Numerical Study of Thermal Stresses in a Planar Solid Oxide Fuel Cell Stack

A three dimensional numerical model of a practical planar solid oxide fuel cell (SOFC) stack based on the finite element method is constructed to analyze the thermal stress generated at different uniform temperatures. Effects of cell positions, different compressive loads, and coefficient of thermal expansion (CTE) mismatch of different SOFC components on the thermal stress distribution are investigated in this work. Numerical results indicate that the maximum thermal stress appears at the corner of the interface between ceramic sealants and cells. Meanwhile the maximum thermal stress at high temperature is significantly larger than that at room temperature (RT) and presents linear growth with the increase of operating temperature. Since the SOFC stack is under the combined action of mechanical and thermal loads, the distribution of thermal stress in the components such as interconnects and ceramic sealants are greatly controlled by the CTE mismatch and scarcely influenced by the compressive loads.

Modeling of Heat Removal in a Single-Channel Microscale Fuel Cell

Considerable waste heat is generated via the oxygen reduction reaction in polymer electrolyte membrane fuel cells. Consequently, heat generation and removal in conventional fuel cell architectures has been carefully investigated in order to achieve effective thermal management. Here we present a novel microscale fuel cell design that utilizes a half-membrane electrode assembly. In this design, a single fuel/electrolyte stream provides an additional pathway for heat removal that is not present in traditional fuel cell architectures. The model presented here investigates heat removal over a range of inlet fuel temperatures. Heat generation densities are determined experimentally for all inlet fuel temperatures. The simulations presented here predict thermal profiles throughout this microscale fuel cell design. Simulation results show that the fuel stream dominates heat removal at room temperature. As inlet fuel temperature increases, the majority of heat removal occurs via convection with the ambient air. The model also shows that heat transfer through the oxidant channel is minimal over the range of inlet fuel temperatures.

Active Control of Stack Inlet and Outlet Coolant Temperature for the PEM Fuel Cell System

With the advantages of high power density, rapid startup, low operating temperature and no emission of pollutants, proton exchange membrane (PEM) fuel cell is considered to be the most promising candidate for the next generation power source of Clean Energy Automotive. PEM fuel cell operation necessitates thermal management to satisfy the requirements of safe and efficient operation by keeping the temperature within a certain range independent of varying load conditions. As for a high power PEM fuel cell system (eg. 80kw) without the external gas to gas humidifier, the temperature of the stack inlet coolant had better track to a time-varying curve produced by the working condition, which introduce the temperature difference between the cathode inlet and outlet, and thus it improves the relative humidity of the inlet air of the cathode. Compared to the traditional stack outlet coolant temperature regulation problem, the new plant is a two inputs and two outputs system, furthermore, the stack inlet coolant temperature control is a tracking problem which is different to the outlet coolant temperature regulation (regulation problem). Considering that the PEM fuel cell without the external humidifier is a promising scheme which has been adopted by the Mirai fuel cell vehicle [1], we actively aim to control both the inlet and outlet coolant temperature as desired simultaneously. In this paper, a two inputs and two outputs decouple control scheme is developed to achieve our aim. Firstly, based on the energy conservation and continuity equation, we establish a dynamic thermal model for the cooling system consisted of a water circulation pump and a radiator coupled to a fan, integrated with the fuel cell stack. Secondly, the static coupling characteristics of the control variable is analyzed according the relative gain matrix method. Then two specific control strategies are designed. One is based on frequency domain pure PID control technique. Considering the coupling phenomenon between two control channels, another technique is based on decouple theory feed-forward decouple control technique. Both of them try to regulate the outlet and inlet coolant temperature through tuning mass flow rate of water circulation pump and duty ratio of radiator. Finally, all the control strategies are demonstrated on the platform of Matlab / Simulink. The results show that both of them can control the stack inlet and outlet coolant temperature simultaneously, but the second strategy has much better performance than the first.

Transient Analysis of Simultaneous Multivariable Signals on Fuel Cell/Gas Turbine Hybrid to Define Control Strategies for Cathode Parameters and Compressor Stall

Transients in a hybrid system composed of a solid oxide fuel cell (SOFC) and a gas turbine (GT) were evaluated during simultaneous manipulation of system airflow bypasses and turbine electric load. The three airflow bypass valves selected for study were chosen for their potential application in controlling dynamic excursions of the main fuel cell and gas turbine parameters in the system. The objective of this work was to understand the physical behavior by the simultaneous operation of the bypass valves along with the turbine electric load in order to formulate scenarios of control on the key parameters relevant to system failure, specifically from compressor stall and surge. Empirical data was collected using the National Energy Technology Laboratory Hybrid Performance project hardware simulation of a SOFC/GT hybrid. Step changes were implemented in all three valves for various open/close valve commands and increase/decrease of the turbine electric load simultaneously. The transient response of process variables was analyzed to determine the potential for mitigating or aggravating compressor stall and surge during load excursions.

