Energy efficiency is the efficiency of using energy. In the narrow sense of the term this efficiency is the ratio of output energy to input energy, often expressed as a percent. For example, if 70% of the electric energy used to power an electric motor is turned into useful work at the rotating shaft of the motor (and 30% is wasted as heat) we say that the motor has an efficiency of 70%. In a broader sense, energy efficiency is the output (often in non-energy units) per unit input of energy. For example, the miles per gallon which an automobile gets is a measure of energy efficiency since it is the number of miles (output) obtained per gallon of energy input. While a gallon of gasoline is not strictly a unit of energy, it does represent a given amount of heat energy that can be determined by burning it (or by looking up the value in a reference book). In a still broader sense, energy efficiency is a topic of study regarding the numerical energy efficiency of various activities and the costs and benefits of increasing (or possibly decreasing) energy efficiency. Also included is a study of the physics and/or engineering principles involved.
The inverse of energy efficiency in called "energy intensity", for example: gallons per mile (= 1/30 for an auto which gets 30 miles per gallon). Increasing energy efficiency (or what is the same thing: reducing energy intensity) in most cases, can help reduce energy consumption and thus conserve energy. It often will reduce costs and save money.
The "energy crisis" of the 1970's resulted in increased concern for energy efficiency. In those days there were sometimes long lines of cars waiting to buy gasoline at the few service stations that had not run out of gas. In the US prior to this, it was only during wartime that there was significantly increased concern for energy efficiency. Increasing energy efficiency is an important aspect of energy conservation. Saving energy reduces the bills that consumers and businesses have to pay for energy. But there are usually other benefits as well such as reduced pollution, including reduction of carbon dioxide gasses which cause the earth to become warmer (the global warming problem). Another benefit is less depletion of non-renewable fuel resources so that they may be available for future generations. Saving energy usually incurs costs as well as benefits and one will often decide to save energy only if the benefits exceed the costs.
The energy which is used by a certain activity is the product of 3 factors. For example: for the activity of driving an automobile (and considering only the direct energy input of gasoline fuel) the amount of gasoline used per year is the product of 3 numbers: 1. The population. 2. The average number of miles driven per person each year (the per-person use of the driving activity). 3. The average gallons per mile (energy intensity). For other activities, the analysis is similar: The energy used is the product of 1. The population. 2. The average use (or consumption) of that activity per person. 3. The energy intensity of that activity.
Thus for a certain activity there are 2 other ways conserving energy besides increasing the energy efficiency. One way is to reduce the population. The other is to decrease the consumption of that activity and replace it (if it need be replaced at all) with an activity that consumes less energy. For example, gasoline consumption by automobiles may be reduced by reducing population and/or reducing the miles driven per year --all without any improvement in energy efficiency.
However reducing one of these 3 factors may tend the increase the values of the other 2 factors. For example, if people drive less and as a result save money, they might spend some of this saved money on buying larger and less fuel efficient autos. If population is reduced, the decreased demand for gasoline might result in a reduction in its price which would encourage people to use more gasoline.
The side effect examples above show how reducing one factor may cause an increase in another factor (a side effect) which cancels out some (or even all) of the benefits. But sometimes the side effects tend to decrease energy consumption still further. For example, decreasing population tends to make cities less crowded and thus makes it easier for people to live nearer their jobs, thus resulting in shorter trips to work and less gasoline consumption.
An automobile directly uses the energy of gasoline fuel. However it took energy to manufacture the automobile. Thus as one consumes the automobile by driving it and wearing it out, some of this extra energy needs to be taking into account. Calculating indirect energy turns out to be a very difficult problem because there is such a huge amount of information that is needed, much of which is not publicly available.
Take the case of the automobile. One needs to know the energy content of every one of its parts so that they can be added up. But there's also the energy use, including indirect energy use of all the specialized machinery and tools used to assemble the automobile. Freight transportation energy enters the picture since parts were shipped to the assembly plant from various locations. The amount of paper used for various purposes including computer print-outs needs to be accounted for since it took energy to make paper. Then one needs to look at all these industries and determine what inputs they use from other industries, and so on.
The solution to the indirect energy cost may be obtained by the use of mathematics known as "linear algebra". Computer programs exist to solve such problems. But no solution is possible since no one has the data (information) needed by the computer to solve the problem. Even if the data were available, the millions of different parts and goods in the world would make the size of the problem so huge that even the fastest computers might have trouble solving it. Spending such a huge effort to solve it may not be worthwhile since the same effort put into improving energy efficiency could be more beneficial.
The gasoline and diesel engines convert the heat energy of petroleum fuels into mechanical work. They are heat engines and the maximum possible energy efficiency which may be obtained by such an engine is the Carnot cycle efficiency. The hotter the burning fuel is above the outside air temperature, the higher the efficiency. This shows that heat supplied at a high temperature is more valuable than heat supplied at a lower temperature.
