Use of  Organic Rankine Cycles in Waste Heat Usage: Adding Value to Waste Heat

Most thermal processes involve the rejection of 'waste' heat. Making effective use of this heat is becoming increasingly important, as waste energy usually represents carbon dioxide emissions. This heat can be used directly, as heat, or to generate electricity. These uses add value and reduce global warming and thermal pollution. Using waste heat for other duties on process plants (i.e. “heat integration”) is the best option, but the heat sources and sinks in a process are not usually a perfect fit. If the remaining waste heat is significant, electricity generation should be investigated. Steam Rankine cycles (SRC), the Kalina cycle and Organic Rankine Cycles (ORCs) can be used to convert waste heat to electricity. The niche of the ORC is operation at lower temperatures, from 400oC down to 90oC, where the heat is difficult to recover by conventional means. This “low-grade” heat can be produced by not only industrial processes, but also solar, geothermal and biomass. Over the past thirty years ORC usage has increased significantly in many industries due to their capability to recover low-grade waste heat and profitably convert it to useful power. 

So far, debate has been about the best way to apply ORCs in industry in order to achieve optimum thermodynamic and economic performance. This debate has covered selection of working fluids and ORC configuration. Research to date has focussed on improving the performance of ORC by optimising working fluids and investigating utilization of regenerators, internal heat exchangers, recuperators and various types of turbine to achieve maximum power and minimum cost.

The choice of working fluid is challenging, as there are a wide variety of working fluids available, and selection criteria include ORC capital cost, toxicity, flammability, availability, efficiency, environmental acceptability and operational safety. Furthermore, the process of choosing the working fluid is becoming more difficult because the working fluid options are reducing, as various fluids are progressively banned or phased out due to their adverse effects on the ozone layer and/or global warming.

This research is aimed at solving these working fluid and configuration selection problems by developing an ORC 'expert system' which can identify when ORCs should be used, and, once identified, determine the optimal configuration, working fluid and operating conditions.

This will involve:

• A comprehensive survey of the literature to create a database

• Data validation using simulation software

• Data validation using experimentally determined values (when simulation data is absent or insufficient

• System development by trialling on expert users

Most process engineers do not have the time to become experts in this area, nor the expertise to recognise when and where an ORC could be implemented. This novel expert system will allow this and replace commonly-used procedures that consume significant time and effort.

For more information please contact Prof Adam Harvey or Prof David Reay.