Enhance Your System With a Windmill

     My neighbor has an electric power generating windmill. Most days I see it working. I especially notice it working when the sun isn't shining. There is a wind farm in West Virginia and one is planned for Bland County, VA. This evidence and a wind map and identifying
wind classes leads me to believe that a windmill might be successful in my part of the Blue Ridge Mountains. 

Windmill Site Location
     First thing is to identify a location that is free from obstructions for at least 200 feet around it. Then make sure you can place your windmill at least 20 or more feet above the tallest obstacle that might interfere with wind flow. Preferably you would place a wind meter in that location to test that the wind map information is correct for your micro location. The Wind profile power law, which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%. You must make sure your chosen location is feasible before you ever put up a windmill. 

     Locating a site near your charger-inverter is critical because transferring AC power has less power loss than transferring DC power.

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Vertical-vs- Horizontal Axis

     Horizontal axis wind turbines have the advantage that much research has already been done on propeller design. With an added tail fin, like a weather vane the propeller is always facing the wind.

     With a vertical design, only half of the drive system is using the wind flow and the other half is fighting against it as the as the drive blades rotates. Most produce energy at only 50% of the efficiency of HAWTs in large part because of the additional drag that they have as their blades rotate into the wind.

Horizontal Axis Wind Turbine Design

    The aerodynamics of a horizontal-axis wind turbine are not straightforward. The air flow at the blades is not the same as the airflow far away from the turbine. The way in which energy is extracted from the air also causes air to be deflected by the turbine. No only that but the aerodynamics of a wind turbine at the rotor surface exhibits phenomena that you don't usually see in other aerodynamic fields.

     In 1919 the physicist Albert Betz showed that for a hypothetical ideal wind-energy extraction machine, the fundamental laws of conservation of mass and energy doesn't allow but 16/27 or (59.3%) of the kinetic energy of the wind to be captured. Modern turbine designs may reach 70 to 80% of this theoretical Betz' law limit.

Wind Map

      One of the first things in planning for a wind turbine is to look at a wind energy map of your area to see if the energy output from your local winds are usable to generate electricity. In addition you need to record the local conditions at you probable site to see what minimum and maximum winds occur and how often. This will determine the strength of the mounting structure for your wind turbine. ("Never design a dwarf!") I had 22 trees blow down over my fence line when  hurricane Hugo came through here. The electric power grid was out in my area for five straight days. Replacement expenses after extreme conditions occur can be prohibitive.

Calculating Wind Turbine Power Output

From the 50 meter wind map of my area your get 200-300 watts per sq meter
times Betz's constant (59.3%)
times pi (3.14) times (blade length 2.134 m (squared)
times blade design efficiency (70-80%)

250 x .593 x 3.14 x 2.134 x 2.134 x 0.75 = 1589.9 watts (1.59KW)

     That's the probable optimum out put from a 7 ft, 3 bladed wind turbine mounted 20 feet higher than any wind obstacles in optimum wind conditions.

Power control

     A wind turbine is designed to produce a maximum of power at wide spectrum of wind speeds. The wind turbines have three modes of operation:

  • Less than rated wind speed operation
  • Close to rated wind speed operation
  • Greater than rated wind speed operation (this is a real strength of materials problem)
  •      When the rated wind speed is exceeded the power has to be limited. There are a number of  ways to achieve this.

    Blade Stall

         Stalling works by increasing the angle at which the relative wind strikes the blades (angle of attack), and it reduces the induced drag (drag associated with lift). Stalling is simple because it can be made to happen passively (it increases automatically when the winds speed up), but it increases the cross-section of the blade face-on to the wind, and thus the ordinary drag. A fully stalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind.

         A fixed-speed HAWT inherently increases its angle of attack at higher wind speed as the blades speed up. A natural strategy, then, is to allow the blade to stall when the wind speed increases. This technique was successfully used on many early HAWTs. However, on some of these blade sets, it was observed that the degree of blade pitch tended to increase audible noise levels.

    Blade Pitch control

         Furling works by decreasing the angle of attack, which reduces the induced drag from the lift of the rotor, as well as the cross-section. One major problem in designing wind turbines is getting the blades to stall or furl quickly enough should a gust of wind cause sudden acceleration. A fully furled turbine blade, when stopped, has the edge of the blade facing into the wind.

         Standard modern turbines all pitch the blades in high winds. Since pitching requires acting against the torque on the blade, it requires some form of pitch angle control. Many turbines use hydraulic systems. These systems are usually spring loaded, so that if hydraulic power fails, the blades automatically furl. Other turbines use an electric servomotor for every rotor blade. They have a small battery-reserve in case of an electric-grid breakdown. Small wind turbines (under 50 kW) with variable-pitching generally use systems operated by centrifugal force, either by flyweights or geometric design, and employ no electric or hydraulic controls.

    Other controls

    Horizontal Nacelle Yawing

         Modern large wind turbines typically use wind vanes located on the back of the nacelle to  keep the turbine facing into the wind. By minimizing the yaw angle (the misalignment between wind and turbine pointing direction), the power output is maximized and non-symmetrical loads minimized. However, since the wind direction varies quickly the turbine will not strictly follow the direction and will have a small yaw angle on average. The power output losses fall approximately with cos3of the yaw angle.

    Vertical Nacelle Pitching

         By use of a horizontal vane and a hinge to the rear of the center of gravity, the wind turbine speed is limited by increasing the vertical yaw angle of the nacelle.

     Electrical braking

         You can slow down a small wind turbine by dumping energy from the generator into a resistor bank, converting the kinetic energy of the turbine rotation into heat. This method is useful if the kinetic load on the generator is suddenly reduced or is too small to keep the turbine speed within its allowed limit.

         Cyclically braking causes the blades to slow down. Slowing increases the stalling effect thus reducing the efficiency of the blades. This way, the turbine's rotation is kept at a safe speed in higher winds conditions while maintaining (nominal) power output. You don't usually see this method used on large grid-connected wind turbines.

    Mechanical braking

         A mechanical drum brake or disk brake is used to hold the turbine at rest for maintenance. Don't try to apply the mechanical brakes until after blade furling and electromagnetic braking have reduced the turbine speed, as the mechanical brakes would wear quickly if used to stop the turbine from full speed. There can also be a stick brake.

         For HAWTs, tower heights approximately two to three times the blade length have been found to balance material costs of the tower against better utilization of the more expensive active components.

    Trade Offs

         Wind is a weather phenomena and is therefore intermittent. It can be a supplement to your power source but cannot be stand alone because it is not predictable. Solar power is more predictable but it too is dependent on the weather (cloud cover). Used in tandem, the combination would be more reliable that one or the other.

         The expense of adding a wind turbine without fully investigating site characteristics is pretty chancy. Unless you do the investigation and know what you're doing, hiring a professional you can trust to investigate may save your money in the long run but not in the short run.

    Distance to Batteries, Inverter and Grid Connection

        Power output from your wind turbine can be in the form of AC or DC depending upon you wind turbine design. The least expensive way to transport this power over distance is to increase the voltage and therefore decrease the current. The higher the current the greater the power loss in transmission.

         The best windmill sites are usually some distance from the location of your house or business. The wind turbine output (usually variable frequency AC) needs to be changed from low voltage to higher voltage at the windmill in order to move it economically to the battery and inverter
    location. The smaller the current the smaller and cheaper the transmission wire.There the AC is converted into DC of the proper voltage to charge the battery and run the inverter.

         For a 220 volt system, 1.59kw needs wiring to handle 7.23amps. A 110 volt system would need to handle 14.46 amps
    . Make sure you know the the max capacity of your wind turbine before you plan the wiring.

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