STATE OF THE ART OF FUEL CELL TECHNOLOGY IN AUTOMOTIVE INDUSTRY

 

ABSTRACT

Vehicles with electric propulsion system are becoming increasingly popular due to a number of advantages they possess. However, there are some drawbacks, like long charging time and low energy density of battery packs that create inconveniences for the customers. Hydrogen fuel cells are implemented in addition to battery packs that can tackle these issues of pure electric drives and generate clean energy. This article illustrates the general concept of fuel cell technology, explains the working principle and describes in details different technologies, auxiliary systems as well as on-vehicle applications. In addition, differentiation is made between fuel cell electric vehicles (FCEV) and fuel cell hybrid electric vehicles (FCHEV). Furthermore, different control strategies for energy management system of FCHEV are described. Finally, the list of mass-produced FCHEVs are listed and future development questions and open research topics are defined.

АННОТАЦИЯ

Автомобили с электрической силовой установкой становятся все более популярными благодаря ряду преимуществ, которыми они обладают. Однако есть и недостатки, такие как длительное время зарядки и низкая плотность энергии аккумуляторных батарей, которые создают неудобства для покупателей. Водородные топливные элементы внедряются в дополнение к аккумуляторным батареям, которые могут решить проблемы электрических приводов и генерировать чистую энергию. Эта статья иллюстрирует общую концепцию технологии топливных элементов, объясняет принцип работы и подробно описывает различные технологии, вспомогательные системы, а также применение на транспортных средствах. Кроме того, проводится различие между электромобилями на топливных элементах (FCEV) и гибридными электромобилями на топливных элементах (FCHEV). Кроме того, описаны различные стратегии управления для системы управления энергопотреблением FCHEV. Наконец, приводится список серийно выпускаемых FCHEV, а также определяются вопросы будущего развития и открытые темы исследований.

  

Keywords: Fuel cell stack, Fuel cell system, Polarization curve, fuel cell electric vehicles (FCEV), Fuel cell hybrid electric vehicles (FCHEV), Toyota Mirai.

Ключевые слова: Блок топливных элементов, система топливных элементов, кривая поляризации, электромобили на топливных элементах (FCEV), гибридные электромобили на топливных элементах (FCHEV), Toyota Mirai.

 

1. Introduction  

The modern world's transportation sector is primarily reliant on fossil fuels, and its large usage is one of the main reasons of global warming, air pollution, and ozone layer depletion. In addition, the excessive usage of fossil fuel in vehicles contributes to the depletion of petroleum resources [1]. Therefore, global efforts are focused on producing clean and renewable energy sources and reduce the amount of pollutant and greenhouse gases. Different low polluting drive units have been proposed to improve the environmental situation. Some of them include advanced spark-ignition (SI) or compression ignition (CI) internal combustion engines (ICE), series or parallel hybrid electric vehicles, equipped with ICEs (HEV), battery electric vehicles (BEV) and fuel cell vehicles (FCV) [2].

The promotion of electric propulsion, which requires overcoming constraints such as limited driving autonomy and the long time required to recharge the batteries, is one of the most effective strategies to accelerate the transport sector's decarbonization process [3]. The gradual shift to fuel cell hybrid electric vehicles is critical for addressing the issues caused by fossil fuel dependency and battery electric vehicle constraints. Fuel cell vehicles, powered by pure hydrogen have zero emissions, with the water vapor as the only local emission. However, by considering not only tank-to-wheel, but also well-to-wheels emissions, the environmental effect of fuel cell vehicles significantly depends on the primary source of hydrogen production, its logistics and delivery [2]. Hydrogen produced from renewable energy, that is used in fuel cells can dramatically reduce overall (well-to-wheels) emissions. For the vehicles, powered by fuel cells, Energy Storage Systems (ESS) are prevalent. Hybridization has a number of advantages, including improved transient power demand, the ability to absorb energy via regenerative braking, and the ability to optimize vehicle efficiency [4]. The coordination of the numerous power sources necessitates a high level of vehicle control.

