Running solar panels backwards

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vibes
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Running solar panels backwards

Postby vibes » Wed Apr 16, 2008 2:05 pm UTC

So I was just staring at my solar powered helicopter (No, it doesn't fly. It's just a model with a spinny rotor on top. Sadly.) and thinking: you can run an electric motor in reverse and make a generator. What happens if you run a solar panel backwards?

To boil the question down further, what happens if you put current into a photodiode? I'm thinking it will get hot. Then probably melt into a sticky mess and/or catch fire, which is the fun part.

And here's where the line of thinking ends: people have been putting a lot of time and money into getting these things super-efficient at absorbing energy... does that mean they'd also make super-efficient heaters?

Looking at the typical materials used on that fount of all reliable information wikipdeia, it doesn't look like it would melt too easily, so I don't see any reason you couldn't run one backwards. Am I missing something on how heat behaves in material like this? Like maybe the materials are efficient at absorbing but not radiating energy? It's been a long time since A-level physics, and even longer since GCSE Chemistry :D

I dunno. I suspect this is one of those idle thoughts that gets blasted away by the blowtorch of further investigation. But hey, why the hell not. I might even learn something.

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antonfire
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Re: Running solar panels backwards

Postby antonfire » Wed Apr 16, 2008 2:21 pm UTC

It's not hard to make a superefficient heater. The inefficiencies in other processes usually occur because some of the energy gets turned into heat, rather than what you want. A light bulb is a pretty efficient heater.
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TomBot
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Re: Running solar panels backwards

Postby TomBot » Wed Apr 16, 2008 2:38 pm UTC

Well it's worth a shot, find yourself a photodiode and try to zap it. I do know that LEDs can work as photodiodes, so I'd think there's a decent chance it would work the other way around, though perhaps not as efficiently.

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Re: Running solar panels backwards

Postby asad137 » Wed Apr 16, 2008 3:19 pm UTC

An "efficient" heater is a ridiculous concept. Anything that dissipates energy is a heater, and everything dissipates energy. The most "efficient" heater is a resistor -- it converts 100% of the power going into it into heat.

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Re: Running solar panels backwards

Postby Arancaytar » Wed Apr 16, 2008 5:04 pm UTC

My uneducated guess is that the diode burns out, and possibly explodes in your face. Our physics class wrecked at least a dozen LEDs by trying to run current against the bias...

Also, a light bulb is a less efficient heater than an electric oven. Although most light bulbs are still a lot more effective as heaters than as light sources.
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Re: Running solar panels backwards

Postby hobbesmaster » Wed Apr 16, 2008 7:03 pm UTC

You (probably) won't get visible light because the photodiode is probably made with an indirect semiconductor like silicon. You'll just get your typical diode characteristics, though LEDs make generally miserable rectifiers (it won't block much in reverse bias. also, they don't take much current in forward before series resistance means they burn up)

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Re: Running solar panels backwards

Postby Mathmagic » Wed Apr 16, 2008 7:12 pm UTC

Arancaytar wrote:Also, a light bulb is a less efficient heater than an electric oven. Although most light bulbs are still a lot more effective as heaters than as light sources.

I've heard that light bulbs are only 5% efficient at turning energy into light; which means it's 95% efficient at generating heat. That seems pretty efficient to me.

EDIT: Make that 10% and 90% - http://en.wikipedia.org/wiki/Incandesce ... efficiency
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zenten
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Re: Running solar panels backwards

Postby zenten » Wed Apr 16, 2008 11:28 pm UTC

mathmagic wrote:
Arancaytar wrote:Also, a light bulb is a less efficient heater than an electric oven. Although most light bulbs are still a lot more effective as heaters than as light sources.

I've heard that light bulbs are only 5% efficient at turning energy into light; which means it's 95% efficient at generating heat. That seems pretty efficient to me.

EDIT: Make that 10% and 90% - http://en.wikipedia.org/wiki/Incandesce ... efficiency


An oven is a much less effective light source.

chionophile
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Re: Running solar panels backwards

Postby chionophile » Thu Apr 17, 2008 6:17 am UTC

vibes wrote:And here's where the line of thinking ends: people have been putting a lot of time and money into getting these things super-efficient at absorbing energy... does that mean they'd also make super-efficient heaters?


The thing is, though, that solar collectors are designed to absorb photons and emit electrons. That's pretty much it. They're the opposite of an LED which absorbs electrons and emit photons. Trying to force a current through a solar cell wouldn't do anything effective. Except destroy it.

