Environmental Engineering Reference
In-Depth Information
transmission links (often DC) move GW-sized quanta of
electricity over distances exceeding 1000 km, and electric
lighting and motors are as indispensable in industries as
they are in households, services, and agriculture.
The extraction of fossil fuels provides high net energy
returns, and in the case of thermal electricity generation
we accept substantial exergy loss in return for a high-
quality, clean, flexible, precisely controllable energy flow.
The highest EROI for Middle Eastern crude oils is about
10
4
; typical rates for old oil provinces are 10-20. EROI
for refined fuel at the consumer level is less than 10, as
are the rates for delivered natural gas. Coal extraction
has a wide range of net energy returns, from more than
200 to less than 50, and virtually all renewable energy
conversions have EROI less than 10. Electricity gener-
ated from fossil fuels contains only 35%-40% of the
energy in the charged fuel, but EROI for nuclear elec-
tricity is definitely higher than for photovoltaic or wind-
based generation.
Rising demand for fossil energies has been accompa-
nied as well as driven by improved performance of new
prime movers: increase in unit sizes, decline in power
intensities, and higher conversion efficiencies. These
advances brought high concentrations of production,
processing, and conversion capacities, unprecedented
personal mobility, the growth of global trade, and a
revolution in agricultural productivity. In turn, these
changes led to rapid urbanization, increasing affluence,
and continuing integration of the global economy. For
millennia animate power limited unit work inputs to 10
2
W, and waterwheels and windmills raised that, locally and
sporadically, to 10
3
W. The steam engine era began dur-
ing the early eighteenth century and lasted for 200 years;
by 1905 there could be no doubt about the supremacy
of steam turbines. Nearly a century later they still remain
the most powerful continuous-load prime movers. The
largest steam turbines, delivering up to 1.5 GW, are
about 20,000 times more powerful than was the largest
prime mover two centuries ago (fig. 13.6).
The evolution of power intensities (weight to power
ratios) progressed in the opposite direction (fig. 13.6).
People and draft animals need at least 500 g/W of useful
power, and the earliest steam engines were no better.
Eventually locomotive steam engines, the prime movers
of the first transportation revolution, rated well below
100 g/W. Their performance was vastly surpassed by
internal combustion engines, whose power intensities
dropped by more than 2 OM in less than a century, to 1
g/W for the best automotive engines and to nearly half
that value for the best aeroengines. These innovations
ushered in the automotive revolution in land transport
and made it possible to realize the ancient dream of pow-
ered flight. As the reciprocating aeroengines neared their
performance limits during the 1940s, gas turbines took
over. Their power intensities fell below 0.1 g/W, open-
ing the way to mass air travel and freight. Gas turbines
powering trains and ships also transformed important
segments of land and water transport.
Improvements of conversion efficiencies raised the per-
formance of steam engines about 40-fold between New-
comen's top model of 1712 and the best triple-expansion
machines of 1900. An efficiency improvement of 1 OM
accompanied the development of thermal electricity gen-
eration, from 4% during the 1880s to 40% for the best
units a century later. Increased unit ratings, lower power
intensities, and higher efficiencies of prime movers led to
impressive growth of machines and industrial plants and
to enormous concentration of extractive and processing
facilities. Coal mines grew from small pits producing a
few hundred kilowatts to surface giants that extract over
