On 23/07/2011 23:28, The Real Doctor wrote:
On Jul 18, 8:26 am, Jeremy wrote:

It also gains them another 80 degrees or so of temperature difference
between the heat source and heat sink, which significantly increases the
maximum possible efficiency of the heat engine, according to the second
law of thermodynamics.

Enthalpy change from superheated steam at 10bar/500C to saturated
water at 1bar/100C: 3052.1 - 417.5 = 2634.6 kJ/kg.

Enthalpy change from superheated steam at 10bar/500C to saturated
water at 0.05bar/32.9C: 3052.1 - 137.8 = 2914.3 kJ/kg.

Extra enthalpy available: 2914.3 - 2634.6 = 279.7 kJ/kg ~ 10%.

Yes, and looking at the Carnot cycle efficiency (i.e. using the second
law of thermodynamics rather than the first law), using your figures:

Theoretical maximum efficiency = 1-(Tc/Th)

For 33 deg C sink temperatu efficiency = 1-(306/773)= 61%
For 100 dec C sink temperatu efficiency = 1-(373/773)= 52%

(Real efficiencies are considerably below the theoretical maximum Carnot
cycle efficiencies, but these indicate the trend).

Getting down the cold sink temperature of a heat engine really improves
its efficiency.
--
Jeremy Double {real address, include nospam}
Rail and transport photos at
http://www.flickr.com/photos/jmdoubl...7603834894248/