Aspects of waste heat recovery and utilisation (WHR&U) in pebble bed modular reactor (PBMR) technology

[摘要] ENGLISH ABSTRACT: The focus of this project was on the potential application of waste heat recovery and utilisation(WHR&U) systems in pebble bed modular reactor (PBMR) technology. The background theoryprovided in the literature survey showed that WHR&U systems have attracted the attention ofmany researchers over the past two decades, as using waste heat improves the systemoverall efficiency, notwithstanding the cost of extra plant. PBMR waste heat streams wereidentified and investigated based on the amount of heat rejected to the environment.WHR&U systems require specially designed heat recovery equipment, and as such the usedand/or spent PBMR fuel tanks were considered by the way of example. An appropriatelyscaled system was designed, built and tested, to demonstrate the functioning of such acooling system. Two separate and independent cooling lines, using natural circulation flow in aparticular form of heat pipes called thermosyphon loops were used to ensure that the fuel tankis cooled when the power conversion unit has to be switched off for maintenance, or if it fails.A theoretical model that simulates the heat transfer process in the as-designed WHR&Usystem was developed. It is a one-dimensional flow model assuming quasi-static andincompressible liquid and vapour flow. An experimental investigation of the WHR&U systemwas performed in order to validate the theoretical model results. The experimental resultswere then used to modify the theoretical heat transfer coefficients so that they simulate theexperiments more accurately.Three energy conversion devices, the dual-function absorption cycle (DFAC), the organicRankine cycle (ORC) and the Stirling engine (SE), were identified as suitable for transformingthe recovered heat into a useful form, depending on the source temperatures from 60 ºC to800 ºC. This project focuses on a free-piston SE with emphasis on the thermo-dynamicperformance of a SE heat exchanger. It was found that a heat exchanger with a copper wovenwire mesh configuration has a relatively large gas-to-metal and metal-to-liquid heat transferarea. Tube-in-shell heat exchanger configurations were tested, with the working fluid flowing inten copper inner pipes, while a coolant flows through the shell tube.A lumped parameter model was used to describe the thermo-fluid dynamic behaviour of theSE heat exchanger. In order to validate the theoretical results, a uni-directional flowexperimental investigation was performed. The theoretical model was adjusted so that itsimulated the SE heat exchanger. It was found that after this correction the theoretical modelaccurately predicts the experiment. Finally, a dynamic analysis of the SE heat exchangerexperimental set-up was undertaken to show that, although vibrating, the heat exchanger setupassembly was indeed acceptable from a vibrational and fatigue point of view.

[发布机构] Stellenbosch University