Pressure test
Giant bags of compressed air on the seabed could be a remarkably cost effective form of energy storage, and might pave the way for the largest wind turbines so far proposed. Jonathan Ward finds out more
Critics of renewable energy often cite the intermittent nature of most renewable sources as a key barrier to their widespread adoption. Today, the inability to match renewable supply with user demand is not really a problem: conventional power can always be flexed to meet demand. As renewable penetration increases, however, there will come a time when the ability to store energy becomes imperative.
Large-scale energy storage solutions exist, of course, and some, such as hydroelectric pumped storage stations, have operated for many decades. According to Seamus Garvey, Neville Rieger Professor of Dynamics in the Faculty of Engineering at the University of Nottingham, the cost of implementing enough of such schemes to allow a 100 per cent renewable energy supply are quite startling. "For the UK to meet its 2020 energy target (20 per cent of all energy from renewables) using offshore wind power, the cost of the turbines and associated infrastructure would be between £80 and £100 billion. The required energy storage, if you used the cost base of current pumped storage technology, would cost about the same again."
Storage
Alternatives to pumped storage are being explored, but few make sense at the large scale required to support a national grid. The cost of electrochemical energy storage, for example, is five times as much as pumped hydro. Thermal storage techniques are under investigation too - particularly as a storage mechanism for solar power, but, says Garvey, they are unlikely to be useful for wind or water energy schemes. "No one is going to convert electricity into heat and back again," he comments.
The final contender in the energy storage race is compressed air. There are already two large-scale pilot schemes, using compressors to raise the air pressure in large underground caverns during times of low demand, then releasing that pressure through turbines when demand increases.
Such an approach usually requires specific geology, however, which may be difficult to find. Garvey, by contrast, proposes an alternative approach to compressed air storage that makes use of a particularly common environment around the UK: the bottom of the sea.
Flexible
Under the proposed scheme, currently being developed by researchers at the University of Nottingham, air would be stored in large flexible bags anchored to the seabed at depths of around 600m. Garvey and his team have carried out cost calculations for several bag sizes and configurations. The most cost effective, he says, is likely to be a dome shaped design around 10m in radius and 8m high, formed of a coated high-strength polymer weave with added reinforcement from marinised steel cables and weighted down with iron-ore ballast. Each individual bag is capable of storing about 6MW hours of energy and a large-scale installation would include several hundred connected together.
The storage capacity of Garvey's proposed system is closely related to the depth of water in which it is installed, while the construction cost remains fairly constant regardless of depth. At depths of 600m - found reasonably close to shore off Irish and Scottish coasts - Garvey estimates his compressed air system could cost as little as £5,000 per MW hour, around one twentieth of the cost of pumped hydrometric storage.
Prototypes
Interest in the concept has been significant. Garvey and his team have secured a €320,000 grant to construct and test small-scale prototypes of the storage bags. The nine month tank tests, due to be carried out this year, should prove that the fabric bags can cope with the pressure changes required for a 20 year life under the sea. If these tests are successful, Garvey plans a real life undersea test with a 5m diameter bag.
The storage element of the compressed air system is only the first stage in an extremely ambitious renewable energy scheme, however. "Once you have the bags," says Garvey. "You need to think about how the compressed air will be generated."
The simplest solution here, he says, is to use electrically operated compressors to absorb excess power from the grid. This approach will be simple to install and integrate, but has inevitable losses in the conversion process. A second approach, already under development commercially in the US, is to replace the electrical generation equipment in the nacelle of a conventional wind turbine with a mechanical compressor. "In fact, we think wave power generating direct compressed air will come first," says Garvey. "We already have designs that can do this using extremely simple mechanisms with only a single moving part."
Ultimately, however, Garvey hopes that direct compressed air generation will enable a radical redesign of wind turbine technology. "Torque is the limiting factor in large conventional turbines, whether their rotating machines generate electricity or compressed air," he says. The largest turbines under construction today are already bumping up against this limit, suggests Garvey.
Alternative
Garvey's radical alternative is to remove the rotating machine from the nacelle altogether and use a different design. In this a very large, relatively slow, multi-bladed turbine is built with pneumatic cylinders running the length of the blades. As the turbine rotates, it lifts a heavy piston in each cylinder, these will fall under gravity, generating a pulse of compressed air as they do so.
As the design does not need to transmit high torques, it could be very large indeed. In fact, says Garvey, it will only make sense as a scale beyond that of largest conventional turbines. He proposes 200m diameter turbine attached rigidly to a floating triangulated framework that would rotate in its entirety to face the wind.
Is the industry ready for such a radical departure? Garvey hopes that it might, in fact, provide a way into the renewables sector for a number of companies in currently operating other areas: aerospace, defence, offshore oil and gas and conventional power-generation for example. "Every key technical problem in the design has already been solved in another sector," he says. "I think that when some of these companies come together and combine their expertise, this will be seen to be an entirely manageable challenge."
Subcribe below for your
FREE newsletter.