Sunday, May 30, 2010

A Primer on Electricity Generation

Most people in the United States take electricity for granted. Whenever we need something we simply plug it in and electricity magically powers whatever instrument we have. Having been in the energy business for my entire adult life, I thought I would write an introduction for the average person on where electricity comes from. Since the country is currently debating items that could drastically change how energy is produced in the US, it is important that people understand the basics how where electricity comes from. After reading this, you will probably know more about electricity production than 90% of the adults in America.

To begin let me start with what everyone does know about. Our homes. Most homes in America have two services of electricity: 240 Volt AC and 120 Volt AC. There are normally three wires that come into your home, two 120 volt wires and one ground wire. Large appliances like oven ranges, dryers, electric furnaces, and air conditioners use 240 volts, while all of the rest of your home uses 120 volts. The first thing to remember is don't ever try to plug a 120 volt appliance in a 240 volt outlet. For one, the plugs are different so it shouldn't work. Secondly, you are going to fry your appliance. If you do try to plug your 240 volt appliance in your 120 volt outlet most likely, you appliance won't work (it is not getting enough electricity), although it probably won't be permanently damaged.

AC stands for alternating current. This means that the direction of flow changes. In the US, we use 60 Hz or 60 cycles per second. So the direction of flow switches 60 times each second.

As an analogy, voltage can be looked at as pressure. Water pressure (from a water tower) pushes water through the pipes which is why it comes out when you turn the faucet on. Voltage pushes electrons (electricity) through the wires so that when you plug something in, it turns on. No voltage, no electricity. The other key term is amps or amperage. For the analogy, amperage is like the amount of electricity that is flowing. When a water faucet is off, there is pressure (voltage) in your pipes, but no water flow. When you turn the faucet on, you get water flow (amps).

Besides your normal appliances and lights, your doorbell runs on electricity as well. This is usually a lower voltage of around 12 volts. To do this, a transformer is connected to a 120 volt circuit. A transformer is a simple device that has two sets of windings, input and output. Based on the ration of loops per winding, will determine what voltage the output will be.

All of the electricity in your house flows in through a meter and a breaker box. The breaker box allows loads (lights, appliances, etc.) to be disconnected without affecting the rest of the house. The meter is where the electric company measures the amount of electricity you are using so that it can bill you.

Working backwards, the 120 volt lines that run to your house are probably coming from a transformer. As before, the transformer steps down the voltage from a higher voltage. Higher voltage allows for more efficient distribution of electricity over long distances. Depending on your locale, the power lines you see overhead could have any number of different voltages and more than likely there are several transformers in between your home and the major power lines. Major power lines have voltages in the neighborhood of 14,500 volts to 345,000 volts or more.

The major power lines run through substations. Substations are sort of like your breaker box for your home. They may include stepdown transformers, but they also include breakers and disconnects so that electricity can be cut off to different areas of a city or county without affecting the entire network. Neighborhood substations feed into major substations.

Major substations are like the clearinghouse for power. Major substations may be near a power plant or not. In all cases, major substations have incoming lines from several sources. Our modern reliance on electricity makes it impossible for a city to lose power just because one power plant is not producing electricity. It is not a matter of backup sources, all of the sources are the primary sources. To return to the water analogy, a city may have only one water tower, however it might use a dozen wells to fill that water tower. The residents of the city never notice that any of the wells are not working as long as the water tower remains full. The major difference, is that a water tower is storing water, electricity in the electrical distribution system is either used or not created. (More on that later.)

Major substations are fed by power plants. Besides some solar power plants (which account for less than 1% of all electricity production in the US), electricity at a power plant is created by a generator. A generator is a machine that rotates magnets passed wires (or wires past magnets). This creates electricity (you can generate your own electricity with a magnet and a piece of copper wire - but don't plan on running anything from it). Electricity and magnetism are related. Electricity creates magnetic fields and magnetic fields create electricity. A generator uses rotational energy to create electricity from wires and magnets (a motor is the opposite, it uses wires and magnets to create rotational energy).

A quick word about solar plants. There are really two types, direct conversion and generator conversion. Direct conversion uses solar cells (like what is found on a calculator) to generate electricity. The materials on the solar cells produce electricity when light shines on them (conversion of visible light to electricity). Generator conversion types use a generator like any other power plant.

