Stress release: Graded buffer layers are the key to combining structurally incompatible semiconductors in high-efficiency solar cells, such as this one from the National Renewable Energy Laboratory. Strain from mismatched crystal structures cracks the cell’s eight-layered buffer (see n=8), relieving strain in the crystal and thereby protecting the active semiconductor layer above.
NREL
The new class of materials enabling the world's best solar cell has a bright future.
A solar cell more than twice as efficient as typical rooftop solar panels has been developed by Spectrolab, a Boeing subsidiary based in Sylmar, CA. It makes use of a highly customizable and virtually unexplored class of materials that could lead to further jumps in efficiency over the next decade, making solar power less expensive than grid electricity in much of the country.
The cell, which employs new "metamorphic" materials, is designed for photovoltaic systems that use lenses and mirrors to concentrate the sun's rays onto small, high-efficiency solar cells, thereby requiring far less semiconductor material than conventional solar panels. Last month Spectrolab published in the journal Applied Physics Letters the first details on its record-setting cell, initially disclosed in December, which converts 40.7 percent of incoming light into electricity at 240-fold solar concentration--a healthy 1.4 percent increase over the company's previous world-record cell. Other groups are developing promising cells based on the new type of materials, including researchers at the Department of Energy's National Renewable Energy Laboratory (NREL), in Golden, CO. The NREL researchers will soon publish results in the same journal showing that their NREL's designs are tracking Spectrolab's, improving from 37.9 percent efficiency in early 2005 to 38.9 percent efficiency today.
Metamorphic semiconductors resemble the high-efficiency cells used in space. Like the cells that grace satellites and planetary landers, they employ three layers of semiconductors, each tuned to capture a slice of the solar spectrum (solar panels have only one active layer). These semiconductor layers are assembled, one upon the next, by altering elements fed to a crystal growing in a vacuum. To avoid growing crystals filled with energy-trapping defects, device designers have until recently employed only a limited repertoire of semiconductors, such as germanium and gallium arsenide, which form similar crystal structures.
Metamorphic materials provide flexibility by throwing off this structural constraint, employing a wide range of materials, including those with mismatched structures. "The parameter space you can explore using mismatch opens up a whole world of possibilities," says NREL principal scientist Sarah Kurtz.
What makes this possible is the addition of buffer layers between the semiconductor layers. This technique was employed in the early 1990s to make high-speed transistors combining silicon and germanium, and then introduced to photovoltaics later in the decade by Cleveland-based semiconductor developer Essential Research. Spectrolab has, however, seen the best results. Its 40.7 percent metamorphic cell improves on Spectrolab's best conventional cells by incorporating new semiconductors in the top and middle layers that excel at capturing infrared light that was all but missed by the cell's predecessors.
Such high output may be just the beginning. Raed Sherif, director of concentrator products at Spectrolab, says there is every reason to believe that these metamorphic solar cells will top 45 percent and perhaps even 50 percent efficiency. Sherif says those efficiencies, combined with the vast reduction in materials made possible by 1,000-fold concentrators, could rapidly reduce the cost of producing solar power. "Concentrated photovoltaics are a relatively late entry in the field, but it will catch up very quickly in terms of cost," he predicts. (See "Solar Power at Half the Cost.")
Exotic materials and cheaper substrates could lead to better photovoltaics.
First Solar's thin-film technology is now challenging silicon panels at large-scale solar-power facilities.
About 5% for the natural process analyzed in this interesting summary by Poles in Krakow:
www.if.uj.edu.pl/Foton/92-special%20issue/pdf/06%20kburda.pdf
However, beware statistics (numbers) can be misleading. In comparisons to nature and her processes the devil is always in the details. Cutting out a little piece of her and analyzing it in detail often leads to misinterpretations and underappreciation of her sublime design perfection.
cellulosic biomass 0.1%; algae 1% to 20% ? efficient
According to a correspondence with Cornell U. Prof. David Pimental, cellulosic biomass captures only about 0.1% of the solar energy, per acre, per year.
Algae are supposed to be 10 to 100+ times as efficient, per acre, per year - meaning 1% to 10% (sometimes to 200 - 20%) and there are numbers of projects active in that vein.
If those PV % are close to correct, these 240 and 1,000 to 1 *CONCENTRATING* collectors would seem to have an advantage at ~ 40%. (FWIW - sounds almost like the Solex device in the James Bond movie "The Man with a Golden Gun - 1974 :) )
The rub, as alway, is $/unit energy, delivered to the end consumers.
Which method(s) will provide the lowest delivered cost KWH or gallon of liquid fuel, as paid for by consumers?
Re: cellulosic biomass 0.1%; algae 1% to 20% ? efficient
Do you have any references on the algae efficiency? I would like to look them up.
