[brian eiland]

Originally Posted by Marmot

....The air car proponents have used efficiency numbers (as well as websites) derived from "compressed air storage" power plants which use large underground caverns filled with compressed air from electric powered compressors which run at night when electrical power is cheap. During the day this air is fed to the burners of a gas turbine that drives a generator that feeds the grid. It's a clever idea similar to the pumped storage hydro plants used in mountain regions and if the energy used to pump the air storage is ignored, the efficiency figures can look pretty good to the gullible investor.

Originally Posted by Grecko

.... If there isn't any heating going on in the latest versions, the efficiency of a compressed gas system is really going to be poor. The reason is that if you don't immediately use the compressed gas it cools and you lose a tremendous amount of energy. This is why compressed gas is really poor energy storage medium, even for relatively short periods of time. Underground storage would only work if you had geothermal heating available to heat the air it might make sense, but if you didn't forget it....

I hate to let this expertise being exhibited here go away without at least asking them to comment on this older energy storage technology that I believe still has some potential, ...FLYWHEEL ENERGY STORAGE...storing energy in a spinning wheel. John Hopkins Lab worked on this idea years ago with the thought that these massive metal wheels (pre carbon fiber) could be located in underground chambers at the power plants and be 'charged up' at night, then linked to a generator during the day when the peak power was needed in lue of the tradition turbine-generators.

Then a most exiting application in cars was going to be the La Mans electric car Chrysler was building...with a flywheel energy storage system.

I mention it here, with a few other links included:
http://www.yachtforums.com/forums/56932-post6.html

It came back to my mind AGAIN when I recently read of the Seakeeper Gyro stabilization system being worked on here in Southern MD. It appears as those these fellows have solved a portion of the bearing/lubrication problem while maintaining the desired vacuum atmosphere for the flywheel to spin in. The March 08 issue had quite a long article on this flywheel stabilizer.

[Marmot]

Originally Posted by brian eiland

... the thought that these massive metal wheels (pre carbon fiber) could be located in underground chambers at the power plants and be 'charged up' at night ...

Maybe they had second thoughts about trying to corral the energy in a multi ton wheel spinning at a rate sufficient to deliver a useful amount of power for a reasonable time.

Besides, converting that power in the days before high power/high voltage industrial IGBTs and computer controlled multi-megawatt power management sytems might have been problematic.

[Grecko]

Originally Posted by Marmot

Maybe they had second thoughts about trying to corral the energy in a multi ton wheel spinning at a rate sufficient to deliver a useful amount of power for a reasonable time.

Besides, converting that power in the days before high power/high voltage industrial IGBTs and computer controlled multi-megawatt power management sytems might have been problematic.

Both true, when metal parts get really big, it becomes difficult to insure that there are no significant flaws in the material that could become cracks and eventually let go. I've seen a six inch diameter rotor come apart and slice through half inch steel plate like it was butter.

Storing energy in flywheels is a fine thing, but most of the composite based systems have not measured up to their theoretical performance capabilities. The primary reason is there are some fundamental problems in distributing the stress in fiber wound composite structures that make it hard to realize the significant gains that these materials should be able to provide. If you don't use very high strength composites then the overall energy density of the system isn't as high (comparatively) and these systems don't make as much sense (like in transportation systems).

As an example of how things work in the real world, as opposed to the theoretical, I was involved with the design of some metal matrix composite compressor rotors for an Air Force sponsored research program some time ago. To give you an idea here, the base matrix material was titanium and the windings were boron fiber. This stuff was way up there in the fundamental strength to weight ratio department, and we had all kinds of coupon testing that guided our design process. While we should have been able to save a lot of weight and improve the structural margins, in the end, that didn't happen. What we found was that, since the reinforcing material had a very high modulus, the stress was then concentrated in the fibers at the very bottom of the windings, and they broke due to overstress early on. Basically, if you can't get the stress distributed in a manner that you can control (like you can do with a monolithic metallic material), then a lot of the gain in basic properties is often lost.

