TECH BUZZ // ENERGY BY MICHAEL E. WEBBER
MECHANICAL ENGINEERING | DECEMBER 2015 | P.16
ENTROPY AND
THE ENVIRONMENT
A fundamental law of engineering can help inform
our future energy decisions.
E
ngineers generally love orderly
solutions to messy problems. But
one of the most powerful insights of
engineering is that disorder naturally increases, and restoring order takes effort.
This revelation is captured by the second
law of thermodynamics.
That disorder, or entropy, manifests itself through inefficiencies, which means
that losses will occur as we use energy.
Such losses are unavoidable and show
up as waste products that impact the environment. The greater the entropy, the
greater the losses, waste, and environmental impact—everything from heated
waterways and degraded air quality to
land contamination.
We may, however, be able use that
fundamental insight to help us plan our
energy future and tackle greenhouse gas
pollution.
For example, processes and energy
conversions seem to have a natural
sequence. A wood log is highly ordered; it
contains energy that is neatly organized
into a compact space. As the wood burns,
its energy disperses as highly disordered
smoke, heat, and soot. That example
illustrates increasing entropy and exemplifies the second law of thermodynamics: turning fuel into heat and smoke is
easy; turning smoke and heat back into
fuel is difficult to envision.
1215MEM_Energy_Webber.indd 16
The processes we use to power society
follow the natural direction from order to
disorder. We burn fuel or run falling water through turbines to turn their stored
energy into power. Doing the reverse—
taking heat and combustion byproducts
to make fuel or pumping water uphill into
a reservoir—not only requires energy, but
more energy than was released in the
first place. Entropy never sleeps.
The lessons of the second law of
thermodynamics can be used to frame
the discussion of environmental impacts.
Reversing a process, or increasing order,
requires work, so reversing environmental impacts takes energy. The higher the
entropy, the greater the environmental
impact and the harder it is to reverse it.
We as engineers can use entropic
flows and measures of entropy to prioritize which impacts to avoid and which to
mitigate after the fact.
Pollution with the highest entropy is
the hardest to remediate. For example,
air pollution is made up of very small,
highly dispersed particles, which makes
it difficult to clean up. Water pollution
can flow with the water, so water pollution is also dispersed, but not as much as
air pollution. Solid pollution is the most
ordered as it does not flow or disperse
without help from air and water.
From the entropic standpoint, then,
avoiding air pollution ought to be the
highest environmental priority, since air
pollution is the hardest to clean once it
has been produced. By contrast, the effects of solid pollution are the easiest to
mitigate.
That means our environmental mitigation efforts should prioritize avoiding air pollution—including emission
of greenhouse gases. At power plants
across the United States, pollutants such
as NOx and SOx that were once released
as gases out of the smokestack are now
captured via scrubbers and turned to
solid-phase compounds. From the lens
of entropic assessment, that seems quite
rational. We could look at doing the same
with carbon pollution, if feasible.
An entropic consideration also supports the cause for nuclear energy and
renewables. Nuclear energy avoids hardto-remediate air pollution, and instead
creates solid waste that should be easier
to contain and manage.
This framework can also be helpful
for considering approaches to climate
change. From an entropic point of
view, avoiding the greenhouse gases
in the first place is cheaper and less
energy-intensive than trying to scrub the
atmosphere after the fact. If we use entropic considerations as our guide, then
nuclear, wind, and solar, which create
finite amounts of solid pollution but avoid
air pollution and heat trapping gases,
are the choices we should prioritize for a
low-carbon energy future. Applying the
concept more strictly means that traditional nuclear energy is still somewhat
problematic because of the thermal
pollution of waterways caused by cooling
power plants.
We can never escape thermodynamic
constraints, but if we use insights about
entropy to our advantage, we can make
better decisions and avoid wasteful energy consumption. ME
MICHAEL E. WEBBER is the Josey Centennial Fellow
in Energy Resources and associate professor of mechanical engineering at the University of Texas at Austin.
10/28/15 2:57 PM