160 comments on Compressed Air Energy Storage - How viable is it?
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160 comments on Compressed Air Energy Storage - How viable is it?
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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.