Hydropower, Storage, Utility Scale, Wind Power

Storing Energy in the Sea — A New Design for Marine Energy Storage

Engineers in Germany are gearing up for pilot-scale testing of a promising new design for marine energy storage.

The Stored Energy in the Sea (StEnSEA) project represents a novel pumped storage concept aiming to facilitate large-scale storage of electrical energy that’s cost-competitive with existing solutions.

Since early 2013, the three-year, consortium-backed project led by the Germany-based Fraunhofer Institute for Wind Energy and Energy System Technology (F-IWES) and supported by Germany-based Hochtief Solutions, has delivered promising results: from concept design and analysis, through to developing a road map for market implementation.

The technology leverages water pressure to drive electromechanical pump components housed within a central tube of submerged spherical storage units. These spheres, constructed of concrete, operate in a manner akin to pumped-hydro storage, as Jochen Bard, Head of Energy Process Engineering at F-IWES, told Renewable Energy World: It’s a straightforward principal — physically speaking, it’s the same as a conventional pumped-hydro scheme featuring upper and lower reservoirs. Naturally, technical realization of these principals is different, however.

Detailing the concept, Bard said: “What we have is a pressure tank that maintains a lower pressure than ambient pressure of the water head above the device, the water column. Assuming the tank is empty, it has a very low pressure.”

He added that, “when you release water into the tank, the pressure of the water column is driving water through a turbine. This process generates energy, and represents the discharge part of the cycle — similar to water flowing through turbines, down into a lower reservoir, in a pumped-hydro system.”

Conversely, Bard said, pumping water out of the system requires energy as the pump is working against the pressure head of the water column.

“This is analogous to pumping water from a lower reservoir up to a higher one,” he said.

For this system, water depth is crucial.

“It’s clear that the deeper you install the system, the higher the pressure, so the more energy you can store within the tank,” Bard said. “But there’s a limit to that — at extreme depths, the system is infeasible. So the highest head we’re using is around 800m. In the end, to remain competitive with existing storage solutions, we’re targeting installment within the 600 to 800m range.”

At commercial scale, StEnSEA envisions arrays featuring 30m-diameter spheres, each with a storage capacity of around 20 MWh at 700m depths. This size, Bard said, “was found to be a reasonable compromise between all design, economic, and manufacturing parameters we needed to take into account.”

StEnSea’s geographical site requirements may appear at first to reduce applicability of the technology to several regions; but the consortium has undertaken comprehensive analysis that serves to relieve such concerns.

“As part of the project we undertook a detailed analysis of eligible sites for the technology, using geographic information systems featuring a list of criteria — distance to shore, distance to ports, sea depth, slope of seabed, exclusions zones etc. — we see there’s great potential for the application of the technology,” Bard said.

Of relevance for European stakeholders, one site in particular is highlighted: the Norwegian Trench off the southern coast of Norway holds technical potential of 8 TWh. Ideal site conditions were also identified in the Mediterranean Sea, the Pacific and Atlantic coast of the U.S., and Japan.

A Cost-effective Solution

Early project work focused on design, cost-efficiency and feasibility of the concept and produced several important outcomes, not least confidence in the physical and financial viability of the system even under conservative assumptions.

We’ve looked at economic variations of the system ranging in scale, from arrays of five to 10 spheres, to up to more than 100. At that latter scale, we’re looking at several hundred MWs, so about comparable with typical pumped-hydro systems,” Bard said.

He added that, “the economics have turned out nicely — all things considered, CAPEX would be very similar to pumped storage; 1,500 euros to 2,000 euros (US $1,675-$2,231) per kW (location dependent). It’s very encouraging for a storage system. Projected efficiency is also very comparable to pumped storage, somewhere in the region of 75 percent cycle efficiency is what we’re expecting.”

Commenting on estimated lifetime costs of storage, he said, “[at] 1000 cycles/year (3 cycles/day), 20 MWh storage per sphere, 5 MW pump turbine, four-hour charge/discharge cycle – [we expect] a levelized cost of storage of about 2 euro cent per KWh. That’s a very competitive price.”

Pilot Testing

The project is currently preparing for its second phase: a small-scale test at Lake Constance, on the Swiss-German border, featuring a fully functional 1:10 scale model of the storage unit installed at water depth of around 100m.

Looking forward to the test — scheduled to begin in the second half of October — Bard says there were several motivations for the pilot, not least the opportunity to gain experience relating to construction and installment of the system. Additionally, he said, “[the test will also] generate field test data to prove our concept and validate our assumptions.” This test is being held prior to a full-scale, open sea pilot of 30-m diameter sphere, a date for which is yet to be confirmed.