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Science Technology

Where are the 70% Efficient Solar Cells? 798

Posted by Cliff from the better-power-from-photons dept.
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?
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Where are the 70% Efficient Solar Cells? More

Where are the 70% Efficient Solar Cells?

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  • Beats me (Score:5, Funny)

    by dmanny ( 573844 ) on Wednesday January 08, 2003 @05:25PM (#5042931)
    They keep me in the dark about these things :-)

  • Rectenna? (Score:5, Funny)

    by medscaper ( 238068 ) on Wednesday January 08, 2003 @05:25PM (#5042933) Homepage
    a patent was granted for silicon chip micro rectennas,

    Anyone else get a sorta shifty feeling when they look at that word and picture the consequences of such an invention?

    • Where the sun don't shine (Score:5, Funny)

      by Arrowmaster ( 635363 ) on Wednesday January 08, 2003 @05:31PM (#5042973)
      I don't think that rectenna is going to be in much sunlight.

    • Re:Rectenna? (Score:3, Funny)

      by jtheory ( 626492 )
      Don't worry, you won't feel a thing after "utilizing an electron beam and suitably coating and doping the rectifying areas" (from the patent description).

      Seriously, I wouldn't be surprised if there are a few grad students who've been toiling away on this project but can't get any funding. Or... maybe the idea just doesn't work out in practice.

      We also can't overlook the fact that lots of important stuff is, well, overlooked. Any comments from anyone in the field?

    • Re:Rectenna? (Score:3, Funny)

      by grub ( 11606 )

      The goatse.cx guy [goatse.cx] can generate over 800 megawatts with his rectenna.

      • Re:Rectenna? (Score:5, Funny)

        by IIRCAFAIKIANAL ( 572786 ) on Wednesday January 08, 2003 @10:26PM (#5044473) Journal
        Heheh, my girlfriend saw me reading and laughing and asked me, "What's so funny?"

        I asked her, "Are you sure you want to know?"

        "Yes, show me."

        So I covered my eyes and shift-clicked.

        "Eeewwwww," she said, "is that real?"

        "Yes dear, it's real and the burning sensation in your eyes will clear in a few days."

        It's scary shared-experiences like these that really solidify a relationship.

    • Why we have to have 80%+ (Score:5, Interesting)

      by cosmosis ( 221542 ) on Wednesday January 08, 2003 @05:48PM (#5043105) Homepage
      I don't get a shift feelign at all. We are already direly close to Hubbert Peak, when oil demand starts to outstrip production. In fact Hubbert, himself an oil man, said that Hubbert Peak, even considering undiscoverd reserves (which is fairly predictable with satellite reconaissance) will come sometime between 2002-2009.

      You can read about here on my website [floatingplanet.net] for more info. Some in the oil industry are thinking that peak will be hit within the next two years. This might explain our rush to invade Iraq.

      Either way, as oil reserve dwindle and demand goes up, it will create a highly destabilized politic - and if you think the repression we've all been feeling lately is bad, it will only get worse... UNLESS:

      We wean ourselves (QUICKLY!) off of Oil. The Hydrogen economy is just waiting in the wings. All of the technology is essentially there. The cost factors will become not only competitive, but cheaper and cleaner than oil, once we start migrating our energy infrastructure over to Hydrogen.
      Lets hope this happens before we end up in some kind of nigthmarish Oil Fedual/Fascist Global New World Order.

      Planet P Blog [planetp.cc] - Liberty with Technology.

      • Re:Why we have to have 80%+ (Score:4, Insightful)

        by evilpenguin ( 18720 ) on Wednesday January 08, 2003 @06:58PM (#5043289)
        Boy, I've never agreed so much with a person on the edge of paranoid hysteria before. It is all economics. Oil is too cheap right now to make investment in alternative technologies attractive. But look at who owns the major PV production facilities: Oil companies. Look at how much effort is going into alternative fuel vehicles even now. The money knows the party will be over in the next generation. My worry isn't a new "oil fascism," because abundant alternatives are more attractive than repressive rationing of ever more expensive oil. When oil gets too expensive, the alternatives become attractive.

