STAYSOLAR

UPWIND or DOWNWIND?

The term "Upwind Turbine" means that the rotor is upwind of the tower.In other words, the wind hits the blades before it hits the tower.A "Downwind Turbine" has its rotor downwind of the tower.The advantage of upwind designs is that the blades operate in undisturbed airflow.The disadvantage of upwind designs is that they are inherently unstable so they need some sort of mechanism for keeping the blades upwind of the tower.The force of the wind on the rotor wants to turn it downwind.Most small, residential sized turbines, use a tail to keep the rotor upwind.Large utility scale turbines use yaw drives to keep the turbine rotor upwind.A yaw drive is an electric or hydraulic motor/gearbox combination that drives the turbine around the tower and keeps it pointing into the wind.Modern yaw drives use small wind vanes and a microprocessor to control turbine yaw and yaw rate.Yaw drives tend to be complicated and expensive so they are usually not found on small, residential sized turbines.One advantage of a tail is that if it is mounted on a pivot or hinge, it can be furled (folded) so that the blades turn out of the wind and stop.This is a convenient way to shut down a turbine in high winds or in case of a malfunction.Downwind turbines don't need a tail or yaw drive becuse the wind itself keeps the rotor downwind.This is a substantial advantage because tails are cumbersome and heavy and yaw drives tend to be complex and expensive.The disadvantage of downwind turbines is that the tower disturbs the airflow so the blades have to pass through this disturbed airflow once every revolution.disturbance is sometimes called "tower shadow".The disturbed airflow can cause cyclic forces which, for rigidly mounted blades, can lead to fatigue unless the blades are carefully designed.The blades can also create a thumping sound as each blades passes through the tower shadow.All wind turbines must have some method of preventing the blades from turning too fast in high winds thereby risking destruction of the turbine.Many residential sized upwind turbines use tail furling to accomplish this.The turbine is designed so that the rotor shaft is mounted off-center from the yaw axis.In strong winds, the force of the wind on the rotor creates a turning moment that wants to turn the rotor out of the wind.The tail is hinged but held straight back by a spring or the force of gravity acting on an inclined pivot.In light to moderate winds the tail remains extended but in heavy winds the force of the wind on the rotor is strong enough to overcome thespring or gravity spring and the rotor turns out of the wind.As it turns out of the wind the projected area of the rotor becomes less so it captures less wind.As the rotor turns out of the wind, the angle of attack of the blade airfoil changes which further reduces rotor efficiency.If the rotor turns a full 90° from the wind, the plane of the rotor is parallel to the wind and the blades are completely ineffective so the rotor will usually come tocomplete standstill or at least turn very slowly.The problem with furling turbines is that they can be very noisy in high winds.When the rotor turns out of the wind (furls), there is a time during each revolution when one blade is moving upwind and the other blade is moving downwind.As a blade revolves it transitions between both extremes.These rapid transitions cause turbulent airflow over the blades.This turbulence generates noise.On some furling turbines the noise is so loudcan be heard a long distance from the turbine.In some cases noise levels may exceed noise regulations set by states and municipalities.Because downwind turbines always operate normal to the wind direction they are not subject to this condition and they tend to be much quieter than furling turbineshigh winds.The Aerostar downwind design completely eliminates these high noise levels.There is no noticable sound level change from light winds to high winds.

