Following on from this question, since cheap energy storage is a big obstacle to using wind/wave/solar energy, why is gravity-based energy storage not used more?
Many coasts have some cliffs, where we could build a reservoir on top of the cliff and pump up seawater to store energy. What is the fundamental problem with this? Efficiency of energy conversion when pumping? Cost of building? The space the reservoir would take (or the amount of water it could hold)?
Actually, this scheme is currently employed by utilities, albeit usually not with seawater. The technique is called pumped storage hydro. Pumped storage hydro accounts for the vast majority of grid energy storage world-wide. Pumped storage hydro is used to by power companies to achieve various goals, e.g.:
flatten out load variations (as you suggested elsewhere in this thread)
provide “instant-on” reserve generation for voltage and frequency support
level out the fluctuating output of intermittent energy sources such as wind and solar (as you suggested above)
Wikipedia states that round-trip efficiency of pumped storage hydro can range between 70% and 87%, making it an economical solution in many cases.
A couple of obstacles to using pumped storage hydro are:
Certain topological/geographic features are needed to make PSH viable
I wonder why there aren’t more of them, or bigger ones. The only seaside-cliff one listed on Wikipedia is the Okinawa Yanbaru station, completed in 1999, which only provides 30 MW.
Apparently the cost/demand situation isn’t favorable.
Yes, it seems like using a seaside cliff would have several advantages over a freshwater solution, not the least of which is an unlimited water supply in the lower reservoir.
I guess the problem is scale, after all. I’m quite bad at physical calculations, so the below may be wrong.
Even a small hydroelectric dam generates gigawatts of power. Assuming a 30 meter tall cliff, each cubic meter of water generates 294 kJ when descending. To produce 1 GW of power, we would need 1,000,000/294=3400 cubic meters of water descending every second (watt = joule/second).
If we build a lake at the top, 10 meters deep and 1 kilometer on a side, it would contain 10 million cubic meters of water. If we run it at 1GW, it would be emptied after 49 minutes. Not very useful, after all.
It makes me really appreciate the scale of natural phenomena like Niagara Falls.
Even a small hydroelectric dam generates gigawatts of power
Actually, multi-gigawatt hydro plant is a large hydro plant, e.g. Hoover Dam has a capacity of 2GW. A medium sized hydro plant might have a capacity of around 200 MW, e.g. Martin Dam in Alabama has a capacity of 182 MW.
Your point is well taken however; scale issues will probably prevent pumped storage hydro from being the one-and-only solution to intermittent energy sources. Just for comparison, the reservoir created by the above-mentioned Martin Dam covers 40,000 acres!
However, pumped storage hydro can still be a useful and economical part of the solution. Other components would be natural gas powered combustion turbines which can be brought online quickly as needed, and a mix of renewable sources. To this latter point, some areas tend to be windier at night than during the day; this suggests that a mix of wind and solar and wind might be a useful combination.
Still, it is hard to imagine that we’ll be getting away from fossil and nuclear any time soon; renewables can help reduce the amount of fossil fuels that we consume, but won’t (for now) be able to eliminate the need for fossil. Pumped storage hydro can be a valuable part of the solution by smoothing over irregularities in the supply and demand while reducing the use of natural gas powered generation.
Apart from what g_pepper has correctly pointed out regarding size/power of hydro plants…
If we build a lake at the top, 10 meters deep and 1 kilometer on a side
With the right terrain, this is pretty trivial, all you need is a relatively small dam wall closing off a small ravine between mountains… here is a nice example:
The operating cost for hydro power plants is very low, so the relevant cost is the initial building. If you dam a river, it just takes one wall, while if you want to create a swimming pool, it takes four. Actually, five, and the floor may be the biggest problem. If you dam a river, you already know that it isn’t easy for the water to flow through the ground, because it isn’t taking that route. Whereas pumping it onto dry ground probably won’t work.
If you dam a river, you already know that it isn’t easy for the water to flow through the ground, because it isn’t taking that route.
Actually, this (water that passes under the dam) is the main problem after water passing directly through the dam. If the bed of the river is ok to hold a height of 10 m of water, it is probably not to hold 20 or 30 or 70 m of water.
Cost. Wind/wave/solar energy is more expensive than fossil-fuel or nuclear energy to start with, and adding not-too-efficient storage mechanisms to even out the supply does not help it at all.
Really, the answer to most questions of this kind is “cost”. It is the default and usually correct answer.
If it were cheap enough, maybe, but at the moment the demand fluctuations are covered by power generating plants which come online in times of high demand (e.g. day) and shut off during low demand (e.g. night). Typically these plants burn natural gas.
Sufficiently cheap storage would be very useful, yes.
Following on from this question, since cheap energy storage is a big obstacle to using wind/wave/solar energy, why is gravity-based energy storage not used more?
