Power and energy management of grid/PEMFC/battery/supercapacitor hybrid power sources for UPS applications

https://doi.org/10.1016/j.ijepes.2014.12.044Get rights and content

Highlights

  • A grid/PEMFC/battery/supercapacitor hybrid UPS system using a DC/DC converter.

  • Modeling and sizing of hybrid power and energy components.

  • Power control and energy management strategies of four operating modes.

  • Experimental verification of effectiveness and feasibility of these strategies.

Abstract

This paper presents a hybrid power and energy source supplied by a proton exchange membrane fuel cell (PEMFC) as the main power source in an uninterruptible power supply (UPS) system. To prevent the PEMFC from fuel starvation and degradation and realize their seamless linking in the hybrid UPS system, the power and energy are balanced by the battery and/or supercapacitor (SC) as two alternative auxiliary power sources. Based on the modeling and sizing of hybrid power and energy components, the power and energy management strategies and efficiency measurements of four operating modes in UPS system are proposed. To evaluate the proposed strategies, an experimental setup is implemented by a data acquisition system, a PEMFC generating system, and a UPS system including AC/DC rectifier, DC/AC inverter, DC/DC converter, AC/DC recharger and its intelligent control unit. Experimental results with the characteristics of a 300 W self-humidified air-breathing of PEMFC, 3-cell 12 V/5 Ah of batteries, and two 16-cell 120 F/2.7 V of SCs in parallel corroborate the excellent management strategies in the four operating modes of UPS system, which provides the basis for the optimal design of the UPS system with hybrid PEMFC/battery/SC power sources.

Introduction

An uninterruptible power supply (UPS) system based on traditional batteries only is hard to provide sufficient backup power to critical loads, especially when relatively long time supply is necessary. Other energy sources and storage technologies, such as proton exchange membrane fuel cell (PEMFC) and liquid-fed direct methanol fuel cell (DMFC), have been investigated to replace the batteries. Since the PEMFCs can provide electrical power with high efficiency, high energy density and no pollution, they are considered as a promising technology for UPS products. Hence, compared with other energy storage devices, such as battery and supercapacitor (SC), the PEMFCs can offer longer continuous run-time of 24 h and greater durability in harsh outdoor environments. Compared with conventional internal combustion generators (ICG), the PEMFCs are quieter and have low or zero emissions depending on fuel source. Because the PEMFCs are modular, UPS systems using them can be more readily sized to fit a wider variety of sites than those using conventional ICG [1].

The PEMFCs are emerging as an economically viable option for providing UPS systems, which play a very important role as the backup and emergency power supply for important applications, particularly for computers, office equipment, communication systems, hospital instruments, industrial controls and integrated data center to supply uninterruptible and reliable power with constant voltage and frequency in case of power failure [2], [3]. For instance, US Department of Energy (DOE) funded 18 fuel cell (FC) backup power systems at 10 installation sites will help accelerate the deployment of clean technology at Federal government facilities and provide valuable data and feedback for FCs [4].

When the utility grid power source is interrupted, the hydrogen will be supplied to the PEMFC stacks in a UPS system. One of the main weak points of the PEMFCs, however, is the slow dynamic characteristics administrated by the fuel transport system, such as the air and water pumps, control valves, mass flow devices, pressure devices, and a hydrogen reformer. During the start-up of PEMFC stack, or a sudden change of external load, the hydrogen cannot be fed in time, and the stack may take a few seconds to reach the required output voltage. As a result, fast load demand for the PEMFCs will lead to a high voltage drop in a short time, which is recognized as fuel starvation and causes the degradation of FC. Therefore, to overcome this issue, a PEMFC should be applied as the main power source in the UPS and vehicle applications, and at least a rechargeable battery or an SC must be employed as an auxiliary power source to improve the performance and prevent the PEMFC stack from degradation when the external loads demand a high energy in a short time. It should ensure that there is enough fuel and battery/SC capacity for providing the power needed by the external load [5].