GENERATOR TYPES

There are two basic types of generators used on wind turbines.The first type is the variable frequency alternator.The second type is the induction generator.The choice of generator has far reaching effects on the design and operating characteristicsa wind turbine.Variable frequency alternators use arotor or permanent magnets to provide the magnetic flux necessary to generate electricity.These alternators are much like the alternator in your car.The faster they turn, the more power they produce.The important characteristic of these alternators is that they allow the turbine rotor to turn at variable speed.As the speed of the wind changes, so does the speed of the rotor.In high winds, the turbine rotor will turn much faster than in low winds.The noise generated by turbine blades is proportional to the speed of the blade tips.In high winds the blades of a variable speed turbine have the potential to generate more noise than in low winds.Electricity provided by utility companies in the U.S. has a frequency of 60 cycles per second.Electricity provided to households is 240 volts AC, single phase.Because variable speed turbines run at different speeds, the frequency, voltage and current of their alternators also varies.If one of these alternators were to be directly connected to the utility grid, the difference in voltage, frequency and phase between the utility grid and the alternator would probably destroy the alternator in a very short time.Because of the inherent incompatibility between these alternators and thegrid, it is necessary to condition the power produced by the alternator before interfacing it with the grid.This is commonly done using a line commutated inverter.An inverter takes the power produced by the alternator, or in some cases a set of batteries, and rectifies the alternating current to direct current - DC.It then takes this DC and constructs a sine wave that is compatible in voltage, frequency and phase with the power supplied by the utility company.Creating this compatible sine wave is a complex process which requires a microprocessor and solid state components capable of handling high power levels.These requirements mean that inverters are usually very expensive.A 10 kW inverter can cost $6,000-$8,000.In many cases, the cost of an inverter represents 1/4 to 1/3 the cost of the wind turbine.Modern inverters tend to be quite efficient.Nevertheless, the solid state power electronics do generate quite a lot of heat so they require large heat sinks and fans, much like the microprocessor on your computer.Heat is wasted energy.So, in addition to unavoidable energy losses in the generator, there are additional energy losses in the inverter.The second type of generator used in wind turbines is the induction generator.Induction generators are essentially the same as induction motors.Induction motors are used by the millions on water pumps, compressors, furnace blowers, fans and just about everyelectrical machine in existence.If you have a water pump, furnace, air conditioner, fan oryou already have several induction motors in your house.Induction motors are very simple, reliable and robust.The motor consists of arotating coreby two ball bearings and a stationary winding.There are no brushes to wear out so these motors often run for 20-30 years or more with no maintenance.If you have a fan run by an induction motor and put this fan in a very strong wind, as the wind tried to turn the fan faster, the motor would begin producing electricity instead of using electricity.In reality, the blades on your fannot designed to extract energy from the wind so they would probably not work very well as a wind turbine.The point is that an induction motor automatically becomes an induction generator when you put power into the shaft, i.e., cause the shaft to turn faster than it turns as a motor.There is no need to change any of the electrical connections.This is such an important concept that it deserves further explanation.An induction motor runs off the 60 Hz power supplied by the utility company.The 60 Hz frequency causes a rotating or pulsating magnetic field in the motor.Because the 60 Hz frequency is fixed, it causes the rotor of an induction motor to run at a more or less constant RPM.The only way to directly alter the RPM of an induction motor is to add to or subtract the number of windings (called "poles")the motor stator.For example, apole motor will run at 3600 RPM and a 4 pole motor will run at 1800 RPM.These speeds are sometimes referred as the "synchronous speed" of the motor.For those who are mathematically inclined the speed of an induction motor is calculated by the equation: 7,200/ number of poles.Now, the rotor of an induction motor doesn't turn at exactly at synchronous speed.The rotating or pulsating magnetic field generated by the stator winding induces currents in the rotor.These currents generate magnetic fields in the rotor which oppose those in the stator and cause the rotor to turn.In order to generate these fields, there must be a slight difference in RPM between the rotor and the synchronous field generated bystator.This difference between the synchronous speed and rotor speed is called "slip".In most induction motors, the slip isn't very much, usually 2-3% of synchronous speed at full load.For a 4 pole motor, this means that the rotor will turn at about 1750 RPM, slightly below the synchronous speed of 1800 RPM.In a motor, the rotor lags the synchronous speed of the stator.When the motor is lightly loaded the slip will be less and the motor will use less power.As the load on the shaft is increased, the slip will increase and the motor will use more power.The important point is that from no load to full load, the speed of the motor will only vary by about 3% or, for a 4 pole motor, about 50 RPM.For all intents and purposes, an induction motor is a constant speed machine.This last statment has very important implications for wind turbines which use induction generators.An induction generator is nothing more than an induction motor.If you were to connect a diesel engine to the shaft of a 4 pole induction motor and opened the throttle, the engine would obviouslly try to turn the motor shaft faster.As the rotor began to turn faster the slip would decrease from 1750 RPM until, atRPM, the slip would equal 0.As the engine caused the rotor to turn faster, the slip would begin to increase above 1800 RPM.The rotor RPM would now be turning faster than synchronous RPM.The rotor would be leading the synchronous speed.As the speed continued to increase, the energy from the diesel engine would cause the motor to begin producing power instead of using power.The motor would turn into a generator.only change that has taken place is the name.now call the motor a generator.Clearly, there has been no physical change in either the motor or its electrical connections.On a more basic level, what happens makes sense.When we take mechanical energy out of a motor, the motor has to convert electricity to mechanical power.The reverse must also be true.When we put mechanical energy into a motor, it must convert this mechanical energy into electrical energy.

Choosing the Right Tower

Choosing the right tower for your location is one of the most important decisions you will need to make before you can install your Aerostar wind turbine.By far, the most important consideration is tower height.It is essential that the turbine be located high enough to be out of wind turbulence.If the tower is too low, power production will suffer, in some cases dramatically.Next on the list is the type of tower.Towers can be guyed, freestanding or monopole types.We will look at each of these and discuss the advantages and disadvantages of each design.

Choosing the Correct Tower Height

The general rule of thumb for siting wind turbines is that the bottom of the blades should be at least 30 feet above any obstruction within 500 feet.Some people use 300 feet but 500 feet isAn Aerostar 6 Meter has a 20 foot diameter rotor so the minimum tower height would be 40 feet above the height of obstructions.It must be stressed that this is a MINIMUM HEIGHT!Higher is better.If you have trees that are 40 feet high, the minimum tower height would be 80 feet.

