The Mercedes-Benz eCitaro fuel cell: More range without recharging thanks to a fuel cell

• eCitaro fuel cell: Consistent, practical and cost-conscious
• Development promoted by German Ministry of Transport
• Fuel cell: Highly efficient, compact and durable
• How the fuel cell drive works
• Modular hydrogen tanks, lightweight and safe
• High-performance batteries of the latest generation
• Long range, high number of passenger seats
• New thermal management uses the waste heat from the fuel cell
• Proven low-floor drive axle
• Cockpit and operating concept do not require any getting used to
• Digital monitoring already integrated
• The key advantages of the Mercedes-Benz eCitaro fuel cell

Leinfelden-Echterdingen / Barcelona – The Mercedes-Benz Citaro is a true classic among city buses. Presented for the first time at the UITP Congress (now GPTS) in 1997, it has also been available as an eCitaro electric bus since 2018. To date, well over 60,000 Citaro and eCitaro buses have been delivered. The low-floor bus bearing the three-pointed star is now once again setting standards. At the Global Public Transport Summit (GPTS) 2023 in Barcelona, Mercedes-Benz will be showcasing the latest addition to the Citaro model range: the eCitaro fuel cell. A fully electric solo bus with fuel cell can achieve a range of around 400 kilometers without recharging in urban traffic, while a corresponding articulated bus can cover around 350 kilometers.

eCitaro fuel cell: Consistent, practical and cost-conscious

The name says it all: The eCitaro fuel cell is the first Citaro to be equipped with a fuel cell. Vehicle drives with a fuel cell are considered the answer when very long ranges are required for fully electric city buses. Ultimately, the fuel cell generates electricity for the electric motors using hydrogen and oxygen from the air while driving. However, fuel cells must operate more continuously and less dynamically, as is typical for city bus operation with its frequent acceleration and braking maneuvers. For this reason, fuel cell drives always require a buffer battery. The possible combinations range from a high-performance fuel cell with a compact buffer battery to a drive system with a large battery capacity and a compact fuel cell.

Daimler Buses has opted for the latter configuration for good reason. On the one hand, electricity from the grid as the basis for power will be significantly cheaper than hydrogen in the foreseeable future. The fuel cell of the eCitaro fuel cell therefore does not serve as the main source of energy but is only used to extend the range. In addition, unlike a fully fledged hydrogen vehicle with a small buffer battery, the eCitaro fuel cell is significantly better at storing the energy recovered during braking by recuperation in the large batteries completely and beneficially. This contributes to the overall economy of the eCitaro fuel cell, especially in urban traffic with its frequent deceleration processes and especially in challenging topography. Last but not least, the large battery capacity enables the release of high drive power for longer periods of time, for example on inclines in mountainous terrain, without the fuel cell having to operate in the top, and therefore, inefficient power range.

With intelligent control of the energy flow from the battery and fuel cell and optimum weight distribution of the batteries as well as the fuel cell and hydrogen components, the fully electric eCitaro fuel cell achieves a maximum range without recharging and with the greatest possible passenger capacity. This makes it not only a practical but also a particularly economical solution for urban routes with long route cycles. The eCitaro fuel cell is therefore the ideal addition for transport companies with an existing battery-electric fleet that want to operate their long routes without having to recharge and without having to put on additional buses.

Development promoted by German Ministry of Transport

The German Federal Ministry of Digital Affairs and Transport is supporting the development of this technology with a total of 3.3 million euros as part of the National Innovation Program for Hydrogen and Fuel Cell Technology (NIP). The funding guideline is coordinated by NOW GmbH (NOW = National Organization for Hydrogen and Fuel Cell Technology) and implemented by the project executing agency Jülich (PtJ).

Fuel cell: Highly efficient, compact and durable

For the fuel cell, Daimler Buses relies on a tried-and-tested module, but in its latest version. It is supplied by Toyota and is already in its second generation. It is a heavy-duty unit with a maximum output of 60 kW. In the eCitaro fuel cell, the unit is operated very efficiently at around 20 kW at its optimal point. It operates in a voltage range of from 400 to 750 volts.

