Nénuphar's vertiwind concept was launched as a pilot project last year by Technip, Converteam & EDF Technologies

By Andrew Williams

Interest in vertical axis wind turbines (VAWTs) is growing and several design concepts including DeepWind and VertiWind are undergoing development and testing. So, how do they compare with horizontal turbines - particularly in terms of performance, costs and ease of installation?

According to Uwe Schmidt Paulsen, Senior Scientist at the Technical University of Denmark and Manager of the DeepWind project, one advantage of VAWTs is that they have an 'omni-directional' operation and low centre of gravity - leading to good structural stability.

The DeepWind concept also dispenses with struts, simplifying structural complexity compared with previous designs. Paulsen argues that large-scale VAWTs could be better at avoiding the 'gravity issues' that plague large horizontal-axis turbines (HAWTs), which can quickly exhaust blades.

DeepWind uses the water as a large bearing, supporting the spar buoy and simplifying the structure from an engineering point of view, which is expected to reduce costs. The use of blade pultrusion reduce the costs of a 1m blade to around a third of those for HAWT blades, at least on the drawing board.

Paulsen is under no illusion that structural VAWT design is a challenge but is confident that technology has improved and can adopt much from other fields of technology development within HAWT aero-elastic design.

"We think that [our] simple design is a first approximation of a real offshore wind turbine where dedicated technological solutions have been used and not just moving the onshore turbine to [an] offshore environment," he says.

Meanwhile, Dominique Roddier, CTO at Principle Power explains that one of the disadvantages of VAWTs is that it can be difficult to access machinery, which is below the main turbine axis and often dug into the ground. However, in floating foundations, like their own WindFloat design, the hull can be hollowed to enable access.

"[Limited] experience in VAWTs makes it difficult to bring forward as a reliable concept. We might start seeing a shift to more integrated systems over the next ten years as the industry becomes more mature," he says.

Performance and costs

In terms of output capabilities, the general consensus is that VAWT efficiency is currently either the same or lower than HAWTs. Charles Smadja, Co-founder and General Manager at Nenuphar Wind explains that the VertiWind VAWT has been designed for a 2MW output, with a power curve comparable to HAWTs of the same rated power.

Meanwhile, Walt Musial, Principal Engineer - National Wind Technology Center at NREL, argues that VAWT efficiency is maybe 20% less 'give or take a little' - whilst Paulsen says that it is approximately 10% less (based on 'classical' profiles used in VAWT wind turbines, like Naca 0018).

"We are investigating new airfoil designs with the potential to boost the efficiency positively - a similar trend [to that seen] with HAWTs," says Paulsen.

As far as costs go, Roddier argues that CAPEX is 'probably higher' for VAWTs now but should 'be about equal in the long run.' His view is that OPEX costs can go both ways - but are likely to be higher at present, again because of a lack of experience.

"[VAWTs] could also be more expensive due to the complexity of the machinery. On land you have to disassemble the whole thing to get to some components. Offshore, the cost really [depends] on the transport and cranes required - changing a gearbox would be very expensive no matter what," he adds.

Smadja says that CAPEX and OPEX costs for VertiWind are comparable with a HAWT bottom-mounted solution in shallow waters - even though they operate in deeper waters. For him, grid-connection costs are 'related to distance to shore' and are therefore more difficult to compare like-for-like. He also explains that VAWTs do not present different health and safety issues to HAWTs.

"Offshore maintenance is indeed much easier [because] the parts to be maintained are located at the bottom, which reduces both the costs of maintenance and the risk of accidents," he says.

Easy to install

According to Smadja, the VertiWind design lends itself very well to high-volume manufacturing. Moreover, transportation is 'much easier' because the unitary blade length is just 30m.

"Installation costs are also reduced [since] VertiWind does not require any offshore craning [sic], just a tug boat - as assembly operations are completed onshore," he says.

Paulsen predicts that innovative VAWT designs could lead to lower installation costs - for example, 'self-erecting' (ballast controlled) VAWTs could be simply towed directly from production facilities to installation sites.

Looking ahead, it seems that VAWTs have a number of novel advantages and could emerge as an interesting and viable alternative to HAWTs. However, a good deal of work remains to be done to convince the wider industry that they can compete in terms of efficiency and costs.

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