Hybrid fuel cell and battery propulsion system modelling and multi-objective optimisation for a coastal ferry

Efforts from all sectors of society including the shipping industry are needed to limit the overall global temperature rise to within 2°C of pre-industrial levels by 2050. The hybridisation of Proton Exchange Membrane Fuel Cells (PEMFC) and Lithium-ion batteries for coastal ship propulsion systems may potentially offer beneficial emission performance. However, such hybrid systems are constrained by power and energy density limitations, lifetime; and costs as well as life-cycle emissions of alternative fuel/energy. There is a lack of holistic design methodology dealing with these uncertainties in the literature. This paper proposes a holistic design methodology for coastal hybrid ships based upon a developed model. The power source sizing problem is solved using constrained mixed-integer multi-objective optimisation in the external layer. The global optimum energy management strategies for an averaged operating profile are obtained from deterministic dynamic programming in the inner layer, while considering power source degradations in the sizing algorithm. The proposed methodology was applied to a coastal ferry to investigate the feasibility and benefit potential of adopting the hybrid PEMFC and battery propulsion system in Matlab. The case studies indicate that the proposed propulsion system can achieve at least a 65% life-cycle greenhouse gas reduction for the considered two cases.     

A dynamic model for high temperature polymer electrolyte membrane fuel cells

A dynamic one-dimensional isothermal phenomenological model was developed in order to describe the steady-state and transient behavior of high temperature polymer electrolyte membrane fuel cells (PEMFC). The model accounts for transient species mass transport at the bipolar plates and gas diffusion layers and the electric double layers charge/discharge. To record the impedance spectra, a small sinusoidal voltage perturbation was imposed to the simulator over a wide range of frequencies, and the resultant current density amplitude and phase were recorded.The steady-state behavior of the fuel cell, as well as the impedance spectra were obtained and compared to experimental data of two different fuel cells equipped with different MEAs based on phosphoric acid polybenzimidazole membrane. This approach is new and allows a deeper analysis of the controlling phenomena. The model fitted quite well the I-V curves for both systems, but fairly well the Nyquist plots. The differences observed in the Nyquist plots were attributed to proton resistance in the catalyst layer and the gas diffusion limitations to cross the phosphoric acid layer that coats the catalyst, phenomena not included in the proposed phenomenological model.     

A review of polymer electrolyte membrane fuel cell stack testing

This paper presents an overview of polymer electrolyte membrane fuel cell (PEMFC) stack testing. Stack testing is critical for evaluating and demonstrating the viability and durability required for commercial applications. Single cell performance cannot be employed alone to fully derive the expected performance of PEMFC stacks, due to the non-uniformity in potential, temperature, and reactant and product flow distributions observed in stacks. In this paper, we provide a comprehensive review of the state-of-the art in PEMFC testing. We discuss the main topics of investigation, including single cell vs. stack-level performance, cell voltage uniformity, influence of operating conditions, durability and degradation, dynamic operation, and stack demonstrations. We also present opportunities for future work, including the need to verify the impact of stack size and cell voltage uniformity on performance, determine operating conditions for achieving a balance between electrical efficiency and flooding/dry-out, meet lifetime requirements through endurance testing, and develop a stronger understanding of degradation.     

Modelling and evaluation of heating strategies for high temperature polymer electrolyte membrane fuel cell stacks

Experiments were conducted on two different cathode air cooled high temperature PEM (HTPEM) fuel cell stacks; a 30 cell 400 W prototype stack using two bipolar plates per cell, and a 65 cell 1 kW commercial stack using one bipolar plate per cell. The work seeks to examine the use of different heating strategies and find a strategy suited for fast start-up of the HTPEM fuel cell stacks. Fast start-up of these high temperature systems enables use in a wide range of applications, such as automotive and auxiliary power units, where immediate system response is needed. The development of a dynamic model to simulate the temperature development of a fuel cell stack during heating can be used for assistance in system and control design. The heating strategies analyzed and tested reduced the start-up time of one of the fuel cell stacks from 1 h to about 6 min.     

Adaptive operation strategy of a polymer electrolyte membrane fuel cell air system based on model predictive control

The present article investigates a model predictive control-based operation strategy of an automotive fuel cell air system. For this purpose, a nonlinear model of a fuel cell system is derived, which is linearized and discretized around the current operation point during each time sample. This model is combined with a cost function taking into account power reference tracking and hydrogen minimization. Additional system constraints ensure a safe and robust operation. Subsequently, the adaptive and efficiency-optimal behavior of the model predictive controller is demonstrated based on a simulation study of different scenarios with varying power profiles. Furthermore, the thermal derating behavior of this control is studied using an exemplary situation with critical thermal conditions. Finally, the model predictive control approach is compared with a validated map-based operation strategy highlighting the potential of reducing the hydrogen consumption by 3% while decreasing the risk of harmful operation conditions.     

Fuel cell systems optimisation - Methods and strategies

This paper reviews the current state of modelling and optimisation with regard to fuel cell systems design. The existing models for portable, stationary and transportation fuel cell systems are identified and characterised by approach, state, system boundary, spatial dimension and complexity or detail. The different model-based design approaches such as parametric study, single-objective optimisation and multi-objective optimisation performed using fuel cell system models are summarised. A case study on the design of a fuel cell micro-cogeneration plant is presented to illustrate the use of modelling and optimisation in generating different design alternatives that contain trade-offs between competing objectives.     

Two strategies for multi-objective optimisation of solid oxide fuel cell stacks

This paper focuses on multi-objective optimisation (MOO) to optimise the planar solid oxide fuel cell (SOFC) stacks performance using a genetic algorithm. MOO problem does not have a single solution, but a complete Pareto curve, which involves the optional representation of possible compromise solutions. Here, two pairs of different objectives are considered as distinguished strategies. Optimisation of the first strategy predicts a maximum power output of 108.33 kW at a breakeven per-unit energy cost of 0.51 $/kWh and minimum breakeven per-unit energy cost of 0.30 $/kWh at a power of 42.18 kW. In the second strategy, maximum efficiency of 63.93%at a breakeven per-unit energy cost of 0.42 $/kWh is predicted, while minimum breakeven per-unit energy cost of 0.25 $/kWh at efficiency of 48.3% is obtained. The present study creates the basis for selecting optimal operating conditions of SOFC under the face of multiple conflicting objectives.     

New materials for polymer electrolyte membrane fuel cell current collectors

Polymer Electrolyte Membrane Fuel cells for automotive applications need to have high power density, and be inexpensive and robust to compete effectively with the internal combustion engine. Development of membranes and new electrodes and catalysts have increased power significantly, but further improvements may be achieved by the use of new materials and construction techniques in the manufacture of the bipolar plates. To show this, a variety of materials have been fabricated into flow field plates, both metallic and graphitic, and single fuel cell tests were conducted to determine the performance of each material. Maximum power was obtained with materials which had lowest contact resistance and good electrical conductivity. The performance of the best material was characterised as a function of cell compression and flow field geometry.     

A novel membrane transport model for polymer electrolyte fuel cell simulations

This work presents the development of a 1D model describing water and charge transport through the polymer electrolyte membrane (PEM) in the fuel cell. The considered driving forces are electrical potential, concentration and pressure gradients. The membrane properties such as water diffusion and electro-osmotic coefficients, water sorption and ionic conductivity are treated as temperature dependent functions. The dependencies of diffusion and electro-osmotic coefficients on the membrane water concentration are described by linear functions. The membrane conductivity is computed in the framework of the percolation theory under consideration that the conducting phase in the PEM is formed by a hydrated functional groups and absorbed water. This developed membrane model was implemented in the CFD code AVL FIRE using 1D/3D coupling. The simulated polarization curves at various humidification of the cathode are found in good agreement with the experiments thus confirming the validity of the model.     

