Naked Science Forum
Non Life Sciences => Technology => Topic started by: chiralSPO on 10/09/2014 15:30:43
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I was reading about the environmental impact of mining rare-earth metals for permanent magnets used in wind turbines, and wondered why we need to use magnetic induction for generating electrical energy from kinetic energy. Couldn't the same task be accomplished by purely electrostatic methods, thereby obviating the use of rare materials?
I propose the following apparatus (see attached figures):
Figure 1: Two parallel plates with a large potential between them. They are held some fairly large distance (d) apart (so there is some energetic fixed cost of assembly). A second set of (smaller) parallel plates, each neutral, conductive and polarizable are held a fixed distance (d2 <d) from each other, between the outer plates (within an effectively uniform electric field) Of course these plates are at different potentials--if a wire were to connect them, there would be a flow of electrons from one to the other.
Figure 2: If these inner plates were rotated about an axis (between them and parallel to all 4 plates), and a pair of stationary conductive brushes made contact with the internal plates when they are closest to the external plates, and a circuit (load) was used to connect the two brushes, the system would essentially be an electron pump, mechanically moving charges against a stationary electric field, and depositing them on the brushes, only to relax back to lower potential through the circuit (load). Because all of the charges make a complete cycle, it can be repeated as many times as the mechanical energy is provided (assuming no charge leakage from the external plates... is that a far-fetched assumption?)
Figures 3-5: I think that several pairs of inner plates arranged in roughly cylindrical symmetry about the axis of ration would be able to supply a fairly constant DC current, where the voltage is defined by the distances d1 and d2 and the strength of that static external field; and the amperage is determined by the surface area of the inner and outer plates, and rotational frequency of the inner plates.
So here's the question: to first approximation, with an ideal system (spherical cow in vacuum), what sort of kinetic energy to kWh efficiencies could one expect, and how would they vary with applied field strength and rotational frequency? Or essentially, the same question: how fast (and hard) would one have to crank a feasible system to get a useable amount of electrical current at a reasonable voltage?
My hope is that the magnetic complications can be discounted at high voltage/low current regimes. I also don't know how hard it will be to maintain a static external field once it is established (if it would require a constant power supply that is anywhere close to the output, this would be a futile... If it needs to self sustain this charge, it's essentially a Wimshurst machine--any idea what the efficiency of one of those is?)
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As you imply, electrostatic generators are good for high voltage and low current, and in practical terms we are talking about a couple of megavolts and a few milliamps from a couple of tons of Van der Graff machinery. The efficiency can be quite high as long as you stick to DC but in any practical application the capacitive losses when dealing with high AC voltage are horrendous, and the difficulty of insulating large static voltages explains why we use linear accelerators, synchrotrons etc to drive particles above about 5 MeV. 10 MV DC is just manageable and has been suggested for power transmission but even 1 MV lines run into spectacular atmospheric problems, and in order to generate useful currents we tend to use "simple" rotating electromagnets (which are delightfully controllable compared with electrostatics) and transformers.
Physics works well, as you say, for a spherical cow in a vacuum, but if you are dealing with bilateraly symmetric lactating herbivorous quadrupeds in wet air, you need to think about engineering.
And the real problem with wind power is that it is useless. The point of electricity is power when you want it, not when the weather gods offer it.
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What other antique power schemes will be dug up will we have slave rowed liners or horse powered corn grinders ?
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Students at Cornell College have come up with an electrostatic motor.
http://www.cornellcollege.edu/physics/courses/phy312/student-projects/electrostatic-motor/electrostatic-motor.html
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And the real problem with wind power is that it is useless. The point of electricity is power when you want it, not when the weather gods offer it.
That's funny, the Germans appear to have missed this "fact." http://www.nytimes.com/2014/09/14/science/earth/sun-and-wind-alter-german-landscape-leaving-utilities-behind.html?hpw&rref=science&action=click&pgtype=Homepage&version=HpHedThumbWell&module=well-region®ion=bottom-well&WT.nav=bottom-well&_r=0
They do point out in this article that at this point in time, the variability in wind and solar output is still a problem, but it still appears to be cost effective.
I suspect that once energy storage technology catches up with energy capture technology, oil- and coal-fired power plants will be fossils themselves. Nuclear (fission) energy also suffers from a similar problem--their output is too constant to keep up with daily energy peaks without employing some kind of energy storage.
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I suspect that once energy storage technology catches up with energy capture technology, oil- and coal-fired power plants will be fossils themselves.
This approach is known as "ignoring the weight of the elephant". I would have no objection to anyone building a windmill with his own money as long as (a) he also builds at least 5 days' energy storage at maximum rated output (like a coal, gas or oil station) and (b) he only gets paid the market rate for the electricity he produces (ditto). Fat chance.
Nuclear (fission) energy also suffers from a similar problem--their output is too constant to keep up with daily energy peaks without employing some kind of energy storage.
Not really a problem. Small nuclear reactors in ships can be powered up and down as required. It just happens that large capital plant, whether fossil or nuclear, is more costeffective if it is run at near-constant loading of 80 - 90%. The advantage of a centralised (and preferably multinational) grid is that the peaks are considerably smoothed out and demand is fairly predictable from hour to hour, so the actual amount of storage required is very small: the UK grid presently handles some 80 GW with only about 20 MWh storage (less than 1 second's worth!).
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Fat chance also that nuclear power stations only get market rate for the energy they produce. I understand that the UK government has had to guarantee market price x 2 for energy generated by the next batch of nuclear to be built here (by foreign companies). Also, quote from "New Scientist" 20th September 2014:- "Global subsidies - In 2012 fossil fuel subsidies far outweighed those for renewables, $544 billion to $101 billion".
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"Students at Cornell College have come up with an electrostatic motor. "
Our 1920,s vintage school physics primer pointed out that Wimshurst machines could be run as motors but suggested removing the belt to reduce friction
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Fat chance also that nuclear power stations only get market rate for the energy they produce. I understand that the UK government has had to guarantee market price x 2 for energy generated by the next batch of nuclear to be built here (by foreign companies). Also, quote from "New Scientist" 20th September 2014:- "Global subsidies - In 2012 fossil fuel subsidies far outweighed those for renewables, $544 billion to $101 billion".
Now the global production of energy is around 90% fossil fuel and 0.9% unreliable renewables (i.e. excluding hydroelectricity, 6.5%) so you can divide your $544 billion by 100 to get the ratio of subsidies per unit energy generated. From your own figures, therefore, the subsidy on renewables is nearly 20 times that on fossil fuels. Hence, in order to provide a useable electricity supply and meet EU emissions targets, it is cheaper and safer to offer twice the market rate for nuclear power than to subsidise unreliables.
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I suspect that once energy storage technology catches up with energy capture technology...
One energy company in Australia is promoting themselves with the slogan "electricity - produced fresh every day!".
This takes one of the major disadvantages of electricity (lack of cost-effective storage technology) and tries to pass it off as an advantage, by implying an analogy with bread and tomatoes sold in the local supermarket.
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Hang on...
"I was reading about the environmental impact of mining rare-earth metals for permanent magnets used in wind turbines, and wondered why we need to use magnetic induction for generating electrical energy from kinetic energy."
Now they may well use rare earth magnets in wind turbines- but they don't really need to.
http://en.wikipedia.org/wiki/Shunt_generator
or, if you prefer
http://nuclearpowertraining.tpub.com/h1011v2/css/h1011v2_93.htm
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Thanks Bored Chemist! This looks like exactly the type of lead I was looking for. Another rabbit hole to explore...