Unit
- Wind energy conversion: The kinetic energy of moving air is transformed into rotational mechanical energy by the turbine rotor, then into electrical energy by the generator. The power available in the wind is proportional to the cube of wind speed, meaning a doubling of wind speed increases available power by a factor of eight.
- Betz limit: No wind turbine can capture more than 59.3 percent of the kinetic energy in the wind passing through its swept area. This theoretical maximum, known as the Betz limit, sets an upper bound on turbine efficiency regardless of design improvements.
- Power curve characteristics: Each turbine model has a unique power curve that defines electrical output at different wind speeds. Three critical wind speeds define operation: cut-in speed (typically 3-4 m/s) where generation begins, rated speed (12-14 m/s) where maximum power is reached, and cut-out speed (25 m/s) where the turbine shuts down to prevent damage.
- Air density effects: Power output is directly proportional to air density, which varies with altitude, temperature, and humidity. A turbine at a high-altitude site produces less power than the same turbine at sea level under identical wind speeds due to lower air density.
- Turbulence and wind shear: Variations in wind speed and direction caused by terrain, obstacles, and atmospheric conditions affect turbine loading and energy capture. Wind shear describes the increase in wind speed with height, influencing tower design and rotor placement.
- Weibull distribution: Wind speed frequency at a given site is modeled using the Weibull probability distribution. The shape factor indicates whether winds are consistently moderate (higher shape factor) or highly variable (lower shape factor). This distribution is used to estimate annual energy production.
- Wake effects: Turbines downstream of others experience reduced wind speeds due to energy extraction by upstream rotors. Proper spacing and layout minimize wake losses, which can reduce total farm output by 5 to 15 percent depending on turbine density and wind direction.
- Capacity factor: The ratio of actual energy generated over a period to the maximum possible if the turbine ran at full rated power continuously. Capacity factors for modern onshore wind farms range from 25 to 40 percent, while offshore sites can exceed 50 percent due to stronger and more consistent winds.
Process Flow
- Step 1 - Site measurement: Deploy meteorological masts with anemometers at hub height to collect wind speed and direction data over at least one full year.
- Step 2 - Data analysis: Calculate average wind speed, Weibull parameters, turbulence intensity, and wind rose to characterize the resource.
- Step 3 - Power curve selection: Match turbine power curves to site wind distribution to maximize energy capture and minimize curtailment.
- Step 4 - Energy yield estimation: Use software tools to simulate annual energy production accounting for wake losses, availability, and electrical losses.
Quick Revision
- Wind power is proportional to the cube of wind speed, making site selection critical.
- The Betz limit caps turbine efficiency at 59.3 percent of available kinetic energy.
- Power curves define cut-in, rated, and cut-out wind speeds for each turbine model.
- Air density, turbulence, and wake effects significantly influence actual energy output.
- Weibull distribution and capacity factor are key metrics for resource assessment.