a battery charged. In that type of a system, when the sun isn't shining and the wind isn't blowing you would need a fueled generator to fall back on to charge up the batteries.

[NNadir]

10. The system you describe has extremely high external costs.

Distributed energy sucks because it is mass intensive and necessarily redundant.

Further, almost by definition, batteries are point source pollutants, even when recycling is attempted.

The distributed energy fantasy reified is the automobile, which has been an unparalleled environmental disaster.

[izquierdista]

11. When you are stuck in the middle of nowhere

Sometimes you have no alternatives.