Where are the 70% Efficient Solar Cells? 798
VernonNemitz asks: "Back in 1984 a patent was granted for silicon chip micro rectennas, which would convert visible photons into electricity in the same way that ordinary rectennas convert microwaves into electricity, at perhaps 70% or greater efficiency. Nobody could make such solar cells back in 1984, but we certainly can today, with sizes of antennas that would capture everything from infrared to the edges of UV -- and the patent has expired. So, where are they?" Currently the most popular type of solar technology is photovoltaics, however PV technology only has an efficiency of about 7-17%. With the potential gains claimed by the technology in the cited patent, has anyone even tried to build one of these units to see if it can live up to the given promise, or at least prove to be a technology than we should be exploring?
Goverment rebates (Score:3, Informative)
It seems someone is working on them... (Score:5, Informative)
Re:It seems someone is working on them... (Score:3, Informative)
They look neat. The researchers think that they'll be cheap and efficient. But where are the microarrays of silicon rectennas?
Perhaps impractical to actually build? (Score:4, Informative)
Assuming the applicant built a prototype and proved this device works, creating metal coatings in the exact thicknesses he mentions with the detail he describes is still something that would be very expensive to do now. That technology hasn't improved very drastically in the last decade or so.
Not with semis (Score:5, Informative)
E=hw
Silicon is actually a rather poor photomaterial, being an indirect material, it's limited to about 60% effeciency at any wavelength. The electron must not only gain energy, but also move a slight bit within the crystal in order to reach the conduction band. Direct materials, such GaAs, being direct, can be > 95%
Perhaps the are other techniques??
Spectrolab Holds Title for Most Efficient (Score:1, Informative)
ANYway...
The current record for a terrestrial cell is from Spectrolabs @ 34%. For spacebased cells it is around 32%.
The company is located here:
http://www.spectrolab.com
and products located here:
http://www.spectrolab.com/prd/prd.asp
Here it is. The inventor calls it lepcon. (Score:5, Informative)
It looks as if this is all still very blue-sky, and there is no reason to think that these nifty ideas will work in practice. The first page I linked to tells us: ``Dr. Marks is in negotiation with U S. government to fund Lepcon research and development.'' Probable translation: ``He's applying for grants, because he can't sell it to the venture capitalists.''
Re:Heres a company - up to 80% efficiency. (Score:4, Informative)
Re:Anyone know the energy in sunlight? (Score:1, Informative)
This is a hell of a lot of energy, and it's damn shameful that we are still burning dinosaur juice just to make some wealthy bastards richer.
Oh yeah, fuck you Bush.
The folks at NREL are fixing photovoltaics... (Score:3, Informative)
But as to making micro-antennas that work in a broad enough bandwidth to beat solar cells, no I don't think they're playing with that. But you're talking something painfully small, even at current technologies, I would think. Those 17% efficient solar cells work up at the blue/ultraviolet range. If you look at the spectrum [electric-words.com] that tells you the wavelength of UV is 10**-8 meters, or 10nm. Now a current transistor on a chip is about 180nm; so to get that bandwidth with an antenna you need a component about 1/10th the size of current chip components, and if you want to do one of those fractal, broad-spectrum antennas, you need sections of antenna at least 1/8 that size... So given Moore's law, you're still 10 years off.
In other news (Score:2, Informative)
Re:Not with semis (Score:5, Informative)
Whoops, I think you're confused. w (which is actually an omega) is angular frequency, not wavelength. And h is really h-bar, which is Planck's constant over 2 Pi, not Boltmann's constant.