Other issues, such as low loss bearing support systems are also potential issues, but frankly I haven't been involved in them lately so I can't comment on them. I have done air bearings in high speed turbomachinery (50,000 rpm plus) and while it was successful, it wasn't as easy as we thought it was going to be. Magnetic bearings work, but not as well as the proponents would have you believe (been there done that too) and they weren't cheap either.

The bottom line is that these systems can easily get very expensive, and in many cases there are less expensive ways to store and retreive energy. Remember that any energy storage system has to be considered based on simple economics. What does it cost, how efficient is it, and how long does it last. With that in mind it gets a lot easier to see why they store water upstream of Niagara Falls every night and let it out during the day. Storage of water at an elevation is a great energy storage concept, and I'm sure there are plenty of others. My take on it is that flywheel storage is simply too expensive a proposition and that's why it isn't being (to my knowledge) used in any material way.

[brian eiland]

Originally Posted by Marmot

Besides, converting that power in the days before high power/high voltage industrial IGBTs and computer controlled multi-megawatt power management systems might have been problematic.

No doubt the idea of this method appeared before the technology to really control it in big scale.

But interestingly it was a bus (electric I believe) in Switzerland that first utilized an 'automotive' flywheel energy device to collect regenerative power from braking, and then utilized that recovered energy to help with acceleration from the stop.

Originally Posted by Marmot

Maybe they had second thoughts about trying to corral the energy in a multi ton wheel spinning at a rate sufficient to deliver a useful amount of power for a reasonable time.

Originally Posted by Grecko

Both true, when metal parts get really big, it becomes difficult to insure that there are no significant flaws in the material that could become cracks and eventually let go. I've seen a six inch diameter rotor come apart and slice through half inch steel plate like it was butter.

I believe they realized the potential danger of this spinning metal wheel, and that was the reason for placing it in a concrete structure well below ground that could absorb that disintegration.

[brian eiland]

Originally Posted by Grecko

Storing energy in flywheels is a fine thing, but most of the composite based systems have not measured up to their theoretical performance capabilities. The primary reason is there are some fundamental problems in distributing the stress in fiber wound composite structures that make it hard to realize the significant gains that these materials should be able to provide. If you don't use very high strength composites then the overall energy density of the system isn't as high (comparatively) and these systems don't make as much sense (like in transportation systems).

There was quite a bit of experimentation with carbon fiber wheel optimization, and of course being a much lighter material the speeds that needed to be attained in order to store significant energy were approaching 100,000rpm. For the most part when these composite flywheels came apart they sort of 'fuzzed' rather than broke off in pieces. This made it a little safer.

Taken to another step, there were experiments that took one larger wheel system and broke it down into 2 or 4 counter-rotating smaller flywheels, such that they would utilize a smaller diameter wheel for these very high rpms, and they would have a gyro canceling effect.

Other issues, such as low loss bearing support systems are also potential issues, but frankly I haven't been involved in them lately so I can't comment on them. I have done air bearings in high speed turbomachinery (50,000 rpm plus) and while it was successful, it wasn't as easy as we thought it was going to be. Magnetic bearings work, but not as well as the proponents would have you believe (been there done that too) and they weren't cheap either.

Bearings and seals were a stumbling block to maintaining a real good vacuum chamber. Interestingly , the friction created by the air through which the flywheel moves is the reason so much power is required to spin up the gyro stabilizer flywheel. In space craft this isn't an issue, as space is a vacuum. But still you need bearing lubrications that will stand up to this usage.

While ordinary grease may work fine for most bearing arrangements, it literally boils or evaporates in a vacuum. So you need spacecraft grease. While the air is a liability to the high speed rotor, its an ally for bearings Air conducts heat and cools the bearings. In the vacuum you need to employ other methods.

That is some of the problems that Seakeeper has sought to solve, and that might be applicable in future flywheel storage devices. If I remember correctly several experiments had these flywheel devices storing 10-12 times the energy of the best batteries at the time. And they were forecasting greater gains.

