DC/DC converter topologies for electrolyzers: State-of-the-art and remaining key issues

In recent years, the use of electrolyzers to produce cleanly and efficiently hydrogen from renewable energy sources (i.e. wind turbines, photovoltaic) has taken advantage of a growing interest from researchers and industrial. Similarly to fuel cells, DC/DC converters are needed to interface the DC bus with the electrolyzer. Usually, electrolyzers require a low DC voltage to produce hydrogen from water. For this reason, a DC/DC buck converter is generally used for this purpose. However, other DC/DC converter topologies can be used depending on the feature of the electrolyzer and electrical grid as well. The main purpose of this paper is to present the current state-of-the-art of DC/DC converter topologies which can be combined with electrolyzers. The different DC/DC converter topologies are compared in terms of output current ripple reduction, conversion ratio, energy efficiency, and power switch fault-tolerance. Besides, remarks on the state-of-the-art and remaining key issues regarding DC/DC converters are provided.     

Integrated modeling and control of a PEM fuel cell power system with a PWM DC/DC converter

A fuel cell powered system is regarded as a high current and low voltage source. To boost the output voltage of a fuel cell, a DC/DC converter is employed. Since these two systems show different dynamics, they need to be coordinated to meet the demand of a load. This paper proposes models for the two systems with associated controls, which take into account a PEM fuel cell stack with air supply and thermal systems, and a PWM DC/DC converter. The integrated simulation facilitates optimization of the power control strategy, and analyses of interrelated effects between the electric load and the temperature of cell components. In addition, the results show that the proposed power control can coordinate the two sources with improved dynamics and efficiency at a given dynamic load.     

A generic FPGA-based PWM generator with automatic device fault recovery for fuel cell,interleaved, multi-phase and multi-switch DC/DC boost converters

This paper describes the simulation and implementation of a generic FPGA-based PWM generator with automatic device fault recovery. This PWM generator is applied to multi-switch DC/DC boost converters for fuel cell applications; these converters have gained attention due to its reliability, equally-distributed power losses and reduced part count. In this paper, three different driving schemes for one phase and different legs are described and one driving scheme has been finally chosen. The operation of the proposed PWM generator with simulations and experimental results are shown. This PWM generator with automatic fault recovery, brings more reliability and fault robustness to fuel cell boost converters, making suitable to use in electric vehicle and uninterruptible power supplies (UPS). The method used in the PWM generator described on this paper, allows to change the number of devices working in DC/DC conversion, increasing the switching frequency and the duty cycle of the power switches, ensuring a DC/DC conversion without interruption.     

Non-isolated multiphase boost converter for a fuel cell with battery backup power system

This work describes a step-up non-isolated DC/DC converter aimed for fuel cell stand-alone power systems. The proposed converter has the following features: simple structure based on the basic boost topology that reduces the number of components; it uses the interleaving technique in order to reduce the current ripple at the input and output sides, reduction of the inductors size, higher frequency that reduce the output filter capacitor and easier power losses management. In addition, the use of an inner current control loop in the input side assures power sharing and easy module parallelization. The converter feeds a backup battery that maintains a DC voltage level at the main bus. An outer battery-charging loop controls the converter. Experimental validation is given for a four-phases 1 kW prototype at 100 kHz PWM switching frequency connected to a Nexa Ballard (1.2 kW-46 A) PEM fuel cell.     

Interleaved,switched-inductor,multi-phase,multi-device DC/DC boost converter for non-isolated and high conversion ratio fuel cell applications

Fuel cell power conditioners often require high step-up voltage gains to accommodate low input fuel cell voltages into high voltage busses. Traditional non-isolated DC-DC boost converters are unable to offer such as gains because of several parasitic elements and non-ideal behaviour of power semiconductors and driving circuits. Moreover, paralleled converters are also desirable to simplify power-up scaling and to reduce input/output current ripples. In this context, a very versatile non-isolated, high step-up voltage gain, interleaved boost converter is presented in this work. Steady-state analysis, simulation and evaluation of different converter structures are discussed in detail. Finally, a 500-W experimental prototype for Nexa Ballard 1.2 kW fuel cell specifications has been implemented and tested to verify the performance.     

