Note: Updated the calculation, found an error. With that, the concept isn't economically viable any more.
I wanted to share a concept for a possible cheap large scale energy storage concept that has been at the back of my head for quite some time. I couldn't find a reason that would make this concept both technically or financially unviable, so I'm eager to hear any criticism.
Ocean water has a mean salt content of 3.5% per mass (3.5 g/L).
If you put water with less salinity in a bag, it will float, with more it will sink in the ocean.
The maximum possible salt content is 359 g/L. Thus one can calculate the potential energy of a bag of water with 360 g salt dissolved that is at sea level, and sea ground is 300 m below sea level as:
E= (0.36 kg – 0.035 kg )* 300 m * 10 m/s2 = 975 J.
The pure water doesn‘t have any potential energy, and we also need to subtract the salt content of the sea water. Only the difference of the salinity (effective weight) leads to potential energy.
Summary: With water having a higher salinity than ocean water it is possible to store energy at sea level with respect to the ocean floor.
The desalination plant in Carlsbad, California produces 50 million gallons of fresh water a day (Source: Can Desalination Plants Quench California's Thirst For Water In A Clean Way?).
For desalination, they produce high salinity water (20% raised salinity, see article) that is piped back into the ocean. But in this concept, this is an effective weight of 6624 metric tons, or at 300 m height difference, this is 5415.52 kWh of stored energy.
An energy storage system using this high salinity water would need 3 main components.
A tank at sea level. Here one could think of building an artificial lake at the coast using a dam, or putting a large plastic bag into the ocean near the coast, and securing it against the waves.
A tank at ocean floor level. Again, this would be mainly a large plastic bag, but one needs to make sure that it is possible to empty this bag fully for maximal energy storage. A height difference of a magnitude > 100 m can be often found within 10 km of the coast (this is actually the case at Carlsbad)
A pump/turbine + pipelines. In order to use pressurized pipelines, one would need to operate the pump/turbine at the ocean floor tank. Using suction enabled pipelines (for example steel) one could also operate the pump turbine onshore.
Limitations of the system
- need for big amounts of high salinity water. Operation thus only close to desalination plants, or close to potash mines (they often have large salt heaps, at least in Germany)
- Environmental impact (1): Placing the sea level tank on the coast line will have drastic impact on maritime life. It should be also possible to install the tank offshore, but with the additional need for large floating structures keeping the tank adrift.
- Environmental impact (2): The tank on the ocean floor will also impact maritime life. However, in depths of > 100 m, maritime life on the floor is less abundant because of the lack of light.
I‘m not an engineer, and costs clearly depend on the solution, especially where and how to construct the upper tank.
However, I think that the costs should within 1-3 times of that of a usual hydroelectric storage system, with
- costs for power electronics + O&M: 1002.5$/kW (not per kWh! Additionally capacity doesn't affect the price here)
- cost for storage unit + load balancing: 12.5$/kWh
See here for details: Analysis of the Cost per Kilowatt Hour to Store Electricity
(IEEE Transactions on energy conversion, vol. 23, no. 2, June 2008, page 529, PDF)
That means the system is easily competitive with current battery technology, with the further upside of having an infinite number of charge/recharge cycles.
Conclusion I think this concept might be viable both commercially and technically. The environmental impact needs to be verified, but might be similar to hydroelectric pump storage, and could be reduced by installing all tanks offshore with the additional need for floating devices.
Furthermore, desalination plants at the coast might use this concept to generate electrical energy without using any tanks, by just installing a pipeline to a deep spot at the ocean floor, and letting the high salinity water sink down. But in that case, it needs to be checked if the diffusion of the salt might be too fast, so that no actual water is moved. This could be solved by small temporary tanks on the ocean floor that can be used as a one way sluice.