        My worry is the health of the planet in the mean time. I live in central Minnesota, USA. Our average high temperature this time of year is about 9 degrees F. Yesterday it was 55 degrees F. I know full well one warm winter does not global warming make, but we've had several bizzarely warm years lately. We're s--tting where we eat and it worries me.
        • I live in Northern Canada, and believe it or not, some lakes aren't even frozen over yet, and by this time of the year it's -40C. We havent even got below -20C for more than a couple of days. In my youth, I remember 4' - 5' high snowdrifts. So far this "winter" you'd be hard pressed to get your boots covered. Yesterday and today were record setting days across the province. Some places broke their all time high for the month of January, by several (10) degrees C.

          We're suffering a major drought now and people who study the environment tell us that we'll need 2 years of record hard precipitation to get the ground moisture back to normal.

          I'm not a big believer in global warming being all our fault, but I do know it's going to be a real ugly summer on the western prairies.

          • El Nino (Score:3, Insightful)

            by freeweed ( 309734 )
            We already went through this during the last big El Nino, then we (well, us in Manitoba anyway) had several really shitty/cold/wet years.

            Hell, there was no Spring 2002 here - the trees didn't bud until well into June - THAT'S how cold it was.

            Then again, in 1997 we had the 'blizzard of the century' followed by the 'flood of the century', but suddenly no one can remember any years with snow since they were a kid? Give me a break.
        • I remember on Winter in MN when the snow never completely covered the ground. Other years there were -90 degree windchills. We have a picture from another year with a snowman holding a sign with the date: April 26th. I believe that was the year we moved to Arizona.

          The weather not consistant any where. Globably, the client changes every year. There's no reason to panic or think something screwy is going on. When did people start getting the idea that every year should be predictably the same? I would guess probably around the same time global warming and global cooling wackos starting showing up.

          Shock of all shocks we have hot and cool summers in AZ. There's nothing to see here, move along.

          Ben

      • Hydrogen? (Score:3, Informative)

        by dmaxwell ( 43234 )
        Hydrogen is only an energy storage mechanism. It is an attractive one to be sure but it is not itself an energy source. What to you propose to generate Hydrogen with once the hydrocarbon tap is turned off? To be sure, Hydrogen's role as an energy transport and storage mechanism will be important whenever oil does run out but that isn't what I'm asking. What fills the large petroleum shaped hole in energy production once it's depleted?

        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:Hydrogen? (Score:3, Interesting)

          by Dastardly ( 4204 )
          I suspect that once we have employed solar, wind, geothermal and etc to limits of any forseeable technology there will still be shortfall.

          Not likely.

          According to this site: http://www.nmsea.org/Curriculum/7_12/The_Solar_Res ource.htm

          You need 33,400 square km to produce enough energy, now their efficiency estimate is about 2x what we are acepting here. So, double that to 66,800 square kilometers. The US has 9,158,960 of total area. So, to produce all of our electricity at 17% efficiency requires 0.7% of the total land area. Which is of course a meaningless number without some other reference. So, the mojave desert is about 65000 sq km, the sonoran desert in Arizona, California, and Mexico is 310,000 sq kilometers.

          So, figure between rooftops, and god forsaken places in the middle of nowhere there is definitely enough room to put enough solar to power the entire US. Storage is a problem, but that is what hydrogen is for.

          Of course, without fossil fuels to rail against rabid environmentalists will be pissed about covering large stretches of land with PV.

      • 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)


        • 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

          The problem is that it doesn't matter how much per gallon you get. If it takes more energy than is in a gallon of oil to extract a gallon of oil from the shale, for example, you are not producing energy. The same problem exists for coal: At some point, it takes more energy to dig it out of the ground than you get from burning it. When this happens, you can continue to produce at a net energy loss, but you need to get energy from somewhere to do it. Where are you going to get that energy without oil?

