Numerical investigation of liquid injection and bearing leakage in industrial heat pumps
Process heating accounts for 53.5% of the total industrial energy demand, and thus contributes significantly to industrial greenhouse gas emissions. 37% of this heat demand is required at temperatures below 200 °C, which means high-temperature heat pumps (HTHPs) have significant decarbonization potential. These heat pumps have been demonstrated up to 160 °C, but are available up to 200 °C only as concept or at lab scale. Recently, natural refrigerants are favoured to synthetic ones as they are more environmentally friendly. A 30 kWe HTHP demonstrator using water-ammonia mixture as refrigerant is currently being commissioned at Ghent University. It is designed to heat up oil to 200 °C. The demonstrator is equipped with refrigerant injection to avoid excessively high compressor discharge temperatures. Moreover, it uses an oil-free twin-screw compressor. The hydrodynamic bearings are lubricated using the refrigerant mixture itself, meaning bearing leakage provides additional cooling effect. This contribution presents a dynamic model of this experimental setup to study the system performance and control strategy. In particular, the optimization of the liquid injection and bearing lubrication loop pressure are addressed, and the potential impact of the currently unknown bearing leakage is studied to support the commissioning efforts. The study shows that liquid injection effectively limits the compressor discharge temperature when adequately controlled. The lubrication loop pressure on the other hand has negligible impact on the system operation and is thus not a priority during commissioning. Finally, it is shown that limited bearing leakage can benefit the heat pump performance, as long it does not become excessive.