INTRODUCTION

Part of the Government's energy policy is to increase the contribution of electricity supplied by renewable energy to 10% by 2010. Initiatives intended to help achieve this target include the promotion of locally based small-scale electricity generation (so called micro-generation), such as solar photovoltaic (PV) cells and small wind turbines. Government-sponsored programmes, such as the Low Carbon Building Programme¹, offer grants for the installation of micro-generation systems, including small wind turbines. These grant programmes, along with media interest in a new generation of micro-wind turbines specifically designed to be attached to buildings, are leading to an increase in the installation of domestic wind turbines.

It is clearly important to ensure that the measures being promoted will generate sufficient energy during their service life to compensate for the environmental impact related to their manufacture, installation and maintenance, and hence that they will be beneficial in reducing greenhouse gas emissions.

A number of studies and reviews have been conducted into to the effectiveness of wind generation, in terms of energy balance, and also its ability to help meet the UK's drive to reduce greenhouse gas emissions. However, these studies have focussed on larger scale wind generation schemes, and are not specifically relevant to ‘micro-wind' generation installations, in particular to installations on typical houses, which are generally rated at 1.5 kW or less.

Clearly, the output from wind turbines is dependant on the local wind conditions. The power in the wind is due to its kinetic energy and is proportional to the cube of the wind speed. Hence, relatively small reductions in wind speed greatly reduce the available power (e.g. halving the wind speed reduces the available power eight times). This is especially significant for small-scale domestic installations in built-up environments where surrounding buildings can dramatically reduce the prevailing wind speeds. The presence of other buildings close to a micro-wind turbine will also increase the turbulence of the wind, which will further reduce the turbine output.

For large-scale wind turbine installations extensive wind monitoring is conducted before sites are selected. Such assessments are rarely possible for small urban installations and predictions are often based on wind speed data, such as that from the Department for Business, Enterprise & Regulatory Reform (BERR) (formerly Department of Trade and Industry) wind speed database, commonly known as the NOABL database. This gives estimates of the annual mean wind speed throughout the UK on a 1 km grid. However, the data is the result of an air flow model that estimates the effect of the gross topography on wind speed but does not account for effects such as the surface roughness caused by the buildings in an urban environment or the local effects around individual buildings. As a result estimates based on this data tend to give very optimistic results in urban locations.

To investigate the effectiveness of building-mounted micro-wind turbines in urban environments the performance of some typical installations of this type has been modelled in Manchester, Portsmouth and Wick. Manchester was chosen as a large inland city in an area of relatively low mean wind speeds (see map in Appendix B) and is considered representative of other such conurbations (e.g. Birmingham, Nottingham and to some extent London). Portsmouth was chosen to represent medium sized conurbations in coastal areas and areas with moderate mean wind speeds and Wick was chosen to represent small towns in areas with high mean wind speeds. London was considered atypical due to its large size but it is anticipated that the attenuation of wind speed due to surface roughness found for Manchester would be exacerbated for London and hence the wind resource would be even less.

This study includes:

• estimates of the environmental impacts, including embodied energy and consequential greenhouse gas emissions (as equivalent carbon dioxide (CO₂) emissions), related to the construction, installation, maintenance and disposal of some typical micro-wind turbine systems
• the development of a method for estimating the wind resource in urban environments, which has been used to estimate the wind resource at locations in and around three towns (Manchester, Wick and Portsmouth)
• a wind tunnel study, reported separately, to investigate the effect of the house on which the turbine is mounted, and any surrounding buildings, on the wind speed at the turbine
• estimates of the expected annual electricity generation for some typical installations of these turbines, taking account of the power curves for the turbines, the wind resource, and building/mounting effects.

These have been used to investigate the likely financial and CO₂ payback periods required to balance the environmental impact of these typical turbine installations, over their anticipated service lives.

¹ The Department for Business, Enterprise and Regulatory Reform (BERR) has established a third party certification scheme for microgeneration technologies and installers. This scheme requires installers to sign up to an Office of Fair Trading code of conduct and to provide homeowners with information on the potential energy generation but does not currently consider the CO₂ payback. Details of the scheme can be found at www.ukmicrogeneration.org.