The only thing you can say for certain about British weather is that it’s unpredictable. One minute the sun is shining. The next, you’re soaked through by April showers. Given our eclectic weather and island status, you’d think that we’d be an ideal location for wind power. But despite leading the world in offshore wind capacity, wind contributes less than three per cent of the UK’s energy needs. The power these sources generate is intermittent, and that’s a big sticking point for investment in the technology.
If, however, we could store that energy on a large scale and release it when needed, our low-carbon future would look far brighter. The problem is that electrical energy can’t be stored directly at an economic rate. It first has to be transformed into another form, and doing so causes losses in efficiency. Engineers are working hard to address this problem. The current front runners for energy storage are pumped hydro plants, batteries, thermal and compressed air plants. Of these, compressed air energy storage (CAES) is now being backed by growing numbers as showing the greatest potential for large-scale, cost-effective storage. Proponents say CAES could also help solve the problem of intermittent energy.
The basic idea of CAES is that excess power on the grid is used in an electric motor to drive a compressor. The compressed air is cooled and stored at pressures of typically 60-70 bar. At times of high electrical demand, air is drawn back from the store, heated and then supplied to a modified gas turbine. The energy from this high-pressure air, together with some thermal input, drives the turbine stage. The energy is then converted by an electrical generator and re-supplied to the grid.
The concept isn’t new. A compressed air storage system with an underground cavern was patented back in 1948, and the first CAES plant with 290MW capacity has been operating in Huntorf, Germany, since 1978. A further 110MW CAES plant has also been operating in McIntosh, Alabama, since 1991. But despite ambitions for being a ’green’ solution, both of these plants currently use carbon-based fuels as part of the process. During the compression stage, a large amount of energy is lost as heat. This heat has to be restored before the compressed air is expanded in a modified gas turbine, and so gas fuel is used to raise the temperature of the air. This means that both plants lose a certain amount of efficiency.
If the technical challenges facing AACAES can be solved, it could pave the way for large-scale energy storage
To increase the efficiency of CAES, there is potential to capture the heat produced when the compressors are running, store the heat and then recycle it when the cool air has to be warmed up again. However, this version of CAES Advanced Adiabatic-CAES (AA-CAES) has some major technical hurdles, particularly in developing a heat-storage system. If these can be overcome, AA-CAES could pave the way for a large-scale energy storage system, and the challenge has been taken up by researchers worldwide.
In Germany, researchers at Braunschweig University are looking at storing compressed air in salt caverns while maintaining constant pressure using brine fed in from a shuttle pond at the surface. Another project is being run by RWE Power, the German Aerospace Centre, Züblin and General Electric on heat-storage technology for CAES plants. The University of California, San Diego, is working on subsea structures for CAES. Meanwhile, work is being undertaken at Edinburgh University to assess ways of making AA-CAES viable.
’The overall process for adiabatic compression sounds simple, but the difficulty is how you handle the heat when you compress the air to 60 and 70 times atmospheric pressure,’ said Edward Barbour, a researcher in CAES at Edinburgh University. ’There are two main problems. One is that the heat is generated very quickly and that’s difficult for the machinery to handle, and the second is that your energy density suddenly becomes very low.’
At these pressures, the heat from compressed air can reach temperatures of 650°C. Seamus Garvey, a professor of dynamics at Nottingham University, believes he has come up with a solution that will allow for cost-effective heat storage. Garvey’s idea is to compress air in containments called Energy Bags held down on the sea bed in deep ocean water. ’Underwater bags are an attractive option because the sea acts as the pressure vessel,’ he said. ’You don’t have to pay for the full structure, just the structural material required to hold the bag down. No matter how full or empty your container is, the pressure stays the same, and that’s lovely for the machinery at the sea surface.’
In Garvey’s solution, heat is contained within a nine-layer floating nested thermal store. Three layers on the outside are made up of mainly sea water, suitable for temperatures of up to 100°C. Three further layers contain mineral oil as a heat-transfer fluid in a porous bed of rock fragments and can handle temperatures of 250°C. The three innermost layers use molten salt as a heat-transfer fluid in more rock-fragment beds. Temperatures of up to 450°C can be managed as part of the system and, according to Garvey, studies show that it can deliver effective turnaround efficiencies of more than 85 per cent.
