A feasible way to handle the heat management of direct carbon solid oxide fuel cells

doi:10.1016/j.apenergy.2018.06.039

Applied Energy

Volume 226, 15 September 2018, Pages 881-890

https://doi.org/10.1016/j.apenergy.2018.06.039 Get rights and content

Highlights

•
A novel system is proposed to handle the heat management of DC-SOFC.
•
Three operation strategies are presented for different operation conditions.
•
Power density and efficiency of the proposed system could reach 8100 W m⁻² and 60%.
•
Effects of some important parameters on system performance are revealed.

Abstract

A novel integrated system consisting of an external heat source, a direct carbon solid oxide fuel cell (DC-SOFC), a vacuum thermionic generator (VTIG) and a regenerator is proposed to handle the heat management of the DC-SOFC. The electrochemical/chemical reactions, ionic/electronic charge transport, mass/momentum transport and heat transfer are fully considered in the 2D tubular DC-SOFC model, which shows that the overall heat released in the cell is always different from the heat required by the internal Boudouard reaction. Three different operation strategies of the proposed system are presented, and accordingly, analytical expressions for the overall power output and efficiency for the proposed system are specified. The results show that the VTIG could effectively recover the waste heat for additional power production at a large operating current density, and the maximum power density and efficiency of the proposed system could reach more than 8100 W m⁻² and 60% at 30,000 A m⁻² and 1173 K, respectively. The effects of the operating current density, the operating temperature and the distance between the carbon layer and anode of the DC-SOFC, and the size, anode temperature and work function of the VTIG on the performance of the proposed system are discussed through comprehensive parametric studies.

Introduction

Despite the tendency in decreasing the reliance on fossil fuels and the development of alternative renewable energy technologies due to energy crisis and relative environmental problems, solid carbon remains a main resource in the coming decades because of its abundant storage and low price [1]. However, the utilization of solid carbon in conventional thermal plants for electricity generation is low-efficient due to the limitation Carnot cycle and complex intermediate processes [2]. In addition, the pollution from thermal plants cause various environmental problems, e.g. acid rains and global warming. Therefore, an alternative high-efficient and clean energy conversion device for electricity generation from solid carbon is urgently needed, such as solid oxide fuel cells (SOFCs) [3], [4].

An SOFC is a whole solid-state device with a dense electrolyte sandwiched between two porous electrodes. As one of the most attractive energy conversion devices, SOFCs can directly convert gaseous fuels, such as H₂ and CO, into electricity through electrochemical reactions. The fuel flexibility characteristic of SOFCs also allows the utilization of other fuels, such as methane and solid carbon [5], [6]. Solid carbon is an attractive fuel since it has a high volumetric energy density compared with gaseous fuels. Moreover, solid carbon is cheap and abundant, bringing huge economic advantages in exploring new markets [7], [8], [9]. However, the large particle size of solid carbon limits its direct contact with the triple phase boundaries (TPBs) in porous anode, resulting in a low output power density of direct carbon solid oxide fuel cells (DC-SOFCs). To overcome this problem, in situ solid carbon gasification has been proposed. Through in situ gasification, solid carbon is converted to gaseous fuel (e.g. CO) before the electrochemical reaction, which keeps the high volumetric energy density of solid carbon and expands the electrochemical reaction area simultaneously. A number of studies have been conducted to further improve the performance of DC-SOFC by adopting catalysts for faster carbon gasification kinetics [10], [11], [12], [13]. Moreover, it has been found that DC-SOFCs can co-generate fuel and electricity power, which further increases their economic advantage [14], [15], [16], [17], [18]. The thermal effect in DC-SOFC has been also studied to examine its potential for combined heat, gaseous fuel and electricity power generation [19]. It was found that the DC-SOFC requires heat input at a small current density due to the endothermic carbon gasification reaction, while the cell releases waste heat at a large operating current density. Initial studies combining conventional Stirling cycle and Otto heat engine with the DC-SOFC for its performance improvement have been conducted [20], [21]. However, a system combining DC-SOFC with more novel and advanced heat-to-electricity conversion device is still needed to examine the potential performance improvement.

A vacuum thermionic generator (VTIG) consists of an emitter (i.e., cathode) and a collector (i.e., anode) with a vacuum gap between them [22], [23], [24]. A VTIG can directly convert thermal energy into electricity using electron gas as working fluid based on the principle of thermionic emission [25], [26], [27], [28], [29]. The VTIG offers many advantages, such as being quiet, efficient, compact and environmental-friendly, but it requires a high-temperature heat source to generate an electrical current for practical usage [30], [31]. VTIGs normally operate at a high temperature (above 1600 K) with high efficiency (above 50%) and are often used as topping cycles for power generation [32], [33]. It is difficult to operate the VTIG at a low temperature because the high work function of common metallic cathodes cannot produce sufficient electron emission at a low temperature. Recently, Liang et al. [34], [35] proposed a single-layer graphene cathode-based VTIG and found that it was possible for the cathode to emit a sufficiently high current density at 700–1000 K. Obviously, the reduced operating temperature provides an opportunity for VTIG to act as a bottoming cycle for waste heat recovery for DC-SOFCs.

The literature survey shows that no research has been reported on the heat management of the DC-SOFCs using the VTIG yet. In this work, an external heat source and a VTIG are adopted and integrated to handle the heat management problem of a DC-SOFC. The thermal characteristics of the DC-SOFC are revealed using a 2D tubular model, in which the electrochemical/chemical reactions, ion/electronic charge transport, mass/momentum transport and heat transfer are fully considered. The operation modes of the proposed system are specified under different operating conditions, and accordingly, the mathematical expressions for output power and efficiency of the proposed system are given, respectively. The feasibility and effectiveness of the proposed system are also shown through numerical calculations. Finally, the effects of several design parameters and operation conditions on the performance of the proposed system are discussed.