          That's the real, serious problem we're going to be facing in the short term. It's not going to be lifethreatening in my productive lifetime. Sucks to be growing up now though. The sad fact is people won't change, but hey, I get to fiddle while it goes down! (sarcasm)

      • You aren't nearly paranoid enough -- don't you wonder why the Hydrogen economy is so ready for us?

        You are right that hydrogen technology is nearly there, and you're absolutely right about the need to wean ourselves off of oil. They're starting to get the kinks out of the development cars. And Every day we rely on oil, we are putting money in the pockets of dictators in the Middle East and other countries.

        Hydrogen, as a general rule, likes to bond to things. As a result, it always takes more energy to separate Hydrogen from what it's bonded to compared to the energy you get when you put it back together.

        And the energy for that first process comes from fossil-fuel burning power plants.

        Right now, it takes less energy to pull oil out of the ground compared to what we get out of it.

        What this means is that the Hydrogen economy is a losing proposition. We will have to pull even more oil out of the ground -- or rely on nuclear power -- and since producing Hydrogen fuel cells costs more energy than it generates, the end result is that this Hydrogen economy will fail. Or it will end up using more petroleum products in the end than if we just dump the stuff directly into our cars.

        You can't defeat the laws of thermodynamics with any amount of technology, particularly when the technology is based on the very thing you're trying to replace. The facts suggest that Hydrogen is not the answer, and would only worsen our situation. Hydrogen will ironically hasten our pace towards the Oil Feudal/Fascist Global New World Order you fear. So I am not looking to Hydrogen to be the all-powerful Saviour from our oil dependency.

        Let's look to technologies that do work. Even PV solar panels are more efficient in the end than Fuel Cells. It's good to look to the end of the oil cartels; but why abandon knowledge and reason in favor of pseudoscience to do it? Doesn't that just defeat the cause in the end?

      • Re:Why we have to have 80%+ (Score:5, Insightful)

        by blair1q ( 305137 ) on Thursday January 09, 2003 @03:39AM (#5045629) Journal
        Hubbert was missing a few cogs.

        Oil demand grows at 1-2% per year. Oil production capacity (current and estimated undiscovered) grows at about the same rate, because that's how economics works. They work just hard enough to keep the demand supplied.

        However, at some point the reserves (current and estimated undiscovered) run out. At current rates of growth, that's expected to be somewhere between 2039 and 2066, with a 65% confidence level.

        There's no peak. We just run into a wall and it's over.

        But that assumes demand will be constant. It won't. As the visible end approaches, the surviving producers will start gouging on the scarcity. Consumption will drop, stretching out the date at which the oil dries up. Possibly indefinitely, since at that point other energy technologies become feasible. But still at a much higher $/kWh than we now pay for oil. There's your peak: when the consumption curve turns down due to market economics and substitution of alternatives and deprivation.

        Petroleum will become a minor energy product, like whale oil is now (whale oil used to be a major energy product; go look it up).

        Which is something the oil companies see coming, but know they can't profit from, so they deprecate alternatives and then secretly work to develop them, so they can continue to control your energy supply after the oil finally becomes deprecated itself.

    • Re:Rectenna? (Score:5, Funny)

      by frankie ( 91710 ) on Wednesday January 08, 2003 @06:11PM (#5043164) Journal
      silicon chip micro rectennas
      picture the consequences of such an invention?

      Here's the best that Google could come up with for picturing a rectenna [google.com], but the exact image I wanted wasn't found. As demonstrated in episode 101, rectennas are powerful, but also put out a lot of heat.

    • Re:Rectenna? (Score:3, Funny)

      by Guppy06 ( 410832 )
      Cartman! There's an 80-foot satellite dish sticking out of your ass!

  • Goverment rebates (Score:3, Informative)

    by Bellator ( 519329 ) on Wednesday January 08, 2003 @05:26PM (#5042936)
    There are government rebates for people who use PV as well. So if anyone gets interseted, look into it. You can also find some good sources at renewables.com.

