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Underwater Pumped-Storage Hydroelectric Project Completes Its First Practical Test (forschung-energiespeicher.info)
What if you built massive concrete spheres -- 98 feet in diameter, with 10-foot walls -- under the ocean to help generate electricity during peak periods? Slashdot reader nachtkap reports that German researchers just finished testing their 1:10-scale prototype StEnSEA:
It was retrieved from Lake Constance, where it was submerged at a depth of 100 meters [328-feet] since November. The system was developed by the Fraunhofer-Institut IWES in Kassel, Germany in collaboration with its inventors... The German Trade Department and Department of Education and Research as well as the German construction company Hochtief are also involved with the project.
The system's hollow concrete spheres are intended to be used in conjunction with off-shore wind-farms to serve as energy storage for peak hours. The spheres are ultimately supposed to be submerged near off-shore wind-farms and pumped free of water with excess energy. When additional energy is needed during peak hours the system goes into reverse and water rushes in, driving a turbine... At 700 meters the system has a capacity of 20MWh, with a linear capacity increase as depth increases.
The system's hollow concrete spheres are intended to be used in conjunction with off-shore wind-farms to serve as energy storage for peak hours. The spheres are ultimately supposed to be submerged near off-shore wind-farms and pumped free of water with excess energy. When additional energy is needed during peak hours the system goes into reverse and water rushes in, driving a turbine... At 700 meters the system has a capacity of 20MWh, with a linear capacity increase as depth increases.
In what ways is this better than simply pumping water uphill
It's better if you don't have hills.
Pumps are very inefficient. I wonder why they wouldn't just use the excess energy to drive a motor/generator to pull an empty sphere towards the bottom with a cable and then generate energy in reverse as it rises up?
Conventional pumped storage systems have about 75-80% round trip efficiency, which is not that bad. One reason for the loss is evaporation from the upper reservoir, which would not be a problem for this system, so round trip efficiency in the 80+% range is realistic. That is not to bad if you have free electricity to begin with.
Stephan
After a certain height, the hanging weight of the water at the bottom causes the pressure at the top of the water column to drop below the vapor point, and all you get is near-vacuum water vapor going into the pump.
You've just reinvented hydroelectric power stations ;)
The practical problem to extracting a useful amount of energy from water is that you have to restrict its flow. You'd end up with a giant lake like every other hydroelectric system, except it would flood the city.
There's no getting around the fact that extracting kinetic energy from water makes it slow down. When it slows down it backs up. Its level raises as upstream flow is converted to gravitational potential energy in the form of increased head height while it is "waiting" to flow through the restriction.
If you want to allow the water to flow mostly unimpeded, you could only extract a fraction of a percent of the available kinetic energy.