Going bananas over wind farms

In Climate Spectator on Tuesday (Queensland Health rebukes The Australian on wind farms, May 29) I highlighted a number of problems with an article in The Australian. As well as trying to beat-up the health effects of wind farms, it left the impression that Australia's entire banana crop could be threatened. Citing horticulturalist Steve Lavis, The Australian said,  

He was also worried about the impact on the region's banana industry, which relied on aerial spraying that would not be possible within 5km of wind turbine towers.

"All of Australia's bananas are grown within 200km of here," he said. "There is no way you can control the diseases without aerial spraying because of the wet season and the inability to get to the crop with a tractor," he said.

A reader of Climate Spectator was sufficiently concerned about this article to complain to The Australian and received the following reply from Clive Mathieson, editor of the paper:

“…an aeronautical assessment prepared for the developer itself acknowledges the wind farm could cause a “critical hazard” to aircraft involved in aerial spraying in the vicinity. It also notes that wakes from wind farm clusters can extend as far as 5km downstream. To quote from the report: “It should be noted that low level flying operations such as agricultural aerial spreading and spraying operations or power transmission line inspections may be affected on the downwind side of the turbines over land on which the turbines are directly positioned, or over portions of some adjoining properties that are sited downwind from the turbines. This is due to wind shear, turbulence and downdrafts in the wake of the turbine rotors presenting a critical hazard to aircraft such as agricultural aircraft operating at low level and high weights during application of chemicals and seeding.” 

Now it is indeed true that wind turbines induce a turbulence wake for some distance downwind from the turbine. But this is heavily dependent on the prevailing wind speed as well as rotor blade diameter (generally around 90-126 meters). The aeronautical assessment report Mathieson quotes from states that:

“A wake length equivalent to 6 times the rotor diameter is considered a minimum in wind conditions of 10-15 knots (18-28 km/h). When the wind turbines are operating in winds of 15 knots (28 km/h) or greater the wake from a single turbine is still prevalent at 10 blade diameters and can persist for up to 16 blade diameters downwind of the turbine. The majority of modern wind turbines reach their maximum output, and in theory, generate the strongest wake turbulence in wind speeds of approximately 47km/h. At this speed, and in combination with the wake produced by other turbines, the wake may exist up to 5km downstream from a large turbine cluster of several rows.”

So that's where The Australian got the 5km figure.

But what Mathieson conveniently left out in his quote from that report were the sentences that immediately followed, which state:

“However, agricultural spraying operations are normally conducted at very low levels and often require calm or very light wind conditions of less than 8 knots (15km/h). At these wind speeds it is reasonable to assume the wake can extend for a distance of 6 rotor diameters or 600m downwind of the nearest turbine based on the proposed rotor diameter of approximately 100m. Given the distances from wind turbines to cultivated areas of land on adjacent properties outside the wind farm boundary there should be minimal impact on agricultural aerial operations.”

I think we can all sleep easy knowing our bananas aren't at risk from wind energy.

And a response to those Landscape Guardians

A number of anti-wind farm campaigners responded to my prior article by pointing out that the association representing agricultural aerial sprayers has a public position that suggests wind farms are a serious safety risk. This really comes down to three issues:

1) Wind turbulence. While wind turbines do induce turbulence effects, the areas of land currently subject to aerial spraying that might be affected is likely to be small. This is because the number of turbines involved is small relative to the area of agricultural land, and the fact that much of the wind farm development occurring in Australia is in areas such as barren ridgelines and pastoral areas that are not near land subject to aerial spraying.    

2) Visibility of wind monitoring masts. The association does have a legitimate concern, which is that wind monitoring masts can be quite hard for pilots to see and are high enough that they pose a collision risk. This needs to be fixed by wind farm developers putting in place measures to make sure aerial sprayers are easily able to identify and avoid these masts. 

3) Collision with wind turbines. Collision risk with wind turbines is low. Already as part of the planning process prospective new wind farms undertake an aviation hazard risk assessment. Where there are noticeable risks, the developer is required to put in place mitigating measures such as installing hazard lighting and ensuring the wind farm is marked on aeronautical maps, so pilots can readily identify and avoid them. Aviation consultants, Hart Aviation in a report for the Victorian government, observed:

“Available records indicate that, worldwide, there are over 75,000 wind turbines of various sizes in operation, or planned to be in operation, and the number is quickly expanding. This includes both onshore and offshore facilities. There is clear evidence that these wind turbines / wind farms can coexist successfully with aviation operations. Indeed, no evidence could be found of any aircraft collision with a wind turbine, or any other related incident.”