2. Fuel cell stack

2.1 General description

Fuel cells are devices, that are able to directly convert chemical energy into electricity, without neither combustion, nor moving parts through the electrochemical process of combination between hydrogen and oxygen, producing water, electricity and heat [6]. Fuel cells use reactants coming from outside, they continue to run as far as they are supplied with hydrogen and oxygen [5]. Two electrodes ad electrolyte are present in single unit of fuel cell. Negative electrode is called anode and the positive electrode is called cathode. Hydrogen is oxidized as soon as it contacts with anode. The chemical reaction that take place in anode is

                                                                      (1)

Electrolyte allows the passage of ions ( ions), while at the same time blocks the passage of electrons. Electrons produces as a result of oxidation, can travel only through an external circuit, thus transferring electric energy to a generic load connected to the circuit. Finally, oxygen at the cathode combines with hydrogen ions and electrons to produce water. This reduction chemical reaction is

The scheme of generic fuel cell unit is represented in Figure 1.  

Fuel cell stack consist of a number of individual cell unit series connected. Scheme of a fuel cell stack is illustrated in Figure 2.  The intrinsic characteristics of fuel cells are

• Relatively high electrical efficiency, which ranges from 40% to 60% and more
• The ability to use different reactants (hydrogen, methane, methanol, ethanol)
• Small environmental impact and high modularity [5].

2.2 Polarization curve

Equivalent circuit of fuel cell stack consist of an ideal voltage source and a resister series connected. The value of open circuit voltage is always lower than the theoretical maximum potential due to intrinsic losses and conditions (reactant concentration, temperature, pressure). When a load is connected, terminal voltage drops even more and strongly depends on current density [6]. The graphical representation of this dependence is called Polarization curve and is illustrated in Figure 3.

The energy conversion efficiency of a fuel cell can be calculated through the ratio of actual voltage in a given working condition and an open circuit voltage.

                                                              (3)

There are different sources for the losses in a fuel cell stack: activation losses, ohmic losses, concentration losses, fuel cross-over losses [5].

Activation losses: reduced reaction kinetics on the surface of electrodes. Part of the voltage is spent in driving the chemical reaction that transfers the electrons. Cell

temperature, efficiency of the catalyst, concentration of the reactants and electrode roughness strongly depend on the amount of activation losses.

Ohmic losses: resistance to the flow of electrons through the material of the electrodes. This resistance linearly varies with current density.

Concentration losses: due to the concentration change of the reactant at the surface of the electrodes. It depends mainly on the conductivity of the electrodes.

Fuel crossover losses: due to a portion of a fuel which can flow through an electrolyte and a small number of electrons which can also pass through an electrolyte. These losses become more relevant in lower temperature cells.

Share of different losses and the dependence of current density are illustrated in Figure 4.

2.3 Fuel cell auxiliaries

Additional devices are required to keep the continuous working of fuel cell stack, which are powered by the energy of the stack. We define a fuel cell system, that includes all the auxiliary components and the stack itself. As a result, the effective power of fuel cell system is lower than that of fuel cell stack (Figure 5). The scheme of fuel cell system is illustrated if Figure 6. Main auxiliary systems are

• Hydrogen supply system: A recirculating pump is used to run an excess of hydrogen.
• Oxidant supply system: Compressor is used to drive air from the environment.
• Humidification system: Hydrogen and electrodes should be humidified to avoid the membrane from drying out.

Cooling system: Cooling circuit is required to remove the heat generated as a result of intrinsic losses of a fuel cell stack [8]. 

On-board hydrogen is mainly stored in either liquid or gaseous form. Hydrogen gas itself is in gaseous form at ambient conditions, thus has very low density at atmospheric pressure. Therefore, it should be stored in compressed form [Figure 7]. On the other side, the mass per unit volume of liquid hydrogen is much higher than the gaseous one, but the temperature should be kept far below the external temperature (around ) to maintain its liquid phase. This process requires an additional refrigeration system, which is powered by fuel cell system, thus reducing the effective output power. In addition, hydrogen tank must be strongly insulated in order to reduce the heat transfer from the ambient to the hydrogen and so prevent hydrogen from boiling.

2.4 Fuel cell technologies

Alkaline fuel cell (AFC)

 This technology is the most efficient among the others, having the potential to reach 70% due to fast kinetics due to low activation losses. At the anode, hydrogen is oxidized, according the following reaction

At the cathode, oxygen is reduced, at the same time, electrons flow through an external circuit

These cells use an electrolyte consisting of an alkaline aqueous solution of potassium hydroxide.

