Experimental development and modelling of a novel auxiliary power unit for heavy trucks
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This thesis identifies the key technical requirements for a heavy truck auxiliary power unit (APU) and explores a potential alternative technology for use in a next-generation APU that could eliminate key problems related to emissions, noise and maintenance experienced today by conventional diesel engine-vapour compression APUs. Through evaluation of alternative technologies, the work identified a free-piston Stirling engine coupled to a zeolite-water adsorption chiller as being an effective technical solution to the range challenges faced by the industry. A prototype test rig of this Stirling-adsorption system (SAS) was constructed and experimentally characterised to investigated system integration dynamics and overall performance. The adsorption chiller achieved an average COP of 0.42 ± 0.06 and 2.3 ± 0.1 kWt of cooling capacity at the baseline test condition. The behaviour of the Stirling and adsorption subsystems were investigated through semi-empirical reduced order sub-models calibrated by measured experimental test data. These were combined with fundamental physics-based sub-models of other components in the Mathworks SimScape® environment. Using this system-level model, a series of duty cycle test scenarios were simulated, which showed that the SAS has overall average electrical and cooling efficiencies of 8.7% and 27.1%, respectively, compared to values of 4.7% and 11.0% for incumbent technology. The model was also used to explore the impact of thermal coupling between the engine and chiller. The work proposed a basic control scheme that dynamically prioritizes cooling or electrical demand in order to meet the overall system requirements. Furthermore, the work identified that using the main truck engine’s coolant volume as a thermal buffer tank could significantly reduce the negative impacts on performance of low thermal buffering in the SAS architecture. The results and experience obtained from the prototype SAS test rig demonstrates that there appear to be no major technology barriers remaining that would prevent adoption of the SAS concept in a next-generation APU. Although there are likely still commercial challenges facing the SAS architecture relating to system capital cost and larger size and weight (albeit still feasible). Nonetheless, such a system could offer reductions in exhaust emissions of greenhouse gases (GHGs), and ozone-depleting substances, while producing less noise and requiring lower maintenance than incumbent technologies. A system payback period is estimated to be 4.6 years.
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