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Sources of Error
Most wind turbines are the single rotor, classic Danish three-blade
design. These blades are often more than 50 meters in length, requiring towers
that are 100 meters in height. It is obvious that unused wind passes between the
blades of these three-blade systems. Smaller diameter multi-blade rotors, such
as those used on farms for irrigation, will turn at lower wind speeds but are
subject to higher stress and unable to withstand extreme wind conditions. My
design is to utilize multiple three-blade rotors. Multiple small rotors weigh
less than a single large rotor, are less costly and easier to produce and
transport and less subject to fatigue.
This project, "Watts up
with Torque!" is Phase 2 of my 2003 project "Torque it Up!" The
purpose of Phase 1 was to determine if multiple rotors would increase the torque
of a horizontal axis windmill. Torque is the force created by a rotating shaft.
In Phase 1, force in Newtons was measured using a spring scale and
torque in Newton-Meters calculated using the formula:
Torque (N-M) = Force (N) x Radius (M)
The revolutions per minute (RPM) of the rotor was used to calculate blade
tip speed and mechanical energy (power in Watts) was calculated using the
Mechanical Energy (Watts) = Torque x Blade Tip Speed
The results from Phase 1 strongly supported my hypothesis. Every test
using multiple rotors produced more torque than a single rotor and the
mechanical power produced was dependent on rotor size, number, blade
orientation, rotor spacing and wind speed.
Subsequent to the completion of Phase 1, I found that researchers in
California have been testing a multiple rotor windmill. Preliminary results from
this testing indicate that their design, the "Quadrunner", using
multiple rotors, coupled to a single shaft, will harvest more wind and energy,
at less cost than current models using a single rotor.
This further convinced me that my research of a multiple rotor
windmill design had merit and was worthwhile pursuing.
Phase 2: Watts Up With Torque!
The overall efficiency of a windmill is the amount of electricity
that can be generated over time on a cost basis. Two important factors that
determine overall windmill efficiency are the ability to use low velocity wind
and ability of the windmill to convert the kinetic energy of the wind into
electrical energy (conversion efficiency).
In Phase 2, I wanted to
further my research of multiple rotor windmills and assess the conversion
efficiency utilizing a more direct approach. This project required a device to
generate electrical energy from the rotating horizontal windmill axis. Direct
current (DC) motors are readily available in various sizes. They work as motors
when you apply electricity to them, but they also work as generators when you
turn the motor axle. Lower voltage motors such as 3 Volt (V), are easier to
start up but have a lower electrical output. Higher voltage motors, such as 12
V, require more torque to start the rotation, but produce more electricity.
The common units used to measure the quantity of electricity are:
Volts: electrical force or pressure behind the electrons in a current Amps:
number of electrons flowing past in a second
Watts: total amount of
electrical energy per second and is equal to Watts = Volts x Amps
For Phase 2, I modified the laboratory scale horizontal axis windmill
model so that the axles of various sizes of DC motors could be coupled to the
windmill axis. The electricity generated by rotor combinations that were able to
start and continuously turn the motor was measured and conversion efficiency
assessed at two wind speeds.
What is wind?
Wind is air in motion, caused by the uneven heating of the Earth by the
sun. Wind occurs when warm air rises, and cooler air moves in to fill the space.
It is estimated that 2% of the solar energy reaching the earth is converted into
wind energy. Air is constantly being interchanged between the warm tropics and
the cold polar caps. The rotation of the Earth also produces wind.
sun radiates the most heat over the equator and therefore the air there is
warmer. Air from both hemispheres is constantly moving toward the equator. The
rotation of the Earth causes the cool winds to be deflected from east to west.
As the surface of the earth heats and cools unevenly, pressure zones are created
that make air move from high pressure to low pressure areas.
What is wind energy?
process by which the kinetic energy of wind is used to generate mechanical power
or electrical energy is known as wind power or wind energy. Kinetic means being
related to or produced by motion such as the blowing wind.
windmill converts the force of the wind into a turning force acting on the rotor
blades. The strength of this turning force is known as torque.
Wind speed and energy:
The amount of energy that can be captured from the wind is exponentially
proportional to the speed of the wind. If a windmill were perfectly efficient,
the power generated is approximately equal to:
P (watts) = 1/2 D
(air density) x A (area of rotor) x V cubed (wind velocity)
density at sea level and 14 degrees C = 1.225.
Therefore, if wind speed is doubled, the power in the wind increases
by a factor of eight, i.e. 2 x 2 x 2. In reality, because wind turbines are not
perfectly efficient, changes in wind velocity do not have such a dramatic effect
on wind power. Betz' Law states that you can only convert approximately 59 % of
the wind energy to mechanical energy using a wind turbine. However, small
changes in velocity do impact on available energy, making wind speed an
important factor to consider in the placement of a wind turbine.
The chart below illustrates that a doubling of wind
velocity increases power available by a factor of eight.
History of Wind Power:
Wind has been used for centuries to propel ships and the wind routes
were well known and used by explorers such as Magellan and Columbus.
power was used as a source of mechanical energy on land for thousands of years.
The Babylonians constructed windmills for irrigation as early as 1700 BC and
Europeans were using windmills by 1000 AD.
Dutch used windmills to drain the land and used eight basic types. Dutch
settlers introduced windmills to the United States in the early 1600s.
Halliday invented a new style of windmill, which many believe encouraged the
rapid settling of the American West. More than 6.5 million windmills were sold
in the US between 1880 and 1935. They were used to pump water, grind grain and
cut lumber. Some small electrical generating systems were used to produce direct
current by 1900. Cheap electricity was introduced in th 1940s and most of the
wind powered generating systems in rural areas were considered obsolete and fell
turbine is the name given to a complete, electricity generating windmill. In its
simplest form, it consists of a tower, blades, generator and, if electricity is
to be stored, batteries. There are large windfarms in many areas of the world.
Wind Turbine Rotor Design:
There has been a great deal of research on rotor design including whether the
turbine will be upwind (rotor facing the wind) or downwind (rotor on the lee
side of the tower), the number, size and shape of blades, the load (forces
acting on the rotor in high wind) and other rotor aerodynamic considerations.
Generally speaking, larger windmill rotors and higher wind speed,
produce more power. The old Western windmills had many, wide blades. During
very high winds, they were exposed to extremely high forces known as loads and
were often damaged. Modern wind turbines by law, have to be able to withstand
extreme winds that may only occur once every 50 years.
Most wind turbines are the classic Danish three-bladed design with the
rotor positioned up-wind (facing the wind). Even numbers of blades cause
instability. Some designs are two bladed, saving the cost of a blade and
reducing rotor weight. They need higher rotational speeds to produce the same
amount of power as a three bladed design. These speeds produce more noise. There
are one bladed designs that require a counter-balance on the other side of the
hub. They also require higher rotational speed.
Rotor blades act like airfoils. An airfoil is a structure around which air
flows creating lift. Rotor blades have a special shape so that when the wind
passes over them, it moves faster over one side. Bernoulli's Principle states
that increased air velocity produces decreased pressure.
When the wind blows there is a pocket of low pressure formed on the
downwind side of the blade. The blade is pulled toward the low pressure making
the rotor turn. This is called lift. The lift force is stronger than the force,
known as drag, acting on the front side of the blade. The combination of lift
and drag causes the rotor to spin like a propeller, and the turning shaft spins
a generator to make electricity. In wind turbine design, the objective is to
have a high lift-to-drag ratio. This is accomplished by twisting the blades. The
blades are twisted so that the wind hits them at the correct angle of attack.
This twist is known a pitch.