Vaisala lidar User Manual

Nacelle-mounted lidar:

New advances in Power Performance Testing, optimization, and financial outcomes in wind energy

As wind energy continues to evolve, so do the financial stakes in maximizing power output and profit.

Today, even a minor difference between estimated and actual energy output creates substantial financial impacts. While developers and operators expect to get the energy output promised by manufacturers, they can’t know if they are succeeding without accurate Power Performance Testing (PPT) data. They are also unable to identify and take corrective actions that can extend the life of the turbine and reduce annualized cost of energy.

Enter nacelle-mounted lidar. With its exceptional accuracy and range, economical design and small footprint, nacelle-mounted lidar is one of the most reliable and affordable ways to assess a turbine and enable IEC-compliant PPT. Recent advances in the lidar itself and the software that interprets its data are revolutionizing wind farm operations.

This eBook explains how nacelle-mounted lidar works, how it is changing PPT, and why it is such a powerful solution for today’s ever-growing wind turbines and farms.

$500,000

Revenue loss from a 1% decrease in annual production (100 two-megawatt turbines)1.

© Vaisala 2021

Contents

How does nacelle-mounted lidar work?	4

Power Performance Testing in detail	6

Leosphere, a Vaisala company's, Windcube suite	10

Case study: Offshore PPT	12

Further applications for nacelle-mounted lidar	13

© Vaisala 2021

How does nacelle-mounted lidar work?

Wind speed measurement and the Doppler effect

Lidar emits light pulses at a very high frequency, and these light pulses are reflected back by aerosols in the atmosphere. The Doppler effect allows the lidar to accurately compute the speed of those particles — and, thus, the speed of the wind carrying them through the atmosphere. Today, wind lidar is used for a wide range of applications in wind energy, aviation, and meteorology.

Notably, WindCube© Nacelle operates on a pulsed lidar principle, which provides several important advantages over other lidar types. Pulsed lidar maintains constant accuracy over the entire measurement range, and it offers the highest accuracy, data availability, and sample rates regardless of weather conditions.

Nacelle-mounted lidar has evolved substantially in recent years. For example, earlier generations of WindCube Nacelle (formerly called Wind Iris) measured up to 450 meters at 10 measurement distances. This was extremely useful, but today’s version covers a measurement range from 50 to 700 meters at 20 separate distances, dramatically increasing PPT quality and making it suitable for even the largest turbines likely to be deployed now or in the future.

WindCube Nacelle measures from 50 to 700 meters at 20 separate distances, dramatically increasing PPT quality and making it suitable for even the largest turbines likely to be deployed now or in the future.

© Vaisala 2021

The best technology for the job

Once a wind turbine is deployed, its operators depend on its performance to match manufacturer expectations. Underperformance can be extremely costly — and without good wind data, it can go unobserved for months or years.

Back when turbines were smaller, this was less of a problem. But now that the scale of wind turbines has increased so much, so has the scale of the financial losses when they aren’t operating at maximum capacity. Even slight underperformances are no longer acceptable.

Traditionally, the way to assess power performance was to use a met mast. But because of their costs and logistical challenges, many wind operators simply skipped using them. One reason why is that you need to have both the met mast and the turbine aligned with the wind direction — meaning that frequent changes in wind direction can invalidate the measurements. Another is that the

costs and logistical constraints are often too high for such a short measurement period. Nacelle-mounted anemometers are cheaper to use, but they are not accurate enough for reliable PPT.

Nacelle-mounted lidar, on the other hand, is much quicker and easier to deploy, and it collects data much faster. It is always aligned with the wind turbine because it moves with it, and it is

typically half the cost (or less) of using a met mast, particularly for taller and offshore wind turbines. Lidar also uses multiple laser beams to cover the whole rotor area, creating more representative wind data that accounts for shear values and other influences acting in three dimensions. (This data, while not mandatory, can now be included in PPT for improved accuracy and higher certainty following IEC rules.)

Expert groups from IEC and independent consultants are also preparing guidelines for using nacellemounted lidar in complex terrain for the next revision of IEC standards.

The bottom line is that nacelle-mounted lidar is here to stay. It is the best solution for PPT — both onshore and offshore — and its continual improvements are making wind farms more profitable, reliable, and efficient.

© Vaisala 2021