Where does wind come from?
Band of higher and lower pressure formed around the earth.
- high pressure produced bu cold dry air descending - adiabatic heating creates warm dry air at ground level
- low pressure areas experience rising air, which cools on expansion, creating rain
In circulation about the earths atmosphere, how many cells are formed in each hemisphere?
Three
What are the Coriolis effects?
- owing to the rotation of the earth, the trajectories of objects in the northern hemisphere will deflect to the right
- this causes prevailing westerly winds above 30N as air moves north from high to low pressure regions
- results in vorticity-cyclones (anti-clockwise rotation) around regions of low pressure
Where does higher wind speed occur?
Around regions of lower pressure
Wind speed measurement devices: Anemometers
- cup anemometers
- propeller anemometers
- drag devices (ERA gust anemometer)
- air-pressure devices (pitot tube)
- sonic devices
- doppler devices (LIDAR and SONAR)
- calibrated kites
- tatter flags
Where are anemometers usually placed?
Towers form 10m to 200m
- at lower heights models of wind speed distribution can be used to determine speed at other heights (Prandlt / Power law modelling)
Does the mean wind speed over a day give a good indication of the available energy in the wind?
No, periods of high wind speed and thus power are obscured when measuring a mean.
What do is necessary to model the power in wind in a useful way?
A frequency distribution.
What are two functions used to model the probability distribution of wind speed?
- Rayleigh function
- Weibull function
Wind and energy peak at different speeds. What speeds should a turbine be designed for?
Design for greater than the average wind speed.