Now consider a home water heater (gas) that is claimed to be nearly 100% efficient because almost all of the heat from the gas flame goes to heat the water. But the gas flame that heats up the water is much higher in temperature than the hot water temperature. Thus we are using high temperature heat to heat up something that could be done by a heat source at much lower temperature (just slightly higher than the hot water temperature). The heat we have put into the water is now capable of doing far less work in a Carnot engine since it is at a much lower temperature than the flame heat that produced it. Thus one may claim that since ability to do work (using a Carnot engine) has been lost, the water heater is far from 100% efficient. This method of calculating energy efficiency is rarely seen.
There are many statistical errors and misleading claims regarding energy efficiency. For example some light bulbs claim to be energy saving. While they do use less energy they also may output less light. Thus they may not be as energy efficient as other light bulbs. It can be shown on paper that a train full of passengers traveling at constant speed on level track is extremely energy efficient, but the typical situation of partly-filled trains slowing down and speeding up are much more energy intensive than the ideal situation.
Estimates of the number of persons per auto or bus have sometimes been in error. In some cases the energy losses at power generating plants have been omitted from energy accounting but need to be included since only about 1/3 of the energy of the fuel burned is actually turned into electricity.
There are so many different ways of increasing energy efficiency that only a few of them can be mentioned here. Since the 1970's there have been many significant improvements in energy efficiency, mostly by doing things that we long knew how to do all along. As fuel costs increased, it became more feasible to increase energy efficiency. Some improvements in energy efficiency resulted from new laws or resulted from tax incentives. Also, laws requiring efficiency labels helped direct consumers to buy more energy efficient products. See fig. 1 for the energy use of 4 sectors of the US economy.
Many heavy industries use fuels for heating metals, ores, chemicals, and other materials. Almost all the metals we see and use were once in a very hot molten state in an industrial plant somewhere. Much of the heat that was once exhausted up smokestacks or otherwise wasted can be used. For example, it can be used to preheat the air which is mixed with fuel before the fuel is burned. To recover heat, a heat exchanger is used. A household water heater is an example of a heat exchanger where hot air (actually the combustion products) flows up a pipe and heats up water in a tank. There are many types of heat exchangers and many different uses for waste heat.
Another way some industries save energy is by generating their own electricity. Although they don't generate it any more efficiently than a large power generating plant does, they save energy by utilizing the waste heat. This is called "co-generation".
The automobile today gets many more miles per gallon than the automobiles of the early 1970's and earlier. Cars are lighter in weight, have smaller engines, and are more streamlined. They don't accelerate as fast or have as much room inside as formerly. Due to both increases in population and more driving, there has been no decrease in gasoline consumption.
For freight, the energy efficiency of trains is usually a few times better than trucks due in part to the low rolling resistance of the steel wheels of trains. There is also less wind force for a train since a train car is partly shielded from the headwind by the car in front of it.
Passenger energy efficiency (in passenger-miles per gallon) is usually (but not always) about the same for most modes of transportation. The most energy-efficient are the privately operated intercity buses like Greyhound. Mass transit, as currently operated, is not much more energy efficient that the automobile. Much depends on what percentage of the seats are occupied. If the speed of a vehicle is doubled the wind force increases 4-fold. Thus a short high speed train may be less energy efficient than a small automobile at lower speed. Airplanes fly high in the sky where the thin air helps reduce the wind force but their high speeds cancel out this advantage.
Bicycles and walking use energy we obtain from food. The human body is roughly no more efficient than an automobile engine in converting "fuel" into mechanical work. These human powered means of transport are more energy efficient mainly because the speeds are low. If an automobile were designed to move only at walking speed, it could be so small and light that it would be about as energy efficient as walking. Also it takes a lot of fossil-fuel energy to grow the food people use to obtain human muscle energy. Some have roughly estimated that to grow, process and distrubute a calorie of food, it takes about 10 calories of fuel. As a result, human energy under these conditions is not very energy efficient either, although there are health benefits from exercise.
Most of this energy is for heating, lighting, and cooling of buildings. Better insulation in new buildings (required by law) and installing insulation in the roofs and walls of old buildings has improved energy efficiency. Weather-stripping has decreased air leakage into buildings but in some cases has decreased the quality of indoor air. Fluorescent lighting is more efficient that incandescent lighting.
Solar energy can heat water (or swimming pools) by having the sun heat water in pipes on rooftops. Solar collectors that do this may sometimes be seen on the rooftops of buildings. Having trees shade a home in warm climates can help keep a home cooler and reduce the need for air conditioning.
Hu, David. Handbook of Industrial Energy Conservation. New York: Van Nostrand Reinhold, 1983.
Lenssen, Nicholas. Energy Efficiency: The Key to International Growth. USA Today (Mar. 1994): 82-84.
Robinson, Steven. The Energy Efficient Home. New York: New American Library, 1978.
Rocky Mountain Institute Staff. Energy Efficient Homes. Amherst, New Hampshire: Brick House, 1994.
Schipper, Lee and Stephen Meyers. Energy Efficiency and Human Activity: Past Trends, Future Prospects. Cambridge, England: Cambridge University Press, 1992.
Carnot engine--An ideal engine that operates at maximal efficiency.
Energy efficiency--The amount of output one obtains from a unit input of energy. Example: miles per gallon.
Energy intensity--The inverse of energy efficiency. Example: gallons per mile.