A suitable model plant for control of the set fuel cell−DC/DC converter

In this work a state and transfer function model of the set made up of a proton exchange membrane (PEM) fuel cell and a DC/DC converter is developed. The set is modelled as a plant controlled by the converter duty cycle. In addition to allow setting the plant operating point at any point of its characteristic curve (two interesting points are maximum efficiency and maximum power points), this approach also allows the connection of the fuel cell to other energy generation and storage devices, given that, as they all usually share a single DC bus, a thorough control of the interconnected devices is required. First, the state and transfer function models of the fuel cell and the converter are obtained. Then, both models are related in order to achieve the fuel cell+DC/DC converter set (plant) model. The results of the theoretical developments are validated by simulation on a real fuel cell model.     

A family of high gain fuel cell front-end converters with low input current ripple for PEMFC power conditioning systems

Since the output voltage of the proton exchange membrane fuel cell (PEMFC) is relatively low and load-dependent, a high-performance fuel cell front-end converter is required to achieve boost and power regulation in PEMFC systems. In response, a novel family of high gain fuel cell front-end converters with low input current ripple is proposed. The proposed topologies can substantially improve the voltage gain through the expansion and combination of active switched-inductor networks and passive switched-capacitor units. The introduced interleaved parallel structure is convenient to limit the current ripple on the input side to prevent accelerated aging of fuel cells, which is another prominent advantage. Meanwhile, the converters can achieve the automatic current sharing between parallel inductors and the low voltage stress on active switches and diodes. In this paper, the fuel cell model and topology derivation of the high gain fuel cell front-end converters are first analyzed. Then, it further describes the operating mode and steady-state performance of converters under the inductor current continuous conduction mode. The comparison with other converters shows that this converter is suitable for connecting the PEMFC to the high voltage DC bus. Finally, a 200 W, 20/180 V converter prototype is implemented, and the simulation and experiment prove the theoretical correctness and validate the superior performances of the proposed converters.     

Design and experimental validation of a high voltage ratio DC/DC converter for proton exchange membrane electrolyzer applications

This paper deals with hydrogen production via water electrolysis, which is considered the most attractive and promising solution. Specifically, the use of renewable energy sources, such as wind electric power generators, is hypothesized for supplying the electrolyzer, aiming to strongly reduce the environmental impact. In particular, micro-wind energy conversion systems (μWECSs) are attractive for their low cost and easy installation. In order to interface the μWECS and the electrolyzer, suitable power conditioning systems such as step-down DC-DC converters are mandatory. However, due to the requested high conversion ratio between the DC bus grid, i.e. the output of a three-phase diode rectifier connected to the output of the electric generator, and the rated supply voltage of the electrolyzer, the classic buck converter alone is not suitable. Therefore, a converter is proposed and designed, consisting of a buck converter, a full-bridge IGBT converter, a single-phase transformer, and a diode bridge rectifier; LC filters are also included between buck and full-bridge converters, and at the output of the diode bridge rectifier with the aim of reducing the ripple on currents and voltages. The components of the described physical system from the output of the three-phase rectifier up to the electrolyzer are then modeled assuming the transformer as ideal, and the model is employed for designing a PI-type controller. Experimental results are provided in order to demonstrate the effectiveness of the developed converter and its control for these applications.     

