Optimal integration of organic Rankine cycles into process heat exchanger networks: A simultaneous approach

Organic Rankine cycles can produce power from various low-to-medium temperature heat sources efficiently, and may be integrated into industrial processes to recover low-grade waste heat for improved energy efficiency. This paper presents a mathematical programming model for the synthesis of organic Rankine cycle-integrated heat exchanger networks. The model is based on an organic Rankine cycle representation of four alternative configurations and a modified stage-wise superstructure, where heat exchange between process streams and organic Rankine cycle streams can take place in all the stages. In addition, the flowrate and operating temperatures of the working fluid are treated as variables, and its thermodynamic properties (e.g. enthalpies and pump/ turbine outlet temperatures) are correlated as functions of the temperatures. This allows the organic Rankine cycle and the heat exchanger network to be optimised simultaneously, with the objective of maximising the net power output or minimising the overall energy cost. Two literature examples are used to illustrate the proposed approach. The results show that apart from producing power, the organic Rankine cycle can reduce the cold utility consumption by up to 26% when integrated with a process. Furthermore, the overall energy cost can be minimised by increasing the net power output optimally (by 54.6%), despite increased hot (+38.9%) and cold utility requirements (+19.5%). The proposed approach is thus useful in assessing the benefits from process integration of organic Rankine cycles.