All About Power
January, 2001. While preparing dinner on the space station, Cosmonaut Ivanovich floats into the Zvezda service module where the galley is stocked with prepackaged food and drinks, a table, eating trays and utensils, a food warmer, and a water dispenser. Opening the food storage bin, Ivanovich looks through his personal stash of food. Everything is labeled with color-coded stickers. After a moment he selects dehydrated shrimp cocktail, macaroni and cheese, irradiated beefsteak, thermostabilized fruit cocktail, and strawberry punch. He takes out his color-coded eating tray, knife, and fork, attaches the tray to the table with Velcro straps and secures his metal utensils to the magnetized tray. He then opens a small hole in the shrimp and macaroni packets and inserts the water nozzle. The macaroni and beefsteak are placed into the food warmer. Cosmonaut Ivanovich eats his shrimp and waits for his food to warm.

When the space station is completed, it will have a shiny new galley with an oven, a freezer, and two refrigerators. Until then the astronauts must make do with a single food warmer. The food warmer is a small, portable unit that plugs into a special outlet in the galley. Its warming plate can heat fourteen food packages at once. When Ivanovich flips the switch, he doesn’t think about the electricity that heats the warming plate. Just like most of us, he takes electrical energy for granted. However, Cosmonaut Ivanovich is prepared to act in case of an electrical power emergency.

The food warmer, like the microwave oven and food blender in your home, is connected to the space station’s electrical circuits by an electric cord consisting of two insulated wires. The cord runs from the appliance to a connection in one of the circuits in the space station. When it is turned on, it "draws" electricity from the station’s electrical power circuit. Electricians use the term "draws" to describe the watts of electrical power required to run an electrical appliance.

This article will help you learn some of what Commander Ivanovich already knows about the electrical power systems on board the space station. You will become familiar with:

      • the fundamentals of electricity
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      • the three components of an electrical circuit: power source, conductor, and load
      • the term "watts."

Fundamentals of Electricity
Electricity, electrons, and atoms
As you may already know, everything is made of atoms. All atoms but one have three basic components, neutrons, protons, and electrons. The hydrogen atom has no neutrons. The smallest nuclear particle is the electron. Electrons have a negative electrical charge. Electrons surround the nucleus of an atom in what today’s physicists call energy levels, or electron clouds. Another way to envision the atom’s structure is to use the Bohr model of the atom, illustrated below. At the time it was conceived, the Bohr model suggested that the atom’s electrons orbited the nucleus as the planets orbit the sun. Neils Bohr was a Danish nuclear physicist, a pioneer in studying the atom. He received the Nobel Prize in 1922 for his theoretical atomic model. Today, physicists have created far more sophisticated models based upon almost 100 years of research.

The nucleus of an atom consists of neutrons and protons. Neutrons have no electrical charge. Protons, however, have a positive electric charge, a positive charge equal and opposite to that of the electron. In atoms, the electrical charges of the protons in the nucleus and the electrons spinning around the nucleus counterbalance each other.

Under certain natural or manmade conditions, when exposed to chemical, magnetic, or solar energies, for instance, it is possible to separate an electron from some types of atoms. When this happens, the atoms become positively charged because they have one more proton than electrons. An atom with an electrical charge, either positive or negative, is called an ion. An atom that loses or gains an electron is said to be "ionized."

Copper, silver, and gold atoms, used most commonly in electrical wires, give up their electrons more easily than other known elements. A copper wire is full of literally trillions of copper atoms. When we attach a source of electricity to one end of a wire, we initiate an instantaneous flow of electrons throughout the wire. This flow of electrons is called electricity.

Electrical Circuits
Every circuit has three elements. The first element is an electrical source. Electricity can be generated using chemical, photovoltaic, or magnetic/mechanical devices. Examples of these electrical sources would be a battery, a solar cell, and a spinning electromagnetic generator. There are many types of batteries and many ways to make an electromagnet spin including nuclear power, coal, wind, and water. Regardless of the methods used each electrical source has both a negative and a positive terminal. The electrons generated by the source move from the negative terminal through a wire or cable and back to the positive terminal. The electrical force created by the source is called the electromotive force, or emf. The emf of a circuit is measured in terms of volts.

The second element in a circuit is a conductor. The wire or cable mentioned above is an example of a conductor. Wires are normally made of copper or some other appropriate material. The conducting material is normally wrapped in an insulating material. When a conductor links the two terminals of the electrical source, the electricity begins to flow from the negative terminal to the positive terminal.

The third required element in a circuit is called the load. A circuit without a load accomplishes nothing. Add a load, which might be a light bulb, a toaster, a warming plate, or a hair dryer, and the electricity goes to work. The work done by the appliance can take many forms. Two of the most common forms of electrical "work" are the creation of heat or light and the energy needed to spin a motor.

