WIND FARM OPTIMALISATION: Accurate wind measurement for optimal wind farm design

In the design phase of an offshore wind farm, met data will be one of the influencing factors that play a part in the choice of WTG. The meteorological services that can be involved in the decision at this stage are detailed model data sets of wave and wind conditions to input into Computational Fluid Dynamic (CFD) models that are used by the farm developer to locate and determine the exact position and height of the individual WTGs.

The use of detailed accurate data-sets for calculations of power output in the development phase ensures that the performance estimation, which is essential for the financing of the wind farm, is robust and accurate. Offshore WIND asked Michelle Spillar, Head of Renewables at the UK Met Office, to illustrate the importance of using meteorological data in the development of an offshore wind farm. The Met Office is an expert in numerical weather prediction and weather forecasting and has been providing services to both government and business customers for over 150 years. For the past five years, they have been investing on focussing their services on the wind industry as the renewable energy market grows. They have developed weather and climate services to meet the requirements of offshore wind, throughout all stages of the project lifecycle, from early development through to operations and re-powering.

What has changed?

The big change in the wind industry over the last five years is the increased accuracy of offshore site assessment. The industry is increasingly using site specific modelled reference data which comes from reanalysis or downscaled Numerical Weather Prediction (NWP) techniques, such as the Virtual Met Mast™ which has been developed by the UK Met Office.

There is increased use of mesoscale models and computational fluid dynamic (CFD) simulations to predict variation in wind resource across large offshore developments. This is enabling developers to design wind farms which will generate maximum power from the wind resource at a specific location. With increased knowledge of the site’s wind resource and variation, the financial risk is reduced and output potential maximised.

Buoys versus Virtual Met Mast™

Floating met buoys are the equivalent of weather stations on land. However, the parameters collected by each type of observing equipment are different. A buoy will be making estimates of wave and ocean parameters and will be recording different atmospheric parameters compared to the land based stations and will have different sources of error and uncertainty, due to the different equipment used on and offshore. However, all of this is known, understood and can be quality controlled with compensation applied before the observations are passed on to the customer as raw data or already processed into specific models. This data source, if well maintained, has the advantage that it is accurate at the specific site.

The advantages of using a Virtual Met Mast™ data set containing a model hind cast, a predictive model using existing historic data, are several-fold as opposed to one from observations. The data coverage from observation equipment such as weather stations and buoys is sparse, so having an accurate measurement for your site or farm is reliant on installing equipment or using a nearby station/ buoy if there is one. You will then only have information for the period that the equipment was installed and for the exact characteristics of the equipment (e.g. a 10m wind measurement).

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Using model climatology has the advantage that it can be created for any site, farm, height or time period. This removes the risk of having measured during ‘a windy period’ and gives a solid climatological average of the conditions over 25 to 30 years. Once created the climatology can be available anywhere, reducing the risks and costs associated with installing measuring equipment, and for large offshore farms information can be provided for many points on the farm, again enabling a high degree of precision in wind resource assessment.

Wind in the Boundary Layer

In general in the atmospheric boundary layer the wind speed will increase as you move away from the surface, because the effect of the surface on the wind flow reduces as you get higher up in the atmosphere. To explain in a little more detail, very near the surface the air encounters surface obstacles that greatly reduce the wind speed (effectively to zero right at the surface). Interaction with the air flowing above in turn induces turbulent mixing that causes a drag higher up. That leads to further turbulence that progressively propagates the drag higher until eventually you reach a height where the surface drag can no longer be felt -this marks the top of what is called the ‘boundary layer’.

Under steady state conditions, a balance between the turbulent drag, the pressure force driving the wind and the effects of the Earth’s rotation is reached which, in turn, leads to the typical wind profile that increases with height above the surface. However, in the real world there are many ways in which these rather idealised steady-state conditions can be disrupted and the resulting adjustment in the flow can easily lead to more complex vertical profiles of wind speed. In particular, the degree to which these turbulent motions can distribute the surface drag vertically is strongly dependent on the static stability of the atmosphere (i.e. the degree to which the buoyancy of the air increases with height).

If the surface is warm this will help propagate the turbulence, and hence the surface drag, upwards. Conversely, for example, if our steady-state logarithmic wind profile flows over a colder surface, this will restrict the height to which the drag can be propagated. This then results in an imbalance in the forces acting on the air just above the new (lower) boundary layer top that causes the air at this level to accelerate. This can particularly happen over land under clear skies as the surface cools off at night, and leads to the formation of a low level jet, i.e. a local maximum in the wind speed, typically at around 100 to 200m above the surface.

Size and location factors

The number of forecasts required for a particular wind farm really depends on the uniformity of the atmospheric conditions at the farm itself and the size of the farm. Offshore, the weather occurring at a particular farm will be affected by the wind flow from off the nearest coast together with the prevailing weather regimes. More than one forecast may be wise for the largest farms as some weather features (e.g. showers/squalls), may be smaller in size than the farm and therefore only affect part of the farm at any time.

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The effect of the wind on the waves

Wave height is influenced by the strength of winds over the ocean and the area and duration over which those winds blow (often termed the ‘fetch’). The stronger the winds and the longer the fetch the higher waves become, up to a limiting condition (that is dependent on wind speed). Once established but no longer forced by the wind, wave energy disperses as swell. This means that it is not uncommon for exposed sites to experience moderate wave conditions without moderate or strong local winds. Secondary effects on wave height relate to the water depth and strength of local currents, which may cause changes in wave height and steepness due to shoaling and refraction effects.

Wave conditions primarily affect build and maintenance activities. Wave data is generally used in planning these activities, but the extent to which downtime due to sea-state is factored into cost-benefit analysis at the resource assessment stage is not clear.

Challenges and achievements

At present, the greatest challenges are in understanding the needs of, and providing services for, the new Round 3 offshore wind farms in the UK and other offshore windfarms in the planning stage across Europe. The challenges faced by developers and future operators of these farms are significant. This, in turn, provides the ‘Met’ industry with a challenge in terms of ensuring that the appropriate weather and climate information is developed and provided to meet the needs of the future operators. This will help the industry to plan and operate efficiently, and in turn contribute to reaching the cost reduction targets that are being placed on offshore wind.

At the Met Office we are currently gathering requirements to identify the detail of the challenges being faced and the weather and climate information that is needed to form accurate judgements and ensure sound investment in offshore environments. This is particularly important offshore, where weather risk can impact construction costs by up to 40% and becomes increasingly significant as the industry moves out further from the coast.

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We are working closely with the industry to understand the weather and climate challenges faced by offshore wind farms and are doing this by consulting with a number of stakeholders and a keygroup of customers and major developers.

The new Virtual Met Mast™ Time Series marks a great achievement. This exciting European, 30 year, 4km, re-analysis dataset is designed for experts in the industry to use for site assessment across the continent. Outputs from this data-set can be provided to wind farm developers across Europe for use in their own modelling process and input into onward CFD models.

With thanks to Michelle Spillar, Head of Renewables at Met Office