Naked Science Forum
Non Life Sciences => Technology => Topic started by: thedoc on 11/07/2014 08:30:02
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Wilf James asked the Naked Scientists:
There has been a lot of talk about the fact that wind farms and solar cells produce useful but unreliable electricity. I think that the problem could be partially solved with "Flywheel Farms"
The Swiss company Oerlikon produced buses that were powered by flywheels. I thought that if a bus can be powered by a flywheel, almost any machine that needs a lot of power could be powered by a flywheel. The advantage of a flywheel system is that it could be energised by relatively low power from solar cells or a wind farm using an efficient hydraulic motor. The flywheel's output could be taken by a hydraulic pump that powers a conventional electric generator. I know that this would be a bit of a lossy system but it would have the advantage that it could provide emergency power at a high level at very short notice.
The advantage of a flywheel system is that it can be fed power at a low level over a long distance to keep transmission losses low. The flywheel (or flywheels) can be sited near to where short term high level power is needed, keeping transmission distances short and transmission losses very low. I think that a flywheel system could be more energy efficient
than a pumped water storage system and could be sited almost anywhere. Once built such an energy storage system would have very low running costs.
I got some of my ideas when I worked for a short time with JCB, the digger company. JCB (and other comparable companies) produce what they call 360 machines. The diesel engines for these machines are in the rotating parts and their caterpillar tracks are powered entirely by a hydraulics trasmission system. If a hydraulic transmission system is
good enough for a heavy earthmoving machine it could be made more efficient for a stationary installation.
If I were 18 and not 78 I would be thinking of starting a company to develop the flywheel system for temporarily storing power..
Wilf James
What do you think?
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The Swiss company Oerlikon produced buses that were powered by flywheels. I thought that if a bus can be powered by a flywheel, almost any machine that needs a lot of power could be powered by a flywheel.
The buses got a recharge at each bus-stop ... http://en.wikipedia.org/wiki/Gyrobus
i.e. the energy was dissipated in a few minutes, rather than stored for months in a reservoir.
Flywheels can be used in regenerative braking (http://en.wikipedia.org/wiki/Kinetic_energy_recovery_system) in vehicles.
[ but the storage of energy is only a very few minutes ].
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There are always a few more generators attached to the power grid than are currently needed, just in case a generator fails, or a transmission line fails.
The immense mass of these turbines and generators forms a "rotating reserve" that can hold over a short-term power shortfall until the valve on the turbine opens up to pour in more power.
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Flywheels are an excellent way of storing energy, but from what I know, they are best suited for quick release of the energy, like maintaining frequency for an AC power grid. Overall, their energy density is not great compared with chemical storage, but for mag-lev flywheels in vacuum, the efficiency is great, especially for AC input and AC output (chemical storage is better suited for DC like solar panels).
At this point, one of the limiting factors of increasing the percentage of green energy in the power make-up is the difficulty of storing excess energy for when the sun stops shining or the wind stops blowing.
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I always think of flies running inside tiny wheels, like miniature hamsters...
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There are flywheel based UPS systems, and I think they have fairly high efficiency, although it may depend on whether they have mechanical or magnetic bearings, as well as air friction.
It certainly would be useful for brief surges in demand, and perhaps help cover some of the off-hours, for example extending solar power from 6:00 PM till midnight (what about the morning power requirements?)
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The problem with unreliable energy sources isn't to cover short-term outages but how to supply peak load when the wind isn't blowing, which it sometimes doesn't for several days (and always the hottest and coldest days) or when the sun isn't shining (50% of the year) or when the tide happens not to coincide with demand (at least 75% of the time).
Wind farm proposals are always quoted in press releases as "able to power 1000 homes". True, a 1.5MW turbine can supply power equal to the average consumption of 1000 houses, provided that (a) the wind blows steadily in the optimum speed range of the turbine and (b) the homeowners take turns to use peak electricity. Alas, neither statement holds true in practice: (a) optimal wind speed rarely occurs more than 10% of the time, and this is not correlated with peak demand and (b) most people like to cook at roughly the same times each day, with very little power consumption between those times: the peak-to-mean ratio for domestic power consumption is around 20:1.
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The problem with unreliable energy sources isn't to cover short-term outages but how to supply peak load when the wind isn't blowing, which it sometimes doesn't for several days (and always the hottest and coldest days) or when the sun isn't shining (50% of the year) or when the tide happens not to coincide with demand (at least 75% of the time).
Wind farm proposals are always quoted in press releases as "able to power 1000 homes". True, a 1.5MW turbine can supply power equal to the average consumption of 1000 houses, provided that (a) the wind blows steadily in the optimum speed range of the turbine and (b) the homeowners take turns to use peak electricity. Alas, neither statement holds true in practice: (a) optimal wind speed rarely occurs more than 10% of the time, and this is not correlated with peak demand and (b) most people like to cook at roughly the same times each day, with very little power consumption between those times: the peak-to-mean ratio for domestic power consumption is around 20:1.
Precisely! Energy storage is at least as important as energy capture in these systems. Currently, our capture technology far outpaces our storage technology, and it is only through advances of the storage that renewable energy can be effectively utilized. Right now pumped hydro and warehouses full of car batteries are the most often employed grid storage options for the huge amounts of energy needed for peak shaving and peak shifting (long-term averaging of supply to match demand). People are working on developing technology for splitting water into hydrogen and oxygen (splitting one liter of water can hold about 16 MJ, or 4.5 kWh, or the equivalent of pumping 1500 L of water up 105 meters), but there are many technological barriers before any implementation is truly feasible (round-trip efficiencies of 50–60% is not very good, and storing hydrogen is very difficult). I am an optimist though (except when I'm a pessimist...), and I believe that we will have this problem solved within the next 15 to 20 years.
