Advanced exergy and graphical exergy analyses for solid oxide fuel cell turbine-less jet engines
Introduction
Performance improvement and emission reductions for aeroengines are important in the aviation sector to save energy and reduce environmental pollution [1]. Progress in combustion engines such as gas turbines and internal combustion engines has been limited due to the inherent Carnot limitations [2]. Aircraft powered by fuel cells were focused on [3]. The thermal efficiency of fuel cells is far higher than that of combustion engines, and allows reduced carbon emissions [4]. Solid oxide fuel cells (SOFC) fueled by hydrocarbon fuel are promising as a power source for aircraft [5].
Energy analyses of SOFC systems for aircraft have been widely conducted [6]. Himansu [7] first showed that a SOFC gas turbine hybrid system can provide propulsive force for high-altitude long-endurance unmanned aerial vehicles (UAVs). The hybrid system provides benefits in terms of fuel consumption for the UAVs, and can increase their range. Commuter airplanes have strict limitations for noise, exhaust emissions, and thermal efficiency. The SOFC hybrid system, fueled by liquid hydrocarbon fuel, offers high efficiency and is capable of providing power for commuter airplanes. This was studied by Stoia et al. with the National Aeronautics and Space Administration (NASA) [[8], [9], [10], [11]]. Their results showed that the steam reformer provides the ability to utilized heavy hydrocarbon fuels with SOFCs on board. The baseline aircraft for the SOFC power system is the X-57 ″Maxwell” MOD II all-electric airplane. In addition, the fuel consumption of distributed-propulsion airplanes can be reduced if they are equipped with SOFC hybrid power systems [[12], [13], [14]].
Exergy analysis is a useful tool for the performance assessment of gas turbine engines. Dong [15] and Sohret [16] reviewed the advantages of conventional exergy and advanced exergy assessments on aerospace power systems. Turan and Aydin [17] showed that the exergetic efficiency of turboshafts is 27.5%. The component with the highest exergy efficiency is a gas turbine generator unit. Yucer [18] analyzed the effects of the load types on the exergy efficiency of jet engines and their components. The highest exergy destruction rates take place in the combustion chamber for all load types. Balli [19] et al. showed that the exergy efficiency of a turbojet engine decreases from 15.40% to 14.33% by replacing jet fuel with hydrogen fuel. Yalcin [20] analyzed a turbofan engine based on the exergetic approach. The results showed the exergy efficiency changes from 80.77% to 84.45% when thrust varies from 86.53% to 142.58% of its nominal rated value.
Advanced exergy analyses are conducted by splitting the exergy destruction into endogenous and exogenous parts. This provides useful information about the interaction between the components of the thermodynamic system [21]. Also, the potential for performance improvement can be determined by splitting the exergy destruction rates into avoidable and unavoidable parts. Balli [22] analyzed a military aircraft turbojet engine with an afterburner system using an advanced exergy method. The results showed that the engine has little improvement potential because the unavoidable exergy destruction rate is 93% with afterburner use. The connection between the components is weak because the endogenous exergy destruction is 83%. For a turboprop engine, the advanced exergy analysis results are similar to that of the turbojet engine [23].
Graphical exergy analyses are carried out by combining the first law of thermodynamics and the second law of thermodynamics. The variation of the energy level of the components is shown while energy variation occurs. Zhang [24] et al. applied graphical exergy analysis to a scramjet under cruise conditions. Their results indicated that the scramjet exhaust process produces the most exergy destruction. Jin and Ishidda [25] showed that problems and potential for substantial improvements of existing thermal power systems can be revealed by the graphical exergy analysis. Correa and Gundersen [26] proposed an alternative representation of exergy, similar to the graphical exergy analysis, which can be used for process design.
A novel aeroengine was proposed in our previous work. The engine is composed of intakes, compressors, SOFC power systems, combustors, motors, and nozzles. There are no turbines in this engine. The compressors are driven by the motor, which is powered by the SOFC. Combustor exhaust directly expands in the nozzle. Energy analyses for the SOFC turbine-less jet engine were carried out. If the flight conditions, combustor exit temperature, and compressor pressure ratios of the SOFC turbine-less jet engine are kept the same as those of traditional turbojet engines, the specific thrust of the former will be 1.27–1.55 times that of the latter [27]. In addition, the performance of the SOFC turbine-less jet engine will be affected significantly by the total pressure recovery coefficient of the SOFC, if anode exhaust recirculation is adopted [28]. The specific thrust is almost unaffected by fuel types [29]. Moreover, the engine can be operated at a high velocity up to Mach 5 with high specific impulse, if the water is injected in the intake stage [30].
The SOFC turbine-less jet engine equipped with an air exhaust heat exchanger has hardly analyzed by an exergy analysis method. This paper is one of the first attempts to fill this gap by revealing sources of thermodynamic irreversibility and identifying components in SOFC turbine-less jet engines. Exergy analyses, particularly advanced exergy analyses, are excellent methods for the study of new engines and thermal systems. Using advanced exergy analyses, the order of importance of components’ improvement, their real performance potential, and the interactions between the components can be quantitatively shown. Also, the graphical exergy method can be used to reveal the amount of energy transformed and the energy level. In this paper, the advanced exergy and graphical exergy analysis methods are both used to show the irreversibility and the variation of the energy processes of SOFC turbine-less jet engines. Additionally, the results for SOFC turbine-less jet engines are compared with that of traditional turbojet engines to demonstrate the advantages of the novel engine.
Section snippets
Engine description
The diagrams of the SOFC turbine-less jet engine and turbojet engine are shown in Fig. 1 (a) and (c). For the traditional turbojet engines, the compressors are driven by turbines. The high-temperature exhaust expands and produces thrust in the nozzle. For the SOFC turbine-less jet engine, the compressors are driven by the motor instead of the turbines. The motor is powered by SOFCs. Thus, there are no turbines in this system. The fuel cell exhaust enters the combustor. The combustor exhaust
Thermodynamic models
The detailed thermodynamic model of SOFC turbine-less jet engines has been laid out in our previous papers [[28], [29], [30]]. They are composed of several sub-models such as reformer models, SOFC models, compressors models, and so on. The main equations and comments for the models can be found in Appendix A Section 1, the model verification and validation are shown in Appendix A Section 2.
Conventional exergy analysis models
Exergy is defined as the maximum work gained from a system when it reaches equilibrium with the surrounding
Model verification
The calculated results for the jet engine model in this paper are compared with those of Abbas [38]. The input parameters are shown in Table 3 and the results are in good agreement with Abbas's work [38], as shown in Table 4. The SOFC model was verified in our previous work [28]. Thus, the models developed in this paper accurately describe the thermodynamic processes.
Engine performance and conventional exergy analysis
The chosen parameters for the SOFC turbine-less jet engines are shown in Table 5. The flight conditions are representative of
Conclusions
This study presents conventional, graphical, and advanced exergy analyses of a SOFC turbine-less jet engine to be used on unmanned aerial vehicles. Moreover, the results of the exergy analysis are compared with the results for the traditional turbojet engine. The remarkable results and useful information from the present study are summarized as follows:
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There are two energy paths and a matter circulation path in the SOFC turbine-less jet engine. The exergy efficiency of the engine is 49.83%,
CRediT authorship contribution statement
Zhixing Ji: Conceptualization, Methodology, Software, Validation, Writing - original draft. Jiang Qin: Supervision, Writing - review & editing. Kunlin Cheng: Visualization, Investigation. He Liu: Data curation. Silong Zhang: Investigation. Peng Dong: Writing - review & editing.
Declaration of competing interest
We have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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