Environmental Engineering Reference
In-Depth Information
desired kind achieved by a converter to the initial energy
input to the device, organism, or system is by far the
most commonly used value. This first-law efficiency (e
1
)
has at least three important drawbacks: its maximum
values may be less than, equal to, or greater than 1 (in
the last case, it is usually called a coefficient of perfor-
mance); it does not capture the efficiency limitations
imposed by the second law; and it is not readily applica-
ble to systems whose desired output is a combination of
work and heat.
The second-law (or exergy) efficiency (e
2
) is the ratio
of the least available work that could have performed
the task to the available work actually used in performing
the job with a given device or system (Ford et al. 1975).
Clearly, this efficiency cannot surpass unity. It offers di-
rect insight into the quality of performance relative to
the ideal (minimization of inputs as the goal of energy
management), and it focuses attention on the desired
task, not on a device or a system currently used for that
purpose. For example, overall energy and exergy efficien-
cies will both be about 40% for the best coal-fired
electricity-generating plants, but while steam generation
has very high e
1
(with some 95% of the input energy
transferred to water), it is only about half as efficient in
exergy terms, e
2
(M. A. Rosen 2004). Similarly, the e
1
of standard household natural gas furnaces is about 75%,
but the e
2
will be only about 10% because the equivalent
heating could have been done much more efficiently by a
heat pump. All conversions where high-temperature
combustion is used to provide low-temperature heat will
be exposed as similarly wasteful using e
2
.
Calculations of e
2
, always more difficult than appraisals
of e
1
, often run into social and behavioral concerns. For
example, the e
2
of cars will not be calculated with respect
to the broadly defined task of moving people speedily
and comfortably around but strictly with respect to the
narrowly defined task of supplying kinetic energy at
the drive wheels. How then to factor in the unoccupied
seats, or what is indeed the true meaning of e
2
in the
case of completely frivolous cruising? And using an
equivalent to e
2
in comparing the efficiencies of photo-
synthetic or heterotrophic conversions is even more
questionable. Because of different photosynthetic path-
ways, wheat is a less efficient converter than sorghum,
but sorghum does not produce bread-making flour. And
conversion of grasses to beef shows inferior e
2
compared
to bleeding the animals, but are Americans willing to be-
have like Maasai?
As for the quest for maximum efficiency, practical
limits were already recognized in Carnot's writings, and
the subsequent history of engineering design of both
prime movers and converters has been always marked by
compromises taking into account bearable costs, accept-
able reliability, and desired power outputs. Infinitesimally
slow conversions produce maximum possible efficiencies,
but transformations done at socially and economically
rewarding rates inevitably carry high heat waste penalties.
Similarly, Lotka's (1925) law of maximum energy recog-
nized that optimal efficiencies are considerably lower
than the maxima that would limit the power output nec-
essary for growth and maintenance of biota. Moreover,
in many instances lower efficiencies are preferable to
practices whose energy conversion efficiencies are higher
but whose utility, manageability, and ensuing productiv-
ity or comfort are greatly inferior. And what is the effi-
ciency of hydroelectric generation? Is the initial input
the potential energy of the precipitation in the entire
watershed, of the water behind the dam, of the storage
above the penstock intakes (including all evaporation
losses), or only of the water actually used in generation?
