Wind farm control (WFC) is a family of methods to operate the individual wind turbines within a wind farm in a coordinated way to achieve a common target. Targets can be, (1) to mitigate wake losses in order to increase the power production of the wind farm, (2) to mitigate structural loads on some or all of the turbines in order to reduce maintenance costs or increase the lifetime of the assets, (3) to ensure that the active and reactive energy production comply with the infrastructural and contractual limitations, (4) to match the energy demand at minimum cost, or (5) to support the grid stability by means of active and reactive power control.
In a wind farm, turbines affect each other through their wakes. Wakes hitting turbines located downstream decrease their power production and increase the structural loading (wear and tear) of their components. It is still a common practice to operate wind turbines individually, each maximizing its own power production ("greedy" control), not considering the wind farm as one unit. Such strategy ignores the wake effects and is thus not optimal for maximizing power output at wind farm level. This triggered TNO researchers to develop a new, cooperative approach called active wake control (AWC). Patents for the potentially game-changing technology were granted already in 2003.
AWC deals with mitigating the wake effects by coordinated control at farm level. AWC boosts the annual energy production (AEP), elongates turbine lifetime, and hence contributes to lowering the levelized cost of wind energy. AWC can be achieved by either down-regulating upstream wind turbines to increase the wind velocity in their wakes (called induction control), or by misaligning the rotors with respect to the wind direction to steer the wakes aside from downstream turbines (called wake redirection). Wake redirection is usually more beneficial than induction control, but also has a more pronounced impact on the turbine loading. Therefore, TNO has been studying in depth the impacts of AWC on both AEP and structural loads. The results are quite optimistic: the fatigue loads tend to decrease under wake redirection control, while at the same time an upside of up to 2% AEP gain is estimated.
For the accurate calculation of the wake effects in large offshore wind farms, TNO has developed the cutting-edge software tool FarmFlow: a 3D parabolised CFD code, that achieves very accurate results at acceptable calculation time. In various benchmark studies in the past, FarmFlow proved to provide often the highest accuracy for large offshore wind farms in comparison with other wake models. The model is validated against wind tunnel measurements and full scale wind farms. Recent model improvements brought FarmFlow predictions even closer to real-life measurements.
While on the shorter term the focus is on boosting up the power production by reducing wake losses with AWC, TNO is currently also investigating the challenges that will come on the longer term, as those related to the implementation of a high share of wind and other renewable energy sources into the energy system. Wind farms will need to be flexible enough to provide active and reactive power control to secure revenues in volatile markets and support grid operation. Furthermore, integration of wind with other renewable sources, energy conversion systems and energy storage systems will be needed to meet the demand variations as well as to comply with the electricity transport limitations. Therefore, TNO is developing innovative controllers for combined production, storage and conversion, that operate renewable power plants to match the demands of the different energy markets and the grid requirements.