"The system can be flexibly used"
StEnSea works like a conventional pumped storage power plant, but does not operate with two basins but with hollow spheres on the seabed. Inlet water drives a turbine that generates electricity. If there is excess electrical power, the water is partly or completely pumped out of the hollow sphere, whereby a sphere can store up to 20 megawatt hours of electricity. The researchers recently tested a concrete prototype at a 1:10 scale in Lake Constance. In the interview, the head of the project, Matthias Puchta from Fraunhofer IWES, talks about the test results and the possible locations for such storage systems.
forschung-energiespeicher.info: You have sunk a three-metre concrete hollow sphere in Lake Constance and have done a whole series of tests. What exactly did you investigate during the one-month testing phase and what results are already available?
Matthias Puchta: We could actually successfully store energy and drive a variety of different cycles. It worked just as we had imagined. An important question that we also pursued during the test was: Does the entire thing only work with a pressure equalisation line to the surface? This ensures that the air pressure above the water column in the sphere always remains at one bar until complete filling. Or is it possible without a pressure equalisation line? If this connection to the water surface is not needed, this would save considerable effort and costs. We were able to show that both operating modes – with and without pressure equalisation – actually work.
In the concept without pressure equalisation, you would therefore be working with negative pressure?
Puchta: Precisely. In the concept without a pressure equalisation line, a small amount of air would be left in at the top. This expands and the pressure lowers to the water vapour pressure. If the water is then allowed to flow in, the air compresses again. If the air volume is not too large, there are only small losses compared with the pressure equalisation line concept. In Lake Constance we also varied the air volume and recorded corresponding data while operating the storage system.
Were there exciting moments during the test?
Puchta: In addition to achieving the first storage cycle shortly after the installation, the most exciting thing for us was that we were able to switch between the two operating modes. We have provided a pressure equalisation line in advance. We also now want to validate simulation models for the large sphere with the data obtained from the model experiment. This allows us to reliably simulate the 30-metre sphere afterwards.
You are transferring the results of the three-metre sphere to 1,000 times the volume and six times the water depth. How do you dimension these?
Puchta: You have to imagine it as follows: The buoyancy of a body is determined by the displaced volume. For a specific sphere size the volume is fixed first. To enable the concept to be realised without anchoring on the seabed, the concrete sphere must have a greater weight than the corresponding buoyancy so that it stays securely on the seabed and does not float.
If the concrete was dimensioned only according to the pressure load, then the wall thickness at a depth of 750 metres corresponds almost exactly to the weight that the sphere must have to stand on the seabed. To compensate for the buoyancy, the sphere in Lake Constance has a wall thickness of 30 centimetres at a water depth of 100 metres. Mathematically, the sphere at a depth of 100 metres would only need a smaller wall thickness in order to just meet the mechanical loads, which would also make it lighter. We see an optimum for the material application in a water depth of 600 to 800 metres. If the distance was more than 800 metres, the wall would have to be thicker so that the sphere could withstand the pressure.
The global potential is 817 terawatt hours
The optimal hollow sphere therefore needs a water depth of 600 to 800 metres. Such circumstances are rather remote from the mainland and also away from offshore wind energy installations. Where do you see the application areas of your concept?
Puchta: There are several ideas. Ultimately, these are always very location-specific. There are areas that have a steep fall of the sea near to possible offshore wind turbines. For example, Japan offers many potential locations. Japan is also intensively researching floating platforms. On the other hand, there is also the option of using such systems at locations near the mainland as pure storage systems. This does not necessarily have to be associated with offshore wind, the electricity can also come from other renewable sources. For example, we are also considering an option in front of the Norwegian channel or integrating it into an offshore network. The StEnSea system can therefore be used flexibly and flexibly configured through different numbers of spheres. There is a worldwide potential of 817 terawatt hours of installable storage capacity.
Did you determine the storage capacity of 817 terawatt hours in view of its achievability?
Puchta: Precisely. We worked with a geo-information system. There we have adopted various parameters, such as ground inclination, soil structure, bottom current, geology, distance from the port, distance to the mainland and water depths. Unsuitable areas are excluded or the areas are joined together. Large areas and thus potential are available, for example, in Europe, USA and Japan. Germany could also benefit from storage capacities in EU countries such as Spain or Italy in a closely linked EU electricity grid.
