Dynamic Characteristics & Perspectives on Control of a Supercritical-CO2 Closed Brayton-Cycle Power-Loop in a Geothermal Power Plant

by Rajinesh Singh, Andrew S. Rowlands, & Peter A. Jacobs
Queensland Geothermal Energy Centre of Excellence, The University of Queensland

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The prospect of enabling the production of baseload electricity utilising heat from Hot-Dry Rock (HDR) geothermal sources at temperatures of up to 280°C in Australia, has prompted the need to reduce the costs of producing electricity from geothermal heat resources by improving the overall efficiency of the employed power-loop. Power generation using a closed Brayton-cycle power-loop with Supercritical Carbon Dioxide (S-CO2) as the working- fluid, can potentially offer compact cycle components and efficiency improvements if large increases in transport properties close to the critical point of carbon dioxide can be taken advantage of. Here we present the transient analysis of the operation of a closed S-CO2 Brayton-cycle power-loop, in the context of a geothermal power plant.

Initial dynamic simulation results of the response of a laboratory-scale power-loop during startup and heat addition, as well as during changes in cooling-medium temperature are presented. The control of compressor inlet pressure and temperature in the event of a decrease in cooling-medium temperature is investigated using mass addition and the regulation of cooling-medium mass flow, with the aim of maintaining steady supercritical operation with net power output at the nominal design value.

A control-oriented modular dynamic model of the power-loop has been constructed in the DYMOLA simulation environment using zero and one-dimensional thermal-hydraulic mathematical models of components, which describes the time-dependent cycle-fluid related loop behaviour. Models of components that comprise the power- loop include that of the compressor, heat-exchangers, pipes, and the expander. Proportional-Integral controllers have also been implemented in the power-loop for the control of compressor inlet pressure and temperature. The results of the dynamic simulation of the power-loop include time-dependent mass flows, pressures, temperatures, and net generated power.

Variations in power-loop pressures and temperatures can occur with fluctuations in cooling-medium temperature, resulting in both deviations in net generated power from the nominal design value and in two-phase flow in the compressor. The control of compressor inlet temperature using cooling-medium mass flow regulation is found to be a suitable control strategy to maintain steady supercritical operation with net generated power close to the nominal design value, in the advent of a decrease in cooling-medium temperature. The one-dimensional thermal-hydraulic modelling and simulation of fluid-component dynamic interactions in the power-loop in the event of disturbances and during transient events expected in a HDR geothermal power plant, gives insight into closed-loop thermal- hydraulic stability and aids in understanding the performance deviations for the development of control strategies for high output and stable power generation.