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The word efficiency describes how much of a given amount of energy can be converted from one form to another useful form. The amount of usable energy that results from the conversion process (electricity generation, lighting, heating, movement, etc.) is significantly less than the initial amount of energy. In fact, of all the energy that is incorporated into technologies such as power plants, furnaces, and motors, only about 16 percent is converted into practical energy forms or used to create products. Where did the other 84 percent go? Most of this energy is lost as heat to the surrounding atmosphere.

Light Bulbs

The light bulb is one example of a conversion device for which a couple of more efficient alternatives have been developed. One alternative, the compact fluorescent light bulb (CFL), was commercially introduced in the 1980s, and LEDs have become more popular in recent years (first invented in 1962).

vs. vs.
  Compact Fluorescent Lamp (CFL)   Light Emitting Diode (LED)

Comparison: A single 20-watt compact fluorescent bulb, compared to a 75-watt incandescent light bulb, saves about 550 kWh of electricity over its lifetime. If the electricity is produced from a coal-fired power plant, that savings represents about 500 pounds of coal. 

Instead of using an electric current to heat thin filaments, the CFLs use tubes coated with fluorescent materials (called phosphors) that emit light when electrically stimulated. Even though they emit the same amount of light, a 20-watt compact fluorescent light bulb feels cooler than a 75-watt incandescent light bulb. The CFL converts more electrical energy into light and less into waste heat. CFLs have efficiencies between 15 and 20 percent, making them three to four times more efficient than incandescent bulbs. NOTE: There are other considerations with developing and using efficient conversion devices, such as costs and government subsidies. 

Energy Conversion

Many power plants in Wisconsin use coal to generate electricity. It is not possible to take a chunk of coal and use it directly to light a bulb. The chemical energy in coal first needs to be converted to electricity (this is true for other resources such as wood, oil, and natural gas). Therefore, which energy resources are used also affects the efficiency of modern electrical appliances (or conversion devices).

This conversion process requires several steps. The coal is mined, crushed, and transported to the power plant. Then it is burned to generate electricity. Finally, the electricity is delivered to our homes through transmission wires and is then put to some end use, such as lighting a bulb. Each of these steps in the coal-fired electrical system uses energy from other sources (e.g., gasoline for transportation) or involves an energy conversion. Therefore, the efficiency of converting electricity to light depends on the efficiency of each step. The total efficiency of the whole process is called system efficiency. It is equal to the product of the efficiencies of the individual steps. The system efficiency for an incandescent bulb is only 1.3 percent.

Energy Efficiency Today

Even though the process of coal-to-electricity has low efficiency, it is an improvement over earlier electrical systems. The efficiency of power plants has risen from 3.6 percent in 1900 to about 33 percent today. Scientists and engineers are developing new technologies to make power plants even more efficient and to improve electrical transmission. Another approach to generating electricity more efficiently is to use alternative energy sources and conversion systems. Hydroelectric plants are 90 percent efficient at converting falling water to electricity, and the best efficiency of a wind turbine is 47 percent.

The efficiency of any system is affected by the efficiency of every conversion within the system. Therefore, an individual — although at the “end” of an energy conversion system — CAN make noteworthy contributions to the efficiency of the whole system. An example of this is replacing incandescent light bulbs with compact fluorescent (CFL) or light-emitting diode (LED) bulbs. This simple change raises the overall efficiency of a coal-fired electrical system from 1.3 percent to five percent. This may not seem like much of an improvement, but the cumulative results of many people doing this are massive. For example, if every household in Wisconsin replaced one 75-watt incandescent light bulb with a 20-watt compact fluorescent bulb, enough electricity would be saved that a 500-megawatt coal-fired plant could be retired.

Installing efficient light bulbs is just one action people can take to improve system efficiency. Other efficient electrical appliances, such as water heaters, air conditioners, and refrigerators, are available and becoming more affordable. You can easily recognize energy-efficient appliances by looking for the Energy Star® label. Turning off lights and other devices when not in use also creates less demand for the system. Therefore, individuals — whether they are engineers improving an energy conversion device or they are children turning off lights around the home — can make significant contributions to energy conservation (Excerpt from KEEP Energy Education Activity Guide “Diminishing Returns“).

The following tips were taken from the Consumer Guide to Home Energy Savings, Seventh Edition, written by Alex Wilson, Jennifer Thorne, and John Morrill and published by American Council for an Energy-Efficient Economy (ACEEE). 

To-Do-List for Today:

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In the Classroom: