Introduction
Imagine that someone offers you a $100 bill that you can use for whatever you want. That
would be a pretty good deal, wouldn’t it? Now imagine that the person attaches a condition
to their offer: in order to get the $100 bill you have to pay them $75. You would still come
out ahead, but this time you would only be getting $25. Does it make sense to spend money
to get money? That depends on how much you get back for what you spend.
Getting and using natural energy sources is a lot like spending money to get money. We use
a lot of energy just to get energy (Figure20.8). We have to find an energy source, extract
it from the Earth, transport it to the places where it will be used, and often process or
convert it into a different form of energy. All of these steps of getting energy require energy
use themselves. For example, we use petroleum to make gasoline for our cars. To get the
petroleum, we often have to build huge drilling facilities and drill down hundreds of meters
into the Earth. It takes energy to do this. We then use trucks or ships to transport the
oil all over the world, which also takes energy. We then have to heat the petroleum to its
boiling point to make different products from it, like gasoline and automotive oil and this
takes even more energy.
In this lesson, you will learn that different sources of energy all require adding some other
energy before they can be made useful. You will be able to compare various sources of energy
in terms of their usefulness. You will also learn some ways that we can conserve energy or
use it more efficiently.
Obtaining Energy
It takes energy to get energy. Net energy is the amount of useable energy available from a
resource after subtracting the energy used to extract it from the Earth and make it useable
by humans. We just discussed someone giving you $100 but requiring you to pay them back
$75. In this case, your net pay would be $25, or $100 minus $75. Net energy is calculated
the same way. For example, for every 5 barrels of oil that we take from the Earth, we have
to use 1 barrel for the extraction and refining process. This leaves us a net supply of only 4
barrels (5 barrels minus 1 barrel).
Remember that oil is a non-renewable resource. Imagine what would happen if the energy
needed to extract and refine oil increased. What might happen if it took 4 barrels of oil
being used to get 5 barrels of new oil? Then our net supply would only be 1 barrel. Our
supply of oil would begin to dwindle away even faster than the current rate.
We sometimes use the expressionnet energy ratioto demonstrate the difference between
the amount of energy available in a resource and the amount of energy used to get it. If we
get 10 units of energy from a certain amount of oil, but use 8 units of energy to extract,
transport, and refine the oil, then the net energy ratio is 10/8 or 1.25. A net energy ratio
larger than 1 means that we are still getting some usable energy. A net energy ratio smaller