Re: OT: Waste heat

"Siderius Nuncius" <matron.nuncius@xxxxxxxxx> wrote in
news:6n023hFiskbaU1@xxxxxxxxxxxxxxxxxx:

I can't find a reliable site to cite, but the Second Law of
Thermodynamics gives the maximum efficiency of energy conversion as
(T2-T1)/T2,
where T2 is the temperature of the hot steam entering the turbine from
the boiler and T1 is the temperature at which the cool steam/water
leaves the turbine. The temperatures are in Kelvin, so the cool steam
is at about 300K, and the hot (superheated) steam at a maximum (I
think) of about 600K. The *maximum* efficiency is therefore roughly
300/700 - about 50%. And, of course, it's impossible to get *all*
the energy from the gas or other fuel into heating the water/steam - a
fair percentage goes up the chimney even in the most efficient
boilers. Then there are then the usual losses in the turbines,
generators, transformers and so on.

Someone will be along with proper citations and figures shortly, but
35% sounds about right to me. Mind you, I may have made a complete
@rse of myself again, as I did in the Infinity Debacle.

IIRC the (T2-T1)/T2 is for a Carnot cycle - i.e. a steam engine as used
in coal fired power stations. When I learnt that 30 years ago a modern
coal fired power station was quoted as achieving 40%.

Combined cycle gas turbine power stations use a gas turbine internal
combustion engine and run a steam engine off its "waste" heat. They can
achieve 60%.
http://en.wikipedia.org/wiki/Combined_cycle

Whilst I am, by inclination, an environmentalist (and have been since the
60s) I cannot support any of the current green lobby groups because of
their prediliction for repeating utter bilge without doing the most basic
fact checking. A few back of the envelope calculations undermine most of
the claims about TVs on standby consuming the output of a large power
station.
--
Jim <http://www.jim-easterbrook.me.uk/>
1959/1985? M B+ G+ A L I- S- P-- CH0(p) Ar++ T+ H0 Q--- Sh0
.

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