The bottom line is that these systems can easily get very expensive, and in many cases there are less expensive ways to store and retrieve energy. Remember that any energy storage system has to be considered based on simple economics. What does it cost, how efficient is it, and how long does it last. With that in mind it gets a lot easier to see why they store water upstream of Niagara Falls every night and let it out during the day. Storage of water at an elevation is a great energy storage concept, and I'm sure there are plenty of others. My take on it is that flywheel storage is simply too expensive a proposition and that's why it isn't being (to my knowledge) used in any material way.

Pumping water back up to a higher level reservoir is great device, but it doesn't fit in our new battery word, nor our hoped for electric commuter car. A motor/generator unit that could spin up the flywheel at night, and then supply electric for the several in-wheel electric motors and give a range of 300miles would be fantastic. And this flywheel might be infinitely rechargeable

[Grecko]

Originally Posted by brian eiland

A motor/generator unit that could spin up the flywheel at night, and then supply electric for the several in-wheel electric motors and give a range of 300miles would be fantastic. And this flywheel might be infinitely rechargeable

If you do a quick calculation on how much power it takes to drive even the most rudimentary car for 300 miles, even at low speed, you will find that the amount of energy you need to store is way way beyond anything that any known material could supply in terms of strength to weight ratio when configured as a flywheel. When I was working on EV's in college, the Allied Signal folks were pushing this technology and it was better, but not hugely so than batteries in terms of energy density. Batteries have gotten a lot better lately, but real high strength materials (at lest in terms of usable strength) aren't that much better, so I'm not sure that this technology is really keeping pace with batteries in terms of energy density. Think 30 to 50 miles, maybe, if the car is really light and small and the flywheel is a big portion of the overall vehicle weight.

Where this technology is superior is in terms of power density. Energy density is how far you can go. Power density is how fast you can accelerate. That is, high power density is the ability to deliver or absorb lots of power in a short period of time. With high power density you can recharge fast and accelerate fast without high losses. For things like regenerative braking you need to dump a lot of energy into the storage medium quickly, and flywheels are good for that. In terms of miles traveled, when you consider the containment requirements, it doesn't fare quite as well.

[brian eiland]

MIT electric car aims to rival gas version

In its bid to make an electric car every bit as convenient and powerful as a gasoline model by 2010, a team of MIT students is currently faced with challenges like wiring together about 8,000 battery cells and using 350 kW to quick-charge its prototype, which uses a 250-hp 187-kW AC induction motor originally designed for an electric bus. Some (very) creative thinking and shrinking just might get them to their end goal. A great report from Network World.

http://www.networkworld.com/video/?bcpid=1343712625&bclid=1363192037&bctid=303137790 01

This stuff fascinates me, and I'm trying to pass that enthusiasm onto my Thai stepson who won a scholarship to study physics at a school in Boston that is only second to MIT

[Marmot]

The air car saga continues ... The post linked below is in Italian but Google offers a translation for the linguistically challenged like me.

http://www.giornaletecnologico.it/hi...43932a0507aae/

[brian eiland]

Originally Posted by Grecko

If you do a quick calculation on how much power it takes to drive even the most rudimentary car for 300 miles, even at low speed, you will find that the amount of energy you need to store is way way beyond anything that any known material could supply in terms of strength to weight ratio when configured as a flywheel. When I was working on EV's in college, the Allied Signal folks were pushing this technology and it was better, but not hugely so than batteries in terms of energy density. Batteries have gotten a lot better lately, but real high strength materials (at lest in terms of usable strength) aren't that much better, so I'm not sure that this technology is really keeping pace with batteries in terms of energy density. Think 30 to 50 miles, maybe, if the car is really light and small and the flywheel is a big portion of the overall vehicle weight.

It has been a long time since I spent time on this flywheel subject, but I thought I remember quotes of 10x energy density of flywheel storage over best batteries of that day.