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

*Jenica-Ileana Corcau, Liviu Dinca and Ciprian-Marius Larco*

#### **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.

**Keywords:** auxiliary power source, hybrid source, nonpolluting fuels, Dc to Dc converter

#### **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 [1–3] (**Figure 1**).

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 [6–11].

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 *Modeling and Simulation of APU Based on PEMFC for More Electric Aircraft DOI: http://dx.doi.org/10.5772/intechopen.105597*

#### **Figure 2.**

*Electric generation systems' evolution.*

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. Ecoefficiency 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 [12–16].

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

#### *Modeling and Simulation of APU Based on PEMFC for More Electric Aircraft DOI: http://dx.doi.org/10.5772/intechopen.105597*

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:

$$\mathbf{H}\_{\texttt{z}} + \frac{1}{2}\mathbf{O}\_{\texttt{z}} \rightarrow \mathbf{H}\_{\texttt{z}}\mathbf{O}.\tag{1}$$

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**.

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

**Figure 3.** *Schematic diagram of a fuel cell [17].*


#### **Table 1.**

*Types of fuel cells.*

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%.
