Performance assessment of a hybrid system integrating a molten carbonate fuel cell and a thermoelectric generator
Introduction
Due to their potentially lower greenhouse gas emissions and primary energy consumptions, fuel cells have been considered as next generation power sources to replace conventional thermal power technologies [1], [2], [3], [4], [5], [6], [7]. Based on electrolyte types, fuel cells can be classified into five major categories: proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), solid oxide fuel cell (SOFC), and molten carbonate fuel cell (MCFC) [8]. Among the various fuel cells, MCFC is fuel flexible, long-term stable, and suitable for stationary applications [9], [10], [11], [12]. The MCFC is a natural carbon capture system that enables to separate carbon dioxide from the original cathodic flow via the CO32− transit through the electrolyte [13]. Unlike some carbon capture and storage technologies such as calcium looping [14], [15], the MCFC does not consume but provide power while operating as a CO2 separator and concentrator [16]. Nowadays, MCFCs are commercially available in some countries such as USA, Germany, Japan, Italy and South Korea [17].
Although progress has been made in MCFCs to prolong the lifetime, improve the stack capacity and lower the manufacturing and running cost [18], [19], [20], the relatively low power density is still a major drawback that restricts the wide commercialization of MCFCs [21]. The power density of MCFC can be improved by optimizing the operating conditions or by developing more effective catalyst materials to reduce the activation loss. From engineering point of view, the MCFC performance can also be improved by recovering the waste heat released in MCFC for additional power generation or for combined heat and power (CHP) cogeneration [22], [23], [24], [25], [26], [27], [28]. Zhang et al. [22], [23] proposed two MCFC-gas turbine (MCFC-GT) hybrid systems with different fuels and investigated effects of some operating conditions and irreversible losses on the performance of the hybrid systems. Sánchez et al. [24] presented a reciprocating engine with three bottoming thermal cycles to recover the waste heat from an MCFC and showed that the Stirling engine was an alternative choice for conventional gas turbines. Angelino et al. [25] considered an ORC (Organic Rankine Cycle) with different working fluids to recover the waste heat from an MCFC. The power and efficiency gain of about in the range 10–15% can be obtained with reference to MCFC plant in the capacity range of 2–5 MW. Baronci et al. [26] studied the integration of an MCFC and a Brayton cycle using supercritical carbon dioxide as working fluid. The boost in electrical efficiency was found to be twice that of a recovery system based on an ORC. Mamaghani et al. [27] proposed an MCFC-GT and ORC hybrid plant, 4E (energetic, exergetic, economic and environmental) analyses were performed and a multi-objective optimization method was utilized to build a set of Pareto optimal solutions. The optimum solutions showed that the exergetic efficiency of the MCFC-GT-ORC system was about 55%, about 5% larger than that of the sole MCFC-GT system. Silveira et al. [28] carried out energy, exergy and economic analyses of an MCFC integrated with an absorption refrigeration system for cold water production. A global efficiency of 86% and the payback period of about 3 and 5 years for investments in MCFCs of 1000 and 1500 US/kW were estimated.
Thermoelectric generators (TEGs) use thermal energy to create a temperature gradient across the hot and cold junctions of a thermoelectric material which is used to generate voltage as per the principle of Seebeck effect [29], [30], [31]. TEGs can be operated in a quiet way compared with other power generation technologies since TEGs are solid-state heat engine and do not contain any moving part. In addition, TEGs offer many other advantages such as compact, environmentally friendly, highly reliable and adapting to different kinds of heat reservoirs [32], [33]. Owing to these advantages, TEGs have drawn more and more attention for waste heat recovery [34], [35], [36], [37]. With the rapid developments of thermoelectric materials, the working temperatures of some thermoelectric generators beyond 600 °C have already been reported [38], [39], [40]. It creates an opportunity to directly integrate a TEG with an MCFC for converting the waste heat into electricity to increase the power output and efficiency of the MCFC system.
In this work, a new hybrid system integrating an MCFC and a TEG is proposed to improve the energy efficiency for power generation. The layout of the hybrid system will be described and each component in the hybrid system will be numerically characterized. Some important parameters for assessing the performance of the hybrid system will be formulated by taking the main irreversible losses of the system into account. The general performance characteristics and optimum criteria for the hybrid system will be investigated and discussed. Effects of some designing parameters and operating conditions on the hybrid system performance will be specified. Some possible problems for the practical operation of the hybrid system are also discussed.
Section snippets
Model description
The hybrid system composed of an MCFC, a TEG and a regenerator, as schematically shown in Fig. 1 (a). The MCFC converts the chemical energy in reactants into electricity and waste heat, and the electricity is delivered to external loads. The waste heat, one part is transferred to the bottoming TEG for additional electricity generation, another part is used to compensate the regenerative losses in the regenerator, and the last part is dissipated into the environment. In Fig. 1 (a), PMCFC stands
General performance characteristics and optimum criteria
As described by the mathematical model in Sec.2, the performance characteristics of the MCFC-TEG hybrid system not only depends on the parameters such as operating current density j, operating temperature T, operating pressure p and partial pressures of species (i.e., , , pH2O and pCO2) of the MCFC, electrical resistance R, thermal conductance K, geometric parameters (i.e., l and S) and dimensional figure of merit ZT0 of the TEG, but also depends on some parameters related to
Effects of operating temperature T
The operating temperature is an important operating condition that not only affects the performance of the MCFC and TEG but also affects the thermodynamic losses within the hybrid system. As shown in Fig. 4, both P∗ and η increase as T increases, and this effects become more significant with increasing T. At a larger T, the ionic conductivity in the electrolyte is more efficient and the electrodes are more reactive, which leads to smaller overpotential losses and a larger cell voltage, and
Conclusions
An MCFC-TEG hybrid system is presented to effectively harvest the waste heat released from the MCFC. The operating current density interval of the MCFC enables the TEG to exert its function is determined. Numerical expressions of two important parameters for assessing performance of the hybrid system are specified under different conditions. The maximum power density and the corresponding efficiency of the hybrid system allow over 30% larger than that of the stand-alone MCFC. Comparisons
Acknowledgments
This work has been supported by the National Natural Science Foundation of China (Grant No. 51406091), the K. C. Wong Magna Fund in Ningbo University, and The Hong Kong Polytechnic University Research Project (Grant No. 1-YW1F).
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