(738e) Energy and Exergy Analysis of the Novel Supercritical CO2 Brayton Cycle Using Different Designs of the Precooler

As part of the nationally determined contributions (NDCs) defined within the United Nations Framework Convention on Climate Change (UNFCCC), each country's ambitious approach is required to decrease the world's greenhouse gas emissions owing to their obligations towards the Paris Agreement. The major segment of these greenhouse gas emissions comes from the power generation and industrial sector, which alone comprises 69.8% of the total gas emissions worldwide. Given this, a novel supercritical power cycle, namely the carbon dioxide Brayton cycle (sCO₂-BC), could be of great value. The cycle with supercritical carbon dioxide (sCO₂) as a working fluid combines the best feature of both Rankine and Air Brayton cycles. This cycle operates near the critical point of sCO₂,resulting in significantly higher thermal efficiency than the cycles mentioned above [1] even at moderate values of turbine inlet temperature. Also, due to the high density of the working fluid in the supercritical region, heat exchangers and turbomachinery components are up to ten times smaller in comparison with the Rankine cycle, consequently reducing the overall system size up to four times and the environmental footprint of the planet by 25% at least [2]. Furthermore, as the critical temperate of sCO₂ is very close to ambient, this makes (sCO₂-BC) well suited for dry-air cooling, reducing the cycle's water consumption by 100% [1]. To meet the tight emission standards being imposed, an urgent need to enhance renewable energy sources has risen. In this regard, integration of the (sCO₂-BC) with concentrated solar power plants (CSP) is highly pertinent and cost effective for the next-generation CSP plants operating at temperature above 600^O C [3]. Additionally, (sCO₂-BC) has been accounted for as an ideal power block for the next generation nuclear reactor [4] and industrial waste heat recovery system [1]. Visibly it is the future of the power cycle.

In the novel supercritical carbon dioxide Brayton (sCO₂-BC), the pre-cooler (Fig. a) role is critical [1,5]. It serves not only as a sink to the power cycle but it also regulates the conditions at the compressor's inlet. The compressor's inlet temperature is intended to be maintained close to the critical temperature of carbon dioxide (CO₂) to achieve greater cycle efficiencies[6]. However, exceptionally higher values of the specific heat capacity of near its critical point (up to 40 times higher than water) requires exceedingly high water flow rates [7] on the cold side to achieve the desired exit temperatures of CO₂. Consequently, the pre-cooler's pumping power requirements become high enough to deteriorate the cycle's performance. This problem can only be mitigated by exploring new channel geometries with enhanced thermohydraulic characteristics. Moreover (sCO₂-BC) currently lacks investigations on enhancing the thermohydraulic performance of pre-cooler using efficient channel geometries. Currently installed (sCO₂-BC), facilities and available numerical work on the topic focus on printed circuit heat exchangers (PCHEs) with straight channel geometries. However, it has been reported [8,9] frequently in the literature that thermal and hydraulic characterizes of the PCHEs with straight channels exhibit significantly low thermal and hydraulic characteristics compared with zigzag and airfoil channel-geometries. One of the reasons why pre-cooler's designs with the aforementioned channel-geometries were never explored was the unavailability of their thermal and hydraulic characteristics in the pre-cooler's operating regime due to limited lab facilities and intricacies linked with abrupt variations in the thermophysical properties near the critical point.

Regarding the discussion above, the current work deals with the numerical evaluation of pressure drop and heat transfer characteristics of the pre-cooler with zigzag channels and airfoil channels. Computation geometries are shown in Fig. a. Hexahedral mesh was generated using ICEM-CFD that was solved employing ANSYS-CFX. Real gas property tables through RGP files were utilized to implement the abrupt variations in the working fluid's thermophysical properties. Furthermore, a pitch-averaged data post-processing methodology is introduced and opted for the precise post-processing of the data[10]. Computed data was used to train the machine learning model based on the artificial neural network (ANN) to predict Nusselt number (Nu) and friction factor (f) as shown in Fig. d. Later, an in-house pre-cooler design and analysis code (PCDAC) was developed and coupled with cycle design point code (CDPC). The precooler code (PCDAC) employs (Fig. b and Fig. c) the developed Nusselt number (Nu) and friction factor (f) correlations based on the CFD data to achieve different precooler designs using various design values of heat exchanger's effectiveness (ϵ_D), and design value of the inlet Reynolds number (Re_D). Later an impact of the different designs of the heat exchanger was accessed on the cycle's performance using both energy and exergy analysis. Moreover, a multi-object optimization study was conducted to locate the best compromise between the cycle's performance and the pre-cooler's size.

Results suggest that the performance of (sCO₂-BC)can be enhanced substantially by employing zigzag and airfoil channel geometries for the pre-cooler design (Fig. e). Cycle's highest efficiency was found with airfoil channel geometry of the pre-cooler at 20k > Re_D > 25k and 0.92 > ϵ_D > 0.95. However, the pre-cooler's most compact designs were found with zigzag channel geometry with design values of Re_D and ϵ_D ranging from 35k to 40k and 0.98 to 0.99, respectively. Optimization results suggest that replacing straight channels with airfoil channels could reduce the pre-cooler's size up to 2.3 times and bring about a considerable enhancement in the power cycle's efficiency.

References

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