Caltech Makes Flexible, 86% Efficient Solar Arrays 439
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by
kdawson
from the bend-me-shape-me dept.
from the bend-me-shape-me dept.
strredwolf writes "Caltech has released a flexible solar array that converts 95% of single-wavelength incandescent light and 86% of all sunlight into electricity. Instead of being flat-panel, they stand thin silicon wires in a plastic substrate that scatters the light onto them. The total composition is 98% plastic, 2% wire — the amount of silicon used is 1/50th that of ordinary panels. So as soon as they can get these to market, solar could be very viable and cheap to produce." Update: 03/01 21:02 GMT by KD : Reader axelrosen points out evidence that the 80%+ efficiency figure is wrong. MIT's Tech Review, in covering the Caltech announcement, says that the new panel's efficiency is in the 15%-20% range — which is competitive with the current state of the art. And the Caltech panel should be far cheaper to manufacture.
NOT incandescent light! (Score:5, Informative)
Re:Absorbed not necessarily equal to electricity (Score:5, Informative)
Massive typo in summary. (Score:4, Informative)
From TFA:
The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight.
Better Article... (Score:5, Informative)
http://www.rsc.org/chemistryworld/News/2010/February/14021001.asp [rsc.org]
'We have shown the optical absorption efficiency and charge carrier collection efficiency of a silicon wire array cell is comparable to a conventional silicon cell, but a wire array cell uses up to 100 times less silicon due to enhanced light-trapping effects,' says Atwater. Significantly, the wire arrays absorb infrared light more efficiently that conventional silicon surfaces, further improving the performance of the new device.
So the gist is that it's more efficient because it converts infrared, uses some type of clear polymer with alumina "reflector particles" in place of 99% of the expensive (doped) silicon, and is flexible and therefore easier to manufacture.
Re:I think its entirely reasonable to say... (Score:5, Informative)
The original article is poorly written. MIT's Technology Review has an article that includes information about efficiency of generating electricity, and it says 15%-20%. http://www.technologyreview.com/energy/24665/?a=f [technologyreview.com]
So the story is really that there might be a way to make cheaper, flexible solar panels by mixing silicon and polymers.
Re:Efficiency (Score:4, Informative)
Collection efficiency (which is what TFA is claiming to be 86%) vs. conversion efficiency (that 40% number you remember) is what you're missing, but from other articles on the technology it appears that the conversion efficiency for these cells should be higher than existing designs:
The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons--in technical terms, the wires have a near-perfect internal quantum efficiency. "High absorption plus good conversion makes for a high-quality solar cell," says Atwater. "It's an important advance."'
Which could give them ~78% conversion efficiency, still nearly double over the best cells currently.
This is way over-hyped (Score:4, Informative)
Re:Absorbed not necessarily equal to electricity (Score:5, Informative)
Yes, just like any other dark panel you leave in the sun. Except not as hot, because some of the energy is being exported as electricity. So unless they're flammable at really low temperature we'll probably be okay.
Predicted photovoltaic efficiency only 14.5% (Score:5, Informative)
Here's the actual scientific paper, "Predicted Efficiency of Si Wire Array Solar Cells". [caltech.edu] That's by the same authors mentioned in the press release. While the thing does trap most of the light hitting it, only a fraction of the energy in that light is converted to electricity. In fact, this thing is currently less efficient than the better commercial solar cells.
From the paper: ... simulated photovoltaic efficency of 14.5%. ... Conclusion: ... "Si wire array solar cells have the potential to reach efficiencies competitive with traditional Si crystalline solar cells."
So, an interesting development, but no big breakthrough. There's a claim that it might be a cheaper way to make solar cells, but everybody who comes up with a new design makes that claim. (Nanosolar comes to mind; their technology is supposed to be cheaper, but so far they've spent half a billion dollars and apparently have only produced sample panels.)
Re:In requires polymer to make... (Score:3, Informative)
Last time I checked you're a blithering idiot who talks out of his ass.
Does silicon grow on a beach? In a manner of speaking...
