Urban Wind Energy Resource Evaluation
N. McMahon, R. Byrne.
The difficulties with small wind turbines in built-up areas include:
- Performing meaningful resource assessment before installation.
- Significant, practically unpredictable, turbulence created by buildings.
- Structural damage to buildings arising from wind turbine vibration.
- Most critically, the lack of wind.
For these reasons, it's generally held that small wind turbines do not work if placed close to buildings in urban areas. Surprisingly, few systematic assessments of wind resources at rooftop level in urban areas have been conducted. Nevertheless, theoretical considerations aside, there is much evidence that small wind is not viable in built-up areas.
One recent, rare, example of a systematic study of urban wind, the Warwick wind trial, was completed in 2007. Its results indicate that small wind turbines, placed without a tall tower on low buildings, are unlikely ever to be cost effective in an urban area. As you might expect, small wind turbines positioned on very tall buildings performed better, but still poorly alongside well-sited machines.
At altitudes much higher than rooftop level, the urban wind resource is better understood. This is because the airflow at higher altitudes is less chaotic; turbulence from buildings dissipates with distance. Consequently, urban wind resources are more easily estimated for commercial scale wind turbines.
There are many examples of successful commercial scale machines operating within town limits. Beside the Centre for Renewable Energy, in Dundalk, in Ireland, for example, a Vestas V52 generates, economically, the majority of the electricity used by the campus.
Resource Assessment at Rooftop Level
In principle, if we could understand better the wind resource at rooftop level, and below, then it may be possible to identify specific locations where the installation of a small wind turbine is of some value, e.g. on tall buildings; in relatively exposed urban locations close to the sea. In practice, it will be next to impossible to do this with sufficient accuracy. Additionally, it is likely that very few such sites exist.
Nevertheless, it may be interesting to establish a protocol, or a set of guidelines for identifying, in theory, the most likely sites in a real city. With the aid of computer based work and experimental data, estimates, with error values, for the viablility of various real locations could be determined and compared with the output from and cost of, say, a solar electric installation. While the results are likely to be inaccurate and in any case disappointing for proponents of urban small wind, it may be useful to have a reasonably authoritative body of work about the wind resource in built-up areas.
As a commercial prospect for small companies, however, urban wind resource assessment will almost certainly prove to be too expensive to conduct and of limited practical use; there is little prospect of near-term profit. Nevertheless, as a public good there may be value in this work.
From the final report:
It is the opinion of the authors that, at present, beyond using common sense rules of thumb or protocols, e.g. those reported by the Met Office, urban resource assessment requires a considerable investment. CFD (computational fluid dynamics) work still requires software and experienced workers and results need to be carefully validated against experimental data; this is also costly to acquire. However, although it may be a stretch to say that easy-to-use computational fluid dynamics has arrived, new technologies and the rapid development of online data amalgamation services mean that user-friendly, useful, wind resource assessment services may not be far off. The Carbon Trust Wind Yield Estimation Tool is a good example of progress in this direction. CFD will surely progress and will be a part of a future solution.
This project was funded by Enterprise Ireland.