Learning Objectives Self-Check
Read through the following statements/questions. You should be able to answer all of these after reading through the content on this page. After going through the content, check the boxes next to the questions/statements that you feel at least somewhat confident answering. I suggest writing or typing out your answers, but if nothing else, say them out loud to yourself. This is to help you reflect on important content, and will help you prepare for this week's quiz. It will also help lay the foundation for future course content.
Explain what it means when an appliance is 40% efficient, in terms of input and useful output.
Though many forms of energy can be converted to many others, it is important to consider how efficient the conversion process is. Energy efficiency is the percentage of "useful" energy that is converted from another form.
For example, have you ever thought about what it means to have an "efficient" light bulb, like a light emitting diode (LED)? Think about it - the purpose of using a light bulb is to provide light. Seems obvious enough, but did you know that about 90% of the energy used by an incandescent light bulb is actually converted to heat? Only about 10% is converted to light, which means that incandescents are about 10% efficient. If you are still using these old-style light bulbs, you are wasting about 90% of the money you spend on the electricity used, unless you are purposefully using them to heat your house (this is a very expensive way to heat your house, by the way). This is one reason why CFLs have become so common, and now LEDs ("light emitting diodes") - both of them are around 40% - 45% efficient, which is 4 - 4.5 times as efficient as an incandescent.
Good to Know
Consumers have a wide array of energy efficient lamps available to them. In addition to using electricity more efficiently, CFLs last about 10 times longer than incandescents, and LEDs around 25 times longer.
Efficiency considerations can be made for anything that uses energy. An efficient car is one that gets a lot of miles (useful "output") per gallon (energy input). An efficient home heating system, such as an electric heat pump, releases a lot of heat energy (output) for each kilowatt-hour of electric input. TVs, cell phones, airplanes, refrigerators, you name it - all have a certain efficiency. It can be used in other contexts as well. If you are efficient at work, you get a lot done (output) in a short period of time (input). In an efficient outing by a baseball or softball pitcher, not many pitches (input) were required to retire the batters (getting outs is the useful output).
This leads us to one aspect of the Second Law of Thermodynamics. A full explanation of the 2nd Law goes beyond the scope of this course, but you are welcome to watch the video below (9:29) from the Kahn Academy for a short explanation. One application of this law is that it is impossible to convert energy into a more dense, useful state without adding energy to the system. As Dr. Eric Zencey of the University of Vermont describes it, "the capacity of the energy to useful work is diminished" whenever it is transformed from one form to another (source: Is Sustainability Still Possible? p. 73). In other words, when energy is converted from one form to another, it is impossible to convert all of it. Some is "wasted" in another form, usually heat.
Let's continue with the lighting example to illustrate this. When using a light, electrical energy is converted almost entirely to light and heat (there may be a little sound energy thrown in there, but not much). Electrical energy is relatively dense, useful, and easy to control. You can store electrical energy in a battery. It is relatively easy to transport across distances without losing much. It can be used for many different things. But what about light and heat? Both of them are relatively diffuse and difficult to control. Neither is particularly useful for converting to other forms. It is very difficult to convert heat or light into another form with any kind of efficiency. Sure, you can convert heat back into electricity. In fact, this is exactly what happens in a typical power plant. But this process is very inefficient. Going further back, it is impossible to convert light, heat, or electricity back into coal (or oil, natural gas, or nuclear energy). Fossil fuels are very energy dense, and the molecules and atoms are neatly organized. Once the bonds are broken and the energy is released, there is no way to put it back together. That's the 2nd Law in action.
To Watch Now
The video below provides a very good explanation and animation of how a coal-fired power plant works. Think it's as easy as dumping a bunch of coal into a furnace and turning a turbine? Watch the video to find out. (9:28 minutes)
Optional - Explanation of the Second Law of Thermodynamics
The second law can be confusing, but the narrator in the video below does a pretty good job of explaining some aspects of it. Watch the Second Law of Thermodynamics (12:40 minutes) from Kahn Academy.
Clearly, a lot of engineering goes into building a power plant. Despite the technical prowess required to convert coal into electricity, the process is extremely inefficient, as are all of the major forms of electricity generation in the U.S. and the world. Take a look at the chart below to see just how inefficient this process is for different fuels.
As you can see, as the most efficient fuel, natural gas-fired power plants are just above 40% efficient on average. Coal is closer to 30%. This, of course, means that around 70% is wasted as heat. 70%! And this does not take into consideration the losses associated with transporting the electricity across long power lines, which in the U.S. averages around 5%.
Power plants are not alone in their inefficiency. The typical internal combustion engine of a car only provides around 20% - 25% of the energy from gas to move the car. New natural gas furnaces are very efficient (95%+), but many older ones operate at lower than 80% or even 70% efficiency. This is all poor energy management in principle - it's just plain wasteful - but it is also important for a couple of other reasons, one in particular. Specifically, there is a limited amount of all of these sources, and yet they are essential for modern society. In other words, coal, oil, natural gas, and nuclear are non-renewable energy sources. (To be fair, all indications are that the world will not run out of coal, natural gas, oil, or nuclear energy terribly soon, but no one knows when it will become too expensive to use. More on that later.)
The "Fifth Fuel" (Or Perhaps the "First Fuel")
One last note before moving on to renewable and non-renewable sources. Energy efficiency is sometimes referred to as the "fifth fuel." Why do you think that is? (Hint: coal, oil, natural gas, and nuclear are the four primary fuels used globally, though that is changing as renewables are used to a greater extent.)
Increasing efficiency reduces the use of other sources of energy. Efficiency is on the demand side of energy use because it affects energy demand (think of this as how much energy is "demanded" for use.) Energy sources are the supply side of energy use because they supply the energy. By reducing demand through energy efficiency, you reduce the need for supply, which is almost like having more supply, to begin with. Hence, it is sometimes referred to as the "fifth fuel." There are tremendous opportunities for energy efficiency improvements worldwide.
Some energy efficiency advocates refer to efficiency as the "first fuel," because they feel that it should be the top priority in terms of energy management. There is some strong validity to this. Consider that a report from the American Council for an Energy Efficient Economy found that it is cheaper to reduce energy use through efficiency than it is to supply energy by any other source. Very interesting reading, if you are so inclined (and only a few pages long).
Check Your Understanding - Efficiency
Optional (But Strongly Suggested)
Now that you have completed the content, I suggest going through the Learning Objectives Self-Check list at the top of the page.