Multi-objective optimisation analysis and load matching of a phosphoric acid fuel cell system

Based on the current model of a phosphoric acid fuel cell (PAFC) system, the electrolyte concentration is optimised. New analytical expressions for the power output and efficiency of the PAFC system are derived by considering the effects of multi-irreversibilities resulting from the activation overpotential, concentration overpotential, ohmic overpotential, and leakage resistance on the performance of the PAFC system. These parameters are used to evaluate the general performance characteristics of the PAFC system. Accordingly, the upper and lower limits of the optimised values for some main parameters, such as the current density, power output, and efficiency, are determined. Moreover, a multi-objective function, including both the power output and efficiency, is introduced and used to further subdivide the parametric optimum regions according to different requirements. In addition, a general formula for the load of the system is derived. The relations between the power output and efficiency of the system and the load are discussed in detail, and the optimum matching conditions of the load are obtained.     

Numerical modeling of an automotive derivative polymer electrolyte membrane fuel cell cogeneration system with selective membranes

Cogeneration power plants based on fuel cells are a promising technology to produce electric and thermal energy with reduced costs and environmental impact. The most mature fuel cell technology for this kind of applications are polymer electrolyte membrane fuel cells, which require high-purity hydrogen.The most common and least expensive way to produce hydrogen within today's energy infrastructure is steam reforming of natural gas. Such a process produces a syngas rich in hydrogen that has to be purified to be properly used in low temperature fuel cells. However, the hydrogen production and purification processes strongly affect the performance, the cost, and the complexity of the energy system.Purification is usually performed through pressure swing adsorption, which is a semi-batch process that increases the plant complexity and incorporates a substantial efficiency penalty. A promising alternative option for hydrogen purification is the use of selective metal membranes that can be integrated in the reactors of the fuel processing plant. Such a membrane separation may improve the thermo-chemical performance of the energy system, while reducing the power plant complexity, and potentially its cost. Herein, we perform a technical analysis, through thermo-chemical models, to evaluate the integration of Pd-based H₂-selective membranes in different sections of the fuel processing plant: (i) steam reforming reactor, (ii) water gas shift reactor, (iii) at the outlet of the fuel processor as a separator device. The results show that a drastic fuel processing plant simplification is achievable by integrating the Pd-membranes in the water gas shift and reforming reactors. Moreover, the natural gas reforming membrane reactor yields significant efficiency improvements.     

A novel electrocatalyst support with proton conductive properties for polymer electrolyte membrane fuel cell applications

The objective of this study is to graft the surface of carbon black, by chemically introducing polymeric chains (Nafion^® like) with proton-conducting properties. This procedure aims for a better interaction of the proton-conducting phase with the metallic catalyst particles, as well as hinders posterior support particle agglomeration. Also loss of active surface can be prevented. The proton conduction between the active electrocatalyst site and the Nafion^® ionomer membrane should be enhanced, thus diminishing the ohmic drop in the polymer electrolyte membrane fuel cell (PEMFC). PtRu nanoparticles were supported on different carbon materials by the impregnation method and direct reduction with ethylene glycol and characterized using amongst others FTIR, XRD and TEM. The screen printing technique was used to produce membrane electrode assemblies (MEA) for single cell tests in H₂/air (PEMFC) and methanol operation (DMFC). In the PEMFC experiments, PtRu supported on grafted carbon shows 550 mW cm⁻² gmetal⁻¹ power density, which represents at least 78% improvement in performance, compared to the power density of commercial PtRu/C ETEK. The DMFC results of the grafted electrocatalyst achieve around 100% improvement. The polarization curves results clearly show that the main cause of the observed effect is the reduction in ohmic drop, caused by the grafted polymer.     

Controller design for polymer electrolyte membrane fuel cell systems for automotive applications

Continuous developments in Proton Exchange Membrane Fuel Cells (PEMFC) make them a promising technology to achieve zero emissions in multiple applications including mobility. Incremental advancements in fuel cells materials and manufacture processes make them now suitable for commercialization. However, the complex operation of fuel cell systems in automotive applications has some open issues yet. This work develops and compares three different controllers for PEMFC systems in automotive applications. All the controllers have a cascade control structure, where a generator of setpoints sends references to the subsystems controllers with the objective to maximize operational efficiency. To develop the setpoints generators, two techniques are evaluated: off-line optimization and Model Predictive Control (MPC). With the first technique, the optimal setpoints are given by a map, obtained off-line, of the optimal steady state conditions and corresponding setpoints. With the second technique, the setpoints time profiles that maximize the efficiency in an incoming time horizon are continuously computed. The proposed MPC architecture divides the fast and slow dynamics in order to reduce the computational cost. Two different MPC solutions have been implemented to deal with this fast/slow dynamics separation. After the integration of the setpoints generators with the subsystems controllers, the different control systems are tested and compared using a dynamic detailed model of the automotive system in the INN-BALANCE project running under the New European Driving Cycle.     