Many coasts have some cliffs, where we could build a reservoir on top of the cliff and pump up seawater to store energy. What is the fundamental problem with this? Efficiency of energy conversion when pumping? Cost of building? The space the reservoir would take (or the amount of water it could hold)?
Actually, this scheme is currently employed by utilities, albeit usually not with seawater. The technique is called pumped storage hydro. Pumped storage hydro accounts for the vast majority of grid energy storage world-wide. Pumped storage hydro is used to by power companies to achieve various goals, e.g.:
flatten out load variations (as you suggested elsewhere in this thread)
provide “instant-on” reserve generation for voltage and frequency support
level out the fluctuating output of intermittent energy sources such as wind and solar (as you suggested above)
Wikipedia states that round-trip efficiency of pumped storage hydro can range between 70% and 87%, making it an economical solution in many cases.
A couple of obstacles to using pumped storage hydro are:
Certain topological/geographic features are needed to make PSH viable
Social and ecological concerns
Yes! Thank you.
I wonder why there aren’t more of them, or bigger ones. The only seaside-cliff one listed on Wikipedia is the Okinawa Yanbaru station, completed in 1999, which only provides 30 MW.
Apparently the cost/demand situation isn’t favorable.
Yes, it seems like using a seaside cliff would have several advantages over a freshwater solution, not the least of which is an unlimited water supply in the lower reservoir.
I guess the problem is scale, after all. I’m quite bad at physical calculations, so the below may be wrong.
Even a small hydroelectric dam generates gigawatts of power. Assuming a 30 meter tall cliff, each cubic meter of water generates 294 kJ when descending. To produce 1 GW of power, we would need 1,000,000/294=3400 cubic meters of water descending every second (watt = joule/second).
If we build a lake at the top, 10 meters deep and 1 kilometer on a side, it would contain 10 million cubic meters of water. If we run it at 1GW, it would be emptied after 49 minutes. Not very useful, after all.
It makes me really appreciate the scale of natural phenomena like Niagara Falls.
Actually, multi-gigawatt hydro plant is a large hydro plant, e.g. Hoover Dam has a capacity of 2GW. A medium sized hydro plant might have a capacity of around 200 MW, e.g. Martin Dam in Alabama has a capacity of 182 MW.
Your point is well taken however; scale issues will probably prevent pumped storage hydro from being the one-and-only solution to intermittent energy sources. Just for comparison, the reservoir created by the above-mentioned Martin Dam covers 40,000 acres!
However, pumped storage hydro can still be a useful and economical part of the solution. Other components would be natural gas powered combustion turbines which can be brought online quickly as needed, and a mix of renewable sources. To this latter point, some areas tend to be windier at night than during the day; this suggests that a mix of wind and solar and wind might be a useful combination.
Still, it is hard to imagine that we’ll be getting away from fossil and nuclear any time soon; renewables can help reduce the amount of fossil fuels that we consume, but won’t (for now) be able to eliminate the need for fossil. Pumped storage hydro can be a valuable part of the solution by smoothing over irregularities in the supply and demand while reducing the use of natural gas powered generation.
Apart from what g_pepper has correctly pointed out regarding size/power of hydro plants…
With the right terrain, this is pretty trivial, all you need is a relatively small dam wall closing off a small ravine between mountains… here is a nice example:
http://www.iwb.ch/media/de/picdb/2012/366/nant_de_drance_stausee_vieux.jpg
http://www.iwb.ch/media/de/picdb/2012/367/nant_de_drance_stauseen_vieu.jpg
The operating cost for hydro power plants is very low, so the relevant cost is the initial building. If you dam a river, it just takes one wall, while if you want to create a swimming pool, it takes four. Actually, five, and the floor may be the biggest problem. If you dam a river, you already know that it isn’t easy for the water to flow through the ground, because it isn’t taking that route. Whereas pumping it onto dry ground probably won’t work.
Actually, this (water that passes under the dam) is the main problem after water passing directly through the dam. If the bed of the river is ok to hold a height of 10 m of water, it is probably not to hold 20 or 30 or 70 m of water.
Cost. Wind/wave/solar energy is more expensive than fossil-fuel or nuclear energy to start with, and adding not-too-efficient storage mechanisms to even out the supply does not help it at all.
Really, the answer to most questions of this kind is “cost”. It is the default and usually correct answer.
But “cost” isn’t very specific. Is the major problem that it’s too inefficient? That it takes too much to build? Too much to maintain?
Doesn’t the energy grid need good storage anyway, to even out differences between day and night?
If it were cheap enough, maybe, but at the moment the demand fluctuations are covered by power generating plants which come online in times of high demand (e.g. day) and shut off during low demand (e.g. night). Typically these plants burn natural gas.
Sufficiently cheap storage would be very useful, yes.
Actually, pumped storage hydro is used for the purposes than DanArmak describes; see my post elsewhere in this thread.