Most small systems have a direct-drive permanent-magnet generator which limits mechanical transmission losses. Systems under 2 kW usually have a 24-48 V 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 than 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
• automatic start-up
The disadvantages are:
• the necessity of a high mast
• 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. This type has even been the basis for an art work as in a sports stadium at Yurigoaka, Japan.
A problem with some very small vertical-axis machines is that they need mechanical start-up, which can be achieved either by an electric motor or a link to a Savonius type rotor. The most common vertical-axis machine is the helical turbine as seen at the former Earth Centre, Doncaster (see Fig. 5.3). 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 (see Fig. 5.4).
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.
A variation of the genre is the H-Darrieus-Rotor with triple vertical blades extending from the central axis. Yet another configuration is the Lange turbine which has three sail-like wind scoops (see Fig. 5.4). Last in this group is the 'spiral flugel' turbine in which twin blades create, as the name indicates, a spiral partnership (see Fig. 5.5).
Returning to the horizontal-axis machines, 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% 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 at low air speeds. This amplification of wind speed has its hazards in that blades
Figure 5.5 Spiral Flugel rotor
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 stabilize 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% more expensive than conventional rotors but the efficiency of performance is improved by a factor of five as against a conventional horizontal-axis turbine (see Fig. 5.6).
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