Micro-wind turbines on high rise and commercial buildings
Paul Blackmore, associate director of BRE's building technology group, gives guidance on siting.Wind turbines are fast becoming a feature of many new tall buildings, south London's Strata crowned by its three 9 metre diameter turbines being the most prominent example. The greater wind resource on tall and large buildings, and reduced shelter and turbulence from surrounding structures, makes them a more appropriate location for wind turbines than the average two-storey house. But there are challenges: determining the most effective siting can be difficult and incorrect siting can result in underperformance of the turbine.
Until quite recently, limited advice was available on the optimum height and location for wind turbines on tall buildings. The IHS BRE Press publication, Building-mounted micro-wind turbines on high rise and commercial buildings, describes research to characterise the wind conditions over a range of building heights from 15 metres to 80 metres. It also provides guidance on the most effective height and location for wind turbine installations on the roofs of tall buildings to maximise their potential for wind power generation.
The objectives of the study were to:
- measure at model scale in the wind tunnel the wind speed-up factors over high-rise and large plan-form buildings typical of commercial and industrial building forms commonly found in the UK
- use the results from the study to develop a simplified methodology that can be used by building owners and developers to arrive at the most efficient positioning of wind turbines, both in terms of position and height of the building.
Among the points and conclusions in the research are:
- Wind turbines will generate most power in smooth flow conditions, but within the built environment smooth flow will almost never occur. There will always be some turbulence: it occurs naturally and at high levels jet streams and convective mixing due to heating from the sun can cause large-scale turbulence. Free-stream turbulence decreases with increasing height above the ground and decreases in smoother terrain such as open country where there are fewer obstacles.
- When wind blows over buildings, additional turbulence is created by flow separation at the buildings' windward edges. Building-generated turbulence is often significantly higher than the free-stream turbulence in the approaching wind, typically by a factor of 3 or more.
- Free-stream mean wind speed increases with height above the ground, hence the higher the wind turbine the greater the wind resource. As wind blows over a building the mean wind speed within the separated flow region will tend to be less than the free-stream values. Just above the shear layer, the wind speed will be greater than the free-stream values due to flow acceleration over the building. The mean wind speed will return to free-stream values quite quickly with increasing height above the shear layer.
- When siting micro-wind turbines on buildings they should be located in regions of high mean wind speed but low turbulence to maximise energy production. This means they should never normally be sited below the separated shear layer.
- There is a clear connection between the height of a building and the size of the region of disturbed air flow above the roof. As a result, when identifying the best place to site a wind turbine it is helpful to be able to visualise mean wind speed and turbulence intensity patterns across and above the roof
Full guidance is given in Building-mounted micro-wind turbines on high rise and commercial buildings, by Paul Blackmore, associate director of the building technology group at BRE. The publication can be purchased online at the BRE Bookshop.