I recently joined a discussion about how gravity might be used to generate and store energy. One of the comments provided a link to Gravity Power, a company that has proposed a modified take on "pumped storage" whereby a vertical water reservoir is used with a heavy piston. During the discussions a few variations on this technology were proposed. I suggested that abandoned open pit mines might represent a good starting point for very large facilities.
As in my earlier posting on Funicular Power the principle behind Hydraulic Energy Storage is to use excess electricity generated mainly from wind farms when demand is low (for example at night) to raise the potential energy of a mass by moving it to a higher elevation. In this case the means to do that is a relatively standard hydro turbine in a very non-standard configuration.
In energy storage mode a massive solid piston is raised by increasing the water pressure below it by running the turbine in reverse, acting as a pump to force water down the penstock.
In generation mode the piston is allowed to sink forcing water back up the penstock and through the turbine.
The piston would be a large concrete "cup" filled with as heavy a material as could be justified by the economics of the project. This could be rock debris, dense concrete, or even iron ore. The denser the material the better.
The containing cylinder would also have to be reinforced concrete. Between the cylinder and the piston there would have to be a pressure seal. This could be a large rubber or plastic tube such as that used to contain oil spills.
The advantage of using hydraulic storage is that it can be scaled up to a truly massive size. A large hole, such as that left behind after an open pit mine has been abandoned, would accommodate a gargantuan cylinder and piston (for example the Marmora Iron mine shown below);
The facility described below would use only a portion of the Marmora pit;
200 Diameter of piston (m)
50 Height of piston (m)
2200 Density of concrete (kg/m3)
1,570,795 Volume of piston (m3)
3,455,749,000 Weight of piston (kg)
1,884,954,000 Bouyant weight of piston = concrete - water (kg)
150 Piston movement = Mine depth - piston height (m)
2,770,882,380,000 Energy (Joules)
769.69 Energy (MW-Hours)
76.97 Energy in MW if generated over 10 hours
The concrete pour required to line the hole and create the cylindrical "cup" is not overly large compared to a major hydro dam. A solid concrete piston would be rather expensive - on the order of $150 million in this example. It would be much cheaper to fill the "cup" with rock debris although this would be less dense. Increasing density by adding iron filings or using "dense" concrete would be useful but expensive.
Based upon other large engineering projects and mining operations this facility could probably be constructed for less than $1 billion - possibly less than $500 million. While that is a large amount of money it would provide 86 times the energy storage capacity compared to the largest battery complex in North America for less than 20 times the price. The Notrees facility completed in December, 2012 by Duke Energy cost $44 million to construct and the battery performance will degrade over time. Hydraulic Energy Storage, which uses exactly the same components as a hydro dam, would have a useful life of as much as 100 years.
Rather than trying to use an abandoned open pit mine which might be a long distance from transmission facilities Hydraulic Energy Storage could also be located close to a wind farm although that would involve additional costs associated with excavating a new hole.
When it comes to long-term, dependable and reliable energy storage there are not a lot of options available. Creative use of existing technologies (see unpumped storage) or investigation of untested concepts such as Funicular Power and Hydraulic Energy Storage have to be on the agenda if we are serious about transitioning to a sustainable energy environment.