Abstract
<jats:p>The decarbonisation of the energy sector requires efficient strategies to reduce fuel consumption and greenhouse gas emissions. Organic Rankine Cycles (ORCs) have emerged as a promising technology for waste heat recovery due to their flexibility and ability to operate with low- and medium-temperature heat sources. This work develops a general mathematical framework, grounded in the Helmholtz energy function, to characterise the limiting and optimal efficiency of ORCs equipped with an Internal Heat Exchanger (IHE) when operating with dry and isentropic working fluids. The framework is exemplified using the van der Waals equation of state and extended to real fluids through the PC-SAFT model. Results show that integrating an IHE significantly enhances efficiency for drier working fluids, which expand deeper into the superheated vapour region, enabling greater internal heat recovery. Efficiency gains diminish at condenser temperature extremes, defining operational boundaries where IHE integration is less effective. From a practical perspective, minimising the temperature difference at the IHE outlet (ΔTmin) is critical to maximise performance. The proposed framework provides theoretical insight and practical guidelines for fluid selection and operating strategies in ORC-based waste heat recovery systems.</jats:p>