| Before
we proceed to the final stages of the home energy audit, we need to briefly
revisit the idea of efficiency. In week one of this lab module, we
discussed the Second Law of Thermodynamics. At that time, we noted
that the Second Law puts limits on the amount of usable energy that
can be transferred, whereas the First Law only stated that the total amount
of energy must be conserved in any transfer. One of the consequences
of the Second Law is that the total amount of usable energy that comes
out of any process will be less than the total amount of energy that went
into the process. The difference between the total amount of energy
input and the usable energy output is expended as waste heat.
This
brings us, once again, to the issue of efficiency. It is defined
as the amount of usable energy output from a transfer divided by the total
amount of energy that went into the process (see diagram at right).
One type of device for which efficiencies are often computed is a heat
engine. This is a device that takes in heat from a hot reservoir
and produces useful work while outputting waste heat to a cold reservoir.
An example of this is an internal combustion engine. In this example,
the hot reservoir is the ignited gasoline in the piston chamber.
The heat from this ignited gasoline causes the gases in the chamber to
expand, pushing the piston back and performing work as it does so. In order
for the piston to return to its original position so that the process can
begin again, the gas within the chamber must be cooled so that it will
contract. Since it would take to long for the air to cool via conduction,
the process is enhanced by venting the hot air to the cold reservoir of
air outside of the engine and replacing it with cold air from this reservoir.
By measuring the total amount of work that is done on the piston and dividing
it by the total amount of energy in the gasoline before it was ignited,
we can calculate the efficiency of this process. As an example, the
average efficiency of the internal combustion engine in passenger autos
is about 25%.
Heat
engines, though, are not the only type of device for which efficiencies
are measured. Any process or device which transfers energy from one
form to another has an efficiency associated with it. |
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