Energy management of fuel cell/battery/supercapacitor hybrid power source for vehicle applications

Abstract

This paper proposes a perfect energy source supplied by a polymer electrolyte membrane fuel cell (PEMFC) as a main power source and storage devices: battery and supercapacitor, for modern distributed generation system, particularly for future fuel cell vehicle applications. The energy in hybrid system is balanced by the dc bus voltage regulation. A supercapacitor module, as a high dynamic and high power density device, functions for supplying energy to regulate a dc bus voltage. A battery module, as a high energy density device, operates for supplying energy to a supercapacitor bank to keep it charged. A FC, as a slowest dynamic source in this system, functions to supply energy to a battery bank in order to keep it charged. Therefore, there are three voltage control loops: dc bus voltage regulated by a supercapacitor bank, supercapacitor voltage regulated by a battery bank, and battery voltage regulated by a FC. To authenticate the proposed control algorithm, a hardware system in our laboratory is realized by analog circuits and numerical calculation by dSPACE. Experimental results with small-scale devices (a PEMFC: 500-W, 50-A; a battery bank: 68-Ah, 24-V; and a supercapacitor bank: 292-F, 30-V, 500-A) corroborate the excellent control principle during motor drive cycle.

Introduction

High prices for gasoline and oil are here to stay. As China, India and other nations rapidly increase their demand for fossil fuels, future fighting over energy looms large. In the meantime, power plants that burn coal, oil and natural gas, as well as vehicles everywhere, continue to pour millions of tons of pollutants and greenhouse gases into the atmosphere annually, threatening the planet.

Well-meaning scientists, engineers, economists and politicians have proposed various steps that could slightly reduce fossil-fuel use and emissions. These steps are not enough. Therefore, this convinces us to dramatically change to hydrogen power, which would be the reasonable answer to this energy crisis problem.

Furthermore, beyond finding new alternative fuels for internal combustion engines (ICEs), researchers are working on hydrogen fuel cells that offer another path toward environmentally acceptable power [1], [2], [3], [4]. To produce electricity, most PEM fuel cells must be supplied either with hydrogen or with hydrocarbon compounds that can be catalytically decomposed into hydrogen.

There are many types of FCs characterized by their electrolytes. One of the most promising ones to be utilized in electric vehicle applications is the polymer electrolyte membrane FC (PEMFC) because of its relatively small size, lightweight nature, and ease of construction [5], [6]. In addition, PEMFC may also be used in residential and commercial power systems [7].

For the past 10 years, many works have been done on the utilizations of FCs in high power applications. Nowadays, the required FC power is in the range of 0.5-kW to 2-MW:

• 0.5-kW to 2-kW for unmanned aircrafts [8] and 40-kW to 700-kW for manned aircrafts [9], [10];

• 50-kW to 100-kW for urban cars [11], [12], [13], [14];

• 100-kW to 200-kW for buses and light tram [15], [16], [17];

• 600-kW to 1-MW for tramways and locomotives [18], [19], [20];

• 480-kW to 2-MW for distributed generation systems (grid parallel connection) [21], [22], [23], [24].

The relatively short life of PEM fuel cells is a significant barrier to their commercialization in stationary and mobile applications. A longer life span for fuel cell components should be achieved to ensure high reliability, low maintenance costs and to justify fuel cells as economical alternative energy systems. Currently, the lifetime target requires PEM fuel cells to achieve 5000 h for mobile and 40,000 h for stationary applications [2].

The overall fuel cell performance decay rate, measured during continuous and uninterrupted operation, is the sum of both the stability and durability decay rates. Normal degradation targets require less than 10% loss in the efficiency of the fuel cell system at the end of life, and a degradation rate of 2–10 μV h⁻¹ is typically accepted for most applications [2].

According to Thounthong et al. [25], [26] experimented on a 0.5-kW PEMFC (ZSW Company, Germany) and a 1.2-kW Nexa™ PEMFC (Ballard Power System Company, Canada), Corrêa et al. [27] experimented on a 0.5-kW PEMFC (BCS Technology Company) and 0.5-kW PEMFC (Avista Company), Zhu et al. [28] experimented on a 0.5-kW PEMFC (Avista Company), Yoneyama et al. [29] experimented on a 100-kW PEMFC for railway vehicles, and Gaynor et al. [30] experimented on a 350-kW solid oxide FC, they point out the fact that the FC time constants are dominated by temperature and fuel delivery system (pumps, valves, and in some cases, a hydrogen reformer). As a result, fast energy demand will cause a high voltage drop in a short time, which is recognized as fuel starvation phenomenon [30], [31]. Fuel or oxidant starvation refers to the operation of fuel cells at sub-stoichiometric reaction conditions. When starved from fuel or oxygen, the FC performance degrades and the cell voltage drops. This condition of operation is evidently hazardous for the FC stack [31], [32].

