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.

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.

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!

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 EnergyStar® 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 Energy Education Activity Guide"Diminishing Returns.")


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. 

    • Let's say you are using the energy you gained from food to operate a pair of scissors. Heat is transferred (lost) during this activity, too. There is friction when the blades of the scissors slide against each other to cut paper. Friction, the resistance to sliding, rubbing, or rolling of one material against another, requires extra work to overcome and results in energy loss through heat. This thermal (heat) energy escapes into the environment. (Taken from KEEP Energy Education Activity Guide "Station Break.")

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. 


The Second Law of Thermodynamics

During energy transfers, it might seem that energy does go away or become reduced. For example, a bouncing ball stops bouncing, a battery dies, or a car runs out of fuel. The energy still exists but it has become so spread out that it is essentially unavailable. Burning a piece of wood releases light and thermal energy (commonly called heat). The light and heat become dispersed and less useful. Another way to describe this process is to say the energy is concentrated in the wood (chemical energy), and becomes less concentrated in the forms of thermal and light energy.

Let's return to the frantic cat in the room with the puzzle. Although you might be able to find all the pieces of the puzzle after the cat's actions, you cannot put the puzzle completely back together. Some pieces have been bent, others torn, and some the cat, well, use your imagination. In other words, although the quantity of the puzzle remains the same, its quality has been compromised. This cat story is a rough analogy to the second law of thermodynamics.

The following set of statements are various ways of expressing the second law of thermodynamics:

* With each energy conversion from one form to another, some of the energy becomes unavailable for further use.

* Heat cannot flow from a cold object to a hot object on its own.

* It is impossible to convert heat energy into work with 100 percent efficiency.

* You cannot break even.

* It is impossible to build a machine that produces as much energy as it uses. (This type of machine is called a perpetual motion machine of the second kind.)

* The entropy of the universe tends to a maximum. (Rudolf Clausius, 1865)

It is much easier to illustrate examples of the second law of thermodynamics. Simply turning on a light bulb shows that in addition to light, heat is generated. Also, try recapturing the light or the heat to do additional work. Tough, isn't it?

Consider this quote by Paul and Ann Erlich:

"Energy is most usable where it is most concentrated--as in highly structured chemical bonds (gasoline, sugar) or at high temperature (steam, incoming sunlight [sic]). Since the second law of thermodynamics says that the overall tendency in all processes is away from concentration, away from high temperature, it is saying that, overall, more and more energy is becoming less and less usable."

Scientists and inventors over the years have recognized this trend of energy "loss" and have strived to overcome it. They have always failed. A common attempted invention to resist the laws of thermodynamics is called the perpetual motion machine. The idea behind this machine is that the motion of the machine provides the energy to continue the motion of the machine. (Huh?) In other words, once the machine starts running, no additional energy is needed (the machine provides its own energy). Think it'll work? The next section, Energy Rules! Section E. Activities and Experiments, will provide a discussion of  perpetual motion machines.


Final Thoughts about Energy Rules

Energy has often been called the currency of life. It flows through Earth's processes creating wind, providing light, and enabling plants to create food from water and air (carbon dioxide). Humans have tapped into this flow to generate electricity, fuel our cars, and heat our homes. The sun provides Earth with most of its energy. It is important for students to recognize and appreciate this source of energy and to explore the transformations that bring the sun's light into their home in the form of light, heat, food, and fuel. We are fortunate to have many "concentrated" sources of energy. Besides the sun, there is chemical energy found in fossil fuels such as coal and oil and in nuclear resources.

While the amount of energy in our world remains constant, as we use it (transfer it to one form to another), it becomes spread out and less useful.  Energy also gives us the ability to work. Through education and becoming aware of what energy is and how we use it, we can learn (i.e., work) to use our concentrated resources more wisely, and ensure that they will be available for future generations.



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Go back to Section C: Energy Conversion










Continue on to Section E: Activities and Experiments