There is much debate about how to define and distinguish renewable energy from non-renewable,
and the terms and definitions chosen can have huge impacts on policy and regulatory efforts aiming
to promote clean energy resources. TREIA's definition of renewable energy has been adopted by the
 Texas legislature, and is as follows:

"Renewable energy: Any energy resource that is naturally regenerated over a short time scale
and derived directly from the sun (such as thermal, photochemical, and photoelectric), indirectly
from the sun (such as wind, hydropower, and photosynthetic energy stored in biomass), or
from other natural movements and mechanisms of the environment (such as geothermal and tidal
energy). Renewable energy does not include energy resources derived from fossil fuels, waste
products from fossil sources, or waste products from inorganic sources."

A definition of wind:
A natural movement of air, especially the earth's air or the gas surrounding a planet

 WHAT CAUSES WIND?
Air is set in motion by the Pressure Gradient Force (PGF). This, as you well know, is a flow from
 high pressure to low pressure. In nature, regions of excess move toward regions of deficit.
This comes from the second law of thermodynamics that states higher energy states move
 toward lower energy states. Nature is always trying to balance. The result of trying to balance
 and equalize pressure results in wind. The pressure gradient force acts on a horizontal plane.

What causes the pressure changes in the atmosphere? The primary reason is through the
warming and cooling of air. The pressure exerted by a gas changes as it becomes more dense
or less dense. Since a cold air mass is denser and has a lower thickness than warm air, pressure
decreases more rapidly with height in cold air than in warm air. When cold dense air is placed
next to warm less dense air, wind results by nature trying to balance the pressure differences at
each level in the atmosphere between the two air masses. This can occur on all scales of motion.
Examples:
On the planetary scale, temperature gradients create the polar jet stream;
On the synoptic scale it creates jet streaks;
On the mesoscale it creates sea breeze circulations and the low level jet;
On the microscale it creates lake breezes.

It was the ancient Greek philosopher, Anaximander, who first suggested that there was a scientific
explanation for wind. Many of his peers were content with the belief that wind was a result of
the breath of gods. 

What is Wind?

In very simple terms, wind is the result of air movement over the Earth. Air moves as a result of
pressure  systems (those “H”s and “L”s on the weather news maps):
from areas of high pressure to areas of low pressure.

There are two reasons that the atmospheric pressures vary around the Earth:

1) The Earth rotates. As it rotates, it drags the atmosphere around with it causing the air to mix
with the higher level atmosphere resulting in turbulence and pressure systems.

2) The sun heats. The heating of air around the Earth varies by latitude and time of day.
At the equator, for instance, large amounts of sun warm the air causing it to rise.
Conversely, the more acute angles of sunlight over the polar regions spread the rays over
a greater area resulting in less warming of the air. The resulting effect is that the air over
the poles is more dense than the air over the equator.

Local variations also account for pressure variances. Close to the sea or ocean, for instance,
on and off-shore air flows will warm or cool quickly as a result of the warmer or cooler land mass.

As air rises, low pressure areas are created below. Air then rushes from the areas of high pressure
to fill in the gap. The larger the differences between the pressures, the greater the flow of air between
the two points. These differences in air pressure, which cause wind formation, can be easily quantified…
and the wind records will amaze you!

Winds are primarily measured two ways: direction and speed.

Direction is measured using your everyday, wind vane. Whether it’s a flag blowing in the wind,
a fanciful steeple-sitting rooster, sailboat vane, or a sock, the main purpose of the instrument is
 to resist the flow of air. This motion causes the instrument to point in the direction of wind movement.
Alternatives include watching clouds drift, licking your finger, determining which way smoke is
blowing,  letting a balloon fly away… you get the idea.

Wind speed is measured with an anemometer. Invented in 1667 by a British physicist named Robert Hooke,
the classic model possesses three or four cups used to catch wind. By attaching these cups to a rotating
calibrated shaft, wind speed can be easily measured. Today’s scientific methods include more sophisticated
technologies including radar and lasers.  

The pressure plate anemometer is the earliest devised (1450). It was made by the Italian mathematician
 Leon Battista Alberti. He described it in his book: On the pleasures of mathematics. Leonardo da Vinci
 made one around 1500. Robert Hooke's 1667 swing plate anemometer had a very nice scale to help in
getting a good reading.

Units of measurement for wind speed vary. The local news shows speak a language we can all related to:
“tomorrow’s winds will be in the 17-20 miles per hour range.” Professions dealing with navigation, such
as the National Weather Service, the Federal Aviation Administration and the Maritime Divisions deal in
 knots. And scientists, trying to standardize to SI units deal in meters (or kilometers or centimeters)
per hour (or second).

