University of Wisconsin-Stevens Point > CNR Associated Programs > Wisconsin K-12 Energy Education Program > Energy Education: Concepts and Practices > Section D: Energy Efficiency and the Second Law of Thermodynamics
​Unit 2: Energy Rules!

Section D. Energy Efficiency and the Second Law of Thermodynamics

The First Law of Thermodynamics states that energy can neither be created nor destroyed, it can only be converted from one form to another. This may suggest that we could always convert energy to whatever forms we need without ever worrying about using up our energy resources.

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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.

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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 device is 100 percent efficient. Even natural systems must comply to this law (see Energy Through Our Lives - Section D. 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.

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 more efficient alternatives have been developed. One 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 CFL bulb feels cooler than a 75-watt incandescent light bulb. The CFL converts more electrical energy into light, and less into waste heat. Typical CFLs have efficiencies between 55 and 70 percent, making them three to four times more efficient than typical incandescent bulbs with efficiences under 20 percent. Another alternative, the light emitting diode (LED), has become more mainstream and affordable in recent years. LED's bring currents with a positive and negative charge together to create energy released in the form of light. LED's have efficiencies between 75 and 95 percent, making them four to five times more efficient than incandescent bulbs. LED bulbs can also last anywhere from 20,000 to 50,000 hours, or up to five times longer than any comparable light bulb.

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A single 20-watt compact fluorescent light bulb (CFL), 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. 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 mega-watt coal-fired plant could be retired. Imagine what replacing them all with LED's would save!

Heat is transferred to the surrounding environment during all energy conversions.

Examples include:

The chemical energy in food that is converted to mechanical energy (moving our muscles) by a process similar to burning called respiration. Energy is needed to break apart the food molecules, and during the process, thermal (heat) energy is generated. Feel your arm; this warmth is the energy that is released by respiration within your cells.