Well, the last post about weights was interesting, and shows how much weight you need to store the power of a single wind turbine. This got me thinking about the old theme of using water to store energy.
Now, what if you put a big water tank on top of a hill, and another at the bottom. How much water would you need to store 1 MWh?
Let's assume a fairly big hill, say 500 meters tall (1500 ft or so). Weight of water is 1000kg/m3. Using the same calculations as in the previous post,
Raising 1 kg 500 meters stores 10*500 = 5000J = 5000/3600 = 1.4Wh. To store 1MWh, we would need 1000000/1.4 = 720,000 kg of water = 720 cubic meters of water. From here:
http://heartlandtankservices.com/building_new_tank.html
Converting cubic meters to gallons, 750 m3 = 200,000 gallons, smaller than Heartland's smallest tank. Their largest tank holds 2 million gallons, so it would store enough power for 10 hours of wind production.
So if you have tall hills around, it might be useful to store electricity using two water tanks. I recall reading somewhere that stored water is about 80% efficient round-trip. Pretty reliable, and low cost as well, just fairly ugly, two tanks plus the pipes connecting them.
Aug 15, 2009
Storage of Wind Energy using Weights on the Tower
A thought experiment. What if you suspended a large mass from a wind tower, cranking it up when there is surplus wind, and letting it fall when electricity is needed. How much weight would you need to make this useful?
Well, from http://en.wikipedia.org/wiki/Kilogram-force, we get the measure of magnitude of the force exerted on one kilogram of mass by a 9.80665 m/s2 gravitational field = 9.8 Newtons (N), or to make things easier, 10 N. The potential energy stored by raising this one meter is F * D = 10 Nm, or 10 Joules (J). Now, there are 3600 J in a Watt-hour (since 1 Watt-second = 1 J).
Wind towers are typically 60-90m tall ( http://en.wikipedia.org/wiki/Wind_turbine ). Let's assume 75m for this calculation. Raising our 1kg mass 75 meters would store 750J, or 750/3600 = 0.208 Wh, not a heck of a lot. Another way to phrase this is we would need 4.8kg to store 1 Wh. Assuming a 1MW turbine, to store one hour of 1MW output would require a weight of 4.8 million kilograms - or 4800 metric tons.
Say that we wanted to use concrete for this. Density of concrete is about 2400kg/m3. (http://www.simetric.co.uk/si_materials.htm). We would need then:
4,800,000/2400 = 2000m3 of concrete, or a block about 10m x 10m x 20m, or about as big as my house. Probably not practical, but you never know...
I guess this is why batteries, air compression, etc are more practical.
Well, from http://en.wikipedia.org/wiki/Kilogram-force, we get the measure of magnitude of the force exerted on one kilogram of mass by a 9.80665 m/s2 gravitational field = 9.8 Newtons (N), or to make things easier, 10 N. The potential energy stored by raising this one meter is F * D = 10 Nm, or 10 Joules (J). Now, there are 3600 J in a Watt-hour (since 1 Watt-second = 1 J).
Wind towers are typically 60-90m tall ( http://en.wikipedia.org/wiki/Wind_turbine ). Let's assume 75m for this calculation. Raising our 1kg mass 75 meters would store 750J, or 750/3600 = 0.208 Wh, not a heck of a lot. Another way to phrase this is we would need 4.8kg to store 1 Wh. Assuming a 1MW turbine, to store one hour of 1MW output would require a weight of 4.8 million kilograms - or 4800 metric tons.
Say that we wanted to use concrete for this. Density of concrete is about 2400kg/m3. (http://www.simetric.co.uk/si_materials.htm). We would need then:
4,800,000/2400 = 2000m3 of concrete, or a block about 10m x 10m x 20m, or about as big as my house. Probably not practical, but you never know...
I guess this is why batteries, air compression, etc are more practical.
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