Compressed Air Energy Storage - How viable is it?
Posted by Libelle on July 27, 2008 - 9:00am in The Oil Drum: Canada
Topic: Alternative energy
Tags: compressed air, energy storage, thermodynamics [list all tags]
One of the most critical aspects of the implementation of renewable
electricity is the ability to store electricity. If a good
solution existed right now, our situation would be a good deal
easier. On the face of it, compressed air seems a likely
candidate: relatively easy to make, store and use - so what is the
problem? Why isn't it used routinely?
More Thermodynamics than You Ever Wanted to Know?
We usually speak of storing and using energy without being very
precise about what we mean. That ends forever if you take a few
chemistry or engineering courses. Thermodynamics rules everything.
Let's start with the usual definition of work - using a force to
push something a given distance (in the direction of the force).
The amount of work is the force multiplied by the distance, and has
units of energy. If we lift a 1 kg mass by 1 metre in the earth's
gravitational field on the surface of the earth, then the work done on it is the force required:
1kg x 9.8 m/s2 (9.8 Newtons), times 1 metre, or 9.8 Joules. Since
a Watt is 1 Joule per second, then in principle (no friction), this
lift could be carried out in 9.8 seconds by a 1 Watt electric
motor. At the end of the process, the weight has acquired 9.8
Joules of gravitational potential energy.
We just constructed an energy storage device. The weight we
lifted could now be allowed to descend, giving its potential energy back
to an electrical generator and making electricity in the process.
This is in fact the basis of possibly the most effective existing way of storing
electricity. Water is pumped from a low reservoir to a high one
at times when there is a surplus of electricity, and then allowed to
flow back when there is a shortage. For useful amounts of energy
storage using reservoirs that are not too large, one generally requires
reservoir height differences of a hundred metres or more, which limits
this to suitable terrain.
So what about compressed air? Surely a cylinder of compressed air contains energy that could be used to drive something?
This is where it all becomes a little strange. The energy
content of compressed gas isn't very different from that of
uncompressed gas at the same temperature. For an ideal gas, the energy contents are identical.
How come we can get work from the compressed gas?
The answer is that compressed gas has a lower entropy than the
uncompressed gas, and that the amount of useful work you can get out of
something when it changes depends both on the change in energy content
and the change in entropy. We usually focus so much on the energy
side of things that we ignore the entropy side.
If the compressed gas has no more energy than the uncompressed gas,
where did the energy used to compress it go? The answer can be
found in the old bicycle pump experiment. When you compress a gas
it becomes hot. In fact all the work put into an ideal gas to
compress it is turned into heat. If that heat is thrown away, the
same amount of energy as was in that work is thrown away with it.
To look at a definite example, if we take 1 cubic metre of air at 1
atmosphere pressure and 20C and compress it to 10 atmospheres pressure,
its temperature will increase very considerably - to 293C. If we
want to store this compressed air at 10 atmospheres pressure and 20C,
then more compression will be needed as we cool the gas, or its
pressure will drop as its temperature does. The total work done
on the gas, and the total heat lost are both about 91.7 Watt-hours
(Wh). (This assumes that the air is an ideal diatomic gas.)
This gas would now have a lower entropy than the same amount of uncompressed
air. The entropy change is 796 J/K (Joules per degree
Kelvin). Note the units are energy per degree. This gives a
hint of how the entropy change is related to the work that can in
principle be extracted from the compressed air. That work can be
calculated by multiplying the entropy change by the temperature of the
environment in degrees Kelvin. 20C is 293K, so the amount of work
that can in principle be extracted is 233 kJ, or 64.8 Wh. If we
compare this with the work done compressing the gas, we see that the
efficiency of the process is about 71%, even if the compressor is
perfectly efficient.
Looking at the expansion of the same air back to 1 atmosphere, using
a motor to do work in the process, we can work out that the temperature
will fall to -121C, and that the work that is done would be
47.5Wh. The efficiency of ths process is thus 47.5/64.8 = 73%,
even with a perfect motor. The round-trip efficiency for energy
storage and use would then be just 52%. With real compressors and
motors it would clearly be considerably worse. These numbers
above are for a compression ratio of 10. If we instead use a
compression ratio of 100, things get worse still, with a round-trip
efficiency of 27%.
This actually gives a clue as to how to improve the situation.
