Air mass flow and pressure optimisation of a PEM fuel cell range extender system

In order to eliminate the local CO₂ emissions from vehicles and to combat the associated climate change, the classic internal combustion engine can be replaced by an electric motor. The two most advantageous variants for the necessary electrical energy storage in the vehicle are currently the purely electrochemical storage in batteries and the chemical storage in hydrogen with subsequent conversion into electrical energy by means of a fuel cell stack. The two variants can also be combined in a battery electric vehicle with a fuel cell range extender, so that the vehicle can be refuelled either purely electrically or using hydrogen. The air compressor, a key component of a PEM fuel cell system, can be operated at different air excess and pressure ratios, which influence the stack as well as the system efficiency. To asses the steady state behaviour of a PEM fuel cell range extender system, a system test bench utilising a commercially available 30 kW stack (96 cells, 409 cm² cell area) was developed. The influences of the operating parameters (air excess ratio 1.3 to 1.7, stack temperature 20 °C–60 °C, air compressor pressure ratio up to 1.67, load point 122 mA/cm² to 978 mA/cm²) on the fuel cell stack voltage level (constant ambient relative humidity of 45%) and the corresponding system efficiency were measured by utilising current, voltage, mass flow, temperature and pressure sensors. A fuel cell stack model was presented, which correlates closely with the experimental data (0.861% relative error). The air supply components were modelled utilising a surface fit. Subsequently, the system efficiency of the validated model was optimised by varying the air mass flow and air pressure. It is shown that higher air pressures and lower air excess ratios increase the system efficiency at high loads. The maximum achieved system efficiency is 55.21% at the lowest continuous load point and 43.74% at the highest continuous load point. Future work can utilise the test bench or the validated model for component design studies to further improve the system efficiency.     

Study of gas pressure and flow rate influences on a 500 W PEM fuel cell,thanks to the experimental design methodology

The behaviour of a 500 W PEM fuel cell stack, fed by pure hydrogen and humidified compressed air, is currently investigated on the fuel cell test platform of Belfort.In this paper, the influences on fuel cell performance of gas pressure and flow rate parameters are studied. The fuel cell is operated in the pressure regulation mode: the gas flow rates are regulated thanks to mass flow controllers placed upstream of the stack and the gas pressures at stack inlets are controlled by regulation valves located downstream of the stack. The choice of the various tests to perform is made thanks to experimental design methodology, which is a suitable technique to characterise, analyse and to improve a complex system such as a fuel cell generator. In this study, the four physical factors considered are both hydrogen/air pressures and anode/cathode flow rates. Each factor has two levels, leading to a full factorial design requiring 16 experiments (16 current–voltage curves). The test bench developed at the laboratory allows setting the other factors (for instance: stack temperature, relative humidity and dew point temperature of the air at stack inlet) at fixed values. The test responses are the maximal output power and the efficiency computed for this power. Statistical sensitivity analyses (ANOVA analyses) are used to compute the effects and the contributions of the various factors to the fuel cell maximal power. The use of fractional designs shows also how it is possible to reduce the number of experiments. Some graphic representations are employed in order to display the results of the statistical analyses made for different current values.     

Maintaining equal operating conditions for all cells in a fuel cell stack using an external flow distributor

This paper presents a novel fuel cell stack architecture that allows each fuel cell to work at the same condition, maintaining the same performance from each individual cell and creating a maximum power output from the cell stack. A fuel cell stack having four PEM fuel cells was fabricated to experimentally compare its performance when fuel and air supplying/distribution schemes are different. The performance of the fuel cell stack and individual cells in the stack is measured to achieve a detailed evaluation of the effect of the different fuel and air supplying schemes. Experimental data shows that non-uniform flow distribution to individual cells has a considerable influence on individual cell performance, which affects the power output of the fuel cell stack. The fuel cell stack with the novel approach of fuel and air feeding shows a better power output performance compared to a different fuel and air feeding approach to the fuel cell stack.     

