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EFFICIENCY FEATURE Major step forward for magnetic gears


Magnomatics has recently tested a 500kW direct drive generator, which it says looks like becoming the best technology for the next generation of offshore wind turbines


T


he generator combines a traditional permanent magnet generator with


Magnomatics’ patented magnetic gear. Such gears have already been in continuous operation, albeit at a smaller scale, in arduous oil and gas applications. With no gear teeth and fewer touching parts, these direct drive generators are efficient, reliable and very compact. Offshore wind is forecast to be one of the highest growth energy sectors globally. Latest forecasts from Bloomberg New Energy Finance are that the global offshore wind generating capacity will expand to nearly 120GW by 2030. The forecast for the UK alone by this date has recently been revised upwards to over 30GW. Other key countries for offshore wind development are likely to include Germany, China and the United States. In many of these territories, floating offshore will become a key requirement, given the depth of the accessible ocean. Most of this expansion is driven by the low cost of levelised energy from offshore wind. Much of the cost reduction is driven by increasing turbine size, with machines now being commissioned of 9.7MW. As well as scaling up, there are a number of technologies being considered that may further lower the cost of energy. The INNWIND.EU project, with a budget of nearly €20 million and with 28 partners, considered many of these technologies. Its objectives included the conceptual design of 10-20MW offshore wind turbines and hardware demonstrators of their critical components. The project developed several innovative designs for rotors, drivetrain components, and fixed and floating substructures that greatly reduce


the Levelised Cost of Energy (LCOE) for 10-20MW offshore wind turbines. An assessment of the entire wind


turbine with the proposed innovations was made at the 10MW and 20MW scales, by applying performance indicators and a comprehensive cost model developed in the project. Moving from conventional 5MW offshore turbines to lightweight 10MW-20MW turbines allows a reduction in LCOE due to the larger size. The LCOE can be improved further with the use of an efficient lightweight rotor and the shift from traditional three-stage geared drivetrains to a direct drive magnetically geared generator. The INNWIND project also considered superconducting generators. Significant further reductions in LCOE can be expected for both 10MW and 20MW designs, due to the advanced concepts researched in the INNWIND.EU project, reducing LCOE to a level where it will offer even more competitive energy costs. The partial load efficiencies of the


Magnomatics Pseudo Direct Drive (PDD) and the two super-conducting direct drive generators at the 10MW level are shown in Figure 1 as a function of wind speed. The partial load efficiency includes the efficiency of the power electronics, but the cryogenic cooling losses of the superconducting generator must be subtracted. In case of the 10MW MgB2 generator this is about one per cent. It is clearly seen that the PDD is predicted to have a superior efficiency, resulting in a decrease of the LCoE of the 10MW INNWIND.EU offshore turbine in the order of per cent compared to the reference drivetrain, which in this case was a


Figure 1 Figure 2


mechanical two-stage gear and medium speed generator. Results for the 20MW generator were


similar, with the PDD again showing the best efficiency across the full operating range (Figure 2). The PDD also shows better mass scaling than the reference drivetrain and was therefore selected as the INNWIND.EU candidate for an innovative non-contact drivetrain. The PDD had the lowest mass of any of the technologies considered. This lower mass substantially reduces the mass of the rotor nacelle assembly, which in turn simplifies site assembly. All these machines are very large,


making manufacture and installation a challenge. The lower mass and compactness of the PDD generator go some way to easing these challenges. For example, smaller vacuum impregnation plants and cranes with a much lower capability can be employed. This means that a factory making generators with traditional technology could effectively upgrade by almost 40 per cent in terms of generator capacity but still use the same plant and equipment.


500kW PDD generator on test


 Magnomatics magnomatics.com ENERGY MANAGEMENT | AUTUMN 2019 21


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