But the actual equation is correct :-)
Re:Rectenna? (Score:3, Informative)
4Rectena.jpg [univ-reunion.fr]
Re:It seems someone is working on them... (Score:1, Informative)
http://www.lbl.gov/Science-Articles/Archive/MSD-fu ll-spectrum-solar-cell.html [lbl.gov]
l l_Solar_Spectrum.html [lbl.gov]
http://www.lbl.gov/msd/PIs/Walukiewicz/02/02_8_Fu
http://www.lbl.gov/Science-Articles/Archive/MSD-Al ivisatos-solarcells.html [lbl.gov]
e w/Highlights/2002/stories/nano/small_world.html [lbl.gov]
http://www.lbl.gov/Science-Articles/Research-Revi
Re:Anyone know the energy in sunlight? (Score:5, Informative)
This is from Here [electrochem.org]
Re:Where are the 70% Efficient Solar Cells? ask GW (Score:4, Informative)
What would happen if all the major automakers decided tomorrow to start building electrics?
We would burn about the same amount of oil, and increase our use of coal.
We would burn about the same amount of oil, because you wouldn't be replacing very many gasoline powered cars on the roads; electrics are still too small and have too short a range to be useful for the majority of Americans. None of this is going to change until there is a dramatic improvement in the stored energy densities of batteries, and/or a reduction in the toxic waste produced in the creation and disposal of the batteries themselves. The last time I saw statistics, the sum total of all the "alternative fuel" vehicles sold in the world over any time period you choose to look it was LOWER than the increase in the number of vehicles in the world ... that is, even with increased sales, we continue to fall further behind.
We would burn more coal because electric cars need to get the electricity to recharge their batteries from somewhere, and the cheapest source of electricity generation (that can be built today in North America and Asia (and even Europe, I believe)) is coal.
This is not to say that there aren't loads of technologies available to improve efficiency of fossil fueled vehicles, but most of them make vehicles MUCH more expensive (by almost any metric you choose) ... and the vast majority of people (Americans AND non-Americans) have little incentive to spend more when they can get the same capabilities for less, EVEN IF it would be to their benefit in the long run (why else would people be willing to lease instead of buy vehicles? It is far far more cost effective in the long run to buy than to lease ... ). Some of these technologies include hybrids, light composite frame and body materials, ceramic and aluminum engine blocks, high efficiency diesels, exhaust scrubbers, biofuels, superconducting electricity distribution grids, etc. etc. etc.
But none of them are perfect, and none of the forseeable technologies will eliminate our reliance on petroleum ... not even that "holy grail" of environmentalists, the "Hydrogen Economy". Hydrogen isn't free after all ... there are no large supplies of the stuff to drill or mine for, and there is none in the atmosphere to distill. You have to generate it by cracking water ... using electricity, that you have to generate by some other means. And currently, the only good way to do THAT is to produce the electricity using nuclear (which the environmentalists ALSO hate and also has a time horizon before the exhaustion of the fuel), hydropower (environmentalists hate this too) or fossil fuels ... and the inefficiencies involved in the seperation, storage, shipment, and sale of hydrogen currently would would require just about the same amount of fossil fuel usage as currently for the same energy extracted by the automobile (although we might be able to use different forms, such as more coal and less oil, and there would be far fewer plants to police). In other words, we'd be burning the same amount of fossil fuel to make the hydrogen as we currently burn to make the cars go in the first place.
There are no simple answers and very few real conspiracies, and I don't understand why otherwise intelligent people continue to believe that there are.
Hydrogen? (Score:3, Informative)
I suspect that once we have employed solar, wind, geothermal and etc to limits of any forseeable technology there will still be shortfall. Once it sinks in that the 15 minute hot showers and the SUV will are out, a new energy supply debate will ensue: When is the uranium going to run out?
Re:Heres a company - up to 80% efficiency. (Score:3, Informative)
Have a look here [lbl.gov]. It even comes with references to papers (not that I've read them, but y'know).