Section snippets

System description

As shown in Fig. 1(a), the proposed system mainly consists of a DC-SOFC, an external heat source, a VTIG and a regenerator. The DC-SOFC generates electricity (P_SOFC) by consuming inlet solid carbon. If the DC-SOFC operates at an endothermic mode, an amount of heat |Q_SOFC| should be provided from the external heat source to ensure the normal operation of the DC-SOFC, as shown in Fig. 1(b). If the DC-SOFC operates at an exothermic mode, excessive waste heat generated in the cell Q_SOFC is

Results and discussion

Based on the above mathematical models and relevant parameters given in Table 1, Table 2, the performance characteristics of the proposed system can be analyzed. The parameters are taken as default ones unless mentioned specifically.

The equivalent power density and efficiency of the VTIG within the system are shown in Fig. 4, where $P_{TIG}^{*} = P_{TIG} / A$ . $P_{TIG}^{*}$ almost linearly increases as $i$ increases, while $η_{TIG}$ first quickly and then slowly increases as $i$ increases. Both $P_{TIG}^{*}$ and $η_{TIG}$ increase as $a$ or

Conclusions

A new system consisting of a DC-SOFC, an external heat source, a VTIG and a regenerator is proposed to handle the heat management of the DC-SOFC. A previously developed 2D tubular DC-SOFC model shows that the overall heat generated in the cell could be smaller than, equal to or larger than the heat required by the internal Boudouard reaction. According to the thermal characteristics of the DC-SOFC, three operation modes are presented. The analytical expressions for power output and efficiency

Acknowledgement

This research is supported by the National Natural Science Foundation of China (Grant No. 51406091), a grant (PolyU 152127/14E) from Research Grant Council, University Grants Committee, Hong Kong SAR, and a grant from Environment and Conservation Fund (ECF 54/2015), Hong Kong SAR.

References (43)

L. Santini et al.
On the adoption of carbon dioxide thermodynamic cycles for nuclear power conversion: a case study applied to Mochovce 3 Nuclear Power Plant
Appl Energy
(2016)
N. Mahato et al.
Progress in material selection for solid oxide fuel cell technology: a review
Prog Mater Sci
(2015)
J. Hanna et al.
Fundamentals of electro- and thermochemistry in the anode of solid-oxide fuel cells with hydrocarbon and syngas fuels
Prog Energy Combust Sci
(2014)
H. Xu et al.
Modeling of all porous solid oxide fuel cells
Appl Energy
(2018)
S. Giddey et al.
A comprehensive review of direct carbon fuel cell technology
Prog Energy Combust Sci
(2012)
Y. Wu et al.
A new carbon fuel cell with high power output by integrating with in situ catalytic reverse Boudouard reaction
Electrochem Commun
(2009)
C. Li et al.
Performance improvement of direct carbon fuel cell by introducing catalytic gasification process
J Power Sources
(2010)
Y. Tang et al.
Effect of anode and Boudouard reaction catalysts on the performance of direct carbon solid oxide fuel cells
Int J Hydrogen Energy
(2010)
H. Xu et al.
Modeling of direct carbon solid oxide fuel cell for CO and electricity cogeneration
Appl Energy
(2016)
Y. Wu et al.
Elementary reaction modeling and experimental characterization of solid oxide direct carbon-assisted steam electrolysis cells
Solid State Ionics
(2016)

H. Xu et al.

Modeling of direct carbon solid oxide fuel cells with H2O and CO2 as gasification agents

Int J Hydrogen Energy

(2017)

Y. Xie et al.

Electrochemical gas-electricity cogeneration through direct carbon solid oxide fuel cells

J Power Sources

(2015)

H. Xu et al.

The thermal effect in direct carbon solid oxide fuel cells

Appl Therm Eng

(2017)

H. Xu et al.

Performance improvement of a direct carbon solid oxide fuel cell through integrating an Otto heat engine

Energy Convers Manage

(2018)

A.D. Post et al.

Computational model and optimisation of a vacuum diode thermionic generator for application in concentrating solar thermal power

Appl Therm Eng

(2017)

K. McEnaney et al.

Aerogel-based solar thermal receivers

Nano Energy

(2017)

Y. Wang et al.

Performance evaluation and parametric optimum design of an updated thermionic-thermoelectric generator hybrid system

Energy

(2015)

Y. Wang et al.

An efficient strategy exploiting the waste heat in a solid oxide fuel cell system

Energy

(2015)

G. Xiao et al.

Thermionic energy conversion for concentrating solar power

Appl Energy

(2017)

G. Xiao et al.

Thermodynamic assessment of solar photon-enhanced thermionic conversion

Appl Energy

(2018)

Z.M. Ding et al.

Optimum performance analysis of a combined thermionic-thermoelectric refrigerator with external heat transfer

J Energy Inst

(2015)

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A feasible way to handle the heat management of direct carbon solid oxide fuel cells

Highlights

Abstract

Introduction

Section snippets

System description

Results and discussion

Conclusions

Acknowledgement

Appl Energy

Prog Mater Sci

Prog Energy Combust Sci

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Prog Energy Combust Sci

Electrochem Commun

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Int J Hydrogen Energy

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Solid State Ionics

Int J Hydrogen Energy

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Appl Therm Eng

Energy Convers Manage

Appl Therm Eng

Nano Energy

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J Energy Inst