Page 209 - Advanced Gas Turbine Cycles

174 Advanced gas turbine cycles
FESR’ = 1 - (z)

FREF

I J

(eN&- is usually limited by the allowable stack temperature Ts. As a fraction of the heat
supplied to the cogeneration plant it remains constant in this application.
For an unmatched gas turbine CHP plant, meeting a power load (WCG = WD = 1) but
not the heat load (Qu)cG < AD, increased useful heat may be obtained by firing the WHB,
as explained in Section 9.2.3, and illustrated in Fig. 9.3~.

9.4. Range of operation for a gas turbine CHP plant

We now illustrate numerically the full range of operation of a gas turbine CHP plant,
(i) with a recuperator (unfired) and
(ii) with a WHB (fired).
A gas turbine plant with an overall efficiency qcG = 0.25 matching a heat load
kG = 2.25 is again considered as the ‘basic’ CHP plant; also implied is a non-useful heat
rejection ratio (QNu)cG/FcG = [l - (qc~)(&G + l)] = 3/16. For FESR calculations, we
again take the conventional plant efficiency as 0.4 and the conventional boiler efficiency as
0.9. At the fully matched condition these assumptions previously led to EUF = 0.8 125 and
FESR = 0.2.
We next calculate EUF and FESR over a range of heat to power ratios AD # kG.
(i) For the plant with a WHR only, for AD < kG, the power is taken via the grid from a
conventional power plant. Thus Eqs. (9.17) and (9.18) yield

0.3(1 + AD)
(EUF) = (9.19)
0.75 + 0.2A~ ’
0.12h~
(FESR) = (9.20)
0.675 + 0.3AD ’
EUF and FESR are plotted against AD on the left hand side of Fig. 9.5. (QNu)CG/FCG
is constant at 3/16 over the range from AD = 0 to 2.25, since the operation of the CG
plant remains the same.
(ii) For the plant with a WHB, and for the demand Ab exceeding 2.25, Eqs. (9.10) and
(9.16) give the values of EUF’ and FESR’ as follows:
for + = 1.2,

1.2(1 + Ab)
EUF‘ = (9.21)
4.8 + Ab ’