A Revolutionary Concept Featuring a RingShaped Generator
Members of the MegaWindForce (MWF) team developed a highly efficient variable transmission system back in 2012. While researching whether this invention would contribute to the efficiency of wind turbines they became aware of the catch 22 situation of the wind industry: rotors need to be bigger to harvest more energy, resulting in lower numbers of revolutions, which makes the design of generators more complex and relatively more expensive. A new concept was developed, where the main shaft was replaced by a ring. This revolutionary concept resulted in several patents.
Published in June 2017 in: //http://www.windtech-international.com/editorial-features/the-megawindforce-turbine
By Ton Bos, cofounder and shareholder of MegaWindForce, The Netherlands
One of the most severe challenges in designing a traditional horizontal axis wind turbine is optimising the blade joints to the rotor hub. With the growth of the size of turbine rotors, the driving torque at the blade root section increases more than linearly with the length of the blade (with the rotor radius cubed). When the same materials are used, the weight increases cubically with size, demanding heavier constructions to withstand forces. The low number of revolutions made it necessary for the nacelle to grow for the direct drive generators or introduced heavy gearboxes. By replacing the main shaft by a ringshaped generatorsupport combination the disadvantages of classical upscaling are eliminated.
The rotor area close to the rotor centre contributes only a small part to the energy production of a ‘classical’ wind turbine, where this area plays a crucial role in supporting the blades and absorbing the driving forces of the generator system. In the MWF concept, forces are spread equally over the large surface of the ring, avoiding stress concentrations. It leads to savings of weight without exceeding the strength allowances of the materials used. Weight reductions up to 50% are achieved. The ringshaped rotor allows the MWF turbine to harvest energy in the optimum range of the rotorswept area with lightweight components. Pitching and yawing is conventional. The design was assessed fully by a certification body for wind turbines, which resulted in an approval on C level. This implies the next step, the building of a test turbine, has started.
The MWF wind turbines are mainly constructed of carbonfibre reinforced polymer (CFRP), a composite material consisting of carbon fibres in combination with a thermoset resin. CFRP gives four times the Emodulus and two to three times the tensile strength of traditional glassfibre reinforcement at 40% less weight. Using CFRP techniques enables us to add strength where needed and to diminish weight where possible. The low weight creates the possibility to work with high rotor speeds to improve the capacity of the turbine at all wind speeds.
In the patented and fully automated production process a filamentwound spaceframe is created for each component to provide a strong and stiff base after which a carbon skin is applied. All components are produced automatically and are identical and reproducible. Huge and expensive moulds for the blades and the labour intensive application of materials belong to the past. Also tower elements are created out of carbon with this special production technology. The aircraft industry is perhaps the best example of successful appliance of carbon fibre.
The two biggest aircraft producers, Airbus and Boeing, are successfully replacing aluminium with carbon fibre in their latest models in order to save weight (= fuel) and maintenance costs.
The use of carbon for the rotor, stator and tower makes the turbine well equipped for offshore and cold climate areas; the generator concept also is very opportune for use both in offshore and cold climate environments. In particular, cold climate areas are an interesting market with favourable wind conditions. These cold climate areas could represent 50% of the future onshore market.
The generator of the turbine is housed in the ring. At the stator side approximately 360 generator units (housings containing multiple coils and a rectifier) are positioned. A large number of magnets are positioned axially at the rotor side. Due to the high path velocity this combination generates an AC voltage in the kilohertz range. The high frequency provides the possibility to work with lowweight electric components, significantly lower than the actual (60–50Hz and below) generators, and peripheral components. The controller is able to add or switch off polepairs during operation, providing an additional control option. This option creates opportunities to adjust the rated power upwards as wind farm controllers sometimes demand an increased rated power level beyond the conventional name plate power, depending on the location of a wind turbine in a wind farm, wind speed and direction, and grid operators’ demands and allowances. This enables wind farm and grid operators to further optimise the integration of wind turbines in the grid system.
The low weight of the generator allows for higher power levels. These are merely determined by the maximum allowable mechanical rotor loads rather than the generator power.
From the point of view of mechanical loading and energy output, nowadays wind turbine systems are optimised per wind class. Below the rated power generators rotate more slowly, and above not all available wind energy can be used in order to avoid overloading of the generator system. In the latter situation the rotor is operating at reduced efficiency. With the large overshoot of capacity in the MWF generator, both suboptimal situations can be avoided and maximum efficiency operation in a wider range than in a conventional manner is created. The generator capacity is no longer the limit for harvesting electricity. Economy becomes the driver.
Installation and Maintenance
As indicated before, components can be made relatively light as forces and moments are low due to the turbine design. This structural concept makes CFRP affordable, consequently resulting in more options for lighter construction. Wear and tear are reduced significantly, which, understandably, reduces maintenance cost. Combined with a large overshoot in generator rating and a multiply redundant aviation type controller setup this results in high uptimes and a widely schedulable maintenance framework.
The carbon elements overcome many of the actual installation challenges. Turbine parts are designed from the start to fit in 40foot containers; after factory assembly and testing they are divided into logical and transportable elements. Heavy transporters, road modifications, special transport equipment and complex permit processes no longer are needed. Lifting equipment on location can be of the prevalent type.
MegaWindForce will install the first test turbine onshore in 2018, after which an extensive test programme will start. The offshore tests will start in 2023. The turbines will be produced in the coastal area of the Eemshaven in the northern part of the Netherlands, where a new factory will be erected.
Biography of the Author
Ton Bos, cofounder and shareholder of MegaWindForce, was educated at the Catholic University of Tilburg. In the past he has worked as a CFO in several companies and managed large projects at companies like Wyeth, Shell and IBM. Some of those projects were built from scratch to 100 million plus activities.