# Photovoltaic cell efficiency question.

Discussion in 'Photovoltaics' started by gary0232, Mar 9, 2009.

1. ### gary0232Guest

If current pv efficiencies are 18% where does the other 82% go. I'm sure the
answer is reflection and heat, but I was curious about the percentages. It
appears most of the sunlight just reflects off those shiny surfaces.

2. ### EeyoreGuest

Some reflects but plenty will turn into heat which sadly reduces the efficiency
of the panel.

Graham

3. ### KToGuest

Hay Gary,
about 10% of the 82% is reflected, the other 70% is turned into heat.
On a good sunny day the temperature of the solar panel will be as high as
80°C.
cheers
Kees

4. ### gary0232Guest

I wonder if anyone thought to modify the panel so water going to your hot
water heater could flow across the underside of the panel.
Not only would this same on your hot water heater costs, you would be
increasing the efficiency of your pv's by keeping them cool.

5. ### EeyoreGuest

Just forget the PV solar bit and use solar thermal to heat your water.

Graham

6. ### Guest

Just trickle a bit of water over the face.

Yup. At the same time, you can also double sun power with a mirror.
Just a fact of life.
OR with a tiny water flow Try numbers...
Does it ever rain on your planet?

Nick

7. ### daestromGuest

==============================================
What determines the peak frequency of the silicon freq resp? The
dimensions of the silicon molecules? In other words, how do you dope
up the molecule to move the resonance from 800nm to 500nm? Smaller
wavelenth, faster frequency... you'd need to make it lighter somehow.
Dope it with something lighter like Boron? The PV wafer already has a
P layer and an N layer. How about turn it over, so the other layer is
exposed to the sun?

I don't believe its the atomic weight but the energy needed to 'lift' an
electron out of its orbit. Once freed from an atom, the electric field of
the depletion zone at the junction moves it across to the other side so it
can't recombine directly back to the atom it came from. So it has to flow
through the external circuit to get back to the atom it came from. (well,
actually it just bumps another electron that bumps another, and another...
until finally one bumps back into the original atom of silicon).

The energy to 'lift' an electron has to come in one 'packet' of light, it
can't work it's way out gradually (quantum mechanics). So a single photon
has to have that much energy. (Photons with shorter wavelength (towards the
blue end of the spectrum) have more energy.) This is known as the
'photo-electric effect'. If the photon is only slightly more energy than
needed, the electron is 'lifted' but with a bit extra 'speed' when it's
free. The extra energy is quickly dissipated as heat as the electron
'collides' with other atoms.

If a photon has a lot more energy than needed, then it may 'lift' the
electron out of the atom's orbit and re-emit a low energy photon that
carries off the left-over energy. This is 'Compton scattering'.

So the issue is to find a way to make it take less energy to free an
electron from silicon. Kind of against 'natural law' of silicon. Of course
different doping can change the electric field in the depletion zone and
that can help a bit. Or try something besides silicon. But the next larger
electron 'shell' gets pretty vague and messy.

daestrom

8. ### EeyoreGuest

I vaguely recall this at school. I think it's bandgap energy dependent on
the material used and the wavelength of the light shining on the surface.
Shorter wavelengths have more energy in each photon, hence why UV would be
great if it came though the atmospohere ( but not so good for us of course
).

http://en.wikipedia.org/wiki/Photo_voltaic#Simple_explanation

Graham

9. ### EeyoreGuest

It's not a resonance.

Graham

10. ### EeyoreGuest

http://en.wikipedia.org/wiki/Planck's_constant

Graham

11. ### MikeGuest

So if you make your pv's track the sun, precisely what issue stops you
cooling the rear face with water?

I certainly can't think of one.

--

12. ### EeyoreGuest

Depends if you have to pay for the water and where does it go afterwards ? Down
the drain ?

You have inadvertently just made the case for solar thermal water heating. A far
more attractive source of 'green energy'.

Graham