Modeling and Simulation of APU Based on PEMFC for More Electric Aircraft

Abstract

The current challenge in aviation is to reduce the impact on the environment by reducing fuel consumption and emissions, especially NOX. An open research direction to achieve these desideratums is the realization of new electric power sources based on nonpolluting fuels, a solution being constituted using fuel cells with H2. Reducing the impact on the environment is aimed at both onboard and aerodrome equipment. This paper proposes the simulation and analysis of an auxiliary power source APU based on a fuel cell. The auxiliary power source APU is a hybrid system based on a PEM-type fuel cell, a lithium-ion battery, and their associated converters. The paper presents theoretical models and numerical simulations for each component. The numerical simulation is performed in MATLAB/SimPower Sys. Particular attention is to the converter system that adapts the parameters of the energy sources to the requirements of the electricity consumers on board the MEA-type aircraft. Power management is performed by a controller based on fuzzy logic.

1. Introduction

Aeronautics has become an important tool for economic growth, leading to an overall increase in demand for air transport services. This increase is accompanied by operational hazards, adverse environmental effects, and unsustainable operating expenses. Air transport is estimated to carry more than 2.2 billion passengers a year and the current fleet of commercial aircraft will be doubled by 2050. In addition, the demand for air transport is expected to increase by 4–5% per year over the next 20 years. This projected increase in aeronautics has significant effects on the global environment. Noise, local air quality, and climate change are one of the key areas to be addressed in aeronautics and their impact on the environment [1, 2, 3] (Figure 1).

Figure 1.

The evolution of air travel [2].

The expected annual growth rate of 4.7–4.8% over the next 20 years in air transport would mean that, in the future, aeronautics could have a greater negative impact on the environment. The challenge will be for more aircraft to operate longer, but to have a lesser negative impact on the environment compared to the current situation. This is how the project Advisory Council for Aeronautics Research in Europe (ACARE) became a priority, which has the following objectives: besides the year 2000, there exists a 50% reduction in the noise level, there is also a reduction in CO₂ emissions per passenger kilometer of 50%, and an 80% reduction in NOx emissions too. All these factors do not affect just the operation way of the aircraft but the design and the construction of the aircraft too. An essential role in improving the air traffic, determined by the following conditions: more efficient aircraft, efficient engines, and the improvement of air traffic management, was detected by ACARE [4, 5]. To achieve the conditions above, new technologies in order to fulfill key functions on the aircraft are introduced, with the review of the entire system of aircraft architecture being required. Nowadays, conventional civil aircraft is characterized by four different secondary energy distribution systems: mechanical, hydraulic, pneumatic, and electrical. This involves a complex onboard power distribution network and a necessity for adequate redundancy of each. To reduce this complexity and improve efficiency and reliability, the aircraft manufacturers’ trend is toward the concept of More Electric Aircraft (MEA) and All Electric Aircraft.

Recently, with the increase in fuel costs and the emphasis on greener aircraft technologies, there has been a particular emphasis on the design and production of more MEA (More Electric Aircraft). An aircraft with all electrically powered secondary power systems can be considered an All Electric Aircraft (AEA). Nowadays, MEA aircraft use electricity in the drive systems of aircraft subsystems, which were previously powered by pneumatic, hydraulic, or mechanical power systems, including flight control systems, air conditioning, anti-icing, and various other small systems. Figure 2 shows the evolution over time of the electrical system on board aircraft [6]. In modern electrical systems, various voltage values are preferred, which consist mainly of the four voltage classes: 235 V VF (variable frequency), 115 VAC CF (constant frequency), 28 VDC, and ± 270 VDC. Many electricity distribution units are also needed to supply electricity to aircraft. Such systems further reduce the mass by reducing the size of electrical wiring [6, 7, 8, 9, 10, 11].

Figure 2.

Electric generation systems’ evolution.

The goal of the MEA concept is to replace nonelectric power with electrical energy. This idea was first applied to military aircraft. Over time, the issue of implementing this concept for civil aircraft has also been raised. The rapid development of power electronics has led to a flexible transmission of electricity from sources to loads. Power electronics are used throughout the electrical system, including power generation, conversion, and distribution. The trend in the construction of such complex electrical power systems on aircraft is to obtain more and more efficient subsystems and integrate them into the energy system of the aircraft. This can reduce design costs as well as design time.