Several factors can cause reactant starvation. A poor water management with flooding and a poor heat management during sub-zero temperatures and cold start-ups with ice within the cell can block the pores of the gas diffusion layers. A poor gas feeding management can lead to non-uniform distribution of the reactant gases resulting in partial or complete fuel and/or oxidant starvation or in sub-stoichiometric operation in individual cells, as already demonstrated in [32]. Reliability and lifetime are the most essential considerations in such power sources. Taniguchi et al. [32] clearly presented that hydrogen and oxygen starvation caused severe and permanent damage to the electro-catalyst of the fuel cell. They have recommended that fuel starvation must absolutely be avoided, even if the operation under fuel starvation is momentary, in just 1 s. In addition, an imperfect stack and cell design with an uneven distribution of mass in the flow fields, a poor stack assembly as well as quick load demands can be reasons contributing to gas starvation.

Thus, to utilize a FC in dynamic applications (such as in a cars, tramways or trains), its current or power slope must be limited to circumvent the fuel starvation problem, for example, 4 A s⁻¹ for a 0.5-kW, 12.5-V PEMFC [33]; a 2.5 kW s⁻¹ for a 40-kW, 70-V PEMFC [34]; and 5 A s⁻¹, 10 A s⁻¹ and 50 A s⁻¹ for a 20-kW, 48 V PEMFC [35]. Then, the electrical system must have at least an auxiliary power source (energy storage device), such as battery or supercapacitor, to improve the system performance when electrical loads at a dc bus demand high power in a short time (for example, vehicle acceleration and deceleration).

To illustrate vehicle characteristics, Fig. 1 depicts a speed and power profile of a European urban tramway (weight: 40–60 tons) during a drive cycle for a 500-m course. The acceleration and deceleration of the vehicle is sustained by electric motor drives with large power. One can observe the following:

• The vehicle positive peak power is around 600, and the negative peak power is around −800 kW.

• The positive and negative peak power durations are around 15 s and 10 s, respectively.

• The average power is between 100 kW and 200 kW according to the auxiliaries (heating or air conditioning).

• The duration of drive cycle is 67 s.

Thus, one can say that the drive cycle is with a high level of peak energy, a relative low average power, and duration around 67 s. Overall, the main power source operates most of the time at lower load. So, the hybridization consists in replacing the bulky generator of 600 kW, for example, with a smaller capable of providing the average power of 100 kW, and in coupling it with at least energy storage devices (typically batteries and supercapacitors) to provide the fluctuating power [36]. So, many recent works have already reported the structures of an FC/supercapacitor hybrid source [37], [38] and an FC/battery hybrid source [39], [40] for vehicle applications.

Energy storage technology is a main device in harvesting the kinetic energy that is wasted whenever vehicles or large machines must be slowed or stopped, called “regenerative braking energy”. Although batteries have been successfully used in light-duty vehicles, hybrid platforms for trucks, buses, tramways and trains will require storage and delivery of much higher powers than can be accommodated readily by batteries. Unlike batteries, new technology storage device of electrochemical capacitors (ECs) can operate at high charge and discharge rates over an almost unlimited number of cycles [1] and enable energy recovery in heavier duty systems.

Like all capacitors, ECs (also called supercapacitors “SuperC” or ultracapacitors because of their extraordinarily high capacitance density) physically store charge. Conventional electrostatic and electrolytic capacitors store charge on low-surface-area plates, but supercapacitors store charge in an electric double layer set up by ions at the interface between a high-surface area carbon electrode and a liquid electrolyte. Supercapacitors first appeared on the market in 1978 as farad-sized devices to provide computer memory backup power [1].

The very high capacitance of supercapacitors comes at a cost. The operating voltage of a supercapacitor cell cannot exceed the potential at which the electrolyte undergoes chemical reactions (typically 2.5–3 V per cell). For high voltage applications, supercapacitor cells, like batteries, can be series-connected.

One of the most important advantages of batteries over supercapacitors is their high energy density. They can store 3–30 times more charge. However, supercapacitors can deliver hundreds to many thousands of times the power of a similar-sized battery. Besides, the highly reversible electrostatic charge storage in supercapacitors does not produce the changes in volume that usually accompany the redox reactions of the active masses in batteries. Such volume changes are the main cause of the limited cycle life of batteries (around 1000 cycles for a lead-acid battery), compared to demonstrated full charge–discharge cycles for supercapacitors into the many millions.

Presented here is a perfect hybridization of the batteries and supercapacitors as energy storage devices with a PEM fuel cell as a main source. It deals with the conception and the achievement of a regulated dc bus voltage hybrid power. Its interest is focused on an energy management in system, presented in Section 2. To authenticate the proposed hybrid structure, a small-scale hardware system is realized by analog circuits and numerical calculation (dSPACE). Experimental results in Section 3 will illustrate the system performances.