Fuel Cell Systems For Passenger Cars Opportunities And Requirements

Joachim Tachtler BMW

D-80788 Munich Germany

Carl Bourne Rover Group Coventry CV4 7 AL United Kingdom

Possible forms of energy for the propulsion of passenger cars

From the point of view of energy density, handling and economy, present-day motor fuels are superior to all known alternatives. The internal combustion engine powered by them satisfies the requirements of customers to an excellent degree.

The search for alternatives can therefore only be justified if emissions can be avoided totally and non-fossil primary energy sources can be used or at least partially our dependence on mineral oil can be reduced.

What was long suspected has been increasingly confirmed, not least by developments at BMW: electricity (stored in batteries) and hydrogen offer the best prerequisites for achieving these goals in the long term. These forms of energy can be produced in sufficient quantities and with relatively little effect on the environment. They promise to produce an absolute minimum of pollutants when used in vehicles.

Natural gas, which is veiy similar to hydrogen, and hybrid systems, that would compensate for batteiy risks, could perform a valuable fonction in the transitional phase.

Significant improvements in the performance of fuel cells

Fuel cells produce electric current direct from chemical fuels, preferably hydrogen. They offer the potential of powering vehicles with electric drivé system with an increased operating range. Of >•

the various types of fuel cells, the most interesting for use in passenger cars is the PEM (polymer electrolyte membrane) fuel cell. Today it can already be operated at temperatures between 4 and

80°C. Oxygen, necessary for the reaction, is obtained from pressurized ambient air [1], *

For several years, specialists in electrochemistry - also in-collaboration with car development engineers - have been trying to make fuel cell technology a useful source of motive power. Considerable improvements in performance have been achieved in recent years. The main motivation for these developments is the high potential efficiency and the possibility of the classification as a "Zero Emission Vehicle" (ZEV), at least if hydrogen is carried onboard.

At Solar-Wasserstoff-Bayern GmbH (SWB), a company in which BMW has been an active partner since 1987, tests have been carried out starting in 1990 on various fuel cells for stationary and mobile applications [2]. A ground conveyor vehicle equipped with the first Siemens air/hydrogen fiiel cell system will start trials at SWB in Neunburg in late 1996. Siemens has '

several decades of experience derived from developing fuel cell drive systems for submarines [3].

The energy consumption of the complete fuel cell system is an especially important consideration. Although the efficiency of single cells is theoretically very high, the additional energy for the cooling system, the air compressor and the controller have to be taken into account. It therefore seems more sensible to develop fuel cells that can be operated at low pressures than to aim for high power density at the cost of high pressures. This is made all the more important by the need to use excess air (X approx. 2). The new Siemens system is particularly interesting, as its maximum operating overpressure is only 0.5 bar.

Besides the need for efficiency and low pollution, fuel cells must prove their worth in everyday use. The critical performance factors are rapid start-up, fast response to changes in power demand, such as we are used to with internal combustion engines, and also reliable operation, under all environmental conditions. Temperatures below freezing could be a problem here. There also exists no experience of how fuel cells cope with real conditions such as sustained road induced vibration or long periods at rest.

A significant problem faced by the automotive sector is the issue of cost. Key requirements for the reduction of the production costs - currently several million DM for each fuel cell system - are the development of mass productionable bipolar plates, inexpensive and environmentally compatible membranes and the minimization of platinum in the electrodes. Present fuel cells contain so much platinum that there are not sufficient known world reserves to equip all the vehicles currently in use. The amount of platinum per vehicle must therefore be reduced to the level of present-day exhaust catalytic converters with a similar level of platinum recoverable through recycling.

Zero-emission vehicle with a high range

Although fuel cell technology will probably be initially commercialized for stationary electrical power generation, it is interesting to know that it is now technically feasible to power passenger cars with fuel cells - albeit with certain restrictions. On the subject of this fascinating challenge, BMW presented at the World Hydrogen Energy Conference "Hydrogen '96" its initial study for an experimental ZEV with a range of 1000 km. The vehicle concept is based on a 3 Series BMW with liquid hydrogen tank, fuel cell system and electric motor.

Controller (electric drive motor, Batteries (start, fuel cell and hydrogen tank) ,on-board network)

Controller (electric drive motor, Batteries (start, fuel cell and hydrogen tank) ,on-board network)

(appr. 30kW) system (LH2) drive motor

Fig. 1: Basic concept for an experimental "ZEV with a range of 1000 km" [4]

(appr. 30kW) system (LH2) drive motor

Fig. 1: Basic concept for an experimental "ZEV with a range of 1000 km" [4]

The current state of development of fuel-cell systems leaves several disadvantages: apart from the high costs and low power output, the weight and volume are still too high. Besides the fuel-cell stack and hydrogen supply, the vehicle must also house various auxiliaries needed for operation. Their size is largely determined by the nominal power rating, operating pressure, and maximum operating temperature. For example, the cooling system must be significantly larger than that of an internal combustion engine, owing to the relatively low permissible temperature for the cooling fluid and the reduced quantity of heat which can be dissipated via the "exhaust'.

