The first known windmills were developed in Persia between 500 and 900 AD to pump water and grind grain. They consisted of vertical sails rotating round a central shaft. The first documented example of the technology in Europe dates from 1270. It shows a horizontal-axis machine mounted on a central post with four sails, known predictably as a 'postmill' machine. It took until the nineteenth century for the windmill sails to achieve peak efficiency. These sails had some of the crucial features which helped in the design of present-day turbine blades.
Energy consultants BTM produced a report in 20051 International World Energy Development -Word Market Update 2004. It showed that, on the world scale, 2004 saw 8154 MW of new installed capacity bringing the global total capacity to nearly 48 GW This represents an increase of 20%. Over the years 2000-2005 growth has averaged 15.8% per year. Europe accounted for 74% of the total in 2004. In that year Spain was the leading market with 2064 MW replacing Germany as the leader with its 2054 MW Growth stalled in the Americas in 2004 with only 516 MW as against 1818 MW in 2003. The report predicts that, as a result of the Kyoto ratification, there will be increasing world interest in wind power. Assuming that the manufacturing base keeps pace with demand, annual installation could reach 29 GW by 2014 with a total accumulated capacity of 235 GW
China increased its capacity from 98 MW to 198 MW over the same period. It plans to builds its first offshore project in the Bohai Sea off the northern province of Hebei. It will generate 1000 MW when completed in 2020. The first phase will be installed in 2007 generating 50 MW. The Chinese government claims the country has the potential to generate 250 GW from wind energy. So far only a tiny fraction of this capacity has been exploited.
Even more optimistic is the EWEA in its Windforce 12 plan which demonstrates how up to 12% of global electricity demand could be met by wind power by 2020. Time will tell.2
Compared with other renewable energy technologies, wind energy is the closest to being competitive with fossil-based systems. The technology is mature and robust, with offshore installations set to take off in Europe. These facts should dispel any uncertainty about the future role of wind in the energy scenarios of the twenty-first century.
This chapter is mainly concerned with small-scale wind generation which can operate as embedded generation in buildings and this is where some of the most interesting developments have taken place. 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 renewables. 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 is not easily 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 (see Fig. 5.1).
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 is located beneath the rotors and therefore can be located within the envelope of the building.
Figure 5.1 Helical side mounted turbine on oil platform
Wind generation can be complemented by PVs by the system patented by Altechnica (see the section Building integrated systems, p. 60). The wind generators continue operating at night when PVs are in retirement.
A projection in WIND Directions, March 2001, estimated that the global market for small turbines by 2005 would be around €173 million and several hundreds of millions 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. Four 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 'wind dreamer', which relates to low-rise developments (see Fig. 5.2).
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 for miniturbines in various configurations on offices, housing blocks and individual dwellings.
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