Design of multi-cycle organic Rankine cycle systems for low-grade heat utilisation

Organic Rankine cycles (ORCs) facilitate the utilisation of low-grade heat sources (e.g., geothermal and industrial waste heat) for power generation, thereby improving energy efficiency in industrial processes and expanding the application of renewable energy. Using multiple ORCs instead of a single cycle provides more flexibility in heat integration and can increase the power output. This paper presents a mathematical model for designing multiORC systems; the design task involves the determination of ORC configurations and operating conditions whilst synthesising the associated heat exchanger network. Two case studies on geothermal and industrial waste heat ORC applications illustrate the developed optimisation formulation. In the geothermal case study, the maximum net power output for a single regenerative n-butane cycle can increase by 11.2 % as a result of optimising the ORC operating conditions. With two independent n-pentane cycles, a 7.6 % increase in the maximum net power output can be reached by optimising the ORC configurations. In the industrial waste heat case study, a 14.3 % increase in the maximum net power generation is found with a second n-butane cycle, and a further 5.7 % increase with a third. For comparison, the total annual cost and the payback period are also calculated in both case studies.