NUMBER OF BLADES

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Small-scale, multi-bladed turbines are still in use for water pumping.They are of relatively low aerodynamic efficiency but, with the large blade area, can provide a high starting torque (turning force). This enables the rotor to turn in very light winds and suits a water pumping duty.  

Most modern wind turbines have three blades, although in the 1980s and early 1990s some attempt was made to market one and two-bladed wind turbine designs.  

The single-bladed design (Figure 1) is the most structurally efficient for the rotor blade, as it has the greatest blade section dimensions with all the installed blade surface area in a single beam. It is normal to shut down (park) wind turbines in very high winds, in order to protect them from damage.  This is because they would generally experience much higher blade and tower loads if they continued to operate.  The one-bladed design allows unique parking strategies – with the single blade acting as wind vane upwind or downwind behind the tower – which may minimise storm loading impact.  However, there are a number of disadvantages.  With a counterweight to balance the rotor statically, there is reduced aerodynamic efficiency and complex dynamics requiring a blade hinge to relieve loads.  The designs of Riva Calzoni, MAN, Messerschmidt and others were of too high a tip speed to be acceptable in the modern European market from an acoustic point of view. However, just when it seemed that the era of single bladed turbines had ended, the Spanish company, ADES, has announced the development of a single bladed, pendular wind turbine in which the cyclic torque variations of the single bladed turbine are compensated by allowing the generator to swing like a pendulum on the gearbox output.  Moreover a new small scale single bladed design, the Thinair 102, rated 2 kW, is being marketed for home applications by the New Zealand company, Powerhouse Wind. 

Figure 1: Single-Bladed Wind Turbine

The two-bladed rotor design (Figure 2) is technically on a par with the established three-bladed design.  In order to obtain a potentially simpler and more efficient rotor structure with more options for rotor and nacelle erection, it is necessary either to accept higher cyclic loading or to introduce a teeter hinge, which is often complex.  The teeter hinge allows the two blades of the rotor to move as a single beam through typically ±7° in an out-of-plane rotation.  Allowing this small motion can much relieve loads in the wind turbine system, although some critical loads return when the teeter motion reaches its end limits.  The two-bladed rotor is a little less efficient aerodynamically than a three-bladed rotor.  

In general, there are small benefits of rotors having increasing number of blades.  This relates to minimising losses that take place at the blade tips.  These losses are, in aggregate, less for a large number of narrow blade tips than for a few wide ones.  

In rotor design, an operating speed or operating speed range is normally selected first, taking into account issues such as acoustic noise emission.  With the speed chosen, it then follows that there is an optimum total blade area for maximum rotor efficiency.  The number of blades is, in principle, open but more blades imply more slender blades for the fixed (optimum) total blade area.  This summarises the broad principles affecting blade numbers.

Note also that it is a complete misconception to think that doubling the number of blades would double the power of a rotor.  Rather, it would reduce power if the rotor was well designed in the first instance.

Figure 2: Two-Bladed Wind Turbine

 

It is hard to compare the two- and three-bladed designs on the basis of cost-benefit analysis.  It is generally incorrect to suppose that, in two-bladed rotor design, the cost of one of three blades has been saved, as two blades of a two-bladed rotor do not equate with two blades of a three-bladed rotor.  Two-bladed rotors generally run at much higher tip speed than three-bladed rotors, so most historical designs would have noise problems.  There is, however, no fundamental reason for the higher tip speed and this should be discounted in an objective technical comparison of the design merits of two versus three blades. 

The one-bladed rotor is perhaps more problematic technically, whilst the two-bladed rotor is basically acceptable technically.  The decisive factor in eliminating the one-blade rotor design from the commercial market, and in almost eliminating two-bladed design, has been visual impact.  The apparently unsteady passage of the blade or blades through a cycle of rotation has often been found to be objectionable.