An integrated power generation system combining solid oxide fuel cell and oxy-fuel combustion for high performance and CO2 capture

An integrated power generation system combining solid oxide fuel cell (SOFC) and oxy-fuel combustion technology is proposed. The system is revised from a pressurized SOFC-gas turbine hybrid system to capture CO₂ almost completely while maintaining high efficiency. The system consists of SOFC, gas turbine, oxy-combustion bottoming cycle, and CO₂ capture and compression process. An ion transport membrane (ITM) is used to separate oxygen from the cathode exit air. The fuel cell operates at an elevated pressure to facilitate the use of the ITM, which requires high pressure and temperature. The remaining fuel at the SOFC anode exit is completely burned with oxygen at the oxy-combustor. Almost all of the CO₂ generated during the reforming process of the SOFC and at the oxy-fuel combustor is extracted from the condenser of the oxy-combustion cycle. The oxygen-depleted high pressure air from the SOFC cathode expands at the gas turbine. Therefore, the expander of the oxy-combustion cycle and the gas turbine provides additional power output. The two major design variables (steam expander inlet temperature and condenser pressure) of the oxy-fuel combustion system are determined through parametric analysis. There exists an optimal condenser pressure (below atmospheric pressure) in terms of global energy efficiency considering both the system power output and CO₂ compression power consumption. It was shown that the integrated system can be designed to have almost equivalent system efficiency as the simple SOFC-gas turbine hybrid system. With the voltage of 0.752 V at the SOFC operating at 900 °C and 8 bar, system efficiency over 69.2% is predicted. Efficiency penalty due to the CO₂ capture and compression up to 150 bar is around 6.1%.     

Heat and power management of a direct-methanol-fuel-cell (DMFC) system

In this paper, we describe the heat and the power management of a direct methanol fuel cell system. The system consists mainly of a direct methanol fuel cell stack, an anode feed loop with a heat exchanger and on the cathode side, a compressor/expander unit. The model calculations are carried out by analytical solutions for both mass and energy flows. The study is based on measurements on laboratory scale single cells to obtain data concerning mass and voltage efficiencies and temperature dependence of the cell power. In particular, we investigated the influence of water vaporization in the cathode on the heat management of a direct-methanol-fuel-cell (DMFC) system. Input parameters were the stack temperature, the cathode pressure and the air flow rate. It is shown that especially at operating temperatures above 90 °C, the combinations of pressure and air flow rate are limited because of heat losses due to vaporization of water in the cathode.     

Exergy analysis of a fuel cell power system for transportation applications

An exergy analysis of a solid polymer fuel cell power system for transportation applications is reported. The analysis was completed by implementing the fundamental governing second law equations derived for the system into a fuel cell performance model developed previously. The model analyzes all components of the system including the fuel cell stack and the air compression, hydrogen supply, and cooling subsystems. From the analysis, it was determined that the largest destruction of exergy within the system occurs inside the fuel cell stack. Other important sources of exergy destruction include irreversibilities within the hydrogen ejector and the air compressor, and the exergy associated with the heat rejected from the radiator. The results may aid efforts to optimize fuel cell systems.     

PEM stack test and analysis in a power system at operational load via ac impedance

This paper presents a method for collecting ac impedance data from a commercial PEFC power system at operational loads. The PEM fuel cell stack in the power system, including 47 MEAs, was operated using room air and pure hydrogen (>99.99%). For a stack test in the power system, the power source for the embedded controller board was simultaneously switched from the fuel cell stack to a similar external power source after the system reached a steady temperature. The PEM fuel cells in the stack were tested by collecting the ac impedance data at different current levels. By using ac impedance, a single fuel cell, a group of fuel cells, and a complete stack were then tested without the embedded control devices for ohmic, activation, and mass transport losses. The ohmic resistance for each cell component in the stack was obtained as 117 mΩ cm² at operational loads from 2.5 A to 35 A. The membrane thickness was further estimated as ca. 51–89 μm. Resistances from ohmic conduction, anode/cathode activation, and mass transport were measured and discussed using the Nyquist plots from the ac impedance spectra.     

