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3rd Thermal and Fluids Engineering Conference (TFEC)

ISSN: 2379-1748

THERMODYNAMIC ASSESSMENT OF ORGANIC RANKINE CYCLE SYSTEMS DURING OFF-DESIGN OPERATION IN COMBINED HEAT AND POWER (CHP) APPLICATIONS

Maria Anna Chatzopoulou
Clean Energy Processes (CEP) Laboratory, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom

Christos N. Markides
Clean Energy Processes (CEP) Laboratory, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom

DOI: 10.1615/TFEC2018.aes.021587
pages 39-50


KEY WORDS: Advanced energy systems, CHP, Combined heat and power, Heat exchanger design, Off-design, Organic Rankine cycle, ORC, Thermodynamic model.

Abstract

Organic Rankine cycle (ORC) engines are a promising technology for recovering waste heat and generating power. In real applications, such as combined heat and power (CHP) systems, ORC engines typically operate under variable heat source conditions and load (demand side). Maximising efficiency, both at nominal and off-design conditions, is crucial for the wider adoption of these systems. A thermodynamic model is developed that predicts the impact of varying heat-source conditions on ORC operation, based on a subcritical lumped model. Alternative working fluids are studied, including low GWP refrigerants and hydrocarbons. The system is first optimised for maximum power output, while recovering heat from the exhaust gases of an internal-combustion CHP engine. A tube-in-tube heat exchanger (HEX) sizing model is also developed, and used for sizing the ORC evaporator and condenser, at nominal conditions. The CHP engine is then run at part-load, and the varying temperature and mass flow rate of the exhaust gases are fed to the ORC engine. The model predicts the new heat transfer coefficient (HTCs) in the HEXs, and the resulting heat input to and output from the cycle, allowing the operating characteristics to be defined. Preliminary results indicate that the U-values in the HEXs drop by up to 34% while the CHP load decreases from 100% to 60%. However, the HEX effectiveness increases by up to 15%, due to the improved mean logarithmic temperature difference across the HEX. HEX and ORC performance maps are then generated, which can be used to predict the system performance over an operating profile and allows selecting the optimal size of ORC HEXs.

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