Interleaved soft-switched active-clamped L–L type current-fed half-bridge DC–DC converter for fuel cell applications

In this paper, an interleaved soft-switched active-clamped L–L type current-fed half-bridge isolated dc–dc converter has been proposed. The L–L type active-clamped current-fed converter is able to maintain zero-voltage switching (ZVS) of all switches for the complete operating range of wide fuel cell stack voltage variation at full load down to light load conditions. Active-clamped circuit absorbs the turn-off voltage spike across the switches. Half-bridge topology maintains higher efficiency due to lower conduction losses. Soft-switching permits higher switching frequency operation, reducing the size, weight and cost of the magnetic components. Interleaving of the two isolated converters is done using parallel input series output approach and phase-shifted modulation is adopted. It reduces the input current ripple at the fuel cell input, which is required in a fuel cell system and also reduces the output voltage ripples. In addition, the size of the magnetic/passive components, current rating of the switches and voltage ratings of the rectifier diodes are reduced.     

Analysis and comparative study of pulsating current of fuel cells by inverter load with different power converter topologies

Fuel cells are being increasingly used for stand alone and grid connected systems in wide range of applications due to their high efficiency and low emissions. Because of unregulated nature of fuel cell voltage a power conditioning unit, consisting of DC-DC converter and an inverter, is invariably used as an interface between the fuel cell and the load in a typical fuel cell system for ac applications. Major issues with the use of fuel cells for ac applications are the low frequency pulsating current propagation on to the fuel cell side and dynamic response to various loads during transient conditions. Low frequency pulsating currents are reported to affect reactant utilization, degrade the performance and life of fuel cells. These current ripples can be reduced by filters with passive elements having bulky inductor and capacitor in the dc-link between the fuel cell and the inverter but, it will add to the weight and cost. DC-DC converters of different configurations are being used in the power conditioning unit of fuel cell systems. These converters are operated at high frequencies and the filtering units of these converters have minimal effect on low frequency ripple. But, it is observed that different configurations of power conditioner with same filtering components perform differently for the low frequency current ripple of the inverter load by mitigating the power mismatch between load and source at the DC link. This paper investigates and compares the low frequency current ripple mitigation by cascaded converters with conventional push-pull and also with series connected converters in the power conditioning stage of fuel cell system for ac applications. Parameters such as peak switching currents, the percentage of peak to DC level of low frequency current ripple are analyzed using these conversion topologies in power conditioning unit. The analytical and simulation results related to the study are presented. Key results are verified with experimental work.     

Energy management of fuel cell/battery/ultracapacitor in electrical hybrid vehicle

This paper describes an energy management algorithm for an electrical hybrid vehicle. The proposed hybrid vehicle presents a fuel cell as the main energy source and the storage system, composed of a battery and a supercapacitor as the secondary energy source. The main source must produce the necessary energy to the electrical vehicle. The secondary energy source produces the lacking power in acceleration and absorbs excess power in braking operation. The addition of a supercapacitor and battery in fuel cell-based vehicles has a great potential because it allows a significant reduction of the hydrogen consumption and an improvement of the vehicle efficiency. Other the energy sources, the electrical vehicle composed of a traction motor drive, Inverter and power conditioning. The last is composed of three DC/DC converters: the first converter interfaces the fuel cell and the DC link. For the second and the third converter, two buck boost are used in order to interface respectively the ultracapacitor and the battery with the DC link. The energy management algorithm determines the currents of the converters in order to regulate accurately the power provided from the three electrical sources. This algorithm is simulated with MATLAB_Simulink and implemented experimentally with a real-time system controller based on dSPACE. In this paper, the proposed algorithm is evaluated for the New European Driving Cycle (NEDC). The experimental results validate the effectiveness of the proposed energy management algorithm.     

Improved performance and energy management strategy for proton exchange membrane fuel cell/backup battery in power electronic systems

The objective of this paper is to propose an approach to analyze the reliability of proton exchange membrane fuel cell and backup battery in the power electronic systems by improving the strategy of energy management. This approach is based on the research of critical causes generating the degradation of power sources in the power electronic application and their undesirable effect. The analysis of potential failure modes that affect the fuel cell and the auxiliary source is developed by using analysis tools such as the Failure Mode and Effects Analysis (FMEA). The undesirable factors have to be taken into account in designing the topology of power converter in order to improve the performance of a fuel cell power system. In this context the authors propose to integrate a multiphase converter to minimize the current ripple and ameliorate the dynamic response of exchange membrane fuel cell. The improvement of battery lifetime is also ensured by the incorporation of an appropriate charge cycle that promises the full state of charge and avoids the overcharge.     

