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22 Antonino La Rocca et al.
for cogeneration of electric energy and heat to be supplied at LNG to be
regasified. The exergetic efficiency ζCHP for the cogeneration system can be
defined by
η
CHP
Peltop + Pelbottom + ExQreg
= .
(8)
Ex
NGburned
(b) S e c o n d d e f i n i t i o n : Exergetic efficiency of the CHP cycle is
defined by
ζ
CHP
Pel + Pel + Ex
=
Ex + Ex + Ex
top bottom Ng
NGburned air LNG
where ExLNG=ExcoldLNG+WpLNG; Exair is the exergy of air going to the suction side
of top cycle compressor; air is cooled by sea water coming out from OR units.
The definition of Eq (9) properly accounts for all kind of exergy items
involved in such a complex process. The parameters defined by the
relationships of Eqs (8) and (9) for the CHP plants analysed are:
ζCHP ζ * CHP
Working fluid in bottom cycle: Helium 0.51 0.49
Nitrogen 0.53 0.47
The exergy efficiency defined by the Eq (9) is the most appropriate,
because it accounts for all kind of exergy item involved in such a complex CHP
plant. The results show that a plant working with Helium has a higher exergetic
efficiency.
4.5. Results of Exergetic Analysis for the Plant Working with Helium.
A detailed analysis has been performed, considering all items which
compose the whole top cycle process. There is a need for this approach to
derive a comprehensive synthesis of exergy losses and exergy recovered. Fig.
11 shows results obtained for the top cycle of a CHP plant working with
Helium. Input exergy is given by
the exergetic efficiency is defined as
.
(9)
ExNGburned+ Exair= 169.975.79 kW; (10)
ζ
topCHP
Pel + Ex
=
Ex + Ex
top Qrec
NGbumed air
,
(11)
the input item includes the exergy of heat power furnished in top cycle (gas
generator CC, Fig. 9) and exergy of air going to the suction side of top cycle
compressor; air is cooled by sea water coming out from OR units. In Fig. 11
items are reported in detail of exergy losses in main plant components and