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Where are the 70% Efficient Solar Cells?
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?
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.
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??
This company actually seems to be developing the very technology, or very similar, in the patent posted in the story. At This page on the Lumeloid website, they list the technology coming from Alvin Marks, the same person listed in the patent.
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.
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There are 4 boxes to use in the defense of liberty: soap, ballot, jury, ammo. Use in that order. Starting now.
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.
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).
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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.
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.