A recent procedure provides an industry-wide best-practice for verifying a wind turbine’s power curve using nacelle-mounted Lidars, laser-based instruments that measure wind speed and direction. The procedure has been submitted to the standardization groups Measnet and IEA Task 32 for an upgrade to an internationally recognized standard. It’s good news for the wind industry because it opens new possibilities for less expensive and more frequent power-performance measurements. The project has been conducted by a consortium made of DTU Wind Energy (formerly Risø Wind Energy Department), DONG Energy, Siemens Wind Power, and Avent Lidar Technology, and funded by the Danish Energy Technology Development and Demonstration Program.

Conducting power performance verifications with met-masts is often difficult and expensive. For instance, many wind farms do not have a met mast or one in IEC configuration. When there is one, it will be fixed, so only a limited number of turbines can be tested with it. Installing or moving an existing met-mast to test other turbines adds administrative burdens and costs. The difficulty and expense in offshore or in complex onshore terrains often means verifications and measurements are sometimes skipped alltogether.

“Measurements of wind turbine power curves are a key issue for the wind industry,” says Thomas Velociter, CEO of Avent Lidar Technology (www.Aventlidartechnology.com). “OEMs are also interested in measuring the performance of their prototype turbines. We see a common interest in sharing the best practices for power curve measurements, rather than for contractual conflict,” he says.

Obtaining the curves has been an issue. “For instance, there must be an agreement on how to measure power performance. This implies a synergy between the OEM and owner. Turbine buyers and OEMs also have an interest in working together to get the most from their turbines. In addition, owners want to use the curves as operational tools to accurately monitor turbine performance and detect potential under-performance to improve their production,” says Velociter.

For power performance measurements, accuracy is provided by the wind measurement instrument and a method described in the IEC 61400-12-1 standard which allows estimating the measurement uncertainties. The edition 2005 standard allows measurements using a met-mast and cup anemometers, while a new revision including ground-based Lidars is near validation stage. The work successfully achieved in this three-year project was to develop an equivalent procedure guarantying the same level of accuracy and repeatability by using a nacelle-Lidar rather than an IEC met tower with a best in class cup anemometer. The project completed successfully.

“Nacelle-mounted Lidars present a cost-effective solution to the problem presented by conventional instruments. They allow for accurate and affordable measurements, resulting in reduced cost of energy and increased competitiveness,” adds Velociter.

The project started with a ground-based Windcube Lidar from Leosphere (www.leosphere.com) which was adapted for nacelle mounting and forward looking, horizontal measurements rather than straight up. Over the course of the project, several prototypes were developed, tested, and evolved into a product called Wind Iris, which combines all elements of the procedure IEC 61400-12-1. “For example, it specifies how to measure at hub-height ±2.5%, a distance of 2.5-rotor diameters in front of the turbine, and account for deviations inherent to the movements a turbine and tower. With a 400-m range, the Lidar can work even on the largest wind turbines,” says Velociter.  The Wind Iris is in operation on a number of projects, including DONG Energy’s 367 MW Walney offshore wind farm in the Irish Sea. WPE