The fuel cell used has numerous advantages. Its slim and compact design makes it ideal for mounting on the roof of the eCitaro. The fuel cell itself operates with high efficiency. This results in a comparatively low consumption of hydrogen for power generation. A voltage converter has already been integrated. The very long service life of around 40,000 hours in use as a range extender is also remarkable, which corresponds to a useful life of seven to ten years, depending on use. Even after that, the fuel cell does not suddenly cease operation. Instead, its efficiency slowly decreases, and aging of the components can be expected.

In the solo bus, the fuel cell module, weighing around 240 kilograms, is mounted on the roof in a position just behind the front axle. On the articulated bus it is positioned on the roof at the front of the rear section.

How the fuel cell drive works

The fuel cell generates electrical current and heat from the reaction of hydrogen and oxygen. The individual cell consists of a polymer electrolyte membrane (PEM), a negatively charged and a positively charged electrode as well as two separators. Fuel cells are galvanic cells.

The hydrogen passes from the tanks on the roof via lines to the negatively charged electrode where it reacts with oxygen from the air. Electrons are released and migrate to the positive electrode. This results in an electrical current. The hydrogen atoms are transformed into hydrogen ions by the release of the electrons. They flow through the polymer electrolyte membrane to the negative electrode. At this point, water is the only emission resulting from a chemical reaction between oxygen, hydrogen ions and electrons. Several hundred fuel cells are required to power an eCitaro fuel cell. Together they form a fuel cell unit, the so-called fuel cell stack.

Modular hydrogen tanks, lightweight and safe

In the eCitaro fuel cell, the hydrogen is used in gaseous form and at a pressure of 350 bar. This means that there is no need for a high degree of compression and correspondingly high expenditure at filling stations. The hydrogen tanks for supplying the fuel cell each hold five kilograms of usable hydrogen. With an inner container made of plastic and an outer casing made of carbon fiber, they comply with the so-called Type 4. This makes them both lightweight and highly durable.

The arrangement transverse to the direction of travel enables easy access to the valves and sensors. Safety is key: Each individual vessel has its own pressure and temperature sensor. This means that any leaks can be reliably detected, which is why additional hydrogen sensors and thus possible sources of errors can be dispensed with. The tanks already comply with the UN ECE-R 134 standard that will only be binding as of 2024. Certification in accordance with this standard requires a successfully completed stress test.

The fuel system has a modular design. In the solo bus, five tanks with a total of 25 kilograms of hydrogen are used. The articulated bus can be equipped with six or seven tanks with 30 or 35°kilograms of hydrogen, as desired. They are each mounted on the roof above the front axle and the front overhang, covered at the top with covers and thus protected from dirt and sunlight. The trim panels can be opened wide to allow easy access for service. All high-pressure lines are rigid. Between the filler neck and the roof, they even have a continuous design without screw connection to ensure maximum safety. The fuel cell is supplied with gaseous hydrogen from the tanks via a low-pressure line. This is a major advantage for the articulated bus in particular, as the fuel cell is located on the rear section of the bus and therefore a flexible high-pressure line around the articulation unit can be dispensed with.

Refueling is carried out consistently in the direction of travel on the right above the second axle. Under optimum conditions and depending on the refueling infrastructure, the refueling time in the solo bus, for example, is around ten minutes.

High-performance batteries of the latest generation

The latest generation high-performance batteries NMC 3 are also used in the eCitaro fuel cell as is the case with the current eCitaro. An outstanding advantage is their enormous capacity, which enables a long range without opportunity charging. In addition, there is the modular design of the battery equipment. The new generation also uses lithium ion batteries. To be precise, these are cells with a new high-energy NMC cathode (NMC: lithium-nickel-manganese-cobalt oxide), a liquid electrolyte and an advanced graphite anode. The common abbreviation NMC (nickel, manganese, cobalt) also stems from battery chemistry. Instead of the previous prismatic battery cells that are the size and shape of a paperback book, ultra-compact cylindrical cells with high-energy cell chemistry are used. Improved cell chemistry combined with an optimized battery pack result in an increase in the overall gravimetric and volumetric energy density of the battery pack. The capacity of 4.93 Ah per battery cell results in a considerable increase in capacity of around 50 percent at the same weight.

