Master's thesis and Internship : Experimental Analysis of a Thermal Compressor in a Supercritical CO2 Waste Heat Recovery Cycle

Abstract

The increasing demand for energy efficiency and the urgent need to reduce greenhouse gas emissions have intensified interest in the recovery and valorisation of low-grade waste heat. Although heat sources below 100 °C represent the largest share of global waste heat, their low thermodynamic quality severely limits the performance of conventional heat-to-power technologies such as steam Rankine cycles and even advanced Organic Rankine Cycles (ORCs).

In this context, this master’s thesis investigates an innovative low-grade waste heat recovery concept developed by Cixten, based on a Brayton-like thermodynamic cycle using supercritical carbon dioxide (sCO₂) as the working fluid. The originality of the system lies in the use of a thermal compressor, which exploits the strong pressure–temperature dependence of sCO₂ under near-constant-volume conditions to achieve a pressure rise primarily through heat input rather than mechanical compression, with the objective of reducing auxiliary power consumption for heat sources in the 60–100 °C range.

A dedicated experimental test bench was developed jointly by Cixten and the University of Liège to enable controlled experimental investigations. The thesis reviews the thermodynamic background and state of the art of low-grade waste heat recovery, with particular emphasis on ORC and sCO₂-based systems, and describes the operating principles of the Cixten concept and of the thermal compressor.

Experimental results are presented from multiple test campaigns conducted on the sCO₂ loop. A dedicated investigation of the Coriolis mass flowmeter demonstrated that the instrument was operating correctly from the beginning of the experimental campaign. The observed measurement inconsistencies were traced back to a controlled valve that was not properly sealed, leading to unintended leakage. After corrective actions, the reliability of the mass flow measurements could be confirmed within the validated operating conditions.

A separate experimental campaign was dedicated to the investigation of the thermal compressor, which constituted the core focus of this work. The results showed that the thermal compressor did not operate as initially expected and was unable to achieve stable and repeatable thermal compression under the tested operating conditions. Critical limitations related to the proper filling of the test bench and to the behaviour of the internal moving components, suspected to be partially or fully immobilised, were identified.

Rather than providing performance validation, the experimental investigation enabled a detailed diagnosis of the thermal compressor behaviour and identified key technical and operational issues that prevented the system from operating as intended. This analysis provides essential guidance for future troubleshooting, corrective actions, and subsequent experimental validation of thermal compression in the test bench.