[Marxism] privatizing the grid--a local perspective

DW dwaltersmia at gmail.com
Sun Feb 5 09:11:42 MST 2012


Just on the bugaboo of "efficiency". I think Rod went some of the way in
explaining this.

Thermal efficiency is the ability to covert thermal heat to mechanical
energy, measured at the output in this case, of a generator.

It is widely understood that traditional thermal efficiency is '33%'
plus/minus a percentage point. That is in the traditional boiler of a
"Rankine" cycle steam turbine/generator set. The other 'cycle' is the
Brayton cycle which is what gas turbines use. In theory the Brayton cycle
is more efficient. In practice, because they are used in "open cycle" gas
turbines the efficiency is the same.

Gas turbines are able, however, to capture the heat and reuse it in a
traditional Rankine cycle boiler/turbine/generator and some of the newest
can achieve thermal efficiency up to 60% (the large GE "H" Frame single
shaft GT for example, one of which I believe is deployed in Wales, the
other in Riverside, CA). They all, however, produce lots and lots of GHG
emissions, not to mention the usual escapte of pure methane when shutting
down, starting up, etc.

But this 'thermal efficiency' is only important in terms of comparing
similar fuels, usually, always, the most expensive part of any generation
operation. It does no good comparing the this "33%" thermal effiency of
nuclear with these other fuels as it has no meaning. The fuel is the
*least* expensive part of a nuclear operation. That gas, or coal, or other
form of fossil fueled generation can achieve a slightly higher thermal
efficiency is quite unimportant in terms of planning.

To clear up another point, generally, in all current Gen II and
coming-to-a-grid-near-you Gen III nuclear reactor runs at much *lower* not
higher temperatures than GTs, coal or thermal gas plants. The steam
pressures are generally lower and the output of the steam generator run at
about half the temperatures of the conventional thermal units. If they
could run at higher temps, a few more % points of thermal efficiency could
be extracted. But they dont.

Future Gen IV reactors (they are building the first commercial Pebble Bed
Modular Reactor in China right now) will run at very high output
temperatures and can achieve a 50% thermal efficiency if gas cooled instead
of water/steam cooled.

David



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