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Re: Running solar panels backwards

Postby Izawwlgood » Thu Apr 17, 2008 6:21 am UTC

Don't LED's effectively do this? Turn current (electrons) into light (photons)?

EDIT: I apologize, I didn't read the whole 8 other posts first.
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Re: Running solar panels backwards

Postby Arancaytar » Thu Apr 17, 2008 8:47 am UTC

zenten wrote:
mathmagic wrote:
Arancaytar wrote:Also, a light bulb is a less efficient heater than an electric oven. Although most light bulbs are still a lot more effective as heaters than as light sources.

I've heard that light bulbs are only 5% efficient at turning energy into light; which means it's 95% efficient at generating heat. That seems pretty efficient to me.

EDIT: Make that 10% and 90% - http://en.wikipedia.org/wiki/Incandesce ... efficiency


An oven is a much less effective light source.


Some ovens do emit a bit of visible red light when the metal starts glowing, but, yeah. I'd say it's closer to 1% : 99%...
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Re: Running solar panels backwards

Postby Nimz » Thu Apr 17, 2008 12:05 pm UTC

Arancaytar wrote:
zenten wrote:
mathmagic wrote:
Arancaytar wrote:Also, a light bulb is a less efficient heater than an electric oven. Although most light bulbs are still a lot more effective as heaters than as light sources.

I've heard that light bulbs are only 5% efficient at turning energy into light; which means it's 95% efficient at generating heat. That seems pretty efficient to me.

EDIT: Make that 10% and 90% - http://en.wikipedia.org/wiki/Incandesce ... efficiency


An oven is a much less effective light source.


Some ovens do emit a bit of visible red light when the metal starts glowing, but, yeah. I'd say it's closer to 1% : 99%...

Good old blackbody radiation curve. For both.
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BeerBottle
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Re: Running solar panels backwards

Postby BeerBottle » Thu Apr 17, 2008 1:15 pm UTC

Most solar panels are p-n junctions, which is to say they are two semiconductors - a p-type and an n-type - joined together. They work by separating the electron-hole pair formed by absorbing a photon - the hole goes into the p-type part (where there are lots of excited holes but no excited electrons) and the electron goes into the n-type part (where there are lots of excited electrons but no excited holes). Since they are separated, they cannot recombine. LEDs are also p-n junctions, but they work the other way round - they push excited electrons from the n-type part into the p-type part where they recombine with the holes and emit light (radiative recombination). So, essentially, one process is the reverse of the other.

However there are complications. As hobbesmaster said, most solar cells are made of Si, which is an indirect semiconductor - that means the electrons and holes are more likely to recombine non-radiatively - i.e. without emitting light. So chances are, your solar cell won't act as a very good LED. More advanced solar cells are now being made from direct semiconductors (as used in LEDs) which would favour radiative recombination.

rdub
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Re: Running solar panels backwards

Postby rdub » Thu Apr 17, 2008 4:03 pm UTC

I guess I will repeat what has been said a little here and say that current run through a solar panel won't readily turn into light because they are typically silicon, but even if you had a really expensive solar cell made of a direct bandgap semiconductor, you still probably wouldn't get light because the design of LEDs and solar panels are quite different. LED structures are designed to optimize light output by trapping holes and electrons in the same area, whereas solar cells don't want to create light (it would lower efficiency), so they are designed to separate holes and electrons as soon as they are formed.

vibes
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Re: Running solar panels backwards

Postby vibes » Thu Apr 17, 2008 6:40 pm UTC

BeerBottle wrote:Most solar panels are p-n junctions, which is to say they are two semiconductors - a p-type and an n-type - joined together. They work by separating the electron-hole pair formed by absorbing a photon - the hole goes into the p-type part (where there are lots of excited holes but no excited electrons) and the electron goes into the n-type part (where there are lots of excited electrons but no excited holes). Since they are separated, they cannot recombine. LEDs are also p-n junctions, but they work the other way round - they push excited electrons from the n-type part into the p-type part where they recombine with the holes and emit light (radiative recombination). So, essentially, one process is the reverse of the other.

However there are complications. As hobbesmaster said, most solar cells are made of Si, which is an indirect semiconductor - that means the electrons and holes are more likely to recombine non-radiatively - i.e. without emitting light. So chances are, your solar cell won't act as a very good LED. More advanced solar cells are now being made from direct semiconductors (as used in LEDs) which would favour radiative recombination.


awesome answers, thanks all:)


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