A stacked interleaved DC-DC buck converter for proton exchange membrane electrolyzer applications: Design and experimental validation

Since the two last decades, hydrogen production has been attracting the attention of the scientific community thanks to its inherent very low pollution when energy coming from renewable energy sources (RESs) are used. However, it implies the use of DC/DC converters to interface source and load. These conversion systems must meet several requirements from current ripple point of view, energy efficiency, and performance to preserve the sustainability of hydrogen production. This article proposes the design and realization of a stacked interleaved buck converter to supply a proton exchange membrane electrolyzer. The converter is designed to ensure a low output current ripple and a suitable dynamic response to guarantee the reliability of the electrolyzer. A theoretical analysis of the converter, taking into account the dynamic model of the electrolyzer, and the design of the control system based both on feedforward and a feedback action is provided. The stability of the control system is discussed as well. The effectiveness of the model and the control algorithm has been verified by simulation and experimental results on a PEM electrolyzer at laboratory scale; the extension to higher power levels is discussed at the end.     

Comprehensive analysis and design of multi-leg fuel cell boost converter

This paper presents the design and development of a practical multi-leg fuel cell boost converter. The applied multi-leg topology and the interleaved switching method can effectively reduce fuel cell current ripple to meet the suggested limitation of 4%. Input inductors play a critical role for ripple current reduction, while output capacitors are used for specified output voltage performance. In the selection of the input inductors and the output capacitors, this paper uniquely presents an analytic method for obtaining optimum parameter values. The design considerations include leg number, switching frequency, output loads, and dynamic response. Novel equivalent models with equivalent series resistors for multi-leg boost converters are proposed. This model can simplify the analysis of multi-leg converters and is thus used for compensation design. To verify the analytic results, Intersil ISL6556B integrated circuit is used to implement two-, three-, and four-leg fuel cell boost converters rated 1000 W. Finally, the experimental results show that the four-leg one can have lower ripple factor than its two- and three-leg counterparts, and meet the limitation of fuel cell output current ripple factor of 4%.     

An utility interactive power electronics interface foralternate/renewable energy systems

The foreseeable shortages in conventional sources of electric power has increased the emphasis on the research and development of alternate sources of energy. In order to make a noteworthy impact, the alternate sources of energy need to be utility interactive by means of a power electronic interface (a DC to AC converter). The inherent assumption in the control of DC to AC converters is, that the DC voltage available at the input of the converter is constant. However, when the input is an unregulated DC source such as a battery, fuel cell, photovoltaic cells or any other form of alternate source, maintenance of the constant DC voltage at the input of the converter is often impossible. A modified inverter switching technique is proposed for the interface with the utility grid such that the AC output of the inverter becomes immune to fluctuations in the unregulated input DC obtained from alternate energy sources     

Highly efficient maximum power point tracking using DC–DC coupled inductor single-ended primary inductance converter for photovoltaic power systems

Solar photovoltaics (PVs) have nonlinear voltage–current characteristics, with a distinct maximum power point (MPP) depending on factors such as solar irradiance and operating temperature. To extract maximum power from the PV array at any environmental condition, DC–DC converters are usually used as MPP trackers. This paper presents the performance analysis of a coupled inductor single-ended primary inductance converter for maximum power point tracking (MPPT) in a PV system. A detailed model of the system has been designed and developed in MATLAB/Simulink. The performance evaluation has been conducted on the basis of stability, current ripple reduction and efficiency at different operating conditions. Simulation results show considerable ripple reduction in the input and output currents of the converter. Both the MPPT and converter efficiencies are significantly improved. The obtained simulation results validate the effectiveness and suitability of the converter model in MPPT and show reasonable agreement with the theoretical analysis.     

