Compressed Air Generators: Good or Bad for Home Energy Storage?

[rghusted]

Have you heard of those cars that are going to utilize a compressed air generator? Well, it seems to me that compressed air is a good and cost effective medium to store energy. Am I wrong? I’m wondering if you think whether a device that can be plugged into your home outlet that stores excess electric capacity at night as compressed air and redistributes it the next day into the home is one, feasible and two, would it be cost effective. En masse, this would effectively increase our capacity without building new power plants. Good idea? Bad idea?

 

[mgb_phys]

Feasible yes - efficent probably not.

The difficulty with using compressed air is that as you compress it you heat the air up (if you double the pressure you also double the absolute temperature).
If you just release this heat then you have wasted a lot of energy, when you expand the air again to generate power it will get extremely cold. In some applications this might not matter - such as in a car, where it is going to be released into the atmosphere.
The alternative would be to keep the heat in the gas so when it is released it is returned to room temperature, but now you have the problem of building a tank that can hold very high pressures at very high temperatures.

Finally compressed air doesn't store very much energy, a 12 litre tank (typical scuba cylinder) at 200bar is only about 1MJ (0.3KWhr) by comparison 12litres of gasoline would be 500MJ

It's not a complete scam - compressed air engines are used in industry where you have a lot of compressors anyway and especially where a spark is dangerous such as in mining.

 

[binzing]

I believe there's at least one company (from France IIRC) that has a car that runs on compressed air.

 

[mgb_phys]

I don't know if they have actualy built/sold any - but it's not a completely silly idea.
The energy density is lower than gasoline, but not far off batteries - they are using carbon fibre tanks so the energy/mass might be better than a lead acid battery.

They (aircar) seem to be using a piston engine - they describe something about having twice the efficiency of expansion with quasi equal temperature which is either a dodgy translation or a scam.

One issue you are going to have is heating - not only do you not have any energy to warm the car, but you must have quite a temperature drop when the 300bar air expands into the cylinder. So in England you would have a large block of ice attached to the car - perhaps in the south of France the refrigeration might be welcome.

As I mentioned, compressed air trains have been used for >100years, it's not much of a modification for a steam train.

 

[Cyrus]

I use compressed air in my car all the time. Compress, inject fuel, burn. Repeat ad naseum.

 

[tribdog]

My car uses compressed air in its tires.

 

[rghusted]

Do you think it merits any bonus points for utilizing a 100% renewable resource and is also zero-emission?

Here is a link to the AirCar website: http://zeropollutionmotors.us/"

Here is a link to an animation of how the engine works: http://zeropollutionmotors.us/video/CAE_animations.wmv"

 

[Hurkyl]

I’m wondering if you think whether a device that can be plugged into your home outlet that stores excess electric capacity at night as compressed air and redistributes it the next day into the home is one, feasible and two, would it be cost effective. En masse, this would effectively increase our capacity without building new power plants.[/color] Good idea? Bad idea?

Are you sure about your claim I have marked in red? We already do store excess electric capacity at night, en masse; why would your scheme be more efficient?

 

[edward]

Are you sure about your claim I have marked in red? We already do store excess electric capacity at night, en masse; why would your scheme be more efficient?

I think the claim is in reference to using off peak electricity during the night. We still have a surplus at 2:00 AM. I really have never seen an electrical storage facility except for hydroelectric.

Wiki's explanation of temporary electrical storage sounded a bit iffy except for hydroelectric.

http://en.wikipedia.org/wiki/Energy_storage

The Salt River project east of Phoenix buys off peak electricity off of the grid to pump water back up into Roosevelt lake during the night.

 

[mgb_phys]

Some windfarms use compressed gas storage - they were built on an exhausted salt mine/natural gas well so there is a vast air tight underground tank ready built.

 

[binzing]

Some windfarms use compressed gas storage - they were built on an exhausted salt mine/natural gas well so there is avast air tight underground tank ready built.

Thats interesting, I'd like to find out more about that.

 

[mgb_phys]

I think I saw it on daily planet / new scientist or similair

There is www.generalcompression.com[/URL] ,looks like it's more of an idea looking for money than an implementation.

Nice if it worked - even without the storage I could see the advantage of having the windmill power an air/hydraulic pump and have all the generating plant at ground level - it would also solve the problem of matching turbine to constant generator speed.

 

[russ_watters]

Do you think it merits any bonus points for utilizing a 100% renewable resource and is also zero-emission?