  • It seems someone is working on them... (Score:5, Informative)

    by jobeus ( 639434 ) <jobe-slash@@@jobeus...net> on Wednesday January 08, 2003 @05:26PM (#5042940) Homepage
    There's supposedly some work being done on Berkeley for cells up to 70% efficient: http://www.scienceagogo.com/news/20021020210743dat a_trunc_sys.shtml [scienceagogo.com]
    • The patent would have us etch tiny dipoles on a semiconductor: The present device differs from the prior art devices in that it utilizes a plurality of dipole antennae for absorbing light photons, ... , while the ones you refer to are modern, much improved examples of what the patent calls prior art.

      They look neat. The researchers think that they'll be cheap and efficient. But where are the microarrays of silicon rectennas?

  • I know where they are (Score:5, Funny)

    by muyuubyou ( 621373 ) on Wednesday January 08, 2003 @05:26PM (#5042942)
    They're next to that new compression algorithm, that one claiming a 90% improvement over Huffman.

  • Is it here yet? How about now? How about now? (Score:5, Funny)

    by Chris_Stankowitz ( 612232 ) on Wednesday January 08, 2003 @05:28PM (#5042952)
    Acording to this [scienceagogo.com] article it will be here soon.

  • But is it real? (Score:3, Interesting)

    by Fished ( 574624 ) <amphigory@gma[ ]com ['il.' in gap]> on Wednesday January 08, 2003 @05:29PM (#5042963)
    Looking at the patent application, I saw absolutely nothing to convince me that this idea makes any sense. I see no company, no diagrams, etc. Is it real? I'm not qualified to say.

  • Research (Score:5, Insightful)

    by crumbz ( 41803 ) <[moc.liamg>maps ... uj>maps_evomer> on Wednesday January 08, 2003 @05:29PM (#5042964) Homepage
    What the US needs is a Manhattan Project for alternative energy to oil. Solar, wind, geo, fusion, whatever. Something but burning simple chain hydrocarbons and because the waste product is mostly invisible, pretending it doesn't exist.

    Who elected George Bush anyway?

    • Re:Research (Score:4, Funny)

      by BabyDave ( 575083 ) on Wednesday January 08, 2003 @05:41PM (#5043050)
      What the US needs is a Manhattan Project for alternative energy to oil.
      They should threaten their enemies with windmills?

    • Re:Research (Score:5, Insightful)

      by iocat ( 572367 ) on Wednesday January 08, 2003 @05:51PM (#5043125) Homepage Journal
      The problem is that *no* alternative to oil will enable people to live with the same wasteful energy useage that oil does. The EROI (energy return on investment) for oil is just way, way higher than for geo, wind, solar, etc.

      So even a "Manhattan Project" style affair will be worthless unless we also make a concerted effort to dramatically improve the energy efficiency of our society -- our cars, our appliances, our homes, etc.

      With not much effort, by not a huge percent of the population, California was able to fairly significantly reduce its energy needs during the whole Enron-initiated "power crisis." Not to sound polyannaish, but just imagine what would happen if we all actually did some simple, painless, things that saved energy.

      The problem is that most people need a real incentive -- dramatically higher costs -- before they will conserve.

      • Re:Research (Score:3, Insightful)

        by Elwood P Dowd ( 16933 )
        Well. Aren't nuke reactors just as cheap per kilowatt hour as natural gas plants?

        Nuclear reactors + fuel cells seem like a solution that would allow a similar level of consumption. It just wouldn't work for Berkeley.
    • Who elected George Bush anyway?

      These guys [archive.org] did.

      (And I STILL think it's funny that the guy who did the map colored the Bush counties red, rather than the Gore counties. B-) )

  • silicon chip micro rectennas? (Score:3, Funny)

    by JeanBaptiste ( 537955 ) on Wednesday January 08, 2003 @05:30PM (#5042967)
    I dont want to 'explore' any such technology if a rectennas are what I think they are

  • Production all goes to... (Score:3, Funny)

    by SuperKendall ( 25149 ) on Wednesday January 08, 2003 @05:30PM (#5042968)
    They're all being used to power the flying cars [hovertech.com] you can't get either.