Single-switch boost-buck DC-DC converter for industrial fuel cell and photovoltaics applications

The realization of dc-dc converters performs a vital function in exploiting renewable energy sources such as solar photovoltaic (PV) and fuel cell applications. This paper demonstrates a single-switch unidirectional buck-boost dc-dc converter for continuous power flow control, excluding the hybrid switched-capacitor. The proposed converter utilizes a limited number of passive components, only four diodes and three inductors required, in addition to six capacitors. The converter can operate at a wide input voltage range with continues input current. The converter has experimented under real-time conditions with 660 W PV system. The obtained efficiency ranges from 93% to 98%. Furthermore, the converter has interfaced with 550 W fuel cell operated under different fuel pressure. The realized efficiency ranges from 91% to 97%. The maximum measured inductance current ripple is limited to under 0.70 A in both scenarios, whereas 0.16 V is the maximum output voltage ripple.     

Time delay control for fuel cells with bidirectional DC/DC converter and battery

Transient behavior is a key property in the vehicular application of proton exchange membrane (PEM) fuel cells. A better control technology is constructed to increase the transient performance of PEM fuel cells. A steady-state isothermal analytical fuel cell model is constructed to analyze mass transfer and water transport in the membrane. To prevent the starvation of air in the PEM fuel cell, time delay control is used to regulate the optimum stoichiometric amount of oxygen, although dynamic fluctuations exist in the PEM fuel cell power. A bidirectional DC/DC converter connects the battery to the DC link to manage the power distribution between the fuel cell and the battery. Dynamic evolution control (DEC) allows for adequate pulse-width modulation (PWM) control of the bidirectional DC/DC converter with fast response. Matlab/Simulink/Simpower simulation is performed to validate the proposed methodology, increase the transient performance of the PEM fuel cell system and satisfy the requirement of energy management.     

直流微电网系统的DC/DC变换器控制系统设计

全桥DC/DC变换器由于具有高功率密度、高效率、高变压比及电气隔离的特点,成为直流微电网系统中重要的电力电子接口。而移相全桥DC/DC变换器具有高阶时变非线性的特点,具体应用时较难建立其精确的数学模型,这影响了传统PID控制性能,因此设计了模糊自整定控制器。通过对峰值电流模式下变换器的小信号建模,在传统PI控制的基础上,给出了基于模糊理论的PI参数在线自整定方法。同时,利用Matlab仿真工具给出了模糊控制器的设计方法,并通过DSP对其进行软硬件实现。仿真和试验结果表明,模糊自整定控制与常规PI控制相比,提高了系统的抗扰动能力,改善了系统的动态性能,从而提高了整个微电网系统的可靠性。     

A maximum power point tracking control strategy with variable weather parameters for photovoltaic systems with DC bus

In photovoltaic (PV) system, the most commonly used DC/DC converter is the basic buck or boost circuit to implement the maximum point power tracking (MPPT) due to their simple structure and low cost while there are some MPPT constraint conditions. By contrast, the conventional buck/boost DC/DC converter without MPPT constraint condition is seldom used because of its high cost or poor performance. To keep the advantages of these three DC/DC converters while overcoming their shortcomings, in this paper, the constraint conditions of capturing the maximum power point (MPP) of PV systems with direct-current (DC) bus are found out. Then, on the basis of this work, a MPPT control strategy with variable weather parameters is proposed. In this strategy, a new buck/boost DC/DC converter is proposed, which not only avoids the MPPT constraint conditions of basic buck or boost DC/DC converter but also overcomes the shortcomings of conventional buck/boost DC/DC converter. Finally, lots of simulated experiments verify the accuracy of MPPT constraint conditions, test the feasibility and availability of proposed MPPT control strategy, analyze the MPPT performance of proposed PV system and compare the output transient-state performance with conventional perturb and observe (P&O) method.