Only if it actually does use 100% renewable and zero-emission energy. AFAIK, there is nowhere in the world that can claim that for a large population - France would be the closest, being that it is all nuclear.

See, air isn't a fuel, these devices are basically just very inefficient batteries.

 

[tribdog]

Some windfarms use compressed gas storage - they were built on an exhausted salt mine/natural gas well so there is avast air tight underground tank ready built.

arrgh!

 

[redargon]

Do you think it merits any bonus points for utilizing a 100% renewable resource and is also zero-emission?

Here is a link to the AirCar website: http://zeropollutionmotors.us/"

Here is a link to an animation of how the engine works: http://zeropollutionmotors.us/video/CAE_animations.wmv"

ummm... you have to get the energy to compress the air from somewhere, ie a powerplant. therefore, the engine may be 100% emmission free and use a renewable resource, but the powerplant used to generate the energy to compress the air is probably not, unless it is run by a wind turbine, solar panel, etc. And even these are made from non renewable resources.

 

[rghusted]

Well, it looks like this technology exists. Russ Waters is right in that it is basically a battery. It would take what is essentially electric capacity that is lost at night and store it in an environmentally friendly way. While the original source of electricity may not be clean, the ability to add capacity to an electric grid without adding to the waste produced makes this zero-emission. I guess the question lies in whether this approach of power management offers any cost benefit. It does seem that while the US may have some capacity to store excess electricity, other countries where rolling blackouts continue to occur, this may be a very cost effective way to effectively increase the electricity available within the country. I seems the answer lies in the world of number crunching.

 

[mgb_phys]

it is basically a battery. It would take what is essentially electric capacity that is lost at night and store it in an environmentally friendly way

Still not sure it's viable as a storage scheme.
Energy density is close to simple battery (lead acid) technology.
Safety / inspection costs are similar, replacing batteries every few years vs removing and hydrostatic testing cylinders. Recycling gas cylinders might be better than lead acid batteries.

The big loss is the inefficiency and cost of turning compressed air back into electricity via a motor and generator. Data centers looked at using massive flywheels instead of batteries as power backup, although the power density is higher and they last for ever the cost and complexity of generating the electricity was too high.

 

[edward]

The Fench vehicle obviously does work. But there are always drawbacks. It requires the air pressure to be very high.

The e.Volution is powered by a two-cylinder, compressed-air engine. The basic concept behind the engine is unique (see this page for details) -- it can run either on compressed air alone or act as an internal combustion engine. Compressed air is stored in carbon or glass fiber tanks at a pressure of 4,351 pounds per square inch (psi). This air is fed through an air injector to the engine and flows into a small chamber, which expands the air. The air pushing down on the pistons moves the crankshaft, which gives the vehicle power.

http://auto.howstuffworks.com/air-car1.htm

 

[mgb_phys]

The Fench vehicle obviously does work. But there are always drawbacks. It requires the air pressure to be very high.

For a vehicle it has big advantages over lead-acid, it's lighter if you use CF tanks and you can drive a piston engine directly (the previous comment was to the OP about using it for large scale grid power storage)

The pressure is 'only' 290Bar - a bit higher than a scuba cylinder but about the same as an industrial gas bottle.
Carbon-Fibre tanks are the break through. They should be cheaper to make and ship than steel / aluminium once you get the production volumes up, they don't corrode and if the did fail they should just rip open rather than throwing shrapnel everywhere.
Not sure how easy it is to inspect them - metal cylinders get periodic induction/ultrasound crack checking.

People have looked at CF before, but for scuba they have the disdvantage of being too light when empty so causing buoyancy control problems. For industrial gas cylinders they aren't as robust when being thrown around. I think some emergency services are using them for breathing apparatus.
With no mass market there wasn't the scale economies and so they remained very expensive.

 

[edward]

For a vehicle it has big advantages over lead-acid, it's lighter if you use CF tanks and you can drive a piston engine directly (the previous comment was to the OP about using it for large scale grid power storage)

The pressure is 'only' 290Bar - a bit higher than a scuba cylinder but about the same as an industrial gas bottle.
Carbon-Fibre tanks are the break through. They should be cheaper to make and ship than steel / aluminium once you get the production volumes up, they don't corrode and if the did fail they should just rip open rather than throwing shrapnel everywhere.
Not sure how easy it is to inspect them - metal cylinders get periodic induction/ultrasound crack checking.

People have looked at CF before, but for scuba they have the disdvantage of being too light when empty so causing buoyancy control problems. For industrial gas cylinders they aren't as robust when being thrown around. I think some emergency services are using them for breathing apparatus.
With no mass market there wasn't the scale economies and so they remained very expensive.

