By Dave Elliott
In my previous post, I looked at airborne wind power devices, which some see as a big new energy option. One major attraction is that tapping the high speed jet streams offers access to a much more continuous and reliable energy flow than using surface level winds. It means that the problems of intermittency can be resolved without having to resort to energy storage or complex grid balancing systems. But if ‘flying wind turbines’ sound too much like ‘Blue Sky’ thinking, then, coming down to earth, or rather under the sea, there are some new large-scale storage ideas, although they too are quite exotic. They involve giant underwater compressed air storage systems.
For example, MIT researchers suggest that electricity generated by floating offshore wind farms could be stored in huge hollow concrete spheres on the seafloor under the wind turbines. These structures, weighing thousands of tons, could, they say, serve both as anchors to moor the floating turbines and as a means of storing the energy they produce. ‘Whenever the wind turbines produce more power than is needed, that power would be diverted to drive a pump attached to the underwater structure, pumping seawater from a 30 meter diameter hollow sphere. Later, when power is needed, water would be allowed to flow back into the sphere through a turbine attached to a generator, and the resulting electricity sent back to shore.’
They claim that a 25 meter sphere in 400 meter deep water could store up to 6 MWh of energy, so that 1000 spheres could supply as much power as a nuclear plant for several hours, enough to make them a reliable source of base-load quality utility-scale renewable energy. The system would be grid-connected, and could also be used to store energy from other sources, including PV arrays on shore, or from base-load power plants.
The concept is detailed in an IEEE Transactions paper by Alexander Slocum, Prof. of Mechanical Engineering at MIT; Brian Hodder, MIT Energy Initiative; three MIT alumni and a former high school student who worked on the project.
Preliminary estimates indicate that one such sphere could be built and deployed at a cost of about $12 m, Hodder says, with costs gradually coming down with experience. This could yield an estimated storage cost of about 6 cents per kWh. Hundreds of spheres could, the team says, be deployed as part of a far-offshore installation of hundreds of floating wind turbines. They calculated the optimal depth for the spheres would be about 750 meters, though as costs fell over time they could become economic in shallower water. Analysis indicated that storage could be economically feasible at depths as shallow as 200 m, with cost/MWh of storage falling until 1500 m before beginning to rise. They say the weight of the concrete in the spheres’ 3 meter thick walls would be sufficient to keep the structures on the seafloor, even when empty. The spheres could be cast on land and then towed out to sea on a large specially built barge.
Slocum and some of his students built a 30 inch diameter prototype in 2011, which functioned well through charging and discharging cycles. The team hopes to extend its testing to a 3 meter sphere, and then scale up to a 10 meter version to be tested in an undersea environment, if funding becomes available. They estimate that an offshore wind farm paired with such storage spheres would use as much concrete as was used to build the Hoover Dam, but would supply comparable amounts of power. While cement production is a major source of CO2 emissions, the team calculated that the concrete for these spheres could be made, in part, using large quantities of fly ash from existing coal plants, material that would otherwise be a waste product, instead of cement. The team calculated that during a decade of construction and deployment, the spheres could use much of the fly ash produced by US coal plants, and create enough capacity to supply a third of US electricity. http://web.mit.edu/newsoffice/2013/wind-power-even-without-the-wind-0425.html
Researchers in Norway, working with SINTEF, have come up with a similar undersea storage sphere idea, with a 80% round trip efficiency claimed from spheres at 400-800 metres depth. They suggest that the system can be scaled according to users’ requirements, both as regards the turbine size and the number of water tanks, but ‘a plant of normal size will produce roughly 300 megawatts for a period of 7-8 hours’.
It sounds an intriguing concept. But 400-800 meters (or even 200m) is much deeper than any offshore floating system yet planned. Even so, using hydrostatic pressure is a clever idea, although Nottingham University Prof. Garvey’s version of this idea, using very large tough inflatable air bags, mounted underwater around the base of a conventional fixed wind turbine, might be cheaper, if perhaps less robust over time. Garvey has formed a university spin-out company NIMROD Energy to commercialize the technology, which he calls Integrated Compressed Air Renewable Energy Systems (ICARES) on which he’s been working since 2006. He claims 90% round trip efficiency.
It will be interesting to see which, if any, undersea system wins out. Localized energy storage, onsite near offshore wind turbines, has its attractions, since then the power transfer down the expensive to-shore grid link is more continuous. However, if you want big energy stores, it may be easier to build them on shore, e.g. using pumped hydro reservoirs or cavern-based compressed air storage or liquid air storage. Or conversion to hydrogen, for interim storage and then distribution on the gas main. It might even be worth looking at compressed air storage, or perhaps hydrogen storage, in old empty undersea oil or gas wells, if these have not already been refilled with captured carbon dioxide. That’s a bit of a long shot, but conventional on-land stores, perhaps in central locations near loads, could be fed via the grid with the varying inputs from a range of renewables at various, possibly remote, sites. There are certainly some interesting ideas emerging, including combining compressed air cavern storage with geothermal heat collection.
Which might be best? It all rather depends on what sort of energy distribution and grid-balancing system is envisaged for the future – but undersea stores do offer one possible option.