In a heat engine, heat (q_h) is absorbed by a working substance (such as water) at a high temperature (T_h). Part of this heat is converted to work (w), and the rest (q₁) is released to the surroundings at the lower temperature (T₁) The efficiency of a heat engine is the ratio w/qh. The second law of thermodynamics establishes the following equation for the maximum efficiency of a heat engine, expressed on a percentage basis.

In a particular electric power plant, the steam leaving a steam turbine is condensed to liquid water at 41 °C (T₁) and the water is returned to the boiler to be regenerated as steam. If the system operates at 36% efficiency, 

(a) What is the minimum temperature of the steam [H₂O(g)] used in the plant?

(b) Why is the actual steam temperature probably higher than that calculated in part (a)?

(c) Assume that at T_h the H₂O(g) is in equilibrium with H₂O(l). Estimate the steam pressure at the temperature calculated in part (a).

(d) Is it possible to devise a heat engine with greater than 100 percent efficiency? With 100 percent efficiency? Explain.

Transcribed Image Text:

efficiency W qh X 100% Th – Ti Th X 100%