vapors, hydrogen rises. This actually makes the spill somewhat less of a hazard, since burning takes place somewhere above the spill, rather than on the ground (where people tend to be). Flywheels High-speed flywheels are now being developed for energy storage applications. Most of the work being done is for flywheels small enough to be used in electric vehicles. US Flywheel and Satcon both have demonstrations of these types of flywheels. The basics of flywheel storage are to spin up a wheel (actually a donut shaped ring) to high speed. The less mass the ring has, the faster it has to spin to store the same amount of energy (KE=1/2Ir2 and I is a function of mass). If a light-weight flywheel is desired, then it has to spin very fest. If the ring has to spin at several thousand RPMs, materials such as steel are not strong enough to hold together. Magnetic bearings are required to reduce wear and the spinning ring will have to be contained in a vacuum chamber to prevent energy loss due to drag. US Flywheel is building a flywheel which is capable of storing 4 kWh. This flywheel is small (about 10 inches in diameter) and the ring is made of carbon or glass composite. It can spin at 100000 RPM. It is contained in a vacuum chamber, has magnetic bearings and is directly attached to a motor/generator. To charge it (spin up) external power s applied and the motor turns the spindle. To get the energy back out, the motor is “run backwards” as a generator. The output is very high frequency AC. The output is conditioned for a specific application (converted to 60 Hz AC for example). The momentary power it is capable of putting out is several kW. The SatCon flywheels are similar, spinning at 65000 RPM, they can store 2 to 4 kWh of energy. The energy density of flywheel storage is higher than batteries by a fector of 3. For example the SatCon flywheel stores 30 Whr/pound, while a battery of the same capacity would store 10 Whr/pound. The power density of a flywheel is 100 times higher (at 2500 W/pound) than a battery, meaning that very high momentary power can be obtained from a flywheel This is very good for industrial applications which have momentary high peak loads. Flywheels have another large advantage over batteries, in that they will survive for 10000 cycles, whereas batteries survive only 1000 to 2000 cycles. Therefore a flywheel system will be good for about 20 years if used in a residential solar electric system. The main failure mode of a flywheel is believed to be disintegration of the ring after buildup of stress over its life. The bearings are probably not going to feil first. Tests are underway to find out just how long the ring will last under various conditions and how to make failures safe. This is a relatively minor technical problem, which can be solved by simply replacing the ring after a certain number of “miles” just as you would a tire or timing chain in your car. So far, though I have only talked about relatively small flywheels that are usefill only for small remote power applications. What about for large scale storage? Is there any way to accomplish that without acres of flywheels? Flywheels are moving parts and if we had to make large arrays of them, would be continually fraught with maintenance problems. One idea is to build a large-scale flywheel, which is essentially a large mass going around on a magnetic rail The mass could be very large and the system could be underground. The mass would be accelerated when there is excess power and its deceleration later would give up power for use. It would be like a maglev car which was accelerated by a linear motor and decelerated by a linear generator. At present, a 2 kWh flywheel costs about $10000. However, according to the manufacturers, the cost of flywheel storage is likely to be the same as for battery storage, when flywheels are mass produced (by the hundred thousands). Also, flywheels for stationary' storage are going to be cheaper, earlier than the ones for electric vehicles, since the requirements are different. An inventor in Vestal NY has built a flywheel
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