The heart of a wind energy system is a wind turbine. Modern
wind turbines have become remarkably sophisticated and are now capable
of high performance in a variety of wind conditions. Let's
explore the different parts of the wind turbine in order to get a
complete picture as to how wind energy works:
Every wind turbine has a rotor or propeller with blades which
harvest the energy from the wind. The rotor is connected to a hub
which in turn is connected to the main drive shaft. The drive
shaft connects to the generator which generates the electricity.
The rotor blades are usually made of plastic, fiberglass or wood
covered with an epoxy or urethane coating. The type of blade that is used in the rotor is
different depending upon the type of work you want the rotor to do.
In a Drag Design Blade the blades are designed to be pushed by
the wind rather than lifted. These types of designs cause the
rotor to turn slower but with more force or torque. This type of
blade is well suited to work such as pumping, sawing or milling and is
the type of blade you might find in a Dutch windmill or in a farm-type
windmill used for irrigation. However, for generating
electricity you need a rotor which turns at very high speeds so modern
electricity generating wind turbines do not use this type of rotor.
Today's wind turbines use what is known as a Lift Design. These
types of blades use the same principle of lift as you would see in an
airplane. The rotor blade edge is similar to that of an airplane
wing and creates lift because of the differential air pressure between
the flat side and the rounded side of the blade. However, since
the blade is turned at an angle the lift causes the blade to turn
rather than rise. Lift-powered wind turbines have much higher rotational speeds than drag types and therefore
are well suited for electricity generation.
One of the questions people often ask is why do wind turbines only
have 2 or 3 blades. The reason is that number of blades that make up a rotor and the total area they cover affect wind turbine performance.
Rotors that use the lift-principle need for the wind to flow smoothly
over the blade. If the blades are too close together the
turbulence from one blade can disrupt the flow of air to the blade
next to it. So the blades must be far enough apart given there
overall size so that this does not happen.
Blade size is important in determining the amount of energy you
can generate. The larger the blade size the more of the kinetic
energy of the wind you can capture. Often it is difficult to do
comparison shopping for wind turbines given the way different
manufacturers rate their systems. One way to cut through some of
this is to compare the actual area the blade for each wind turbine can
cover. This area is known simply as the Wind Swept Area and can
be calculated with the formula Π
x Radius2. You just take the length of a single
blade, which is usually provided by the manufacturer, square it, and
then multiply the result times the constant pi. The bigger the Wind
Swept Area the more power the turbine will generate.
Another factor that is looked at in rotor design is what is called
the Tip Speed Ratio, the tip being the tip of each blade on the
rotor. The larger this ratio, the faster the rotation of the wind turbine rotor at a given wind speed. Lift-type wind turbines have maximum tip-speed ratios of around 10, while drag-type ratios are approximately 1.
The tips of a wind turbine rotor can reach speeds of up to 300 mph.
Since electrical generators require the shaft to turn at a high speed,
high tip speed ratios are needed.
The generator is the device inside the turbine that actually
generates the electricity. It can also be referred to as a permanent
magnet alternator. It takes the kinetic energy generated by the
rotor and translates it into electricity. Inside the generator, coils of
copper wire called the armature are rotated in a magnetic field to produce electricity.
Generators can be designed to produce either alternating current (AC) or direct current (DC), and they are available in a large range of output power ratings.
If the turbine is designed to produce DC current it will have an
additional component inside the housing called a rectifier which will
convert the originating AC current to DC current. Another approach for
getting DC current is to pass the AC current to a separate device
called an inverter which can convert the AC current to DC current.
Grid-tied and off-the-grid systems have different requirements
when it comes to wind turbines. In a grid tied system you will want AC
output which matches that of the electrical grid itself. Most home and
office appliances operate on 120 volt (or 240 volt), 60 cycle AC. if
you are using your wind turbine as part of an off-the-grid standalone
system you will need to store the power it generates in batteries when
not using it directly. Batteries run on DC current. Battery voltages
are voltages of between 12 volts and 120 volts. DC generators are
normally used in battery charging applications and for operating DC
appliances and machinery in a business.