Guest (rhapsodyinglue)
I would say you can take this result to be accurate. They have a long history of producing cells for satellites and the efficiency of the cells has been creeping up by a percent or few each iteration. No out of blue claimed breakthrough... just consistent sound engineering over a lot of years.
As for the expense... I guess at this point it isn't cost competitive with the other solar concentrating technology. Recently there have been announcements of utility scale solar troughs and stirling dishes... with the largest commitment seeming to go to stirling dishes. So I'd assume they have the current cost advantage.
Given that these cells require concentrated sun there is the expense of some form of optics and a tracking mechanism. One company is developing a very novel flat panel solar concentrator that may be a good match for these ultra efficient PV cells...
http://www.solfocus.com/technology_gen2.html
Stirling dishes? $/KWH to consumer
I haven't come across any stirling dish stories, recently. Gotta' handful of URLs to share?
Care to consider Green and Gold Energy's SunCube:
http://www.greenandgoldenergy.com.au/ ?
I'm not doubting the laboratory efficiency claims.
But that % is always the very highest the efficiency can possibly be, for systems that use that style of device.
Rather, there are so many other factors (focusing, tracking, reflection, DC to AC inverter, dirt / aging, transmission distance to market, ... efficiency losses, not to mention capital and operating costs, longevity, reliability, availability ...) that weigh heavily on the final, most relevant $/KWH price to consumers. In essence the net PV %, so to speak.
So, I should have said if those PV % are close to what the end user finally gets - meaning bare minimum losses elsewhere and bare minimum additional expenses.
Re: Stirling dishes? $/KWH to consumer
There are two Stirling Energy Systems, stirling dish power fields that are waiting final permitting to begin construction. Approval for both projects have already been granted.
http://www.energy.ca.gov/sitingcases/all_projects.html
They're clear down at the bottom in the section for renewables. Final construction certification is supposed to be in Aug, for stirling one, and Sept for two.
SES dish isn't as efficient as 40%, more like 20%, and has some operating expenses. I still like the tech, and think the biggest advantage is that it requires no exotic materials.
240 concentration of sunlight to a 40 % eff cell means 60 % of that energy has to go somewhere...
like melt the cell?
Some of the 60% represents light that was not absorbed but some, as you suggest, represents light that is absorbed and converted to heat instead of electricity. Keeping the produced heat from melting the equipment, for example by ensuring air flow over the device, is an important component of the engineering of a solar concentrator system.
Putting the solar cells on aluminium square pipes and cooling by circulating water enables much of the 60% heat to be harvested (or stored. If we can utilise 50% of the heat then we now harvest up to 70% of the energy falling on an area. This starts to change the economics.
Yes, the necessity of cooling really changes economics of PV. I wonder, why TFA doesn't mention it...
Guest (CarlHitchon)
A common technique is to use very small cells with heat sinks that use radiational and convective cooling.
I would be nice to harvest the heat left after conversion for hot-water, dryer hot air, and heating. A possible limitation is that these cells have to be kept fairly cool or efficency degrades rapidly.
developing solar tech seems to me to be on of the greatest ways of saving the earth
So far nuclear still is the cleanest energy we have. If you calculate the entire life cycle polution including the process of making and recycling the panel, solar power is comparable to wind power, but far behind nuclear.
I don't see how you can say nuclear power is clean when we don't even have a place to put the waste products not to mention mining all the uranium. You have to mine a lot of uranium just to get a miniscule amount of plutonium and then you have to put all the work into refining it. That just doesn't sound all that clean compared to growing some crystals?
Nuclear power plants do not run on the Plutonium. They use enriched Uranium isotope, and Plutonium is a by-product formed during fission in the nuclear reactor.
Looking generally, in the nature there is plenty of the radioactive elements, but they are almost evenly distributed all over. That makes intensity of the localized radiation to be below limit to which life has been genetically accommodated.
Problem with radioactive waste are isotopes produced by nuclear reaction, which can be more radioactive than starting material. Now, if all this waste is evenly distributed over the earth, local radiation will be really negligible. But this is not at the moment possible.
There are many other more dangerous materials, which human race is throwing around, but we do not recognize its dangers, because general public is not informed, or the facts are kept hidden by governments. Just to mention Chemical weapons (neurotoxins, neural toxic gases), like VX, which even US government has enclosed in the cement encased barrels and disposed huge amounts in the oceans. Just imagine, what can happen after several decades, when see water “eats” through these containers, and VX starts leaking under a see.
