Physics: Principles and Problems

Chapter 22: Current Electricity

In the News

The biggest machines in the world: Electric power systems

December 2004

What keeps the lights on:

Now that you are familiar with electric circuits, you'll be able to understand the vast electric power 'grid' that keeps our lights lit and our industries healthy. The power system in North America is by far the largest in the world. Except for fuel used in transportation and heating, all our energy resources are converted to electric power.

An electric circuit that's the size of North America

Our electric power grid is a circuit with many resistances and many sources, all of which are connected in parallel. In electric power work, the resistance of your house or of your city is known as a 'load,' because the generators supply energy to it.

All of the electrical appliances in your home are connected in parallel. Every home, business, and industry in a city is connected in parallel with every other. All cities are connected in parallel with each other. And every generator is connected in parallel with every city and with every other generator. This way, a disabled generator will not prevent the other generators from supplying the loads, nor will the switching of a load affect the service to others. There are about 16,000 generating stations in the US, ranging in capacity from 200,000 watt diesel units to nuclear plants generating over 1000 megawatts. Generators in Canada and Mexico connect with ours.

It takes energy to crank the generator. Here's where it comes from:

Since the mountains in the United States are mostly dry, only 6% of our electric power is produced by hydroelectric facilities like dams and waterfalls. 1% is supplied by wind or solar energy, and another few percent by diesel engines and geothermal sources. The remainder is supplied by generators powered by steam that is generated in great boilers heated by fossil fuels or by a nuclear reactor. 20% of our power comes from nuclear reactors and about 50% from coal. It is far cheaper to carry energy over a power line than it is to carry coal. Therefore, most coal-fired power plants are located next to coal mines.

Heat is created by a stream of natural gas, powdered coal, or oil spray that is blasted into the furnace by compressed air. The intense flames play on heavy pipes, flashing the water in the pipes into steam at 1000 psi and 1000 deg F. So energetic is this steam that an iron bar exposed to it will turn red hot, and even a small steam leak can sever nearby steel pipes. The steam rotates a great turbine engine that is connected through a thick shaft to the generator.

A one-million horsepower engine. Really.

Steam from the boiler strikes the smallest set of blades in the turbine and expands, forcing the blades to turn. The expanded steam then rushes through nozzles onto a larger set of blades to spin them, and onto a stage of larger blades. Water cooled in a huge, spool-shaped cooling tower condenses the steam back into water to be recirculated back to the boiler.

Delicate steam control valves ensure that the turbine and generator rotate at precisely 3600 revolutions per minute. The generator and turbine rotate in bearings cushioned with high-pressure steam; the entire assembly can be turned with one finger.

Look up at those power lines with respect. They keep everything running.

Power lines connect the generators to the loads and to other generators. The voltage used on a power line depends on its length. Some run for hundreds of miles. In general, any power line supported by steel towers has a voltage of 138,000 volts or more, up to 765kV. The lines are kept high on towers to protect people and to provide insulation from the earth. The conductors, insulated from the tower by long strings of insulators, are up to three inches thick, bare aluminum strands wound around a high-strength steel core. Special high voltage power cables can also be buried: their carefully-engineered insulation withstands up voltages up to 300kV.

Getting power to your town, and protecting the power line that does it:

If you look at a major power line from an airplane you'll note that it occasionally dips down to supply large step-down transformers at each town or neighborhood. At each of these 'substations' is also a set of great switches, or 'circuit breakers.' These are controlled by sensitive instruments that monitor the voltage across the line and the current through it. If a lightning stroke or other disturbance affects the power line, the current is instantly shut off. When the instruments detect that the disturbance has passed, the line turns back on once again. This is why your lights flicker on and off in a thunderstorm.

Activity:

Learn to identify the electric power lines that supply your house and your street. They are the top-most wires on the pole. If there is a streetlight on the pole, they'll be above the streetlight.

Find and visit the electric generation station that is closest to your home or school (ask your electric company.)

Resources:

http://science.howstuffworks.com/power.htm

http://en.wikipedia.org/wiki/2003_North_America_blackout

Gasoline prices and environmental concerns have raised interest in hydrogen-powered automobiles. How practical is the concept?

March 2005

Where do we find hydrogen, anyway?

When hydrogen gas is burned in an engine or used to generate electric power in a fuel cell, nothing is left over but water vapor. But hydrogen gas is not a practical source of energy because we do not have hydrogen wells. We find little H2 in the atmosphere because it tends to float off into space. Moreover, hydrogen is a very reactive element and so always combines with other elements and so is not available as a gas in nature. Its most common compounds are water and the hydrocarbons.

Water contains hydrogen, and there's plenty of water. But:

You already know how to obtain hydrogen from water. Electric current is passed through water between two electrodes. Oxygen gas bubbles up from one electrode, hydrogen gas from the other. In commercial practice, the hydrogen is gathered, dried, and compressed into tanks at high pressure. It can then be burned or used in a fuel cell.

Note, however, that electrical energy is required to pull the water molecules apart. The energy we obtain from burning the hydrogen can be no greater than the electrical energy used in the separation process. In fact, the dissociation of water requires more energy than we get back because the water is unavoidably heated by the electric current. This heat cannot be recovered.

Where the energy really comes from:

Thus hydrogen obtained through dissociation cannot be considered an energy source, but only a means to store energy. The electric power for the dissociation process is not an energy source, either: it is a means to transmit power from the engine which turns the generator. The engine is usually powered by steam; which is in turn generated by heat from a fire—fueled by oil, gas, or coal—or from the core of a nuclear reactor. These resources are limited for various reasons, and their use unavoidably affects the environment.

The price of alternative energy:

Even so, science writers have long predicted a “hydrogen economy,” in which cars would be fueled by hydrogen gas dissociated from water at fossil-fuel or nuclear electric generating stations. This could work if we wished to spend the money: we would have to expand our electric power system and accept the handling difficulties inherent with hydrogen gas, including the rebuilding of our present motor fuel distribution network. A process in which ethanol is converted into hydrogen gas is being developed, but it is expensive; at present it takes more energy to distill the ethanol than can be recovered from the hydrogen.

Where get the hydrogen to show that hydrogen cars could work:

It takes far less energy to remove hydrogen from hydrocarbon compounds like natural gas. It is only necessary to heat the gas in the presence of a catalyst separate the hydrogen from the carbon. While this catalytic process is quite practical—it supplies all of the hydrogen used by industry, including that used for the demonstration of hydrogen-powered cars—it requires natural gas, which is one of the fuels we strive to replace.

Activity:

Would an electric car save energy and prevent pollution? (Consider the sources of the energy.) Why might people think that hydrogen-powered cars would be completely 'clean?' If hydrogen cars turn out to not be practical, will the effort that went into research and development have been wasted?

Reference:

http://www.emagazine.com/january-february_2003/0103feat1.html

http://www.alternet.org/story/15239

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