Effects of Long-Distance Wakes Between Projects

1. Effects of Long-Distance Wakes Between Projects

2. DNV KEMA Energy & Sustainability – who we are • Formed from the merger of DNV and KEMA in March 2012 • More than 2,300 experts in 30 countries around the world advising and supporting organizations along the energy value chain

3. Introduction Purpose of presentation: Give an overview of DNV KEMA’s research and recommendations on long-distance wakes First part of presentation: Measured long-distance (2+ km) wakes at operational wind farms Second part of presentation: Recommendations and conclusions regarding siting

4. Overview – how we measure wakes between projects • General methodologies • Option 1: Compare turbine production or wind speeds before and after installation of upwind projects, relative to an unwaked control • Option 2: Compare turbine production across a partially waked project, controlling for wind direction to change the waked and control turbines (provided you can distinguish between wakes and topographic effects)

5. Overview of sites with observations • Variety of sites that have been looked at (all North America): • Pacific Northwest – unidirectional winds, rolling hilly terrain, low turbulence • Upper Midwest – omnidirectional winds, flat terrain, generally low (but variable) turbulence • South-central – one predominant direction, projects on mesas, generally low (but variable) turbulence • Types of sites not yet looked at • Offshore – Have not (yet) had data from multiple nearby projects • Hilly, forested terrain – Not many cases of projects close to each other, turbine arrangements often don’t facilitate analysis, and topographic effects and land cover often confuse matters

6. Example 1: Unidirectional site, before/after comparison Winds unidirectional from west to east Neighbouring project installed approximately 20 RD upwind, waking half of the front row of the original project Downwind turbines operational for about 2 years without wakes

7. Example 1: Overall project performance before/after upwind project

8. Example 2: Varied wind rose, wakes compared by direction Winds from all directions Neighbouring project installed approximately 50 to 70 RD upwind, depending on wind direction Depending on wind direction, either the north or south half of the downwind project is waked.

9. Example 2: Wakes under different conditions (speed, direction dataset, etc.) measured compared to eddy viscosity model 30-50 RD modelled: 8.3% 30-50 RD measured: 18.0% (Factor of 2.2) 50-70 RD modelled: 4.6% 50-70 RD measured: 11.9% (Factor of 2.6)

10. Main conclusions from measurements Significant wakes observed to travel >50 RD downwind under stable conditions Many wake models tested against measurements (e.g., eddy viscosity) do not accurately characterize long-distance wakes – models consistently underestimated effects by about a factor of three Not a “deep array effect” – large wake deficits were observed at long distances from only one or two upwind turbine rows DNV KEMA has improved models incorporating wake and atmospheric observations – but the models are still dependent on having good data and knowing where future turbines will be

11. Recommendations for siting: Site characterisation Remote sensing/tall tower measurements up to and above top of rotor are useful for accurate wake modelling Stability measurements are also useful (but not as much so)

12. Considerations for siting: How far away are there “zero” wake effects from upwind projects? No “wake-free” safe distance has been found – significant wakes were observed at 70 rotor diameters, so it’s some distance beyond that (at least at some sites) “Safe” distances should consider sizes of future turbines – your project may have 70m rotors, but another developer’s upwind project could use 120m+ rotors: 70 rotor diameters = 8.4+ km Land rights/setbacks of this distance are unlikely at many sites (and may be inefficient)

13. Expected magnitudes of external wakes • Highly dependent on layouts, distances, turbine sizes, stability, atmospheric profiles, etc., so impossible to generalize, but… • To generalize: • Worst case with project 20 RD completely upwind (e.g., unidirectional wind rose) and low-moderate wind speeds, external wake losses >10% on AEP are likely • Projects 40-60 RD upwind with varied wind roses/partial overlap could have approximately 1-2% effect on AEP • Large downwind projects with large internal wakes will have less average incremental effect • …and keep in mind that 1% energy loss can have a huge effect on a project economics

14. Recommendations for siting: establishing project setbacks Requiring “zero” wakes is not really plausible at sites in regions conducive to large-scale wind development Tolerance for external wakes is a project-specific decision – different lenders and equity partners will have different opinions Calculating a “maximum external wake” loss assuming other developers build projects everywhere that’s feasible will probably give a grim result – but it is useful to know Recommend tiered siting guidelines: “no-action” zone, “test” zone, “no-build” zone

15. Tests to measure wakes – how to plan Depending on the site layout and surrounding terrain, it can be difficult to plan ahead for measurements of wakes from as-yet-unknown turbines Permanent mostly-unwaked meteorological masts can be useful, particularly in pairs (or more) to compare; remote sensing also useful Wake measurements can’t be done quickly – may need a full year to capture varying atmospheric conditions and wind directions

16. Example test case on mesa Permanent measurement Wind turbine – existing project No-build zone: 2 km to south, 1 km other directions Test required out to 10 km south, 5 km other directions Wind Rose Existing project on mesa

17. Conclusions • There can be large impacts from external wakes on projects – much more than many current models indicate • Given modern turbine sizes, 8 km or more distance may be needed to avoid impacts • Good upfront data collection will help estimate potential effects and establish setbacks • External wakes can be measured effectively – with the right planning

18. www.dnvkema.com Visit us at Stand C1-C9 Holly Hughes holly.hughes@dnv.com M +44 7817 639369