Less Bird Deaths with Black Wind Turbine Blades

An American research report led to the elimination of eagle deaths on the Norwegian island of Smøla. A review demonstrates that with today’s larger wind turbines, three black turbine blades per wind turbine can give at least as good results as the Norwegian experiment with one black blade. Given the importance of the appearance of the landscape to those viewing it, the former option is preferable.

 

 

Although not threatened globally, golden and white-tailed eagles in Sweden and some other countries are classified as ‘near-threatened’ and thus justify a radius of 2 to 3 kilometres free from wind turbines around each nest. For Sweden’s slightly over 1,000 breeding eagle pairs this equals an area the size of Belgium. Since there are also other species that need protection, a means that reduces the threat would be most welcome for the wind community.

 

On the Norwegian island of Smøla, 68 large wind turbines were completed in 2005, in an area with 45 nesting white-tailed eagle pairs. The killing of eagles came as no surprise, although the rate has been fairly low at 0.1 eagle for each wind turbine per year. However, the turbine owner Statkraft considered it rather embarrassing and has supported efforts to find means to reduce the killing.

 

In the USA, laboratory experiments were carried out in 1999–2002 by Professor William Hodos at the University of Maryland with the aim to reduce bird deaths at wind turbines. As recommended in the study, one of the three turbine blades of four wind turbines within the Smøla wind farm in 2013 was painted black, which resulted in a 70% reduction in the total number of birds killed and no more dead white-tailed eagles. The other two turbine blades had the usual white-grey shade. Comparison was made with the outcome of a control group consisting of four turbines with untreated blades.

 

This solution may be perfect for the birds, but for the humans viewing the landscape, the impression can be dubious. The black blade will dominate. The turbine will thus appear unbalanced and as if it is moving up and down towards the horizon. A solution where all three blades are painted will be more acceptable. Since the conclusion from the laboratory experiments was to paint just one of the blades, it remains to be proved that the solution with three black blades is technologically feasible. A proof will necessitate an analysis of the problem. In this case we are happy to be in a situation where the information needed is available in the earlier reports, although this information might not have been fully evident to the authors.

 

The Problems
In the Hodos 2003 report, the problem of motion smear is described in roughly the following way. Birds, like humans and other animals, are not able to perceive objects that move too fast. When a stationary eye looks at a moving object, the image of the object will move over the retina in the back of the eye globe. It is the retina that registers the image. The speed at which the image moves is determined by the angular velocity of the object, which in turn depends on its linear velocity and distance. At a certain speed, the retina cells do not have time to perceive the object, which disappears in a haze.

 

The report also deals with the phenomenon that the retina cells need a certain amount of time to recover after being stimulated. Here it is called the problem of recovery. The implication of this is that the visibility may improve if you only paint one of the turbine blades, since the time between each clearly visible blade passage then increases by a factor of three. Another way of influencing the phenomenon is to paint the turbine blades with a staggered pattern, which is not further described here.

 

Since these two phenomena occur simultaneously, one cannot directly determine their mutual strength. As will be shown later, they scale in different ways, which is important for the assessment of larger wind turbines.

 

Method
For the experiment, 15 American kestrels (Falco sparverius) were borrowed from a wildlife research centre. Before each test, the lightly anaesthetised bird was fixed in a position 57 centimetres in front of a test device with a rotating disc (diameter 68 centimetres) to which models of turbine blades with different painting and patterns could be attached. A lens system was positioned between the bird and the disc, and compensated for the eye’s distance setting being put out of play by an injection into its accommodation muscles. With the help of electrodes attached to the eye, a pattern electroretinogram, PERG, could be obtained. A PERG is a measure of the eye’s reaction to a stimulus, such as looking at a certain object. The PERG obtained was compared with that obtained from the eyes when covered with patches.

 

Results
In the Hodos report, the most important experiments are summarised. The first is the recording of the reference value, with the eyes of the bird covered. Experiment 2 is with all white blades. In experiments 3–7, one or more white blades have been replaced with blades with different black stripes. They are omitted in Table 1. In experiment 8, the turbine contains two white and one black blade. No attempt was made with three black blades.

 

The most striking result is the high value of visibility obtained with one black blade: a 130% better PERG value than with all white blades.

 

In Table 2, values have been entered from the laboratory test from Smøla and from a possible large-scale demonstration of the technology. In Smøla, 2MW turbines with a diameter of 80 metres are used. These, like the new wind turbines of today, have a tip speed of around 80m/s at rated power. A large-scale demonstration plant is assumed to consist of today’s commercial turbines with a diameter of 150 metres.

