Wind Flashcards
True or False: Hot air is more dense than cold air
False - less dense
What is the Coriolis Force based on? In northern hemisphere, how will air move?
Coriolis force is the apparent bending force that can be seen due to the earth rotating on its axis.
The movement in the northern hemisphere moves to the right because the earth rotates anticlockwise. So air rises up towards the north pole then to the right and back down to the equator.
True or false: In the Southern hemisphere, particles bend towards the right and wind tends to rotate anticlockwise from high to low pressure.
False - northern hemisphere.
What is geostrophic wind?
Global wind
True or False:
1) Geostrophic wind is driven by temperature differences, and thus pressure differences.
2) Geostrophic wind is very much influenced by the surface of the earth.
3) Geostrophic wind is found at altitudes above 1000 metres (3300 ft.) above ground level.
4) Geostrophic wind speed cannot be measured.
1) True
2) False - it is NOT very influenced
3) True
4) False - it CAN be measured by air balloons
What are prevailing winds determined by?
Geostrophic (global) winds
How does surface friction affect local winds?
It affects wind strength and direction at altitudes near the Earth’s surface - up to 100 metres.
The wind will be slowed down by the earth’s surface roughness and obstacles. It has the greatest influence on local winds.
When do local winds dominate?
When large scale global winds are light.
Describe a local wind - sea breeze?
1) The daytime sun heats the land more quickly than the sea (due to heat capacity).
2) The hot air rises and creates a low pressure at ground level which attracts the cool air from the sea.
3) At dusk there is often a period of calm when land and sea temperatures are equal.
4) At night the opposite occurs, but usually to a lesser extent because the temperature difference between land and sea is smaller at night.
Describe a valley wind.
1) In the northern hemisphere, south-facing slopes & air are heated.
2) Hot air rises up the sides of the valley.
3) Cooler air from above falls down the valley sides to replace the hot air.
4) This generates an uphill wind.
5) At night, this is reversed as the wind direction is reversed & turns into a downhill wind.
What is a canyon wind?
Similar to a valley wind but if the valley floor is sloped, the air may move down or up the valley, as a canyon wind.
What is the tunnel effect?
When air becomes compressed increasing the air pressure on the windward side of obstacles like buildings or mountains. As the wind is channelled between such obstacles its speed increases.
How does the tunnel effect influence wind power?
The wind is too turbulent and is difficult to harness. It also would inflict wear & tear on the wind turbine.
Why is it good to locate a wind turbine at the top of a hill? What should be considered?
Because wind speeds are higher.
Considerations:
- Wide view on prevailing wind side to maximise power output.
- However this is highly visible - social impact.
What is the hill effect?
When wind speeds are often higher than in the surrounding area. This is due to the fact that the wind becomes compressed on the windward side of the hill, and once the air reaches the ridge it can expand again as its soars down into the low pressure area on the leeward side of the hill.
What important factors should be used to determine local wind conditions? Where can data be collected?
Factors:
- Surface roughness
- Obstacles
- Terrain contours
Data:
- Meteorological - rarely collected & may not be available for specific sites.
- Nature (deformity of trees, etc.)
- Turbines in the local vicinity.
What 6 things should be considered when locating a wind turbine?
1) Local wind conditions
2) Undisturbed wind access in the prevailing wind direction
3) Low surface roughness
4) Distance to nearby electricity grid connections
5) Are grid reinforcements necessary?
6) Local soil conditions – foundations and road access
True or false: Wind speed increases with height from ground level.
True
Which wind velocity model, the US (NREL) or Danish (DWIA) suggests a greater power output for any given height?
Danish (DWIA)
What does the gradient in the velocity vs height curve show?
That surface friction causes wind velocity to lower when closer to the surface
Why are turbines located on land so tall (boundary layer & vertical wind shear)?
On land, faster wind speeds are located at greater heights. This is due to that, during the daytime, solar heating due to insolation mixes the surface air with the atmospheric air, thickening the boundary layer depth and reducing vertical wind shear.
Whereas, at night it is much cooler and the boundary layer decouples the surface and atmospheric winds, therefore increasing vertical wind shear.
Taller towers also minimise the bending moment that results from decreased speed closer to the ground.
Why can wind turbines be located at smaller heights when out at sea?
Less surface roughness (no land) and less daytime solar heating because the ocean takes a longer time to heat up than land.
Describe wind gust?
Gust is turbulent eddies that are superimposed on the average flow. Gust velocities are typically 30-50% higher than the mean, and also produce directional oscillations.
What are 2 ways the intermittency of wind can be managed?
Other energy sources like pumped storage and interconnectors
How does the intermittency of wind benefit the electricity demand seasonally?
Demand is higher in winter and more wind is produced in winter
What does the Weibull distribution describe?
It is a probability density distribution that describes the wind variation at a specific site.
What is the Rayleigh distribution and what is it’s significance?
When the shape parameter of the Weibull distribution is exactly 2, it is known as a Rayleigh distribution. Wind turbine manufacturers often give standard performance figures for their machines using the Rayleigh distribution.
What does the median wind speed describe on a Weibull/Rayleigh distribution?
The wind speed where half the time the wind will be stronger than and half the time it will be weaker.
True or False: The UK is the windiest country in Europe.
True
True or False: Wind became the largest renewable energy source in the UK & overtook hydro in 2009
False - 2007
What is the relationship between wind velocity & power output from a wind turbine?
Power is directly proportional to the velocity cubed
What is a capacity factor used for?
To convert peak power to the typical power generated (based on country).
Why can’t all the power generated by the wind’s velocity be used?
The turbine deflects and slows down the wind before it reaches the turbine (but it still leaves with some velocity).
What did Albert Betz observe?
He found that if the wind speed is reduced to ⅓ of its initial speed when approaching a turbine, 59% of the total power is extracted.
List the 3 assumptions made about the wind turbine in Betz Law.
1) The rotor does not possess a hub, i.e. it is an ideal rotor, with an infinite number of blades which have zero drag (drag would only lower this idealized value)
2) The flow into and out of the rotor is axial. This is a control volume analysis, and to construct a solution the control volume must contain all flow going in and out, failure to account for that flow would violate the conservation equations.
3) This is incompressible flow. The density remains constant, and there is no heat transfer from the rotor to the flow or vice versa.
What does Betz Law tell us about the velocity at the rotor?
The velocity at the rotor is equal to the average of the upstream and downstream flow
What is the maximum theoretical efficiency of a wind turbine (based on Betz Law)
16/27
~ 59%
