Most small systems have a direct drive permanent magnet generator which limits mechanical transmission losses. Systems under 2 kW usually have a 24-48 volt capacity aimed at battery charging or a DC circuit rather than having grid compatibility.
Up to the present, horizontal axis machines are much more in evidence that the vertical axis type even at this scale. These machines have efficient braking systems for when wind speed is excessive. Some even tip backwards in high winds adopting the so-called 'helicopter position'. There are advantages to horizontal axis machines such as:
• the cost benefit due to economy of scale of production;
• it is a robust and tested technology;
The disadvantages are:
• mounted on buildings they require substantial foundation support;
• in urban situations where there can be large variations in wind direction and speed, this necessitates frequent changes of orientation and blade speed. This not only undermines power output, it also increases the dynamic loading on the machine with consequent wear and tear;
• there are noise problems with this kind of machine especially associated with braking in high winds;
• they can be visually intrusive.
As stated earlier, vertical axis turbines are particularly suited to urban situations and to being integrated into buildings. They are discrete and virtually silent and much less likely to trigger the wrath of planning officials.
The most common vertical axis machine is the helical turbine as seen at Earth Centre, Doncaster (Figure 9.3). In that instance it is mounted on a tower but it can also be side-hung on a building.
Another variety is the S-Rotor which has an S-shaped blade (Figure 9.4).
The Darrieus-Rotor employs three slender elliptical blades which can be assisted by a wind deflector. This is an elegant machine which nevertheless needs start-up assistance (Figure 9.4).
A variation of the genre is the H-Darrieus-Rotor with triple vertical blades extending from the central axis (Figure 9.4).
Yet another configuration is the Lange turbine which has three saillike wind scoops (Figure 9.4).
Last in this group is the 'Spiral Flugel' turbine in which twin blades create, as the name indicates, a spiral profile (Figure 9.5).
Produced by Renewable Devices Ltd, the Swift wind turbine claims to be the world's first silent rooftop mounted wind turbine (35 dB) by incorporating silent aerodynamic rotor technology coupled with a revolutionary electronic control system. Care has been taken to provide a secure mounting system which will not transfer vibrations. Its peak output is 1.5 kW and it is estimated that avoided fossil fuel generation produces a saving of 1.8 tonnes per year of carbon dioxide (CO2). The first unit was installed in Collydean Primary School, Glenrothes, Scotland, and there are plans for installations in four other primary schools. It is regarded as an ideal system for residential developments (Figure 9.6).
A development from the 1970s has placed the turbine blades inside an aerofoil cowling. A prototype developed at the University of Rijeka, Croatia, claims that this combination can produce electricity 60 per cent more of the time compared with conventional machines. This is because the aerofoil concentrator enables the machines to produce electricity at slower wind speeds than is possible with conventional turbines.
The cross-section of the cowling has a profile similar to the wing of an aircraft which creates an area of low pressure inside the cowling. This has the effect of accelerating the air over the turbine blades. As a result, more electricity is produced for a given wind speed as well as generating
Helican turbine on a column at Earth Centre, Doncaster
Helican turbine on a column at Earth Centre, Doncaster
Left: S-Rotor; top centre: Darrieus-Rotor; bottom centre: Lange turbine; right: H-Darrieus-Rotor
Spiral Flugel rotor
Spiral Flugel rotor at low air speeds compared to a conventional rotor. This amplification of wind speed has its hazards, for example blades can be damaged. The answer has been to introduce hydraulically driven air release vents into the cowling which are activated when the pressure within the cowling is too great. They also serve to stabilise electricity output in turbulent wind conditions, which makes them appropriate for urban sites.
This technology can generate power from 1 kW to megawatt capacity. It is being considered for offshore application. The device is about 75 per cent more expensive than conventional rotors but the efficiency of performance is improved by a factor of five as against a conventional horizontal axis turbine (Figures 9.7 and 9.8).
A mini horizontal axis turbine was introduced in late 2003 called the Windsave. It can generate up to 750 watts at an installed cost of £1 per watt. Its manufacturers claim it could meet about 15 per cent of
Figure 9.8 Wind turbine with cowling wind
Simulation of wind turbines on the Vivo shopping complex, Hamburg concentrator
Simulation of wind turbines on the Vivo shopping complex, Hamburg concentrator the average household electricity demand. It starts generating at a wind speed as low as 3 mph but is most efficient at 20 mph. Producing AC power it can be linked directly to the grid and the householder credited under the Renewables Obligation charges which currently pay a green electricity provider 6 p per kilowatt hour. By using remote metering, each unit can be telephoned automatically each quarter to
Windsave rooftop wind energy system
Windsave rooftop wind energy system assess the amount of electricity generated. The power company then collects the subsidy and distributes it back to the home owner on the basis of the total generated. It is this subsidy which justifies a claim that the payback time can be as short as 30 months (Figure 9.9).
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