From a distance, wind turbines don’t appear to move very fast at all. You may wonder how something that moves that slowly can generate any power. But the reality of these feats of engineering is far different.
Wind turbines can spin at an impressive 180 miles an hour at a maximum speed, and that is impressive indeed. They achieve this speed through a series of gears that amplify the wind force acting on the blades and accelerate them to generate the power required.
Some of the key points we will discuss are :
- How wind turbines generate speed
- What happens when the wind gets too strong
- Safe speeds vs survival speeds
- Factors that influence turbine speed
Let’s take a look at the fascinating facts about wind turbine speeds.
How Do Wind Turbines Generate Speed?
Because they are designed to harness the power of the wind, the turbine blades catch the air as it moves over them, which causes them to rotate and so start to generate power.
But the wind is often not enough to get the blades spinning fast enough to create electricity. Hence, the turbine relies on gears to utilize the initial wind speed and then increase the revolutions on the blades to produce electricity.
Wind turbines are most effective when placed in areas with constant wind at regular speeds rather than wind bursts at higher velocities. This makes them more effective and consistent in power generation.
The Physics Of Wind Turbine Speed
Within the design of the turbine are the mechanisms that allow the turbine to generate speed and electricity. The turbine itself, the rotor, and the blades are the three components in a wind turbine that work to create momentum and power.
Each turbine has a nacelle located on the top of the turbine and allows it to automatically orientate itself to catch the wind direction regardless of how it is moving.
Cut-In Speed
You might think that any wind will get the giant blades moving, but they need a minimum wind velocity called the ‘cut-in’ speed to get going. The ‘cut-in’ wind speed is usually from 6mph-10mph.
The blades are connected to a rotor which in turn is connected via a shaft to a gearbox. The rotor will convert a turbine blade speed of 13-20 rpm to between 1500rpm and 1800 rpm!
This massive increase in revolutions is necessary to generate the electricity needed to power the grid or property. The gearbox then transfers this kinetic energy to the generator, which then produces the electrical power.
What Happens When The Wind Becomes Too Strong For A Wind Turbine?
On average, a wind turbine would spin at around 100mph, which is considerable when you look at its size. While they can reach speeds of 180mph, any faster than that will damage the rotors and gearbox, so they have systems in place to cut operation if the wind gets too intense.
Wind turbines are designed to operate to a maximum speed delivering maximum efficiency, and this is known as the rated speed or survival speed. If a storm or similar severe high-wind weather event occurs, a fail-safe system kicks in to stop the turbine and prevent damage.
Anemometer
The wind speed is measured by a vane anemometer attached at the top of the turbine. It measures the wind speed and indicates the wind direction, which is essential so the nacelle can orientate itself to the optimum position.
When this happens, the turbine has two mechanisms to stop the operation: the automatic cut-off and ‘feather blades.’ The auto-cut off is triggered when the wind speed achieves survival speed, and the motor is automatically shut down.
At the same time, there are feather blades mounted to the outer edge of the blade. This is done during the construction phase, and they are designed to reduce excess rotational torque during high winds and then resume normal function once the wind dies down.
What Are The Safe Speeds And Survival Speeds?
The rated safe speed for a wind turbine is between 89mph and 161mph, but the survival speed is usually calibrated at 134mph. The motor is shut down as soon as the anemometer mounted on the top of the turbine registers survival speed.
Factors That Influence Wind Turbine Speed
Three factors influence power generation and blade speed in a wind turbine, and they are :
- Wind power
- Air Density
- The Blade Radius
- Tip Speed Ratio
- Wind Power
This is the most obvious factor as the stronger the wind, the faster the blades will spin, and the more rotational acceleration can be imparted to the blades through the gears.
- Air Density
Air density is a combination of factors, including pressure, temperature, and altitude. The greater the air density, the more pressure is exerted on the blades, and the faster they will spin in the wind.
- Turbine Blade Radius
The bigger the turbine blades are, the larger the surface area is, and this captures more of the wind’s kinetic energy than smaller blades or bladeless turbines. Bigger blades need faster currents to get them moving, while smaller blades can move in lighter winds.
- The Tip Speed Ratio (TSR)
The TSR is a formula used by turbine designers to ensure their blades capture enough air during operation and is defined as the ratio between the wind velocity and rotational speed of the blade tip.
So, to take a simple example, if the wind speed is 30mph and the blades travel at 120mph, the TSR = 4. This means that the blade is traveling four times faster than the wind.
The more efficient blades have a TSR of 6-7, and while the higher TSR ratios are favored, they cannot be too high, or they could damage the motor.
If the TSR is too low, the blade moves too slowly, and a lot of the potential wind energy is being lost as it bypasses the blade.
When the TSR is too high, the blade is then moving through the residual turbulence of the last blade; this would decrease the blade’s efficiency in operation and reduce the efficacy of the turbine to generate power efficiently and consistently.
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