Concrete spheres could deliver feasible energy storage for offshore wind turbines

The intermittent nature of wind and solar power generation is one of the biggest challenges facing these renewable energy sources. But this isn’t likely to remain a problem for much longer with everything from flywheels to liquid air systems being developed to provide a cheaper form of energy storage than batteries for times when the wind is blowing or the sun isn’t shining. A new concept out of MIT can now be added to the the list of potential solutions. Aimed specifically at offshore wind turbines, the concept would see energy stored in huge concrete spheres that would sit on the seafloor and also function as anchors for the turbines.

The MIT concept works by using excess energy generated by the wind turbines to pump seawater from a hollow concrete sphere sitting on the seafloor that measures 30 meters (98 ft) in diameter. Then, when the wind dies down and power is needed, a valve is opened to let the water back into the sphere through a turbine that drives a generator to produce electricity.

The MIT researchers say that such a sphere positioned in 400-meter (1,312 ft) deep water could store up to 6 MWh of power, meaning that 1,000 spheres could supply as much power as a nuclear power plant for several hours. They claim this is enough to transform offshore wind turbines into a reliable alternative to conventional on-shore coal or nuclear plants.

Additionally, since the system would be connected to the grid, the spheres could also be used to store energy generated from other sources, such as on-shore wind and solar, or from base load power plants that are most efficient when operating at steady levels. Such a system could reduce the reliance on the generally less efficient peaking power plants that kick in when there is a high electricity demand that base load plants can’t meet.

The spheres with their 3-meter thick concrete walls would weigh thousands of tons each, which would also make them suitable to anchor the wind turbines in place. However, because there is currently no vessel with the capacity to deploy a load of their size and weight, a specially built barge would need to be constructed to tow them out to sea after being cast on land.

This contributes to the preliminary cost estimates of about US$12 million to build and deploy one sphere, with costs gradually declining from there. The team estimates the technology could yield storage costs of around six cents per kilowatt-hour, which is considered viable by the utility industry.

While the team’s analysis indicates the technology would be economically feasible at depths as shallow as 200 m, with costs per megawatt hour of storage dropping as depth increases to 1,500 m before rising again, 750 m is seen as the optimal depth for the spheres. However, Brian Hodder, a researcher at the MIT Energy Initiative, says as costs are reduced over time, the system could become cost-effective in shallower water.

Alexander Slocum, the Pappalardo Professor of Mechanical Engineering at MIT, and his students built a prototype 30-inch (76 cm) diameter sphere in 2011 to demonstrate the feasibility of the system. The team now hopes to scale testing up to a 3-meter sphere and then, if funding becomes available, a 10-meter version that would be tested in an undersea environment.

They estimate an offshore wind farm using the technology could supply an amount of power comparable to the Hoover Dam, while using a similar amount of concrete. The team says that some of the concrete for the spheres could be made using fly ash from existing coal plants to cut the amount of carbon dioxide emissions resulting from production.

The MIT team has filed a patent for the system, which is detailed in a paper published in IEEE Transactions.