Page 130 - Advanced thermodynamics for engineers

116 CHAPTER 5 RATIONAL EFFICIENCY OF POWER PLANT

Temperature, T 3

2 4
2 2s 2s
4s 4s

W net
1 1 (rejected)
T 0
T (s -s )
0
2
2s
Entropy, S
FIGURE 5.7
The effect of inefficiency in the compressor.

This equation shows how the inefficiencies reduce the net work obtainable from the cycle.
However, an additional quantity of net work has been lost which is not evident from the equation, and
that is the reduction in maximum net work caused by the inefficiency of the compressor. The latter
causes the temperature after compression to be higher than the isentropic value, and hence less fuel is
necessary to reach the maximum cycle temperature.

5.5 CONCLUDING REMARKS
The concept of the Second Law, or Rational Efficiency, has been introduced. This provides a better
measure of how closely a particular power-producing plant approaches its maximum achievable ef-
ficiency than does the conventional ‘First Law’ thermal efficiency.
It has been shown that devices in which the working fluid changes phase (e.g. steam plant) can
achieve rational efficiencies of 100%, whereas those which rely on a single phase fluid (e.g. gas
turbines) can never approach such a high value. The effect of irreversibility on rational efficiency has
also been shown.

5.6 PROBLEMS

P5.1 In a test of a steam power plant (Fig. P5.1), the measured rate of steam supply was 7.1 kg/s
when the net rate of work output was 5000 kW. The condensate left the condenser as saturated
liquid at 38 C and the superheated steam leaving the boiler was at 10 bar and 300 C.

Neglecting the change in state of the feed water in passing through the feed pump, and
taking the environment temperature as being equal to the saturation temperature of the