Storing Green Energy

It looks interesting. So it'll be work as something like as jet engine.

But I'm puzzled the estimated efficiency of such a way to storage energy - up to 20% for worst case. Of course 20% is better than nothing, but wouldn't be more efficient to use surplus of energy at low-demand hours for producing hydrogen by means of electrolysis?

I thought hydroelectric power stations during off peak times, used the turbines to pump water UP to the reservoir again.

I should imagine that is quite an efficient storage system, maybe 50% efficiency?

Up to 85% is the best case I've heard claimed for off peak hydro-electric. The low efficiency of the compressed air system is due to heat lost from the compressed air. If you pumped water to compress the air and then drew water to run the turbine the efficiency would be much higher, an oil layer would prevent the trapped air from dissolving into the water. By compressing trapped air the process becomes nearly isothermal and the net efficiency would be much better. Air will dissolve or disperse into rock though. I don't think much of these schemes but they may be best for some locals where the elevation needed to make hydraulic off peak work is just not available.

In order to use pumped storage, the hydro plant must have two reservoirs at significantly different levels. There are at least a couple places here in California that do that, and I'm sure there are others elsewhere.

I have often wondered about their efficiency. Anybody have any solid data?

This came up years ago with the concept of delivering high pressure compressed air to deep underground cavities to be used later in reverse to supply the stored energy to recover as electricity.

I can't much remember the details but the concept of using the deep underground natural vaults as compressed air storage chambers.

Nothing new under the sun?

Efficiency and cost effectiveness as we know it are not considerations once the government starts subsidizing projects. All they are looking for is "renewable" Of course it is woefully inefficient and ghastly expensive, but where does that fit into the politically correct equation?

There are some significant restrictions for building two separate-level reservoirs of water for storing energy:

1) there are only a few places on the globe where such projects could be implemented;

2) ecological influence on environment of artificial erected reservoirs and canals is the controversial issue itself;

3) this kind of storing energy is believed to be very pricey.

On the contrary, proposed here the air pre-compressed approach likely:

1) could be applied everywhere. [Even if its implemented on the ground placed facility it's thought to be more compact insulated construction to compare with huge lake]

2) is to considered as minimal in terms of ecological risks [on my opinion];

3) wouldn't exceed significantly two-reservoir project [i have not any idea about of a complete cost, but I guess].

Beyond any doubts, the pre-compressed air is to deliver more effective conditions for jet/fanjet engine operating.

One more analogy: turbine which is used in car's engines. Isn't it effective for short time "off-peak demand" mode when driver is compelled to drive fast.

I'm not clear supporter of such technology, I simply tried here to weigh out all pluses and minuses with calm mind.

The "key inventor" should have done a patent search. U.S. Patent Number 5,832,728 shows how to store nergy with compressed air with a very high efficiency. 20% is the number you get from unimaginative bureaucrats at the Dept. of Energy. Based on experimental work in Germany, 80% should be easy.

How many times do we have to tell everyone how inefficient compressed air energy storage is?????!!!!! Sheeesh!

And then, according to the article, after they store the compressed air, they are going to heat it with natural gas before they run a turbine with it so they save natural gas over using the ng straight for running a turbine? Ludicrous! Where do these bozos come from?! A natural gas turbine is 85% efficient. Using the compressed air, after everything is factored in will be LUCKY to acheive 8%. A TEN TIMES ENERGY LOSS!!!! During off-peak hours you simply burn less ng. That is where the energy saving comes from. Do any of you actually believe that any utility company is going to blow billions of dollars on something like this so they can happily throw away 90%+ of their energy output?

So. That still leaves us with the question of how can you make 'green' utility output more continuous by storing surplus energy during peak production hours. Right now you can't and maintain any kind of reasonable utility costs. Currently, these resources are toned down at peak production times or low usage times to prevent overload - this also aids in maintenance and reduces replacement costs. Is compressing air better than just turning things down and lowering production to meet demand? Unfortunately, due to physics and reality, this compressed air concept is such a waste of energy that by the time it is used to regenerate electricity, it is simply not worth the time or the cost. The electrical production would be a pittance compared to the basic output and would hardly be noticed. It would take centuries to recoup the investment if it could even be done. This concept would actually increase the cost of these already more costly forms of energy production.

It is time for the 'green energy advocates to stop being stupid and look at the simple facts and costs. I don't want to have to pay good tax dollars for someones stupid idea that only ends up costing everyone even more and provides no measurable benefit. Frankly the way it is being handled right now is far better than doing this idiotic compressed air thing. Even battery storage is better. It would be far more efficient, far cheaper and far safer. There are people working on real solutions, but we are some years away. Be patient and be smart.

Thankfully the utility companies are not staffed by idiots. This is the reason why they did not start using the compressed air idea decades ago (the technology was there back then). However, the same cannot be said for politicians - most especially the left wing dolts who have no rooting in reality and are blown about by every light-headed idea.

Most the physicists I know like to look at the real world evidence before saying something can't be so.

