Elsevier

Applied Thermal Engineering

Volume 99, 25 April 2016, Pages 572-582
Applied Thermal Engineering

Research Paper
Experimental and numerical investigation of supercritical CO2 test loop transient behavior near the critical point operation

https://doi.org/10.1016/j.applthermaleng.2016.01.075Get rights and content

Highlights

  • A test was performed to simulate reduction of cooling in a S-CO2 power system.

  • S-CO2 system transient analysis was performed with a gas system analysis code, GAMMA.

  • The code was upgraded to have better code prediction near the CO2 critical point.

  • The code prediction showed reasonable agreement with the experimental data overall.

Abstract

Despite the growing interest in the supercritical CO2 (S-CO2) Brayton cycle, research on the cycle transient behavior, especially in case of CO2 compressor inlet condition variation near the critical point, is still in its early stage. Controlling CO2 compressor operation near the critical point is one of the most important issues to operate a S-CO2 Brayton cycle with a high efficiency. This is because the compressor should operate near the critical point to reduce the compression work. Therefore, CO2 compressor operation and performance data from the S-CO2 compressor test facility called SCO2PE (Supercritical CO2 Pressurizing Experiment) were accumulated. The data are obtained under various compressor inlet conditions. Furthermore, in this study, the validation of the gas system transient analysis code GAMMA was carried out by utilizing the experimental data of SCO2PE. To simulate the data by the GAMMA code, the code was revised to model the compressor performance. A transient case for reduction in cooling event was simulated with the facility and the experimental data were compared to the revised GAMMA code. The revised GAMMA code showed a reasonable performance and demonstrated the potential of the code for being used in a larger scale S-CO2 power system.

Introduction

A supercritical CO2 (S-CO2) Brayton cycle is receiving attention as a promising power conversion system for various heat sources because of its high thermal efficiency, simplicity of the cycle configuration and compact turbomachinery compared to other power cycles: air, water, helium [1], [2], [3], [4], [5], [6], [7]. The main reason why the S-CO2 Brayton cycle can achieve high efficiency is because the compressor work is lower than other gas Brayton cycles due to its high density and low compressibility near the critical point [1]. However, most of the existing property databases are not proven to be accurate near the critical point [8], [9]. Moreover, in case of the S-CO2 Brayton cycle, it is not appropriate to use the ideal gas assumption for predicting the cycle performance. Therefore, controlling the compressor operating near the critical point of CO2 is one of the key technical issues for operating the S-CO2 Brayton cycle with a high thermal efficiency.

Due to the growing interest on the S-CO2 Brayton cycle, research on the transient analysis of the S-CO2 system has been conducted by a few researchers previously [10], [11], [12]. However, previous studies showed some limitations in the modeled experimental facility, and the experiment was not performed to observe specific scenario.

To accumulate experiences in control by obtaining experimental data of the CO2 compressing system, KAIST research team has constructed and operated the CO2 compressing test facility, SCO2PE (Supercritical CO2 Pressurizing Experiment). The SCO2PE can be divided into two systems: a primary CO2 system and a secondary water system. The CO2 loop consists of a canned motor type compressor, a spiral type heat exchanger, a globe valve, some control valves, etc. Using the SCO2PE, a compressor under various operating conditions can be tested.

The GAs Multidimensional Multicomponent mixture Analysis (GAMMA) code is a transient analysis code for analyzing hypothetical transient cases in a high temperature gas-cooled reactor system [13], [14], [15]. To simulate the SCO2PE with the GAMMA code, some updates in the code were necessary. A transient scenario was selected to compare the transient analysis code performance and the experimental data. The selected transient scenario is the reduction of cooling event, which the test was conducted in SCO2PE by decreasing the mass flow rate of the cooling water loop. The selected transient is of interest since the compressor inlet conditions start to move away from the critical point of CO2. This can cause degradation in the compressor and heat exchanger performances as well as the system performance. The transient analysis code is to predict the overall system behavior while the event is taking place. Major components, the compressor and the heat exchanger, are separately modeled before the whole SCO2PE loop was simulated, and then the transient simulation is finally conducted with the updated GAMMA code.

Section snippets

The S-CO2 system test facility review

The S-CO2 Brayton cycle benefits from small compression work than other gas Brayton cycles by operating the compressor near the critical point of CO2. However, properties of CO2 near the critical point vary substantially and also it is not appropriate to utilize the ideal gas assumption for predicting the cycle performance unlike other gas Brayton cycles. Therefore, the CO2 compressor data from the S-CO2 compressor test facility called SCO2PE (Supercritical CO2 Pressurizing Experiment) are

The system transient analysis code review

It is known that the testing and modeling of the S-CO2 Brayton cycle require a great deal of technique because the compressing process is conducted near the critical point featuring a large uncertainty [8], [9], [18]. Especially, modeling a S-CO2 compressor is a challenging issue due to the significant gradients in the fluid properties near the critical point and significant windage and other parasitic losses in the compressor which are difficult to model [17], [18], [19]. This issue was well

Validation of GAMMA code with SCO2PE data

Prior to the simulation of whole SCO2PE loop, the major components, the compressor and heat exchanger, were separately modeled and simulated with the GAMMA code.

Conclusions

The main reason why the S-CO2 Brayton cycle can be expected to show high performance is because it consumes smaller compression work than other Brayton cycles due to high density and low compressibility near the critical point of CO2. However, it is difficult to clarify fluid characteristics near the critical point and also it is not appropriate to use the ideal gas assumption for predicting the cycle performance unlike other gas Brayton cycles. Therefore, a S-CO2 pressurizing experiment loop,

Acknowledgements

The authors gratefully acknowledge the support from the National Research Foundation (NRF) (NRF-2013M2A8A1041508) and funded by the Ministry of Science, ICT and Future Planning (MSIP) of the Republic of Korea.

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