Waste heat to power via process integration of organic Rankine cycles

Significance 

Energy efficiency plays a fundamental role in developing a sustainable global energy system, mitigating climate change and reducing energy-related CO2 emissions. Heat recovery through heat exchanger network (HEN) synthesis has emerged as a promising strategy for improving energy efficiency in industrial processes. Mathematical programming and pinch analysis approaches have been widely used in HEN synthesis and heat integration. While the former provides process visualization to facilitate efficient energy analysis, the latter is mostly used to address network design issues and cost trade-offs. A major problem in most cases is the rejected low-temperature heat that is difficult to recover even at maximum heat recovery.

Lately, organic Rankine cycle (ORC) has been integrated into industrial processes to improve overall energy efficiency by recovering and converting into power the low-to-medium grade waste heat. The integration process has been extensively studied using different approaches. Recently, superstructure optimization strategies have been extended to optimize ORCs, HEN synthesis and multi-period operations. Most industrial heat recovery processes based on integrating ORCs use sequential approaches where ORC operating conditions are optimized first before HEN synthesis, resulting in suboptimal solutions. Additionally, these processes restrict heat exchange between the process and ORC streams to the regions below the process pinch. However, integrating ORC with the process over a wide temperature range could improve energy efficiency and generate more profit.

To address these shortcomings, Mr. Sung-Ta Li and Dr. Jui-Yuan Lee from National Taipei University of Technology together with Mr. Supaluck Watanapanich from King Mongkut’s University of Technology Thonburi developed a new mathematical programming model for effective synthesis of ORC-integrated HENs. This model was based on a modified stage-wise superstructure and ORC representation considering four different configurations: basic, with regeneration, with turbine bleeding or with both regeneration and turbine bleeding. The work is currently published in the journal, Energy Conversion and Management.

In their approach, the benefits of the proposed model were demonstrated. Heat exchange between the ORC and process streams could occur in all the stages of the superstructure, allowing proper handling of the condensation and evaporation of ORC streams. Regression equations were adopted to determine the thermodynamic properties to avoid highly nonlinear models and violation of overall energy balance. The operating temperatures and the flow rate of the working fluid were treated as variables, while its thermodynamic properties were correlated as functions of the temperature. Further ORC modifications were considered to improve thermal efficiency. The proposed approach was illustrated using two literature examples.

The research team achieved simultaneous optimization of the ORC configuration, HEN structure and operating conditions with maximum net power output and minimum overall energy cost. Besides, integrating the ORC with the process reduced the cold utility consumption by 5% and 25.6% in examples 1 and 2, respectively. Despite an increase in cold utility consumption by 19.5% and an increase in hot utility consumption by 38.9%, minimizing the overall energy cost was still achieved by optimally increasing the net power output by 54.6%. This applied to scenarios with relatively higher electricity prices like example 1 or scenarios where maximizing power output at constant hot utility consumption is a priority like example 2. In both scenarios, however, the simpler HEN designs were obtained at lower capital costs by minimizing the number of heat exchange units.

In summary, a simultaneous optimization approach for effective synthesis of ORC-integrated HENs was developed in this study. The trade-offs between the heat exchange units and energy costs were also explored via sensitivity and Pareto analysis. This model applies to both cases where utility costs dominate the total HEN cost and those where energy efficiency is of key priority. It can also be extended to solar thermal and geothermal ORC systems. In a statement to Advances in Engineering, the authors explained that the new presented approach would contribute to an effective and efficient assessment of the benefits of integrating ORCs with industrial processes.

About the author

Dr. Jui-Yuan Lee is an associate professor in the Department of Chemical Engineering and Biotechnology, National Taipei University of Technology. His research centers on chemical process integration and optimization, with particular research interests in energy systems. He is the author of more than 100 Scopus-indexed publications, with a total of about 1,500 citations and an h-index of 23. Dr. Lee is working with collaborators in Japan, Malaysia, the Philippines, Mainland China and South Africa.

Reference

Watanapanich, S., Li, S.-T., & Lee, J.-Y. (2022). Optimal integration of organic Rankine cycles into process heat exchanger networks: A simultaneous approachEnergy Conversion and Management, 260, 115604.

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