In the US, a few utilities have been using compressed air storage for decades. They use the compressed air to augment or bypass the compressor of a gas turbine, allowing more of the energy extracted by the turbine to be used to generate electricity at times of peak demand, instead of using some of it to compress air. Of course it's ludicrous, but the accountants and stock holders haven't found out yet.

If a compressed air system is only 8 per cent efficient (the U.S. Dept. of energy thinks 15 per cent), a physicist should ask himself where 92 per cent of the energy went. The answer is, usually, it was dissipated in the atmoshere as a result of cooling the compressor and the compressed air. Does that impress you as dumb engineering? Sometime before 1930, some clever Germans ran an experiment to compare diesel-electric power transmission with diesel-pneumatic power transmission. They found the air compressor-air motor was so much more efficient than the electric generator-electric motor that they saved more than 25 per cent in fuel for the diesel. I have not been able to find the details (probably held as a trade secret), but it is asserted they used heat from the diesel exhaust to reheat the air, and it is suspected they cooled the compressor by injecting water, which turned to steam as it cooled the compressor and then reheated the air as the air expanded and the steam condensed. The only substantial loss of heat (the only visible radiator in photos) was for cooling the diesel engine and, of course, in the diesel exhaust, after it had warmed the air. Reference U.S. Patent Number 5,832,728.

Since the Germans did not store the compressed air (it was used to power a "steam" locomotive), they didn't worry about heat loss by conduction, but that need not be a big deal if the air-steam mixture is stored hot. If the storage vessel is big enough, in terms of surface to volume, losses will be manageable, and for smaller scale applications, such as road vehicles (an xprize entry?), we know how to insulate tanks. One can store about 30 KW-hr in a cubic meter, so a compressed air vehicle with a range of 1-200 miles is entirely conceivable. If the compressor is powered by electricity, the over-all efficiency should be higher than a gasoline powered hybrid or a even a battery-driven car, and if the compressor is driven by a "greener source", such as a wind turbine, the whole system is "earth friendly". As an energy storage medium, air has several economic advantages over electrochemical batteries. It scales up nicely without disrupting commodity markets for scarce and toxic materials. No expensive copper need be used. It need not be recycled, nor cleaned up after a "spill." Compressed air tanks are not bombs; they leak rather than explode, and, of course, the leaking air is non-toxic. Would firemen wear them on their backs if it were otherwise?

If I had a dirty coal-fired generating plant and wanted a "greener" source of electricity, I would look to wind turbines. Use the turbines to compress air/steam and pipe the mixture, with or without some storage for windless days, to the boiler of the old plant. The turbine, generator, switch gear, etc. are already paid for. The coal pile and steam condensers/cooling tower are no longer needed. The fuel (wind) is free and the noxious emissions zero. If there is a nearby deep lake or a flooded mine, the air could be stored in a big "balloon", contained by the hydrostatic pressure of the water. By the way, if a similar plant is located in a desert or on an island, the condensing steam can be filtered out and used as potable water.

If there are those of you out there who persist in thinking air is somehow an inherent destroyer of energy, I will be happy to correspond with you about it.

"Most the physicists I know like to look at the real world evidence before saying something can't be so."

Well I am not a physiscist but a mechanical engineer and almost every manufacturing and research facility has air compressor(s), driers, and storage tanks for compressed air. We are not discussing such storage here.

The discussion centers on mass storage of compressed air to store the electrical energy from wind turbines when it is not needed by the grid to hold that energy for later release to generate power for the grid when needed.

Will you please identify all of the CAES plants/facilites that you know of in the US. There is suposed to be one in AL but have not been able to find any information on it or its location. Allegedly there is also one in TX.

A second CAES facility was to have been built in IA as of early 2007 but inquiries to the contact person have not been answered nor have they been returned.

Compressed air is a versatile power transmission system for special and limited applications but it is NOT a substitute for the grid and/or electrically operated equipment.

If you have something new and pertinent to share with us please do so. Otherwise please refrain from off topic comments.

Gentlemen,

This string has hit upon the essence of the greatest renewable energy stumbling block. It is a wall that contemporary engineers have encountered for thirty years - Storage of energy for when the "sun don't shine" and the "wind don't blow."

R&D engineers and inventors have worked with solar power experimentation significantly since the seventies. we are all searching for the Holy Grail of "green," that is alternative energy that burns nothing. Many new ways of collecting the energy have been offered. But there is a great need and reward for a significant discovery of storage medium.

Electricity can only be stored with batteries. Unfortunately there have been few significant breakthroughs in battery power. Though few understand why, one presidential candidate has even offered a reward for a new battery. Investors abound for those that can come up with an innovative battery or storage medium. The funding is there if one can show a working model.

But it has not happened. The problems have been the realities of power storage. Even with the potential for reward, have you seen the solar products offered? They are mostly lame with little light and less staying power. And they all need batteries. The overall return on our dollar is poor, as is the addition of toxic waste with battery disposal.

And this is the problem for most home alternative generation; the products so far make little sense to the consumer – The real judge. They vote with their checkbook and it isn't opening up without a product that makes sense. That means a short payback with a profit.

