Wind Turbine Selection & Siting Criteria
Wind Turbine Definitions:
Wind Power Factors: The power available from wind is found from the equation, P = ½•d•A•V3 where P = power, d = density, A = area, V = wind speed. Thus, the most important factors for energy production from a wind turbine are the swept area (size) of the rotor, and, especially, the wind speed. If the rotor diameter is doubled the available energy increases by a factor of 8. If wind speed doubles the available energy increases by a factor of 8. For example, a 25% increase in wind speed, say from 10 mph to 12.5 mph, will result in approximately twice the wind power available.
Wind Tower Height: As a general rule, small wind turbines should be located so that the lowest part of the rotor is a minimum height of 10 m (30 ft) above an obstacle within a 100 to 150 m (300 to 500 ft) radius, in order to reduce wind shear and turbulence, and increase wind speed. A taller tower is always the best investment in a wind energy system. For example, an increase in tower height from 100 to 120 ft may result in a 20 to 25% increase in energy production. Increasing tower height from near ground level, say from 30 ft to 100 ft, will result in a more dramatic energy production increase, possibly as much as 100%.
Energy Production vs. Peak Power: Power is an instantaneous measurement, in kilowatts (kW). Energy, in kilowatt-hours (kWh), is what does the work and what is purchased from (or sold to) the utility company, and what really matters in a wind energy system. The peak power rating of a wind turbine has little importance compared to its energy production over time. A wind turbine that is “rated” at a high kW power output in high winds may be very poor at producing energy over an entire year, as the high winds at which it is rated do not occur over a long period of time. Thus, the peak power rating of a wind turbine should not be used as selection criteria. If peak power ratings between different wind turbines are compared, it should be done at a standard wind speed, typically 11 meters per second (m/s), or 25 mph.
Power curve and energy production estimates for wind turbines should be used with caution. Turbine manufacturer estimates of energy production are often not based on real world testing and are often overly optimistic. Only power and energy curves based on testing certified by an industry-accepted, qualified, independent, test facility should be trusted.
Turbulence: Turbulence reduces effective wind speed available to the wind turbine, as well as the efficiency and reliability of wind turbine. Locating a wind turbine as recommended in the Tower Height section will minimize turbulence. In addition, it is recommended to separate adjacent wind turbines by a distance of at least 3 times the rotor diameter (or width), and to not align multiple wind turbines in prevailing wind directions.
Lift vs. Drag: Most modern power-producing wind turbines employ the aerodynamic lift effect, utilizing rotors blades with an airfoil shape. Such rotors typically use 2 to 3 blades and operate at a high rpm speed. Lift-style wind turbines have a theoretical maximum conversion efficiency of wind energy to mechanical energy, called the Betz Limit, of 59%. Practically, the conversion efficiency limit for small lift wind turbines, from wind power to electrical power, is near 30%.
Wind turbines that do not use airfoil blades employ the drag effect, i.e. the wind simply pushes the rotor. Wind turbine rotors using the drag effect turn more slowly (lower rpm) than lift rotors, and often have many blades, or a solid rotor surface. The theoretical limit for conversion efficiency from a drag-style wind turbine is 15% (compared to 59% for a lift turbine), with actual conversion efficiencies often much lower. Wind powered water pumpers are one example of drag wind turbines in use.
A type of rotor commonly used in VAWT’s is the Savonius design, which is a modified drag-style rotor, with higher efficiency than a simple drag rotor. 30% maximum conversion efficiency is possible with a Savonius rotor in wind tunnel tests. However, in practice, Savonius turbines have conversion efficiencies of less than 15%.
VAWT vs. HAWT: HAWT’s are the most widely utilized style of wind turbine currently employed for power production. HAWT designs have the best conversion efficiency in utilizing the lift effect, due to the constant angle of attack of the rotor blades with respect to the wind direction. Because of the relatively constant and centrifugal forces on the rotor, HAWT’s are also the most reliable in using a lift rotor. HAWT designs are more complex than VAWT’s, due to the need for the turbine to yaw when the wind direction changes so that the rotor maintains the same relative position with respect to the wind. Also, lift-style HAWT’s must employ a governing mechanism to prevent the rotor from spinning too fast in high winds. Because of the sensitivity to wind direction the performance of lift-style HAWT’s is significantly affected by turbulence. Several, established, recognized small wind manufacturers offer HAWT’s.
VAWT’s can use lift or drag rotors. Their main advantage is that the design can be simpler, as the rotor is omni-directional (no yaw). Because of this VAWT’s are less sensitive to turbulence than HAWT’s. Also, VAWT’s are inherently self-governing (no over speed control required). A VAWT rotor always has one half going downwind and one-half returning upwind, which limits the rotational speed (and efficiency). This same attribute, however, also results in differential stresses applied to the rotor each revolution, which affects rotor reliability. This is especially true for lift-style VAWT’s, which operate at higher speeds and typically have vertical airfoil blades mounted at the end of support struts. The blade support struts often experience failure after extended operation. Also, VAWT’s are typically supported only from the bottom. The rotational force of the rotor combined with the leveraging force of the wind can apply severe stresses to the generator bearings. There are many wind turbine manufacturers that offer VAWT’s. None, in our opinion, are as yet recognized, established, and known to offer reliable products.
Turbine Mass: A heavy wind turbine can be an indicator of stout construction and a sufficiently sized alternator (more copper windings are heavy) for good power production.
Certified Testing: The Small Wind Certification Council (SWCC) is an independent, industry-accepted body organized to certify small wind turbines tested for power output, energy production, reliability and sound level. As of the date of this writing 8 turbines have completed and received full SWCC certification. Several more wind turbines are under testing. See www.smallwindcertification.org for current status information.