Analysis Rooftop wind turbines are actually net carbon emitters for most British properties, according to new research. Worse still, it appears that even if small turbines became common they could produce only a tiny fraction of the UK's energy requirements.

The new report (pdf) is titled Small-scale wind energy and is issued by the Carbon Trust, a quango dedicated to reducing greenhouse gas emissions. The research was carried out with the assistance of the Met Office and consulting engineers Arup.

According to the report's authors, the most commonly used UK windspeed database is highly optimistic in the case of urban areas. They suggest that one should be wary of believing any figures offered by the wind-turbine industry.

NOABL [Numerical Objective Analysis of Boundary Layer] is a public domain reference dataset used widely in the UK wind industry ... analysis by the Met Office suggests that NOABL tends to ... over-predict the amount of power it is possible to generate with small turbines in built-up areas ... The [free from the government biz department] Wind Speed Database (created using the NOABL model) does not reflect the effects of urban areas (the wind speed values are representative of open, level terrain) ...

There is also a technical supplement (pdf) intended for engineers and designers, which goes even further:

There are a variety of sources of wind speed and direction data available ... All of these data types have limitations, either in terms of their temporal or spatial extent, or in terms of their representativity of urban areas ... There are a number of proprietary systems used by the wind energy industry ... Tools such as WindFarmer, WindFarm and WindPRO do not include any functionality designed specifically for siting turbines in urban areas.

The researchers go on to model the expected yields of small wind turbines using a more realistic Met Office database which you have to pay for, rather than the free one, and they allow for the serious reductions in windspeed to be found close above urban rooftops. The results make depressing reading for microgeneration fanciers.

Because small turbines are mounted at relatively low heights, their mean hub height wind speeds may be close to their cut-in speeds. The implications are that, for long periods of time, a small turbine may not operate at all, or if it does operate (and visibly spin), it may not generate much electricity. Practically speaking, small wind turbines require ... locations which are open, exposed and experience high wind speeds, which generally tend to be found in rural areas.

Because the output to be expected from urban turbines is so low, the cost of the resulting energy is very high. According to the Carbon Trust analysts, electricity prices would need to be double their present levels before any urban turbine could earn its keep. Even if electricity soared to eight times its current price, economically viable urban turbine sites would still be a rarity:

Practically no urban sites have costs of energy below 25p/kWh ... At 100p/kWh, the energy that could be generated at rural sites is about nine times that of urban sites; i.e. the split is 90 per cent rural to 10 per cent urban.