The use of high-power electric motors and the addition of new loads have greatly increased the demand for power. In addition, the increase in household load on transport aircraft by almost 500 W per passenger and on entertainment by almost 100 W per seat has reached a total of up to 350 kW for a transport aircraft, which is also added to the power demand.

It is well known that several aircraft incorporate MEA models; however, it is widely acknowledged that the two programs, which have really integrated the MEA concepts, are the Boeing 787 and Airbus A380 commercial aircraft [6]. These aircraft are characterized by intensive electrification because loads such as the Environmental Control System-ECS (for B787) and electro hydrostatic flight control actuators (for A380) are powered. As a result, their power generation capacity is an order of magnitude larger than all other aircraft. Both the B787 and the A380 have replaced the traditional generating system that uses the Integrated Drive Generator-IDG with Variable Frequency Generator-VFG coupled directly to the motors. The B787’s main power generation is based on four 250 kVA VFGs (two for each main engine), while the A380 uses four 150 kVA VFGs (one for each engine) [6].

The transition to a more electric architecture, the adaptation of energy-efficient engines and the rigorous use of lightweight composite materials bring the contribution to a substantial reduction in B787’s operating costs compared to its predecessor, B767-300/ER. Especially (based on airline data), the reduction in block hourly operating costs is around 14% [6].

The increasing pressure to reduce fuel consumption, noise emissions, and pollution, along with the new requirements for aircraft operating systems, has led directly to the search for new cleaner technologies, so the fuel cells have great potential. To increase efficiency, flexibility, and interoperability, aeronautics engineers have made great efforts in recent years to make the transition from pneumatically and hydraulically operated systems to electrical systems. Due to the increasing consumption of electricity during the flight, conventional electric generators have undergone a change, becoming larger and more powerful to compensate for the excess energy needed for the flight [12].

Several studies have shown that the conventional auxiliary power unit (APU) plays an important role in aircraft pollution emissions. This has led to the replacement of fuel cell APUs, with special focus on proton exchange membrane fuel cells (PEM-FC) and solid oxide fuel cells (SOFCs). By using this technology, the ecological efficiency can be increased, knowing that these fuel cells do not pollute the environment. Eco-efficiency is one of the main goals of the aerospace industry. First, a “green” aircraft saves the money for the airlines, due to the forecasts of the increase in the fuel prices in the future; second, environmental pollution will become a growing problem for human society. Therefore, the importance of reducing emissions is not only affected by financial reasons. During ground operations, auxiliary power sources, classic APUs, or turbogenerators (TGs) as they are also known in the literature, generate electricity used for the automatic start of aircraft engines, thus resulting in pollution gases. Much of the emissions from airports are produced by these auxiliary turbogenerators. In the future, airlines will have to pay taxes for polluting emissions from airports, according to European Union regulations.

The use of fuel cells in aircraft has attracted the attention of the aeronautics industry, they have formed working groups for the direct development of lines using fuel cell systems for civil aircraft applications. The Society of Auto Engineers (SAE) has collaborated with the European Organization for Civil Aeronautics Equipment (EUROCAE) In 2008, to form the WG80/AE 7AFC Working Group, this group contributed to the support and development of hydrogen fuel cells for large civil aircraft, through standardization and certification. The WG80/AE 7AFC Group has implemented two standard documents to support the use of PEMFC systems [12, 13].

The first document was published in 2013, and it provides basic information on the use and installation of hydrogen PEMFCs on board aircraft for the purpose of generating auxiliary power without the use of separate ground supply systems [12]. And the second document followed 4 years later in 2017, which defined technical guidelines for testing, integration, validation, certification, and development in a secure environment of PEMFC systems for high-capacity civil aircraft, including storage and the distribution of fuel, and the integration of electrical systems in aircraft. In 2015, US Federal Aeronautics Administration of the US Department of Transportation funded The Fuel Cells-Energy Supply Aeronautics Rulemaking Committee. This team concentrates on the use of PEMFCs and SOFCs. The principal objective of this team is to reduce and even eliminate the uncertainties surrounding the safety and application of hydrogen fuel cells in commercial aircraft [12, 13, 14, 15, 16].

Here, this chapter discusses the simulation and analysis of an auxiliary power source APU based on fuel cells. The following sections dealt in detail with fuel cell, modeling of APU based on the fuel cell with its simulation, and, finally, with the energy management system that can be used successfully for the applications with the high pulsed loads and transient power requirements.