However, the factories that process raw silica into high grade silicon for semi-conductor production are in short supply, and this has driven up the price of silicon. Silica is cheap, and every where. Silicon is manufactured, and currently not cheap (enough for widespread solar panels).
Re:Absorbed not necessarily equal to electricity (Score:5, Informative)
conversion effiency (Score:2, Informative)
reading these slashdot comments reveals a whole lot of confusion about solar cell efficiency.
photons with energy less than the bandgap of the conversion material will not be converted to electrons. photons with energy greater than the bandgap will only convert at the bandgap energy. the high effieincy multijunction cells attempt to address this. multi-exciton generation can happen if the photon is several times the bandgap energy, and there is some hope that quantom dot cells will be able to achieve high efficiency this way though the most effienct qd cells currently get like 5% effiency.
electron-hole recombination happening within the material instead of through the cathode and anode will cause a photon to be released. the higher absorption of the cells in TFA will help keep this photon trapped in the cell, but if it loses energy, it will be less than the bandgap and not be converted again. multijunctions can address this effect somewhat by absorbing the new lower energy photon. high temparature operation also helps increase the likelihood of the electron-hole pair making it to the cathode-anode. the highest effiency cells so far have been achieved by concentrating the lightbeams and then splitting the beams into different colors to be absorbed by different sections.
if you put panels on your roof and you live in a place with cloud cover then it won't be worth using a heliostat system to track the sun so you will suffer loss of the cosine of the angle between your panel and the sun. without tracking the sun you aren't going to be using rainbox concentrators either so you are necessarily using lower efficiency cells too.
Re:Plastic? 10 years under the sun? (Score:4, Informative)
My plastic garbage bins have spent at least a decade out in the Aussie sun. A lot of plastic that you find in throw away stuff these days has been deliberately engineered to be bio-degradeable due to pollution concerns in the 80's. The older non-biodegradable stuff has formed a large "islands" in the North Pacific and North Atlantic.
Re:I think its entirely reasonable to say... (Score:5, Informative)
The title of this post and the article is incredibly misleading and very annoying/frustrating to someone who's been working on solar technologies for a while. Don't get me wrong, I think this is a very cool thing, sounds like they have to potential to make very cheap cells, but approach, let alone surpass, current multijunction cells (30-40% eff.) they will not.
Disclaimer: University of Michigan Solar Car Team alum
Re:Predicted photovoltaic efficiency only 14.5% (Score:5, Informative)
The take-home message from the paper, as far as I can tell, is that the researchers showed that one can achieve performance comparable to commercial solar cells by using 1% of the expensive ultrapure silicon used in current PV's.
Re:I think its entirely reasonable to say... (Score:3, Informative)
The lowering of cost would actually be the most important impact. Current solar panels would cost too much no matter how efficient they were.
Re:I think its entirely reasonable to say... (Score:5, Informative)
Re:Absorbed not necessarily equal to electricity (Score:3, Informative)
The quantum efficiency of existing solar cells is also very high - approaching 100%. But a large fraction of those electron-hole pairs quickly recombine within the semiconductor. In order to create usable electricity, the electron-hole pairs must be separated and collected before they recombine. In a conventional solar cell, the separation is done by the internal semiconductor junction, and the collection is done by the metal electrodes on one or both faces. This team, as far as I can tell from the press release, hasn't published numbers for this step of the process.
One can make a sort of Drake Equation [wikipedia.org] concerning the conversion of incident photons to usable electricity:
Number of incident photons above a minimum energy,
times the percentage of photons that are absorbed by the active area of the solar cell,
times the percentage that create electron-hole pairs (the quantum efficiency),
times the percentage of electron-hole pairs that separate and make it to the electrodes before recombining.
