Site assessment is the process of quantifying the wind and weather conditions on a wind energy site. This is typically done before the wind turbines are installed, but may also occur after. The objective of a site assessment is to quantify the wind speed and direction at multiple heights, and also the temperature, pressure, and precipitation at the site. This information is used to quantify the conditions that the wind turbines operate in and may be used for turbine selection and energy estimates before the site is built. Site assessment takes place both on land and offshore.
The state of the art in site assessment on land varies by country and the stage of plant development. Most developers use lidar to supplement measurements from fixed, 60-80-m tall meteorological (met) towers, particularly to confirm hub-height wind speeds based on vertical extrapolation from lower elevations. Some developers use lidar to confirm the accuracy of their horizontal extrapolation process, where by data from the towers are extrapolated to cover gaps between the met towers. Even fewer developers (representing the true state of the art) have replaced met towers with profiling lidar that are used to measure the wind speed at multiple heights across the potential turbine rotor disk, possibly supplemented by scanning devices. In some cases developers or their consultants may use very specific and focused lidar measurement campaigns to probe regions where models and measurements show high discrepancy. In the majority of cases lidar are not used to measure turbulence, which limits the utility of the data for turbine selection.
Floating lidars (i.e. lidars integrated or placed on top of floating platforms as buoys) were recently introduced as a cost-effective alternative to offshore met masts. Today there are several system suppliers on the market, and four of the developed systems have already reached the ‘pre-commercial’ maturity stage . The first offshore wind projects have already been planned on the basis of floating-lidar data. Despite this fast integration into current practice, floating lidar technology faces a lot of challenges – as e.g. the harsh offshore conditions, the impact of the buoy motions on the measurements or the need for a sufficient autonomous power supply – that have not yet been fully understood and solved.
Current issues with the use of lidar for site assessment can be broken into three categories. These categories are concerns by end users about cost, accuracy, and reliability.
- Lidars currently cost more than traditional met towers on land but can measure at greater heights above ground. In contrast, the use of floating lidar systems offshore may bring a cost benefit in comparison to the installation of met masts.
- Also, lidar accuracy may be a weak function of environmental conditions and flow conditions, and so there are some industry concerns about accuracy. As well, turbulence intensity measured by lidar is not directly comparable to a cup, meaning that data cannot be used directly for turbine selection.
- Finally, lidars are complex devices and are not always as reliable as met towers, and thus there are concerns about the cost of ownership and missing data impacting results. For offshore application, this weakness is even more critical since the devices are exposed to even more extreme conditions and a failure may result in a longer downtime than on land.
- Revise the IEA Recommended Practices for ground-based remote sensing for wind resource assessment and the IEA Recommended Practices for floating lidar systems.
- Explore ways to improve lidar systems regarding cost, reliability and accuracy.
- Workshop #1 on Floating Lidar System: Current Technology Status and Requirements for Improved Maturity
- Carbon Trust, “Carbon Trust Offshore Wind Accelerator roadmap for the commercial acceptance of floating LIDAR technology”, CTC819 Version 1.0, November 2013.