Thermal and electrical experimental characterisation of a 1 kW PEM fuel cell stack

The present work describes the experimental characterisation of a self-humidified 1 kW PEM fuel stack with 24 cells. A test bench was prepared and used to operate a PEMFC stack, and several parameters, such as the temperature, pressure, stoichiometry, current and voltage of each cell, were monitored with a LabView platform to obtain a complete thermal and electrical characterisation. The stack was operated in the constant resistance load regime, in dead-end mode (with periodic releases of hydrogen), with 30% relative humidity air and with temperature control from a cooling water circuit. The need to operate the stack for significant periods of time to achieve repeatable performance behaviour was observed, as was the advantage of using some recuperation techniques to improve electrical energy production. At low temperatures, the individual cell voltage measurements show lower values for the cells nearer to the cooling channels. The performance of the fuel cell stack decreases at operating temperatures above 40 °C. The stack showed the best performance and stability at 30 °C, with 300 mbar of hydrogen and 500 mbar of air pressure. The optimised hydrogen purge interval was 15 s, and the most favourable air stoichiometry was 2. Between 15 A and 32 A, the maximum electrical efficiency was 40%, and the thermal energy recovery in the cooling system was 40.8%; these values are based on the HHV. Electrical efficiencies above 40% were obtained between 10 and 55 A. The variation in the electrical efficiency is explained by the variation in the following independently measured factors: the fuel utilisation coefficient and the faradic and voltage efficiencies. The deviation between the product of the factors and the measured electrical efficiency is below 0.5%. Measurements were taken to identify all the losses from the fuel cell stack; namely, the energy balance to the cooling water, which is the main portion. The other quantified losses by order of importance are the purged hydrogen and the latent and sensible heat losses from the cathode exhaust. The heat losses to the environment were also estimated based on the measured stack surface temperature. The sum of all the losses and the electrical output has a closure error below 2% except at the highest and lowest loads.     

Experimental analysis of dynamic characteristics on the PEM fuel cell stack by using Taguchi approach with neural networks

This study determines the optimum operating parameters for a proton exchange membrane fuel cell (PEMFC) stack to obtain small variation and maximum electric power output using a robust parameter design (RPD). The operating parameters examined experimentally are operating temperatures, operating pressures, anode/cathode humidification temperatures, and reactant flow rates. First, the dynamic Taguchi method is used to obtain the maximum and stable power density against the different current densities, which are regarded as the systemic inputs considered a signal factor. The relationship between control factors and responses in the PEMFC stack is determined using a neural network. The discrete parameter levels in the dynamic Taguchi method can be divided into desired levels to acquire real optimum operating parameters. Based on these investigations, the PEMFC stack is operated at the current densities of 0.4–0.8 A/cm². Since the voltage shift is quite small (roughly 0.73–0.83 V for each single cell), the efficiency would be higher. In the range of operation, the operating pressure, the cathode humidification temperature and the interactions between operating temperature and operating pressure significantly impact PEMFC stack performance. As the operating pressure increasing, the increments of the electric power decrease, and power stability is enhanced because the variation in responses is reduced.     

The compression of hydrogen in an electrochemical cell based on a PE fuel cell design

The compression of gases in conventional compressors is often combined with low efficiency and the contamination of the compressed gases. In this paper, a system is described that uses an electrochemical cell for the compression of hydrogen. This electrochemical hydrogen compressor operates at lower hydrogen flux than mechanical hydrogen compressors. Further development of polymer electrolyte membrane (PEM) fuel cells afford a good prospect of realizing the electrochemical compression on a competitive basis. A specialized gas diffusion layer established the possibility to pressurize the cathode volume up to 54 bar. The ohmic resistance and pressure stability have been rescued by an improved membrane electrode assembly (MEA). A three cell stack on a laboratory scale has been designed. Data of single cell and stack experiments will be discussed in this paper. The advantages of the electrochemical system, apart from the efficiency, are: noiseless operation, purified hydrogen and simplicity of the system cooling.     