“Underwater bags are an attractive option because the sea acts as the pressure vessel”
SEAMUS GARVEY, NOTTINGHAM UNIVERSITY
’The reason we do not store compressed air in pressure tanks at the surface is mainly cost,’ he explained. ’CAES has the potential to be the lowest-cost mass-energy store with costs per unit of energy stored in the order of £1/kWh-£10/kWh if you use nature to help you store the air.
Pumped hydro typically comes in above £50/kWh and most electrochemical stores are more like £500/kWh of capacity. Using surface pressure vessels to store the high-pressure air in a CAES system typically brings the costs similar to that of electrochemical storage.’
Designed and developed for Garvey’s project by Canadian firm Thin Red Line Aerospace, the bags use a butyl rubber bladder and a polyester-reinforced fabric for the outer surface. Special coated steel or aramid straps provide the main structural strength. At depths of around 600m, there will be enough pressure in one 20m-diameter bag to store around 70MW hours of energy. That’s around the same as 14 hours of energy generation from the largest offshore turbines currently in operation.
’From an engineering perspective, the exciting stuff happens when we think about energy storage as an integrated part of a system, rather than a bolt-on subsystem,’ said Garvey. ’Think about natural hydro power. Rain is the power source but you do not have to make electricity when it rains because the energy can be stored in an intermediate form and converted to electricity when it is needed’. Garvey says the same paradigm can be applied to offshore wind.
His research received a three-year grant from E.ON in 2008 and was recently boosted by another grant through the Research Fund for Coal and Steel as the potential for using mines to store compressed air is recognised. There are two small prototype Energy Bags undergoing trials in Nottingham and a further Energy Bag has been shipped from Canada to be put into the sea at the end of May.
By 2020, Garvey believes the UK should aim to install 200GWh of CAES, with a cost for the energy storage alone of between £0.2bn and £2bn depending on what compressed air stores are adopted. He believes a further sum of up to £10bn may be required for the associated energy conversion equipment. ’Without that kind of storage, the jobs that the government hopes will emerge around a wind-turbine engineering industry will be redundant again by 2021 as the grid struggles to remain stable,’ said Garvey.
As momentum picks up in CAES research, Garvey’s concept is gaining attention. It remains to be seen whether adiabatic compressed air energy storage will be viable, and whether Energy Bags are the right way forward. But without someone thinking outside the box, the concept of AA-CAES is likely to remain firmly on the drawing board.
Going Deep
The use of salt caverns for CAES is expected to place in competition with natural gas storage and carbon capture and storage, where caverns close to energy sources are required.
Garvey admits while the use of salt caverns for CAES would be cheaper, his energy bags can be far more flexible in location.
‘In the UK, we don’t have a lot of very deep water close to shore. But interestingly, the places where that water is are the same places where the wind power is best. So there is a beautiful symmetry there between where you would like to collect your wind power and where you would like to store your compressed air.’
Garvey added that the west coast of America, north coast of Spain, west coast of Portugal and the entire Mediterranean would be ideal sites for his Energy Bags.
Other research - water works
Researchers across the world are looking at using undersea structures for compressed air storage. The concept of storing compressed air underwater has been appreciated in other research groups. Prof Richard Seymour from the Scripps Institute of Oceanography at University College San Diego advocates using rigid concrete structures on the sea bed in the nature of upturned cups to achieve constant pressure stores.
In Canada, Hydrostor also aims to deploy flexible air containments deep underwater although this time in lakes. The company plans to pump energy generated from wind turbines into marine salvage bags moored to the bottom of the ocean with granite slabs.
The energy bags, which are about the size of a bus, have a modular design and can be scaled up using 2MW generating units. It is said to be able to operate at an efficiency of more than 70 per cent without adding heat. A project to install 225 bags has been planned at a Lake Ontario offshore wind farm. In the meantime, Hydrostor has conducted tests of its energy bags at a pool at Windsor University.
Elsewhere, Dr Ole Hededal at the Technical University of Denmark also advocates the use of bags, but ones that can be inflated deep under sand. If the sand behaved similarly to a liquid, the pressure in these containments would also be fairly constant.
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