  • Where are they? (Score:5, Interesting)

    by nuggz ( 69912 ) on Wednesday January 08, 2003 @05:31PM (#5042978) Homepage
    I would guess the inventor couldn't get it to work, which is why they never took advantage of it.
    The patent just expired. It takes a while to develop and prove out a new technology.

    I see lots more work on windmills for alternative power. Many have shelved solar panels because the current ones have poor performance.

    • Re:Where are they? (Score:5, Interesting)

      by rodgerd ( 402 ) on Wednesday January 08, 2003 @05:36PM (#5043013) Homepage
      Living in a rainy country, something which has caught my attention are small hydroelectric units that can be dropped into streams and rivers crossing one's land (for that matter, any fall of water will get you power, so water collected from the roof of a tall building during monsoon season in the tropics may be viable).

      • Re:Where are they? (Score:5, Interesting)

        by stilwebm ( 129567 ) on Wednesday January 08, 2003 @07:09PM (#5043371)
        I first learned about these microhydroelectric generators from another rainy country, Columbia. There is a great book about it, Gaviotis: Village To Reinvent The World [barnesandnoble.com], along with many other sustainable energy projects. The book is also interesting because it discusses the use of solar collectors for heating water, which is quite widespread in Latin America and South America, including Columbia. I found this interesting due to the amount of year-round cloud cover in much of Columbia. Anyway, slashdot readers interested in sustainability or even just community problem-solving might find this book interesting.

      • Re:Where are they? (Score:5, Interesting)

        by Waffle Iron ( 339739 ) on Wednesday January 08, 2003 @07:19PM (#5043465)
        The Hoover Dam, one of the largest civil engineering projects in U.S. history, sports a not-so-huge 2000 megawatt power station.

        The downspout on an office building is going to produce proportionally less electricity. Let's assume you have a 4000 square meter roof 100 m high and you get 100 cm per year of rain. That's 4 million kg of water dropping 100 m. Since E = mgh, you get a potential annual power yield of 4 gigajoules. Over the whole year, this comes out to an average of 126 watts. Factor in unavoidable generation losses, and you'd be lucky to power a single light bulb with the rainwater from an entire office building.

  • cost? (Score:5, Insightful)

    by Ashish Kulkarni ( 454988 ) on Wednesday January 08, 2003 @05:31PM (#5042980) Homepage
    Even if someone does find a way to implement such high-efficiency converters, the cost has to be low for wide-scale use...otherwise it'll just be something used in specialized applications (eg. space)

    • Re:cost? (Score:3, Insightful)

      by Drakula ( 222725 )
      Cost is always an issue for optoelectronic based technology. The overheard for production of the devices is large, from the the equipment required for processing right down to the materials themselves. However, the cost comes down when and if a large number of consumers by them. Look at CD/DVD players. Their high cost was due to the lasers involved, not the rest of the component. Once everyone and their uncle started buying them the cost came down, way down. These components are dirt cheap now.

      Now these converters are a different story. Even if they were that efficient, they cost would be so large, like you pointed out, it would be prohibitive for the average consumer. However, with efficiencies that high, eventually lots of people would want one and probably bring the cost down.

  • Check your math. (Score:3, Insightful)

    by gurps_npc ( 621217 ) on Wednesday January 08, 2003 @05:31PM (#5042981) Homepage
    The Patent said it could reach 70% (Theoretical, as they did not build) It also said that current state of the art (in 1970) was 20% Theoretical, 10% practical. (1/2) using that same ratio, the 70% Theoretical technology should be producing 35% pracitcal. We currently have 17% practical, about 1/2 of what their technology could do. Frankly, the 7% increase is reasonable for most technologies, excluding the computer chip (which contrary to Moores law has been doubling every 3 years or so)

  • Ask yourself... (Score:3, Insightful)

    by anthony_dipierro ( 543308 ) on Wednesday January 08, 2003 @05:37PM (#5043022) Journal
    Why haven't you built one of these things? Chances are that's the same reason that they haven't yet been built.