Obstructions (Courtesy Iowa Energy Center)

The effect of the building extends to a distance of 20 times the height of the building and to an elevation of 2 times the height of the building.Afoot high building could create turbulence up to 60 feet high and for a distance of 600 feet from the building.A few general rules of thumb are:1.When siting a turbine upwind of a building the minmumfrom the buildingbe twice the building height.2.When siting a turbine downwind of a building the minimum distance should be 10 times the building height and preferably 20 times the building height.3.When siting a turbine downwind of a building the minimum turbine height should be twice the height of the building.The power in the wind increases with the cube of the windspeed.A change in windspeed from 10 to 12.5 MPH doubles the power in the wind.Because even small changes in windspeed have dramatic effects on power production, turbulence has the potential to substantially reduce the effectiveness of a wind turbine.Obstructions sometimes cause the wind to make rapid and dramatic shifts in direction.A wind turbine will try to follow these changes.It is usually easy to tell when a wind turbine is running in turbulence because it will be constantly yawing in an attempt to follow the wind.Obstructions also cause wind shear.Wind shear is a large change in wind velocity with height.When a turbine operates in high wind shear, the upperwill be subjected to a different wind velocity than the lower blade.Both turbulence and wind shear can place heavy loads on a turbine and tower, potentially shortening the life of the components.It has been said that placing a wind turbine on too low a tower is like putting solar collectors in the shade.Don't do it!

Guyed Towers

Guyed towers typically consist of a slender structure supported and stiffened by cables called "guy wires".The tower can be fabricated as a lattice type structure, typically solid round rod or tubing or it can consist of a single tube such as the one shown on the image at the right.Permanent guyed towers have 3 sets of guy wires arranged 120° apart."Tilt Down" guyed towers must have 4 sets of guy wires arranged 90° apart.Guyed towers are very efficient from a design point of view.This efficiency results in a simple tower which uses a minimum of steel.The cost of a guyed tower is generally less than for other types of towers.Tilt down guyed towers have a hinge at the base.winch or tractor is used to raise and lower the tower.advantage of tilt down towers is thatcrane is required for assembly, so installation costs can be lower.Another advantage is that the turbine can easily be serviced without having to climb the tower.The tower can also be lowered in situations where severe weather, such as a hurricane, is expected.Aerostar tilt down towers are available with an electrickit so that raising the tower is so easy anyone can do it.Pull out a single pin and press the button on the winch and the tower is on the ground in a few minutes.Guyed towers take more room that other types of towers because the guy wires are spread out from the tower base.A typical 100 foot tilt down tower has about a 70 foot spread at the base.

Lattice Freestanding Tower

Freestanding towers consist of 3 or 4 legs with diagonal bracing.Three leg towers usually have legs made from pipe or round tubing and angle steel braces connecting the legs.The braces give the appearance of "lattice work", hence the name.Four leg towers are usually made entirely from angle steel.The advantage of freestanding towers is that they take up less room than guyed towers.For a turbine the size of the Aerostar 6 Meter,typical foundation for a 100 foot freestanding tower is a concrete block about 4' deep and 16 feet square.tower base itself is only about 12 feet across.Because the tower is so compact, the force from wind loads results in higher forces in the legs and braces than on a guyed tower.The tower must therefore be built using larger members.This results in a heavier and more costly structure.The lack of guy wires makes it much more difficult and expensive to design a tilt down structure so a crane is typically needed for installation.

Monopole Tower

A monopole tower consists of a heavy large diameter tube.The primary advantage of monopoles is that they take very little space.For this reason, many large turbines are installed on monopole towers.In the case of large turbines, the tube is large enough in diameter to provide internal stairs or an elevator to service the turbine while remaining out of the weather.Monopoles for small turbines such as the Aerostar 6 Meter are too small in diameter to offer this advantage.The primary disadvantage of monopoles is their cost.They tend to cost 50% to 100% more than a lattice type freestanding tower.Another disadvantage is that because of their large surface area they tend to be more effective at radiating sound than other types of towers so noise from the turbine can be amplified.For a tower large enough to support an Aerostar, a crane is required for installation.

Tower Installation

Tower Installation usually requires a permit from your local building department.In order to secure the necessary permit your dealer must provide the building department with plans showing the tower and foundation.In most municipalities, the tower and foundation plans must be stamped by a Professional Engineer licensed to practice in your state.Having the plans and foundation reviewed and stamped by a local engineer could cost you thousands of dollars.Aerostar can provide you with stamped engineering drawings for our towers and the foundation.

Tower Vibration

Towers, like all physical objects vibrate at certain frequencies.These frequencies are called "Natural Frequencies".A tuning fork is an often cited example.Another example is a playground swing (although technically a pendulum, the example serves to illustrate the point).It takes very little energy to excite an object at its natural frequency.Once a swing is moving it doesn't take much energy to keep it moving or to increase the arc.In order to keep the swing moving it is necessary to push it at the end of its arc.The "pushes" must occur at just the right time.Obviously, if you were to push the swing while it was still coming toward you, you would slow the swing not keep it moving.Another way of saying this is that the "push" must be in "resonance" with the frequency of the swing.The blades of a wind turbine are a large heavy rotating mass.If the rotational speed of the blades happens to correspond to the natural frequency of the tower, the tower will begin to vibrate.If the blades were to continue to be in resonance with the tower, the amplitude of the vibrations could increase and, if there were not enough damping in the tower,damage the structure.It is therefore important to ensure that the natural frequency of the tower does not correspond to the rotational frequency of the turbine rotor.

For more informations please use our contact page