Today there are three forms of commercially available solar cells - monocrystalline, polycrystalline and thin-film. Each of these technologies has its benefits and demonstrated strengths. Due to their higher efficiency and longer useful life monocrystalline cells have been the technology most favoured by industry, however, the trend and future seems to lie with thin-film which typically has demonstrated lower cost per watt, cell efficiencies and life expectancy.
The wind is a renewable energy source, continuously generated or replenished by the forces of nature. Renewable energy technologies, such as wind energy systems and solar photovoltaic (PV) systems, which use sunlight, convert renewable resources into usable forms of energy that can complement or replace conventional energy sources.
Wind is very complex process, which can be described very simply.
The sun heats the earth at different rates depending on weather an area is below clouds, in direct sunlight, or covered with water. The air above the warmer areas heats up, becomes less dense, and rises. The rising air creates a low-pressure area. Cooler air from adjacent higher-pressure areas moves to the low-pressure areas. This air movement is wind.
The wind turbine rotor is one of the most visible parts of a wind energy system, but there is more to the turbine than just the rotor.
The most familiar turbine is the horizontal axis wind turbine, known as a HAWT. The main propeller-like rotor has an axis that is parallel to the ground, and therefore horizontal to the wind. Horizontal wind turbines are most common in small applications, and can be placed on a tower which does not require a large area. If servicing has to be done to a horizontal wind turbine, however, the tower either has to come down, or the service technician has to go up.
A vertical axis wind turbine, known as a VAWT, has an axis perpendicular to the flow of the wind. The generating equipment in a vertical axis wind turbine is at ground level, but vertical axis wind turbines require a lot more space to be cleared for guy wires.
Because any wind turbine may be exposed to high winds, rain, snow, ice, sun, and
even salty air, its parts should be made of tough, durable and corrosion-resistant materials.
A well-built and well-maintained turbine should have a life expectancy of 20 years or more.
One of the first steps in determining if a wind energy system is feasible is finding out how much wind energy is available.
To do this, wind speeds are measured over a period of time, making note of the amount of time the wind blows at various speeds. From this, an average annual wind speed is calculated. A wind energy system usually needs an average annual wind speed of at least 15 km/h to be practical.
It is also important to know the variation in wind speed.
As it turns out, the wind is almost never calm, and rarely exceeds twice the annual average speed, and then only briefly.
Air movement is slowed by friction close to the ground. As you move higher, wind speed increases. For most open spaces, wind speed increases 12 percent each time height is doubled.
Locating a wind energy system on a hill, and on a tower will increase the amount of wind energy available.
A small increase in wind speed leads a large increase in the amount of energy available (because volumes of air are being moved, the energy available in the wind is proportional to the cube of the wind speed).
A wind energy system is simply a method of extracting the energy from the wind and converting it into useful energy. This conversion can be to mechanical energy, where the wind turns a rotor which drives a mechanical device such as gear or lever system running a water pump. The convert ion can also be to electrical energy, where the rotor runs a generator.
A basic wind energy system consists of a turbine (a propeller like rotor, a gear box and a generator), a tower, and a Balance of System (BOS) package.
Typically, wind speeds greater than 15km/h are needed before a wind energy system can begin to generate electricity. This is known as the "cut-in" speed.
The "cut-out" speed, usually around 70km/h, is where the system stalls to protect itself from damage.
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