[Caoimhan]

I understand that the highest efficiency any PV cell has gotten is slighty more than 20%.

What happens to the remaining 80% of the sunlight?

Assuming 1000 W/sq. meter insolation, how many Watts are reflected? How many are absorbed thermally (and not converted to electricity), and how many are lost to heat due to electrical resistance?

Anyone know the answers to this?

Thanks,
Caoimhan

[MicroHydro]

For home use the critical number is price per watt peak, efficiency doesn't really matter very much. There is a lot of data at the weblink.

http://www.solarbuzz.com/

[Frontierenergy1]

That solarbuzz is a good site. Thanks


I think that the highest efficiency achieved is closer to 30% but average is around 10-12% for single crystal silicon. The cells actually are more efficient during cooler weather due to lower resistance. The IV curves are at a standard temperature.

Most of the energy is lost as heat with the electons recombining with the silicon. Only the electrons that find the past of least resistance being the silkscreens grids are usable energy.

I am not an expert at the physics of photovoltaics- Im sure you can find some fascinating reading on the subject by looking into Einstiens theories on the subject.

[I_Like_Plants]

The 80% goes to heat and light reflected off.

My understanding on PVs is, just like the batteries in your transistor radio, it takes far, far more energy to make 'em than you ever get out of 'em, but they're still made and sell well because they're a source of energy you can take with you.

Hm, how to make this REALLY simple....

My Fellow Republicans: Eating stuff you find when out hiking is much more energy-effecient than eating processed "food bars" you bought at the supermarket, but those food bars taste SO good! And they were 2 for $3, for goodness' sake! So, people still take 'em on their fishing trips....

[Devil]

$this->bbcode_second_pass_quote('I_Like_Plants', '
')My understanding on PVs is, just like the batteries in your transistor radio, it takes far, far more energy to make 'em than you ever get out of 'em,

[I_Like_Plants]

I was hoping you were right and I was wrong, but on an interesting site called "dieoff", I find:


"Calculations show that solar cells consume twice as much sej as they produce. dieoff.com/pv.htm So even if all the energy produced were put back into production, then one can only build half as many cells each generation -- they are not sustainable. Even if the sej efficiency of solar cells doubled, ALL of the energy produced would have to be used to manufacture new cells, which still leaves a zero net benefit to society! "

Man that dieoff site's big! http://dieoff.com/pv.htm

[Devil]

Corrigendum:

I apologise: when I wrote that, I must have been running out of caffeine! I guess the 200 figure I had in mind must have been the local currency price less the subsidies! OK, the going price for 165 W is nearer the $700 mark RETAIL. There are many places on the web at the lower price, such as http://www.partsonsale.com/solarex.html

Nevertheless, my argument is still valid. If it costs $5500 to make in electricity costs alone (assuming everything else is free), it could not be sold retail below about $10,000, not $700.

[I_Like_Plants]

Oh, no problem! I had to dig around to find the data I put on there, and while I'd heard "PV cells take more energy to make than you get out of them" years ago, I was not all that sure myself, absolutely, if that was true.

Good old dieoff, they're just a mine of good info.

[Devil]

$this->bbcode_second_pass_quote('I_Like_Plants', 'O')h, no problem! I had to dig around to find the data I put on there, and while I'd heard "PV cells take more energy to make than you get out of them" years ago, I was not all that sure myself, absolutely, if that was true.

Good old dieoff, they're just a mine of good info.

[Caoimhan]

Okay, my question was being asked because I was thinking that if a significant portion of the light is reflected from the surface of the PV cell, one could actually design a solar concentrator from PV cells. For instance, arranging the cells into a fresnel reflector design, with another group of cells at the focus, one could theoretically enhance the efficiency of the cells.

Alternately, if a significant portion of the energy was lost to heat, one could theoretically capture the heat through an aluminum substrate, and use either thermo-electric generators (using the Seebeck effect) or a Stirling motor to reclaim some efficiency through heat - electic conversion.

Using combined systems like this, I wonder if we could reach an efficiency of >40-50%.

Of course, the costs might be prohibitive...

Caoimhan

[pup55]

$this->bbcode_second_pass_quote('', 'O')K, the going price for 165 W is nearer the $700 mark RETAIL

[Frontierenergy1]

Fresnel lense PV concentrators have been tried and there are some efficiency calculations available. Sorry I dont have them with me at present but maybe within the next few weeks I can get them for you via email. Concentrating systems with sterling cycle engines have also been tried. Once again I dont have specific information with me but maybe in my files.

The glass used in most PV panels are low iron glass with a surface that reduces reflection. Not only is this used for greater efficiency but imagine if we become a solar society the amount of reflection that airplanes would be subject to. This was an actual consideration.

The key to making PV more economical is in material processing and using low grade silicon for use in them. Much of the cost and energy used is in making 99% pure silicon from silicon dioxide- quartz crystals or sand. The melting point is about 1800 deg F and requires tremendous energy to process- although conceivably this energy could be provided by solar furnace. There was such a furnace in Europe that could provide much higher temps than this.

Amorphous solar solved some of the more expensive material processing by eliminating the diamond saw slicing of wafers - however they are somewhat lower in efficiency.

Solar Thermal is much more efficient in conversion at about 50% to 70% for flat plate solar and up to 90% with applications such as pool heating. The lower the temperature through the panels the greater the efficiency.

For in depth analysis, there is a book available "Solar Engineering of Thermal Processes" by Duffie and Beckman. It is kind of expensive but it is the best reference that I am aware of.