A novel cesium hydrogen sulfate-zeolite inorganic composite electrolyte membrane for polymer electrolyte membrane fuel cell application

A new type of CsHSO₄-HZSM-5 inorganic composite electrolyte membrane is prepared by mechanically mixing CsHSO₄ (CHS) and nanometer-scale HZSM-5 zeolite powders. The effects of HZSM-5 on the crystallite structure, proton conductivity, and thermal stability of the CsHSO₄ electrolyte are investigated. Incorporation of HZSM-5 is found to significantly increase the low-temperature proton conductivity of the CsHSO₄ electrolyte, extending its operating temperature down to 100 °C. The composite electrolyte with 40 mol% HZSM-5 shows the highest proton conductivity in the measured temperature range. The low-temperature activation energy of the composite with 40 mol% HZSM-5 is lower than that of the CHS-SiO₂ composite. The improvement of the proton conductivity can be attributed to the enhanced interfacial interaction between the two phases. And the small HZSM-5 particles lead to a change in the bulk properties of the ionic salts. The melting point of the CHS-HZSM-5 composite electrolyte is lower than that of the pure CHS electrolyte. The CHS-HZSM-5 composite electrolyte is suitable for polymer electrolyte membrane fuel cells operated at 100-200 °C.     

A model-based parametric analysis of a direct ethanol polymer electrolyte membrane fuel cell performance

In the present work, a model-based parametric analysis of the performance of a direct ethanol polymer electrolyte membrane fuel cell (DE-PEMFC) is conducted with the purpose to investigate the effect of several parameters on the cell's operation. The analysis is based on a previously validated one-dimensional mathematical model that describes the operation of a DE-PEMFC in steady state. More precisely, the effect of several operational and structural parameters on (i) the ethanol crossover rate from the anode to the cathode side of the cell, (ii) the parasitic current generation (mixed potential formation) and (iii) the total cell performance is investigated. According to the model predictions it was found that the increase of the ethanol feed concentration leads to higher ethanol crossover rates, higher parasitic currents and higher mixed potential values resulting in the decrease of the cell's power density. However there is an optimum ethanol feed concentration (approximately 1.0 mol L⁻¹) for which the cell power density reaches its highest value. The platinum (Pt) loading of the anode and the cathode catalytic layers affects strongly the cell performance. Higher values of Pt loading of the catalytic layers increase the specific reaction surface area resulting in higher cell power densities. An increase of the anode catalyst loading compared to an equal one of the cathode catalyst loading has greater impact on the cell's power density. Another interesting finding is that increasing the diffusion layers’ porosity up to a certain extent, improves the cell power density despite the fact that the parasitic current increases. This is explained by the fact that the reactants’ concentrations over the catalysts are increased, leading to lower activation overpotential values, which are the main source of the total cell overpotentials. Moreover, the use of a thicker membrane leads to lower ethanol crossover rate, lower parasitic current and lower mixed potential values in comparison to the use of a thinner one. Finally, according to the model predictions when the cell operates at low current densities the use of a thick membrane is necessary to reduce the negative effect of the ethanol crossover. However, in the case where the cell operates at higher current densities (lower ethanol crossover rates) a thinner membrane reduces the ohmic overpotential leading to higher power density values.     

