Environmental Engineering Reference
In-Depth Information
of temperature, and a constant dependent on the molec-
ular composition of the gas. Its use also illustrates the
limits of high conversion efficiencies. Transformation of
chemical energy into electricity in fuel cells is more than
70% efficient, but the low diffusion rates in electrolytes
limit the power density to some 200 W/m
2
of the elec-
trode, too low for a centralized base load supply in a
high-energy society (Brandon and Thompsett 2005).
The second and broader meaning of power density—
the rate of energy flow per unit of surface area (rather
than per unit of the working surface of a converter)—is
perhaps the most critical parameter determining both
the structure and the operating modes of energy conver-
sion systems. I use it for systematic comparisons of all
important inanimate fluxes (from solar constant to geo-
thermal energy), for autotrophic and heterotrophic con-
versions (from photosynthetic performance to predator
feeding), and for all principal modes of past, present,
and contemplated anthropogenic energy conversions
(from biomass to electricity generation and including
waste heat rates).
Specific power (W/g) is used to characterize intensities
of autotrophic and heterotrophic metabolism (adult basal
metabolic rate is@1 mW/g) as well as those of inanimate
transformations (car engines develop @1 W/g). When
looking at the performance of prime movers, from steam
engines to rockets, it is better to use the reverse (g/W)
in order to emphasize the importance of lightweight con-
version devices in transportation. For their first flight the
Wright brothers built a four-stroke engine (with alumi-
num body and steel crankshaft) that weighed 8 g/W;
the most advanced turbofan engines that power wide-
bodied jets weigh less than 0.1 g/W. Finally, metabolic
intensities (g/J), whether of recyclable nitrogen in do-
mestic animals or SO
2
emissions by different combustion
1.9
Pyotr Kapitsa (1894-1984), Nobel prize in physics,
1978. Photograph
8
The Nobel Foundation.
replacing electromagnetic generators by the electrostatic
ones for large-scale electricity production. Two clear
advantages would be simpler construction and the possi-
bility of direct feeding of high voltage to transmission
lines. But to avoid sparking, the electrostatic field is
restricted by the dielectric strength of the air, and to pro-
duce 100 MW the electrostatic rotor would have to
cover about 400,000 m
2
, a clear impossibility compared
to about 10 m
2
for an identically rated electromag-
netic generator with a power density vector of about
1 kW/cm
2
.
Where combustion is involved, the Umov-Poynting
vector is the product of gas pressure, the square root
