From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Flywheels Not Common? Date: Sun, 04 Mar 2007 18:01:44 -0500 Message-ID: <rbimu2dhmeqg51b165hvj46fo9ohgeh4d0@4ax.com> On Sun, 04 Mar 2007 09:00:07 -0500, "(PeteCresswell)" <x@y.Invalid> wrote: >Per Anthony Matonak: >>I suppose you could just forget about energy density and build it >>large and slow. > >That's what came to my own mind as soon as I saw the OP. For residential use, >the size/density issue seems to be mitigated to a great extent. Something 10 >feet high could easily be housed in something like a garden shed or other >outbuilding. No. first off energy storage goes up as the square of velocity but only linearly with mass so increasing mass is a loser. Second, any flywheel with useful energy storage capability will operate in a vacuum. Otherwise windage losses rapidly dissipate the stored energy. Small little concrete example. Awhile back I bought one of these high tech gyro toys: http://www.gyroscope.com/d.asp?product=SUPER2 Slick little booger. Streamlined shape, nice polish, close fitting band to reduce boundary turbulence, instrument-grade ball bearings. The supplied little motor spins the thing up to about 15,000 RPM. The blurb says that it'll spin for 7 minutes and technically it will but the gyro action is mostly gone in about 2 minutes. Put that same gyro in a bell jar and pull a vacuum on it and it'll spin for hours. Mine was still going strong after 4 hours when I let the air back in. In a vacuum only bearing losses are present. Magnetic bearings can reduce that to essentially zero. All this is old news to those familiar with flywheels which is why efforts are expended toward stronger materials that can be spun faster and the device still be small enough to operate in a reasonably sized vacuum chamber. With due regard for making blanket statements :-), I seriously doubt that flywheels will ever compete with batteries for long term energy storage. Even if material science comes up with something strong enough to let the flywheel spin fast enough to equal the energy density of plain old lead-acid batteries, there remains the practically insurmountable problem of explosive energy release during failure. Unlike a battery that gives up (part of) its energy over several seconds or minutes even when directly shorted, a failed flywheel releases the stored energy practically instantly as anyone who's experienced a flywheel explosion in a dragster knows. For the few thousand joules of storage the currently available flywheel surge batteries are capable of storing and releasing, the manufacturers are taking the expedient (and perhaps only) course of making the housing heavy enough to contain the fragments and dissipate the energy as heat. Reach the hundreds of thousands or millions of joules stored and the situation becomes quite serious. To put this in perspective, a 100 amp-hour, 12 volt storage battery (1200 watt-hours) stores 4,320,000 joules. Another point of perspective, a pound of exploding TNT releases about 2 million joules of energy. How many pounds of TNT do you want in your back yard? :-) John |