BASIC WIND ENERGY TECHNOLOGY

The amount of wind you have at a wind energy site determines how much power you can expect from
a wind turbine of a given size.

Air density varies with temperature and elevation. Wam air is less dense than cold air.

Any given wind turbine will produce less in the heat of summer than it will in the dead of winder
with winds of the same speed.

There is less power in the wind for a specific wind speed at a site in Idaho han near sea level.

Changes in  air density relative to standard conditions at sea level can cust power production
10 to 20%  or more.

Power in the wind varies with the cube of the wind speed.  Double the speed of  the wind and
you increase the power of the available by eight times.

A 20 % increase in wind speed can give you a 70% increase in available power.

Small changes in wind speed can have a profound effect on the power of the wind. 
Measuring the wind speeds for a year to have an accurate count may  be offset by
actually installing a micro turbine. The mast cost, measuring devices and the professional
analysis needed can be compared to a micro turbine where if the test comes out negative
 from a cost return on investment  can be  removed and sold. There is a bigger  market for used
 wind turbines than for used anemometers.  You can also  locate owners of small wind turbines
 in  the area to determine performance data. a small wind turbine can be installed for about
the same cost as a recording anemometer.

Power is also directly related to the area intercepting the wind, that is the area swept by the wind.
When you double this area you double the power available.

The relationship between  the rotor's radius (or diameter) and energy capture is
 fundamental to understanding wind turbine design. Relatively small increases in blade length
or in rotor diameter produce a corresponding large increase in swept area, and thus power. 
A   20% increase in diameter can increase the capture area 44%.

The exponential relationship between swept area and the power available also explains a crucial wind
energy axiom:  Nothing tells  you more about a wind turbines potential than a rotor diameter.

Streamers attached to a kite will fly smoother the higher they get.  Where these streamers fly smoothly
is where you want  your wind turbine. Because obstructions near the ground disrupt the flow of the wind,
wind speeds typically increase with the height, especially over rough terrain. Wind is often measured
about 33 feet off the ground.

Wind turbines can be small as a 1.7ft diameter  20-watt Marlec  500 to Vestra's 200 ft diameter
1,650 kW giant wind turbine.

There are more than fifty manufacturers of small wind turbines worldwide producing over
100 different  models.

Wind speed typically increases with height as there are less obstructions to disrupt the
flow of the wind.

Small Wind  turbine sizes  measured by the size of  rotor which range  from the Micro turbines 
 of  2 to 4 feet,  to the Mini Wind Turbines  4 to 9 feet and  then household size wind turbines 
from 9 to 23 feet.

When charging batteries, the load they can receive may be less than what is available.
 When they become fully charged the turbine must spill  or dump the excess energy that
is available.

Rotor diameter usually refers to the rotor's  swept area. It is on of the critical factors in
determining how much energy a wind turbine can capture.

The more aerodynamically sophisticated rotors can capture, at most,  40 percent  of the
energy in the wind. Small turbines can seldom deliver more than 30%  of the energy of the
 wind. Often a small turbine can convert only  10% of the energy of the wind.

There are not government agencies ensuring the accuracy of published power curves,
nor is there an agreement on how power curves for small turbines should be measured.

Permanent-magnet alternators produces voltage as soon as they begin turning.
The voltage increase with the increasing wind speed until the voltage needed  for
the applications, such as battery charging, is reached.

When a wind turbine is providing energy only for a local use then it is called  "off grid".
If a system is connected to spill excess energy to the local electrical power company
it is considered to be " on  the grid" .

Wind turbines with  3 blades run smoother than turbines with just 2  blades. That means they will usually last longer.

Aluminum is never used for turbines blades because of it's propensity  for metal fatigue.

Nearly all small wind turbines use tail vanes to point the motor into the wind.

Heavier small wind turbines  are more rugged and dependable than lightweight machines.

A wind turbine furl is the ability to fold about a hinges so that the rotor wings toward the
tail vane. IN some models during height winds the rotor tilts into the air where it resembles
a helicopter rotor.

Most small turbines use permanent-magnet alternators. The case to which the magnets are
attached, sometimes called the magnet can, rotates outside the stator or stationary part of
the generator. In this case the blades can be bolted to the case.
Centrifugal force presses the magnets against the wall of the magnet can.The magnets
attached to  the shaft in a  conventional  shaft-driven alternator are thrown away from
the spinning shaft. Some batter -charging  models rectify the AC to ?DC at the generator:
others rectify it at the  controller that can be some distance from the generator.

On a percentage basis, wind energy has been the world's fastest-growing energy source
over the past decade. Wind Energy Weekly provides detailed coverage of developments
in the U.S. wind energy market as well as other regions around the world.