Question of the week: Do we need vertical-axis turbines?

As horizontal-axis turbines increase in size, some are looking to vertical-axis machines as an offshore solution. Windpower Monthly asked Andrew Scott of the Energy Technologies Institute and Philippe Veyan of EDF Energies Nouvelles whether the technology has a future.

A 2MW prototype of the Vertiwind is operating in France
A 2MW prototype of the Vertiwind is operating in France

Question: Are vertical axis turbines a necassary and practical objective for the wind industry?

Andrew Scott - Programme manager offshore wind ETI

The ETI has looked into vertical axis turbines in order to try and get to the bottom of whether they have any potential as a form of offshore generation. And it's a very sensible question to ask, as they do have some advantages that might be very useful when you go offshore.

One can't help feeling that there has to be a limit to the size of horizontal axis turbine that can be built. A 10MW horizontal axis machine is likely to have a 200-metre diameter swept diameter; that means the blade tip will be around 230m above sea level. That's going to be a big machine. Perhaps moving to vertical axis turbines could allow us to reduce energy costs even more, though economies of scale, by moving to bigger generators; we could be talking 20 or 30MW machines.

If you think of a horizontal axis you have lump at the end of a long stick that's being pushed by the wind. That's not the case for vertical axis. Vertical axis turbines have a very low over-turning moment. Their low or even positive over turning moment keeps them more stable.

But there are also big drawbacks. Vertical axis turbines are much less aerodynamically efficient. With horizontal axis machines, all of the blades are always pointing into the wind and are therefore working optimally. A vertical axis means the blades are only working fully at certain points in the rotation, making it a much less efficient structure.

They also tend to rotate much more slowly. And because the drive train size is inversely proportional to the speed of the rotation, the slower rotation means you need a much bigger drive train to extract energy from the wind. Larger drive trains substantially increase capital and maintenance cost.

By our analysis, a 10MW vertical axis turbine would be a little more expensive in terms of cost of energy than a similar sized(10MW) horizontal axis machine. If we look to the future, it is possible that the ability to build a 20 or 30 MW vertical axis machine will deliver scale and engineering benefits that overcome the drawbacks of vertical axis, delivering competitive energy costs. More work is need to establish if this is possible. This is a potential technology for the 2030s and beyond, rather than the 2020s.

Philippe Veyan - Project manager EDF Energies Nouvelles

Floating wind turbines are destined to grow in addition to standard offshore turbines which technical and economic reasons cannot be installed at depths beyond 40 to 50 meters. Floating turbines are ideal for depths between 50 and 200 metres. This innovative concept would thus multiply the implementation of offshore wind on coastlines with significant depths in areas such as the Mediterranean Sea, but also beyond Europe, Japan and North America.

The technology allows floating turbines to get rid of heavy work at sea, since the structures are simply anchored to the ground. Installation work will therefore be limited in time and be less detrimental to the environment.

It is this reason that prompted EDF EN has joined with other companies to develop a doubly innovative concept in that it is a new generation of wind turbine, vertical axis, specifically designed for integration on a float. We believe that vertical axis turbines are better suited to being integrated with floating platforms. A first prototype is being land being tested in port area of Fos sur Mer now and a second prototype will be tested at sea in 2016.

The industrial development of this new technology is planned from 2020, with a first pilot park being developed by EDF EN, under the NER 300 program. This technology has potential for optimisation and a reduction of cost which could make it competitive vis-à-vis other forms of energy by the end of the next decade.

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