Small wind turbines

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In this context 'small' means wind machines that are scaled from a few watts to 20 kW. Machines between 1 and 5 kW may be used to provide either direct current (DC) or alternating current (AC). They are mainly confined to the domestic level and are often used to charge batteries. The larger machines are suitable for commercial/industrial buildings and groups of houses.

Small-scale electricity production on site has economic disadvantages in the UK given the present buy-in rates for small operators. Currently the government is considering how to redress this inequity and thereby give a substantial boost to the market for small-scale renew-ables. Wind generation will do well if this happens since it is much less expensive in terms of installed cost per kilowatt than PV which makes it an attractive proposition as a building integrated power source.

Wind patterns in the built environment are complex as the air passes over, around and between buildings. Accordingly a wind generator introduced into this environment must be able to cope with high turbulence caused by buildings. Such conditions tend to favour vertical axis machines as opposed to the horizontal versions which have proliferated in wind farms. This is because the vertical versions may be able to operate at lower wind speeds and they are less stressed mechanically by turbulence. In addition, horizontal axis machines mounted on roofs tend to transmit vibrations through the structure of the buildings. Because of the bending moment produced by the tower under wind load, measures must be taken to provide adequate strength in the building structure. This may not easily be achieved in retrofit situations.

By their very nature the vertical axis machines are not affected by changes in wind direction or turbulence. They can be sited on roofs or walls. They have been particularly successful mounted on the sides of oil platforms in the North Sea (Figure 9.1).

The machines are well balanced, transmitting minimum vibration and bending stress to walls or roofs. They also have a high output power to weight ratio. A further advantage is that the electricity generator can be located beneath the rotors and therefore can be located within the envelope of the building.

Wind generation can be complemented by PVs as illustrated below (p. 114) by the system patented by Altechnica. The wind generators continue operating at night when PVs are in retirement (see Figure 9.9).

A prediction in 'WIND Directions', March 2001, estimates that the global market for small turbines by 2005 will be around Euros 173 million and several hundreds of million by 2010. For example, in the Netherlands alone there is the potential for 20 000 urban turbines to be installed on industrial and commercial buildings by 2011.

The increasing deregulation of the energy market creates an increasingly attractive proposition for independent off-grid small-scale generation insulating the operator from price fluctuations and reliability uncertainties, with the proviso that there is a level playing field.

Currently there are several versions of vertical axis machines on the market. However, they are still undergoing development. When it is fully appreciated that these machines are reliable, silent, low maintenance, easy to install and competitive on price, it is likely the market will expand rapidly. At present the regulatory regime for small turbines is much less onerous than for >20 kW machines. It is to be hoped that the bureaucrats fail to spot this red tape opportunity.

Research conducted by Delft University of Technology and Ecofys identified five building conditions to determine their effectiveness for wind turbines. They are described as 'wind catchers', 'wind collectors', 'wind sharers' and 'wind gatherers', terms which define their effect on wind speed. The wind catcher is well suited to small turbines being usually high and benefiting from a relatively free wind flow. Small horizontal axis machines could be satisfactory in this situation.

The wind collector type of building has a lower profile and can be subject to turbulence. This is where the vertical axis machine comes into its own. The third type, wind sharers, are found in industrial areas and business parks. Their relatively even roof height and spaced out siting makes such buildings subject to high winds and turbulence. Ecofys has produced a diagram which depicts how four urban situations cope with varying wind conditions. There is a fifth category, the 'winddreamer' which relates to low rise developments (Figure 9.2).

Development work is continuing on designs for turbines which are suitable for the difficult wind conditions found in urban situations. This is appropriate since climate change predictions indicate that wind speeds will increase as the atmosphere heats up and so becomes more dynamic. There is growing confidence that there will be a large market

Ecofys Delft Urban Wind Study

Figure 9.1

Helical side mounted turbine on oil platform

Figure 9.1

Helical side mounted turbine on oil platform

Figure 9.2

Categories of building cluster and their effectiveness for wind generation (courtesy of Ecofys and REW)

Figure 9.2

Categories of building cluster and their effectiveness for wind generation (courtesy of Ecofys and REW)

Wind Catchers

for mini-turbines in various configurations on offices, housing blocks and individual dwelling.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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