“Storing Energy at Sea (StEnSea)” is a novel pumped storage concept for storing large amounts of electrical energy offshore. In contrast to well-known conventional pumped-hydro power plants, this concept greatly expands the siting possibilities, and allows for modular construction and ease of assembly. Instead of two separated water reservoirs of different heights, the StEnSea concept makes use of the static pressure of the water column in deep waters. In order to use this potential, a hollow concrete sphere is installed in deep water. A pump-turbine in the hollow sphere enables the electrical energy to be stored as mechanical energy. When the water is flowing into the sphere, the storage is generating. In this case the pump-turbine is running in turbine mode, generating electricity. In order to re-charge the storage system, the water is pumped out of the sphere against the pressure of the surrounding water column. A schematic cross-sectional view of an energy storage sphere is presented in Fig. 1.
Figure 1: Schematic cross sectional view of the StEnSea concept
The Investigation
The concept was investigated by Fraunhofer IEE and project partners between 2014 and 2016 in Germany. A detailed system analysis was carried out including construction, manufacturing and logistics concepts of the pressure reservoir, and development and detailed design of the pump/turbine unit. In addition, grid integration on the basis of load calculations, market analyses and economic viability calculations for an international market, the development of a commercialization strategy and a roadmap for technical implementation were developed.
Phase 1
In the first phase of the project, a feasibility analysis resulted in the physical design parameters for the energy storage system. An advantage of the system is that the power and energy can be designed separately. While the power increases with the installation depth, the energy for one given installation depth can be increased by scaling up the inner volume of the sphere. The diameters depend primarily on two different design criterion. The first is that the gravitational force of the device has to be higher than the buoyancy force of the device to ensure that it stays on the seabed. The second is that the sphere must withstand the force of the pressure from the surrounding water.
There is an optimum between those two design criterions. While the pressure on the sphere increases with the installation depth, the buoyancy force is not dependent on depth. Obviously, the most straight- forward approach to maximize the power and capacity of the storage system for a given diameter is to install it as deep as possible. For the project and the first Full-Scale prototype, the diameters were the results of an optimization. Based on the fact that higher volumes lead to higher capacity, it may be assumed that it is advisable to build the spheres as large as possible, but in contrast to that, the size of the sphere has a big impact on the logistics of a project. Therefore, it has advantages to install several smaller spheres instead of one big sphere. The feasibility analysis lead to a compromise. The StEnSEA system was designed with an electrical power of 5 MW. The feasibility analysis lead to a corresponding inner diameter of 28.6 m with a wall thickness of 2.72 m, providing a volume (V_inner) of 12,000 m3. The analysis showed a commercial optimum as follows:
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- Diameter: 29 m
- Water depth: 700 m
- Wall thickness: 2,7 m
This results in a storage capacity of about 18 MWH.
Next Step
In the 2nd phase of the project, a field test of a 1:10 scale model with a diameter of 3 m was conducted in a lake in Germany. Details regarding construction and manufacturing, installation and logistics as well as operation & maintenance of the storage system were investigated. After this successful test, a follow-up demonstration phase with a larger model (e.g. 1:3) is being planned. As soon as suitable offshore test sites have been identified and the funding is secured, the demonstration in deep water will be started.
Bernhard Ernst, Jochen Bard, Matthias Puchta, Christian Dick
Fraunhofer IEE
Very interesting technology. Is there a large difference in equipment and/or maintenance costs if the system is placed in seawater vs. fresh lake water?
Interesting concept. What will the maintenance program look like for this type of power generation model? At 700m depth, it will be expensive to maintain. How will maintenance inspections be performed? Will you solely rely on condition-based monitoring? How will any corrective maintenance take place? Will you pull the whole assembly up from the seabed and conduct repairs on the sea surface?
The system is designed to run in seawater. So far we have not considered locations in fresh water. The test took place in a lake just for logistic reasons.
The maintenance concept foresees to lift just the pump-turbine-generator unit. The concrete sphere will stay on the seabed. With the unit at the surface all maintenance and repairs can be made on a ship or on land.
This is a very interesting concept, especially given that pump hydro is currently the backbone of existing energy storage solutions.
I wonder if the authors did investigate what would be the impact of bio-fouling on the system, it’s maintenance and durability particularly ?
Also is this concept adaptable to shallower water depth (200 – 500 meters), maybe at the cost of a modification of the geometry and size of the hollowed structure ?
This is a really nice concept, it has everything: high capacity, reliability, safety, invisibility (unlike wind turbines) and reasonable prices. The first time I read about it was in December 2016. After that I didn’t hear much more about it, hopefully it is still being worked on behind the scenes.
Looks like a really good idea,
I am sure with modern oilfield subsea technology and ROVs the unlatching and retrieval of rotating equipment for maintenance would be no problem.
Also the addition of a chlorine cell would be worth considering to inhibit marine growth.
Best Regards
Bob
Handling toxic chemicals on the ocean bed is likely a bad idea. Can’t the pumped out water not simply be put into a bag? Then it would be a closed system that can be filled with clean water initially.
Very interesting.
What’s the performance of the charge/discharge process? What is the CBA (Cost Benefict Analysis) outcome considering all the immediate cost?
Amazing! In the past i was thinking about giant submerged cyclinders, which could be filled from the bottom with big pressure and then drained from the top (surface) for less power usage. I thought that perhaps we could “cheat” and gain free energy, but probably that doesn’t work 🙂 . These spheres are a great idea, and i wonder why it took humanity so long to think about something like this? Batteries in the bottom of otherwise useless sea floor-brilliant! Perhaps these spheres could be made to float for the “charging” cycle and then to be pulled back on the floor for discharging, making the charging/discharging cycle more efficient? I guess i am still looking on cheating physics laws :D. Amazing. Looking forward to see it used in large scale.
I think it’s a great idea. I wonder how efficient it would be to do something like this but with flexible air filled spheres instead of compressed water. Assuming there’s materials in existence that could handle the stress of expansion and contraction process for a sufficient amount of time, you could anchor some to the sea bed and then connect them via tubes to floating platforms at the surface. There, the pumps/turbines and monitering hardware could be housed safely above the water. That would allow them to be visited for inspection or towed in for servicing when needed. Even better, it might be possible to step such a system up by combining it with existing or future energy storage systems. If their installed into pumped hydro reseviors or in costal areas with high depth contrast from natural tidal activity, it could greatly help minimize the costs pressurizing the sphers and thus the cost of storing energy. Additionally, working with air would make it all less susceptible to any internal build up that you’d have to deal with when using seawater as the working fluid. Anyone tried something like that yet?
this seems like a really good idea charles. The only issue I think with your point would be that how could you possibly integrate it with something like a fuel cell. What extra benefits would it give us by combining it with lets say a pumped storage system, (and would costs and difficulty go up?) Very good points though!!
Would it be possible to combine this concept with wind energy, with excess energy from the wind turbine beings used to run the system, so that in effect it serves as a PSH