600 battery cells are mounted within each battery module. They have been integrated into the cooling circuit to ensure the battery’s ideal temperature of around 25 degrees Celsius. Temperature control ensures maximum service life and efficient charging. Nine battery modules combine to form a battery pack with 5400 cells. This works out at a rated energy of 98 kWh per battery pack.

Similar to the fuel tanks, the batteries are also scalable. Three battery packs with a combined capacity of 294 kWh are used for the solo bus. For the articulated bus, there are either three or four battery packs with an energy capacity of 392 kWh. These battery packs alone ensure a considerable range.

As both the battery capacity and the quantity of hydrogen carried can be selected, each public transport company will receive the eCitaro fuel cell tailored to its individual usage profile.

Thanks to the use of the fuel cell to extend the range, opportunity charging of the batteries en route is not necessary and also not envisaged. Charging takes place consistently by plug at the depot with a maximum charging capacity of 150 kW. As usual with the eCitaro, there are three plug positions to choose from, at the left and right above the front axle as well as at the rear. Up to two plug positions are possible per vehicle for flexible positioning in the depot or vehicle hall.

Long range, high number of passenger seats

Intelligent control takes over the energy management and regulates when the fuel cell starts to operate, with what kind of performance and for how long it produces electricity, which is additionally made available to the vehicle, according to the respective requirement and the chosen operation strategy. Transport companies can choose between two different operating modes.

In “maximum range” mode, both battery charging process and hydrogen are used to the maximum. The fuel cell always operates in the most efficient operating range. This also applies when operating in “minimum H₂ consumption” mode, in which the battery supplies the majority of the energy for the drive and auxiliary consumers. The fuel cell only contributes as much energy as is necessary to achieve the previously set range.

Depending on the configuration of batteries and hydrogen tanks, the eCitaro fuel cell can achieve a range of around 400 kilometers as a solo bus with average requirements without opportunity charging or refueling. The range of the articulated bus is around 350 kilometers. These outstanding values completely cover the requirements of almost all transport operators.

In addition, the precisely optimized weight distribution of batteries, fuel cells and hydrogen tanks ensures a high number of passenger seats. A three-door articulated bus with one driven axle, three battery packs and seven hydrogen tanks, for example, can provide around 128 passenger seats. As a result, the eCitaro fuel cell even provides a higher passenger capacity than a battery-electric eCitaro with maximum battery equipment.

New thermal management uses the waste heat from the fuel cell

Thanks to the integration of the fuel cell drive, the Mercedes‑Benz Citaro has once again demonstrated its versatile concept. Although extensive adaptation measures were necessary, no complete redesign has been required. Transport companies therefore also benefit from the familiar driver cockpit and passenger compartment as well as from a large number of identical components and parts of the Citaro and eCitaro in the eCitaro fuel cell.

What is new in the eCitaro fuel cell is the software of individual control units, for example. For instance, the heat input from the fuel cell results in a completely new thermal management. The fuel cell's waste heat can be used profitably for the heating of the vehicle interior. As with the battery-electric eCitaro, Daimler Buses also uses the compact air conditioning system with heat pump for the eCitaro fuel cell, however, with R134a refrigerant. In combination with the waste heat from the fuel cell, it also achieves higher efficiency at low temperatures compared with the CO₂ air conditioning system of the eCitaro. The waste heat from the fuel cell can also be used to control the temperature of the batteries.