The generators are turned by a turbine. A turbine is another rotating machine that has blades attached to it. As a fluid (usually steam) passes over the blades, it causes the turbine to rotate (like a pinwheel in the wind) which turns the turbine. Hydroelectric dams use liquid water to rotate specially designed turbines. Wind generators use a wind turbine which is open to the environment because air is the fluid that is pushing the blades. But the blades on the windmill are connected to a generator. All other power plants (coal, gas, nuclear, solar) use the same type of turbine, and in some cases if a company (say GE) designed two different types of plants (say nuclear and coal) at the same time (say 1975), it is very possible that they have the exact same model of turbine and generator. So except for those solar and wind plants I already mentioned, from the turbine out to the electric distribution lines they are all the same (and in some cases exactly the same).

And since all of the electrical lines are interconnected, the electricity is all the same. In other words, lets say that you are a rabid environmentalist and want to get all of your electricity from wind power. If you have a wind turbine in your yard and your house is not connected to the grid, then congratulations. However, if your house is connected to the grid, then you are getting electricity that was made from coal, oil, natural gas, nuclear, solar, and wind. There is no device you can put on your house that is going to filter out the "bad" electricity. 1 kilowatt of electricity from coal is indistinguishable from 1 kilowatt of electricity from solar.

So, why hasn't wind and solar energy taken off, in spite of promises that it would for the last 40 years? In a nutshell, the answer is energy density and reliability. (Coincidentally, the same issue that has kept the electric car from being the savior of mankind since the late 1800's). Energy density could be described as the amount of energy that can be derived from a certain volume. Wind and solar's energy density sucks rocks. Sure it is all over the world, however it is in such small amounts that huge tracts of land are needed to supply energy. For instance, an average size power plant is around 1000 MW. For a coal, oil, natural gas, or nuclear plant all of the equipment can be located on 50 acres or less (with room to spare and build walking paths for the employees at lunchtime). A comparable wind or solar farm would need several thousand acres to produce the same electricity. Wind does have an advantage over solar in this case, because 95% of the land could still be used for farming or other uses.

Energy density has to do with the fuel itself. Nuclear has an extremely high energy density. 95% of the mass of nuclear fuel is made up of non-fuel (uranium 238 that isn't fissile, stainless steel, zirconium, and some other metals). In spite of this, a nuclear plant produces only a few hundred tons of waste each year (which includes the 95% of non-fuels in the fuel). Compared to a coal plant which produces millions of tons in ash alone.

As you can begin to see, every type of fuel has its tradeoffs. So reliability is coupled with energy density to determine what fuels we use to make electricity. We need electricity all day long, although in varying amounts. Simplified, there is a base load and a varying peak load. The base load is the minimum amount of electricity that is needed. Plants that produce baseload need to be the cheapest plants that can be continuously run. Since plants will always be shut down for maintenance on a periodic basis, there is little to no "extra" base load, unless a new plant is started up. Coal, nuclear and hydroelectric plants are the vast majority of this country's baseload. Coal and nuclear plants also cannot just be started and stopped at will. The process takes several hours to days.

Peak load is during those times (daytime, early evening) when more electricity is demanded because of industrial needs. In hot climates, the use of air conditioning can greatly affect the peak demand. Because this electricity is not needed at all times, certain peaking plants are used. Natural gas is the primary fuel for these plants. Gas turbines plants are basically glorified jet engines (really, really BIG jet engines). They can be started up rapidly, and are designed to go through lots of start stop cycles. Solar is a great option for peaking plants since it is available at the prime peaking times. Unfortunately, it is not predictably available at the peaking times. Cloud cover can decrease the effectiveness of solar power to near nothing, so any solar plant is going to have to be backed up by a gas turbine plant.

Wind is even less predictable than solar. While some areas have great wind resources, Mother Nature doesn't always cause the wind to blow like the average of the last 30 years. Sometimes it is significantly below that. And when the wind isn't blowing, there is still the electricity demand. So while wind and solar may someday provide up to 20% of the electricity we use, there will be another 20% of unused fossil fuel capacity just in case. Running two power plants (even though one is inactive) is more expensive than running one.

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