Reply to a non-scientist who asked, at a public discussion
on energy, why the thermodynamic efficiency of a standard power plant is so low
35% is low, for the very oldest coal-fired or nuclear
plants; new coal-fired plants hit 45%. There are fundamental
thermodynamic limitations to efficiency of a heat engine. The limit
discovered by Sadi Carnot about 150 years ago is
firm: the maximum efficiency for a heat engine operating between a high
temperature T2 and a low temperature T1 is (T2-T1)/T2. All
temperatures are absolute (Kelvin, or, if you're stuck in US units,
Reaumur). A few points:
- The practical high
temperature is that of the working fluid (steam, up to about 600oC or 873
K), not that of the combustion stream (max. is about 2000oC or 2273 K;
maximum practical T is 1200oC or 1473 K)).
- The practical low T =
T1 is just above the boiling point of water at normal atmospheric
pressure - maybe 110oC = 383 K. This is a bit higher than ambient
temperature, which averages about 20oC = 293K nationally.
- So, having to work
with a practical fluid (water) cuts our maximum efficiency from
(1473-293)/1473 = 0.80 = 80% to (873-383)/873 = 56%
- Add in losses in
generators plus energy use to run coal conveyors, sulfur-dioxide
scrubbers, etc., and we're down to 45%.
- Nuclear plants have
had to operate at lower temperatures, cutting their efficiencies.
Newer designs work at higher T and higher efficiencies.
- Fuel cells can do better,
maybe 80%, not being heat engines. They work best with very simple
fuels such as hydrogen or methanol.
- We can convert coal
to syngas (hydrogen and carbon monoxide) or
further to methanol. There is a necessary loss of efficiency here
(any process occuring at a finite rate operates
at < 100% efficiency, according to thermodynamics)
- They're easily
poisoned by sulfur --> S removal costs $$ and costs some energy.
- They also carbonize
and stop working if the fuel contains carbon
- They have low power
densities. To make any power plant would need an enormous
investment in $$ and materials. Those materials cost a lot of
energy to make, so it would be a loss, energetically, to use even the
most advanced fuel cells for practical power plants.
- Fuel cells hold more
promise for small-scale use - in fact, they're used in some hospitals and
other medium-sized institutions. They're being developed for cars
but are still far from practical. Of course, once we start
distributing fuel, there are costs in $$ and energy to do this (pumping
costs, trucking costs...)
So, in short, higher efficiencies in central power
generation, or even medium-scale power generation, are a ways off in
time. Many, many people are working to advance this, but the technology
is not there yet. We can't stop pushing hard for this.