Oh, and did I mention that it must be automatic and maintenance free, or the maintenance must be included with the pay back numbers? With an expensive battery bank needed for alternative DC energy in the home, a knowledgeable person and high maintenance is required. Pulling a wind turbine down off a hundred foot tower for maintenance is out and a battery bank can be destroyed with a careless error. Think that will go over with the consumer?

A few years ago many parabolic heliostats were combined with oil for sensible heat storage in a utility size power plant in the Mojave Desert. But less efficient oil as-thermal-mass is being replaced by more-efficient molten but still inadequate and very corrosive salt. Experiments with other forms of storage including kinetic are widespread. But so far they have had modest success even though highly financed by the marketing potential of "Green" trendsetters

The almighty dollar is behind the momentum of the marketing phenomenon created and maintained by the influence-hungry greenies likeAl Gore with his home and leer jet consuming ubiquitous power. But I digress.

The realities of manifest ownership of the private real estate upon which sits an inadequate grid must also be analyzed. If constructed properly, the grid could be a leveling factor or a form of storage. Utility sized solar thermal or wind generation located in ideal parts of the country must be able to send electricity long distances. Concomitant difficulties inherent to direct current and its inversion must be added to the problem.

Compressed air stored in caverns has many inherent problems including the need for grid excellence, the cost of initial tanking, the risk catastrophic loss of compression and inefficiency. Storage of power with kinetic, compressed air or heat with thermal mass (oil and molten salt and aluminum), has been exploited exhaustively. Most are prohibitively inefficient.

Respectfully Cornelius

The first problem with any of these ideas for storing the excess energy is the requirement that there be two complete generating facilities. One uses solar or wind power, the other uses some kind of pressure storage, water storage or hydrogen storage to produce power when the first is not. That is expensive.

Compressing air produces a lot of heat [waste energy], takes a lot of energy to compress enough, requires strong containment, which can be expensive. Making hydrogen using the cleanest method, electrolysis, takes a lot of electricity and water. Hydrogen storage also requires pumping and compressing which also wastes energy as heat. H also does not stay contained well, it leaks and it makes its containers brittle. Brittle containers break and "BOOM!", the whole place is gone.

The only other way to store electricity is batteries and they are really expensive.

So the only ways to store green energy are expensive. Thus "green electricity" is more expensive than electricity from nuclear, coal, hydro or NG plants. Not being independently wealthy I know that if my electricity cost as a percentage of income were to rise, then I would have to lower my standard of living. Millions of people would have to severely cut back on use of electricity in their daily living. I don't think being "green" is worth the extra cost.

What would make sense is to store electricity as electricity in banks of relatively small, cheap, high energy batteries. I don't say all research into other areas is worthless, but we need a program that will emphasize battery or other electrical storage development so that the cost of "green" energy will not harm the way we live. Until we have such a system, shoving "green" energy down our throats for questionable reasons will be counterproductive.

I do support energy alternatives and renewable energy, but it has to make sense monetarily and in its impact on the cost of living.

"The first problem with any of these ideas for storing the excess energy is the requirement that there be two complete generating facilities. One uses solar or wind power, the other uses some kind of pressure storage, water storage or hydrogen storage to produce power when the first is not. That is expensive."

If the wind power compresses air directly, no electricity or expensive generator is involved.

"Compressing air produces a lot of heat [waste energy], takes a lot of energy to compress enough, requires strong containment, which can be expensive."

It's only waste energy if you waste it. It takes a lot of energy to compress it because you are storing a lot of energy. You may use that stored energy to generate electricity, or you can use it directly. Given hot compressed air (no waste heat), you can heat or air condition a building with no electricity involved. The compressed air can power vehicles, elevators, machine tools, etc. "Strong containment" is a lot cheaper than batteries, especially on a large scale. With air, one can store a few megaWatt-hr in a hole in the ground. What would that cost with batteries?

Relying on commercial components, which haven't changed much in a century, is like relying on a horse to pull your carriage. Think how to do it better.

"If the wind power compresses air directly, no electricity or expensive generator is involved." Fine, you have these big storage tanks of compressed air. Do you use it to turn your windmills or do you use it to run a big air motor that turns a generator? You have some wind turbines which produce electricity and some which compress air?

I believe they would all produce electricity and some of the electricity would run electric motors which would power compressors. The compressors might possibly be used as air motors to turn generators. In any case you would need both wind turbines and compressed air powered generators. Which is my point, you need two generating sources instead of one.

The same goes for all the schemes to store solar-electric power as Hydrogen, by pumping water to reservoirs or as compressed air. You need a second system to make electricity from your storage method. Two systems instead of one.

The air compression storage and air motor driven generator could be far from the wind turbines. Yes, some of the waste heat could be used before being let into the atmosphere. "The compressed air can power vehicles, elevators, machine tools, etc." However pneumatics are less efficient than electrics and much more expensive and difficult to install. When you can convert a standard IC gasoline car to travel the same distance on on tank of air as it does on gas, then air powered cars will be practical as something other than expensive toys.

It is not the ability to store energy so much as the efficiency and practicality of the method used, plus its ultimate cost to the consumer.

