Organic Rankine cycle (ORC) engines
are suitable for the
conver
sion of
low
-
grade h
eat into
useful
power.
While numerous
substances
are available as
ORC working
-
fluid
candidates, computer
-
aided molecular design
(CAMD) techniques allow the
rigorous
selection of an optimal working fluid during system optimisation. The aim
of this
present
s
tudy is to extend an existing CAMD
-
ORC framework [1,2]
by incorporating
, in addition to
thermodynamic performance objectives, economic objectives when determining the optimal
system
design,
while maintaining the facility of selecting optimal working fluids
.
The
SAFT
-
γ Mie equation of state is used to
predict
the
thermodynamic properties of
the
working fluids
(here, hydrocarbons)
that are
relevant to the
system
s
’
economic
appraisals
and critical/transport properties are estimated using empirical group
-
contri
bution
methods
. System investment costs are estimated with equipment cost correlations for the key system
components,
and
the stochastic NSGA
-
II solver is used for
system
optimisation. From a set of NLP
optimisations, it is concluded that the optimal molec
ular size of the working fluid is linked to the heat
-
source
temperature. The optimal specific investment co
st (SIC) values were £10,120/kW and
£4,040/kW
when using
heat
-
source in
let temperatures of 150
°C
and
250
°C
(representative of low
-
and medium
-
grade
heat)
respectively,
and
the corresponding optimal working fluids were propane, 2
-
butane and 2
-
heptene.