A better understanding of wind flow through cities will give companies that manufacture and erect turbines, such as Mariah Power, Reno, Nevada, a better idea of where to place units for maximum power production.

Controversy seems to follow the installation of wind turbines on building rooftops, and for good reason. On one hand, there can be considerable wind with harvestable kinetic energy at roof level. Accessing this clean, renewable source of power can be a good way for building owners to save money on their electric bills, reduce their dependency on the grid, and trim their carbon footprint. On the other hand, it has been difficult to understand how the wind behaves in built up areas, resulting in low power outputs and ultimately, bad reviews. Historically, many people have installed rooftop turbines on the assumption that even if the devices don’t work well, they’ll look good.

The challenges
As with traditional tower-mounted wind turbines, if a building-integrated wind turbine (BIWT) is to be successful, it must be assessed in terms of its net economic benefit. To date, BIWT installations have consistently missed power output expectations, as much as 90% in some cases. After careful review of BIWT installations on a global scale, it is clear that a main culprit behind these underperforming wind turbines is their placement. This conclusion is confirmed by a section of the American Wind Energy Association’s 2009 Small Wind Market Study calling for improved assessment technology.

Part of the problem is that urban and suburban areas include considerable turbulence and turbines mounted there have simply not been capturing enough laminar wind. Predicting wind-energy quality and density is more complicated in built up areas, such as cities, than in rural plains where there are few obstacles upwind of the turbine.

Traditional methods of assessing wind-energy density each have drawbacks, especially when applied to more complex areas. Anemometers must be set up on their own meteorological towers and collect data for at minimum 3 months, but really a year or more is required to be accurate. Wind maps are not built for site selection as much as screening, with even the tightest resolutions still overlooking local effects. Computational fluid dynamics studies show promise, but are still quite costly and difficult to set up properly. Existing technology leaves building owners and wind installers ill equipped to properly analyze the complex wind conditions, leading to poor site selection and inadequate power output. As a result, most BIWT’s are commissioned without a serious wind study to determine a best location, or in many cases, even if the site is appropriate at all. So it is not surprising that most building mounted turbines have failed to reach their expected power outputs. With such results, it is also not surprising that many have criticized the use of wind turbines on building rooftops.

BIWTs have yet to prove themselves to the small-wind community accustomed to installing turbines on tall towers in open terrain. However, BIWTs have one advantage over ground-mounted turbines typically positioned in good wind 40 meters up: BIWTs don’t need a costly 40-m tower to reach that height. On a sufficiently high building, typically one will never need more than a 10-m tower to clear turbulence caused by wind hitting the roof edge. Assuming equal installation costs, a 10-m tower on a 10 story building could cost $20,000 less than a 40-m tower reaching comparable heights. Despite this, for BIWTs to be taken seriously though, performance must improve, and for performance to improve, siting must improve.

The way forward
To understand wind in urban areas, it is necessary to consider many things ignored by existing evaluation tools. One must address the timing issues of anemometers, the accuracy issues of wind maps, and the cost issues of CFD studies. It is first important to consider the topography and texture of the area for several kilometers around the target site. This accounts for general turbulence in the local environment, a condition called roughness. Understanding this is required because even in cities like Atlanta or Los Angeles where there are relatively few tall buildings, the air is still quite rough due to the presence and extent of many smaller buildings.

Perhaps most important is the need to consider local effects, factoring for the size, shape, height, and distance of various obstructions. This incorporates the wind energy impacts of nearby trees, buildings, and other structures to understand how much the wind is blocked, what turbulence is created, and in some cases, how much the wind speed is increased. Further, if mounting on a building, it is critical to account for effects from the building, details such as its surface, roof edge, and roof features such as towers or chillers. For example, when wind hits any obstruction, it creates a separation zone arching out from the top of its vertical face. Above this point, the air remains smooth, but below, it becomes quite turbulent. This behavior must be considered in every rooftop-wind project to figure out how much higher the turbine must be mounted to capture energy from the smooth airflow.

So to improve the performance of building integrated wind turbines, one must consider local roughness, blocking, turbulence, and roof dynamics, something most assessment tools do not do. Furthermore, customers want answers quickly regarding wind energy potential at a reasonable price, another thing most tools have not done well.

Analysis services however, are available which meet these market needs – information on wind energy and expected performance can be delivered quickly for a small fraction of installed turbine cost. With better data, installers and customers can make better decisions, which will lead to better performance of building integrated wind turbines. This opens up the urban wind energy market for explosive growth in the coming years.

A few company initiatives are available at www.windanalytics.com.