Ch 7: Energy From The Wind

Question 1

What are the attributes of a HAWT?

Answer 1

1. Requires a relatively constant wind direction.
2. High speed required.
3. High RPM.
4. High vibration.
5. High noise levels.
6. Horizontal plane rotor axis.

Question 2

What are the attributes of a VAWT?

Answer 2

1. Any wind direction.
2. Low speeds required.
3. Low RPM.
4. Low vibration.
5. Low noise levels.
6. Vertical plane rotor axis.

Question 3

What are the main components of a HAWT?

Answer 3

Blades
Hub
Low speed shaft
High speed shaft
Wind vane
Anemometer
Gearbox
Generator
Tower
Rotor
Nacelle

Question 4

What is the equation for kinetic energy?

Answer 4

Ke = 1/2 mv2

Question 5

What does betz limit mean?

Answer 5

It’s an expression for the maximum proportion of energy that can be extracted from the wind in a wind turbine.

Question 6

What is the the underlying principles of the betz limit?

Answer 6

The air coming into the swept area has a high velocity and takes up a small area. When the wind causes the blades and hub to turn, it looses energy and so has less energy downstream, so the velocity of the air downstream must be lower than the velocity up stream.

Question 7

What is the equation for betz limit?

Answer 7

Velocity upstream + Swept area upstream = Velocity downstream + Swept area downstream

Vu + Au = Vd + Ad

Question 8

What happens when a turbine has only 1 blade?

Answer 8

Most of the air would pass straight through the swept area, as it does not collide with the blade. The downstream velocity and swept area are not very different from upstream so this turbine would not be effective.

Question 9

What happens if a turbine has an infinite number of blades?

Answer 9

These blades will collectively act as a solid disk. The velocity of air downstream is now 0. This acts like a blockage preventing any further air from flowing through the system from upstream so there would be no energy extracted from the wind.

Question 10

What is the upper limit of the quantity of energy that can be extracted from wind?

Answer 10

59.3% when theoretically can never be reached.

Question 11

What is the typical efficiency of a turbine?

Answer 11

Between 25% and 45%
As there are always sound and thermal losses from the wind energy.

Question 12

What is the definition of rotor collected energy?

Answer 12

The exact amount of energy extracted from the wind.

Question 13

What is the 1st reason for the upper betz limit not being reached?

Answer 13

Mechanical: Friction
Any moving parts rub against each other, causing some of the kinetic energy to change to thermal energy which increases the temperature of the material.

Question 14

What is the 2nd reason for the betz limit not being reached?

Answer 14

Electrical: Resistance
Any current moving through a cable will encounter resistance. This causes the wire to heat up and so some electrical energy is converted to thermal energy.

Question 15

What is equation 1?

Answer 15

P out = 1/2 x p x A x v3 x COP

Question 16

What variables have direct proportionality to power out?

Answer 16

P out and A
P out and v cubed
P out and d squared
P out and p

Question 17

What is meant by the cut in speed?

Answer 17

The minimum wind speed required to produce power.

Question 18

What is meant by the rated power?

Answer 18

The max power that a wind turbine can produce.

Question 19

What is the relationship between high temperature and air density?

Answer 19

A high temperature means that the molecules of the gases in air have more kinetic energy and so move with a greater speed between collisions. This high temperature gives a low air density as there is less mass per unit volume. So high temperature = low P out.

Question 20

What is the relationship between low temperature and air density?

Answer 20

A low temperature means the molecules of the gases in air have less kinetic energy and so move with smaller speeds between collisions. This low temperature gives a higher air density as there is a higher mass per unit volume. So low temperature = high P out

Question 21

What is meant by the term altitude?

Answer 21

How high a specific point is above the surface of the Earth.

Question 22

What is the relationship between high altitude and air density?

Answer 22

At high altitude the air is thinner and so contains lower amounts of oxygen per cubic metre of air. So higher altitude means lower air density.

Question 23

What is the relationship between lower altitude and air density?

Answer 23

At lower altitude the air density is high and there has been no separation of molecules.