Turning the air green

A Nottingham professor has come up with a way of using wind turbines to produce compressed air directly from falling weight pistons and employing compressed air manifolds to gather energy from a variety of ‘green’ energy sources, wind, wave and tidal, plus solar warming – all of which could conceivably be exploited on a single offshore site. As in the manufacturing industry, compressed air may not be energy efficient, but pneumatics are convenient and cheaper to work with than electrical systems when it comes to energy storage and turning that energy into mechanical work. If the primary energy sources are free, pneumatics can become very economically attractive, because it is the cost of producing usable energy that is crucial, regardless of the efficiency of how that free energy is converted. The ideas come from Professor Seamus Garvey, Neville Rieger professor of dynamics in the school of mechanical, materials and manufacturing engineering at the University of Nottingham. Wind turbines are getting larger, not least because there is more wind higher up, and the ‘Britannia’ turbine to be installed at Blyth will be 150m in diameter and rated at 7.5MW. Tall may be good, but if anything goes wrong with the bearings or gearbox, it is a long way up and a major problem to access, especially if the turbine is offshore. One of Garvey’s ideas is to have a wind turbine with weighted pistons running inside tubes within the blades. As the blades rise towards top dead centre, the pistons fall down the tubes, compressing air beneath them. The obvious snag with this is that the centripetal acceleration exceeds the force of gravity for much of the blade length in current machines. His solution, outlined at a seminar on ‘Sustainable Energy: New Solutions for Physics and Engineering’ at the Institute of Physics in London, is either to have blades opposite each other and mechanically connect the pistons moving in diametrically opposite directions or to re-introduce some air to, as he puts it, “kick off” the downward traverses. Garvey calculates that, for a 200m diameter four blade machine, with wind at 12.5m/s, available power is 18.04MW, and with a tip speed ration of 4.32, the turbine will rotate at just over 5 rpm. He then calculates that the mass of each piston should be 55 tonnes, travelling in a tube of cross sectional area 0.55 sq m. This would mean that the bending moment at each blade root resulting from Coriolis forces would be up to 70MNm, which would require bracing the blades, as was done with some early wind turbines. But one of his strongest argument for pneumatics, whether the air pressure be directly produced by pistons moving within the blades or by a compressor attached to a conventional turbine hub, is that it provides an easy way to store large amounts of energy, in order to even out the delivery of energy in both the short term and longer term. Furthermore, the same facility could also store energy generated by wave power, which is inherently a reciprocating motion lending itself to driving a reciprocating piston air compressor, and tidal power. He has come up with a novel design for a wave-powered compressor, which takes advantage of changes in pressure as waves rise and fall. “The key to success is to allow small pressure ratios per stage,” he explains, with perhaps 10 stages to compress air from 1 bar to 70 bar. It might also be possible to employ the ‘Anaconda’ bulge wave generator, featured in Eureka’s April 2008 edition. For tidal power generation, Garvey has come up with the idea of a ‘Tidal ram’, which he describes as “a relatively minor extension of the well known hydraulic ram, which has been understood for around two centuries”. He even sees a place for solar energy as a means of heating compressed air, so that it expands and thus exerts a higher pressure. However, because waves, wind and sun are intermittent, whereas heavy demands for electricity peak at certain times of day, he argues the case for pneumatics delivering the means to provide sufficient storage to be able to produce several hours of peak power. Currently, he says, the UK consumes about 4000TJ of electrical energy per day and would need to store a significant amount of this, in order to be able to deliver it when most needed: perhaps 1000TJ. Clearly, batteries are not going to provide a solution and the largest energy storage facility in the UK, the Dinorwig pumped storage system in the Welsh Mountains, is only able to store 35TJ, and deliver 1800MW. But this cost more than £500 million in 1980, which is likely to be equivalent to £1.2 billion in today’s money and the UK would need more than 30 such installations, if its power generation was going to come from, say, 50% renewables. There have been trials of large-scale storage of compressed air underground. One - at Huntorf in Germany, operated by Eon - stores air in salt domes. This was commissioned in 1978 and has 310,000m3 of storage capacity. The air is compressed using a 60MW turbine, and energy is recovered using it as the air feed for a 290 MW turbine, fuelled by natural gas. A conventional gas turbine uses around two thirds of its shaft output to compress combustion air. Maximum air pressure is 70bar and normal minimum pressure is 43 bar, although the cavern would not collapse even if it were discharged down to 1 bar. A similar scheme, which went commercial in May 1991, exists at McIntosh, Alabama. In this case, the storage capacity is just below 300,000 m3. At full charge, pressure is 76 bar and air is discharged down to 45 bar. The gas turbine is rated at 100MW. Garvey’s solution is to store compressed air in large, flexible, weighted bags under water. The major advantage here is that the hydrostatic head of water above the bags, which remains much the same as air is pumped in or let out, maintains the pressure. An important aspect of any air storage scheme has to be conservation of heat, which is generated when the gas is compressed. Garvey does not go into details about this, but seems to have in mind a kind of regenerator scheme, where something is warmed by air being pumped in and makes this warmth available to air being released. He estimates that the cost of an underwater compressed air bag should come to no more than about £5,000 per MWh. Pointers * A wind turbine could compress air by having weighted pistons fall down long cylinders within the blades, compressing the air beneath them * Large air reservoirs could store air compressed by wind, wave and tidal power, all at the same time, aided by solar warming to raise the temperature and so volume and pressure * Flexible bag reservoirs under the sea could maintain constant pressure by the hydrostatic pressure of the water above them * Weighted bag cost is estimated at about £5,000 per MWh