Distributed combined heat and power (CHP) systems have the potential to cover a significant amount of the global energy requirements for power, and heating. Small-to-medium scale CHP systems, in the built environment and in the industry (up to a few MWs), are typically driven by internal combustion engines (ICE). In CHP-ICE systems, more than 55% of the energy input is transferred as heat in the exhaust-gas stream and the jacket water cooling circuit. Unless these thermal outputs are utilised,

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... he energy will be released to the atmosphere as waste heat, deteriorating the system's efficiency. Organic Rankine cycle (ORC) engines are a promising heat-to-power technology, for converting waste heat into power. Therefore, coupling ORC engines as bottoming cycles to CHP-ICEs can maximise overall system efficiency, and reduce energy costs. In this thesis, the design of ICE-ORC CHP systems is investigated from thermodynamic, operating and economic perspectives, aiming to fully unlock the potential of such advanced high-efficiency cogeneration systems. An integrated ICE-ORC CHP optimisation tool is developed, which, unlike previous studies, captures the performance trade-offs between the two interacting engines, to optimise the combined system performance. A dynamic ICE model is developed and validated, along with a steady-state model of subcritical recuperative ORC engines. Multiple working fluids are investigated, along with naturally aspirated and turbocharged ICEs. By optimising the combined ICE-ORC CHP system simultaneously: i) the total power output increases by up to 30%, in comparison to the conventional approach where the two engines are optimised separately; ii) the electrical efficiency increases by up to 21%, in comparison to the stand-alone ICE; and iii) in the integrated system the ICE operation is adjusted to promote the ORC power output, which generates up to 15% of the total power, improving fuel efficiency. When focusing on maximising power output only, this comes at the cost of higher fuel consumption. I [...]