You are correct, sort of - nuclear plants in the USA run on refined uranium in a "once through" process. Laws intended to prevent proliferation restrict reprocessing, this adds to the waste problem bu tmore importantly it means that nuclear energy is NOT a long term solution. This is because there are finite quantities of uranium and once we burn through them and bury them, that game is over. With growing world energy demand and the present security considerations, we migth only have a 50 year supply of uranium. Note the run up in both prices and futures in recent years!
If we do change our laws to allow reprocessing, there is a considerable (effectively infinite) energy supply. However, for each gigawatt-year of electricity generated (the size of a typical modern coal fired power plant is aroung a GW) you produce one ton of radioactive cesium and one ton of radioactive strontium - these materials have half lives of about 400 years.
Is a sustainable energy system one that each year produces two tons of radioactive material that needs 400 year stewardship? Do you think stewardship for even a couple hundred years is possible to guarantee? I remind you that the USA, at 231 years old, is the longest standing government presently on the planet.
We have a perfectly beautiful nuclear reactor, one that uses clean fusion power and which is safely located 92 million miles from earth - why not use it?
We also have fusion energy on earth. The ITER experiment, the largest scientific collaboration ever, is a fusion reactor being built in Cadarache , France.
The idea that nuclear energy produces a byproduct that remains forever is incorrect.
This misinformation is most likely sustained by the animosity towards nuclear weapons.
Nuclear fuel residue is unstable, which means it has a half life.
Each unstable element has a different half life but most elements have half lives which are less than a year.
This means if you put the residue in a barrel for ten years most of it is ready to use again.
By processing residue and removing the portion of the material which has reverted back to its original state and replacing the unwanted portion into the ground the nuclear fuel can be reused.
Yes, nuclear fuel is a renewable resource.
In a manner of speaking Japan has reused its reactor material several times.
Some of the material has never reverted to its original state on the other hand some has reverted multiple times.
They just keep retrieving the portions which are ready for reuse.
In the United States it is illegal to reprocess the nuclear fuel because of a bill passed during Jimmy Carters presidency intended to make nuclear power economically undesirable.
For this reason nuclear power is not economically feasible in the United States but many other industrialized nations have used it to reduce dependance on oil.
smart people write fat books about total (lifecycle) cost of nuclear energy. simple claims about it has no place on forums like this...
Is this approach compatible with SOI (silicon on insulator)?
Valikor
Management of radioactive waste
We need to assess the risk of management of radioactive waste by the multi-barrier system. Using knowledge of the chemical properties of the various radionuclides in spent fuel, let follows each of the important radionuclides as it travels through the many barriers placed in its path. It turns out that only two radionuclides are able to reach the biosphere, and they arrive at the earth’s surface only after many thousands of years. A careful analysis of the critical points of the disposal plan emphasizes site rejection criteria and other stages at which particular care must be taken, demonstrating how dangers can be anticipated and putting to rest the fear of nuclear fuel waste and its geological burial.
Re: Management of radioactive waste
The US government is legally entitled, and ultimately also responsible for that waste. There is little hope for privatisation of this, for obvious reasons.
Doing a lifecycle cost analysis is rather difficult if the industry cannot be accounted for all costs and responsibilities. This is also very difficult if these particular costs and responsibilities are handed over to future generations and governments.
Then there are the weapon implications, which only cause more gov't meddling.
No thanks, let's not go the France route. It's bad enough already with current government energy policy.
Guest (CarlHitchon)
Re: Management of radioactive waste
This is just one of many disparaging remarks you have made about nuclear energy. It works fine in France.
Fire can burn, should we give that up? Nuclear wastes are minute in quantity when compared with fossil fuel wastes. Nuclear wastes are manageable. The problem with nuclear energy is political as is this anti-nuke attitude.
If we can cover internal surface of the combustion engine with this new Photovoltaics then we can capture 40% energy as light, 25% as ordinary engine cycle and next 10% in steam cycle (re six stroke engine) 3% leftover with thermoelectric module.
There are a number of schemes for using the wasted heat that normally passes through solar cells. Since they typically pass IR quite efficiently. These processes use the heat in solid state electrolyzer cells to generate hydrogen from water (actually steam because of the temperature). They work like solid oxide hydrogen fuel cells in reverse. The electricity from the cell powers the dissociation and the heat makes the process more efficient.
I'd love to lay my hands on these cells to experiment with. But where can an individual buy such devices?
Let us not forget a great American Ovonski (I spelt his name correctly),who began his work as a mechanic helping his daughter on a science project on the nervous system; from this he invented amorphous semiconductor technology. Now almost a half century later we have the same technology giving high return solarelectrolysis. That all our inventors are respected.Thank you MIT Review.
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1 Comment
How does this compare with nature?
How do these efficiencies compare with that of photosynthesis as a way of capturing solar energy?
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