Towards improvement of waste heat recovery systems: A multi-objective optimization of different organic Rankine cycle configurations

Abstract

In industrial processes, a significant amount of waste heat is produced. However, most of the low-grade waste heat is directly dismissed into the environment without being used. With the benefits of energy efficient, power generation capability and environmentally-friendliness, the Organic Rankine Cycle (ORC) represents an effective approach to recover energy from low-grade waste heat. This work aims at finding the optimal selection of ORC (cycle configuration-working fluid type) by using the thermo-economic optimization with the multi-objective genetic algorithm as the optimizer. The ORC configurations using each working fluid with optimization to its optimum conditions and the associated thermo-economic parameters are compared under every optimum condition. Results indicate that, the selection of electricity production cost (EPC) and net power output as objectives can be more attractive for ORC configurations, due to the full utilization of the possible heat in the exhaust gas and to the low EPC for all working fluids. Comparing the cycle configurations, ORC with IHE exhibits approximately 0.4–5% and 2.53–8.78% higher net power output compared to regenerative ORC and basic ORC, respectively. However, regenerative ORC is mostly suitable for wet fluids. Moreover, the basic ORC configuration has the lowest EPC, followed by the regenerative configuration, while ORC-IHE has the highest EPC; the EPC is highly affected by the cycle configuration, regardless of the working fluid type. In particular, it is found that, the working fluids reaching the optimal performance are those ones that have an optimal evaporation temperature to inlet temperature of the heat source ratio between 0.68–0.75, 0.66–0.73 and 0.64–0.73 for basic-ORC, ORC-IHE and regenerative ORC, respectively.