Sustainable Energy Engineering

Comparison Direct Drive and Gearbox Wind Turbines

 Schagen, November 2021

VerVent made a comparison of direct drive and gearbox windturbines, a study based on professional literature written by scientific and research organizations and on information from suppliers and manufacturers.

The weight of direct-drive wind turbines is likely to grow proportionally faster than the weight of the gearbox turbine. Direct drive needs a larger generator and a bigger and heavier supporting structure. The proportion of active material (copper, neodymium, etc.) will also grow rapidly as the nominal power increases. It is the question whether the already much improved gearboxes can be scaled up to higher torque. Direct drive turbines will undoubtedly retain the advantage of lower regular maintenance costs over a gearbox turbine

With the conclusions drawn about the development of both technologies various calculations have been made.  The Default choice for these calculations was for the same investment in a 12MW direct drive and gearbox turbine. It should be emphasized that in practice this is not the case; an MMW gearbox turbine has a lower purchase price. The scenarios relate to variations in system losses, O&M and investment costs. 

The VerVent Calculations deals with the investment in the wind turbine, nacelle, rotor and blades and the tower. Foundation and grid connection are not included. 

VerVent has made a second calculation for the differences in LCOE direct drive and gearbox in a wind farm using the Pathway study by TNO and BLIX (2021). This Pathway study concerns an offshore wind farm, situated 38  NM from the coast. The investment concerns all hardware needed to get the wind farm operational and feed the electricity produced into the onshore grid. The results are in line with these of the VerVent Calculations. 

In scenarios 4 and 5 of both scenarios the LCOEs of the gearbox turbine are lower than those of direct drive. But that does not necessarily mean that the gearbox turbines makes more profit. Because the system losses of the direct drive turbine are lower than those of the gearbox turbine, the efficiency is higher and thus the turnover. The tabel gives the result of the VerVent Pathway Calculations.

The difference in LCOE between the two must be large enough in favour of the gearbox turbine to make up for this increased output/profit.

With increasing rated power, the weight and investment costs of direct drive will grow proportionally faster than that of gearbox turbines. That works in favour of the gearbox turbine. It is more about the confidence that an investor will have to have in these technologies and the guarantees that the supply chain partners give. The general conclusion is that the growing nominal power of wind turbines calls for a rethink of the choice between direct drive and gearbox technology.

The shareholders of the OEM are also confronted with continued product innovation and incremental improvements.  The lifetime of individual turbines need to be ensured from a project perspective, while at the same time the turbine platform should enable upscaling and margin improvements as well. Increasing the nominal power of currently available turbines are or will be confronted with limits to increase the rated power on the same platform. 

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