Even then, this product isn't the same as the overall efficiency of the solar cell. That just tells you the conversion of photons to electricity. That's not the same as conversion of sunlight to electrical power. This is because the energy contained within an electron-hole pair is a fixed quantity for a given solar cell construction. Consider three photons: a red photon, a green photon, and a blue photon. The red has the least energy; the blue has the most. Let's say the solar cell's threshold energy is greater than the red photon's. In this case, the red photon will generate no usable electrical energy. The green photon and the blue photon will both create an electron-hole pair. Here is the key point, however: although the blue photon is more energetic, it will create no more useful electrical energy than the green one. The extra energy of the blue photon above and beyond the solar cell's threshold energy is, essentially, wasted. This is a key limiting factor in photovoltaics today. Some folks have gotten around this with multi-junction solar cells [wikipedia.org], which can tune their (multiple) threshold energies to better utilize the solar spectrum. But these are more exotic than silicon solar cells and find use almost exclusively in space applications (or solar racing vehicles) because of their cost.
So figuring out the "incident solar power in to useful electrical power out" efficiency of a solar cell is much, much more complicated than just absorbing photons.
This is not to say that the innovation in the article is worthless. Far from it - it's a pretty neat and new development that will likely have good application. But it isn't the "85% efficient solar power!1!!" that some posters are jumping at.
Re:It's plastic ! (Score:3, Informative)
Re:Absorbed not necessarily equal to electricity (Score:3, Informative)
They've already invented inexpensive lightweight solar panals (see Nanosolar).
As a result, their entire production for the next few years has already been sold to a solar power plant in germany.
I figure I was blowing $2k for computers every 3 years. If I extend that by a year or two, I get two solar panals and support hardware with the savings. A cost turns into a profit center.
Re:I think its entirely reasonable to say... (Score:3, Informative)
If a solar solution appears that costs less than oil, they will pour money into it and beat the others to market with it. That's the way that capitalism works.
Re:It's plastic ! (Score:1, Informative)
Eh, plastic is a pretty cool guy. He's quite durable and doesn't afraid of anything.
Re:Godwin's Law! (Score:4, Informative)
Right you are. Ugh I feel dirty. I thought only morons did that! Maybe it is still true ... maybe I am a half-wit!
Re:I think its entirely reasonable to say... (Score:2, Informative)
To hit 25+% efficient cells, in essence 3 cells were combined to create a multijunction cell that has 3 layers in series. To get to 30+%, refined deposition methods and clever semiconductor tricks were used. To get to 40+%, you need concentrated light and, in some cells, more layers of different materials.
The journal paper describes a simulated cell with a 14.5% efficiency and that it will compete with crystalline silicon cells - the middle of the pack for silicon efficiency and cost. The big advantage here is that the amount of semiconductor used is very low, which could make these cells very cheap. Another thing to keep in mind is that this method can potentially be applied to other, more efficient technologies, although there will be a few more hurdles should they go that route.
I very much like the idea of cheap solar cells, but it needs to be clear that this is not an efficiency breakthrough. But let me tell you, as a solar car alum, the idea of even a 40+% efficient non-concentrator cell gets me excited, even though this is not that.
Re:I think its entirely reasonable to say... (Score:2, Informative)
A second method not often mentioned here is called thermophotovoltaics. The idea is that you use concentrated light to heat up an element that then emits light at a different spectrum. To me, the cool idea about this is that if you can emit at the right range and get really high efficiency cells, somewhere down the line you might be able to replace the steam part of current power plants. Although that goes outside my realm of knowledge.
The issue with most cells' spectrum is that you need to shift the light up in frequency, not down, which is harder/more expensive. However, it could be useful if you could target the frequency at which the cell is most efficient.
Re:Absorbed not necessarily equal to electricity (Score:3, Informative)
most poeple dont stay in the one home for 20+ years, so it's very hard to justify the investment.
Except installed solar increases the value of the house. If you live in a house a few years and have solar panels installed when you move in, when you leave it will be mostly paid for and you get more from the sell. This is even more true in California with it's high electricity costs.
If they can get the costs down, more poeple will buy this, just like solar how water and insulation.
Actually of these the first step should be to insulate more, increasing insulation has the quickest payback. And in many places solar thermal or hot water also pays back faster than solar PVs. Solar power may heat water someplace that doesn't get enough sunlight to provide electricity.
Falcon