The best discussion of PV that I have seen is the Photovoltaic Training Manuals that were produced by Siemens Solar. Im not sure if they are still availble but they were very detail oriented design manuals.

[DriveElectric]

$this->bbcode_second_pass_quote('pup55', '')$this->bbcode_second_pass_quote('', 'O')K, the going price for 165 W is nearer the $700 mark RETAIL

[pup55]

d'oh again!

Still, at 23 kw, I think we know the real problem.......

[Devil]

The theoretical maximum efficiency that can be obtained is a tad over 30%. Why? Because solar energy has a very broad spectrum, covering wavelengths from about 0.20 µm in the UV to 20 µm in the IR (this range covers about 99.99% of the irradiance). Roughly 50% of this is in the single octave representing visible light, but this is not a flat, uniform curve but a very jaggy and peaky one with a max at ~0.475 µm (ie, towards the violet end). This means the the highest efficiency could be obtained by having the PV cells with as wide a spectral response as possible, but peaking in the violet end. Solar cells operate using a bandgap to capture the energy in the photons and converting it to electricity. If the light and the response curve were strictly monochromatic, then high efficiencies would be possible. In practice, it is necessary to restrict the bandwidth, so that the response to red-orange light is very low, which means that the full visible spectrum is not used and only about 30-35% of the total irradiance can theoretically be converted, assuming that the cell sensitivity matches the radiation peak (in itself, not a mean feat which is why current practical maxima are in the 20% region. This is a physical limitation.

Most of the remaining radiation is in the UV (~8%) and IR (~40%). The UV energy is too small to be exploitable, although UV sensors exist. The IR is spread over an enormous bandwidth and is difficult to exploit without cryogenic sensors. The amount of energy contained in a bandwith of 0.1 µm in the near IR is about 0.1 units whereas in the blue-violet, the same bandwidth contains about 4 units (the units used involve differential calculus, but are essentially proportional to W/unit area). It is therefore easy to see that the restrictions due to the bandgap would lead to little electricity production.

It would not be productive to use spectral splitting: most of the light in the active end of the spectrum is absorbed, so that any reflected energy would not be economically exploitable for PV work.

OTOH, if dichroic concentrators were used that reflected the useful part of the spectrum onto the PV cells, letting the IR pass straight through, this means the the latter could be used for e.g. water heating. However, it would be far cheaper and more efficient to just use an ordinary PV array plus a separate solar water heater panel so, once again, the ideal must give way to hard economic facts.

[Caoimhan]

Devil,


Thanks for the explanation on that. I knew about the spectral frequency issues re: PVs.

I know there's a lot of recent buzz about IR PVs. Because the earth re-radiates IR energy, these cells could theoretically work through the night. Imagine a heliostat PV array being pointed at a large asphalt parking lot after the sun goes down.

Other means of converting heat to electricity seem also possible. For instance, a Concentrated Solar Power (CSP) PV array usually implements some sort of heat-sink on the back of the PV cells to help keep them under their operating temperature limits. What if you used heat-pipes to pull away this thermal energy and concentrate it onto the hot side of a Stirling Cycle engine? You turn the heat in the PV cells from an enemy into a friend.

Caoimhan

[Devil]

I can't see IR radiation becoming very useful, in practical terms. A car park is hardly a black body radiator (if it were, the cars would sink into the asphalt up to the hub caps on a hot, sunny day!) and large areas would be shielded from the sun by the cars! Yes, a road surface can reach 50°C or maybe even 60° on a sunny day. I know I cannot walk in my bare feet on even a relatively light-coloured one in the early afternoon. But I also know I can in the evening, even before sunset, so I presume the temp has already dropped to ~40°C. Question: is the heat conducted downwards or radiated upwards or convected into the air (aided by summer zephyrs, no doubt). I would say all three, but if I were forced to guess I would say the last-named would be the most important and the radiation the least important because of the low delta-T. I honestly don't think that the non-focussed radiation that could be captured from such a surface would be very significant. Then the radiation spectrum would be very broad and shallow, over, say, 1 to 15 µm and no electronic captor could reasonably operate over such a bandwidth (a thermal captor like a thermopile would be better, but the efficiency of such a device would be horrendously low).

[blumfeld]

Hi there,

I'm a physicist working in a solar energy lab in Germany and just felt obliged to set the record straight on the energy balance of photovoltaic panels:

$this->bbcode_second_pass_quote('', ' ')My understanding on PVs is, just like the batteries in your transistor radio, it takes far, far more energy to make 'em than you ever get out of 'em

[EnergySpin]

$this->bbcode_second_pass_quote('blumfeld', 'H')i there,

I'm a physicist working in a solar energy lab in Germany and just felt obliged to set the record straight on the energy balance of photovoltaic panels:

$this->bbcode_second_pass_quote('', ' ')My understanding on PVs is, just like the batteries in your transistor radio, it takes far, far more energy to make 'em than you ever get out of 'em [\QUOTE]

This is a myth! According to the US Department of Energy, with current multicrystalline silicon technology it takes less than four years to get the energy out of a solar panel that was needed to produce it. In the near future, this "energy payback time" could go down to less than two years for thin film modules. Don't take my words for it, read it yourself:

NREL fact sheet on PV energy payback

[Frontierenergy1]

A question for the physicists:

Some years back there was talk of a polarized conductive polymer that was being researched for solar electric applications. It was conjectured at the time that one film could be placed 90 deg in respect to the other, with a possible conversion of near 50% of incident solar radiation.

Was this all B.S. or was there anything to this?