September 14, 2007 General Motors have released details on stage four of their HydroGen fuel cell vehicle development program. Over the next 18 months, over 100 fuel-cell powered HydroGen4 vehicles will be tested internationally in a range of driving environments and conditions. The question remains though, is Hydrogen likely to become a viable alternative to petrol?
Despite well-understood difficulties in producing and distributing hydrogen as a replacement to petroleum fuels, and the inherent energy inefficiency of fuel cells as a storage medium, hydrogen-electric vehicles are still viewed as one of the more feasible alternatives to the internal combustion engine as the sun begins to set on the oil era.
Batteries are a vastly more efficient option, but unsuitable for car applications because they take some time to charge, where Hydrogen can be pumped into a tank like petrol. Many major vehicle manufacturers are thus very publicly developing their ability to deal with hydrogen energy – such as General Motors (GM), whose HydroGen program has just reached its fourth stage. Perhaps as much an enviro-friendly PR exercise as a genuine path forward, the HydroGen 4 is a significant improvement on its predecessors and will be tested throughout 2008.
With the HydroGen4, GM presents the fourth generation of its fuel cell technology. "Fuel cell propulsion with hydrogen as a fuel highlights General Motors’ commitment to take the car out of the environmental debate and reduce our dependency on oil," says Carl-Peter Forster, President of GM Europe." HydroGen4 is powered by GM’s most advanced fuel cell system and marks an important milestone on the road toward completely emission-free, competitive fuel cell technology in the automobile. The HydroGen4 features considerable progress in everyday usability, dynamics and system durability compared to its predecessor."
Fuel cell development at GM is also entering a new organizational era. "The Fuel Cell Activities (FCA) research division with over 600 employees is currently being integrated into regular series development, giving it key importance within the concern," adds Carl-Peter Forster. "We are thus preparing for the series production of fuel cell technology." More than 400 engineers will now drive the development forward within the Powertrain organization, with a further 100 moving into global product development to begin the integration of fuel cells into upcoming GM models. More than 100 fourth-generation vehicles ready for global deployment
The GM HydroGen4 (length/width/height: 4796/1814/1760 mm) is the European version of the Chevrolet Equinox Fuel Cell. As early as fall 2007, the first of these fuel cell prototypes – a global fleet of more than 100 vehicles is planned – will be on the roads in the USA. They will take part in an extensive testing and demonstration program called "Project Driveway". The vehicles will be given to customers so that GM can contain all aspects of their use of the car and how they handle filling it with hydrogen. The findings will then be included in the further development. From mid-2008, a total of ten HydroGen4 vehicles will take part in day-to-day testing within the framework of the Clean Energy Partnership (CEP) in Berlin. In the second phase of CEP, various customers with different driving profiles will operate the fuel cell vehicles day after day to test the cars’ everyday usability.
The HydroGen4’s fuel cell stack consists of 440 series-connected cells. The entire system produces an electrical output of up to 93 kW. With help from a 73 kW/100 hp synchronous electric motor, acceleration from zero to 100 km/h takes around 12 seconds. The front-wheel driven vehicle’s top speed is around 160 km/h.
The HydroGen4 is designed for a lifecycle of two years/80,000 kilometers, and can start and run at sub-zero temperatures – a considerable advancement over the predecessor HydroGen3 and an important characteristic with regards to the everyday usability of fuel cell vehicles. This improvement is possible thanks to an intelligent combination of measures including thermal insulation, water management and operating strategy.
The four-seater offers the comfort, spaciousness and high safety level of today’s conventional cars, and includes driver and front passenger airbags and side airbags. ABS, Traction Control and ESP are also fitted.
4.2 kg of pressurized hydrogen provides an operating range of up to 320 km
During the HydroGen4’s development, scientists and engineers from GM fuel cell centers in Honeoye Falls (New York), Torrance (California) and Mainz-Kastel (Germany) were able to make use of a wealth of knowledge and experiences that were gathered during the extensive and rigorous practical testing of its predecessor introduced in 2002 (see corresponding chapter).
There were two versions of HydroGen3, for example. While one variant operated on liquid hydrogen at -253°C and another on compressed hydrogen, the decision has now been taken to focus on gaseous hydrogen. "The main reason for this is the unavoidable ‘boil off’ that occurs with liquid hydrogen," explains Dr. Udo Winter, Director, GME Fuel Cell Activities. "Even with optimum insulation, the tank’s contents warm up slowly, so that the liquid hydrogen vaporizes and the pressure in the tank increases. After a few days, gaseous hydrogen has to be released from the parked vehicle, leading to a loss in fuel. There are no such vapor losses ("boil off") with compressed gas, however."
The HydroGen4 has a tank system with three, 700-bar high-pressure tanks made from carbon-fiber composite material, which can hold 4.2 kg of hydrogen. This provides an operating range of up to 320 kilometers.
Buffer battery enables regenerative braking
The new fuel cell propulsion system also has a nickel-metal-hydride buffer battery and a capacity of 1.8 kWh. The battery ensures improved driving performance and covers the system’s performance peaks. The efficiency of the entire propulsion system has also been improved, as the buffer battery enables regenerative braking in the HydroGen4. When braking or overrunning, the electric motor switches to generator operation and uses the electrical energy produced when braking to charge the battery.
If the driver has to brake harder, the car will also be decelerated hydraulically, as is the case in a conventional car. This combination of regenerative and hydraulic brake performance is called "brake blending". It is applied by driving stability programs such as ABS or ESP, or when the required deceleration exceeds the maximum regenerative braking performance. This is determined by the size of the generator and battery input capacity.
Battery and braking technology are also important links to the innovative GM E-Flex electric vehicle architecture that the company is also working on.
Electric turbo compressor provides air to fuel cells
The heart of the HydroGen4 is its fuel cell stack. Fuel cells convert chemical energy into electrical energy without combustion. Via an electro-chemical reaction, they combine hydrogen and oxygen to form water, and produce electricity at the same time.
The electro-chemical process in a fuel cell works as follows: Hydrogen on the anode catalyst splits into protons and electrons. The positively-charged protons pass through the membrane to the cathode, while the negatively-charged electrons travel in an external circuit, producing electricity on the way. On the cathode catalyst, oxygen reacts with the electrons and protons to form water. A stack connecting a large number of individual cells can thus produce enough power to drive an electric motor.
Unlike its predecessor, the individual cells of the new stack are positioned horizontally – as opposed to vertically – for packaging reasons, i.e. for optimal distribution of the individual components in the vehicle. The gas supply to the stack is also different in the HydroGen4 compared to the HydroGen3: instead of a screw-type compressor at the cathode, an electric turbo compressor provides the fuel cells with air. This increases efficiency and acoustics.
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