Organic Rankine cycle
(ORC)
engines in waste
-
heat recovery applications experience variable heat
-
source conditions (
i.e.
temperature
and mass fl
ow rate
variations
). Therefore
maximising
the ORC system performance under off
-
design conditions is of
key importance, for the financial viability and wider adoption of these systems.
In
this paper
,
the off
-
design performance of
an
ORC
engine
is investigated,
with
screw expander and
two
heat exchanger (
HEXs
)
architectures,
while recovering heat from an
internal combustion engine (ICE)
.
Unlike
previous
s
tudies
where the ORC expander
and
HEX
s
performance is assumed fixed
during
off
-
design operation,
in this work
we consider the time
-
varying characteristics of the system components.
Firstly, nominal
system
sizing r
esults indicate that the screw expander isentropic efficiency exceeds 80%
,
while
th
e plate HEXs (PHEXs) heat
transfer
area requirements
are 50% lower
,
than the respective
ones for
double pi
pe (DPHEX) design
. Next, the ORC engine
operation is optimised at part
-
load
(PL)
ICE conditions.
Although, the HEXs
heat transfer coefficients
decrease
by 30% with
part
-
load, the
HEX
effectiveness increases
by up to 20%
, due to higher temperature difference across the working fluids.
Findings
also
reveal that t
he PHEX performance is less sensitive to the off
-
design operation.
Optimum o
ff
-
design power output
maps
indicate
that
the
ORC e
ngine
PL
reduces to 72%, for ICE PL of 60%
, while
ORC
engines with PHEX
s
generate
slightly
more power
,
for the same heat source conditions.
Overall, th
e modelling tool developed
predict
s
the
ORC performance over an
operating envelope and allows the
selection of optimal designs and sizes of ORC HEXs
and expanders
.
The findings can be used
by ORC plant operators to optimise the ORC engine power output, given the varying heat source conditions observ
ed on their
site
,
and by ORC vendors to inform HEX and
expander design decisions.