The maximum efficiency of the cycle depends on the pressure ratio, and
rises to 100% as that ratio approaches 1. The answer is to use
staged compression, with cooling back to ambient temperature between
the stages, and staged expansion, with reheat back to ambient
temperature between stages. If we get the 100 times compression
by two stages of times 10 each, then half the work goes into the first
stage and half into the second, with efficiencies as for 10 times
compression - a huge improvement. If we use four stages (ratio
3.17), then the maximum effficiency would be 72%. If we take into
account that real compressors and engines are not perfect, and neither
are coolers and reheaters, we can see that real overall efficiencies
achieved are never likely to be very good, even with very complicated
equipment.
Whether technology is useful depends, though, on comparison with the
alternatives. The overall efficicency of a compessor train and a
compressed air car may not look all that high, but an internal
combustion vehicle engine can look pretty inefficient, even with North
American fuel prices. This means that an air-powered
car may make some sense. For more details on the MDI air car, see
some MDI engine tests. Notice that in a conventional car you get free heating, but in a compressed air car you get free cooling.
Bulk power storage is another matter. Large reservoirs of
compressed air can be and have been constructed, but they are not used simply
to drive engines to regenerate power. Building large heat
exchangers to warm the air in a power generating unit would be very
costly and not very efficient, so the air is instead heated to a much higher temperature before the expansion turbine by
burning natural gas in it. The whole installation is thus a sort of gas turbine, with the difference that the compressor and power
turbines are run at different times instead of together. This is no longer a straightforward energy storage device.
Libelle -
Nicely done treatment of the subject.
It should be clear by now that compressed air storage is analogous to a very inefficient spring, with considerable losses in both compressing and releasing the spring.
Heating the compressed air prior to expanding it through a turbine is just a way of putting back some of the energy that was lost during compressive heating. As such, most compressed air torpedoes of the WW II era employed such heating to increase their speed and improve their range.
As I see it, the only way to make large-scale compressed air storage even halfway viable would be if one had a ready use for the wasted heat of compression (such as for space heating or process heating perhaps). Or possibly if one could store the some of the heat of compression and then give some of it back during the expansion part of the cycle. Either way, we're talking about high capital cost in relation to what would be gained.
Compressed air storage hardly looks like a winner.
many small towns in the us and canada have water towers. some of these towns are dying a slow death.
so it would seem that they might have excess capacity of stored water.
just wondering if any could be used to store potential energy, at night say, and used to generate electricity during the day ?
Pumped water has been used for years to store energy. As the article states, it's not about efficiency, but about alternatives: What are the options, what are the characteristics and costs of these options?
A water tower is not very big when talking about municipal power generation.
but the potential energy can be used:
Granted, they are using excess pressure within the system to run the generators. Nonetheless, I have seen city water delivered at 80 PSI (had to install a pressure reducer)!
We can rapidly calculate the gravitational energy stored in the water using the following equation:
Energy Stored = mgh
m = mass of water
g = acceleration due to gravity, 10 is a reasonable approximation
h = height water is stored above your energy extraction device, e.g. turbine.
A typical power station has a rated output of 1000 MW, if we assume it runs at an average of half capacity (500 MW) for 24 hours, this is a total energy output in this time of 43.2 GJ. Lets say we have a 200m high tower. Using the above equation, we would need to pump 21600000000 kg (21.6 million metric tonnes)of water into the tower to store the same amount of energy. For reference, 1 kg of water is 1 litre, 1 american gallon is 3.78 litres, making it 5714 million american gallons. This is impractical, even if we have many towers.
P.S. if anyone notices a mistake in this quick calculation feel free to correct me!
500 MW running for one second is simply 500 M joules.
Running for 24 hours it is:
500 M x 24 x 60 x 60 = 43,200,000 M joules = 43.2 T joules
If the tower is 200 meters high then you need to store this much water in one day:
43.2 T / (200 * 9.8) = 22 G kg
Since one cubic meter contains 1000 kg water, therefore it means:
22 G/1000 = 22 M cubic meter water
I assume that you need to store in water half the energy you need and the other half is used right away. Then you need 11 million cubic meter water.
If you have a dam as big as an approx cricket/football/hockey stadium (which I take as a square with each side one stadia or 200 meters) then you need a depth of these many meters:
22,000,000/(200 * 200) = 550 meters
I don't know the construction cost of that big a stadium. In Pakistan the cheapest brick is in villages, its size is 6" x 4" x 3" and come at a price of atleast Rs. 5 per piece. That is when its made on stoves that are heated by burning tires which is a very polluting way but we get along with that in village where air is very clean. We also have the advantage of having low prices. The labour of the brick factories hardly earn enough to feed themselves.
