Lecture 21 Wind : Part 1

What Wind Is and What Causes It

• Wind is moving air, generally flowing from a region of higher pressure toward a region of lower pressure.
• The main driver is the Sun’s uneven heating of Earth’s surface. Differences in temperature create pressure differences, which set air in motion.
• The lecture identifies several influences on wind:
  • Solar heating and pressure differences
  • Earth’s rotation, which causes the apparent deflection of moving air through the Coriolis effect
  • Topography, such as mountains, valleys, and water bodies, which redirects or alters wind
  • Seasonal changes, which can produce regional and monsoon winds
• The lecturer notes that a substantial share of incoming solar energy is ultimately converted into wind energy.

Three Broad Types of Wind

Primary or Planetary Winds

These winds circulate on a large scale and tend to blow in consistent directions throughout the year. Examples include:

• Trade winds, flowing from subtropical high-pressure belts toward the equatorial low-pressure belt
• Westerlies, which generally blow from west to east in the mid-latitudes
• Polar easterlies, which generally blow from east to west in polar regions

Secondary or Periodic Winds

These winds change direction periodically. Examples include:

• Monsoon winds, which reverse seasonally
• Sea breezes, which typically blow from sea to land during the day, when land heats more quickly
• Land breezes, which typically blow from land to sea at night, when the sea remains warmer

Tertiary or Local Winds

These winds occur in particular locations and are shaped by local geography and temperature or pressure conditions. Examples named in the lecture include Loo in northern India, the Mistral in France, Chinook winds in the Rocky Mountains, and Bora winds in Europe.

History and Potential of Wind Power

Wind has been harnessed for a long time:

• Before 5000 BC, it was used to propel sailing ships.
• Practical windmills were developed in Persia around the 7th–9th centuries for grinding grain and pumping water.
• Windmills spread through Europe in later centuries.
• Wind-power technology advanced substantially in the late 19th century, including early electricity-generating machines and turbines.
• A megawatt-scale turbine was installed in Vermont in the 1940s, and commercial wind farms expanded rapidly in Europe and the United States in the 1990s.

The lecture describes large untapped potential worldwide, estimating over 70 TW for onshore wind and about 18 TW offshore. It says India’s wind generation is mainly onshore, with Gujarat and Tamil Nadu identified as leading states.

The lecturer gives Indian onshore potential estimates of roughly 3,002 GW and 695 GW at two stated hub heights; the subtitle’s hub-height figures are inconsistent.

Turbines have grown larger over time, with increasing rotor diameters, tower heights, and rated power. Greater height can provide access to stronger winds.

Wind-Power Equation and Energy Conversion

A wind turbine extracts energy from moving air. Its blades rotate a shaft; the mechanical power can then be converted into electrical power by a generator.

The ideal power available in the wind is:

P = ½ ρ A V³

where:

• ρ is air density
• A is the turbine’s swept area
• V is wind speed

The swept area of a horizontal-axis turbine is the area traced by its blades:

A = πr²

A turbine cannot capture all the wind’s energy. The power extracted by the rotor is:

Pₜ = ½ Cₚ ρ A V³

where Cₚ is the power coefficient, representing the rotor’s aerodynamic performance.

Key implications:

• Power increases with the cube of wind speed, making wind speed and site choice especially important.
• Power increases with swept area, so larger rotors can capture more wind.
• Power also depends on air density.
• There is an aerodynamic limit to how much power a turbine can extract; the lecture cites the Betz limit at about 0.59.

Actual electrical output is lower than rotor power because of losses in the turbine, gearbox, generator, and power electronics. The lecture describes overall efficiency as the product of turbine, mechanical, and electrical efficiencies, and gives a typical overall range of 25–45%.

Worked Example from the Lecture

For a turbine with a 40 m blade radius, wind speed of 12 m/s, air density of 1.22 kg/m³, and Cₚ = 0.4:

• Swept area: π × 40² ≈ 5,027 m²
• Power available in the wind: approximately 5.3 MW
• Power extracted by the turbine rotor: approximately 2.13 MW

The lecture emphasizes that this last figure is rotor-extracted power, not the final electrical output after gearbox and generator losses.

Onshore and Offshore Wind

• Onshore wind installations are land-based. The lecture characterizes them as generally less costly and easier to install and maintain, with lower operating costs.
• Offshore wind installations are located in seas or oceans. They can access stronger, steadier winds and support larger turbines and higher energy output, but involve greater initial costs, more complex installation, and often greater distance from electricity demand and grid connections.
• The two settings use different foundations: the lecture names gravity or shallow foundations for onshore installations and monopile, jacket, or floating foundations offshore.

Wind-Farm Site-Selection Method

The lecture outlines the following process:

1. Collect wind data over an adequate period—potentially one or more years—to understand wind availability and patterns.
2. Map the wind resource to identify locations with promising wind potential.
3. Analyze terrain and land to assess the physical suitability of candidate sites.
4. Conduct an environmental assessment to identify potential impacts or constraints.
5. Study grid connectivity, including the distance to the grid and the feasibility of connecting the wind farm.
6. Assess economic feasibility, including the cost of developing and operating the site.
7. Review regulatory and social considerations, such as protected forests, tribal areas, and effects on local communities.
8. Select the site by comparing the findings across these factors.

A suitable site should have high, steady winds; low turbulence; open terrain or a coastal or ridge setting; a short distance to the grid; sufficient open land; limited environmental impact; and good road and transport access for large turbine components.

The lecturer ranks offshore or coastal locations highly, mountains as a favorable onshore option, and plains as less favorable. The Muppandal wind farm in the Kanyakumari region is given as an example.

Speakers and Sources Mentioned

• Speaker: An unnamed lecturer presenting the class; no guest speakers are identified.
• Historical figures or projects cited by the lecturer: Joseph Fredlander (name as transcribed in the subtitles), James Blyth, and the Smith–Putnam turbine.