The development of a hydrocarbon high temperature heat pump for industrial heating applications
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This thesis describes the development of a high temperature heat pump that delivers heat up to 115 oC for industrial applications in a more efficient and environmentally friendly way. The heat pump will increase heat utilization efficiency by the recovery of waste heat in industrial processes. By making use of natural working fluids such as propane and butane, which have zero ozone depletion potential and negligible global warming potential, the heat pump will not only be a cost-effective alternative to conventional heating systems but also a clean energy system for a greener future. Advancing the concept of energy sustainability has become imperative if future generations are to have a conducive environment to live in. The approach for this combines the process of increasing the efficiency of energy use and the avoidance of harmful substances in energy generation to achieve its goals. The production and use of harmful substances that are considered greenhouse gases and halocarbons and their emission to the atmosphere, poses a direct danger to the environment. Also, the installation of systems with poorly integrated processes for efficient heat utilization results in the production of waste heat, which is a loss of potentially useful energy. This has indirect influence on energy sustainability through continuous dependence on energy systems that are not cost effective and are not environmentally friendly. The high temperature heat pump developed in this thesis followed a stepwise procedural methodology to solve the challenge of heat energy utilization and waste heat recovery as observed in industrial processes. The process of development involved the identification of the technological need, the clarification of operating boundaries through a case study, the heat pump concept development, the selection of working fluid and choice of cycle configuration, the theoretical evaluation of models and finally the experimental investigation of a laboratory scale system. A state-of-the-art review was conducted to evaluate existing technology, highlight challenges, and set the focus for the research. This thesis presents the results of the activities done in the research. Early investigation of the temperature of heat demand in the case study showed the possibility to utilize a heat pump if the technology is improved. The abundance of waste heat, which is typically free or in some cases disposed to the environment at a cost, provided an opportunity to develop a cost-effective alternative to conventional heating systems. Further research suggested several fluids suitable in a heat pump for the temperature range considered. The research considered propane and butane as a working fluid in a cascade configuration heat pump as the start point for the development of the technology. Theoretical analysis is used in the research to investigate the operating parameters of the heat pump. This revealed the performance potential of the design, the possibilities for optimization and the challenges for its physical development. The high temperature cycle of the cascade configuration design with butane as working fluid is found to have the compressor operating parameters outside commercially available compressors. This butane compressor formed the limiting component of the technology as other components were within available technology. Modifications to existing compressor technology were proposed and further theoretical analysis showed that with the constraints of 80 oC maximum suction temperature and 140 oC maximum discharge temperature at the compressor, the heat pump can operate effectively and with good coefficient of performance (COP) above 2.0 for a temperature lift over a 100 K. This is achieved by the careful sizing of the internal heat exchanger and operating the cascade heat exchanger within certain parameters. After the theoretical demonstration of the technology, a 20 kW heating capacity cascade configuration heat pump was built in the laboratory to conduct experiments and validate the theoretical results. The heat pump showed an average COP of 2.2 for a temperature lift of 87 – 109 K. The high temperature cycle is installed with the modified prototype butane compressor which showed an average of 74 % total compression efficiency. The compressor discharge and suction temperature had an average of 127 oC and 68 oC across all operating conditions. This suggests the possibility to increase the heat delivery temperature to a value above 115 oC required by the case study. With a heat delivery temperature of 115 oC, the heat pump will meet the heating specifications of industrial applications such as pasteurization, drying, distillation, sterilization, pressurized hot water production and other industrial processes within the same temperature demand. By the recovery of waste heat, the heat pump will not only replace the energy capacities of conventional heating systems such as steam boilers, but also the cooling demand capacities of cooling towers and dry coolers thereby improving the overall heat utilization efficiency of the industrial process.