Since one brick is 24 square inches at maximum area and stadium is:
(200m x 550m x 4) + (200m x 200 m) = 440,000 + 40,000 = 480,000 square meter
Since there are about 40 inches in one meter therefore it means an area of:
480,000 * 40 * 40 = 768,000,000 cubic inches = 32,000,000 bricks = Rs. 160 million
This is when the dam's boundaries that is its walls and floor is just 3" thick. That obviously is not enough to hold a pressure of 550 m high water column at floor and 200 m long horizontal water pressure at walls. I assume we need atleast a ten ft thick wall. 10 ft = 120" = 40 walls in a row at walls and floor. The cost now is:
Rs. 160 million x 40 = Rs. 6.4 billion
I assume it to be Rs. 12.8 billion assuming 50% rise in expense each for labour cost and mortar cost.
Pakistanis on average consumes 220 watt energy and has a per capita gdp of $880. Gdp density is about 0.25 watts per dollar or 8 million joules per dollar. Since the cost of dam is Rs. 12.8 billion or 200 million dollars so it means an energy expense of:
200 M x 8 M = 1600 M M joules
The dam is supposed to provide storage for 250 mega watts for a duration of 30 years, the usual life time of dams. In that time the dam would store:
250 M x 30 x 365 x 86400 = 237,000 M M joules
The amount of money spent to make the storage place is just this much percent of the gdp gained in the process:
1600/237,000,000 x 100 = 0.675% (that is less than one percent)
The base thickness of a dam wall is proportional to height of the wall. For a stable gravity wall you would be looking at the width of the base being a third of the height. This gives you a factor of safety of 1. I wouldn't want to live under a dam which can only just hold back the pressure of the water. FYI the three Gorges dam wall is 101m high and 115 m wide at the base. Essentially a factor of safety between 3 and 4.
FYI - you can't make a dam wall out of bricks. Bricks have no tensile strength which is why brick houses fall down during earthquakes. A brick wall of a dam would burst without being reinforced with steel tie rods.
The most efficient structure to store water is a circular tank. The wall of the tank can be extremely thin because the tank wall works only in tension ie hoop stress. Tanks walls can be steel work or steel reinforced concrete - any material that has a high tensile capacity.
Water as an energy storage device has been around since the first civilisations. It is a common misconception that it needs to be a great differential between the input and output height to actually produce power. A 1m height difference with a very large volume will give the same energy output as a 100 m height difference with 1/100th of the volume.
The Dutch and other low lying countries have worked with this for years. Frankly the world could do a hell of a lot better than looking into the past for energy creation and storage solutions. A low lying paddock, a paddock a little higher, a few windmills, a few inclined screws and and a water source you have yourself a very powerful energy generator and storage device. Even better if the paddocks lie beside the ocean as you can utilise the potential of tidal flows to fill it up when the wind isn't blowing.
The capital costs of CAES are relatively low, and heat storage isn't that expensive either. Cleverly designed AACAES might not cost more than pumped storage and could be almost as efficient.
Libelle,
That is an excellent essay; concise and very informative. I wish you had written my physics textbooks!
I get the feeling that Libelle and I were thinking along the same lines, but she obviously wasn't reading me or this article would have been different.
BTW, one term which wasn't mentioned here but probably should have been is "availability". This is the amount of energy which can be recovered from a system, and it's one of the crucial concepts I took away from my thermo course (which changed the way I saw the world as it did Libelle).
A suggestion. Why don't you or PG or someone do a thermo primer? Libelle did decently, but as you said to analyze this you have to use the Helmholtz free energy and/or exergy concepts.
I always reckoned that almost everyone would switch off when the equations started.
Exactly. To do this in a way that is accurate is not trivial. I've made a couple of presentations to economists, with a positive reception, but I don't know how much stuck.
It's "he", btw.
Sorry. I can't tell the players without a program, even when I'm one of them.
Libelle is a very feminine name, most likely caused by the affix 'elle'. It's the German word for Dragonfly isn't it?
Yes. I used to fly one of these: http://www.planepictures.net/netshow.php?id=222694
Amazing little machines.
One of the original "glass slippers". Part of the Aptera's parentage, from the looks of it.
Well, now, all this is good as far as it goes, but it dropped the real ball early on- water storage. Pumped hydro storage. Been done for centuries. Works just great. DOES NOT NEED A HILL. Look it up.
Please, please, people, I ( and others, too) have said this over and over and it never seems to stick. YOU DON'T NEED A HILL TO STORE PUMPED UP WATER. All you need is an ELEVATION DIFFERENCE between two pools (or, if you want, lakes) of water. Whether the elevation difference is between a lake up a mountain and ground level, or a lake on the ground and another one 200 meters down a hole makes no difference. The energy stored is the same either way- and it's just as accessible and just as efficient.
There, I've said it again.
