Fuel-cell powered uninterruptible power supply systems: Design considerations
Introduction
Conventional uninterruptible power supply systems (UPSs) employ engine generators and/or batteries as their main power sources to provide the electric power for critical functions or loads when the normal supply, i.e. utility power, is not available [1], [2]. Typical UPS systems consist of rechargeable batteries such as valve-regulated lead-acid (VRLA) or nickel–cadmium (Ni–Cd). These batteries, however contain toxic heavy metals such as cadmium, mercury, and lead and may cause serious environmental problems if they are discarded without special care [3].
Fuel cells are emerging as an attractive power source by virtue of their inherently clean, efficient and reliable service [4]. As the demand for various applications such as remote generation, back-up power generation and distributed generation increases, the use of fuel cell is spreading widely. Accordingly, their prices are steadily reducing and this is further accelerating their penetration into market. Among the various kinds of fuel cells, proton exchange membrane fuel cells (PEMFCs) are compact and lightweight. They also provide a high output power density at room temperature, plus ease of start-up and shut-down in system operation [6]. Further, unlike batteries, fuel cells can continuously provide power as long as the reactants are supplied. This feature is especially useful in situations where the duration of the power outage is uncertain.
It is important for the UPS system to be able to take over immediately the full load in power outage or out-of-tolerance situations to avoid any data loss, uncontrolled system shut-down or malfunctioning of the devices. Some critical applications do not even allow power interruptions of only several tens of milliseconds. As is well known, fuel processors have a delay as much as several tens of seconds, and a fuel cell cannot take over the full load if its membrane is not properly humidified. For this reason, a supercapacitor module is employed to compensate for these response delays by supplying the required instantaneous energy, which is stored during normal operation. This energy can also be used to handle overload conditions.
In this paper, the design of a 1-kVA, fuel-cell powered line-interactive UPS system that employs modular (fuel cell and power converter) blocks is discussed (Fig. 1). A design example for the dc–dc boost converter and sizing of the supercapacitor, as well as fuel calculation, is presented and the validity of the design is verified through simulation.
Section snippets
Architecture of proposed fuel-cell powered UPS system
A block diagram of the proposed approach is given in Fig. 1. The design consists of two boost converters with fuel cells and one bi-directional converter with a supercapacitor. Normally, the utility power is transferred to the load through the static switch SSM. At the initial start, fuel cells charge the supercapacitor through the bi-directional converter, and then supply 10% of the rated load along with the utility. In the event of power outage or out-of-tolerance conditions, however, the
DC-Bus control scheme
A block diagram of the parallel dc–dc boost converter control scheme is presented in Fig. 3. The dc–dc converters 1 and 2 are combined with fuel cells 1 and 2, and a bi-directional converter is combined with a supercapacitor module. The control scheme is composed of one voltage control loop and three independent current control loops. The dc bus voltage is controlled by a PI controller to generate the system current command. A signal from the fuel cell indicates the available power from the
Simulation results
Simulation results are shown in Fig. 4 for the dc–dc converters incorporated with the fuel cells and supercapacitor when a power outage occurs. Initially, the dc–dc converter and fuel cell modules power 10% of the load and then the load changes suddenly from 10 to 100%. In this condition, the system is not able to respond fast enough to supply the load. The top trace is the “power available signal”, which indicates the amount of power available from the fuel cells. In this simulation, it is
Specification of proposed fuel-cell powered UPS
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rated power: 1 kVA,
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normal output power: 10% rated power with utility power available,
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fuel reformer time constant: <20 s,
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output voltage: 120 Vac ± 5%,
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output voltage frequency: 60 Hz ± 0.1%,
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total harmonic distortion (THD): <2%,
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overload rating: 200% for 10 s.
In this design example, all the calculations are based on a PEMFC manufactured by Avistalabs (Appendix A).
Required fuel calculation for 1-h power outage and normal mode operation
In this section, hydrogen consumption is calculated for a 1-kW PEMFC stack. The basic chemical equation for a fuel cell reaction can be expressed
Conclusions
A fuel-cell powered, line-interactive UPS system has been discussed in detail. The approach provides stable power to the load when the utility is interrupted. Also, this approach verifies the possibility that the fuel cell can replace conventional UPS power sources such as engine generators, batteries and flywheels. A supercapacitor module is incorporated to overcome transients such as instantaneous power fluctuations, slow dynamics of the fuel preprocessor and overload conditions. In
Acknowledgement
This work was supported by the Soongsil University Research Fund.
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