Upward spiral

IMAGINE this Rube Goldberg-esque scenario: wind turbines in Wyoming create a current that is delivered on transmission lines snaking through Colorado to Utah, where the power is used compress air into salt deposits deep underground. The air is released, driving a turbine that generates electrical energy once more, delivered on yet more transmission lines through neighbouring Nevada to California, where, say, an electric vehicle has just been plugged in.

Last week a consortium of power companies proposed just such a scheme, to drum up 2.1 GW of Wyoming wind power 1,500 km away from where it will be needed. But such a convoluted journey comes at a price: about $8 billion. Through a quirk of geography, America’s strongest winds at the height of current turbines blow across a broad swathe of the Mid-West, far from most centres of population and industry. But if California is to meet its Renewables Portfolio Standard, a requirement for a third of its power to be generated by renewable sources by 2020, the state needs to tap every zephyr it can—even distant, costly ones.

There is a far larger prize to be found; all that Californians and others have to do is look straight up. A report by the US National Renewable Energy Laboratory estimates that raising wind turbines from today’s average height of 80 metres to 140 metres would render wind power cost-effective across 615,000 square kilometres of the country, unlocking up to 1,800 GW of wind power. That is more than America's total, currently installed electricity generation capacity.

But those 60 vertical metres can be harder to span than the many hundreds of kilometres to windy Wyoming. Taller towers and attendant higher-capacity turbines naturally require sturdier bases. But there's the rub: such bases won't fit the transport network. America's bridges and famously ample highways cannot accommodate cylinders wider than about 4.3 metres.

Engineering considerations have it that a welded steel tower 140 metres tall should have a base around seven metres wide, with walls 25 millimetres thick. By constraining the base to a width of 4.3 metres, the walls have to be strengthened, to a thickness of 80 millimetres. That can add 200 tons of steel to each factory-made tower, making them not only heavy but also prohibitively expensive. There are alternatives; some companies build the lower, wider sections of tall towers out of concrete blocks, tensioned with steel cables. Others assemble towers out of square panels and tens of thousands of bolts. Neither method is particularly easy or cheap.

Now Keystone Tower Systems, a start-up based in Cambridge, Massachusetts, thinks it has the cost-effective answer: welding the towers at the wind-farm sites themselves. Eric Smith, the firm's founder, has come up with a way to modify the spiral-welding technology that has been used for decades for the on-site manufacture of oil pipes in remote areas. This continuous process involves feeding flat steel plates into machines that simultaneously roll and weld them into a tube. Mr Smith’s innovation is to introduce a gradual taper that instead creates an elongated cone—perfect for turbine towers.

The approach uses steel directly from a mill, cut into trapezoidal sheets, a process that results in no waste and stackable sheets that can fit on roads and under bridges. Mr Smith estimates his on-site process could produce towers using a tenth of the labour per ton compared with conventional tower factories, and at half the price. Spiral towers could then make financial sense for farms of as few as five towers (the Wyoming project will comprise hundreds).

Two weeks ago Keystone was awarded a $1m grant by America's Department of Energy to complete a tower design for Vestas, the world’s largest turbine maker, and to integrate its mills into the wind-farm construction process. If widely adopted, Mr Smith estimates his site-built towers could knock 10% off the cost of wind-generated electricity. It would also obviate the need for transmission lines that cross several states. Then the decision to increase capacity in places such as California would be a breeze.