Siemens 7MW Offshore Wind Turbine: Internal Changes Lead to Increased Performance

Siemens SWT-7.0-154

Tom Lombardo

Siemens made a few internal changes to its tried-and-true 6 MW offshore wind turbine, turning it into a 7 MW turbine with no changes to the exterior. By upgrading a few internal components, engineers were able to increase the turbine's power output by nearly 10%. (Yes, I know that going from 6 MW to 7 MW is a 16.7% increase; I’ll discuss that later.) The blades, tower, and nacelle remain exactly the same, allowing Siemens to make a more powerful turbine without excessive retooling, testing, and certification costs. Siemens refers to the upgrade as “Thinking Inside the Box.”

Siemens - SWT-7.0-154

The Original Design

Siemens’ SWT-6.0-154 offshore wind turbine has a rotor diameter of 154 m (505 feet). The length of each blade is roughly the same as the wingspan of an Airbus A380. With more than 18000 square meters of swept area, the turbine generates 6 MW in 12 m/s (27 mph) winds. The blades can adjust their pitch in order to regulate the speed at which the turbine spins.

Siemens 7MW Offshore Wind Turbine: Internal Changes Lead to Increased Performance

Inside the turbine there’s no gearbox - it’s a direct drive system connected to a synchronous AC permanent magnet generator. Because a generator’s speed determines both its voltage and frequency, the “wild AC” is rectified to DC and then converted back to AC before going to the grid. The inverter that does the DC to AC conversion synchronizes with the grid frequency, provides voltage regulation, and delivers reactive power as needed.

Meet the New Boss, Same as the Old Boss (on the outside)

The new SWT-7.0-154 is basically the same model with a few simple changes. First, the generator’s magnets are stronger. The heftier magnetic field allows the generator to produce more energy at the same rotational speed. However - and this is where I think the power discrepancy comes from - a stronger magnetic field will also make the turbine more difficult to spin. In order to achieve the same turbine rotational speed, a faster wind is required. Sure enough, their 6 MW turbine produces nominal power at wind speeds of 12-14 m/s, but the 7 MW turbine is rated at 13-15 m/s. So while Siemens is a little generous in its naming, the press release is honest when it says “nearly ten percent more power.”

In addition to the beefier magnets in the generator, Siemens’ engineers upgraded the internal electronics and the transformer so they could handle the increased power output. Other than that, it’s the same turbine as the old 6 MW model.

More of a “Spin” than a Revolution

Okay, so it’s not a revolutionary new design and the 7 MW moniker is a bit of a marketing spin, but a 10% increase in power output is still pretty significant, especially on the scale of a wind farm. Adding 10% to a single 6 MW turbine translates into an extra 14 MWh per day, or more than 5000 MWh per year. That’s enough electricity to power an additional 500 homes per turbine, while preventing 3.6 metric tons of CO2 from entering the atmosphere. Siemens calls it a “minimum upgrade for maximum effect.” I call it a turn for the better.

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