Page 205 - Advanced Gas Turbine Cycles

170 Advanced gas turbine cycles

9.2.2. Artijicial thermal eflciency

A second criterion of performance sometimes used is an ‘artificial’ thermal efficiency
(vA) in which the energy in the fuel supply to the CHP plant is supposed to be reduced by
that which would be required to produce the heat load (eu) in a separate ‘heat only’ boiler
of efficiency (VB), i.e. by (QU/vB). The artificial efficiency (vA) is then given by

(9.4)

where is the overall efficiency of the CHP plant.
For the unfired plant of Fig. 9.2a and taking 178 = 0.90, the artificial efficiency would be

0.25 0.25
--
7)A = 2.25 - 0.375 = 0.666.
I--
(0.9)4
For the supplementary fired plant of Fig. 9.2b, the artificial efficiency would be

9.2.3. Fuel energy saving ratio

A third performance criterion developed for combined heat and power plant involves
comparison between the fuel required to meet the given loads of electricity and heat in the
CHP plant with that required in a ‘reference system’. The latter involves conventional
plants that meet the same load demands (indicated by subscript D), for example, in a
conventional electric power station and in a ‘heat only’ boiler.
Such a ‘reference system’ is shown in Fig. 9.3a. The overall efficiency of the
conventional electric power plant is 7)c (for simplicity the subscript 0 for overall
efficiency is dropped from here onwards); the (demand) electrical load is unity. The ratio
of heat to electrical demands is AD, so that the demand heat load is taken as AD. The
efficiency of the ‘heat only’ boiler is vB so the fuel energy required for the boiler is
(AD/%), i.e. there are heat losses AD[(l/vB) - 11 involved before heat is delivered to
district or process heating.
A CHP system meeting the same power and heat demands (1, AD) is shown in Fig. 9.3b;
it is implied that this cogeneration plant is perfectly matched, delivering the required
(1, AD) precisely, using a WHR.
The total fuel energy required in the reference system is