Hybrid trigeneration - thermally activated heat pump technologies

Abstract

This Thesis describes a theoretical approach to the design and analysis of concepts in the field of trigeneration, which is the combined generation of electricity, hot water and chilled water from a single prime mover. The continuously increasing demand for cooling in modern buildings and cities due to economic expansion, which is further compounded by the effects of climate change, means that there is a tremendous opportunity for new concepts in the fields of trigeneration, refrigeration and heat pump cycles and technologies. It is this opportunity which forms the context and primary aim for the research presented within this Thesis, which is to identify high performance heat pump cycle concepts. A thorough review of trigeneration systems, controls and operational strategies is presented, which demonstrates their energetical benefits in terms of Primary Energy Consumption (PEC) and Energy Utilisation Factor (EUF). This is followed by a review of the state of the art in absorption chiller cycles, adsorption chiller cycles and R744 (carbon dioxide) Mechanical Vapour Compression (MVC) heat pump cycles. To fulfil the main aim of this research, the focus of the review is in multi-stage/multi-effect absorption and adsorption chillers, hybrid compression-absorption chillers and high temperature R744 heat pump and supercritical Brayton cycles. A number of novel absorption heat pump cycle concepts are modeled and analysed, the most advanced of which being: a double-stage/triple-effect ammonia water absorption heat pump cycle, and a triple-stage/triple-effect lithium bromide-water compression-absorption heat pump cycle. Under certain operating parameters, these cycles are capable of achieving a Coefficient Of Performance (COP) in cooling of upto 2.0. However, they are not believed to represent the best opportunity for further research due to: the inability for simultaneous heat and coolth production; potential corrosion problems resulting from the high temperatures; the extremely high volumetric flowrate of steam required through compressors; the significant body of research which has explored almost all avenues for innovations in absorption heat pump cycles. A novel thermally activated transcritical R744 heat pump cycle is developed, which combines the principles of both the Brayton cycle and reverse Rankine cycle. The cycle is designed to utilise relatively hot gas turbine exhaust (∼ 500◦C) gases to produce low temperature hot water and chilled water for domestic hot water and space cooling, respectively. The cycle itself achieves a COP of 1.58 and 2.41 in cooling and heating respectively with a maximum inlet temperature of 350◦C. A Coefficient of Performance in cooling of 1.58 is comparable to a triple-stage/triple-effect lithium bromide-water absorption heat pump cycle. The principal advantage of the novel cycle is that it simultaneously produces hot water (∼ 65◦C), while absorption chillers are not capable of producing heat and coolth simultaneously. Certain components of the novel R744 heat pump cycle are analysed in detail, including: R744 heat exchangers, R744 two-phase ejector, R744 compressor and work expanders. A design and calculation methodology is presented with the purpose of forming the continuation of this research from a Technology Readiness Level of 2 (technology concept and/or application formulated) to a Technology Readiness Level of 3 (analytical and experimental critical function and/or characteristic proof-of-concept).