Optimization Design Research of Supercritical Carbon Dioxide Power Cycle under Low-temperature Heat Sink Environment

Abstract

This paper presents a cold-end optimization design study of a nuclear energy power conversion system in the context of low-temperature heat sink applications. Based on the principles of simple recuperation cycles and recompression cycles, the study focuses on the supercritical carbon dioxide power cycle and its cold-end optimization for application in low-temperature heat sink environments. These strategies are designed to enhance the efficiency and reduce the operating pressure of the supercritical carbon dioxide power cycle under low-temperature heat sink conditions. Three different cold-end optimization approaches were proposed, which include: a non-condensing liquid-phase compressed supercritical carbon dioxide power cycle, a condensing liquid-phase compressed supercritical carbon dioxide power cycle, and a gas-phase compressed supercritical carbon dioxide power cycle. This research established a thermodynamic analysis program, which modeled turbine machinery such as compressors and expanders using isentropic models. Heat exchangers such as recuperations and coolers were simplified and modeled, with pressure drops and end temperature differences assumed. The study’s thermodynamic analysis program was compared with computational results from a Sandia National Laboratories research report, confirming the accuracy of the thermodynamic analysis methodology presented in this paper. For a 100 MW scale power generation unit, the thermodynamic analysis program was used to conduct computational analyses on both simple recuperation cycles and recompression cycles. These cycles were driven by a medium-to-high-temperature reactor heat source with a turbine inlet temperature of 550 ℃, and a medium-to-low-temperature reactor heat source with a turbine inlet temperature of 315 ℃. The study reveals that the optimal cold-end optimization approach varies based on the heat source temperature and cycle configuration. For the medium-to-high-temperature reactor heat source, the gasphase compressed supercritical carbon dioxide power cycle in the simple recuperation cycle configuration achieves the highest efficiency at 39%. In the recompression cycle configuration, the condensing supercritical carbon dioxide power cycle achieves the highest efficiency at 48%. For the medium-to-low-temperature reactor heat source, regardless of the cycle type, the condensing supercritical carbon dioxide power cycle yields the highest efficiency, the highest efficiency is 28% for the simple recuperation cycle and 33% for the recompression cycle. When comparing the cold-end optimized supercritical carbon dioxide power cycle to conventional supercritical carbon dioxide power cycles with the same maximum temperature and pressure, the optimized system exhibits a reduction in operating pressure drop and an efficiency improvement of 2% to 4%. The cold-end optimized configurations of the supercritical carbon dioxide power cycle hold promising applications for nuclear energy power conversion systems with low-temperature heat sink environments.

Keywords

• supercritical carbon dioxide power cycle
• low-temperature heat sink
• cold-end optimization