 

Basic case	Turbine		Motion smear		Recovery
	Diameter	Tip speed	Angular velocity	Distance	Time/revolution	Time/blade passage
	m	m/s	deg/s	m	s	s
Laboratory test	0.68	1.3	130	0.57	1.7	0.55
Smøla	80	80	130	35	3.1	1.1
Demonstration	150	80	130	35	5.9	2.0
Table 2. Basic parameters for laboratory tests, Smøla, and possible large-scale demonstration

 

The results according to Table 2 apply at an angular velocity of 130 degrees per second, which constitutes the limit for good visibility and thus a value free from the previously defined problem of motion smear. This value applies in the eye and thus also between the eye and the object being viewed, i.e. the wind turbine. The bird is assumed to approach the turbine either straight from the front (moving in the same direction as the wind) or straight from behind (towards the wind direction). The angular velocity will increase as the bird approaches the wind turbine. A specific relationship applies between angular velocity, tip velocity and distance. If the angular velocity exceeds the specified limit, reduced visibility occurs due to motion smear, which can cause the bird to continue towards the turbine in the belief that it is a safe area. Note that the size of the turbine itself does not affect the angular velocity.

 

According to Table 2, the same angular velocity is achieved at Smøla when a bird has reached, on approach, a distance of 35 metres from the turbine plane. The good results at Smøla confirm that the birds themselves managed to manoeuvre out of such potentially dangerous situations. Since the turbines in a possible large-scale demonstration are assumed to have the same tip speed, the distance of 35 metres also applies in that case.

 

The problem of recovery means that the retina cells need a certain amount of time to recover after being stimulated. Table 2 reveals that in the laboratory test the time for an entire turbine revolution amounts to 1.7 seconds and for a blade passage to one-third of that, i.e. 0.55 seconds. This means that the visibility may be improved if you only paint one of the blades, because the time between each clearly visible blade passage then increases by a factor of three. So here are two options for what time to use.

 

With increased turbine size, the scaling of the rotational speed is inversely proportional to the turbine diameter, provided that the speed of the blade tips is kept constant. For Smøla, according to Table 2, the time for an entire turbine revolution is 3.1 seconds and for a blade passage 1.1 seconds. If the laboratory test is interpreted so that the time required (for an entire turbine revolution) is 1.7 seconds, the time might have been too short if the turbines had been equipped with three black blades per turbine. But that did not happen, so we do not know what the result would have been.

 

Today’s turbines have further increased in size. The use of 150-metre turbines in a large-scale demonstration means that the rotational speed drops further. Now the time for a full turbine revolution is 5.9 seconds and for a blade passage 2.0 seconds. It is found that the time for a blade passage is clearly greater than the maximum result the laboratory test can justify, 1.7 seconds. This means that you do not have to worry when supplying the demonstration plants with turbines with three black blades.

 

A demonstration park may conceivably consist of an arbitrary number of wind turbines, where some have all black blades, some have one black and two blades with the usual light grey shade, and some have all light grey blades. With black blades only, the contrast increases correspondingly, which should have a further positive effect on visibility.

 

For aviation, the increased visibility will be a benefit. In order to secure visibility in all lighting conditions and backgrounds, it is probably best to keep the light grey colour of the tower. When dark, the ordinary obstruction lights are operated.

 

One site that has been mentioned for a demonstration is Näsudden on the island of Gotland, Sweden. At that site there is already an IdentiFlight camera system, which is used to identify eagles and their flight paths up to 1 kilometre away. Thereby, one will be able to get a qualified evaluation of what deterrent ability the various blade paintings have. The answer should be much more comprehensive than what you get by just counting felled eagles, which may be a few per year for a park of moderate size.

 

Need for Further Research on Bird Vision
Humans and other mammals have vision based on the three colours red, green and blue. Birds instead have vision based on four basic colours and in addition, for some species, extending into the ultraviolet area. They can see many more colours and perceive them differently than we do. Therefore, there is a need for more basic and applied research. Maybe it is possible to create a blade colour that is neutral for humans but easily visible to, and has a strong signal effect on, birds?

 

The report also covers experiments with red, yellow, blue and green blades against backgrounds consisting of colour images of landscapes at or near ground level. The outcome varies greatly with the choice of background, and the Hodos report does not recommend the use of any alternative with coloured blades. One can argue that the use of backgrounds in the experiments did not seem to give realistic pictures of reality as a bird, or a pilot, sees it. It is useful to know that what is on the horizon is at exactly the same height as yourself, provided that the terrain is fairly even. When visibility decreases, the horizon becomes smeared, especially over the sea. Even a bit below the horizon, contrasts are reduced. Together, this means that the sky and surroundings with low contrasts dominate the view.

 

Biography of the Author
Staffan Engström, Master of Science, Mech. Eng., has been working in wind power since 1975 at the National Swedish Board for Energy Source Development, the National Swedish Energy Administration and Nordic Windpower. He is presently Managing Director of Ägir Konsult.