(Chip’s Note: With hurricane season only a month old and one hurricane making landfall, this guest post is a reminder that wind isn’t always damaging; it can be harnessed for good purpose as an alternative energy source. Wind energy is more relevant and necessary now, in light of the environmental catastrophe caused by BP’s Gulf Oil spill. The following is a guest post by Mary Jones).
Wind is caused by the uneven heating of the Earth by the sun and the fact temperatures are always trying to reach an equilibrium (heat is definitely moving to a cooler area). With the rising price of energy and the damage to the environment from non-renewable fuels, it has become cost efficient to harvest this renewable resource.
Some great benefits of wind energy are that it’s virtually free (after you purchase the equipment) and there is no pollution. The disadvantages include the fact that it isn’t a consistent source (the speed varies and many times it is insufficient to generate electricity), and it typically requires about one acre of land.
How Wind Energy Works
The amount of power that is available varies by wind speed. The quantity available is known as its power density, which is measured in watts per square meter. For that reason, the U.S. Department of Energy has separated wind energy into classes from 1 to 7. The normal wind speed for class 1 is 9.8 mph or less while the average for a class 7 is 21.1 or more. For effective power production, class 2 winds (11.5 mph average speed) are generally required.
Generally, wind speeds increase as you get higher above the Earth. For that reason, the conventional wind turbine is a component of a tower at least 30 feet above obstructions. There are two basic types of towers employed for residential wind power systems: free standing and guyed. Free standing towers are self supporting and are usually heavier, which means they take special equipment (cranes) to place them. Guyed towers are supported on a concrete base and anchored by wires for support. They typically are not as heavy and most are tilt down models which may be easily raised and lowered for maintenance.
The kinetic energy from the winds is harnessed by a device called a turbine. This turbine contains airfoils (blades) that capture the power of the wind and use it to turn the shaft of an alternator, like you have on a car– only bigger.
There are two basic kinds of blades, drag style and lifting style. We all have seen pictures of old fashioned windmills with the large flat blades, which are a good example of the drag style of airfoil. Lifting style blades are twisted instead of flat and resemble the propeller of a small airplane.
A turbine is classified according to whether it is built to be installed with the rotor in a vertical or horizontal position and whether the wind strikes the blades or the tower first. A vertical turbine typically requires less land for installation and is a better option for the more urban areas around the globe. An upwind turbine is made for the wind to impact the airfoils before it does the tower.
These units normally have a tail on the turbine which must be pointed into the wind. A downwind turbine does not require a tail, as the wind on the blades tends to keep it oriented properly.
These turbine systems would be damaged if they were to be allowed to turn at excessive speeds. Therefore, units should have automatic over-speed governing systems. Some systems use electrical braking systems while others use mechanical type brakes.
The output electricity from the alternator is sent to a controller which conditions it for use in the home. The use of residential wind power systems requires the home to either remain linked with the utility grid or store electricity in a battery for use when the wind does not blow sufficiently.
When the home is linked with the grid, the excess electricity that is created by the residential wind power system can be sold to the utility company to reduce or even eliminate a utility bill. During periods with not enough wind, the home is supplied power from the utility company.
Small residential wind power turbines can be an attractive alternative or addition to those people needing more than 100-200 watts of power for their home, business, or remote facility. Unlike PV’s, which stay at basically the same cost per watt independent of array size, wind generators get more affordable with increasing system size. At the 50 watt size level, for example, a small residential power wind mill would cost about $8.00/watt compared to approximately $6.00/watt for a photo voltaic module.
This is why, all things being equal, photovoltaic is more affordable for very small loads. As the system size gets larger, however, this “rule-of-thumb” reverses itself.
At 300 watts, the turbine costs are down to $2.50/watt, while the PV costs are still at $6.00/watt. For a 1,500 watt wind system, the cost is down to $2.00/watt and, at 10,000 watts, the price of a wind generator (excluding electronics) is down to $1.50/watt.
About the writer – Mary Jones writes for the residential wind energy systems site, her personal hobby blog dedicated to suggestions to reduce CO2 and lower power costs using alternative power sources.