Energy From Wind Flashcards
Describe two differences between vertical axis wind turbines and horizontal axis wind turbines
A VAWT can operate with wind in any direction whereas HAWT must yaw to face wind.
VAWT can operate at low wind speeds while HAWT requires higher wind speeds
VAWT rotates at low RPM while HAWT rotates at a higher RPM
VAWT is less noisy that HAWT
VAWT has less vibration than HAWT
Calculate the wind speed required to produce 17280 joules of energy from 540 kg of air
K.E. = 1/2mv2
17280 = 0.5 x 540 x v2
V2 = 17280/0.5 x 540
= 64
V = 8m/s
Calculate the wind speed required to produce a maximum theoretical energy of 15680 joules from 640kg of air.
K.E. = 1/2 mv2
15680 = 0.5 x 640 x v2
V2 = 15680/0.5 x 640
V2 = 49
V = 7m/s
Define what is meant by the Betz Limit when applied to a wind turbine and explain how it relates to power efficiencies achievable by wind turbines in the real world
The maximum amount of the winds kinetic energy that a HAWT can convert to mechanical energy turning a rotor
Betz calculated this at 59.3% of the kinetic energy from the wind
Most modern turbines however can only convert 35-45% of the winds energy to electricity
Because of the energy losses in gear boxes etc
For a rotor diameter of this size and with a wind speed of 11m/s the maximum available rated energy in the wind is 16.4 kW. Identify two reasons which explain why there is an energy shortfall between the maximum energy available in the wind and the actual rates energy output of the turbine
Because a significant portion of the available wind energy has to pass through the blades and is unavailable for energy conservation
In addition there will be further energy losses within the gearing and electrical components of the turbine.
Explain the relationship between power output and swept area of a HAWT
The power output is directly proportional to the swept area
If a turbine has a rotor diameter of 6.0m calculate the rotor swept area for the turbine
Radius(r) = 1/2 x diameter = 3m
Swept area(A) = pir2
A = pi x 32 = 28.27m2
Describe how the power output of a wind turbine is affected by the following factors
(1) Air density
(2) Temperature
(1) when air density is lower the power output is less or when the air density is higher the power output increases
(2) when temperature is lower the turbine power output is greater or when temperature is higher the turbine power output reduces
Outline two critical factors that must be taken into account when determining the hub height of a wind turbine installation
Wind resource assessment of the site
Topography of the site
Size of the turbine/ blade length
Visual impact of the turbine
A turbine has a mass of 10 tonnes
Show that the blade length is doubled the new mass of the turbine will be 80 tonnes
Existing mass (10 tonnes) is proportional to R3
If we double the blade length to 2R then
New mass (X) is proportional to (2R)3
Which is 8R3
If R corresponds to a mass of 10 tonnes
Then 2R corresponds to a mass of 80 tonnes
John is considering a wind turbine to power his home. Describe two ways in which the performance of his turbine could be influenced by the following factors
1- blade length
2- strength of materials
3- siting requirements
1- Longer blades could generate more power than short blades due to larger swept area
Longer blades may need stronger wind to generate power
2- Lightweight blade materials may be too weak and may break
Stronger, heavier blades may need stronger wind speeds to generate power
3- Exposed locations provide stronger, more consistent wind
Obstacles can reduce performance
Define the term ‘wind survival speed’
The maximum wind speed that a turbine is designed to withstand before sustaining damage
Wind turbines are designed with a range of power control systems. Name one power control system used in wind turbines
Yawing
Describe the purpose of the yaw mechanism
Yaw mechanism ensures;
Rotor faced the wind at all times
Maximum energy extraction
