Research PaperStudy on CO2 – water printed circuit heat exchanger performance operating under various CO₂ phases for S-CO₂ power cycle application
Introduction
Recently, the efficiency of the power conversion system became more important due to the increasing energy demand and global warming issues. At this point, the supercritical CO2 (S-CO2) power cycle is receiving more attention as a key technology for resolving energy problem. The main advantages of S-CO2 power cycle are having a large potential to be highly efficient and can be constructed in compact system configuration. Due to the remarkable fluid characteristics of supercritical state CO2, e.g. liquid like high density, low compressibility factor and gas like low viscosity, the cycle can achieve high efficiency by reducing compression work [1], [2].
Another advantage is the S-CO2 power cycle can achieve high thermal efficiency at moderate turbine inlet temperature (450–650 °C) compare to the conventional power cycle, so the S-CO2 power cycle technology can be utilized with various heat source applications such as the next generation nuclear reactor, waste heat recovery system, fuel cell bottoming system [3], [4]. For these reasons, the S-CO2 power cycle has been studied as the next generation power conversion cycle which can exceed conventional steam Rankine or gas Brayton cycles [22], [23].
The promising S-CO2 system configuration for the next generation nuclear reactor and concentrated solar power applications was introduced by Dostal [1] as shown in Fig. 1. The temperature-entropy diagram and the recompression S-CO2 Brayton cycle layout show that the compressor inlet condition (5) is near the CO2 critical point and the turbine outlet condition (2) is still high temperature. Due to the high temperature of the turbine outlet temperature, the system requires large amount of recuperation processes to maximize the use of available heat in the system.
One of the challenging engineering issues for developing S-CO2 technology is designing a component which operates near the CO2 critical point (30.98 °C, 7.38 MPa) [5], [8], such as a compressor or a precooler. Because of the dramatic change of the thermo-dynamic properties in vicinity of the critical point, conventional logarithmic mean temperature difference (LMTD) design methods based on constant property assumptions are not applicable. Furthermore computer aided design or computational analysis near the critical point is challenging due to the dramatic change in property with respect to pressure and temperature.
However, experimental and computational studies on the precooler cases near the critical point are not abundant and no study assures that the conventional heat exchanger design methodology can be applied in this area [19], [20], [21]. Thus, the KAIST research team focused the applicability of the PCHE application as a precooler for the S-CO2 power system. Furthermore, the off-design performance of the PCHE was also experimentally investigated to contribute to the future S-CO2 cycle control related research.
In summary, the objectives of this study are the following:
- (1)
Development of PCHE design methodology which accurately reflects the real gas effects near the critical point.
- (2)
Performance test under various CO2 conditions including gas, liquid and supercritical phases.
- (3)
Validation of the developed methodology with the obtained experimental data.
- (4)
Development of friction factor and heat transfer correlations with the assistance from computational fluid analysis.
Section snippets
PCHE design and experiment
A compact heat exchanger, printed circuit heat exchanger (PCHE), was recently developed to reduce the heat exchanger volume while having large heat transfer area and can sustain large pressure load. A large heat transfer area is generally needed for the gas heat transfer due to relatively poor heat transfer characteristics of the gas. The PCHE can achieve high effectiveness due to providing large heat transfer area within small volume and showed the best performance in high pressure operating
Channel corner shape effect
Unfortunately, flow local information of a PCHE micro channel was not able to be measured during the experiment. However, since the detailed channel geometry is known exactly, a CFD analysis was conducted to reproduce the channel internal flow information from the measured inlet and outlet data. At first, the hydraulic performance analysis was conducted without heat transfer. In order to consider the hydraulic characteristic, two different cases were chosen. Case 7 and case 15 in Fig. 7 were
KAIST_HXD code evaluation
In order to verify the developed PCHE design code with the developed correlations, the KAIST_HXD code was modified with the newly developed correlations. The identical internal geometry was implemented into the code to predict the heat exchanger performance for off-design conditions and the results were compared with the data.
However, the measurement in experiment includes additional form losses at the inlet and the outlet due to the header part. The authors modeled four additional form losses,
Conclusions and further works
The printed circuit heat exchanger (PCHE) received significant attention for the S-CO2 power cycle application due to high compactness and high effectiveness. In order to evaluate a PCHE performance, KAIST research team designed and manufactured a lab-scale PCHE. In this study, experiment and CFD analysis were used for developing a PCHE design methodology that can be applied to the S-CO2 system. Due to the lack of experimental and computational studies on the precooler, which operates near the
Acknowledgement
Authors gratefully acknowledge that this research is funded by Saudi Aramco - KAIST CO2 Management Center.
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