2. Fuel cell

Hydrogen will become the raw material for various industries such as petrochemicals, amino acids, methanol, hydrogen peroxide, the food industry, and the transportation industry. But due to its high calorific value, research continues to find new uses. In the 1960s, the hydrogen engine was designed to launch missiles. In 1968, the first model to appear in the United States was launched.

This technology will facilitate the success of the Ariane rocket and is considered a forerunner of the hydrogen energy era.

The electrochemical conversion, i.e., the direct, nonpolluting, and silent transformation of the chemical energy contained in a wide variety of substances, into electrical energy is an alternative direction of obtaining electricity. A class of devices in which this conversion takes place is fuel cells.

After the Volta battery was built, it was used by Nicholson and Carlisle to decompose water into hydrogen and oxygen, and by Davy in 1807 to decompose alkalis. Daniel and Faraday continued their brilliant experiments in these new energy sources in the first half of the last century. Although the first fuel cell was invented in 1839 by WR Grower, the evolution of these devices did not take place until the 1960s, because of the development of space programs and especially after 1980 when programs for the implementation of “clean” technologies were imposed in the production of electricity. The fuel cell is a galvanic cell in which the free energy of a chemical reaction is converted into electricity. In the case of a conventional fuel cell, which runs on hydrogen and oxygen, the reaction that takes place is:

$${\mathrm{H}}_2+\frac{1}{2}{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}\text{.}$$E1

The reaction, a chemical combustion, takes place in a cell composed of two electrodes separated by an electrolyte and takes place in a temperature range between 70 and 1000°C. Whatever the types of batteries studied, the general principle remains the same Figure 3. Only the electrolyte, electrodes, and temperature change. There are currently 5 types of cells, Table 1.

Figure 3.

Schematic diagram of a fuel cell [17].

Cell type	Electrolyte	t[°C]	Field of use
Alkaline (AFC)	Potassium (liquid)	80	in space, transportation; Range: 1–100 kW
Polymer acid (PEMFC and DMFC)	Polymer (solid)	80	Portable and stationary applications, transportation; Range: 1 W–1 MW
Phosphoric acid (PAFC)	Phosphoric acid (liquid)	200	Stationary applications, transports Range: 200 kW – 10 MW
Liquefied carbonate (melted) (MCFC)	Melted salts (liquid)	650	Stationary applications; Range: 500 kW–10 MW
Solid oxides (SOFC)	Ceramic (solid)	700–1000	Stationary applications, transports; Range: 1 kW–10 MW

Table 1.

Types of fuel cells.

PEMFC and SOFC have a much longer lifespan than other types of batteries, much more compactness, moderate cost, and offer interesting long-term prospects. Unlike PEMFC, SOFC is a very underdeveloped type of fuel cells. Having a solid electrolyte, like the first one, it is differentiated by the level of the operating temperature: between 650 and 1000°C. This feature gives it a higher resistance to impurities and especially an overall efficiency (electrical + thermal) of the order of 80%, due to the high-temperature level and heat dissipation that allow recovery in combined cycles.

High-temperature fuel cells offer other advantages, already used in fixed installations; we can mention a few generators delivered by Rolls-Royce, especially in North America. In fact, the large manufacturer has recently set up a specialized subsidiary of Rolls-Royce Fuel Cell Systems Ltd. (RRFCS) and will develop a specialized research Centre at the University of Genoa, Italy. This industrialization will lead to a sharp reduction in the cost of the elements of a high-temperature fuel cell. In the short term, these applications will focus on auxiliary generating sets (APUs) for land vehicles, but also for aircraft. Boeing has adopted this solution for its future 777 model. APU-FC ensures the generation of electricity with a level of safety that allows the elimination of hydraulic systems, thus bringing a gain in terms of weight and a reduction in energy required for engine systems, which translates into a reduction in consumption of the order of 15%.

3. Auxiliary power source based on the fuel cell

The proposed auxiliary power source contains the following energy sources: Proton Exchange Membrane (PEM) fuel cells, battery system, DC to DC boost converter connected to the PEMFC output terminals, DC to DC boost converter connected to the battery package, DC to DC Buck converter used for providing the charging/discharging path between the PEMFC and battery, as in Figure 4.

Figure 4.

Block diagram of hybrid fuel cell/battery.

The dynamic characteristics of the two types of power sources make the hybrid power system more complicated. Therefore, it is essential, very important, and necessary to ensure efficient energy management. Energy management strategies determine the allocation of power between different energy sources and promote the energy efficiency and life of the hybrid power system. The energy stored in the battery systems offers a double benefit, keeping the life of the fuel cell and obtaining a better dynamic response to load variation. The goals of this hybrid configuration are presented in detail in [15].