Fabrication of polymer electrolyte membrane fuel cell MEAs utilizing inkjet print technology

Utilizing drop-on-demand technology, we have successfully fabricated hydrogen–air polymer electrolyte membrane fuel cells (PEMFC), demonstrated some of the processing advantages of this technology and have demonstrated that the performance is comparable to conventionally fabricated membrane electrode assemblies (MEAs). Commercial desktop inkjet printers were used to deposit the active catalyst electrode layer directly from print cartridges onto Nafion^® polymer membranes in the hydrogen form. The layers were well-adhered and withstood simple tape peel, bending and abrasion tests and did so without any post-deposition hot press step. The elimination of this processing step suggests that inkjet-based fabrication or similar processing technologies may provide a route to less expensive large-scale fabrication of PEMFCs. When tested in our experimental apparatus, open circuit voltages up to 0.87 V and power densities of up to 155 mW cm⁻² were obtained with a catalyst loading of 0.20 mg Pt cm⁻². A commercially available membrane under identical, albeit not optimized test conditions, showed about 7% greater power density. The objective of this work was to demonstrate some of the processing advantages of drop-on-demand technology for fabrication of MEAs. It remains to be determined if inkjet fabrication offers performance advantages or leads to more efficient utilization of expensive catalyst materials.     

Hydrogen circulation system model predictive control for polymer electrolyte membrane fuel cell-based electric vehicle application

Polymer electrolyte membrane fuel cell (PEMFC) is one of the promising solutions overcoming future energy crisis and environment pollution in the automotive industry. However, PEMFC is vulnerable to the circulation of hydrogen mass flow rate and pressure, which may cause the degradation of the PEMFC's anode components and reduction of output performance over time. Thus, the control of the hydrogen supply system draws attention currently and is critical for the durability and stability of the PEMFC system. In this study, a model predictive control (MPC) approach for hydrogen circulation system is developed to regulate the hydrogen flow circulating. A model of the hydrogen supply system that contains a flow control valve, a supply manifold, a return manifold and a hydrogen circulating pump is firstly developed to describe the behavior of the hydrogen mass flow dynamics in the PEMFC. Subsequently, a hydrogen circulating pump MPC scheme is designed based on the piecewise linearized model of hydrogen circulation as well as the switched MPC controllers. By predicting the pressure of the return manifold and the angle velocity of the pump, the proposed MPC approach can manipulate the hydrogen circulating pump to achieve efficient and stable operation of the PEMFC.     

A new approach to multi-objective optimisation method in PEM fuel cell

This paper presents an optimisation model for polymer electrolyte membrane fuel cell system based on simultaneous power maximisation and cost minimisation. The results show that, by employing appropriate relation between the objectives, the innovative design could be proposed. Genetic algorithm is applied to solve the optimisation problem. Power maximisation results reveal that at maximum amount of power (1.95 kW), unit cost of energy is $0.64. In contrast, minimisation of cost decreases unit cost of energy to $0.33. In this condition, output power is reduced approximately to 0.93 kW. To consider both optimisation problems concurrently, weighting method and Pareto set are employed. Our outcomes proposed that applying Pareto set to any optimisation problem leads to realistic decision-making. Eventually, sensitivity analysis is done on Pareto set, based on different Nafion membrane types and hydrogen cost variation.     

Demonstration and development of a polymer electrolyte fuel cell system for residential use

Fuel cell systems, especially those fed with hydrogen, have reached considerable performance targets in laboratory conditions with constant loads and conservative environmental conditions. However, a check of the potential of such systems in real conditions is necessary, particularly in terms of varying electrical and thermal loads and of more severe climatic conditions.To determine the state of the art of such technology and to develop systems capable of supporting future national energy scenarios within the PNR-FISR project, “Polymeric electrolytes and ceramic fuel cells: demonstration of systems and development of new materials,” the development of fuel cell systems ranging from 1 to 5 kW of power and based on either solid polymer (PEMFC) or solid oxide (SOFC) technology are in progress. In this paper, the demonstration of a pre-commercial PEMFC system fed with hydrogen and developed in cooperation with NUVERA is described. The system has been developed in order to determine its limits and capacity in relation to start-up time, response time, consumption, efficiency, reliability, etc. It has currently reached 1000 working hours of continuous performance with variable loads that simulate those of a typical residential dwelling.