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An optimally located windfarm such as those located along the Columbia Gorge in Oregon/Washington will have good wind most of the time. It may vary somewhat, but there is almost always some wind. The problem is that not all wind farms have such an optimal location.
Something like the centrifugal UPS would help with leveling off peak and off-peak demands. Switching could be instantaneous, but the capacity would have to be huge for any practical use. A hydroelectric system on a river should be able to be designed to vary output with demand without additionally pumping water uphill, especially if it is coupled with a buffer system to level out surges in water flow.
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Its a bit of "A" level GCE, to show the energy stored in a flywheel, just say if you want the math.
Volvo runs busses with regenerative kinetic potential energy flywheels as the bus brakes.
The flywheel is an energy store but unless you can get wave power to make flywheels revolve and store power, the green bit goes out of the window.
In the 1980's I did go down the flywheel path and adding a battery mass within the flywheel and using the centrifugal effects to move electrodes and hence have a battery/flywheel power source. The patent application number I can give you if you wish
Sadly after 12 years in the advanced battery industry the task of doing the engineering to marry the flywheel to the battery internally was not forthcoming.
BUT it was, and still is a great project if anyone want to find funds but it cant be considered GREEN.... but its just a super project to get efficient vehicle power and the target objective was railways power and possibly cars.
May I close by suggesting you consider a 100Kg flywheel in a vehicle doing say 500 rpm the power is fine but don't dare have a crash in such a vehicle with that potential power ready to come out instantly.
Say Green, Flywheel and wave power and your on the right path. Allan
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Chiral SPO: Storage and conversion efficiency is irrelevant if the input energy is free. And there is no problem storing and handling hydrogen: "town gas" was 40 - 60% hydrogen before it was replaced by methane: the gas grid exists and is already responsible for about 50% of UK energy distribution.
Clifford: "Varying somewhat" is no good! Power output of a turbine varies with the cube of the wind speed, and the safe peak output is limited by sonic shockwaves at the rotor tip, so the economically useful speed erange is very small indeed. Almost no onshore turbines exceed 10% of rated power, and even this is a bit of a con trick: if you build a 2 MW blade and couple it to a 1 MW generator you can easily exceed the rated power of the generator - the trick is always to declare a much lower power rating than you can actually produce, and then sell your windfarm as "generating 25% of rated power over a year" because the declared rating is less than a quarter of the design rating.
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Alan: Even if the input energy is free, the infrastructure to collect it and convert it is still quite expensive. If you have to cover twice as much area with solar panels because 50% of it is going to heat during conversion, that leads to some very serious economic and physical ramifications (PVs are only about 20% efficient, but cutting it down to 10% is not so good.)
Also, while the grid already exists for distributing the hydrogen, that still doesn't account for being able to actually store it (say from summer when solar energy is in excess until winter when heating needs are greatest, and sunlight is scarce). Even though hydrogen has one of the highest energy densities by mass, it has a pretty pathetic energy density by volume at standard temperature and pressure. For example, an average US household uses about 30 kWh of electricity per day, which can be stored by splitting only 2 L of water (4 L if 50% efficient), but that would generate about 2500 L of hydrogen gas at atmospheric temperature and pressure. That's a fairly large volume for one household for one day, try storing 4-month's-worth for a million households. If you want to compress it, that's an additional investment for the compressors and pressure containment as well as a hit on efficiency because of the energy requirements for running the compressors (maybe one could reclaim this energy on gas decompression, but that would require yet more equipment). Liquification is even more energy intensive, plus it requires special insulation and refrigeration (H2 boils at 14 K, and even liquid hydrogen is not that dense (0.07 g/cm3) so it would still require 3 L to store that 30 kWh (solidly in the realm of feasibility, about on par with gasoline energy density, but would still require substantial additional investment in infrastructure).
None of this is impossible to do today. It's just impossible to do it cheaply, and that is the problem.
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I always think of flies running inside tiny wheels, like miniature hamsters...
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fimagecache.w00t.com%2F%3Furl%3Dhttp%3A%2F%2Fartoftrolling.files.wordpress.com%2F2010%2F11%2Ffblvn.gif%3Fw%3D100%26amp%3Bh%3D100&hash=ecd272eb4f65867a20b2d39bd4749752)
Neat. You got me.
I thought it is a real thing on my pc. Wondering why it repeats the same path?
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One can rate one's wind farm based on peak energy, or average energy production.
Even if the average production is only 10% of the peak, that doesn't mean that it is worthless.
In a hurricane, the windmills should be auto-furling.
As mentioned, in the Columbia Gorge, the wind blows quite strongly about 90% of the time (and thus it is a favorite for wind surfers and sail boats). Whether it gets the mills spinning to peak capacity is irrelevant, the blades are turning and power is being generated.
If power generation is too high, then the operators should be able to direct the blades out of the wind, or perhaps send the power into a dump load. Or, perhaps convert the power into something else such as hydrogen or aluminum.
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I thought it is a real thing on my pc.
Yeah, it got me too when I first saw it.
Wondering why it repeats the same path?
Because it's an animated GIF...