Think tank: Redesign our wind turbines

The Sunday Times
January 25, 2009
A novel invention could be more efficient than current systems

Ireland’s west coast is one of the best places in the world for harvesting renewable energy, largely because of the strong winds. It is also an ideal location for large, offshore wind turbines and wave-energy converters.
Ireland has easily enough natural power to supply its entire energy demand several times over until well past 2050. The main obstacles are how to make the systems cheap and unobtrusive, and how to keep the lights on when the wind is not blowing.
I propose a single solution — integrated compressed air renewable energy systems (Icares). The idea is that wind turbines, wave-energy converters and tidal turbines should compress air rather than generate electricity directly. When the wind is strong, some compressed air is used to drive large high-speed generators and the rest is stored in flexible containers under water. When the wind is less strong, the stored air is resurrected and used to keep the generators turning.
Bigger is better for wind turbines, but conventional designs have hit a ceiling in size. Scaling up these machines further will raise the cost per unit power. By contrast, my turbines only become practical at 200m diameter and improve steadily above that.
Being able to store energy was not important in the past because the output from power stations could be controlled. However, when more than a quarter of the total electricity supply is from intermittent renewables such as wind, there will be times when the total power being generated by the wind turbines exceeds demand across the country.
The industry has converged on a consensus that large wind turbines should be three-bladed, horizontal-axis machines on top of a tubular tower.
Fine, up to about 150m in diameter. The problem for larger turbines is that they have to turn very slowly and, in order to get all of the power out, the machinery inside has to have a much larger turning force (torque).
A 150m-diameter machine would generate up to 8 megawatts (MW) of power, enough for 17,000 homes. The torque would be equivalent to hanging 11 tonnes on the end of one of its 75m blades. By contrast, a 300m-diameter machine would generate up to 50MW of power. The torque would be equivalent to hanging 58 tonnes on the end of one of its 150m horizontal blades. Although the power increases by six, the torque increases by more than 10, and most of the cost of the larger machines depends on the torque.
My turbine is dramatically different: a horizontal-axis machine with eight blades — four long and four short. A floating framework replaces the tower, and it converts wind power internally within the blades. Think of a bicycle wheel rotating slowly, and a loose bead on each spoke. The beads represent pistons travelling back and forth inside tubes in the blades, compressing air as they do so.
The baby of the family is a 200m-diameter machine producing 18MW in a decent wind and costing less than 40% of the ¤40m you’d spend on a corresponding set of direct-generating machines. Her big sisters might easily reach 400m in diameter and could be 50% more cost-effective.
Of course, nobody would want such a monster outside their front window. The logical place for these titans is tens of miles offshore where even a structure half a kilometre high is not offensive.
Energy-storage capability comes as an added bonus. I am suggesting “energy bags” held down on the ocean floor with ballast weights about 600m below the surface. Ireland has such deep water in the Atlantic suitably close to shore.
To store 10% of our average total daily electricity generation in this way would would cost less than €300m. That would mean 1,000 bags 25m wide when full.
If you think that all of this is hot air, you would be right — in part. Air compressed to 60 times atmospheric pressure will get hot — up to 630C.
As part of a €60m research initiative, E.ON recently funded my research on this.
The time is right for Icares. Interest rates are low, a highly skilled workforce is under-utilised, all of its components can be sourced in Ireland and the UK, and energy security is more important than ever. Icares could form the basis of a substantial indigenous industry with large export potential. Best of all, there is a ready-made 100% reliable customer base for the energy — ourselves.
Seamus Garvey is professor of dynamics at the University of Nottingham. This is a synopsis of a lecture he will deliver at University College Dublin on Friday