Britain’s Energy Savings Trust Finds Rooftop Mounted Small Wind Turbines Perform Poorly in Tests

By Paul Gipe

Confirms That Rooftop Mounting Doesn’t Deliver

Britain’s Energy Savings Trust published a report in July, 2009, based on field testing of several dozen small wind turbines.

In a play on the old real estate adage, the Trust titled the report Location, location, location: Domestic small-scale wind field trial report.

Significantly, monitoring of the turbines in the Trust’s program began as early as 2007.

Most of the “report” appears to be an attempt to correct misinformation the Trust had been giving consumers for several years about small wind turbines.

Though the Trust claims to be impartial, it has allied itself with Britain’s aggressive small wind turbine industry and their trade association, the British Wind Energy Association. This becomes obvious in the report as the Trust attempts to sugar coat the findings from the field trials.

Britain has become notorious for the promotion of small, rooftop-mounted wind turbines and the often over-the-top hype around them. While Britain has certainly not been alone in this — there have been similar outbreaks in Canada, the USA, and in France — British promoters have put more hardware on rooftops than anywhere else.

The field trials monitored 57 small turbines for an entire year, the world’s largest effort at monitoring machines of this size say the reports authors.

All together, the Trust monitored 38 building-mounted turbines and 19 turbines installed on ground-mounted towers.

Installing small wind turbine on towers mounted on the ground is the standard and recommended method for installing small wind turbines. Rooftop mounting is outside the norm and the number of rooftop wind turbines in the Trust’s monitoring program illustrates the degree to which this phenomenon has overtaken proven practice in Britain.

One of the study’s most important findings, and the most embarrassing to the Trust, was that the method recommended by Trust for estimating potential generation substantially overestimated the site’s actual wind speed for building-mounted wind turbines. According to the report, “actual wind speeds measured at the urban and suburban field trials sites was less than predicted.”

That statement has certainly been massaged many times over. It doesn’t begin to convey the seriousness of the findings.

One indicator of how poorly the turbines performed in the tests was the the Trust’s admission that at most the sites the actual average wind speeds were less than 5 m/s.

The Trust had wanted to compare the turbines’ test results with BWEA’s “reference annual energy” generation. However, BWEA’s standard is generation at an average annual wind speed of 5 m/s and “few sites achieved” this wind speed says the report.

Probably most damning was the Trust’s conclusion that no “building mounted sites generated more than 200 kWh” during 2009. But it gets worse. “In some cases, installations were found to be net consumers of electricity . . .”

The Trust’s results are comparable to those from the Warwick wind trials and in fact the Trust’s report includes data from the earlier study.

The most likely reason for such poor performance, little or no wind, confirms the charge made by critics of rooftop-mounted wind turbines that the obstructions of nearby buildings and the roof itself defeats any presumed benefit of installing a wind turbine on a rooftop.

The Trust admits this in so many words when the report concludes that all building-mounted sites experienced average wind speeds of “less than 4 m/s”.

The best performance of any building-mounted wind turbine was one 1.5 kW turbine in Scotland “corresponding to around 975 kWh . . .”

The turbine was not identified in the report, but there was only one 1.5 kW wind turbine mounted on a building and that was a Swift. The Swift uses a rotor 2.1 meters in diameter and sweeps 3.5 m². If indeed this unit was a Swift, it delivered a yield of 275 kWh/m². For comparison a large wind turbine at a good site should produce 800-1,000 kWh/m²/year and a small wind turbine at a good site should deliver about one-half of that.

Equally significant, the Trust acknowledges in the report that free-standing turbines, that is, those not mounted on rooftops, produced about six times more electricity than those on rooftops. The Trust reported that the average capacity factor of free-standing turbines was 19 percent. For the Proven 6 kW wind turbine used in the trials, this is equivalent to ~10,000 kWh per year. The Proven uses a 5.5 meter diameter rotor and sweeps ~24 m². This gives an annual specific yield of 420 kWh/m²/year, about one-half that of commercial turbines at good sites.

Despite the dismal results, the Trust nevertheless claims there is a market for nearly one-half million small wind turbines in Great Britain.

While the field trials have been welcomed by Britain’s small wind community, the Trust has received withering criticism for withholding data on the program and their sometimes tortured attempt to make the best of a bad situation.

Despite the role of public funds used in the trials, and despite the Trust’s function as a public agency, the Trust has claimed that the data is proprietary and, hence, secret government property, withholding it from circulation.

While Britain may still have an “official secrets act” that shields damning government reports and their supporting data from the prying eyes of the public, it is not in the best interests of renewable energy or the small wind turbine industry itself to hide the data. Only with reliable, third-party testing can the industry improve or consumers learn to demand better, more reliable and more productive products.

The Trust, by feebly attempting to shield the small turbine industry from its own failures, and as a consequence misleading the public about the reliability and performance of small wind turbines in Britain, has done serious, though not irrevocable, damage to the growth and maturity of small wind turbine technology specifically and to renewable energy in general.