Climate change is the single most serious environmental threat facing this planet today and renewable energy sources including wind are a key aspect of combating this threat. Wind turbines are some of the most technologically advanced and cost-effective renewable sources available at this time. Modern turbines are likely to be in operation for about 85% of the year, and have a service life of at least 20 years.
The UK, with its long coastline and North-east Atlantic location, has the greatest potential wind energy resource in Europe with approximately 40% of the European total. Scotland and England have annual mean wind speeds ranging from around 6 metres per second on sheltered terrain to over 11.5 m/s on highlands.¹
If harnessed, this resource would constitute a substantial source of renewable energy that would reduce the UK’s CO2 emissions, helping to combat climate change, and would enhance the security of our energy supply in a future where fossil fuel resources are likely to be increasingly unreliable.
The European Council has imposed a binding target for renewable generation to provide 20% of EU energy consumption by 2020. In the UK, where the figure currently stands at around 4%, Government measures to promote renewable energy include the Renewables Obligation, introduced in 2002, which requires all suppliers providing electricity to end consumers to supply a certain proportion of their electricity from eligible renewable sources – a proportion which will increase each year to 15.4% by 2015–16. The target was extended in 2006 to 20% by 2020–21, although the 2007 Energy White Paper cast doubt on whether this was achievable.²
The Government has also funded two grant schemes for domestic microgeneration and community renewable energy projects. However, both the Clear Skies programme and its replacement from 2006, the Low Carbon Buildings Programme, were insufficiently funded to meet the demand for grants.
Currently wind turbine developments of up to 50MW installed capacity require planning permission from the local authority, while larger developments are considered by the Secretary for State for Energy in consultation with the local authority. Wind turbine applications of various sizes have often been unsuccessful historically, but recent publications such as the 2004 Planning Policy Statement (PPS) 22: Renewable Energy, the 2007 planning white paper and the Draft PPS on Climate Change have indicated central government’s intention to force regional and local planning bodies actively to promote renewable energy projects.
The principle of harnessing wind power is that the kinetic energy of wind is converted to electrical energy by the movement within a generator of a rotor driven by the turbine’s rotating axis. The rotation is caused by the flow of wind around the turbine’s blades, which act as aerofoils.
The efficiency of a wind turbine is the proportion of the power contained in the wind that is converted to electrical power. Another measure of a turbine’s performance is the Capacity Factor, which is the ratio of the actual energy generated to the amount of energy that would have been generated if wind speeds throughout the same period had been between the rated and maximum (‘cut-out’) speeds. Capacity Factors in the UK are generally around 30%. This may seem low but it is due to the fact that most turbines are not running at full capacity all the time even if they are in use 90% of the time.³
Many factors in a turbine’s efficiency are primarily relevant to turbine designers, but a number of factors are related to the appropriateness of a proposed combination of site and turbine.
The power contained in the wind is proportional to the cube of the wind speed, and thus wind speed is a critical factor in a turbine’s viability in a particular site. If the actual wind speeds at the site are less than the rated optimum speeds cited by the turbine manufacturer, the power generated may be significantly below expectation.
Wind turbines contain gear systems to increase the rotational speed of the blades and axis to the kind of frequency needed to generate electricity. Changes in wind speed require gear changes within the turbine, which reduce its efficiency. Steady wind speeds, rather than gusts, are thus also desirable. Constant wind direction is very important. Wind hitting turbine blades at the wrong angle will not generate optimum power, and the rotation of a traditional horizontal-axis wind turbine (HAWT) on its vertical mast to face the wind generates no power, or actually consumes energy in the case of turbines with active yaw control.
These requirements highlight the importance of undertaking a site feasibility study before buying or installing turbines. The minimum requirement is to obtain an estimate of the annual mean wind speed for the location from a resource such as the European Wind Atlas or the UK Wind Atlas, which provides an estimated value for every 1 km grid square in the country. Ideally a further stage would comprise either a computer model wind flow simulation, taking into account local wind speed data and the exact topography of the site, or a series of anemometer measurements taken on site.
Locations likely to meet the requirements described above include offshore sites and those on hills or unobstructed plains. Turbines located on low-rise buildings in built-up areas are considerably less likely to receive the necessary wind in terms of constant speed and direction.
However, there are a number of technical means to optimise the power that can be generated from the wind resource available at a given site. For example, vertical-axis wind turbines (VAWTs) are capable of generating power regardless of the wind’s direction. One example of which is Xco2’s Silent Revolution.
Another strategy is to increase the elevation of the turbine’s blades, because wind speeds generally increase progressively above ground level, and the absence of surrounding obstacles minimizes turbulence. This is often best achieved by increasing the height of the tower on which the turbine hub is mounted or, in urban areas, by locating turbines on tall buildings.
A third solution is to integrate turbines into the design of tall buildings in such a way that the contours of the building envelope focus wind on to the turbine blades, much like the casing around a gas or water turbine. Examples of such designs include the Bahrain World Trade Center, the SkyZED ‘Flower Tower’, Queens Wharf ZED Tower and Marks Barfield’s Skyhouse.
A recent example, in London’s Elephant & Castle is Castle House, a pioneering 43 storey 147m landmark residential building designed by Hamiltons Architects for Client Multiplex Living. The development comprises 408 high quality apartments. Detailed planning consent was granted in March 2006 by the Southwark Council Planning Committee and construction will commence during the third quarter of the 2007.
Three 9m wind turbines integrated into the top of the building are expected to generate sufficient power to drive the energy efficient lighting to the building, an integral part of the sustainable credentials for the building as a whole.
Ben Coleman from Hamiltons Architects (London)
View Castle House project
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