Re: OT: Waste heat

Jim Easterbrook wrote:
"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

That's as maybe - but the power losses in the national grid amount to 50% of the leccy fed in. Not good, is it.

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.

But millions of TVs on standby use quite a bit of energy surely? To meet carbon reduction targets will mean adopting lots of seemingly small changes - unless a global miracle brings sufficient cooperation to generate power by using "concentrating solar" generation.

Quote from http://www.trec-uk.org.uk/csp.htm -

"Every year, each square kilometre of desert receives solar energy equivalent to 1.5 million barrels of oil. Multiplying by the area of deserts world-wide, this is several hundred times the entire current energy consumption of the world. The cost of collecting solar thermal energy equivalent to one barrel of oil is about US$50 right now (already much less than the current world price of oil) and it is likely to come down to around US$20 in future".

Anyone care to guess why we don't hear much about it?

We also need to re-build the grid - globally. Modern high voltage transmission lines lose about 3% per thousand kilometres, about 10% after conversion to A/C.


--

Chris

I am not young enough to know everything.
Oscar Wilde (1854 - 1900)


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