[Gasification] THERMAL EFFICIENCY

[Benjamin Domingo Bof]

Benjamin Domingo Bof <benjaminbof at yahoo.com.ar> escribió: Heat engines When transforming thermal energy into mechanical energy, the thermal efficiency of a heat engine is the percentage of heat energy that is transformed into work. Thermal efficiency is defined as



etha/th = W out/ Q in
 

 
 

 
 [edit] Carnot efficiency The second law of thermodynamics puts a fundamental limit on the thermal efficiency of heat engines. Surprisingly, even an ideal, frictionless engine can't convert anywhere near 100% of its input heat into work. The limiting factors are the temperature at which the heat enters the engine, , and the temperature of the environment into which the engine exhausts its waste heat,, measured in the absolute Kelvin or Rankine scale. From Carnot's theorem, for any engine working between these two temperatures:
 

etha/th (minor or equal)  1- Tc/Th

 
    
 
 This limiting value is called the Carnot cycle efficiency because it is the efficiency of an unattainable, ideal, lossless (reversible) engine cycle called the Carnot cycle. No heat engine, regardless of its construction, can exceed this efficiency.
 
 Examples of  are the temperature of hot steam entering the turbine of a steam power plant, or the temperature at which the fuel burns in an internal combustion engine.  is usually the ambient temperature where the engine is located, or the temperature of a lake or river that waste heat is discharged into. For example, if an automobile engine burns gasoline at a temperature of  and the ambient temperature is , then its maximum possible efficiency is given by:
 
etha/th (minor or equal)  1- 294 K / 1084 K = 73%

 
    
 
 In practice, because no practical implementation of the Carnot cycle exists coupled with other irreversibilities such as the combustion process itself and friction, real engines fall far short of the Carnot efficiency. Real automobile engines are only around 25% efficient, combined cycle power stations have efficiencies much higher however will still fall at least 15% points short of the Carnot value. As Carnot's theorem only applies to heat engines, devices that convert the fuel's energy directly into work without burning it such as fuel cells, can exceed the Carnot efficiency.

       
---------------------------------

Tarjeta de crédito Yahoo! de Banco Supervielle.Solicitá tu nueva Tarjeta de crédito. De tu PC directo a tu casa. 
 Visitá www.tuprimeratarjeta.com.ar
_______________________________________________
Gasification mailing list
Gasification at listserv.repp.org
http://listserv.repp.org/mailman/listinfo/gasification_listserv.repp.org
http://gasifiers.bioenergylists.org
http://info.bioenergylists.org


       
---------------------------------

Tarjeta de crédito Yahoo! de Banco Supervielle.Solicitá tu nueva Tarjeta de crédito. De tu PC directo a tu casa. 
 Visitá www.tuprimeratarjeta.com.ar