As the world’s floating wind power fleet has expanded from prototype to array, with first commercial-scale projects now tantalisingly close to lift-off, the debate in industry has tended to focus foremost on the technology’s hull design – spar, semisubmersible, ‘damping pool’ barge, tension leg platform – and turbine ‘agnosticism’ to platform in shaping the economic case.

But a big part of the sector’s future viability could come from a shift in thinking above the waterline. In the last year, three ‘two-headed’ concepts have moved deeper into the mix, building an argument around a significant boost to installed capacity if these twin-turbine concepts are used instead of single platform units on a wind farm.

Last year, the world’s first twin-rotored floating wind power platform, W2Power’s eponymous design, was brought online off the Spanish island of Gran Canaria by developer EnerOcean.

A triangular steel semisubmersible with angled turbines at two corners that ‘weathervane’ into the wind – was run in 65 metres of water in a fast-track testing programme, as the launch-pad for the developer’s plans for a maiden full-size array in the coming years using “off the shelf” 6MW turbines, giving each unit a 12MW nameplate – as powerful as the biggest single turbine turning today, GE’s Haliade-X prototype in the Port of Rotterdam.

German utility EnBW, meanwhile, recently announced promising test results on a 1:10-scale prototype of Aerodyn Engineering’s Nezzy² design, after trials of a precast concrete version of the innovative two-headed concept – in development since 2014 – near Bremerhaven, with an installation to follow next year in the Baltic Sea.

Much of the hype and hope comes down to the capacity numbers these pioneers are seeing from their prototypes – which closely mirror a DNV GL-anointed software development project from Swedish floating wind technologist Hexicon that modelled its Dounreay Trì site off Scotland to have the potential for a 70% boost to installed capacity “with the same wake assumptions as in a single turbine wind farm” on a per-platform basis.

That is, it had been calculated that there was space on Doureay Trì to moor 14 single turbine floaters or 12 of its twin-rotor designs, meaning that if 8MW machines were used the Hexicon units could produce 80MW more from the same offshore acreage.

Traditional floating wind has already been proved to have a higher average capacity factor than fixed-foundation, with the pioneering Hywind Scotland running at 65% through its first winter compared to average bottom-fixed wind farms.

But add to this greater power output that having the number of floating units on a wind farm would cut in two much of the big ticket componentry for a project – the platforms themselves, 50 versus 100 platforms, for instance, for the same 500MW output using 5MW rather than 10MW machines – but also the high-cost mooring systems needed to anchor and daisy-chain these flotillas together in deep water, and the economics of floating wind is honed further.

The next few years will a period of reckoning for multi-turbine floating wind, with Enerocean-W2Power targeting construction of a 3-5 unit development of up to 60MW, dubbed CanArray, near its test site in the Atlantic, at a capital investment of €140-€170m, with first power as early as 2021.

Aerodyn Engineering says beyond the Baltic deployment, it is planning for another to follow potentially off China. And Hexicon is hoping its once world-leading Dounreay Trì project – which fell afoul of cash flow problems after missing a key government subsidy deadline in 2017 – could be reborn at larger scale for the upcoming ScotWind tender off the UK and is also partnering with Genesis Eco-Energy to explore building utility-scale arrays in the waters off South Africa.

Time will tell, Hexicon CEO Henrik Baltscheffsky said recently, but “the difference [between floating and fixed capacity factors] is vast – and so is the business case”.

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Could floating wind power find two heads are better than one?