OK, so you will say holes are expensive. Well, yes and no. Some are, some are already there ready to get full of water. And people dig big holes all the time and are smart about how to do it.
I envision a sustainable energy system in which every source is a water pump. Windmill water pumps, solar thermal (my personal passion) water pumps, everything pumping water up from down or up from here, storing gobs of energy. This gives us an artificial big powerful river of water. We know how to use big rivers of water, right? A big water turbine (very efficient, very well known) driving a big alternator (very efficient, very well known) 24 hrs/day, all the time.
On my morning walk today I was playing around with the idea of replacing that expensive gear box in those 2megawatt windmills with a water pump. Much cheaper, much more reliable, and lighter by far than the gearbox and alternator that is now up on that stick.
And don't tell me it would freeze. Absolutely no way. Too big, too much heating even tho very efficient.
Your suggestion sounds good in theory, Wimbi, but is there a working example of a pumped water storage scheme using a big hole (as opposed to a mountain reservoir) in the real world? Don't get me wrong, I'm not opposed to the suggestion, but would like to see it being tried and working successfully someplace in the world. It's hard to imagine that it would be entirely overlooked if it was practical. There are flat places in the world that have lots of wind but no reservoirs for pumped storage (ie the Texas panhandle).
I can see some significant problems in trying to implement pumped storage using a hole in the ground:
1) you'll need an awfully big hole, actually, an underground reservoir (cave system?). Otherwise, the hole will fill up with water very quickly, maybe in a matter of minutes.
2) the underground reservoir may fill up with ground water, not just the water you're pouring into it. You'll have to constantly pump out ground water, using energy in the process and negating the whole idea of using this as a pumped storage area.
3) you need an above ground reservoir to store the water that you're pumping up
Just punch in "pumped hydro below ground" to your favorite search engine and you will get the same string of examples I did. Take your pick.
BTW. reason pumped hydro is better is that unlike gas-air- it is incompressible, or nearly so, and the energy you put into pumping it has to go to elevation change, and not partly into bumping molecules around faster at random and then having them go bump something else outside and waste that energy (heat of compression, heat transfer to willynilly).
You can hardly put any energy into water by just merely pushing on it while it is sitting there, any more than you can put energy into a concrete wall by pushing it. It doesn't move. No move, no energy. But you can put a hell of a lot of energy into air by just pushing, because it moves (compresses), as any tire pump with a blocked exit will tell you. And you don't get it all back, not by a long shot.
That's what the post said, of course, more elegantly. Me, I am a rude, crude engineer, totally
inelegant. Just make it work, any which way you can.
AACAES should be able to get 70-80 percent round trip. Good enough I'd reckon. Pumped hydro can be a bit better with modern tech, perhaps 90 percent round trip.
Underground pumped hydro is really an interesting underdog technology. Possibly the cheapest, one of the most efficient, and environmentally friendly storage methods.
90 percent? Single point, yeah, but the reversible pump/turbines have rangebility problems. At least the ones at Power Vista in NF used to.
I'm thinking low head. The Dutch have a plan that envisions damming the inland lake:
http://www.netserver2.net/waterforum/index.asp?url=/template_a1.asp&que=...
Modular pumped hydro would scale the size of each module for effective operation. And, of course, with multiple modules, there is less need for a wide range of operation between minimum and maximum flow for each unit.
Ah, the light bulb comes on...
When is a giant hole in the ground a liability?
Answer: when it is an exhausted open pit mine.
When is a giant hole in the ground an asset?
When you can use it to store/extract kinetic energy.
Thank you wimbi
Actually you need two reservoirs - one substantially above the other - in the real world, not many exist like that.
The problems of storage of energy in any gas/fluid are simmilar to those causing peak oil.
What is required is an adequate and cost competitive flow rate of energy (both charging and discharging), which will be determined by the need for suppliers to make a profit and the abilty of consumers to afford the cost.
This was a good simplification of a difficult subject Libelle, most people don't understand entropy and it's implications. I would like to see somebody fill up an MDI vehicle and drive it continuously at normal highway speeds (not just a demo around the block) and see just how far it gets - not as far as most people might think is my guess.
Just for info, here is a pump storage system in Wales with the delightful name of "Ffestiniog Power Station"
http://www.fhc.co.uk/ffestiniog.htm
Yes, that is a very good real world example of what is required - and why pumped water isn't adequate as a non-FF battery for the UK at least.
360MW (from a store of 2 million cubic metres of water at a rate of 27 cubic metres per second) for a few hours (about 20 by the looks of it) is a small part of 820 MW UK declared net wind capacity in 2006, supposed to be ~25GW by 2020.