    • Re:Ask yourself... (Score:5, Funny)

      by deft ( 253558 ) on Wednesday January 08, 2003 @05:46PM (#5043085) Homepage
      "Why haven't you built one of these things? Chances are that's the same reason that they haven't yet been built."

      90% of slashdot just simultaneously realized that these solar panels havent been developed because they are too busy looking at porn and playing warcraft.

  • Possibly overlapping Patents? (Score:4, Interesting)

    by SpeedBump0619 ( 324581 ) on Wednesday January 08, 2003 @05:39PM (#5043031)
    Could it be that the effect in question has been patented for some other use [foveon.com]? I'm not familiar with the patent quagmire, but multiple similar uses for the same physical phenomenon (light absorbtion into silicon) might be the issue...
    • I thought that patents were only issued for a particular *application* of a technology. That's why overly-general patents like "method of communicating ideas with others" are bullshit. I assume that I could get a patent for "method of using rock as offensive projectile" and it wouldn't affect your "method of using rock to flavor soup" patent. I could be wrong, though.

  • Perhaps impractical to actually build? (Score:4, Informative)

    by ivan256 ( 17499 ) on Wednesday January 08, 2003 @05:42PM (#5043060)
    This device may be fabricated upon a transparent slab by the deposition of one or more metal coatings in a known manner. The various rectifier elements are first prepared by opening appropriate windows in the metal coating utilizing an electron beam and suitably coating and doping the rectifying areas. An electron or ion beam cuts the shape and connections shown. The connections are completed after deposition of the insulating coating 9. The circuit is then the same as that shown in FIG. 1.

    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.

  • What, or rather Who keeps this off the market? (Score:5, Funny)

    by mcmonkey ( 96054 ) on Wednesday January 08, 2003 @05:48PM (#5043101) Homepage
    Who controls the British Crown?
    Who keeps the metric system down?
    We do, we do.
    Who keeps Atlantis off the maps?
    Who keeps the Martians under wraps?
    We do, we do.
    Who holds back the electric car?
    Who makes Steve Guttenberg a star?
    We do, we do.
    Who robs cave fish of their sight?
    Who rigs every Oscar night?
    We do, we do!

  • Not with semis (Score:5, Informative)

    by sirsex ( 550329 ) on Wednesday January 08, 2003 @05:48PM (#5043104)
    Semiconductor photocells can easily be >90% effecient, but over a rather small range of wavelengths. This is due to the bandgap. An electron is freed if the electron gains enough energy from the photon(s) to overcome the bandgap. the energy of several photons can be combined to free and electron, but is lossy. If the photon has more energy than is required to free the electron, the extra will mostly be dumped as heat. The equation governing wavelength, energy, and Boltzmann's constant is

    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??

    • Re:Not with semis (Score:5, Informative)

      by pclminion ( 145572 ) on Wednesday January 08, 2003 @06:52PM (#5043259)
      The equation governing wavelength, energy, and Boltzmann's constant is E=hw

      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 :-)

  • Anyone know the energy in sunlight? (Score:3, Insightful)

    by cornice ( 9801 ) on Wednesday January 08, 2003 @05:59PM (#5043142)
    I know this is a dumb question. I remember hearing the answer back in high school but I have since forgotten it. I want to know the total energy in sunlight. I know it varies widely depending on location and weather but an average or a range per square foot, square yard, acre or whatever would be interesting. I ask because I think some people overestimate the value. If you can produce a 1 inch square solar cell that's 100% effecient but it costs $1000, then it's never going to pay for itself except in space applications. The big payoff for solar cells will come when you can produce them for almost nothing and plaster them over everything. When that cool, one way billboard plastic wrap stuff that covers busses also acts as an 80% effecient solar cell, then we'll see more of this stuff.

