Page 203 - Advanced Gas Turbine Cycles

168 Advanced gas turbine cycles

COGENERATION
REJECTED(Qu)co 1 (CHP) PLANT 1 WORK WCG
t
4
USEFUL HEAT

I NONUSEFUL HEAT

REJECTED (Q&G

Fig. 9. I. Cogeneration (CHP) plant.

It could then be supplementarily fired in the winter when the heat load is heavier, as
in Fig. 9.2b.

9.2. Performance criteria for CHP plants

9.2.1. Energy utilisation factor

For an open circuit power plant, an (arbitrary) overall efficiency has been defined as
W
To= -. (9.1)
F
This criteria of performance has less relevance to a combined heat and power plant
which provides heat and generates electrical power. For an open circuit gas turbine plant, a
more logical criterion is the energy utilisation factor (EUF) which can be calculated as

(9.2)

where (Qu)cG is the useful heat rejected to meet the required heat load, at a temperature Tu
higher than To, the temperature of the environment. It is preferable not to use the term
efficiency for EUF, to avoid confusion with the thermal or overall efficiency.
For the unfired example, Fig. 9.2a, the efficiency is 0.25, and EUF = (W + Qu)/F =
(1 + 2.25)/4 = 0.8125. For the supplementary fired example of Fig. 9.2b, the efficiency
remains at 0.25 but the EUF becomes

It must be remembered that work is difficult to produce and highly priced, whereas the
useful heat is a lower grade, lower priced product from the plant. The energy utilisation