Design masterclass 1 Wind power

Figure 3: Two identical Enercon E-70 turbines, operated by Ecotricity, are shown. The one at Shooters Bottom in Somerset (right), is generating 5,700,000 kWh a year. The other one, beside the M4 at Green Park near Reading, (left) produces just 3,500,000 kWh. The difference is accounted for by the wind energy dissipated through friction and turbulence as a result of the surrounding buildings and trees.

capacity factor of 30 per cent. So, in an unobstructed location, a typical small turbine could be expected to generate around 40,000kWh and the large turbine about 5,250,000kWh. Let’s consider the opposite scaling. The micro-

turbine is about 20 per cent of the size of the small- scale one and is mounted at about the same height above ground. So, based on the size difference, we would expect the output from the micro-turbine to be around 3.5 per cent of our small one, or 1,400kWh. It was estimates like these that led to one micro-turbine manufacturer claiming that their product could meet 33 per cent of domestic electricity demand. However, in order to complete the picture we need

to look at the influence of location in Figure 2. Due to the increased surface friction, the wind velocity at any given height is reduced over suburban or wooded locations compared with open countryside. The difference becomes more pronounced the closer you get to the ground. The wind velocity at 15m elevation drops by half compared with open country, and at 80m it still drops by around 15 per cent. We would therefore expect the capacity of the large

turbine in a suburban location to be around 60 per cent of the open country condition, due to this reduction in wind velocity. Based on our earlier estimate for generation capacity in open country, we can estimate the suburban capacity to be around 3,150,000kWh. Figure 3 shows two such turbines in different locations which exhibit this capacity reduction almost exactly. Now when we come down to 15m and below, the

height of small and micro-scale generators, the wind velocity is reduced by 50 per cent compared with open countryside, which means that there is just 12.5 per cent of the wind power available. The situation is even worse in city centres. That, in a nutshell, is why wind turbines attached to buildings in urban locations achieve little

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more than decoration. Few people install small-scale turbines in urban or suburban locations because of this fact, as the generation capacity of around 5,000kWh or less could not possibly justify the installation cost. However, with the low cost of micro-generators, many are still tempted. Applying the location factor for a suburban micro-

turbine, you would expect an annual output in the region of just 175kWh. In fact, a recent survey by the Energy Savings Trust found no instance of a micro- turbine in an urban or suburban location that generated more than 200kWh per year. In some instances, the mains electricity consumed by the electronic control systems exceeded the annual generation from the wind. Micro-wind turbines are sold for around £1,500 to £2,000 and, in typical situations, may generate about four per cent of domestic electrical consumption. On commercial buildings, with their subsequently higher demands, micro-turbines are rarely justified. Small-scale turbines of the size I have used as an

example cost around £50,000 to £60,000 and, in a good location, will generate sufficient electricity to meet 100 per cent of annual demand for around six to eight homes. However, put them in an urban or suburban location and you will cripple them. Large turbines cost around £1.5m to £1.7m and can generate sufficient electricity for around 1,250 homes. If you have cash to burn, buy a micro-turbine to

make yourself feel good. But if you want to save carbon, invest in large-scale wind power or, even better, on energy efficient fabric and building services. l © Doug King 2010

Doug King is principal of King Shaw Associates and visiting professor of building physics at Bath University. He was author of the Royal Academy of Engineering

report, Engineering a Low Carbon Built Environment.

June 2010 CIBSE Journal

47

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