Tackling the global blockage problem in a wind-farm due to interactions between turbulent wakes

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With rising concerns about the global climate and the impact of continued use of fossil fuels, renewable sources of energy have received much attention of late. Despite this attention, and considerable progress, there remain many challenges to fully exploit the energy available in the wind. A wind turbine produces usable electricity by extracting energy from the wind. This produces a wake, an area of slower moving air immediately behind the turbine. One turbine is good, but to satisfy our energy needs on limited land (or sea) we need many carefully arranged turbines (called a wind farm). This can be a problem, as downwind turbines will sit in the wake of upstream turbines. This lower energy wake means there is less energy to be extracted by the following turbine(s), as well as increased structural loading, reducing energy yield and increasing costs. In the wind industry, it is often useful to think of an individual turbine and base the performance of the wind farm at a specific site on this individual turbine. We know that we must account for the wakes of the turbines when we account for the performance of the wind farm. This, however, is not the complete picture. With the increased installation of wind farms, more information about their operation has become available and a deficit energy production has been identified. This deficit is small, but considering the costs involved in designing, constructing and operating a wind farm for a period of 20 years this small deficit can result in significant financial consequences. The source of this deficit has proven to be subtle and elusive until recently. The influence of the wind farm on the energy yield is more complex than the combination of wakes. The presence of the wind farm itself changes the wind encountered at the wind farm's site. The wind farm acts as an obstacle, a blockage, to the wind, and like the concept of the path of least resistance, the wind in a manner tries to avoid the wind farm. This manifests itself as the leading upwind turbines receiving wind at a lower speed than they would in the absence of the wind farm. This is referred to as Global Blockage. One would expect that global blockage would be subject to considerable academic and industrial interest, which it indeed has received. But this has been completed in a somewhat ad hoc manner, focusing on disparate aspects of the problem. The nature of this project is to complete a thorough and rigorous investigation of global blockage, and to identify and characterise the principal exacerbating aspects of wind farm design that result in blockage. There remains a degree of uncertainty on the precise definition and quantification of wind farm blockage, this project will have to identify from literature or create such definitions. The investigation will be experimental in nature, taking place in the wind tunnels at Imperial College London, namely the 10ft x 5ft Low Speed National Wind Tunnel Facility. As alluded to previously, the energy lost to blockage is only a small percentage (around 2%) of the total energy yield of the wind farm. With the influence of blockage being so small, considerable care must be taken to ensure any results are distinguishable from measurement uncertainty. As the need to go carbon neutral increases and the necessity of wind energy increases, high wind yield locations such as the North Sea will become in increasing demand. With more and more farms being developed, global blockage is only going to play an increasingly less subtle role. The potential results of this investigation could have important bearing on the design of future wind energy projects. Characterisation of blockage may allow for it to mitigated and increase energy yields or if it cannot be mitigated, it will allow for a more accurate picture to be developed of the financial viability of a particular wind energy project.