Transient Modelling of a Positive Displacement Pump for Advanced Power Cycle Applications

Abstract

Compared to the conventional fossil-based steam Rankine cycle, supercritical carbon dioxide (sCO2) power cycles possess the potential to achieve higher thermodynamic efficiencies and lower component costs. As such, they have been proposed for the next generation of renewable energy technologies in an effort to strive for a sustainable future by reducing global greenhouse gas emissions.The improvement in thermal-to-electric energy conversion efficiency in sCO2 power cycles is primarily attributed to the reduction of power required at the compression stage. Although small-scale prototype CO2 compressors have been developed and tested, large-scale commercial units are currently unavailable due to their inherent complexity in design. This is one of the primary drawbacks contributing to the present absence of large-scale operational sCO2 power plants. Fortunately, large-scale CO2 transfer pumps are commercially available today and they possess the potential for direct integration into the sCO2 power system. This presents an opportunity to eliminate the research efforts required to develop a new large-scale CO2 compressor technology.To assess the compatibility and performance of the CO2 transfer pump in the sCO2 power cycle, a detailed theoretical model of the pump system is sought. The current research focus is to establish a numerical model using the Python programming platform for predicting and analysing the transient behaviour of the single-acting positive displacement CO2 pump, in particular, triplex and quintuplex systems were studied. Zero-dimensional (0D) lumped parameter modelling of the fluid system was initially completed to establish a baseline model. Next, compressible quasi-one-dimensional (1D) Euler equations coupled with Helmholtz energy formulation were used to describe the flow behaviour and thermophysical properties of the working fluid accounting for real gas effects. In the detailed model, valve dynamics and the multiplex system response were also investigated.The computer models developed in this thesis provided good insight into to the transient dynamics of a positive displacement CO2 pump operating in a simple environment under ideal inlet conditions. The response characteristics of the transient pump model showed good agreement with experimental data obtained form the literature. The primary benefit of developing an accurate numerical model of the CO2 transfer pump without the use of commercially available computational fluid dynamics (CFD) packages is the significant savings in computational effort and cost. A dynamic model integrated into a power system not only predicts transient system behaviours, but it can also adjust system inputs accordingly to meet the desired operating conditions. Development of a modular code also permits ease of integration and adaptation into more complex systems. The numerical model developed in this study aims to help form the basis of creating a complete transient model of an advanced CO2 based power cycle.