It is said that this unit could supply the needs of the whole of North Wales for a few hours - not surprising since it is one of the most mountainous and least populated parts of the UK - sadly nowhere near what is required if we want smooth non FF power for the whole of the UK.
now that you mentioned it, i dont know why an abandoned oil well wouldn't work for this application. many wells are capable of producing thousands of barrels (of water per day) and they will also accept thousands of barrels per day under injection under just the hyrdorostatic pressure of the water column.
it would seem that the problem arises in designing a pump and electrical system to move that much fluid from a significant depth. one would imagine that to be workable, the pump would have to operate variably, according to the electrical energy available. that problem could probably be worked out, but at what efficiency ?
and an added bonus would be the heat that could be harvested from such a system.
Moving fluid from large depths is best done by placing the pump on the bottom of the resevoir rather than on the surface. This probably makes the water pump-in-windmill idea a bit less attractive.
Variable operation costs a bit in efficiency, but it's quite acceptable.
actually, i was thinking submersible pump and submersible generator. not in the same well though.
Ah, OK. Wimby brought up the pump in windmill idea. I don't think that would be very practical anyways.
NPSH is a bitch.
One thing to think about, through, is that the turbine is at the bottom of the connection between the two reservoirs. In the case of a mountain or foothill, this is convenient ... because generally the bottom of the mountain is more accessible than the top, and the fact that the top is just a passive reservoir and a pipe is how you'd rather have it anyway.
If the top reservoir is at ground level and the bottom at the bottom of a hole 200m deep, then that location at the bottom is an inconvenience.
Mind you, it does not mean its technically infeasible, it just increases the cost.
By the same token, a top reservoir on a tower is certainly feasible ... but putting it on the ground where the ground is at an elevation saves the capital cost of the tower.
And the 100m is not a technical feasibility requirement ... its also an issue of capital cost. If the reservoirs and pipe and turbines are all sized to provide a good balance between volume per unit and scale economies ... then the power storage of that standard unit increases in proportion to the elevation between the top and bottom reservoir, so twice the elevation(NB) is twice the energy storage per cubic meter of water capacity.
That is why the 100m threshold is referring to a purpose-built facility ... if there is an existing hydro generating facility, and the capital cost of the reservoirs are already covered, and the generators can be converted to reversible pump/generator for a reasonable capital cost, then that is a potential target. However, modular pumped storage avoids the problems of interfering with the river ecosystem.
(NB. Strictly speaking that is a slope twice as steep, to get twice the elevation with the same capital cost for pipe.)
What the other poster said, NPSH is a bitch...
But you need a surface body of water at one end or the other (or your cost goes way up), and that's a problem.
Take the Ludington (MI) pumped storage facility as an example. The reservoir is quite large; it's plainly visible on Google Maps. This reservoir is not suitable for fish habitat or recreation because it is filled and emptied rapidly. Yet as large as it is, it can only supply a fraction of the state's electric needs for a matter of hours.
Trying to scale up facilities like Ludington would mean lots of real estate devoted to such single-use reservoirs. In many parts of the country, it would mean large lakes where there are now none, and large evaporation losses where water is already scarce. This does not look like it can or should be our model.
CAES has its issues (mostly related to efficiency), but scalability and siting are not among them; the atmosphere is far more vast than our ability to pump air, and it is not subject to local depletion or scarcity. If the heat of compression can be partially stored and recovered, the efficiency can be improved. Biofuels or biofuel byproducts (e.g. tail gas from "green diesel" F-T plants) can supply the energy for reheating air. The same underground caverns you suggest for pumped hydro will store far more energy as compressed air, and the surface footprint of a CAES plant is minuscule.
I was once a CAES skeptic. I ran the numbers, and now I'm an advocate.
Libelle,
What are we to make of the document at "MDI engine tests"? I have trouble understanding it. I wonder if you know what they mean by 'autonomy' of a vehicle. What is the 'global concept' that permits significant autonomy?
I remember reading somewhere that compressed air energy storage was being used, or maybe being planned to be used, where the pressure vessel was a closed natural cave. The pressure rise was quite small and consequently the efficiency was much higher. But, of course, caves can't be carted about in a one or two passenger vehicle.
I think that by "autonomy", they just mean how far it will go, and by "global concept" they mean the design of the car.
"20C is 297K, so the amount of work that can in principle be extracted is 233 kJ, or 64.8 Wh. "
I couldn't get the numbers to calculate, and then I remembered that 0C=273K, which would make 20C=293K.
293 *.796 = 233
So should it read
"20C is 293K, so the amount of work that can in principle be extracted is 233 kJ,