On Sun, 14 Sep 2003 17:33:24 -0700, mikee wrote:
I don't think that 50% efficiency is thermodynamically possible. The best
designed fossil fuel powerplants barely make thirty percent. Carnot cycle
limitation?

Actually the best fossil power plant, a combined cycle plant, currently
holds the efficiency record at 52%. That's primary energy to electrical
energy, so the prime mover efficiency has to be higher than that to
account for generator losses. Large diesel electric plants typically
run about 45%.

The Carnot cycle limit is

Ec = 100* (Th-Tc)/Th

where Th is the high temperature source and Tc is the low temperature
sink, both expressed in degrees Kelvin, and Ec is Carnot efficiency. As
you can see by inspection, you can only reach 100% efficiency if the
sink temperature is absolute zero. But we don't have access to an
absolute zero sink of sufficient capacity to be useful. 50% efficiency
is possible if the difference between source and sink is half the high
temperature. That's achievable in practical systems.

The Carnot limit actually permits any efficiency up to 99.999999......%.
But to approach that, the Th has to be very high, and the Tc has to
be very low. That's where practical limits come into play. Materials
can't stand extremely high temperatures, and the atmosphere puts
a practical limit on Tc.

Gary