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Power train optimisation for vertical axis wind turbines

13 November 2015

The UK based company, 4Navitas Green Energy Solutions has developed a vertical axis wind turbine that is set to revolutionise the worldwide onshore wind turbine market, currently dominated by horizontal axis wind turbines.

The 4Navitas VAWT

The new vertical axis wind turbine (VAWT) was designed by some of the UK’s leading structural, aeronautical and electrical engineers, and is being manufactured by a supply chain of blue-chip partners. The main focus of the design concept was to avoid fatigue, an ongoing issue in horizontal axis wind turbines (HAWTs) with wind coming from all directions, creating bearing and gearbox stress due to yaw/pitch requirements, plus vibration and ultimately failure.

The VAWT is designed with low drag airfoils and a low rotor speed of 32rpm.  There is no need for a blade pitch control system, and with power, electronics, motor and gearbox at ground level for easy maintenance, wear is considerably reduced and performance improved well beyond the 20-year predicted lifespan that is current for most HAWTs.

The Sandia Report of January 2012* confirmed that VAWT design and philosophy was the preferred type, and one that considerably reduced operating and maintenance costs over the lifespan of the turbine.  In fact, the VAWT has been around since Persian times, when it was used for grinding corn, but the wide adoption of HAWTs over the last decade has led to engineers neglecting the potential of the VAWT.

“Independent studies such as the Sandia Report identified that vertical axis wind turbines have the potential to be more suitable than the current widely adopted HAWT, and our developed model represents the next generation of high performance, low maintenance wind turbines for micro-generation,” says 4Navitas director, Dominic M’Benga.

A fully integrated drive
4Navitas worked with Siemens and partner, HMK, over a period of 18 months to refine the design and engineering of its VAWT, and from this long-term collaboration a robust and efficient turbine providing good power output has been developed. Indeed, the output is already rated at up to 75kW with plans for a 1MW version already in the pipeline.

The drive, motor, coupling and gearbox configuration is based on Siemens’ integrated drive system (IDS). The 4Navitas VAWT houses a Siemens FZG helical bevel gear unit at its base, and the IE3 rated motor is close coupled with the gearbox.  The shaft is supported by bearings located within the mast, so gearbox load and fatigue transfer is significantly reduced - factors designed to extend the lifespan of the turbine and improve its long-term reliability.

Overall control of the system is achieved by a Siemens S7-1200 PLC and a Siemens Sinamics S120 system. The S120 uses Active Front End (AFE) technology for connection to, and synchronisation with, the supply, enabling power to be transferred safely to a 415V, three-phase grid. Simon Grant, business development manager for offshore renewables with HMK, takes up the story:

“By providing 4Navitas with engineering and technological support – including power curve support from our Siemens colleagues in Germany – we were able to develop and refine the function blocks, drive technology and software.  What we were also able to bring was added value and partnership to meet the challenges together and produce the best solution.”

Near-silent running
Another significant advantage of this VAWT is that by effectively earthing through the 4Navitas-designed tower, two buttress legs and grillages, resonance and noise is reduced to just 40dBA – the equivalent of a quiet library.  Noise is an ongoing issue with onshore wind turbines and this reduction in noise means the 4Navitas turbine can be situated in populated areas, near schools, hospitals and within communities.

For micro-generation puposes, the 4Navitas turbine has a height of just 37m, significantly less than that of its HAWT counterparts. Moreover, the structure has a much smaller footprint and there is no requirement for a large concrete base. The unit has undergone extensive trials, including testing to almost three times the normal operating speeds (100rpm at the mill rotor). The 4N-55 series VAWTs are now moving into serial production, following considerable worldwide interest in the technology. Dominic M’Benga again:

“Our turbine can take wind from any direction without suffering the vibration and wear issues of horizontal axis turbines, because it’s designed to turn rather than to hold a hub; there’s no weight transfer onto the gearbox and fewer moving parts. We’ve over-engineered it to last 25 years and in fact the wings and spreaders have reserve factors as high as 64, showing that the mill will survive way beyond its operational life.

“By engaging with Siemens we have been able to bring it to market much sooner and to feel confident in our UK designed and built product with the additional benefit of Siemens’ worldwide local support.  The enquiries we’ve had are testament to this successful partnership.”

4Navitas is currently working on developing a 1MW VAWT – again in partnership with Siemens and HMK – and is aiming to achieve IEC 61400 accreditation in the near future.  The company is also considering international licences for the worldwide manufacture of its VAWT. Siemens IDS business development manager, Richard Fear concludes:

“Working with 4Navitas on this innovative micro-generation project has been an excellent application of our IDS principles, helping develop a highly-efficient turbine with good power output.  The long-term projections for this VAWT are significant, both in terms of applications and enhanced asset life.”

*The Sandia Report of January 2012 - A Retrospective of VAWT Technology – can be read here.

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