    • Re:Anyone know the energy in sunlight? (Score:5, Informative)

      by afidel ( 530433 ) on Wednesday January 08, 2003 @07:02PM (#5043317)
      For simplicity we take the solar energy density falling onto a window to be 1000 kWh/m2 yr. This is regarded as a typical number for a south-facing window, and more correct values for south-facing/north-facing/horizontal surfaces would be 850/350/920, 1400/450/1700, and 1100/560/1800 kWh/m2 yr for Stockholm, Sweden, Denver, USA, and Miami, USA.

      This is from Here [electrochem.org]

    • Re:Anyone know the energy in sunlight? (Score:4, Interesting)

      by bmwm3nut ( 556681 ) on Wednesday January 08, 2003 @07:05PM (#5043334)
      i don't know actual numbers for how much energy we can get from the sun, but i do work with a bunch of people who do research for alternative energy sources. one neat statistic that i was told a while ago is:

      if you take all the unused building space in america and covered them with solar cells (of today's technology and efficiency) you could generate (more than) all the power we use (even if we convert all cars to electric cars). the neat thing is that the "unused building space" doesn't mean "cover the desert with solar cells" is just means to put solar cells on every roof top, on the top of the medians along the highways. granted this is a difficult task, but what if every residential house that was built had to have it's roof coated in solar cells and put on the grid. after a while you'll be generating quite a lot of power.

      another cool thing that is in development is turning the huge windows of office buildings into solar panels. in high-rise office buildings, it's not uncommon to have 10 foot tall windows that span the entire room. usually these windows are tinted and the office generally has blinds or curtains to keep all the sun out (to reduce cooling costs in the summer, and having that much sun in the office would make it to bright anyway). so rather than just using some type of tinting that reflects the light partially, i've seen some cool work with dithering (very) small solar cells in the windows, these will absorb light and turn it into electricity. so you'll get two benefits: tinted window and extra electricity for the building.

      just simple solutions like that could help our upcoming power crisis when we run out of oil.

  • Here it is. The inventor calls it lepcon. (Score:5, Informative)

    by RealAlaskan ( 576404 ) on Wednesday January 08, 2003 @06:07PM (#5043153) Homepage Journal
    Right here [sintef.no]. Scroll right down to the bottom of the page. The patent holder, Alvin Marks, calls this approach lepcon. He's more excited about another approach, called lumeloid. Someone else has already linked to the site [luminet.net] he has for that.

    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.''

  • PVs can exceed 35% efficiency (Score:4, Interesting)

    by leftism11 ( 177941 ) on Wednesday January 08, 2003 @06:43PM (#5043223)
    http://www.spacedaily.com/news/solarcell-01h.html

    This article mentions Tecstar--apparently the leading supplier of high-output PV cells for space missions. They made the cells for the Mars lunar rover, among other projects.

    The difficulty is that manufacturing the cells is very difficult and expensive, requiring multi-million dollar Metal Oxide Chemical Vapor Deposition (MOCVD) machines. Due to variations in the rather elaborate manufacturing process, the yields vary greatly, and also produce cells that have widely varying efficiencies. Some cells fall below a given spec, while others exceed the spec.

    In order to achieve a given power output for a space system, they are able to mix the under and overperforming cells into an array that provides the necessary power.

    So, although some cells may only provide 30% output, a few will exceed specs, and thus provide > 35% efficiency. The challenge is delevoping manufacturing processes that improve yield and thus reduce costs.

    Why aren't we ever going to see these type of cells on earth? Cuz they cost millions of dollars to produce, and there is a multi-year order backlog for them for planned space missions. Unless hundreds of millions of $ gets directed towards making them a commodity, well, you know the story...
  • The folks over at NREL [nrel.gov] are working on [nrel.gov] multijunction photovoltaics. They've gotten 25% efficiency with 2 junction-layers instead of 1, and are going for 33% with a 3 junction-layer cell. I saw a talk by one of the folks doing that work a few months ago. Basically at the moment they're targeting the space program as customers, because the cost of launching solar panels is major.