Solar cells (Score:4, Informative)
The cost and deposition speeds already eliminate a number of materials from practical application; and, there are a number of other issues that must be taken into account (especially among modern solar cell materials):
1) For sunlight, the maximum intensity occurs at 550nm, which means that a device bandgap of 1.35eV is needed to give a maximum electron/hole pair generation rate. One of the best known solar cell materials, c-GaAs, has an Eg = 1.4eV, but its high cost makes it prohibitive. c-GaAs can be grown by epitaxy, but requires high temperatures for deposition -- it can be difficult to grow uniformly over large surface and on more desirable substrates (like plastics).
2) c-Si, which is cheaper than c-GaAs, must be deposited in very thick layers for max efficiency, and has an Eg = 1.1eV, making it difficult to deal with and not that efficient. Like c-GaAs, impurities in the crystalline latice can have large, detrimental performance effects.
3) a-Si is a great material as it can be grown over large areas very inexpensively, but it has a number of disadvantages. Being amorphous, it suffers from metastability (the Stabler-Wronski effect), which means that its performance degrades over time as more light hits it. It also has Eg = 1.8eV, which makes it less than perfect for sunlight-based solar cells.
There are other issues facing solar cell designers, such as:
1) reflection at the surface
2) series resistance due to impurities and scattering within the semiconductor
3) non-radiative recombination (phonon recominbation) in the semiconductor
4) incomplete extraction of carriers in the semiconductor
5) interface and contact losses in the p-i-n or Schottky barrier structure
Triple-tandem solar cells made of p-i-n structures of a-SiGe, uc-Si, and a-Si:H (which have bandgaps like 1.2eV, 1.45eV, and 1.85eV in the same package) can be used to target as much of the solar spectrum as possible, and can still be grown "relatively" cheaply by PECVD reactors. They get somewhere around 17% efficiency in sunlight.
Performance of solar cells would undoubtedly be better if cost were not an issue, and if single-wavelength light were being used, as exotic material devices could then be targeted for these specific conditions. Alas, real life is not so ideal.
-kris
Re:Someone PLEASE answer this. (Score:1, Informative)
You create a chip with very many, very small antennas on it, capable of receiving not in the usual radio and microwave wavelength range (as you probably know, this goes from kilometers down to millimeters) but instead tuned for optical frequencies (which are electromagnetic waves, only differing in wave-length from "usual" radio waves).
What you'll get when you place an antenna into a EM field is an AC current flowing in that antenna.
If you now rectify this and place some connections on the chip that let the resulting voltage get out of the circuit, you have this new,yet-to-be-built solar cell.
It's basically the same as putting two wires at both ends of a flourescent lamp and going near a very powerful radio station. If your antenna (the two wires
If you have a cellular phone, CB transceiver or some similar device and a simple voltmeter with a millivolt range? Try it yourself. Go to the next radioshack, buy the cheapest germanium diode and some nf capacitor and built approximately this:
|>| - Diode
--- - Wire
-||- - Capacitor
v Antenna v Diode v Antenna
----------+----|>|-----+--------------
| |
+-----||-----+
v v
Wire to
your voltmeter.
That ugly semicolon above shouldn't be there. It's a white space I can't find in my posting
You can place the diode serial, parallel as shown, omit the capacitor (the wires are enough), it will probably work anyway. Just play a bit around.
This is a simple field strength meter used by many ham radio activists.
Re:Where are the 70% Efficient Solar Cells? ask GW (Score:1, Informative)
Because the oil from Iraq gets sold to Texas and California based companies. GWB profits from the foreign oil too.
Re:Possibly overlapping Patents? (Score:3, Informative)
Re:Where are they? (Score:1, Informative)
There are also hydraulic rams that, powered by the flow of water in a stream, can pump it vertically to join with that rain water in the tank.
U-235 vs. U-238 (Score:5, Informative)
Then there's a wide variety of other radioactive substances that can be burned in reactors. For example, breeder reactors can actually breed plutonium from the very common U-238 (U-238 is one of the most common elements in the Earth's crust), creating an almost infinite supply of fuel. Military breeder reactors work fine for producing lots of plutonium for atomic bombs. Research on commercial breeder reactors (basically the military reactors tied to turbines to power electric generators) was stopped by worries about arms proliferation (it is much easier to seperate Pu-239 from U-238 than it is to seperate U-235 from U-235 in raw uranium ore, thus makes it easier to get enough fissile material to crete atomic bombs), but could be re-started pretty swiftly if necessary. Which would not be for 50 or 100 years, as you mention.
