There is great interest in distributed combined heat and power (CHP) generation in the built environment due to the higher overall efficiencies
attained in comparison to separate provision of these vectors. Organic Rankine cycle (ORC) systems are capable of generating additional electricity from the thermal outputs of CHP engine
s
, improving
the electri
cal conversion efficiency and power
-
to
-
heat ratio of
such
system
s
. Thermodynamic analysis and technical
feasibility are at the core of the development of these systems
, while
a
critical factor for the wider adoption of ORC
systems concerns their economic proposition.
O
btain
ing
credible estimates of system costs requires correct sizing
of individual components. This work focuses on the thermodynamic optimisation, sizing and costin
g of
ORC units
in
CHP applications, over a range of heat
-
source temperatures. The working fluids examined include R245fa,
R1233zd, Pentane and Hexane, due to their good performance and favourable environmental character
istics.
The
o
ptim
al
cycles
obtained
c
an
increase the power
-
to
-
heat ratio of the complete CHP
-
ORC
system
by
up to 65%.
A
lternative equipment sizing methods are
then
applied for each fluid and the resultant component sizes are
compared. The cost estimates obtained from t
he alternative methods are
also
compared to real ORC application.
Based on this, a hybrid costing method is proposed
and
applied to
an
ORC system design
,
in order to
obtain the
specific investment cost (SIC
). The results indicate that as the heat source te
mperature increases, the power output
increases, result
ing
in larger and more expensive components. Nevertheless, the SIC drops from 1
7
GBP/W for
low
-
power outputs to
1.1
GBP/W for high
-
temperature/high
-
power outputs.