<<Yes, some of the waste heat could be used before being let into the atmosphere. "The compressed air can power vehicles, elevators, machine tools, etc." However pneumatics are less efficient than electrics and much more expensive and difficult to install. When you can convert a standard IC gasoline car to travel the same distance on on tank of air as it does on gas, then air powered cars will be practical as something other than expensive toys.>>

1. There need be no waste heat. Yes, when air is compressed, it gets hot, but there is no need to waste that heat. There is no need to "let into the atmoshere."

2. Pneumatics need not be less efficient than electrics, nor need they be more expensive.

3. The auto xprize wants a four-seat vehicle which can go 200 miles at 100 MPG of "gasoline equivalent," with a max price in production of $80,000. Given the price of batteries, an electric car can't win. However, one could take a production 4-seat pickup truck, fill the back with insulated SCUBA tanks or equivalent, and do a few modifications on the installed IC engine to run on air. Such a vehicle could win the prize ($10,000,000). A purpose-built vehicle could do even better.

Granted, gasoline can give you 400 miles without stopping (if your bladder can stand it), but 100 miles with compressed air is easy, and the tanks can be refilled about as quickly as refilling a gasoline tank, assuming that gas (air) stations are attached to petrol stations. If you use wind turbines or off-peak electricity to compress the air the cost of fuel is small, and it's not imported. Performance, acceleration, with air is unmatched. While the electricity generating plant may generate pollution, the car does not. When compressed air is compared with batteries, air is cheaper.

"If you use wind turbines or off-peak electricity to compress the air the cost of fuel is small, and it's not imported. Performance, acceleration, with air is unmatched. While the electricity generating plant may generate pollution, the car does not. When compressed air is compared with batteries, air is cheaper."

You have made some pretty rash assertions. Can you support them with rational facts, calculations and costs figures?

"Pneumatics need not be less efficient than electrics, nor need they be more expensive." Try running air pipes throughout a multi-storey building for all your pneumatic motors as compared to running wires for electricity. Try looking at the actual total cost of each system. Electric is cheaper and easier.

The auto xprize as I saw it did not consider the cost of the vehicle, just that it could go 200+ miles on the same amount of energy as is contained in 2 gal of gasoline. That energy has been computed for its equivalents in other fuels. In a compressed air engine you must consider the energy needed to compress the air, not just the energy of the compressed air. Using hydrogen as a fuel must also add in the energy used to make the electricity used to make the hydrogen. If it was as simple as you say an air-powered car would already have won the prize.

I will admit that as long as you have a hand-cranked compressor you will never "run out of fuel" although you may run out of oomph. However your car will have to have a generator for the onboard electricals and heat, which means a battery. What is your choice of engine, piston, Tesla Turbine, vane/bladed turbine, Lysholm expander? Are you using a compound engine? What happens as your tank pressure drops? Do you just go slower and slower down the road, becoming an obstacle to traffic?

Consider the size of a service station compressor to repressure just 60 cars per hour in the middle of summer. Pretty big. How do you use the excess heat? No place needs it when it's 90° in the shade. Using it to make AC for buildings still leaves a lot of heat to dump into the atmosphere. When it's -18° them the excess heat is very useful. Also when all the cars run on compressed air, you will need more generating capacity, more plants and still more pollution, just in a different location. Over there where the poor people live, not in your backyard.

I would like to see support for the claim of a hundred miles on a tank of compressed air. What kind of tank how much psi. What kind of vehicle and what is the drive mechanism.

One other point, the wind does not conveniently blow on off peak hours.

There is good evidence that sustainable technology is fertile grounds for the scam. The e.Volution compressed air car . . . http://indicview.blogspot.com/2007/03/evolution-air-car-in-previous-avatar.html

. . . is just one more example. It won an entry on the popular tech site howstuffworks.com before it appeared on any street. The e.Volution made waves back in 2000. Zero Pollution Motors was to manufacture the car in South Africa. Their claim: "There are currently two factories in France, with the first models expected on the streets later this year."

Eight years later, true believers still dub the e.Volution "marvelous" and "creative" alternative engineering." But ten years after its conception, we have yet to see the car. The website for Zero Pollution Motors shows a site under construction, and though there is a listing for an office in New York City it also mentions the same under-construction website.

Just like so many big promises we keep hearing "next year" "soon" and "launch is imminent," but where is the beef? We have yet to see these breakthroughs in stores. Many of alternative energy products are lame and a poor value for the money. People are just not buying unless there is better bang for the buck.

It seems like "green" is a great opportunity for charlatans who see green as their favorite color. We must all be careful.

There seem to be skeptics, which is fine.

Number 20 says: "There is good evidence that sustainable technology is fertile grounds for the scam. The e.Volution compressed air car . . ." Quite so, but that is like saying that Langley proved heavier-than-air flight is impossible. It helps to think the problem through.

#18 and #19 want numbers, size and cost.

First, re-read post #13.