    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.

  • Solar cells (Score:4, Informative)

    by vorwerk ( 543034 ) on Wednesday January 08, 2003 @07:17PM (#5043440)
    I've seen no practical solar cells with efficiency that is much higher than 30-35%. Don't be too surprised by this figure -- remember, we're trying to make practical cells that can handle sunlight (which consists of a variety of wavelengths, not all of which can be absorbed by a single device), and at a cost per area of about $0.02 / cm**2. To achieve this "economical make-it-or-break-it rate", you must use technology that can be cheaply deposited over a large area and at a deposition rate of between 10 and 30 angstroms per second.

    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

  • Comment removed (Score:5, Insightful)

    by account_deleted ( 4530225 ) on Wednesday January 08, 2003 @08:29PM (#5043885)
    Comment removed based on user account deletion

  • Theoretical problems with optical rectennas (Score:5, Informative)

    by Phil Karn ( 14620 ) <karn.ka9q@net> on Wednesday January 08, 2003 @09:07PM (#5044084) Homepage
    I see a serious theoretical difficulty here that may explain why the optical rectenna was never built.

    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.

    • An equally fundamental problem, I think, is due to the skin effect.

      IIRC, the rate of attenuation of an electromagnetic wave in a conductor is

      sqrt(2/wus),

      where

      w = 2*pi*f = frequency of the wave, in radians
      u = permeability of the conductor, maybe 4*pi*10^(-7) Henry/m for copper
      s = conductivity of the conductor, maybe 6*10^7 mhos/m,

      meaning that 1/e or about 37% of the wave decays in this distance into the conductor.

      If my numbers are right, f = 5.2*10^14 Hz for yellow light, and the skin depth in copper for yellow light is then 2.8*10^(-9) m, or 2.8 nm.

      Since the wavelength is 570 nm but the skin depth is only 2.8 nm, it seems that a copper antenna would instead act more like a mirror, reflecting the radiated energy rather than absorbing it.
    • Hmmm. The electric field of EM radiation is transverse (perpendicular) to the direction of propagation (i.e. wavelength) so the potential difference doesn't build up in that direction.

      The problem here is likely timescales: light has a VERY high frequency (10^17 Hertz roughly, if I did my math right). At that frequency one doesn't push the electrons back and forth like a kinetic particle in the usual diode treatments so much as one excites interband transtions, which is how a regular solar cell works.
      • This doesn't make sense. The E field of an EM wave is perpendicular to its direction of propagation, so if you have a properly tuned and aligned conductor that E field will impose a voltage across it. That's a simplified view of how any antenna works, and there's no reason to believe it shouldn't work at optical frequencies (except perhaps for the skin effect mentioned by another poster.)

        Photovoltaic cells work on a very different principle. Because there's a significant amount of energy in each photon, enough to dislodge an electron from silicon's valence band into the conduction band (about 1eV), current can be generated.

        If the photon lacks enough energy (has too long a wavelength), nothing happens except that the silicon heats up. If the photon is too energetic, it will still dislodge the electron but it won't impart all of its energy; the rest will again be wasted. These two effects account for much of the poor efficiency of PV cells on sunlight, which has substantial energy across the visible spectrum and into the near infrared.

  • Oil is solar power! (Score:4, Interesting)

    by Lord Ender ( 156273 ) on Wednesday January 08, 2003 @09:27PM (#5044178) Homepage
    Plants stored energy from the Sun, then died and were burried for a long time. That energy still came from the Sun. If we can just figure out a way to go from plants to oil more quickly, we can plant fields and basically they would be giant solar pannels.

    I think it is a good idea, anyway. The only energy that is not solar is geothermal and nuclear.

  • The real problem with solar cells... (Score:3, Funny)

    by aquarian ( 134728 ) on Wednesday January 08, 2003 @10:03PM (#5044340)
    ...is that they only work when we don't need to turn the lights on anyway.

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