Effects of Long-Distance Wakes Between Projects

Holly Hughes; Gordon Randall18 April 2012, EWEA Copenhagen

Effects of Long-Distance Wakes Between Projects


18 April 2012, EWEA Copenhagen

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

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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

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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)

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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

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Example 1: Unidirectional site, before/after comparison

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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



Example 1: Overall project performance before/after upwind project

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Example 2: Varied wind rose, wakes compared by direction

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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.



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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)



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

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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)

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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)

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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

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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

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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

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Example test case on mesa

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Existing project on mesa

Wind Rose

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



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

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Holly Hughes [email protected] +44 7817 639369

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Effects of Long-Distance Wakes Between Projects