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Mercedes-Benz F125!: fuel cell plug-in hybrid with Li-sulfur battery and structure integrated hydrogen storage with MOFs
13 September 2011
|Mercedes-Benz F 125! research vehicle. Click to enlarge.|
Mercedes-Benz presented the F125! research vehicle (the “125” marking 125 years of the automobile), a fuel-cell plug-in hybrid with a range of more than 1,000 km (621 miles) that anticipates more than two generations of vehicles to 2025 and beyond.
The F125!, intended to demonstrate emission-free individual mobility in the luxury segment in the future, incorporates already well-proven concepts and technologies which are not yet available today, but for which basic research has shown great future promise, and therefore a realistic chance of implementation in future series-production cars, according to Mercedes-Benz. The decisive innovations are:
A logical further development of the fuel cell drive system in combination with plug-in technology.
New hydrogen storage technology: the structure integrated hydrogen composite storage unit leveraging Metal Organic Frameworks (MOFs).
Lithium-sulphur batteries (e.g., earlier post), which Mercedes-Benz is examining in parallel with further development of the current lithium-ion battery and research into lithium-air technology. The high-voltage lithium-sulphur battery has a specific energy density of 350 Wh/kg at the cell level, which allows considerably higher recuperation rates in combination with the e4MATIC all-wheel drive.
|Mercedes-Benz F 125! research vehicle. Click to enlarge.|
Fuel cell system. In the new Mercedes-Benz research vehicle, the fuel cell stack is centrally located under the hood at the front, while the compact electric motors are installed near the wheels in the front and rear axle areas. The composite hydrogen reservoir in the area of the center tunnel, between the front seats and the floor assembly, has a capacity of around 7.5 kilograms and is protected against the consequences of accidents.
The drive system relies upon a further development of the Mercedes-Benz fuel cell stack which has already demonstrated its efficiency and day-to-day suitability in the successful B-Class F-CELL World Drive this year.
The stack in the F 125!, which is further improved with respect to performance, consumption and practical suitability, provides the power for four electric motors installed near the wheels. The modular e4MATIC system, which also uses improved drive components from the SLS AMG E-CELL, generates a continuous output of 170 kW (231 hp) and a peak output of 230 kW (313 hp). This accelerates the F 125! to 100 km/h in 4.9 seconds, with a top speed of 220 km/h (137 mph).NEDC fuel consumption is 0.79 kilograms of hydrogen per 100 kilometers (= 2.7 liters diesel equivalent, or 87 mpg US equivalent).
Hydrogen storage. The structural integrated hydrogen composite unit of the F 125!, which allows the hydrogen tank to be fully integrated into the bodyshell structure for the first time, is based on Metal Organic Frameworks (MOFs). (Earlier post.) MOFs are porous solid bodies which consist of numerous, always identical basic components and can be very variably put together on a modular basis. They are made up of nodal points known as Structural Building Units (SBUs). The connecting elements between these nodal points are formed by organic molecules known as Linkers. This structural principle allows solid bodies with extremely large specific surface areas, which in turn provides the basis for an enormous hydrogen storage capacity.
With inner surfaces of up to 10,000 m2 per gram—the current status of research—MOFs are attractive for numerous applications: they are suitable as gas cleaners for fuel cells, for example, and also, as envisaged for the F125!, as a storage medium for gases. MOFs can be used as pressurized containers (30-80 bar), but for a higher storage density also as low temperature tanks at 77 K (around -196 degrees Celsius), i.e. considerably above the 20 K boiling point of hydrogen.
These attributes and the fundamental variability of the MOFs’ shape allow an installation position suited to the vehicle requirements. This means that future MOFs can be flexibly installed in the body structure. Key advantages of this solution include:
Less installation space thanks to better adaptability means more scope for packaging and more room for the occupants.
The low installed position is conducive to a low center of gravity, with a positive effect on handling and driving dynamics.
Full integration into the bodyshell structure ensures the best possible crash and operating safety.
Using this technology, says Prof. Dr. Thomas Weber, member of the Board of Management of Daimler AG, responsible for Group Research and Mercedes-Benz Cars Development, future vehicles with fuel cell drive systems could achieve the operating ranges of current diesel models with no loss of interior space. Based on the current level of know-how, Mercedes specialists consider it possible that they may develop this technology to series production level from 2025.
The tank integrated into the floor assembly has a capacity of around 7.5 kg of hydrogen. Compared to the high-pressure tanks in use today, the H2 tank potentially requires less installation space. This is because to withstand a pressure of up to 700 bar, current tanks need to be cylindrical in shape, and owing to this round cross-section there are inevitably cavities between tanks installed next to or above each other. In contrast, tanks that can be filled at a pressure of 30 bar or less can be better integrated into the bodyshell. At the same time they are able to act as structural components.
The lithium-sulphur battery for the F 125! has a storage capacity of 10 kWh and is installed behind the rear seats. Combining the fuel cell drive system with the lithium-sulphur battery makes a total operating range of up to 1,000 km possible, of which up to 50 km (31 miles) can be under battery-electric power alone.
The battery pack can be inductively charged at intelligent charging stations, and the convenient charging process can be monitored by smartphone. When designing the F 125!, the developers worked on the assumption that by the time of its introduction into series production, this battery type will be capable of energy densities up to 350 Wh per kg. This would represent roughly a doubling of current performance. The real potentials of this technology are however the subject of basic research, and are still difficult to assess at present, Daimler notes.
Bodyshell. This study combines the advanced electric drive and bodyshell technologies with unique control and display concepts. The bodyshell features lightweight hybrid construction with a high proportion of fiber-reinforced plastics and an intelligent mix of carbon-fiber, aluminum and high-strength steels, which allows a significant weight reduction while offering a further considerable improvement in safety. A high-strength construction with crash-responsive protective systems within the doors allows the omission of B-pillars, as well as the use of wide gull-wing doors which allow convenient access to the four seats.
Telematics and Assistance. The F 125! provides an outlook on future Mercedes-Benz telematic systems and driver assistance system. If the driver requires, the F 125! is also able to carry out frequently occurring driving maneuvers autonomously.
Advanced Driving Assist allows lane-changes on multi-lane, one-way roads, and in a further development stage even automatic overtaking maneuvers. With radio-based networking with the environment (Car-to-X communication), the F 125! is also able to exchange information with other vehicles, a specially equipped infrastructure including traffic lights or warning signs and traffic control centers. Specific applications might include a warning of approaching emergency service vehicles, well before the driver can see or hear them, a reminder that other vehicles have the right of way at obscure road junctions, or obstacles on the road.
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