The Space Station’s Power Source
On board the space station, one source of electrons is the thousands of solar cells organized into what are called the photovoltaic, or PV arrays. It is within the solar cells that sunlight (electromagnetic energy) is converted into a flow of electrons. The electricity from the PV arrays powers the space station while it is in the light of the sun. It also recharges the Nickel-Hydrogen batteries. Batteries are the space station’s secondary source of electricity and take over the job of electrical production when the space station is in the earth’s shadow.

Good Conductors vs. Good Insulators vs. the Semiconductors that Changed the World

In order for electricity to flow from the power source to the load, there has to be something conducting it, something that will allow the free flow of electrons. The following materials have atomic structures that make good conductors.

    • Metals like gold, silver, copper, zinc, steel, etc.
    • Carbons
    • Acids
    • Water (with electrolytes)

Gold is one of the best conductors. It is also very expensive, which is why electrical wires are commonly made of copper. It is far less expensive than gold and has excellent conductive properties.

Some materials consist of atoms that resist giving up their electrons under all but the most extreme conditions. Here is a list of common materials that are considered good insulators.

    • Dry Air
    • Wood
    • Porcelain
    • Glass
    • Rubber
    • Plastic

Insulating materials play a very important role. Plastic, for instance, is used to wrap the copper strands in wires. The plastic separates us from the flow of electrons every time we plug in a hair dryer or an electric razor. Humans, because we are made of a large percentage of water, are excellent conductors of electricity. A car’s rubber tires insulate passengers during a lightning storm. Glass materials are woven into gloves used by electricians who work with heavy power lines. Porcelain is used on telephone poles and power lines to minimize the risk of electric shock.

Semiconductors are materials that conduct electricity under some conditions but not others. This attribute permits scientists to use them to control the flow of minute amounts of electrical current. Semiconductors permit the precise design and control of current within computer chips. The silicon wafers in solar cells and the layered carbon disks used in transistors are all examples of semiconductors. In the last 50 years, semiconductors in computers have changed the course of mankind.

Power Loads
Electricity flows around a circuit like a runner around a quarter-mile track. A running race has a beginning and an end. An electrical circuit begins at the negative terminal of a battery or power plant, and ends at the positive terminal. The circuit’s track is usually a wire, sometimes a heavy cable. A circuit also has to have a load, something for the electricity to do during its journey. This might mean lighting a light, turning a fan, or heating an oven. A circuit doesn’t need a switch, but as in every well-run race, electrons behave more effectively when told when to start and when to stop. A switch is the starter and race judge, all in one.

Different kinds of circuits are designed to handle different types of power loads. A flashlight contains a simple electrical circuit. It uses 1 or 2 batteries as a power source, often has copper strips for conductors, and has a light bulb for its load. This circuit provides a constant source of electrical current to the filament in the bulb. What happens as the batteries run down? The bulb begins to dim.

Electrical engineers design circuits to handle a specific power load. In your home, you have a number of circuits all attached to a breaker box that is attached in turn to a transformer that is in turn attached to the main power lines in the street. Inside your house, some circuits service only one high-load device such as an electric clothes dryer or an air conditioner. Kitchens, bathrooms, and garages, where appliances such as hair dryers and power tools are used, may have only a single circuit because only several appliances may be turned on at any one time.

When you plug something into a socket, you are attaching it to a circuit. When you have too many power loads on a circuit operating at once, you may cause the fuse to blow or the circuit breaker to trip. This emergency switch stops the flow of electricity. This is a safety measure put in place by the home’s designer. This safety measure prevents fires by limiting the flow of electricity through the wires. Wires are designed with specific electrical currents in mind. Too much electricity may cause overheating of the wires, a dangerous source of home fires.

The space station has a circuit breaker system run by computers. One of the critical circuits on the space station is dedicated to the life support equipment. Another is dedicated to Cosmonaut Ivanovich’s kitchen. The appliances in your house and the equipment on the space station all represent the load on the different circuits.

Power is measured in watts (W) or kilowatts (kW). Look at the glass end of a light bulb. It tells you how many watts of power are needed by the light in order for it to shine. A 1000-watt light bulb burning for one hour consumes 1kWhr of electricity. A 100-watt light bulb burning 10 hours consumes the same. A kilowatt equals 1000 watts.

The PV arrays of the space station can produce 24,490 watts of electrical energy or 24.49 kilowatts per hour. This amount of electrical power is divided among all of the circuits on the space station. Computers regulate this division of power. During an emergency such as a solar proton event, Mission Control and the Mission Specialists make recommendations to the astronauts who can manually regulate the distribution of electrical power, or watts, to the station’s various circuits.