Minimization of Stack Mass in Miniature PEM Fuel Cell Systems With DC/DC Converters

Polymer electrolyte membrane (PEM) fuel cells have been explored as a clean battery replacement in portable and miniature applications where total system mass and specific energy density (Wh/kg) are critical design constraints. By coupling a boost (step-up) DC/DC converter with a miniature PEM fuel cell stack, the total power system mass can be reduced while providing voltage regulation capabilities not available with a fuel cell alone. This configuration is applied to the design of a controlled meteorological (CMET) balloon power system as a case-study. In this work, we designed and tested three different micro-power DC/DC boost converters that were deployed in series with a PEM fuel cell stack. Testing of the converters revealed a transition region in which the converter output voltage is hysteretic, not well regulated, and dependent on the input voltage. As a result, it is important to identify the minimal stable and reliable input voltage to a given DC/DC converter in order to minimize the fuel cell power system mass. An optimization strategy is presented here that enables the minimization of PEM fuel cell stack mass by identifying the appropriate DC/DC converter input voltage subject to the dimension constraints of the fuel cell components. Prototype DC/DC converters were then experimentally tested in direct connection to a miniature two-cell PEM fuel cell stack.

Flow Distribution Analysis in the SOFC Stack Using CFD Technique

In this paper, a three dimensional solid oxide fuel cell (SOFC) model is constructed to investigate the gas distribution and the pressure variation in the external manifold stack by a computational fluid dynamics (CFD) approach. Several geometric parameters of external manifold stack, including the position of inlet tube, depth of the manifold and the channel resistance are optimized to achieve uniform gas distributions among the channels. Simulation results indicate that a gas distributor can enhance the flow uniformity effectively. Besides, with the increasing depth of the manifold, the flow tends to be more uniform in T-manifold but has a small impact on C-manifold stack. It is also shown that flow distribution is intensively enhanced with the raise of resistance, especially from 0Pa to 100Pa. Modeling results highlight the importance of manifold and channel structure design for external manifold stack and can be widely applied to design the geometric parameters.

SOFC Micro-CHP System With Thermal Energy Storage in Residential Applications

To improve the energy efficiency and achieve zero-net energy goals, as well as to reduce environmental impacts, we demonstrated and evaluated the use of a 1.5 kW Solid Oxide Fuel Cell (SOFC) with Micro-combined heat and power (Micro-CHP) for powering residential homes. In this study, we designed, tested and demonstrated an SOFC Micro-CHP system as a Distributed Generation (DG) prime mover that has high reliability and availability, high efficiency and ultra-low emissions for steady state operation. Energy balances and dynamic analyses of integrating a thermal storage system with the SOFC Micro-CHP system were carried out using a summer load profile of a residence in Southern California. The thermal storage system was found to mitigate the dynamics introduced from the electric water heater and smooth out the residential load profile. Additionally, the integrated thermal storage system and the SOFC Micro-CHP system was found to reduce the overall electricity import and thus the carbon emissions.

Hydrogen Production From Various Heavy Hydrocarbons by Steam Reforming

The authors develop a small and simple steam-reforming reactor in a home-use size for such various heavy-hydrocarbons fuels as n-octane, n-decane, n-tetradecane and n-hexadecane in addition to n-dodecane, and measure the inside-temperature profile and the molar fractions of main gas components such as H2, CH4, CO and CO2. As a result, the authors successfully achieve suitable inside-temperature profiles. Namely, temperature almost-linearly increases in the downstream direction along a reactor, under such two conditions as 600–950 K at the upstream end of the catalyst-layer bed in the reactor and as less-than 1,070 K everywhere in the reactor. And, the authors reveal the effects of the liquid-hourly space velocity (LHSV) upon the molar fractions, a conversion ratio and reforming efficiencies for various heavy-hydrocarbons fuels. All the molar fractions, which agree well with thermochemical-equilibrium theory, are approximately independent of LHSV. The conversion ratio is about 90 % for LHSV ≤ 0.6 h−1, and monotonically decreases with increasing LHSV for LHSV > 0.6 h−1. Then, each reforming efficiency always attains the maximum for LHSV ≈ 0.6 h−1 being independent of fuels. This suggests the common upper limit of LHSV for practically-suitable operation.

Producing Hydrogen From Jet-A Fuel in a Reactor With Integrated Autothermal Reforming and Water-Gas Shift

Jet fuel is used to produce hydrogen on board of vehicles through integrated reactions of autothermal reforming (ATR) and water-gas shift (WGS). One of the most attractive benefits of autothermal reforming of hydrocarbon fuels is the possibility of self-sustainability of the reaction if the process is carefully managed and designed. In this work, an integrated fuel processor, including a mixing zone, an ATR reactor, a WGS reactor and two recuperators was fabricated and the hydrogen production performance (using Jet fuel) was tested and studied. Analysis of the energy in reactions and heat transfer area in the heat recuperator was carried out in order to obtain some insights on the optimal design and operating conditions. The previous experimental results regarding the reformate compositions and fuel conversion efficiency were used to evaluate some of the parameters described in the energy analysis model. The predicted results show that the overall process heat was negative (exchanging heat larger than preheating heat) under most of the previous experimental conditions, indicating the potential of self-sustainability of reaction in the fuel processor.