My Prius gets about 50 mpg, and the efficiency of the prime mover, a gasoline engine, is probably only 20 per cent or so. So at the hypothetical 100 % efficiency, I could go 100 miles on 0.4 gallons of gasoline. A gallon of gasoline is equivalent to about 35 kWhr, so, in theory, one can go 100 miles with 14 kWhr. Perhaps a realistic figure would be 30 kWhr, allowing that an air car may be heavier, have more drag, etc., and, of course, there is friction, etc., so 100 % efficiency is impossible. 80% seems reasonable, so, again, in input of 30 kWhr is reasonable.

30kWhr is in the air. Let's be generous and assume 35 kWhr of electricity is needed to compress the air. (Remember, we are not throwing away the heat. An air cooled compressor might need 200kWhr) Multiply 35 by the price of electricity per kWhr, perhaps five or ten cents, and the price per hundred miles of "fuel" is $1.75 to $3.50, less than the price of a gallon of gasoline. There is no cost for biannually recycling the batteries, as air tanks have a very long life. OK on the costs?

How do we build the car? One-off cars are very expensive. Let's start with a store-bought vehicle. A pickup truck with four seats (extended cab) would cost what? $20,000? Less used. It doesn't have to be a truck, but it provides a convenient place to place the tanks and would be robust enough to carry the extra weight of the air tanks. It is unnecessary, since the cab is separate, to provide a bulkhead to shield the passengers from high velocity air, in the event a berserk Exxon employee puts a bullet into the tanks. (Air tanks do not explode, no matter what you saw in "Jaws". They leak. Our tanks will leak air and a cloud of moisture at approximately room temperature)

For simplicity, the drive-line remains stock, with a manual transmission needed mainly to get a reverse gear. Air and steam motors develop full torque at starting, so changeable gear ratios are not really needed. If you want to reduce weight, throw away the transmission and simply start the engine in reverse if you need to back up. That requires a more sophisticated valve system on the engine.

The engine which came with the truck is larger than need be, but keep it for simplicity. Sell the fuel tank, fuel pump, catalytic converter, radiator, etc. etc., keeping the alternator (for lights and radio) and possibly the A/C. (Heat can come from the hot air, and cooling can come from over-expanded exhaust, but is it worth the bother?) The most important modification is air-injectors to fit in the spark plug holes. Possibly existing diesel injectors could be made to do, electronically controlled, common rail -- I need to research that -- but at any rate, it is doable. (Rudolf Diesel did it) The "compression ratio" (actually expansion ratio) should be as high as possible. The cam should be modified so that the valves open for roughly half of each revolution, converting the engine into a single-acting "steam" engine. If you want, add a filter and holding tank to the exhaust pipe (maybe use the old fuel tank) to avoid leaving a cloud or snow behind on cold days, as there will be condensed steam in the exhaust. Controls, gauges, etc. are trivial.

The remaining question is more difficult: what is the weight and volume of tanks required? Considering that one is dealing with phase changes (water to steam to water) and such, getting the answer can be complicated. A bit of laboratory experimentation might be the most straight forward way to optimize the system. However, allowing that the compression and expansion of the gasses are nearly isentropic, with no heat lost, and given that the exhaust from the expander will, on average, have the same pressure and temperature as the compressor intake, one can assume that the work done by the expanding gasses, approximately, is the product of pressure and volume in the tank.

SCUBA tanks, readily available, can handle 3000 psi, approximately 200 bar, which is 20,000,000N/m2. A cubic meter would contain 20 million N-m or 20 million joules which is a bit more than 5 kWhr. Allowing that we have cylindrical tanks in a rectangular box, 30 kWhr of stored energy would seem to require more than 6 cubic meters, a rather large box, but possible to fit in the back of the truck. That's a sort of worst case, as doubling the pressure would halve the volume, and, more to the point, steam has a higher energy density than air, so the steam-air mixture would store more energy per cubic meter. Some old calculations, which I cannot reproduce here (no computer), put the energy density at about 30 kWhr per cubic meter (with the temperature limited to 300C, so as not to degrade the strength of the tank material). If so, 100 miles of wet air could fit behind the rear seat of a sedan. Ideally, the car would be built with a platform chassis made of air tanks, topped by a light car body. The vehicle would be immensely strong with a low center of gravity.

I haven't got the wholesale price of good air tanks, but I guarantee the cost would be less than high-tech batteries to store 30 kWhr.

If anyone would care to do the exercise of better computations, please post your results. Remember, as the air is compressed and warms up, it boils the water entrained in it, resulting in a mixture of air and steam. As the air expands, cooling, the steam condenses, reheating the air, so the starting point is ambient temperature air and water and the exhaust is ambient temperature air and water. Thermodynamically, it's a reversible process, "100 % efficient."

"If anyone would care to do the exercise of better computations, please post your results. Remember, as the air is compressed and warms up, it boils the water entrained in it, resulting in a mixture of air and steam. As the air expands, cooling, the steam condenses, reheating the air, so the starting point is ambient temperature air and water and the exhaust is ambient temperature air and water. Thermodynamically, it's a reversible process, "100 % efficient."

First it is painfully obvious that you have no idea of what or how to do the necessary computations.

Second your knowledge of thermodynamics in woefully inadequate to discuss compressed air in any way manner shape or form.

