Environmental Engineering Reference
In-Depth Information
MEASURING ENERGY
Energy can be measured. That is, the amount of energy a thing has can be given
a number. As in other kinds of measurements, there are measurement units. The
units of measurement for measuring energy are used to make the numbers under-
standable and meaningful. The SI unit for both energy and work is the joule (J). It
is named after James Joule, who discovered that heat is a type of energy. In terms
of SI units, 1 joule = 1 newton-meter, and 1 joule = 1 kg⋅m
2
⋅s
-2
. The energy unit
of measurement for electricity is the kilowatt-hour (kWh); 1 kWh is equivalent to
3,600,000 J (3600 kJ or 3.6 MJ). A common way to express energy is in the British
thermal unit (Btu) (see Table 1.1). A Btu is the amount of heat energy it takes to
raise the temperature of 1 pound of water by 1°F at sea level.
•
1000 joules = 1 Btu
•
1000 joules = 1 kilojoule = 1 Btu
•
1 therm = 100,000 Btu
Note that electricity is a secondary energy source. The energy sources we use to
make electricity can be renewable or nonrenewable, but electricity itself is neither
renewable nor nonrenewable. Many of the producers of renewable energy primar-
ily produce electricity. Hydropower, for example, can be converted to mechanical
power via a waterwheel connected to a gear train to power a grinding wheel to
grind wheat or to a pump or other machine; however, today the primary purpose of
hydropower is to generate electricity. Likewise, a windmill can also be connected
to a gear train to power a pump or other machine, but today the primary purpose
of wind turbines is to produce electricity. Solar power can be used to heat water to
purify it and directly to produce steam which in turn powers a turbine to perform
mechanical functions. Solar power can also be used to produce photovoltaic (PV)
electrical energy. Even biomass liquid fuels can be used to power boilers that pro-
duce steam to power turbines connected to generators to produce electricity. In
addition, waste off-gases or exhaust from various machines such as gas turbine
engines can be used to heat boilers.
TABLE 1.1
Btu Conversion Factors
Energy Source
Physical Units and Btu (Weighted Averages)
Electricity
1 kilowatt-hour = 3412 Btu
Natural gas
1 cubic foot = 1028 Btu = 0.01 therms
Motor gasoline
1 gallon = 124,000 Btu
Diesel fuel
1 gallon = 139,000 Btu
Heating oil
1 gallon = 139,000 Btu
Propane
1 gallon = 91,333 Btu
Wood
1 cord = 20,000,000 Btu
