Residential Wind Turbines - Fast Info.

There are two types of residential wind turbines, horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs). A basic differece between them, as ttheir names suggest, is a matter of operating orientation. However, the differnce in orientation gives rise to radically different designs, performance characteristics and application. For VAWT information, see our vertical wind turbines page.

HAWTs

Horizontal axis wind turbines resemble an ordinary windmill in form and operation. The device consists of a rotor (essentially a propeller) attached to the shaft of an electrical generator (sometimes through gearing). Mounted on a tower to give maximum elevation and wind resource exposure, it translates horizontal windspeed into rotational mechanical energy (torque). Generally speaking, they are the best option for residential applications outside of urban areas. With many decades of field operation behind their designs (and sometimes as few as 3 - 4 moving parts), they are efficient reliable performers. A closer look at their basic components will help explain why.

The Rotor

The rotor component (resembling a propeller) actually captures the wind and induces the rotational force on the turbine shaft. Its size is extremely important to power generation. In general, the larger the rotor (measured by the diameter of the circular area traveled by the blades) the more wind it captures. This is also known as the rotor swept area. The swept area (and blade pitch design) determine how much power is extracted from wind moving at a given velocity. All other things being equal, the greater the swept area, the greater the amount of power that can be extracted from the moving air. In fact, swept area is a fairly reliable guide for power output comparisons between residential wind turbines of differing brands and sizes because moving air contains reasonably predictable power constants. This is very useful in understanding the efficiency of different rotor designs in an industry that has not developed reliable standards for comparing turbine performance. In other words, rotor swept area and wind speed are currently the most reliable basis on which to judge comparative turbine performance. For instance, a turbine with a swept area of 150sq. ft. should produce twice as much power as a turbine with a swept are of 75sq. ft.

The physics of rotor operation is captured in its airfoil design (design for how the rotor blade will interact with moving air) and the materials used in its construction. Wood as a component material has given way to reinforced fiberglass to increase strength/mass ratios and reduce maintenance. Reduced mass also means less wind energy needed to force the rotor to start spinning. That energy can be applied to generating electric power instead. Some turbines will produce power in as light a wind as 7 - 9 m.p.h and start spinning at 5 mph.

The foils can be designed to take advantage of varable wind velocities with variable pitch surfaces, allowing the blade to slice through the air more efficiently (and start-up more easily). At the average wind speeds typical of many residential locations, efficiency increases of up to 30 percent or more have been reached with designs that optimize aerodynamic lift and reduced drag factors. In fact, rotor air foil design has been so effective that a certain amount of de-optimisation has to be injected (aka.. air foil stall speed design or passive stall control). Wthout this or some other form of speed governance (e.g. active braking, electrical shutdown, physical shutdown) a rotor can develop speeds high enough to overload and destroy the power unit. Even worse, the vibration and twisting pressures from high velocity wind can be so severe that the corresponding stresses will simply tear the turbine apart. This is why regular inspection and maintenance is a double priority. Flying blades, not a good scenario. Other terms and methods for rotor speed control are:

1) Active stall control - for this type of speed control system, on larger wind turbines (1 megaWatt and larger) the blade pitch is adjustable, like the blade pitch of pitch-controlled systems. However, the active stall system controller will actively pitch the blades to produce a stall (as opposed to simply reducing wind capture rate) when power levels exceed a specified point. This makes active stall control a bit more accurate since the excess wind energy is actively countered almost immediately upon detection.

2) Passive Stall Control - The blades are rotor mounted at a fixed pitch angle but their design is such that the pitch angle changes slightly along its longitudinal axis. This slight "twist" is calculated to disrupt air flow (cancels blade lift) over the blade when wind speed begins to exceed acceptable levels. The twisting shape of the blade induces a gradual stall (as opposed to a sudden active and mechanically induced stall) that automatically distorts optimum air flow over the blade, canceling the lift effects from the additional wind speed. Essentially, the extra wind energy is wasted in turbulence created on the lift side of the blade. Unfortunately, since the shape of the blade is complex and difficult to produce, it is not a very practical design for do-it-yourself turbine building. In addition, if done incorrecrtly, it can also promote vibration to the point where it becomes a real problem. Still, its a design that removes complicated control systems and additional moving parts, thus simplifying maintenance. It is very popular with many manufacturers for residential wind systems as well as some commercial applications.