Thanks for the informative post. I apologize for slipping in the air powered car bit. It wasn't meant to be a reply to the previous post. I was thinking that the car had been mentioned before.

Come to think of it using off peak electricity to produce compressed air for air powered vehicles might be a feasible idea.
OH OH there I go again bringing up that air powered vehicle.

 

[mgb_phys]

Come to think of it using off peak electricity to produce compressed air for air powered vehicles might be a feasible idea.

For any sort of Plug in Vehicle you would want to use off peak power, especially since that is generally overnight, so for a commuter vehicle it's perfect.
You could fit a very small electric pump (or even an electric motor to drive the exiting air engine) and fill it slowly overnight, so you wouldn't have to worry about the temperature rise. It migth be even easier to fill quickly at a filling station than an electrically powered car.

If they can get the CF tanks to work (make them cheap and reliable) I can see it beating Lead Acid in some markets.

The main 'feature' is going to be heating/cooling. You have no spare electrical power for a heater (I assume they will have a small battery for lights/electronics) but you do have potentially a lot of cooling power from the expanding gas which you might be able to use as AC. As I said before, it will work great in the South of France but in Britain you are likely to have a big ice cube form around you!

 

[russ_watters]

Well, it looks like this technology exists. Russ Waters is right in that it is basically a battery. It would take what is essentially electric capacity that is lost at night and store it in an environmentally friendly way. While the original source of electricity may not be clean, the ability to add capacity to an electric grid without adding to the waste produced makes this zero-emission.

You seem to be under the impression that a power plant uses as much energy (ie, burns as much coal) at night as it does during the day, while producing less electricity. This is not correct. The amount of energy produced determines the amount of energy used. So this car will still require more coal, oil, nuclear fuel, flowing water, etc. to be consumed. The efficiency lost by running at part load, is relatively small.

 

[rghusted]

You seem to be under the impression that a power plant uses as much energy (ie, burns as much coal) at night as it does during the day, while producing less electricity. This is not correct. The amount of energy produced determines the amount of energy used. So this car will still require more coal, oil, nuclear fuel, flowing water, etc. to be consumed. The efficiency lost by running at part load, is relatively small.

No, I'm coming from the assumption that electricity is cheaper at night because there is a surplus of it. I think power companies do reduce supply according to demand where it is economical to do so, but their pricing structure seems to indicate that there continues to be a surplus supply of electricity at night. Perhaps I'm mistaken?

 

[mgb_phys]

I think power companies do reduce supply according to demand where it is economical to do so, but their pricing structure seems to indicate that there continues to be a surplus supply of electricity at night. Perhaps I'm mistaken?

Not quite a surplus but it is a problem for power companies.
Most types of power station operate most efficently at their design capacity and the time taken to ramp up or down differs.
Coal fired stations normally need to be big and run full out, it is very inefficent to reduce their output and almost impossible to turn them off (without a rebuild).
Nuclear can be turned on and off quickly but since the cost is all up-front in constructing them (the fuel is cheap) it makes sense to run them full power.
Generally companies use natural gas (methane) for quick response load balancing - but this is the most expensive.
Unless they are lucky enough to have hydro which is ideal for load balancing.

So if you are a power company you have to decide to run wasteful amounts of cheap 'baseline' load or run just enough but have to use more gas to handle peak load.

There are a few ways of storing large amounts of energy (pumped storage schemes) but these are expensive and limited in where you can build them. Renewable power that comes from sources you can't predict (wind) makes it worse.

A widely distributed network of power storage (eg. plugin electirc vehicles) would help the power company - especially if in return for a cheaper rate they could control the supply of power, and even suck power back from them if needed. So you are right, a wide adoption of PEV would reduce power wastage overall.
How much would depend on a whole bunch of technical and market factors

 

[signerror]

Only if it actually does use 100% renewable and zero-emission energy. AFAIK, there is nowhere in the world that can claim that for a large population - France would be the closest, being that it is all nuclear.

Yes there are, if you count only electricity - Norway and Iceland are in excess of 99% zero-emissions electricity. Norway is entirely hydroelectric; Iceland is mostly hydroelectric with a large geothermal fraction. Sweden and France are extremely close, with less than 10% fossil fuels; Sweden is about half each nuclear and hydroelectric, while France is 80% nuclear and 10% hydro.

http://www.iea.org/Textbase/stats/surveys/elec_archives.asp