Development of a HTHP design tool for an industrial waste heat recovery system integrated with a two-phase variable geometry ejector and IHX using natural refrigerants
The accelerated use of thermal energy by industrial sector is accompanied by an increase in waste heat rejection. High-temperature heat pumps (HTHPs) could significantly improve the overall energy efficiency and contribute to energy circularity of industrial processes by enabling waste heat recovery back to process temperatures through temperature upgrade. However, the general requirement for high temperatures in the industrial sectors constitute a technological challenge. The present work focuses on the design and performance assessment of an HTHP in an innovative configuration integrating a two-phase variable geometry ejector (VGE) for expansion work recovery and an internal heat exchanger. A numerical design tool was developed and applied for studying the use of five different environmentally friendly working fluids. VGE features considered allow the active adjustment of the ejector area ratio and the hypothetical throat section using a spindle and a changeable primary nozzle exit position. The model is based on the application of mass and energy conservation principles for the individual system components and for VGE cross-sections, applying real gas thermodynamic properties. The simulations were conducted, in the EES software, for a small-scale unit with a minimal heat output of 20kW at constant temperature of 130ºC and with a gross temperature lift range from 30ºC to 60ºC. The results showed that R601 performed the best for the operating range considered. For R601 and a temperature lift of 60ºC, it was found that VGE integration can lead to electrical consumption by 15% (COP=3.1) lower than without the ejector. Similarly, for the same heat output, compressor flow rate decreased by 23% which also contributes to a reduced sensitivity of the cycle to variable operating conditions. Based on the results, it is concluded that the integration of a VGE into the HTHP systems is highly beneficial both in terms of performance and operation stability.