1D MODELLING OF A WATER-STEAM EJECTOR AS A COMPRESSION STEP IN HIGH-TEMPERATURE HEAT PUMPS

Abstract

High-temperature heat pumps (HTHPs) have emerged as a promising solution for decarbonizing process heat. With natural refrigerants like water, Rankine cycle-based HTHPs can deliver process heat up to 200°C, which covers a wide range of industrial processes. However, achieving these high temperatures necessitates advanced turbomachinery. Current designs processes for steam compressors are intricate and often tailored for specific applications. Furthermore, achieving the desired temperature typically requires multiple compressors, adding to the complexity of the system. To address this, ejectors can be utilized as a secondary steam compression mechanism within the heat pump architecture. By integrating an ejector, a portion of the heat pump condensate can be used and mixed with superheated steam from the compressor to simultaneously de-superheat the steam and raise its pressure. This approach can potentially reduce both compression power and the number of compression stages required. This paper presents a one-dimensional mathematical model of a water-steam two-phase ejector designed for HTHPs. Using thermodynamic 1D modelling, differential conservation equations and the IAPWS-IF97 equations of state are applied across the ejector’s components, accounting for flow compressibility and its two-phase nature. Closing equations are used in the mixing and diffuser sections to simulate the transfer of mass, momentum, and energy between the two streams, assuming homogeneous equilibrium. A specific use case for the ejector’s integration in a high-temperature heat pump cycle identified the boundary conditions for the simulations. The model enables the calculation of the 1D distribution of flow variables and key ejector performance indicators, such as the pressure ratio. This research offers advancements in two-phase water steam ejector modelling, shedding light on their potential as steam compression devices in HTHPs.