Status of the development of a direct methanol fuel cell

In the past, most papers on direct methanol fuel cells (DMFC) reported about systems using pure oxygen instead of air supplied to the cathode. The status of the work on DMFC at Siemens was characterized by more than 200 mW/cm² at a cell voltage of 0.5 V under oxygen operation (4–5 bar abs.) at high temperatures (140°C). High oxygen pressure operation at high temperatures is only useful in special market niches. Low air pressure up to 1.5 bar abs. and therefore low operation temperatures in the range of 80–110°C are necessary technical features and economic requirements for widespread application of the DMFC. Today, our system produces 50 mW/cm² under air operation at low over pressure and at 80°C, while the cell voltage again amounts to 0.5 V. These measurements were carried out in single cells between 3 and 60 cm². First results for a cell design with an electrode area of 550 cm², which is appropriate for assembling a DMFC-stack, are shown. In the new cell it was possible to achieve the same power densities as in the experimental cells at low air over pressure. Also a three-celled stack based on this design revealed nearly the same performance. At 80°C a power output of 77 W at a stack voltage of 1.4 V can be obtained in the air mode. The low pressure air operation results in a lower performance which must be compensated by future improvements of the activity of the anode catalyst and by an adequate membrane with a low methanol and water permeation, which would be a great progress for the DMFC.     

Performance enhancement of air-cooled open cathode polymer electrolyte membrane fuel cell with inserting metal foam in the cathode side

In this study, a novel way to improve performance of the air-cooled open cathode polymer electrolyte membrane fuel cell is introduced. We suggest using a metal foam in the cathode side of the planar unit fuel cell for the solution to conventional problems of the open cathode fuel cell such as excessive water evaporation from the membrane and poor transportation of air. We conduct experiment and the maximum power density of the fuel cell with metal foam increases by 25.1% compared with the conventional fuel cell without metal foam. The open cathode fuel cell with metal foam has smaller ohmic losses and concentration losses. In addition, we prove that the open cathode fuel cell with metal foam prevents excessive water evaporation and membrane drying out phenomena with numerical approach. Finally, we apply the metal foam to the air-cooled open cathode fuel cell stack as well as the planar unit cell.     

Experimental investigation of a liquid cooled high temperature proton exchange membrane (HT-PEM) fuel cell coupled to a sodium alanate tank

In high temperature proton exchange membrane (HT-PEM) fuel cells, waste heat at approximately 160 °C is produced, which can be used for thermal integration of solid state hydrogen storage systems. In the present study, an HT-PEM fuel cell stack (400 W) with direct liquid cooling is characterized and coupled to a separately characterized sodium alanate storage tank (300 g material). The coupled system is studied in steady state for 20 min operation and all relevant heat flows are determined. Even though heat losses at that specific power and temperature level cannot be completely avoided, it is demonstrated that the amount of heat transferred from the fuel cell stack to the cooling liquid circuit is sufficient to desorb the necessary amount of hydrogen from the storage tank. Furthermore, it is shown that the reaction rate of the sodium alanate at 160 °C and 1.7 bar is adequate to provide the hydrogen to the fuel cell stack. Based on these experimental investigations, a set of recommendations is given for the future design and layout of similar coupled systems.     

Numerical analysis of output characteristics of tubular SOFC with internal reformer

In the solid oxide fuel cell (SOFC) system, the internal reforming of raw fuel will act as an efficient cooling system. To realize this cooling system, a special design of the internal reformer is required to avoid the inhomogeneous temperature distribution caused by the strong endothermic reforming reaction at the entrance of the internal reformer. For this purpose, a tubular internal reformer with adjusted catalyst density can be inserted into the tubular SOFC stack. By arranging this, the raw fuel flows along the axis of the internal reformer to be moderately reformed and returns at the end of the internal reformer as a sufficiently reformed fuel.In this paper, the output characteristics of this configuration are simulated using mathematical models, in which one-dimensional temperature and molar distributions are computed along the flow direction. By properly mounting the catalyst density in the internal reformer, the temperature distribution of the cell stack becomes moderate, and the power generation efficiency and the exhaust gas temperature are higher. Effects of other operating conditions such as fuel recirculation, fuel inlet temperature, air recirculation and air inlet temperature are also examined under the condition where the maximum temperature of the stack is kept at 1300 K by adjusting the air flow rate. Under this condition, these operating conditions exert a considerable effect on the exhaust temperature but have a slight effect on the efficiency.     

