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Field trial report on domestic wind turbines

In 2007, the Energy Saving Trust (EST) launched a programme to monitor domestic small scale wind turbines, sited in the UK. The main aim was to determine how the technology performs when installed under ordinary domestic working conditions. As a result, EST can now provide a better indication of the realistic opportunities and savings resulting from installing domestic small-scale wind turbines. Michael Smith, NBS Information Specialist, introduces the report.

The report

A representative sample of participants and products were chosen by EST to participate; sites were identified from a selection of grant recipients who had installed either building mounted or free standing (pole mounted) domestic small scale wind turbines as well as further sites put forward by wind turbine manufacturers. EST maintained a close relationship with these participants throughout the duration of the project.

The report, generated as a result of the trial, illustrates the technical factors that impact on the performance of domestic small-scale wind turbines, including installation, proper siting and turbine performance, factors which are also highly dependent upon the local wind speed. In light of this the research undertook an assessment of the accuracy of wind speed prediction models and recommends the most suitable option for customers. It is essential to accurately predict the local wind speed before installation because performance is shown to be directly dependent upon available wind speeds.

The report also discusses the information manufacturers provide about their turbines to potential consumers. To date, there have been minimal standards in place to regulate the information available to customers, and as such, it has been difficult to determine which model of turbine might be appropriate for a particular customer, or whether a turbine is even the most appropriate technology.

The trial

In early 2007 EST approached Low Carbon Buildings Programme (LCBP) grant recipients who had installed a building mounted "micro" wind turbine. These grant recipients were invited to join the trial as fully monitored sites or to provide monthly meter readings.

Site selection
38 building mounted turbines were chosen to be fully monitored for one full year. The first data collected from these sites was available from November 2007. Additional sites – predominantly free standing pole mounted turbines – were added to the field trial in early 2008. In total 19 free standing turbines were chosen to be fully monitored for one full year. Field trial sites included urban, suburban and residential areas, agricultural farms, and remote community schemes.

In addition, householders at 68 additional sites agreed to provide monthly energy generation data, and a further 29 Warwick Wind Trial sites contributed data. However, the analysis presented in the report focuses mainly on the technical performance of the 57 monitored sites.

Monitored turbines
The turbines were domestic small scale building mounted and free standing turbines ranging in rated power output from 400W to 6000W. Usually turbines are manufacturer rated at wind speeds ranging from 11-12.5 m/s; however, manufacturers are not yet required by an industry standard to rate their product at an agreed wind speed.

Turbine performance calculation methods
Three methods are commonly used to calculate the predicted performance of small-scale wind turbines and provide information to consumers, these are load factor, Annual Energy Production (AEP) yield, and power curve.

Data collected from the field trial was analysed to determine if the manufacturers' claims of annual energy production at average wind speeds were accurate and if this is a suitable wind speed at which to rate turbines for sale in the UK market. The AEP method is part of the British Wind Energy Association (BWEA) standard to determine the "reference annual energy" generation of small-scale wind turbines and provide an indication of a turbine's annual yield at an average wind speed of 5m/s.

Wind speed prediction
A commonly accessible predictive model of wind speed, the Government's Numerical Objective Analysis Boundary Layer (NOABL) database, has until recently been the primary tool used by manufacturers, installers, planning authorities and consultants to determine a site's potential wind speed. However, the model, which provides annual average wind speeds for a 1km UK grid square, has been shown to overestimate the potential wind speed at many sites in the field trial, especially those in urban and suburban locations. This is mainly because the database does not consider the impact of local obstructions, including trees and buildings.

There are now a number of additional methods to predict the local wind speed, including an adjustment to NOABL, which has been adopted as part of the Microgeneration Certification Scheme (MCS) installer standards. The Carbon Trust has also launched a Wind Yield Estimation Tool which takes local topography and obstructions into consideration.

The field trial undertook an assessment of these wind speed prediction models compared with actual on-site measurements from the field trial sites and readings from Metrological Office observation stations, to determine the most accurate model to predict wind speed at a potential domestic location.

Results

Building mounted turbines
Results indicate that building mounted turbines did not approach the commonly quoted load factors of 10%. No urban or suburban building mounted sites generated more than 200kWh or £26 per annum, corresponding to load factors of 3% or less. In some cases, installations were found to be net consumers of electricity.

The highest load factor from a fully monitored 1.5kW building-mounted turbine located in Scotland was only 7.4%, corresponding to around 975 kWh, or £127 of electricity generation per annum. In light of the lower than anticipated figures the Energy Saving Trust recommends that sites achieve a minimum average annual wind speed of at least 5m/s.

Building mounted turbines performed best when mounted on the gable end of a building, positioned above the ridge line and located in remote rural locations with an undisturbed flow from the direction of the prevailing wind.

Free standing turbines
The best performing free standing sites in the field trial were always remote rural locations, usually individual dwellings near the coast or on exposed land such as moors. The results from these sites show the performance of free standing turbines frequently exceeds commonly quoted annual load factors of 17%. In fact, the average monitored load factor was 19%, and the best sites had load factors of 30% or greater. A 6kW turbine with a 30% load factor would be expected to produce approximately 18,000kWh per annum, equating to around £2,300. Additionally, annual generation also agreed closely with the manufacturer's predictions.

Free standing turbines sited in built up areas did not perform as well, once again due to the insufficient wind resource.

Research summary

The turbines monitored in the field trial, both building mounted and stand alone, were primarily located in urban and suburban locations. The key findings and recommendations of the research, as outlined in the report, are summarised below:

Building mounted sites were found to have inadequate wind speeds, which significantly impacted their measured performance
Free standing turbines installed in the appropriate location with an undisturbed wind resource were seen to have very good performance with annual load factors in some instances in excess of 30%
Scotland was found to be a better location than the rest of the UK due to a number of factors including higher wind speeds and more suitable topography
Manufacturers' presentation of their power curves and ratings have been calculated using various methods; a number of these were deemed inaccurate or incorrect. Potential customers should treat published power curves with caution until such time that product receives MCS accreditation
The Microgeneration Certification Scheme (MCS) has developed standards that require manufacturers to publish an accredited power . These standards are likely to be in general force by the end of 2009
The NOABL database was found to significantly overestimate the wind resource at urban sites. It should not be used for wind speed estimation on these sites. The Carbon Trust tool for wind speed estimation was found to be the most accurate method of predicting local wind speeds, for both free-standing and building mounted turbines
Customer feedback indicates that their perception of the technology corresponds closely with technical performance; they were more positive about their wind turbines when receiving demonstrable cost and energy savings
There is a good market potential for small-scale domestic wind turbines in the UK, especially free standing pole mounted turbines installed in exposed rural locations.