3.1 Modeling of PEMFC

A PEM fuel cell stack model was chosen from the MATLAB/Simulink, SimPowerSystems (SPS) Toolbox library. The MATLAB/Simulink model implements a generic hydrogen fuel cell stack. The model has two options: a simplified model and a detailed model.

The simplified model shows a particular fuel cell stack operating at nominal conditions of temperature and pressure. Figure 5 presents the dynamic behavior of a PEMFC and Table 2 shows fuel cell model parameters. The stack is supplied by liquid hydrogen and compressed air [15].

Figure 5.

The dynamic behavior of a PEMFC.

Fuel cell model input parameters
The voltage at I = 0A and I = 1A [V_0(V), V_1(V)]	[52.5, 52.46]
The current and the voltage at nominal operating point [Inom(A), Vnom(V)]	[250, 41.15]
The current and the voltage maximum operating point [Iend(A), Vend(V)]	[320, 39.2]
Number of cells	[65]
Nominal stack efficiency (%)	[55]
Operating temperature (Celsius)	[45]
Nominal Air flow rate (lpm)	[732]
Nominal supply pressure [Fuel (bar), Air (bar)]	[1.16,1]
Nominal composition (%) [H2 O2 H2O(Air)]	[99.95, 21,1]

Table 2.

Fuel cell parameters.

3.2 Modeling of batteries

The batteries chosen in order to realize this study are lithium-ion types as they have proved to exhibit a high energy density and efficiency in comparison to other battery types like lead-acid, NiCd, or NiMH. This makes them more attractive for aircraft applications. The battery output voltage is given by [15].

$$V_{\mathit{batt}}=E_0-K\frac{Q}{Q-\int idt}-R·i+\mathit{Aexp}\left(-B\int idt\right)$$E2

where i is the battery current [A], E₀ is the battery constant voltage [V], K is the polarization voltage [V], A is the exponential zone amplitude [V], Q is the battery capacity [Ah], B is the exponential zone time constant inverse [Ah]⁻¹, ∫idt is the actual battery charge [Ah], R is the internal resistance [Ω], and V_batt is the battery no load voltage [V].

The characteristics of the chosen battery are presented in Figure 6 and the parameters of the above model are shown in Table 3. The state-of-charge (SOC) of the battery is between 0 and 100%. The SOC is calculated as

Figure 6.

The dynamic behavior of a battery.

Battery model input parameters
Maximum capacity (Ah)	[40]
Cut-off Voltage (V)	[36]
Fully charged voltage (V)	[55.8714]
Nominal discharged current (A)	[17.3913]
Internal resistance (Ohms)	[0.012]
Capacity (Ah) at nominal voltage	[36.1739]
Exponential zone [Voltage (V), Capacity (Ah)]	[52.3, 1.96]

Table 3.

Battery parameters.

$$SOC=100\left(1-\frac{Q·1.05}{\int idt}\right)$$E3

3.3 Modeling of a DC-to-DC converter

Relying on load profile, APU consists of the following: 12.5 kW (peak), 30–60 V PEM (Proton Exchange Membrane) FCPM – Fuel Cell Power Module, with nominal power of 10 kW; 48 V, 40 Ah, lithium-ion battery system; 12.5 kW fuel cell DC to DC boost converter, with regulated output voltage and input current limitation; two DC to DC converters for discharging (4 kW boost converter) and charging (1.2 kW buck converter) the battery system.

Two classical types of DC-to-DC converters are selected for the proposed hybrid battery/fuel cell system (Figure 4) to stabilize the output profile of the auxiliary power source system. During transient conditions, the battery supplies electric power for the essential loads on the aircraft electric network until PEM-FC warms up. Moreover, the fuel cell is the one that provides all requested power for the essential loads when the synchronous generator is shut down. Therefore, the DC-to-DC converter of the battery must ensure a bidirectional flow. The battery system during transitional periods provides electric power to the emergency loads, so the converter operates in the boost mode, increasing the output voltage to its standard value at 270 VDC using a feedback control system.

