Rotor Requirements for Wind and Water Turbines

            In recent years a growing percentage of the efforts to utilize renewable energy has been concentrated on the creation of wind farms.  The advancement in the design of the ever-larger wind-powered machines has been truly remarkable, with the most efficient wind-powered generators having carbon fiber turbine rotors with diameters of 80 meters (262.5 feet) or more.  According to data from Crainfield University in the UK, about 500 watts per square meter of sweep area is about the upper limit of what can be expected of any wind machine.  Based on that figure, a wind turbine having a diameter of 80 meters should produce approximately 2,513 kilowatts under ideal conditions.

                As previously stated, the power output of a turbine varies with the cube of the fluid’s velocity.  The “rated wind speed” of a wind turbine is that wind speed which corresponds to the point at which the conversion efficiency nears its maximum.  Each wind turbine is rated for a specific wind speed that is between 15 and 30 miles per hour.  Above the wind-turbine’s rated capacity, the power output is mechanically or electrically maintained at a relatively constant level.

            Wind turbines that generate electric power usually have two or three long, narrow rotor blades.  They have these long blades – not because they can capture the most energy from the wind – but because the blades must be able to survive violent wind conditions.  A wind turbine with many blades or very wide blades (turbines with a solid rotor) would be subject to extremely large forces when the wind blows at hurricane velocities because the energy increases with the cube of its velocity.  Wind turbine manufacturers must certify that their turbines can withstand extreme wind conditions that may occur only rarely.  To limit the impacts from these extreme conditions, the manufacturers of the wind machines prefer that their turbines have only two or three long narrow rotor blades that can be feathered and locked.  

Rotor diameters for wind turbines vary because manufacturers optimize their machines to the local wind conditions where they are to be placed.  A large generator requires more power to turn it.  That means, if you were to install a wind turbine in a low wind area you would actually maximize the annual output by choosing a fairly small generator for a given rotor size – or a larger rotor size for a given generator.  The reason that you could get more output from the smaller generator in a low wind area is that the smaller generator would run more hours during the year.  For a 600 kW machine the rotor diameter may vary from 128 to 157 feet.  The area of the disc covered by the turbine’s rotor (and wind speeds, of course), determines how much energy the generator can produce.  A typical wind turbine with a 600 kW electrical generator will have a rotor diameter of some 141 feet.  If you double the rotor diameter, you get an area that is four times larger and four times the power output from the rotor.  The big problem with the wind turbines is that wind energy is inherently intermittent, very variable, and undependable.  Sometimes it can far exceed that wind required for generating maximum power.  The designers of wind turbines can expect wind speeds that can range from 0 mph to more than 100 mph. Because the wind’s kinetic energy increases by the cube of its velocity, a wind machine designed to generate at its full capacity in a 20-mph wind would be subject 15.625 times that much energy in a 50-mph wind, 27 times as much in a 60-mph wind, and 52.734 times as much energy in a 75-mph wind. 

Unlike the erratic winds, the Gulf Stream’s velocities are much more nearly constant. Because water has approximately 854 times the mass as air, it requires huge amounts of sustained force over a long period to get such an ocean current moving.  Because most of the force driving the Gulf Stream is produced by the steady eastward rotation of our planet, the current’s speeds tend to remain within a relatively narrow range. Very rarely do they exceed 7 mph or drop below 3.5 mph.  If a Gulf Stream water turbine were designed to produce at its full capacity in a 6-mph current, the energy input would increase by a factor of only 1.58 in a 7-mph current. 

Because the water in the Gulf Stream would be nearly a thousand times more dense than air and would be flowing at a much more constant velocity, instead of their rotors having just two or three narrow blades to absorb the kinetic energy from a small percentage of the fluid passing through the sweep area, they can have more or wider blades to extract a larger percentage of the water’s kinetic energy.  Because there would be only relatively small variations in the Coriolis-force-driven current’s velocities, the Gulf Stream Turbines’ rotors can be designed to more nearly maximize their output.