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Concrete spheres could deliver feasible energy storage for offshore wind turbines

Concrete spheres could deliver...
A new system being developed at MIT would store excess energy in concrete spheres on the sea floor
A new system being developed at MIT would store excess energy in concrete spheres on the sea floor
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A new system being developed at MIT would store excess energy in concrete spheres on the sea floor
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A new system being developed at MIT would store excess energy in concrete spheres on the sea floor

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.

Source: MIT

35 comments
Nigel Giddings
Interested to see the suggestion of using these as anchors for wind turbines, surely if they are hundreds of metres underwater it wouldn't make sense to use them as a form of foundation (anchor) for a turbine as it would need hundreds of meters of structure to get the turbine above the water line? Storage of energy appears to be the biggest issue for renewables, especially wind based systems, but being able to store a 'few hours' doesn't deal with the extended periods needed for when the wind isn't blowing. I'm not sure about off-shore wind, my knowledge is based more on the variations on-shore but you can see the variations in UK wind produced energy here http://www.gridwatch.templar.co.uk/ which highlights the fluctuations we currently see. I'm trying to be positive but there is still a long way to go to match the on-demand system that we have today. Maybe we should be looking at people using power when it is available and deal with the issues that creates...
pATREUS
@ Nigel Giddings - There are plenty of seaborne wind turbine designs that can float on the surface. Look up the FLIP research vessel to see equivalent technology or 'floating wind turbines'. Once in place, the turbines are tethered securely in place to the seabed. But your point about storage is absolutely correct. Once we solve this power storage problem, and make it portable, we will start to see the technologies we all dream of =).
Jan Vegt
A rather "heavy-weight" a solution ... I still think the energy bags as proposed by Seamus Garvey (Nottingham University) are a cheap and elegant - thus lightweight - solution. Admittedly you need some water depth, but at some places that's not a problem at all. Nice background article: http://www.theengineer.co.uk/in-depth/the-big-story/compressed-air-energy-storage-has-bags-of-potential/1008374.article
angryman77
I wonder if they considered using a low-density object that could store buoyancy energy when pulled under the surface of the water. The density of concrete is ~2320 kg/m3 and the density of water is ~1000 kg/m3. So, for a 1 meter cube, the downward force, ignoring density changes for water at increased depths, is not much different than the upward force of a void at the same depth. Given that it's less resource intensive to build a void than it is to build a solid, I think they should consider something along those lines.
mystixa
Looking at various reports and studies, we have a lot of information on the intermittent of wind power. Its pretty site specific, as some places are just more windy then others. In general for the united states it appears that wind turbines run at full power about 25-30% of the time. I didn't look deeper to see how much of that 75% was complete dead wind. The thing is the spheres don't have to make up for that entire 75%. They will be needed only when demand is high and wind is low. Then you switch on the spheres, which have a known power period. We already have backup systems in place, with gas turbines. Unfortunately since the power has to be ramped up at a moments notice we have to use open cycle turbines, which are far less efficient then the close cycle versions. Closed cycle can be brought up to 2/3rd power in about 1 hour. So if you have a few hours worth of sphere power online, it gives you time to start up your super efficient backup systems in an organized manner. Also the system can be expanded as you go to allow for long and longer sphere cycles, using the backup generators less.
Max Kennedy
Interesting idea but unless it can be done at about the same cost as conventional anchors it's a very expensive option. Since internal pressures need to be the same for the air to displace the water anyway the storage can be a lightweight structure anchored to the seabed and doesn't even need to be at the turbine site. That would permit it to be situated in deeper water and thus have a higher energy density which means for the same storage capacity you can have a smaller less costly unit.
Robert Moynihan
What a waste of time materials and effort. Alternative energy research will never be what it should be until government investments are driven into low cost low maintenance consumer based systems that individually feed back into a rebuilt grid. The wind is always blowing somewhere and the sun is always shining somewhere in the USA for at least 8 hours. 200 million home based units could power all of the USA and probably Canada and Mexico at the same time
Synchro
There's lots of wild conjecture about intermittent supply, but this is a half-decent study: http://www.stanford.edu/group/efmh/jacobson/Articles/I/Offshore/offshore.html It's no surprise that the straightforward solution to intermittency is scale, but most of the fossil fuel brigade try to make the case that if it's not windy in one place then it's not windy anywhere; this study shows that that assumption is wrong.
StWils
There are at least TWO existing alternative approaches to making these spheres or any similar really large underwater structure. First, electro-deposition can, and has been used to build large underwater structures. A steel wire and rebar structure defines a skeleton through with a current is run which causes mineral ions, ( calcium & magnesium mostly), in seawater to electro-deposit onto the wire form. These can be made arbitrarily large and thick at the intended depth, literally, deposited/built in place. Alternatively this could be an ideal use for a team of robots to place concrete on a form. The form could be the same kind vinyl sheet used for swimming pools only far larger and inflated with sea water to establish the desired shape. Think something on a scale of a blimp only filled with seawater. Concrete is continuously mixed and placed by many small bots centrally controlled so that the leading, or "wet" edge of concrete is always being added to with more concrete. The objective is wind up with a single monolithic seamless structure cast in place. Third, both of these approaches could be combined in each sphere to gain the best of both construction techniques. This would side step having to build and then cart around an object significantly the size of a cruise ship or oil rig. Remember how well that worked from the news about Carnival cruises many big dead buoy ships and also several oil companies such as the oil rig that broke free in the arctic. Jockeying stuff that large against ocean currents provably has NOT worked well.
Adrian Akau
I think that more emphasis should be placed upon improvement of the grid system so that power could be moved from one area to another without the need of too much storage. We have many alternative energy sources and more will be forthcoming such as with wave or current (Gulf of Mexico) energy conversion