Environmental Engineering Reference
In-Depth Information
3.13
FUEL (THERMAL) EFFICIENCY
The energy embodied in fuels can serve many purposes: generating mechanical or electrical energy,
propelling vehicles, heating working or living spaces, creating new materials, refining ores, cooking
food, and so on. In this chapter we have focused on the production of mechanical work as an example
of the constraints imposed on fuel energy use by the laws of thermodynamics. Nevertheless, these
laws apply universally to all energy transactions.
The use of fuel to produce mechanical or electrical power, for whatever purpose, accounts
for more than half of all fossil fuel consumption. It is for this reason that the considerations of
this chapter provide important information needed to evaluate the environmental consequences of
current use of this type and especially of options for reducing fossil fuel use in the future. Whatever
the beneficial use of the mechanical energy produced, be it vehicle propulsion, material processing,
fluid pumping, and so on, the initiating step of converting fuel energy to mechanical form is an
essential ingredient that has important economic and environmental consequences. A measure of
the influence upon these consequences is the efficiency with which the fuel energy is converted to
mechanical form.
A practical measure of the efficiency of converting fuel energy to work is the ratio of the work
produced to the heating value of the fuel consumed, which we may call the
fuel efficiency
η
f
(or
alternatively,
thermal efficiency
). Usually we use the lower heating value
LHV
of the fuel for this
purpose, as this measures the practical amount of fuel energy available. Fuel efficiency is useful
because we can readily calculate the fuel mass consumption rate
m
f
of an engine of given power
˙
output
P
if we know its fuel efficiency,
P
m
f
˙
=
(3.54)
η
f
(
LHV
)
Sometimes the fuel efficiency is expressed differently. The ratio of fuel consumption rate
m
f
to
˙
engine power
is called the
specific fuel consumption
. From equation (3.54), we can conclude
that the specific fuel consumption is the inverse of the product
P
.
Table 3.2 summarizes the range of values of fuel (thermal) efficiencies of current technologies
for producing mechanical or electrical power. It can be seen that none of these exceeds 50%. These
η
f
(
LHV
)
TABLE 3.2
Fuel (Thermal) Efficiencies of
Current Power Technologies
Type
Efficiency
Steam electric power plant
Steam at 62 bar, 480
◦
C
30 %
Steam at 310 bar, 560
◦
C
42 %
Nuclear electric power plant
Steam at 70 bar, 286
◦
C
33 %
Automotive gasoline engine
25 %
Automotive diesel engine
35 %
Gas turbine electric power plant
30 %
Combined cycle electric power plant
45 %
Fuel cell electric power
45 %