The power control and energy management of hybrid power sources have already been studied recently. For instant, Thounthong et al. [6] proposed a perfect energy source supplied by a PEMFC as the main power source and storage devices: battery and SC, for modern distributed generation system, particularly for future fuel cell vehicle applications. Zhang et al. [7] proposed a seamless transfer control strategy by using a power management unit, which is suitable for FC-UPS. García et al. [8] presented a comparative study in order to select the most suitable control strategy for high-power electric vehicles powered by FC, battery and SC, in which each energy source uses a DC/DC converter to control the source power and adapt the output voltage to the common DC bus voltage, from where the vehicle loads are supplied. Torreglosa et al. [9] evaluated a hybrid power-train based on FC, battery and SC for a tramway, which currently operates in the city of Zaragoza, Spain. Kyriakarakos et al. [10], [11] presented an agent system for the multi-generation micro-grid topology which also included the fuzzy logic and gray prediction algorithms for better management respectively. Feroldi et al. [12] presented an energy management strategy for a sustainable hybrid system, which is based on wind-solar energy and bioethanol.

The major topology of parallel structures for a PEMFC/battery/SC hybrid power sources UPS system is shown in Fig. 1, in which the structure of three DC/DC converters in parallel is widely used. In this paper, in order to reduce the cost, improve the performance, and decrease the losses for the UPS system, a structure of grid/PEMFC/battery/SC hybrid power system is proposed in a high-frequency single-phase small-power UPS system as depicted in Fig. 2. In the figure, the outputs of PEMFC, batteries and/or SCs are linked in parallel, and the outputs of power and energy are controlled intelligently by power switches K0–K6 (thyristors) through the energy management and power control system.

Section snippets

Voltage model of PEMFC

Because the PEMFC is a type of electrochemical energy conversion device, if the parameters for each single cell are lumped to represent the stack, the output voltage of the stack can be obtained as [13]VStack=Ereversible-VactLOSS-VOhmicLOSS-VconcLOSS-VleakLOSS=Ereversible-NRTαFlni+ini0+ROhmici+in+RTnFlniLiL-i+inwhere Ereversible is the reversible voltage (V); VactLOSS the activation voltage loss (V); VOhmicLOSS the Ohmic voltage loss (V); VconcLOSS the concentration voltage loss (V); VleakLOSS

Power sizing of hybrid UPS system

In order to meet the demand of the technical reliability and calculate the life cycle costs of the system, the sizing of an UPS hybrid power system will be dealt with the aim of obtaining a cost effective system according to any defined hourly load profile and any defined backup time. A solution is defined by PEMFC, SC and battery sizes and the energy management strategy.

Power and energy management strategy

The purpose of the UPS system with hybrid PEMFC/battery/SC power sources is to provide uninterruptible, reliable, and high-quality power of 24 h to the loads. Energy management and power control strategy for hybrid power system in UPS and automobiles applications are based on the needs of loads and operating modes. The concept of multiple hybrid power sources is to ensure a sustainable power supply for their system. In this UPS applications, the power control and energy management strategy are

Experimental setup

The experimental setup consists of a UPS hybrid power sources system and an intelligent power control and energy management controller, 3-cell deep cycle lead–acid batteries, two 16-cell 120 F/2.7 V of SCs, a 300 W self-humidified air-breathing of PEMFC generating system and the data-acquisition devices including multifunction analog input unit PCI-6036E, analog output unit PCI-6713, parallel digital I/O interface PCI-6503 and analog multiplexer with temperature sensor AMUX-64T (National

Conclusion

The power control and energy management strategies and efficiency considerations of four operating modes for an intelligent network UPS system with backup PEMFC, SC and battery power sources are proposed in this paper, to overcome the slow dynamics when PEMFC starts up or an external load changes suddenly leading to the fuel starvation and degradation of the stack. Based on the designed UPS hybrid system, three stages of theoretical analysis and experimental test are conducted. Firstly, the

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