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Energy Rules!
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Section C. Energy Conversion and the Laws of Thermodynamics
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Most of us don't realize how important energy is in
our lives. Every facet of our life involves energy. One of the reasons
energy is hard to conceptualize is that it is constantly changing from one
form to another. When this happens it is called an energy conversion.
| During these conversions, energy is changing between potential and kinetic forms of energy (For more information refer to What is Energy? Section B. Two Main Forms of Energy.). Potential energy is the energy in matter because of its position or the arrangement of its parts. Kinetic energy is the energy of motion. For example, to operate a wind-up toy, kinetic energy from | |
| winding the toy is converted to elastic potential energy in the toy's spring mechanism. After the spring is released, the elastic potential energy is converted back to kinetic energy when the toy moves. |
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Heat is transferred to the surrounding environment during all all energy conversions. Examples include:
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With each energy conversion, transferred heat leads to a slight increase in the thermal energy in the environment. In other words, this thermal energy is "lost" in to environment (eventually lost in space!) and not useable.
Energy Conversions and the Laws of Thermodynamics
To elaborate more on energy conversions suppose you paid $100 a year to light your home.
What if you found out that only five dollars of this payment went toward
paying for the light? Would you feel shortchanged? What about the other
$95 of energy you paid for? Where did it go?
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If you light your home with incandescent light bulbs, most of your $100 paid for the heat the light bulb generated rather than the light. A light bulb is one of many types of conversion devices. Its purpose is to convert electrical energy to light energy. NOTE: The efficiency of incandescent light bulbs can range from five to ten percent. |
During the conversion process, all the energy that enters a conversion device is turned into other forms of energy. That is, you end up with an equal quantity of energy before and after the conversion. This is another way of stating the first law of thermodynamics that energy can be neither created or destroyed.
However, not all the energy is converted into the desired form of energy (such as light). Although the quantity of energy is the same before and after conversion, the quality is different. An incandescent light bulb has a thin wire filament mounted inside it. When the bulb is turned on, an electrical current passes through the filament, heating it up so much that it emits light. The thermal energy that is produced by the light bulb is often called wasted heat, because it is difficult to use this form of energy to do work.
The energy that is wasted when a light bulb shines exemplifies the second law of thermodynamics that states that with each energy conversion from one form to another, some of the energy becomes unavailable for further use. Applied to the light bulb, the second law of thermodynamics says that 100 units of electrical energy cannot be converted to 100 units of light energy. Instead, of the 100 units that are used to generate light, 95 are needed to heat the filament. NOTE: There are other considerations with developing and using efficient conversion devices, such as costs and government subsidies.
Second Law of Thermodynamics and Energy Efficiency
In terms of energy, efficiency means how much of a given amount
of energy can be converted from one form to another useful form. That
is, how much of the energy is used to do what is intended (e.g., produce
light) compared to how much is lost or "wasted" as heat.
A formula for energy efficiency is the amount of useful energy obtained
from a conversion divided by the energy that went into the conversion
(efficiency = useful energy output / energy input). For example most
incandescent light bulbs are only 5 percent efficient (.05 efficiency =
f units of light out / 100 units of electricity in).
Because of unavoidable compliance with the second law of thermodynamics, no energy conversion devise is 100 percent efficient. Even natural systems must comply to this law (see Energy Through Our Lives - Section B. Energy Flow in Ecosystems)
Most modern conversion devices -- such as light bulbs and engines -- are inefficient. 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, on average 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.
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You might be wondering why improvements have not occurred if there is so much room for increasing efficiency?
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One reason is when light bulbs and other conversion devices were first invented, energy supplies seemed abundant and there was not much concern for the waste heat they generated as long as their primary purpose (light, movement, and electricity) was accomplished. However, as it is becoming apparent that the energy supplies -- primarily fossil fuels -- that we use are indeed limited, one goal of technology has been to make conversion devices and systems more efficient. |
The light bulb is one example of a conversion device for which a more efficient alternative has been developed. This alternative, the compact fluorescent light bulb (CFL), was commercially introduced in the 1980's. 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.
| A single 20-watt compact fluorescent bulb, compared to a 75-watt incandescent light bulb, saves about 550kWh of electricity over its lifetime. If the electricity is produced from a coal-fired power plant, that savings represents about 500 pounds of coal. 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. |
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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 on the system. Therefore, individuals -- whether they are engineers improving an energy conversion device or children turning off lights around the home -- can make significant contributions to energy conservation. (Taken from KEEP Activity Guide "Diminishing Returns.")
Next reading: Energy Rules! -
Section C.2. More about the First and
Second Laws of Thermodynamics.
Return
to Energy Rules! - Section B. Energy
Transfer.
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To return to the
Course
Overview page.
Copyright 2002 KEEP