Turbines

FIELDS OF STUDY: Classical Mechanics

ABSTRACT: Turbines are an important component of many machines, large and small. They use rotational motion to convert energy from wind, water, or steam into electricity. They can also use fuel to power a diverse array of engines, such as those in cars, motorcycles, jets, rockets, and ships.

Principal Terms

• force: the ability to produce motion, often calculated by multiplying the mass of an object by its acceleration.
• impulse turbine: a turbine set in motion by the velocity of a fluid hitting each blade.
• kinetic energy: the work capacity of an object in motion.
• potential energy: the stored work capacity of an object.
• power: the rate at which work is done.
• reaction turbine: a turbine set in motion by the pressure and flow of a fluid.
• torque: a twisting force that produces rotational motion.
• work: the energy transferred to an object by a force, calculated by multiplying the force applied to an object by the distance it has moved the object or the amount of resistance it has overcome.

The Power of Rotational Motion

The word "turbine" comes from the Latin word turbo, which refers to something that spins. First used in the 1800s, the word described machines that harnessed energy from steam. Similar devices, such as water wheels and windmills, have existed since ancient times, but the shift to the term "turbine" corresponded to major advances in technology.

In order for an object to move, something must transfer energy to it. Anything that uses energy to change an object’s state of motion is called a force. If a force moves an object, it has done work. The rate at which work occurs is called power, calculated by multiplying force by speed. Greater force allows work to be done more quickly. Lifting a person straight up off the ground requires a large amount of force. A lever allows force to be applied over a greater distance, which means that less force is required at any given moment. A see-saw illustrates this principle. Two people on different ends of a see-saw can easily lift one another into the air, even if they cannot lift the other person’s weight in their arms. Turbines use torque, a force that produces rotational motion around a central point. Using torque to accomplish a task is similar to using a lever.

Turbines vary greatly depending on their type and purpose, but all turbines have several basic components. Blades (also called vanes) catch the wind or water or respond to the pressure of water or steam. A child’s toy, the pinwheel, demonstrates the way a wind turbine works. A rotor is the central part of a turbine to which the blades are attached. The rotor is connected to a shaft inside the turbine. There may be one or two shafts that spin to power a generator, which in turn creates electricity.

Making Energy More Usable

A turbine is a way to transform energy from an unusable form (such as water, wind, or steam) to a usable form (electricity). Moving water and air have kinetic energy because they are in motion. If water is stored behind a dam, it has potential energy because it is being contained, but it has the potential to move. If the dam is removed, the water’s potential energy becomes kinetic energy as the water flows again. When harnessed by a turbine, both water and air possess the ability to perform work. When water flows through turbines, it powers them with its kinetic energy. All types of turbines are essentially engines that use kinetic energy to cause rotational motion around a central axis. This movement can then power larger systems, such as geothermal or nuclear power plants, as well as water- or wind-powered systems.

The workings of turbines powered by wind and water are visible at wind farms or dams, but it is more difficult to see the workings of turbines powered by steam. As gases heat up, they expand, putting pressure on whatever is containing them. An empty milk jug placed outside on a hot day may pop its top when the air inside it expands too much. This same principle propels the blades of a turbine. Steam entering the turbine is under pressure because it is in an enclosed space, and that pressure forces the blades of the turbine to move.

Steam turbines are used in many types of power plants. In a geothermal power plant, hot water is drawn from underground reservoirs, such as hot springs or geysers, to power turbines that then create electricity. It is also possible to power turbines by concentrating solar energy to heat water and make steam. In a nuclear facility, steam created from nuclear reactions powers turbines to generate electricity.

Turbines may also increase the power of combustion engines that run on gasoline, as in a jet engine or a turbocharged automobile. A gas turbine, like a steam turbine, relies on the ability of gases to expand when heated. Gas turbines are used in jet engines and ships. In an airplane, the action of the turbine creates exhaust that propels the plane forward. In a ship, the turbine drives a propeller, which in turn moves the ship. In these cases, the turbine is not converting moving wind or water to electricity but rather using energy to increase power.

Impulse and Reaction Turbines

Turbines are designed with different environments and needs in mind. A turbine powered by wind is much different than one powered by water. Even among turbines designed for use with water, there is a great deal of variation because water itself varies in volume, speed, and force. When designing hydroturbines (water-powered turbines), engineers consider the water’s flow rate (volume per second) and its head (level or depth). High head is deep water, and low head is shallower water.

The two types of hydroturbine are impulse turbines and reaction turbines. Some turbines, called impulse-reaction turbines, are a mixture of these two types, which means they can adapt to a broad range of conditions.

An impulse is a force that acts for a short time to produce a particular change in momentum. Because of the way an impulse turbine is designed, force hits each blade in sequence, causing it to move. As the force hits each blade, the rotor assembly spins. An old-fashioned water wheel demonstrates the physics behind impulse turbines: the water hits one blade at a time, turning the wheel. In a reaction turbine, the blades move in response to the steady application of force—that is, they react. Instead of water hitting each blade in sequence, it flows over the entire assembly, ideally with constant pressure.

An impulse turbine works best when hit at high velocity. Water hits each turbine blade with a lot of force and propels it. A reaction turbine does not require as much force to be set in motion. An impulse turbine would be suited to harnessing the energy from a small, fast-moving river, while a larger, deeper, slow-moving river would be better served by a reaction turbine. In real life, rivers can change seasonally or in response to storms, so many hydroturbines use combination impulse-reaction turbines in order to work with variable flow.

A wind turbine may be thought of as a reaction turbine because wind moves over the entire turbine, setting it in motion. The two types of wind turbine are vertical axis and horizontal axis. Horizontal-axis turbines resemble pinwheels and operate in much the same way. A vertical-axis turbine operates like a kitchen mixer; both the axis and the blades are oriented vertically, with the blades curving to catch the wind.

Turbines and the Future

Turbines facilitated the advent of the Industrial Revolution, and they continue to serve an important role in industry and technology. They are a part of almost every engine, from jets to cars. Turbines are also an important part of both alternative and fossil-fuel-based energy production. As a result, many innovators are working to streamline turbine operation and expand how turbines are used. For instance, in the wind industry, engineers are developing lightweight turbines that can hover high in the air to catch better air currents. They are also experimenting with adding a second rotor to traditional wind turbines in order to increase efficiency. In the water-power industry, turbines already generate power from rivers and tides and will be a part of generating power from tidal lagoons. In addition to industrial applications, engineers are developing hydroturbines for small-scale individual use, such as generating power for personal electronic devices.

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