Thermodynamically, it's a reversible process, "100 % efficient. TUBS!

"As the air expands, cooling, the steam condenses, reheating the air, so the starting point is ambient temperature air and water and the exhaust is ambient temperature air and water"

If the 'steam' (water vapor) condenses, it experiences roughly 2000:1 reduction in volume. There went your pressure! Its now either pushing way less or possibly even pulling on the piston. The returned heat of vaporization is minor compared to the phase change in volume.

" It helps to think the problem through."

"If the 'steam' (water vapor) condenses, it experiences roughly 2000:1 reduction in volume. There went your pressure! Its now either pushing way less or possibly even pulling on the piston. The returned heat of vaporization is minor compared to the phase change in volume."

It appears that some of the respondents are not reading carefully. Consider the textbook example of the "frictionless" piston in an insulated cylinder. There is air in the cylinder. One does work pushing on the piston, compressing the air, increasing both temperature and pressure. The work done can be recovered by allowing the air to expand, until the temperature and pressure are back to the original. It is a reversible process, with no energy lost through the insulated cylinder. This is approximately what happens with the air springs found on busses, or, similarly, when your tire deforms going over a bump. Will anyone (Stirling?) dispute this?

Now, suppose that there is some water with the air in the cylinder. As the air heats up, water vaporizes (there is a critical temperature at which liquid water cannot exist), adding to the mass of gas and further increasing the pressure but moderating the temperature rise because of the "latent heat" of vaporization. When the mixture expands, the pressure and temperature drop and the process reverses, releasing the heat of vaporization, which warms the air. Neither mass nor heat has escaped the cylinder. At the end of the cycle, the cylinder is again full of air with some water in it. At no point does the condensing steam suck on the piston! Yes, it is a reversible process.

If you think about it, similar processes occur in thunderstorms. Water vaporizes into the air, which rises (the density is less with water in it) and expands. Water vapor condenses, warming the air, which increases the bouyancy and results in the familiar towering cumulus clouds. The condensing cloud droplets do not suck in the air!

Now, let's consider a compressed air vehicle. For illustrative purposes, keeping the engineering challenges within the state of the art, assume that temperatures may not exceed 600K, a value chosen as metallurgically acceptable. Environmental effects must also be considered.
Exhaust temperatures are limited, as a lower bound, to 200K, which is still
frighteningly cold. Common lubricants thicken at such temperatures. Human
contact with such exhaust would result in frostbite, and, as the cold exhaust
mixed with ambient air, fog and/or snow would result. The same size storage tank
is assumed for each system, conveniently 1000 L, 1 m.sup.3. Such tankage would
fit behind the seat of a small car, or under the bed of a pick-up truck. A
specially built vehicle could accommodate more tankage.

The following calculations make certain assumptions as listed. Purists will
argue that they are imprecise, but they are adequate to compare systems.

1. Air is an ideal gas. k=1.4; PV=RT, where R for air is 0.28 KJ/(kg-K) or 2.8
L-bar/(kg-K) ; for isentropic processes, T.sub.2 /T.sub.1 =(V.sub.1
/V.sub.2).sup.k-1 ; P.sub.2 /P.sub.1 =(V.sub.1 /V.sub.2).sup.k ; Work at 100%
eff. is (U.sub.1 -U.sub.2)=mass(C.sub.V)(T.sub.1 -T.sub.2). C.sub.V =0.75
kJ/kg-K (It is not constant, but let's assume it is) The work done to compress
the air isothermally (impossible in practice, but assumed here) is P.sub.1
V.sub.1 In(V.sub.2 /V.sub.1). Ambient air, the intake to the compressor, is at
300K, 1 bar pressure, with a density of 1.18 g/liter. Exhaust pressure cannot be
below 1 bar.

2. Steam behaves as listed in steam tables.

3. Efficient expanders (motors) with variable expansion ratio and no internal
friction are available. (Zero friction is impossible, but assuming it treats all
the systems equally with regard to friction losses)

4. Another assumption, which can be questioned, is that all the stored gas is
useable. Clearly, as the pressure drops, the expansion ratio of the motor must
decrease, but assuming a variable expansion ratio (which may be achieved with
valve timing) for comparison purposes, there is no big error involved in
calculating energy density or efficiency. Batteries, of course, cannot be
totally discharged, either.

Process A: Energy storage with dry compressed air.

The Pneumocon Inc. car, "Spirit of Joplin", now running in Joplin, Mo., uses air
stored at 3000 psi (about 200 bar) and ambient temperature, about 300K. The
output is throttled, to reduce pressure to 33 Bar, still at approximately 300K,
but if expansion is limited by the temperature of the exhaust the power output
and efficiency remain essentially unchanged over a broad range of pressures. A
thousand liters of air at 200 bar would weigh 236 kg. The energy output,
ideally, would be (236 kg.) (0.75 kJ/kg-K) (300K-200K)=17.7 megajoules (4.9
kw-hr). The work required to compress the air isothermally would be P.sub.1
V.sub.1 ln(V.sub.2 /V.sub.1)=(10.sup.5 N/m.sup.2) (200 m.sup.3) (-5.52)=110
megaJoules. Efficiency, useful output divided by energy input, is 17.7/110 or 16
per cent. (Typical industrial compressed air systems rarely exceed 15 percent
overall efficiency)

Suppose, in an effort to improve energy storage density, the pressure is
increased to 600 bar, which is acheivable with available components. The amount
of air stored will triple, as will the weight of the tanks, approximately, as
the walls will be thicker. Now the output will be tripled, to 53 megaJoules,
14.7 kw-hr, and the input will be 384 megaJoules. Efficiency would drop to 14
percent. Since the energy density increase is large for a small drop in
efficiency, high pressures seem desireable.

