A combined heat and power (CHP) plant delivers per cent of its fuel's lower heating…

PHYSICS · Subjective · Fermi University Physics · Thermal, Statistical & Complex Systems · Earth, Climate & Energy Physics · The laws of thermodynamics

Energy from Physics, Ch Heat engines on fuel: exergy, real cycles, biomass and the nuclear heat source

Problem

A combined heat and power (CHP) plant delivers

30

per cent of its fuel's lower heating value as electricity and

55

per cent as useful heat at

350\,\mathrm{K}

. A brochure argues: “Our plant's overall efficiency is

30 + 55 = 85

per cent. A modern coal power station only reaches

40

per cent electrical efficiency. Our plant is therefore more than twice as good.” Find the error in this reasoning. Take the surroundings at

T_0 = 300\,\mathrm{K}

and, for both fuels, the fuel's exergy as

1.04

times its lower heating value. Compute the exergetic efficiency of the CHP plant and of the coal station, and compare them.

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