The DC-to-DC boost converter controls the fuel cell. Bi-directional DC to DC boost converter controls the battery. These converters are also output voltage regulated with current limitations. The fuel cell DC to DC converter system is 30–60 V DC input, 270 V DC, 9.2 A output. The battery DC to DC converter system includes 2, 40–58.4 V DC input, 270 V DC, 7 A output, and these DC to DC isolated boost converters are connected in parallel. Alongside 1, 240–297 V DC input, 48 V DC, 20 A output, DC to DC isolated buck converter. The converters used in this study contain the average value model.

Figure 7 shows DC to DC boost converter model realized in Simulink/SimPowerSystems (SPS) [15].

Figure 7.

DC to DC converter model in Simulink/SimPowerSystems (SPS).

3.4 Power management strategy of APU based on fuel cell

The most common schemes presented in the literature include the following techniques: the state machine control strategy, the rule-based fuzzy strategy, the classical PI control strategy, and the equivalent consumption strategy ECMS [18, 19, 20, 21, 22, 23, 24, 25].

The energy management strategy is designed based on: keeping the fuel cell lifetime by avoiding an insufficient supply of reactants (fuel cell starvation); the fuel cell current slope of 40A/s; fuel cell power: Pfcmin = 1 kW and Pfcmax = 10 kW; battery power: PBattmin = 1.2 kW and PBattmax = 4 kW [15]; also, in order to operate the battery system efficiently, it is requested always keeping the battery SOC above 40%. The fuel cell power is caused by the battery state of charge and the required load power (Pload). The bus voltage is stabilized through the battery converters for energy management system strategies. The output of the algorithm is the reference for the fuel cell power, as it can be seen in Figure 8. This quantity relative to the fuel cell voltage and from the efficiency of the DC-to-DC converter has resulted in the value of the fuel cell reference current [15, 26]. A detailed description of the controller based on fuzzy logic is presented in [15, 26].

Figure 8.

Schematic of the power management strategies [15, 26].

4. Implementation in MATLAB/Simulink APU based on fuel cell

Figure 9 shows the simulation scheme made in MATLAB/Simulink. It contains the sources, converters, and the source management system. The performance management scheme proposed in this paper was tested by numerical simulations. The energy management system block outputs the control signals required by DC-to-DC converter. The fuel cell power is identified by the battery state of change and the required load power (Pload). The bus voltage is stabilized through the battery converters for energy management system strategies. The management system was tested using an electric load profile, presented in the Figure 10. Figure 11 shows the time variations of the voltage and current of the fuel cell, Figure 12 shows the time variations of the voltage and current of the converter related to the fuel cell, and Figure 13 shows the time variation of fuel consumption. Figure 14 shows the time variations of battery voltage and current, and Figure 15 shows the time variations of battery converter voltage and current.

Figure 9.

The APU based on fuel cell model in MATLAB/Simulink.

Figure 10.

Load profile used in numerical simulations.

Figure 11.

Variation in time of the voltage and current corresponding to the output terminals of fuel cell.

Figure 12.

Variation in time of the voltage and current of DC-to-DC converter connected to the fuel cell.

Figure 13.

Variation in time of fuel consumption.

Figure 14.

Variation in time of the voltage and current corresponding to the output terminals of a battery.

Figure 15.

Variation in time of the voltage and current of DC-to-DC converter connected to the battery.

5. Conclusions

Several studies have shown that classical APU plays an important role in pollution emissions. This has led to the search for new cleaner technologies, so fuel cells have great potential. Major aircraft manufacturers have shown great interest in replacing conventional auxiliary power sources with auxiliary power sources based on fuel cells. In this paper, an auxiliary power source based on the PEM-type fuel cell is proposed. This hybrid source contains fuel cells, batteries, converters, and source management system. The results obtained from the numerical simulation of the proposed auxiliary power source show that it offers a higher peak power than each individual component while maintaining a power density, which is vital for an aircraft. It is also observed on time variations of the battery and battery output currents that the power flow is well managed between the fuel cell and the battery. Another advantage of the hybrid power source is that during the process of starting and transient charging of the system the battery will reliably support the electrical network of the aircraft. The slow performance of the PEM fuel cell during start-up/charge/discharge operations is offset by the fast battery dynamics. If the charging power is subject to sudden changes, the battery is the one that responds immediately so that the electrical system remains in normal operating conditions.

The energy management system proposed in this paper can be used successfully for the applications with high pulsed loads and transient power requirements.

Acknowledgments

Source of research funding in this article: Research, program of Electrical, Energetic and Aerospace Engineering Department financed by the University of Craiova and Military Technical Academy “Ferdinand I”.