The example concept envisages an energy park consisting of 80 spheres at a depth of 700 metres. What storage capacity and performance could be expected from such an energy park?
Puchta: We expect an output offive megawatts and a storage capacity of 20 megawatt hours per sphere. Whereby, depending on the application, there are various scenarios for operating the spheres. You don't have to run all of them at the same time. For instance, they could also be shifted time-wise or switched on sequentially.
Comparison with conventional pumped storage power plants
And what's the efficiency like?
Puchta: For the large sphere we expect an efficiency comparable to conventional pumped storage systems, i.e. between 75 and 80 per cent.
While we're talking about conventional pumped storage systems: These have a serious environmental problem because they considerably change the landscape. Is your concept more environmentally friendly?
Puchta: Our concept uses the sea itself as an upper reservoir. This is naturally present. The only thing you have to construct is the lower reservoir. As we have shown in the model experiment, it is even possible to dispense with the pressure equalisation line. The technology itself is essentially composed of concrete, steel for the pump turbine and an electrical cable. Initially, these are not environmentally hazardous substances.
For example, if you take a look at our experiments in Lake Constance: This is a drinking water reservoir in which we have only installed the same concept on a small scale and had to have it approved. Our system therefore does not contain any special components that are hazardous to water.
What needs to be taken into consideration: A specific check of the environmental influences must always be made at the respective location and specific flow velocities maintained at the inlet and outlet. These must not be too fast, so that no fish or other animals are sucked in. That's why we have devised something special for our system that can clean the inlet and prevent marine animals from being sucked in.
Does limiting this flow velocity create an obstacle?
Puchta: No. This can be easily achieved with a diffuser. This is simply done by extending the cross-section of the inlet and outlet openings.
How much does it cost to store one kilowatt hour?
Puchta: We have made a detailed economic assessment. In doing so, we have included the maintenance, the installation costs as well as the costs for the ships and preliminary expenditure. We have varied the storage size between 5 and 120 spheres. The costs vary between 1,500 and 2,000 euros per KW of installed power. This is comparable with today's pumped storage systems. If it is assumed that you are running 1,000 cycles per year, this amounts to 1.6 to 2 cents per implemented KWh.
That sounds good.
Puchta: We were also very positively surprised at first. Of course, we hoped the values would be good. In particular, we have assumed rather conservative cost parameters. If a lower cycle number is assumed, the costs increase correspondingly. If I run more cycles, the price will tend to be smaller.
Surrounding technology a challenge
The concrete sphere is very simply built. But what's the situation with the other components, such as the pumps and turbines, which have to withstand the high pressure? Is there already such equipment on the market?
Puchta: The concrete sphere is not so simply built. Although the concrete itself is not high-strength, the challenge, of course, is how to cast and design a 30-metre sphere. No one has ever done this so far at such a large scale. Another challenge is the pump turbine. This must be specially designed for it. In principle, there are already pump turbines for such pressures. Ultimately, the logistics and the entire peripheral technology is also a technological challenge when constructing a 30-metre sphere.
Where are such pumps used today?
Puchta: There are already pumped storage systems that have a relatively high drop height. Then also in the offshore industry. A challenge – and here we have also learned a lot during the test experiment – will be what we call energy systems engineering. How do I get a whole functioning system consisting of operation management, measurement technology, sensor technology and logistics from individual components? These are a lot of things that have played a major role in the model test and will also play a role when constructing a 30-metre sphere.
How do you imagine maintenance and logistics?
Puchta: The idea with the large-scale system is that we will service the turbines with an underwater vehicle, since we do not want to have to retrieve the sphere for this purpose. Already during the model test we had the entire technology in a pipe provided for this, with the pump turbine unit and the measuring technology. This is also the idea for the large test. This means that later we will only have an electrical cable as a connection to the surroundings. You have to keep in mind that everything that is implemented at a depth of 700 to 800 metres requires considerable effort.
What is the next stage with StEnSea?
Puchta: The currently funded project is scheduled to continue until the middle of 2017. Until then, we will be transferring the results of the model testing to the large sphere. We then want to implement a larger sphere and test it at a suitable location in the sea. The successful trial in Lake Constance was already the first step in this direction to enable the later commercialisation of the technology.