Chapter 4: Why Does a Wind Farm Care So Much About Wind Speed?

The Big Idea

The power in the wind depends on the area the blades sweep and on the cube of the wind speed. If the wind speed doubles, the power available goes up eight times (2 × 2 × 2). That is why engineers spend a year or more measuring wind before building a single turbine, and why turbines keep getting taller with longer blades.

Did You Know?

Gujarat and Tamil Nadu have the most wind power capacity in India. India is also preparing its first offshore wind farms in the sea off these two states, where winds are stronger and steadier than on land.

Story Time — Devraj and the Spinning Cups

Devraj, age 13 · Jaisalmer, Rajasthan

Near Devraj’s village outside Jaisalmer, a team put up a thin metal tower, taller than a ten-storey building. At the top were three little cups that spun in the wind. Devraj rode his bicycle over to find out what it was.

An engineer named Ms. Fernandes waved him closer. “This is a met mast, and those cups are an anemometer. It measures wind speed every few seconds, day and night, for at least a year.”

“A whole year, just to check the wind?” Devraj asked. “Yes,” she said. “Because wind speed matters more than you think. If the average wind here is 7 metres per second instead of 6, a turbine can make about 60% more energy.”

Devraj frowned. “But 7 is only a little more than 6.” Ms. Fernandes wrote on her notepad: 6 × 6 × 6 = 216 and 7 × 7 × 7 = 343. “Wind power goes with the cube of speed. 343 is about 1.6 times 216.”

She also showed him a vane that recorded wind direction. “We need to know where the wind comes from, so we can space the turbines. A turbine standing in another turbine’s wake gets weaker, choppier wind.” Devraj rode home thinking that the desert wind, which he had always found annoying, might be one of Rajasthan’s treasures.

Story Time — Nila Looks Out to Sea

Nila, age 12 · Thoothukudi, Tamil Nadu

Nila’s father is a fisherman in Thoothukudi. One evening, a visiting scientist spoke at the village hall about plans for offshore wind — wind turbines standing in the sea, many kilometres from the coast.

“Why go into the sea when we have so much land?” an elder asked. The scientist explained, “Over open water there are no hills, trees or buildings to slow the wind, so it blows stronger and steadier. Offshore turbines can also be much bigger, because huge blades are easier to carry by ship than by road.”

Nila’s father raised his hand. “What about our fishing grounds and the fish?” The scientist nodded. “That is an important question. Before building, studies must check the sea bed, the fish, the birds and the routes that fishing boats use. Fisher communities must be consulted.”

“And how will the power reach land?” asked Nila. “Through thick cables laid on the sea bed,” said the scientist. “Offshore wind costs more to build and repair, because everything must survive salt water and storms.”

Walking home along the beach, Nila looked at the dark horizon and imagined a line of turbines blinking with red lights. “I hope they build them the right way,” she told her father, “so that the sea gives us both fish and power.”

🔢 Numbers Corner

Cube rule: new power ÷ old power = (new speed ÷ old speed)³. From 5 m/s to 10 m/s: (10 ÷ 5)³ = 2³ = 8 times the power.

Myth or Fact?

Myth: A turbine that is not spinning is broken.

Fact: Often it has simply stopped on purpose: the wind is below cut-in speed, above cut-out speed in a storm, or the turbine is under planned maintenance.

🔬 Try it in the Green Energy Lab

Experiment with these ideas in Lab 2: Wind Speed and the Cube Rule.

Brain Buster

Make a simple anemometer with four paper cups and a pencil. Count the turns in 30 seconds on a calm day and a windy day. How different are they?