Keeping the turbines turning

Renewable resources produce more than one-third of the world’s energy. Wind energy alone accounted for 48GW generated globally in 2018. Movements like Extinction Rebellion and individuals like Greta Thunberg reflect public sentiment and reflect changes at government level. For example, in 2006, the UK government raised the goal of the nation’s energy to be derived from renewable sources to 20% by 2020. The country is on course to hit this target, although recent Government analysis indicates that it could be exceeded to reach 50% by 2025.

Engineers involved in renewable energy projects must work consistently to maximise the efficiency and efficacy of renewable infrastructure, such as photovoltaic panels and wind turbines.

In the UK, the world’s largest offshore wind farm, the Walney Extension, occupies 56-square miles of the shore on Walney Island, off the west coast of England. Another wind farm is scheduled to be built this year at Dogger Bank, the large sandbank in the North Sea. Scheduled to begin operation in 2023, the farm is expected to also house the world’s largest wind turbines.

While wind turbines are undoubtedly effective, they are fundamentally limited by their need for real estate. For example, a typical 2MW turbine installation will require roughly 65,340 square feet of space. However, each turbine must also be spaced a certain distance apart to allow for the blade to turn and to avoid creating turbulence that disrupts the air flow around other turbines, reducing their efficiency.

There is a need for ever-evolving efficiency improvements during the design of wind turbines as the need for efficient wind generation is set to continue for many years to come.

Rotor design

The key to maximising the efficiency of a wind turbine is for the rotor to capitalise on the aerodynamic power of the wind, which varies frequently. Variable speed wind turbines achieve this by using variable speed drives (VSDs) that adjust the rotor speed. This maintains an optimum tip-speed ratio, which is the ratio between the speed of the blade and the wind speed. VSDs also play a key role in pitch control systems that adjust the angle of the turbine, further improving its speed.

Although variable speed turbines can achieve greater efficiency, they do pose a problem of compatibility as the AC voltage frequency is not constant. This requires effective current rectification, from AC to DC, prior to further conversion at the point where the turbine couples with the grid. All of this can produce erroneous electrical frequencies that can develop into issues, such as harmonic currents, unless properly filtered.

To design a variable speed turbine to maximise efficiency, engineers must consider other power electronics beyond rectification and filtering equipment. This means designing in measures to ensure that any surplus energy from turbines is dissipated safely without causing any damage to the VSD. This requires an effective braking resistor.

In wind generation, these resistors must not only be able to handle the immediate increase in energy, but must also do so consistently after frequent use. In addition, they must balance the capacity for high continuous power with a compact profile, to meet the space-limited requirements of wind turbines.

Resistive and inductive wound components manufacturer, REO is increasingly involved in renewable energy technology. It has developed the Reohm 155 series braking resistor for the wind turbine industry. The resistor offers continuous power of up to 3,500W, to drives with medium- and high-power frequency converters, with isolation voltages to 4.4kV. It is supplied in a compact, closed unit that is rated to IP66, offering protection against dust ingress and powerful jets of water. It has also passed salt mist testing, making it suitable for offshore applications.