Regarding 100 and 400 years of oil, my own best estimates are somewhat lower than that. My estimates are that we will experience shortages within 20 to 25 years, and that within fifty years we will have basically exhausted all economically accessible oil resources (i.e., there will be oil out there, but it will take more energy to extract it than can be obtained by burning it). However, hopefully by that time the current taboo regarding nuclear power will have eased, and we will be able to replace the lost petrochemical resources with synthetic hydrocarbons or other such creations. (Don't laugh, we use petroleum as feedstock for chemical plants because it's cheap, available, and readily "cracked", but there are certainly other feedstocks that could be "cracked" into various petrochemicals if necessary, including coal, for that matter -- after all, both the Nazis and the South Africans did it).
Re:Something I've always wondered (Score:2, Informative)
some links:
http://www.stratec.be/EnvironnementGB355.
http://www.eren.doe.gov/power/consumer/rebasi
http://europa.eu.int/comm/ener
http://swco.ttu.edu/Manuscrip
Re:Research (Score:3, Informative)
Nope, breeder reactors [netcom.com] take care of that problem. It'll give quite enough fuel to last much longer than a century
On the hubbert curve and hydrogen (Score:3, Informative)
Pople have been predicting we will run out of oil within 10 years for at least 30 years. It hasn't happened yet and shows few signs of happening too soon. Even if it does happen, hydrogen is unlikely to replace oil because far superior fuels (both renewable and non-renewable) exist
The U.S. Departmetn of energy has a very detailed presentation [doe.gov] describing different scenarios for world oil consumption including the work of Hubbert and of Laherre along with their prediction that we have more than a decade of cheap oil left.
Even if the earlier projections of running out of oil come true, there are enormous amounts of "oil" that can be extracted from oil shale (at $80/bbl) and liquid fuels can be produced from coal (methanol is especially easy) and plants (ethanol, biodiesel). All of these become practical with $3/gallon gas
All this suggests to me that hydrogen, which is difficult and dangerous to handle, and very expensive to produce may not have that much of a future as a fuel in the near term. Even Natural gas is a better choice because it is less expensive and can be used with fuel cells (sort of)
Theoretical problems with optical rectennas (Score:5, Informative)
Sunlight at the earth's surface has a power flux density of about 1 kilowatt per square meter. To convert that to an electric field strength, we take the square root of the power flux density times the impedance of free space, 377 ohms. This gives 614 volts/meter.
Yellow light has a wavelength of 570 nm. That means the electric potential over that distance is only about 350 microvolts. This is approximately the voltage you'd see at the terminals of a 50 ohm half wave dipole, and it's far below the voltage needed to switch a rectifier. Silicon rectifiers take about 600-700 millivolts of forward bias to begin conducting, even if one could be constructed to work efficiently at optical frequencies. Germanium takes about 300 mV, and silicon Schottky diodes take about the same.
It is not possible to construct a diode that doesn't require a forward bias, otherwise we could rectify the noise from room-temperature resistors and convert ambient heat to useful work. This is specifically prohibited by the second law of thermodynamics.
Solar cells aiming for full spectrum efficiency (Score:2, Informative)
Re:What about Biodiesel? (Score:3, Informative)
-ant
--the most common song of all time is not "Happy Birthday", but "king of the castle", or more infamously "nyah-nyah-ne-nyah-nyah".
Re:how about sewage to energy? (Score:3, Informative)
Separating out the useful components of sewage (perhaps waste cellulose in human fecal matter... ewwww) to produce energy probably would undoubtedly use a lot more energy than you could extract from it. Though I think there are anaerobic digestion systems in use in some sewage plants to extract methane and burn it in a turbine, pumping the generated electricity back into the grid.
