When it comes to developing a wind project from A to Z, there is a minefield of potential obstacles, risks to be factored in, and almost certainly some controversy to be faced (these are explored in greater detail in our article here).
Successful wind project development needs cutting edge wind assessment and measurement, the right market conditions in place to make it economically feasible, and a great deal of careful preparation and handling.
Wind assessment is arguably one of the first challenges the developer will face.
Wind assessment - a UK case study
Although various Government wind speed data exists in the form of wind maps, this data is too general to make precise forecasts of energy production from a proposed site. Accurate measurement is carried out using a range of sophisticated instruments combined with technical expertise and knowledge.
The use of met towers equipped with anemometers and wind vanes to assess wind speed, direction, and shear, as well as sensors to measure temperature, pressure and relative humidity are essential to build up a complete picture of the wind environment on the site. Sensors on the met mast must be carefully positioned by technically – qualified experts.
A well established method is used by Wind Measurement International in order to determine the wind vane north offset.
This process is continuously being improved, and currently allows the wind vane setting to be presented to the nearest degree. There is evidence to show that if the separation of the anemometers from the met mast, booms and other sensors is not sufficient then the wind speed recorded by the sensors will be inaccurate.
There are known standards which state the recommended minimum separation and orientation of particular sensors. Full-scale tests and computational simulations should be carried out for each type of mast, in order to minimise the sheltering effects on these sensors.
Increasingly, LIDAR technology (Light Detection and Ranging) is also employed to measure the wind profile from 10m up to 200m according to the proposed turbine configuration. This system is very accurate at measuring wind speed and turbulence. It is therefore a useful tool, when combined with met towers, for predicting the long term energy yield for a particular site.
Incomplete or inaccurate data can have a huge impact on the anticipated energy yields from a proposed development. For example, a site which has an annual average wind speed of just 10% higher than another will produce over 20% more energy. A change of just 1% on the final energy yield can be the difference between a site being viable or not. This shows how critical it is to choose the correct sensors, have sound installation procedures and site reporting for the met masts and LIDAR, and have the knowledge and expertise to produce an accurate final energy yield analysis.
Most planning authorities and banks will require developers to provide at least one year's full data and in some cases up to three years to verify the feasibility of a site. It is vital therefore that data collection and analysis is carried out in the most efficient way possible if project delays are to be avoided.
The first thing to bear in mind is the type of mast required. The height of the met mast should be as close to the proposed turbine hub height as possible in order to reduce the uncertainty of the energy yield calculations. If the height of the top anemometer is below the proposed turbine hub height then extrapolation techniques should be applied. The total uncertainty will increase as a result of this extrapolation.
The level of uncertainty will depend on the accuracy of the extrapolation model. Wind Measurement International has an in-house peer reviewed software package called ShelterMap which minimises the uncertainty associated with this extrapolation process by including the surrounding shelters into the analysis.
Anemometers vary with price and accuracy. Some have known problems which cause errors in their readings. Therefore the tradeoff between price and accuracy has to be considered when choosing suitable anemometers for use on a met mast. The calibration of the anemometers also varies with manufacturer. It is important to check with the banks what criteria they are using to assess the data and ensure compliance with this. Most banks will ask for a specific calibration, which will add a significant cost to a met mast installation.
It makes sense to involve wind measurement professionals from the beginning of the project. Using inexperienced operators can lead to problems further down the line and there could be health and safety issues with an inexperienced crew. An amateur installation can lead to under estimating or over estimating wind resources, which is bad news as it could fail banking due diligence.
At the site survey stage a professional wind measurement company should be employed to carry out desk research to screen the proposed site for potential pitfalls such as proximity to public spaces, paths, power lines and microwave links. In addition, electromagnetic interference with respect to mobile phone, radar and TV transmission needs to be measured. The location, height and characteristics of all significant wind obstacles on the site need to be accurately assessed to optimise wind measurement location and maximise the certainty of the wind speed measurements.
On a large windfarm site a number of met masts must be strategically placed to give accurate data. However, as wind is highly variable year to year this data needs to be correlated with longer term weather stations (such as those based at airports) and necessary adjustments made. Wind speeds can vary enormously according to the type of terrain, vegetation and buildings on the site, so sophisticated wind modelling software is used to calculate the impact these factors will have on future energy forecasts.
You should expect monthly reports from your wind measurement company. This should include correlation with the nearest available meteorological data and detailed analysis of the turbulence intensity and average wind speed. Any site issues should also be highlighted on this report – including high wind shear, low mean wind speed or high turbulence levels. This is a good opportunity to check that all the sensors are in good working order, and the battery is healthy.
Data is captured using loggers attached to the met mast. These loggers must be robust and reliable to function in all types of weather conditions. They should be programmed to send an SMS message in the event of any malfunction or drop in battery voltage, have photovoltaic charging systems and be capable of transmitting data remotely using a GSM modem back to base.
Erection of the met masts should be carried out accurately within the strictest health and safety guidelines. There are a number of proven anchoring systems, which ensure the mast will stay up even in hurricane winds. However, regular inspection and maintenance of the met mast is essential to ensure it is functioning properly. On completion of the wind measurement phase the masts need to be decommissioned.
Once the data is amassed and analysed the last part of the equation is the final resource assessment, which can be used for bank due diligence purposes or to quantify individual investment risks for smaller turbine installations. It should present the calculated energy yield in conjunction with an in-depth uncertainty analysis. This report can also include complex terrain flow analysis, wake analysis, optimisation and noise assessment plus shelter analysis and height extrapolation.
In conclusion, the key to a successful wind measurement campaign is proper planning, choice of the right equipment and employing high level technical expertise.
About the authors:
Dr Steve Ritchie is a director, and Mr Lee Rowberry is a wind energy analyst, at Wind Measurement International.