Phosphoric acid fuel cell (PAFC)

This technology uses liquid phosphoric acid () in a silicon carbide () matrix as an electrolyte. The reaction at the anode is

The reaction is the cathode is

PAFCs generate electricity (>40% efficiency) and nearly 85% of the steam this fuel cell produces is used for cogeneration and have the advantage of having a big choice of fuels usable.

Molten carbonate fuel cells (MCFC)

Usually operates at high temperatures (600-700) and contains an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert matrix of beta alumina solid electrolyte (BASE). The reaction at the anode is

The reaction at the cathode is

The main disadvantages of this technology are high operating temperature and high electrolyte corrosion.

Solid oxide fuel cells

It contains ceramic electrolyte and the has high power conversion efficiency, long-term stability, fuel flexibility. However, these fuel cells operate at extremely high temperatures (around 1000-1200). The reaction at the anode is

The reaction at the cathode is

Due to high operating temperature, activation losses are strongly reduced and the prevalent one becomes ohmic losses. The disadvantage is the brittleness of electrolyte which can be damaged due to thermal stresses, assembling and costs.

Direct methanol fuel cells (DMFC)

Methanol is used as a fuel and polymer is used as a membrane. The ease of methanol transport and high energy density is one of the advantages of this technology. However, efficiency range between 30 and 40 percent. The reaction at the anode is

The reaction at the cathode is

Proton exchange fuel cell (PEMFC)

PEMFCs are fueled by hydrogen and an oxidant, usually air or oxygen. This technology is mainly used in transport application. Solid polymer membrane made of perfluoro sulfonic acid which conducts hydrogen protons. Operating temperature is comparably low (50-100)  and operates on the opposite principle to electrolysis, which consumes electricity [7]. The chemical reaction at the anode is

The reaction at the cathode is

PEMFC technology is used in automotive industry dur to the following advantages [11]:

• Low-cost technology
• Relatively low temperature operating range
• Air can be directly used as an oxidant
• Durability (3000-5000 h as functional life)
• Fast response

3. Hydrogen fuel cell traction systems

In general, hydrogen fuel cell vehicles can be subdivided into two categories: pure fuel cell vehicle and simple series hybrid. Simple series hybrid fuel cell vehicles can be classified as full power, load follower and range extender (Figure 8).  For the full power configuration of FCHEVs, the fuel cell system is designed in a way to be able to provide maximum transient power demand. In case of load follower configuration, fuel cell system is sized to supply maximum continuous power demand. Finally, fuel cell system is designed to provide average power demand for the case of range extender [12]. One of the main drawbacks of pure hydrogen fuel cell vehicles is the absence of an additional reversible energy source, which is present in hybrid vehicles in the form of battery packs or supercapacitors, that enables the energy recovery during braking phases and assist main driving unit during hard acceleration phases. In general, hydrogen fuel cell system is series coupled (electrically in parallel) with a second energy and/or power source.

 

Figure 8. Hydrogen fuel cell vehicle classification

 

3.1 Pure fuel cell propulsion system.

Only fuel cell stack (FCS) is responsible to produce tractive power, no any additional power sources are available (Figure 9). It is a simple solution in which the FCS is sized to supply maximum transient power. Due to irreversibility of FCS, the direction of the power flow cannot be reversed, so regenerative braking is not possible. FCS must fully manage the instantaneous power demand of the vehicle and may face with warm-up issues during cold start.

 

Figure 9. FCEV propulsion system

 

Figure 10. FCHEV propulsion system

 

3.2 Series hybrid fuel cell propulsion system.

 An additional power/energy source is implemented in this solution (Figure 10). FCS is electrically in parallel (series) coupled with either battery or power buffer (supercapacitor) or both. Usually, batteries are applied as a second source. Proper control strategy is required to distribute the power demand between FCS and an additional power source, to optimize the hydrogen consumption of FCS. This done by forcing FCS to work at optimum operating line, i.e. at high efficiency working points, and the rest is supplied by the batteries. At the same time, when the power demand is low, FCS can operate with higher load, yet with high efficiency, charging batteries. Since battery packs and supercapacitors admit bidirectional power flow, some energy could be regenerated during braking or deceleration phases. Moreover, batteries can supply power during start-up, thus simplifying the transient loading of FCS.