Generators typically require 1,200 to 1,800 revolutions per minute
(RPM's) to operate efficiently. However, the RPM's of a wind
rotor are usually more in the range of 40 to 400 RPM's. In order
to make up this difference wind turbines will usually have a
gear-box transmission to increase the rotation of the generator to the speeds necessary for efficient electricity production.
The gear is connected to a second high speed shaft which because of
the gear ratio turns at the higher speed the generator requires.
In any device there is usually a tradeoff between complexity and
maintainability. This is true for wind generators. Some DC-type
wind turbines do not use transmissions but instead have a direct link between the rotor and generator. These are known as
direct drive systems. By eliminating the gearbox the device is
considerably simpler and will require less maintenance. However,
in order to generate sufficient electricity with a direct drive a larger generator is required to deliver the same power output as the AC-type wind turbines.
Types of Wind Turbines
Wind turbines are classified into two general types: horizontal
axis and vertical axis. A horizontal axis machine has its blades
rotating on an axis parallel to the ground. A vertical axis machine
has its blades rotating on an axis perpendicular to the ground. There
are a number of available designs for both and each type has certain
advantages and disadvantages. However, compared with the horizontal
axis type, very few vertical axis machines are available commercially.
This is the most common wind turbine design. In addition to being parallel to the ground, the axis of blade rotation is parallel to the wind flow. Some machines are designed to operate in an upwind mode, with the blades upwind of the tower. In this case, a tail vane is usually used to keep the blades facing into the wind. Other designs operate in a downwind mode so that the wind passes the tower before striking the blades. Without a tail vane, the machine rotor naturally tracks the wind in a downwind mode. Some very large wind turbines use a motor-driven mechanism that turns the machine in response to a wind direction sensor mounted on the tower.
Although vertical axis wind turbines have existed for centuries, they are not as common as their horizontal counterparts. The main reason for this is that they do not take advantage of the higher wind speeds at higher elevations above the ground as well as horizontal axis turbines. The basic vertical axis designs are the Darrieus, which has curved blades, the Giromill, which has straight blades, and the Savonius, which uses scoops to catch the wind.
A vertical axis machine need not be oriented with respect to wind direction. Because the shaft is vertical, the transmission and generator can be mounted at ground level allowing easier servicing and a lighter weight, lower cost tower. Although vertical axis wind turbines have these advantages, their designs are not as efficient at collecting energy from the wind as are the horizontal machine designs.
Grid Connected Systems
The size, or generating capacity, of a wind turbine for a particular installation depends on the amount of power needed and on the wind conditions at the site. It is unrealistic to assume that all your energy needs can be met economically by wind energy alone. As a general rule, a wind system should be sized to supply 25% to 75% off your energy requirements. Most residential applications require a machine capacity of between 1 and 10 kW.
In a grid-connected system excess electricity from the wind turbine is automatically fed to the utility and backup power is automatically supplied. While this does not constitute true storage, it provides power on demand at any time, in any amount.
One of the major advantages of this approach is that batteries are not
needed to store the power. In essence the grid acts somewhat
like a battery providing power whenever your wind turbine is
generating insufficient electricity to meet your needs. On windy days
when you are generating more energy than you can use this energy is
fed into the grid and your power meter runs backwards giving you
credit for the energy you are generating.
Most states are required to provide access to the grid using a
two-way power meter. Usually your local utility company will have to
do the actual hookup. Whenever installing a wind turbine you
should notify the power company well in advance of the installation
and make arrangements for them to do the hookup. As more people
have begun moving to solar and wind energy some utilities have started
to get backed up in doing these hookups so you should plan carefully
this part of your project.
Off-grid power systems can result in higher cost energy, but the high cost of extending a power line to a remote location often makes an independent energy system the most cost effective choice for remote homes and equipment. If the average wind speeds at a location are greater than 12 mph, a wind turbine may provide the least expensive form of energy.
Because wind is intermittent, it is often used in conjunction with batteries or with other energy sources, such as a gas generator or solar electric panels, to make a hybrid system. Battery systems can supply the owner with reserve power whenever energy demand exceeds that delivered by the wind turbine. This reserve power comes in handy during calm spells, but in situations where the storage capacity is taxed beyond its limits, a backup system, such as a portable gasoline or diesel generator, may be necessary. By combining two or more sources of energy, the size of energy storage can be decreased.