Original URL: http://www.theregister.co.uk/2008/08/07/rooftop_wind_turbines_eco_own_goal/

Carbon Trust: Rooftop windmills are eco own-goal

Perhaps we should stop subsidising them

By Lewis Page

Posted in Science, 7th August 2008 14:31 GMT

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.

Not only would rooftop turbines in built-up areas be an enormous money-pit (barring crippling electricity price rises), they would also be bad for the environment. More greenhouse gas would be emitted during their manufacture, shipping, installation and maintenance than would be saved at power stations. The Carbon Trust people considered the case of the SWIFT turbine, a representative model.

Assuming that the entire household population of UK urban areas has this machine installed ... in the majority of cases (over 80 per cent), yields are less than 500 kWh/year ... over 50 per cent of installations have a carbon payback of more than 20 years, which is beyond the expected life of the turbine ...

So an urban rooftop windmill normally produces well under 2 per cent of the energy requirements of an average UK household*, represents an enormous financial loss, and will normally increase the world's greenhouse gas emissions rather than reducing them. So you'd have to be mad to install one.

In fact, no you wouldn't - because the government will give you money or tax breaks for a roof turbine. Even though it's usually expensive and actually bad for the environment, thanks to the various kinds of eco-subsidies (particularly those built into the low- and zero-carbon buildings regs) it may make sound financial sense to buy a windmill.

Some government grants are not conditional on site wind speed and electricity generation potential. This carries the possible risk of grants being awarded to installations which save little carbon ...

Only certain sites amongst all places where small turbines could possibly be installed are actually suitable for installation ...

A range of policies is encouraging the growth of small-scale wind energy in the UK, including the Low Carbon Buildings Programme, Permitted Development Rights (PDRs) for domestic installations and the Code for Sustainable Homes.

It is recommended that ... In any future grant schemes, a criterion is used to measure likely carbon savings ...

You'd think that grant schemes with the phrases "low carbon" and "sustainable" in their titles would have such criteria, but this seems not to be the case.

This is all quite hard for the Trust to say - being eco-quangocrats, they are the kind of people who would love to believe in a Blighty powered largely - or even partly - by renewable microgen. But, through gritted teeth, they admit that "up to 1.5 TWh could be generated [annually] ... these figures are fairly low ... [if the cost of wind turbines halved, this] could change to 3.1 TWh ..."

"Fairly low"?

The UK used 2,700 terawatt-hours of energy in 2006, actually, so we're talking about a thousandth of our energy supplies at the very outside. A better description would be "insignificant" or "microscopic".

It would be nice to think that this finally puts a merciful stake through the heart of the seemingly unkillable home-windmill idea, but it probably won't. Even the Carbon Trust didn't much like reading their own numbers, and it will be surprising if anyone else bothers to heed them. For a lot of people, off-grid living and microgeneration are religious/moral standpoints, not sets of engineering techniques; they won't be swayed by mere numbers, even ones from fairly right-on types like the Trust.

The rest of us, though - unless we happen to live on a windy hilltop out in the boonies - might reasonably file the roof-turbine plans in the bin and think of something else. ®

*According to the UK Office of National Statistics.