WIND MONITORING
IT’S ALL ABOUT THE DATA!
When looking at the financial feasibility of any given renewable energy project, quantifying the available resource is the cornerstone of a successful project. While there are a number of ways this can be achieved, for the vast majority of projects there is no substitution for onsite measurements.
5 FUNDAMENTAL QUESTIONS Ian Ravey, Head of Measurement Services at RES, answers 5 fundamental questions on how to achieve excellent wind data.
WHERE DO I START?
Good question! – having decided to take a project from the pre-feasibility to full site assessment phase, you must define your objectives regarding data quality, availability and uncertainty and plan your measurement campaign (and budget!) around them.
Choosing inappropriate instrumentation, technology, mast or measurement configurations can have a major impact on measurement uncertainty, which in turn directly impacts financial uncertainty. Getting measurements wrong can delay projects, seriously impact on financial return, and in some cases completely kill off the project.
Understanding this at the start of the project planning phase is key and should be the main driver throughout your measurement campaign.
HOW AND WHERE DO I MEASURE? Well, ideally at the proposed turbine hub height in a location central to the planned wind farm layout and at an appropriate distance from complex features such as steep slopes or forestry. Logistically, however, this is not always possible.
Wind speed can vary greatly from one part of a site to another and, unfortunately, it is not economically viable to measure everywhere. Choose a measurement location that best represents the ‘average’ flow conditions on the site and then use an appropriate wind model to assess the wind flow conditions at the proposed turbine locations.
The most common way to measure wind speed is to install a meteorological mast with a set of cup anemometers in a central location away from steep slopes or ridges. However, the size and complexity of the site influence measurement strategy; the bigger and the steeper the site, the more measurement positions you’ll need to avoid uncertainties at individual turbine locations.
Extrapolating wind speed using the shear profile will increase your overall uncertainty too, particularly on complex or thermally driven sites where the shear profile is difficult to model. To reduce uncertainty in long term predictions, measure at hub height.
WHAT IS THE BEST TECHNOLOGY FOR MY SITE?
We firmly believe that Remote Sensing Devices (RSD’s) and meteorological masts are complementary technologies. We own and operate 10 such devices and they form an essential tool in assessing flow at specific turbine locations across large sites or reducing shear uncertainty where installing a hub height mast is not a possibility.
While RSD’s offer many logistical and safety advantages over large masts, and can characterise the wind field up to 300m above ground, there are fundamental differences in how RSD’s and point measurement systems such as anemometers operate.
Unlike an anemometer, which measures the wind flow at a single point in space, LiDARs and SoDARs are volumetric sensors, measuring the flow at points on the outside of a measurement cone at the required heights. Simple trigonometry is used to calculate horizontal and vertical wind speeds directly above the device. This means that the higher the measurement the greater the volume.
Though not a problem when measuring the wind resource in flat homogenous sites where the flow through the volume is fairly uniform, this can be a problem in complex terrain where you can expect highly turbulent non-uniform flow through the measurement volume. This can cause additional uncertainty in the measured data – something we most definitely need to avoid.
DO I NEED TO MEASURE ANYTHING ELSE?
Yes! – although wind speed is the most important measurement to assess how much energy your site will produce, wind direction also needs to be measured in order to optimise your wind farm layout and minimise the energy loss due to turbine wake effects.
Also, as the available power in the wind depends on the density of the air, the measurement system should include temperature and pressure sensors to calculate the site average air density.
HOW DO I BRING IT ALL TOGETHER? There are a number of issues you will need to address during the planning and measurement design phase: power supplies, data management, site access logistics to name but a few! You will want to maximise your return on investment by ensuring high data availability and no gaps.
Keeping sight of your objectives and using qualified, experienced contractors, who fully understand all aspects of the process (as well as the impact of poor installation) from the system design to the mast installation, is a big advantage.
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www.windenergynetwork.co.uk
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