HOW DOES WIND GENERATOR WORK
Before discussing wind turbines choices, selection, and ratings, let’s quickly review how they work. A basic understanding of a system operation and the terminology can help you make the right choice for your application. In any electrical generator, electricity is produced by rotating a magnetic field relative to the coils. In gensets, the rotation is produced by a rotary engine that burns fuel, such as gasoline. The fuel of course costs money and releases unwanted emissions when burned.
In a wind powered generator, the rotation is driven by naturally occurring air currents, so the fuel is free and no emissions are produced. The main part of wind generators that captures the energy of moving air and uses it to drive the alternator is a turbine. It has a specially designed set of blades around a rotor. When the airflow hit the blades, the rotor spins and drives the shaft of an alternator. In the process, the turbine slows down and deflects the air. Note that we can’t capture all of the energy in the wind because that would require that the air molecules come to a complete stop. If that happened, they would not let the air behind them pass through the turbine. There is a physical law known as Betz’ law that states that even theoretically you can’t extract more than 59% of the kinetic energy in the air (read more details about windpower efficiency here).
It is important to remember that residential-grade turbines produce non-regulated AC voltage. Its frequency and magnitude are varying all the time– you can’t use it directly. This voltage has to be rectified and then converted by an inverter to a conventional AC suitable for household use. If the generator is intended to provide power back up, it should also contain a battery bank and a charge controller. A portion of generated energy will then be used to charge the batteries.
TYPES OF WIND TURBINES: HOW TO CHOOSE
There are two basic groups of the turbines based on the orientation of their rotation: the horizontal-axis (HAWT) and the vertical-axis (VAWT). VAWT in turn has two variations: Savnoius (S-type) and the Darrieus (egg-beater). The chart below summarizes their advantages and disadvantages and tells you what you need to know about each type before you buy.
UNDERSTANDING THE TERMINOLOGY
The wind systems are usually listed and advertised by their rated power measured in kilowatt. It may be a misleading characteristic though, which you should use carefully for comparing the products. This is because wind power strongly depends on the air velocity. For example, when wind speed doubles, the available power increases eightfold. The problem is manufacturers usually use arbitrary high air speeds at which they claim their system’s wattage. In addition to this, different manufacturers may use different speeds at which they rate their models. To let you compare apples and apples, American Wind Energy Association (AWEA) proposed in 2009 a voluntary standard for small turbines with rotor swept area under 200 m2. This proposal particularly calls for rating the turbine at a specific air velocity of 24.6 mph (11 m/s). Yet, this speed may be much higher than the one you would normally experience in your site. If for example, a system is advertised as 5,000 watt at 25 mph, and average wind in your location is 10 mph, it will be typically generating for your home only 320 watt!
RATED ANNUAL ENERGY (RAE)
The RAE by definition is expected total kilowatt-hours that would be produced during a year under specific conditions. Sometimes RAE states a monthly energy instead. Since the output depends on the air speed, turbine manufacturers often provide the numbers for various speeds. Note that AWEA proposed to provide RAE at 5 m/s (11.2 mph). The rated energy is a more useful characteristic than nominal power, but may you still need to do some calculations. The same turbine obviously will generate different amounts of electricity in different geographical locations. Therefore, you need to know the mean air velocity in your geographical area. You can find this value from a U.S. wind energy map. Data in the maps are provided for particular heights above the ground (usually 10m or 50m). To find the number for your actual tower you need to apply the so-called 1/7 power law. This empirical law states that wind shear increases at the 1/7 power of the altitude. Having calculated the mean speed at your tower height, you can estimate the RAE, which is proportional to the cube of the speed.
Example. Suppose you are considering a particular model that has a 20 ft tower and claims 2000 kWh annual output at 11 mph. You want to estimate how much kWh it would actually produce for you. First, you need to find out from the above referenced map the wind class for your geographical area. Let’s say it’s class 4. From wind power density table we find that class 4 corresponds to air speeds V from 12.5 to 13.4 mph at 33ft height. Let’s assume V=13 mph. By applying the 1/7 power law you get the average air velocity at your 20ft tower:
V=13×(20/33)1/7 = 12.1 mph. Then expected annual energy will be 2000×(12.1/11)3=2662 kWh. At a typical U.S. residential electricity price of 12¢/kWh, in our example such a wind generator may save you $319 a year. Is it worth it? A 2kW setup may cost about $10,000. If half of this amount will be offset by various incentives, it would still take you 15 years to recover the initial cost, assuming of course the system will work that long.