Process B: Storing compressed air at 600K.

A multistage compressor would compress the air until it reaches 600K, the
maximum temperature allowed, then compress it isothermally until the pressure
reaches 200 Bar. The tank will hold only 118 kg of air at that temperature. The
first compression adds energy to the air (isentropically), 0.75 kJ/K or 225
KJ/kg, which is 26.6 megaJ total for the first stage. The pressure is 11.7 bar
and the volume 17 m.sup.3. The isothermal compression to 1 m.sup.3 and 200 Bar
consumes 56.3 megaJ, for a total of 82.9 megaJ. When the air is expanded with a
temperature drop of 400K, the energy recovered is 35.4 megaJ, 9.8 kw-hr, with an
efficiency of 43 percent. During the isothermal compression, about 56 megajoules
of heat is rejected (since the internal energy, U, of the compressed air was not
increased).

Suppose we prefer to store the air at 450 bar. We can store 265 kg of air, 22
kw-hr, but the energy to isothermally compress the air has risen to 163 megaJ,
189 MJ total input, for an output of 79.6 MJ, or an efficiency of about 42
percent. Again, higher pressures seem better. Since tanks are likely to be
stronger at 300K than at 600K, the 450 bar and 600K process and the 600 bar at
300K process are likely to be comparable in weight. It would seem Process B is
superior, with higher efficiency and higher energy density.

Process C: Energy storage in steam.

The greatest energy density occurs when we fill the tank with saturated steam at
600K (maximum temperature allowed). The pressure is about 125 bar, specific
volume is 13.5 L/kg, U=2500J/kg, entropy=5.47. The mass of steam in the 1000 L
is about 74 kg. Total internal energy, U, is 185 megaJoules. Since we started
with 74kg. of liquid water at 300K, which had an internal energy of 8
megaJoules, the work required to fill the tank with steam was approximately 178
megaJoules or 49 kw-hr.

Now, we need to know how much mechanical energy we can extract from that steam.
The perfect expansion process is isentropic; total entropy remains the same.
However, we know that as the steam does work it will cool, and some will
condense. (Steam is not an ideal gas) At 1 Bar (exhaust), we will have X kg of
steam and (74-X)kg. of water, both at about 373K. Water at that temperature has
entropy=1.30 and steam has 7.35. X(7.35)+(74-X)(1.30)=74(5.47) It follows, after
a bit of elementary algebra, that the exhaust had 51 kg of steam and 23 kg of
water. The total internal energy of the exhaust is 51 kg(2506 kJ/kg)+23
kg(419kJ/kg)=137.4 megaJoules. Subtracting that from the initial enegy, we find
that the greatest possible mechanical work we could get from the steam system is
40.6 megaJoules or 11.3 kw-hr., and the efficiency is only 23 percent. The
exhaust is dangerously hot, and it cannot be exhausted directly from a vehicle
without creating fog and raining on the following vehicles; a big condenser is
needed.

Summary of characteristics of the three ideal processes:

______________________________________
useful output
efficiency
fuel cost
______________________________________
A: 300K compressed air
14.7 kw-hr 14% 7
B: 600K compressed air
22 kw-hr 42% 2.4
C: 600K steam 11.3 Kw-hr 23% 4
______________________________________

Arguably, any of these state-of-the art processes is competitive with
electrochemical batteries. A battery powered car typically stores about 20 kw-hr
of energy; when that is exhausted, it takes hours to recharge the batteries.
Power output is limited to approximately 200 W per kilogram of battery, so the
power to weight ratio is poor. (That was true back in 1995) Compressed gas vehicles can release stored energy at much higher rates and can be recharged from a stationary "filling station" in minutes.

Relatively inexpensive and non-toxic materials are used (mainly ferrous metal
technology) in the three systems described, and there is little to wear out; no
expensive batteries are used which need periodic replacement/recycling. (The
cost of recycling the batteries periodically overwhelms the cost of electricity)
The impracticality of electric cars is nicely described in the February 1995
issue of Popular Science in an article titled, "It's The Battery, Stupid|" A
December 1994 report, "Electric Vehicles", by the U.S. General Accounting
Office, concludes, "The ultimate viability of EVS as a widespread tranportation
option cannot now be ensured." Economics favor compressed gasses over electrics,
both in low operating cost (no recycled batteries) and in first cost; an
electric van from Chrysler (in 1995) is priced at $120,000, while an air-powered vehicle need not be substantially more expensive than current vehicles; it is
mechanically simple (eg. simple or no transmission, no radiator, no ignition
system, no catalytic converter, etc.).

