Organic Rankine cycle (ORC) engines are suitable for heat recovery from internal combustion engines (ICE) in combined heat and power (CHP) systems. However, trade-offs must be considered between ICE and
ORC engine performance in such integrated solutions. The ICE design and operational characteristics influence its own performance along with
the exhaust-gas conditions available as heat source to the ORC engine,
impacting ORC design and performance, while the heat-recovery heat
exchanger (ORC evaporator) will affect the ICE operation. In this paper, an integrated ICE-ORC CHP whole-system optimisation framework is presented. This differs from other efforts in that we develop and apply a fully-integrated ICE-ORC CHP optimisation framework, considering the design and operation of both the ICE and ORC engines
simultaneously within the combined system, to optimise the overall system performance. A dynamic ICE model is developed and validated, along with a steady-state model of subcritical recuperative ORC engines.
Both naturally aspirated and turbocharged ICEs are considered, of two
different sizes/capacities. Nine substances (covering low-GWP refrigerants and hydrocarbons) are investigated as potential ORC working fluids. The integrated ICE-ORC CHP system isoptimised for either
maximum total power output, or minimum fuel consumption. Results
highlight that by optimising the complete integrated ICE-ORC CHP system simultaneously, the total power output increases by up to 30% in comparison to a nominal system design. In the integrated CHP system,
the ICE power output is slightly lower than that obtained for optimal stand
alone ICE application, as the exhaust-gas temperature increases to promote the bottoming ORC engine performance, whose power increases
by 7%. The ORC power output achieved accounts for up to 15% of the total power generated by the integrated system, increasing the system efficiency by up to 11%. When only power optimisation is performed, the
specific fuel consumption increases, highlighting that high-power output comes at the cost of higher fuel consumption. In contrast, when specific fuel consumption is used as the objective function (minimised), fuel consumption drops by up to 17%, thereby significantly reducing the operating fuel costs. This study proves that by taking a holistic approach to whole-system ICE-ORC CHP design and operation optimisation, more power can be generated efficiently, with a lower fuel consumption. The findings are relevant to ICE and ORC manufacturers, integrators and
installers, since it informs component design, system integration and
operation decisions.