Water balance in a polymer electrolyte fuel cell system

Polymer electrolyte fuel cell (PEFC) systems operating on carbonaceous fuels require water for fuel processing. Such systems can find wider applications if they do not require a supply of water in addition to the supply of fuel, that is, if they can be self-sustaining based on the water produced at the fuel cell stack. This paper considers a generic PEFC system and identifies the parameters that affect, and the extent of their contribution to, the net water balance in the system. These parameters include the steam-to-carbon and the oxygen-to-carbon ratios in the fuel processor, the electrochemical fuel and oxygen utilizations in the fuel cell stack, the ambient pressure and temperature, and the composition of the fuel used. The analysis shows that the amount of water lost from the system as water vapor in the exhaust is very sensitive to the system pressure and ambient temperature, while the amount of water produced in the system is a function of the composition of the fuel. Fuels with a high H/C (hydrogen to carbon atomic ratio) allow the system to be operated as a net water producer under a wider range of operating conditions.     

Fuel economy of hydrogen fuel cell vehicles

On the basis of on-road energy consumption, fuel economy (FE) of hydrogen fuel cell light-duty vehicles is projected to be 2.5–2.7 times the fuel economy of the conventional gasoline internal combustion engine vehicles (ICEV) on the same platforms. Even with a less efficient but higher power density 0.6 V per cell than the base case 0.7 V per cell at the rated power point, the hydrogen fuel cell vehicles are projected to offer essentially the same fuel economy multiplier. The key to obtaining high fuel economy as measured on standardized urban and highway drive schedules lies in maintaining high efficiency of the fuel cell (FC) system at low loads. To achieve this, besides a high performance fuel cell stack, low parasitic losses in the air management system (i.e., turndown and part load efficiencies of the compressor–expander module) are critical.     

Modelling and analysis of a solid polymer fuel cell system for transportation applications

This paper presents the development and application of a performance model for a solid polymer fuel cell power system for transportation applications. The model simulates the operation of all the components in the system — including the fuel cell stack, and the air compression, hydrogen supply, and cooling subsystems. Two duty cycles are used to examine the performance of the baseline system, as well as to evaluate the benefits obtained by improving individual system components. The results indicate that the largest improvements in system performance would be obtained by (a) a reduction of the cooling loads, (b) improving the efficiency of the inlet air compressor, and (c) recovery of the thermomechanical energy in the stored compressed hydrogen.     

Parametric analysis of a hybrid power system using organic Rankine cycle to recover waste heat from proton exchange membrane fuel cell

Using fuel cell systems for distributed generation (DG) applications represents a meaningful candidate to conventional plants due to their high power density and the heat recovery potential during the electrochemical reaction. A hybrid power system consisting of a proton exchange membrane (PEM) fuel cell stack and an organic Rankine cycle (ORC) is proposed to utilize the waste heat generated from PEM fuel cell. The system performance is evaluated by the steady-state mathematical models and thermodynamic laws. Meanwhile, a parametric analysis is also carried out to investigate the effects of some key parameters on the system performance, including the fuel flow rate, PEM fuel cell operating pressure, turbine inlet pressure and turbine backpressure. Results show that the electrical efficiency of the hybrid system combined by PEM fuel cell stack and ORC can be improved by about 5% compared to that of the single PEM fuel cell stack without ORC, and it is also indicated that the high fuel flow rate can reduce the PEM fuel cell electrical efficiency and overall electrical efficiency. Moreover, with an increased fuel cell operating pressure, both PEM fuel cell electrical efficiency and overall electrical efficiency firstly increase, and then decrease. Turbine inlet pressure and backpressure also have effects on the performance of the hybrid power system.     

Quasi-stationary UI-characteristic model of a PEM fuel cell–Evaluating the option of self-humidifying operation

This work presents a zero-dimensional PEM fuel cell UI-characteristic model created in MATLAB Simulink® for operation with dry or humidified air supply. It is parameterised and validated based on the results of stack operation by varying stack temperature (50–80 °C), gas pressure (1.0–2.4 bar) and air humidification (0.0–1.0). The model is based on physical and electrochemical correlations and expanded by empirically assumptions concerning the influence of the humidification and limiting current density on the performance. The UI-model is intended to be integrated into a comprehensive zero-emission powertrain model. Since non-humidified operation of PEM fuel cell systems provides benefits for mobile applications by reducing space demand and system complexity, the objective of the model is to relate performance to the operating conditions and underlying physical parameters. Results confirm the feasibility of a self-humidifying stack operation at high performance by optimal parameter setting.