The eCitaro fuel cell also incorporates the heat-emitting components into the vehicle’s heating circuit. The temperature of the passenger compartment and, separately, the driver cockpit, is controlled automatically. As with the eCitaro, air flow rates and the heating energy used also depend on the number of passengers.

As with the eCitaro, the electrical heating and climate control in the eCitaro fuel cell allow the vehicle interior to be pre-conditioned to the desired temperature while the batteries are being charged at the depot. This means that the energy for the required climate control is not drawn from the battery at the start of the trip, which increases the range. In winter, the city bus can therefore be pre-heated in the depot during charging if required. Similarly, pre-cooling takes place in summer.

Proven low-floor drive axle

The power is transferred as usual to the low-floor portal axle ZF AVE 130 with motors near the wheel hub. They generate a maximum output of 125°kW per wheel and achieve a torque of 485 Nm. A fixed gear ratio results in a maximum torque of 11,000 Nm per wheel. In the case of articulated buses, the center and rear axles are powered as standard. In light terrain, the driven axle of the rear section is also sufficient.

Cockpit and operating concept do not require any getting used to

The layout of the eCitaro fuell cell passenger area corresponds to that of familiar Citaro models. The driver does not need to adapt any more than the passengers do: The cockpit and operating concept largely correspond to the usual setup, and the direction of travel is selected as usual with the D N R pushbuttons. The almost identical operation compared to the Citaro with a combustion engine or to the battery-electric eCitaro therefore enables the usual quick change of driver.

The instrument panel also largely corresponds to that of the eCitaro. Here as there, a power availability display shows the current output request. The battery’s state of charge is also displayed. The only difference and evidence for the fuel cell drive concept is the fill level display for hydrogen in the instrument panel. The driver can use the central display to view the range, the power availability, the charge indicator and the available hydrogen quantity.

The eCitaro fuel cell also helps its drivers to save energy with an acceleration control system: When moving off from a bus stop or at traffic lights, whether empty or under full load, the torque is reduced to a comfortable level. This leads to a gentle and passenger-friendly driving style and reduces energy consumption at the same time.

Digital Vehicle Monitoring already integrated

It is not only since the introduction of battery-electric drives that the requirement for digital services to monitor and optimize consumption values has increased in transport companies. However, especially for vehicles with exclusively depot charging – whether with electricity or hydrogen – monitoring the energy consumption, battery charge level, hydrogen supply and vehicle condition can help to ensure the maximum availability and profitability of the buses.

That’s why the Mercedes-Benz eCitaro fuel cell comes with all the components required ex works and allows simple integration of the vehicle into the Omniplus On portal for real-time monitoring of vehicle functions. The Omniplus On monitor can be used at any time to view the fill levels and charge levels of hydrogen and batteries as well as the resulting remaining range. In the event of malfunctions or a dropping below of the minimum charge or fill level values, the driver or fleet manager receive a corresponding warning. If required, this is not only sent to the screens at head office but also SMS text message to the mobile phone.

In addition, Omniplus On monitor shows detailed trends and optimization potential for energy consumption. In addition to diagrams of consumption values per vehicle and route, Omniplus On monitor also differentiates whether the energy was used for the drive or the vehicle air conditioning. This makes it easy to compare the individual vehicles with each other and, if necessary, take suitable measures to reduce energy consumption.

The key advantages of the Mercedes-Benz eCitaro fuel cell

The new and locally emission-free Mercedes-Benz eCitaro fuel cell perfectly complements an existing fleet of electric buses. With its fuel cell technology, it covers challenging routes with long cycles without recharging. Its concept also focuses on the total cost of ownership: The drive’s energy supply is primarily provided by low-cost mains power, while hydrogen, which is currently still costly, is used to extend the range. In addition, the body and chassis of the eCitaro fuel cell are largely identical to those of the Citaro that has proven itself thousands of times over; in case of damage, replacement parts are therefore quickly available. Thanks to the favorable weight distribution of the batteries as well as the fuel cell and hydrogen system, the eCitaro fuel cell offers a high number of passenger seats.