4. On-vehicle applications

Fuel cell technology is realized in several passenger vehicles. Hyundai ix35 FCEV is the first commercially produced hydrogen fuel cell vehicle. Toyota Mirai (1^st and 2^nd generations), Hyundai Nexo, Honda Clarity are the other examples of mass-produced fuel cell hybrid electric vehicles. Toyota Mirai is for sure the most popular and wide-spread FCHEV in the world (Figure 11). This vehicle can travel 500 km on a full tank, according to EPA UDDS driving cycle with an average consumption of 3.6 liters of hydrogen per 100 km of travel distance. FCS accelerates the vehicle from 0 to 100 km/h in 9.2 seconds. Hydrogen refueling takes from 3 to 5 minutes. The first generation of Toyota FC stack achieved a maximum output power of 114 kW [13]. Electricity generation efficiency was enhanced through the use of 3D fine mesh flow channels. Each stack consists of 370 single-line stacking cells. In addition, it is also equipped converter, developed to boost the generated voltage up to 650 volts. Toyota Mirai contains two hydrogen tanks with a three-layer structure made of carbon fiber-reinforced plastic, which is able to withstand 700 bars of pressure. Mirai has 245 V (1.6 kWh) nickel metal hydride traction rechargeable battery pack.

 

Figure 11. Toyota Mirai (1^st generation) [14]

 

Table 1.

Mass-produced fuel cell vehicles technical specifications

 

Vehicle	Travel range [km]	Curb weight [kg]	Motor Power [kW]	Max speed [km/h]	Acceleration time (0-100 km/h) [s]
Toyota Mirai (2nd generation)	647 (UDDS)	1920	136	175	9
Honda Clarity Fuel cell (2016)	589 (UDDS)	1875	120	165	8,7
Hyndai Nexo (2018)	805 (NEDC)	1850	113	177	8,4

 

By considering Kia Optima Lx 2019 , equipped with conventional internal combustion engine, having 137 kW maximum power and 1465 kg of curb weight, it is clear that specific power density for the ICE vehicle is much lower than those, equipped fuel cell technology, being equal 0.0935 kW/kg for Kia Optima Lx 2019 and 0.07 kW/kg for Toyota Mirai. The reason for this phenomenon is the complexity of fuel cell system, power converters and electric motors. Therefore, the total weight becomes larger than the weight of ICE vehicle’s powertrain and drivetrain.

5.Future development and perspectives

As it is mentioned earlier, FCHEV overall (well-to-wheels) emissions significantly depend on hydrogen production methods. The main sources for hydrogen are fossil fuels (coal, oil, natural gas), biomasses and water. The primary energy, needed to be supplied for hydrogen production can be taken from fossil fuels, nuclear energy and renewable energy. Electrolysis of water is the cleanest way to produce hydrogen, yet the most expensive. The cost for the electrolysis might be up to 10-12 times more than the conventional steam reforming process. Therefore, a number of current researches are concentrated around the simplification and price reduction of electrolysis process for hydrogen production [15]. Moreover, the capacity and safety of hydrogen on-board storage system is another issue. One of the main reasons, of why FCHEV are not widely spread in the world is the lack of infrastructure (hydrogen refueling stations), which requires high capital investments.

6. Conclusion

This article demonstrates the capabilities and functionality of hydrogen fuel cell technology, applied in automotive industry. These vehicles promise zero tank-to-wheel emissions, and is an evident solution concerning the environmental concerns of pollutant and greenhouse gases emissions from transportations sector. Pure water () is the only emission, which is produced as a result of the reaction of hydrogen () and oxygen (). Vehicles, equipped with fuel cell technology promise a working efficiency range, that is higher of gasoline and diesel internal combustion engines, but lower than of battery electric vehicles. Fuel cell hybrid electric vehicles can take an advantage of regenerative braking and flexibility in operating conditions of FC system. There are, however, some challenges, such as aiding regenerative braking, maximizing efficiency, increasing the transient performance of FC in the system, and decreasing FC fuel consumption, have still to be resolved. Nonetheless, researchers expect that in the near future, FCHEV will be a strong rival to traditional ICEV, as the cost of FC and related technology continues to fall.

 

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