Re:how about sewage to energy? (Score:2, Informative)
Not Necessarily. You dilute the poop in water, and grow algae in it. The algae perform photosynthesis and produce sugars. Additional bacteria do a little organic chem to process the output of the algae into a form that automobiles can eat. Actually, you hardly need any poop at all, most of the algae's biomass comes from atmospheric CO2.
All that's required is lots of surface area to catch sunlight for the photosynthesis, so essentially, this is mother nature's form of solar energy
Re:Ethanol is not good (Score:4, Informative)
What my post talks about if you read it, is what is generally referred to as "bioethanol". Of course, ethanol from corn sugars is biological in origin, but what most people call bioethanol is ethanol produced from biomass or lignocellulosic feedstock.
That means _waste_ cellulose. Such as corn fibers, not the corn iteself, or pulp/wood chip byproducts from the milling/cardboard industries, and "waste" crops such as bagasse in Louisiana that grow in swamp land (i.e. land not arable for production of more valuable crops and that grow with very little external water and energy input and thus are very low in terms of actual feedstock cost including any energy input).
The cellulosic chains are broken down and the constituent glucose and xylose sugar molecules are fermented - there are several processes such as SSF (Simultaneous Saccharification and Fermentation), steam-cracking weak acid hydrolysis, and recirculating strong acid hydrolysis, which are all more-or-less viable for this process.
I shall not defend the corn ethanol industry - you are correct in saying that they exist because of federal subsidies. I am promoting a process for taking otherwise "valueless" biomass that would end up in land fills or lie unused elsewhere that can be obtained at relatively low cost and converted into a relatively high value energy product.
OK - quick physics lesson on polarization (Score:3, Informative)
Light may be considered to act as both a particle and a wave, but for our purposes we'll stick to waves.
Imagine looking at an unfiltered, unpolarized light source. The light is travelling towards you but each little bit of light is a wave which may be orientated up and down, or side to side, or anywhere inbetween, as it travels. (I'm ignoring what is known as circular polarization for now.) You can consider a polarization filter as a grill. Only the waves which line up with the grill will get through intact. Waves which are partly aligned will pass through the amount that does line up. Waves which are at right-angles to the grill will not get through at all. The light which does not get through is absorbed. If you take evenly distributed unpolarized light, which is what solar cells pretty much get from the sun, then you can only get 50% of the light through into linear polarization with a single filter. You can change one polarization into another efficiently, but the initial conversion from unpolarized to fuly polarized causes 50% loss.
However, a 50% polarization followed by an 80% conversion within the solar cell still gives you an overall 40%, which is much better than current cells.
Here's something to think about: If you shine light through 2 polarizers which are crossed at right-angles then no light gets through. This is because the light coming through the first polarizer is all aligned at 90 degrees to the orientation of the second polarizer and is hence completely blocked. If you now add a third filter at 45 degrees between the other two then you start getting light through. Adding a filter has increased the amount passing through. Explaining why, I'll leave as an exercise for the reader.
Re:OK, which forests do we cut to make solar farms (Score:2, Informative)
Why should these get centralized? Keep the power infrastructure as it is, powered primarily by flow from the solar panels placed on every home on the grid. Every house charges when it can, feeds back energy when fully charged, draws from internal batteries when the sun is obscured, and draws from the grid when the batteries are dead.
Many people already do this - I just helped install one. 70% efficiency photocells would make the house I just installed gather and "bank" 3-5 times it's own expenditure in energy.
It's be nice to have one on the roof of a hybrid gas/electric car (or in my case, a gas/electric truck, once they get around to it). Gas consumption would certainly drop considerably.
I don't know who those folks you refer to that make knee-jerk, thoughtless reactions are, but if you're listening to idiots, that's your own fault.