3) Pitch Controlled System - An electronic controller constantly samples turbine power output. This is usually done several times per second. At any point where excessive power is being produced, the controller will pivot the blade pitch out of optimum position. This reduces the rate of wind capture on the blade surfaces, slowing the rotor. Obviously, pitch-controlled systems require an adjustable rotor blade mounting mechanism and sophisticated hydraulic actuator or servo controls. As wind speed moderates the controller re-optimizes blade pitch to re-establish maximum power output for the current wind speed.

4) Yaw Control - To operate efficiently a HAWT must face into the wind. Some systems use a directional mechanism to maintain an optimum wind orientation. It could be as simple as a vane or as complex as a microprocessor controlled subsystem. Some of the more complex systems take advantage of this capability by using it as a speed control method. When rotor velocity exceeds safe limits a sensor will tell the yaw control system to steer the turbine away from the oncoming wind, thus decreasing rotor velocity,

Power Generation

The component responsible for producing electric power from a wind turbine is its power unit. Some manufacturers use a DC (direct current) producing generator, while others use an alternator to produce single or three phase AC current which is then rectified into DC. Three phase current allows the alternator to be smaller and will result in lower line losses. Which is best, depends on the nature of the application being supported. Whether a generator or an alternator, the device must be matched in size and power production capability to the rotor's swept area as well as be capable of producing power at low rpm. Proper matching is crucial as errors can result in low turbine efficiency (measured by how well the turbine translates wind energy into electrical energy).

Residential wind turbines usually carry power ratings measured in kilowatts (1 - 25 kW is common). Actual output voltages vary though. It's basically a matter of manufacturers design convenience and market requirements. However, common applications are battery charging (12vdc - 48vdc is common) or utility grid-tied applications (some wind turbines have an inverter integrated as part of the power unit).

It is quite possible to adapt power generation units (generators, alternators) from autos, trucks or other devices to use as the power unit in a home built wind turbine. However, to do so successfully requires that the power unit be of low rpm design, otherwise gearing will be required to reach the needed rpm. The disadvantage of gearing is fhat it tends to rob a turbine of its low wind speed startup capability. This is especially critical for highly variable or low wind speed applications.

Towers

Towers come in a number of configurations, but they all have the same purpose - place the turbine and rotor as high above the ground as possible. The taller the tower, the greater the likelihood of avoiding energy consuming ground drag and turbulence caused by ground objects. With tower height comes the high speed steady wind that will produce reliable power. So critical is height that it is common for an 80- to 120-foot tower to be supplied with a wind turbine purchase. Since wind speed increases with height, and modest increases in wind speed increase wind power exponentially, every incremental investment in additional tower height can produce very high rates of return in power production. For instance, installing a generator on a 120-foot tower rather than a 60-foot tower might involve a 12% increase in overall system cost but it can yield a 34% power increase. For optimum height placement, a rule of thumb for getting clear of ground turbulence requires the bottom most edge of the swept area to be at least 30 feet above the top of anything within 500 feet (houses, trees, barns, statues, etc). Towers for residential wind turbines will more than likely be restricted, especially in suburban residential locations.

Several types of towers are available and depending upon the application, each has its advantages.

Guyed tower is an economical and common choice for residential wind turbines.

lattice - resembling a cross-braced three or four sided ladder and having a small footprint, (sometimes slightly wider at the bottom than top) it receives its lateral suppott from the three or four guy wires that have been anchored to the ground. Installation is commonly performed using a crane. Unfortunately, turbine maintenance is performed by climbing the tower to access the hub assembly/nacelle. The guy wires give this tower a relatively larger footprint.

Pipe - Constructed of internally overlapping welded lengths of pipe and guyed from three or four directions this tower can be easily installed.

Guyed hinged towers - a bit more expensive, but easier to install, this tower is laterally supported by a four point guy wire system. Its main advantage is the relatively easy installation procedure that allows a single pole column (or even lattice tower) to be tilted up into place (the wind turbine is pre-installed on top) on the massive hinge integrated into its base, using an ordinary winch (or car) and ginpole. Also, unlike other towers, turbine maintenance can be performed on the ground by tilting the tower down, performing the maintenance and then simply tipping it back into place. The tilting action of this tower makes its footprint much larger (equilvalent to its height and width).

Freestanding - freestanding towers are engineered to support a wind turbine and windshear loads wthout the use of guy wires. Generally they are fairly wide and heavy at their base which makes them stable. In the case of the tapered monopole towers, support comes from a deeply anchored base, which allows for a relatively small footprint. Freestanding towers tend to be more expensive than other tower types.