Most of the disadvantages of batteries in vehicles also apply to stationary
storage schemes. Telephone companies use a lot of batteries, now, but it is hard
to imagine a public electric utility load-levelling with batteries. However,
large storage tanks at elevated temperatures appear to be feasible, perhaps
underground or under water or in remote locations, to allay public fears of an
explosion and to minimize real estate costs.

Parenthetically, hydrogen-air fuel cells might compete as sources of power for
vehicles, but there will be "consumer resistance" to hydrogen in cars. Will they
be allowed in tunnels? A fuel-cell powerplant in New York City was vetoed by the
city fire department.

The operation of the improved process is illustrated by use for a zero-emissions vehicle, though the process is amenable to a variety of uses. Using the same assumptions as described under Prior Art, the superiority of my invention is obvious. It can be called "Process D." The working fluid is air, and the coolant is water,
resulting in what I call "wet compressed air" (WCA).

The use of a mixture of steam and air appears to be novel and not obvious, as
indicated by the lack of literature references to the practice. There are
references to warming air with steam, to ameliorate the chilling effect of the
exhaust of air tools in mines, but the steam was not generated by the
compression of the air. Handbooks and texts on compressed air technology
typically stress the desireability of removing moisture from the air, not adding
it. It appears that the patent classification list has no sub-class for power
plants which mix steam with air in a context which does not involve combustion;
hence a search turned up no relevant patents. (Nor did it turn up the unpatented German trade secrets and actual demonstration with a railway locomotive)

Process D: Energy storage in WCA.

This process resembles Process B (in Prior Art), except that the intercooler is
missing. Instead of throwing away about half the input energy, the heat of
compression is used to make steam, which is conserved. Let's assume water is
mixed with the air at the compressor. As the air is compressed, the temperature increases, vaporizing the water droplets, so that a mixture of steam and air passes to the storage tanks.

118 kg of air is pumped
into the tank at 600 K, with an increase of internal energy of 26.6 MJ. However,
the mass of air in the tank is less (60+%), because there is now steam in the
tank, also, from water boiled during the compression process. The energy used to
compress the air rises to approximately 66.5 megaJoules and that energy is
making about 30 kg. of steam, with the partial pressure of the air approximately
330 bar and the partial pressure of steam 125 bar, for a total pressure of 455
bar, which is feasible. (The mass of the contents is about 115 kg less than
Process B at 450 bar) The total energy in the tank, at 600K, is now about 93
megaJ, the same as the input energy, since no heat was rejected from the
insulated system. Now, when the mixture is expanded, isentropically, the exhaust
at 1 bar is at the original temperature, 300K, and no energy is lost in the
exhaust. The steam condenses to water droplets, and the heat released as it
condenses expands the air, so all the energy is recovered. The
compression-boiling and expansion-condensation is a thermodynamically reversible
process. The energy output is the same as the energy input, 93 MJ or about
26kw-hr. The WCA tank and motor can be in a vehicle, while the compressors and
bulk storage tanks are at various filling stations, with quick-disconnect hoses
to recharge the vehicle, or the vehicle can be a hybrid, with an engine and
compressor in the vehicle.

Summary of characteristics of the processes:

______________________________________
useful output
efficiency
relative fuel cost
______________________________________
A: 300K compressed air
14.7 kw-hr 14% 7
B: 600K compressed air
22 kw-hr 42% 2.4
C: 600K steam

11.3 Kw-hr 23% 4

D. 600K WCA

26 kw-hr 100% 1

E. Electric batteries
20 kw-hr 85% High*
______________________________________
*The cost of parking at a recharge station and of replacing the batteries
overwhelms the cost of the electricity.

Process D, Wet Compressed Air (WCA), is the clear winner in energy density,
cost, and environmentally harmless exhaust (ambient temperature air with
droplets of water which can be filtered out and recycled). It is useful over a
wide range of temperatures and pressures.

Summary and Ramifications

The invention improves the efficiency of compressed-gas energy
transmission/storage by factors of 2-7. The heat normally wasted by cooling the
gas and compressor is retained in a coolant and returned to the gas during
excpansion, approximating an ideal, reversible thermodynamic process. Therefore,
the cost of compressing the gas, the fuel cost, is reduced, making the novel
process economically and environmentally preferable to other processes for
storing energy. It is also, generally, safer to use, as a leak will exhaust
ambient temperature gasses and droplets of coolant, innocuous as compared with
hot steam, cold air, or battery acid.

A few utilities store off-peak energy with caverns full of compressed air and
use the compressed air to generate electricity during periods of peak usage. The
invention would make this much more efficient, therefore more economical.
Because the energy can now be stored efficiently, in relatively low-cost
containers, a number of applications are feasible. Used in a small scale, the
"Process D" could power portable tools, lawnmowers, self-launching gliders,
boats, and the like, with little noise and no pollution. Inconstant sources of
energy, such as wind turbines, can drive compressors.

To those of you who remain convinced that I am ignorant, delusional, and too clumsy to use a slide rule: please do post correct computations.