Modern wind farms consist of an array of wind turbines each with a typical capacity of 1 to 8 MW. Each turbine consists of foundations, tower, nacelle, hub and rotor, drive train (gearbox and generator), electronics and controls. Such wind farms:
- are dependent on the wind which may not be blowing. Under these circumstances the load must be taken up by other power plant
- may be located onshore or offshore. Offshore locations involve expensive platforms and undersea cables but usually benefit from higher average wind speeds
- need to be located at sites where the average wind speed is high. Generally speaking, wind speeds are highest on hills and ridges and lowest in sheltered terrain. The order is typically as follows: hills and ridges > open sea > sea coast > open terrain > sheltered terrain
- produce low capacity factors
- experience seasonal fluctuations in water flow which affects the cash flows
- are maintained twice a year
- performance degrades slowly over time
- undergo major maintenance every 20 years when the sails and machinery are replaced. The performance returns to that of a new turbine
- are subject to straightforward taxation calculations but may receive subsidies.
How Promoter handles Wind Power Projects
Promoter assumes others have carried out the design and optimization of the layout and choice of turbine type.
The user chooses the calculation basis from one of the following options:
a) Promoter generated figures (in the early planing phases)
The user inserts the following information:
For the site, the altitude and temperature, the wind characteristics, in particular the average annual wind speed, the wind shear factor and the Weibull distribution density shape factor.
For the chosen turbine, the turbine characteristics, in particular, the power speed curve
Promoter first adjusts the average wind speed for wind shear and the wind farm height and temperature. It then calculates the capacity factor from the turbine manufacturer and the wind speed distribution from the selected Weibull formula. For each element of the wind histogram, it multiplies the percent occurrence by the corresponding element on the turbine power curve. It adds these figures together to get the mean annual power production.
b) 3rd Party figures (once these are available)
The user inserts the calculated capacity factor supplied by the 3rd party
If required, it will repeat these calculations for each month of the year to produce a monthly power production figure.
If required, it will incorporate a cyclical element into the long term mean wind speeds at the chosen site.
Promoter will also calculate the mean annual production for different mean wind speeds between 6 and 14 m/sec.
Although it is an unusual requirement, the user can add additional wind turbine types and/or wind distributions to a single project.
The power efficiency curve comes from the turbine manufacturer. The capacity factor is calculated from this curve and the histogram of wind speeds. The two are displayed in the chart below.
The average wind speed at many locations displays seasonal variations. A cash flow model should take these into account.
Promoter takes into account seasonal variations and these can be clearly seen when displaying charts and reports on a quarterly basis.
The following chart displays the mean annual production for different mean wind speeds between 5 and 13 m/sec
Typical Project Cash Flows
The following diagram illustrates the cash flows on a wind farm project.
The project has a high capital cost but very low operating costs. The chart also illustrates the gradual decline in efficiency and the need for sail and generator replacement after 20 years.