Optimisat
ion of a small
-
scale bottoming organic Rankine c
ycle (ORC) engine is carried out for a combined
biomass
-
gasifier
-
CHP system, drawing heat from the
syn
gas
conditioning unit of the gasif
ier and the exhaust
gas of the internal combustion
(IC)
engine
. The optimisation considers different working fluids and selection
of a positive
-
displacement expander. Single
-
and two
-
stage screw expanders and single
-
stage reci
procating
-
piston expanders are modelled in order to capture the variation in their performance at a range of design
points.
Double
-
pipe heat exchangers are employed for both evaporator and condenser, le
ading to a low
-
cost
but bulky design.
The system is op
timised first for maximum electrical power output, and second for
minimum specific investment cost
(SIC)
. Cost correlations are used for each of the principal ORC
components.
The optimal design for minimum SIC is found to be a two
-
stage screw expander with
ethanol
as the working fluid, which produces a 13
.6
% increase in the electrical power output relative to the system
without an ORC.
The investment attractiveness of the whole system with and without the bottoming ORC is
assessed and the system is found to
be profitable for avoided electricity costs above 150 $/MWh
el
and
biomass costs of 50 $/t
, with the ORC making the system more attractive in all cases studied.
Discounted
payback periods range from 1
2
years at 150 $/MWh
el
to 3.
5
years at 250 $/MWh
el
for
the system with ORC
.