Smaller-scale wind turbines can be found in a wide range of applications including off-grid power; either directly by charging a storage battery or in combination with another form of generation to cover intermittent periods when there is little to no wind. In such instances small wind may be cost effective depending on the costs of alternate off-grid technologies and fuel prices; however the overall contribution of small wind to climate change mitigation will probably be limited due to the long payback periods required to offset the carbon used in their manufacture.
Wind energy is actually a form of solar energy; the temperature differences caused by the sun shining on the earth act, along with other factors, to cause large bodies of air, winds, to move across the face of the planet. As a result of these factors the highest wind speeds, and thus renewable resource, is found at larger latitudes, however there are many localised regions with excellent resource closer to the equator.
The conversion of the kinetic energy in these winds into electrical power is known as wind energy. There are a number of ways in which this conversion can be done and while a single type of 3-bladed design has come to dominate the large-scale wind energy market there is more diversity seen in the small-scale, or micro, wind market. Most small wind turbines are still premised on the same 3-bladed design seen in their larger counterparts (see picture above) but there are also vertical axis designs based on helical, bladed and savonius rotors. As with larger wind turbines the moving air, wind, acts to turn the rotor which generates an electric current via a generator located in the head/nacelle of the turbine just behind the rotating hub.
The terms ‘small-scale’ and ‘micro’ wind are sometimes used to distinguish between sizes up to 50kW and those up to 3.5kW respectively (BWEA, 2007) while other publications consider small wind to include everything up to 100kW (IPCC, 2010). In this document the words micro and small-scale are used interchangeably and are intended to refer to this last definition of turbines; those up to approximately 100kW in size. Based on the size of the turbine it is possible to define some broad categories of use (see Figure 1)
[media:image:1] Typically those turbines with a nameplate capacity above 10kW use a gearbox to increase the rotor speed to match the generator. For horizontal systems a passive yaw system is provided with a tail vane to point the turbine into the prevailing wind. Alternately the rotor can be downwind of the generator, so it naturally and passively aligns with the wind. Vertical axis designs do not have this need for a yaw system as they are insensitive to the direction of the wind.
Resource and Location
As with large scale wind systems, the primary consideration for the installation of a small wind turbine is the level of resource present at the deployment site. A relatively high average wind speed is important in ensuring that the turbine operates economically and efficiently. In order to determine the level of wind resource for a small turbine it is usual to take onsite measurements using an anemometer with logging device mounted on a mast in a similar position to where the small turbine may eventually be placed. This should ideally be averaged over the course of a year to account for seasonal variations.
The wind modelling software that is normally used to assist in identifying locations for large scale wind turbines is normally less useful for small turbines as their close proximity to the ground, and often to other structures, generally increases the likelihood of non-linear localised effects and makes the results of computer modelling unreliable for a final choice of location.
Typically, and ideally, small wind systems are mounted to masts that are erected in open areas; however there are some instances where it may be necessary or desirable to mount the turbine onto a building or other structure in built up areas. See also 'Building-integrated wind turbines '. Where possible turbines should be mounted 9 metres above any obstruction that is within 100 metres (BWEA, 2007), but with building mounted devices this is often not possible, increasing the risk of exposing the turbine to turbulence and thus reducing its effectiveness and likely lifetime. For this reason it is desirable to place the turbine where there are no obstructions between it and the prevailing wind.
Until recently, data from small wind units installed worldwide shows that the US market was the frontrunner in terms of installed capacity with approximately 80MW. However there has been recent rapid growth in other parts of the world; China is now estimated to have in the order of 90MW of small wind power capacity installed as of the end of 2009 (IPCC, 2010).
Much of the growth in demand in developing countries has been for the electrification of isolated areas where small wind turbines can be used in isolation for battery charging or in combination with other supply options (such as solar or diesel generators) in hybrid systems. Future growth in these countries is likely to continue to be driven by this need for remote power for development reasons or for the displacement of expensive alternatives. However it is unlikely to be driven by reasons of climate change mitigation as the CO2 savings associated with small wind energy are marginal at best (see section below on ‘climate’), nor by economic considerations for typical grid connected customers as the pay-back periods for domestic scale turbines, even at good sites, is more than a decade (BWEA, 2007).
The size of the market is still relatively small in terms of capacity, with the AWEA (2009) estimating installed capacity of small wind turbines from leading manufacturers at under 40 MW in 2008. A significant number of countries have feed-in-tariffs that support, or are aimed specifically at, small wind installations (Fig 2). [media:image:2]
There is very little literature on the environmental impacts of small scale wind turbines. Their small size, relatively high rotation speeds (i.e. high visibility) and lack of significant associated infrastructure makes their likely environmental impacts small or negligible. Most of the concerns around installations centre on human factors such as visual annoyance or noise, though newer designs have eliminated much of the noise associated with older models.
The contribution of small wind energy to climate change mitigations is likely to be very small. In typical urban installations the wind resource is highly site-specific and can often be poor due to turbulence and obstructions. In these instances the CO2 emission reduction due to the replacement of grid electricity can be low or even negative (i.e. the turbine is a net lifecycle emitter of CO2) once the manufacture and installation of the turbines are taken into account (Carbon Trust 2008; Phillips et al., 2007). Hence the careful siting of small wind turbines at good unobstructed sites is critical if they are to have any real contribution to climate change mitigation.
Due to a lack of economy of scale and the lower relative efficiencies, small turbines are more expensive per kW and kWh than large turbines. Small turbines with a power capacity below 100 kW can be purchased at prices in the order of 1,500 to 6,000 USD/kW (BWEA, 2007; AWEA, 2009) but even at these costs the payback periods are typically long at most sites. In spite of these prices, small wind can be economically competitive in remote or isolated areas where the small turbines are replacing an expensive alternative form of supply (Byrne et al., 2007). Small wind turbines require low infrastructure development when they serve off-grid applications and grid connection typically represents a major investment compared to the turbine itself.
[this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group ]
Project developers of small-scale wind projects under the CDM mainly apply the following CDM methdologies:
CDM projects based on wind represent 17.3% of all CDM projects in the pipeline. Recent years have shown a tendency towards a more widening geographical dispersal of CDM wind projects, indicating that countries other than India and China observe the CDM as a tool to support wind projects. [media:image:3]
AWEA (American Wind Energy Association), 2009. AWEA Small Wind Turbine Global Market Study; year ending 2008. American Wind Energy Association, Washington, DC, USA, 24 pp.
BWEA, 2007, BWEA Briefing Sheet - Small Wind Energy Systems, available from: http://www.bwea.com/pdf/briefings/smallsystems.pdf 
Byrne, J., A. Zhou, B. Shen, and K. Hughes, 2007. Evaluating the potential of small-scale renewable energy options to meet rural livelihoods needs: A GIS-and lifecycle cost-based assessment of Western China's options. Energy Policy, 35, vol 8. pp. 4391-4401.
Carbon Trust, 2008. Small-scale Wind Energy: Policy Insights and Practical Guidance. Carbon Trust, London, UK, 40 pp.
CanWEA 2008, Small Wind Turbine Types and Applications, available from: http://www.smallwindenergy.ca/en/Overview/TurbinesApplications/Applications.html 
IPCC 2010. Special Report on Renewable Energy Sources and Climate Change Mitigation, In Press.
Phillips, R., P Blackmore, J Anderson, M Clift, A Aguilo-Rullan and S Pester 2007. Micro-wind turbines in urban environments - an assessment, BREPress, available from: http://www.brebookshop.com/details.jsp?id=287572