Energy-based visualisation of a transcritical CO₂ heat pump cycle

Abstract

The ongoing global phase-out of ozone-depleting substances under the Montreal Protocol has motivated research into alternative working fluids for the use in refrigeration applications. The use of refrigerant grade carbon dioxide (R744) in refrigeration systems as an alternative working fluid shows great promise due to its inert properties and its minor environmental impact. It has an ozone depletion potential (ODP) of zero and global warming potential (GWP) of one. The use of carbon dioxide in heat pump systems has been investigated in the literature. However, limited research has been done in the field of fault identification and the isolation thereof for these systems. The objective of this study is to develop an energy-based representation of a transcritical CO₂ heat pump system, to visualise, identify, and monitor the progression of faults that may occur. The energy-based approach is used as a means of system representation because a heat pump is essentially an energy converting device. A representation using energy characteristics thus allows all the physical phenomena present during operations to be summarised into a compact and easily interpretable form. The use of a linear graph representation, with heat pump components represented as nodes and energy interactions as links, was investigated. Node signature matrices were then used to compile the energy information in a more mathematical compact form. To generate the energy and exergy information for the compilation of the energy-based heat pump representation - a descriptive thermal-fluid model of the heat pump system was developed. The thermal-fluid model was based on the specifications of a transcritical CO₂ heat pump test facility. Cost matrices were generated using an algorithm that computes the difference between healthy signature matrices and those with system energy characteristics under fault conditions. The eigenvalues and eigenvectors of the cost matrices were visualised for fault detection purposes.