Liquid hydrogen (LH2) was chosen in order to achieve the greatest possible range, but also because of the purity required by PEM fuel cells. LH2 has a much higher energy density than other forms of hydrogen storage. The alternatives, high-pressure cylinders or metal hydride storage, are so bulky and heavy that they would compromise the vehicle's performance in terms of range, safety and handling to the extent where their use would offer no advantage over conventional battery vehicles. For short-haul traffic, battery vehicles excel themselves through a higher overall efficiency under certain conditions.

The described vehicle concept represents the practical optimum with the available fuel cell technology, taking into account safety and convenience in everyday use. However, the advances made with fuel cells should not let us forget that with the same basic vehicle equipped with conventional technologies at relatively low costs considerably higher payloads and roughly five times the power are achievable, whilst still remaining below emission limits set for the Ultra Low Emission Vehicle (ULEV).

Emission problems with on-hoard generation of hydrogen

Since there is no existing hydrogen infrastructure and the costs of providing it are high, work is also being carried out on systems for generating hydrogen in the vehicle. Of all the chemical fuels which are liquid at normal temperatures, methanol is the easiest to convert to hydrogen. Experiments with gasoline are also under way, with a view to using the existing network of filling stations. Direct on-board reforming of natural gas would be another way to avoid the energy losses involved in first producing methanol.

The development of the necessary reformer systems for hydrogen production concentrates on its behaviour under start-up and transient loads as well as energy efficiency, but must take also into account the by-products which are produced. Even if no immediate poisoning of the electrodes can be detected, the influence of lowest concentrations of impurities over the life of the fuel cell remains hard to estimate. It is also an open question whether a system consisting of fuel cell plus reformer could meet the requirements to be classified as an "Equivalent ZEV" (EZEV). taking into account the necessary overall assessment of emissions for this classification.

To complement BMW activities, Rover is involved in a joint project funded by the EU to develop a system of steam reforming of methanol suitable for use in vehicles. CJB, the partner responsible for developing the reformer, has been operating a stack of Ballard fuel cells with reformer gas since 1993. The project has also accumulated valuable experience in operating individual cells with deliberately contaminated reactants (air and hydrogen).

Concep studies have shown that a large number of additional components must be taken into account when incorporating fuel cell systems with on-board fuel processing into a car [5], In addition to the required purification and cooling of the hydrogen gas before it reaches the fuel cell, it may be necessary to use a buffer store to cope with the power transients experienced during vehicle operation. Due to the resulting adverse effect on payload and useful space, together with the many still unanswered questions on reforming, on-board storage of hydrogen will remain the ideal way of supplying fuel cell systems for the foreseeable future.

From natural gas to hydrogen

Since the late seventies, BMW has been working on the use of LH2 as a fuel for cars. In cooperation with research partners, certification authorities and other manufacturing companies, the experimental vehicles and storage systems have been continuously improved in terms of function, safety, customer acceptance and cost. The cryogenic tanks built into the present BMW experimental vehicles with hydrogen drive system have proved so effective that no insurmountable technical risks can be expected if they were to be put into series production.

In order to accustom today's public to the idea of hydrogen, BMW has decided to take natural gas as an intermediate step. Natural gas consists largely of methane (CHt). After hydrogen itself, it is the fuel with the highest hydrogen content and its physical properties are similar. Both energy carriers are gaseous at normal temperatures, and are the cleanest fuels.

BMW is the first European manufacturer to offer series-production vehicles powered by natural gas [6], The 316g compact and 518g touring models, adapted for dual-fuel operation on gasoline or compressed natural gas (CNG), are international trendsetters. The cars meet the strictest Californian emission values and can be considered amongst the circle of cleanest cars in the world.

Outlook

Efforts to reduce the carbon content of motor fiiels will open up opportunities for new technologies, not just in the field of energy but also in that of motive power. In addition to the versatile spark-ignition engines and besides fuel cells, gas turbines could also become an interesting alternative, so long as their transient behaviour can be improved while retaining the fuel efficiency.

In terms of efficiency and emissions, fuel cell systems are promising not just for passenger cars but also for commercial vehicles and stationary generation of electricity. Working together with capable system deliveres, BMW and Rover are contributing to solve the key problems of fuel cell systems for passenger cars, finding a possible route into mass production.

"Same" source of energy

Compressed natural gas

(immediate use for ~

specific applications) CNG (200 bar)

Compressed natural gas

(immediate use for ~

specific applications) CNG (200 bar)

Liquefied natural gas (long cruising range)

"Same" technology

Liquefied natural gas (long cruising range)

Liquefied hydrogen (long-term availability) LH2 (-250 °C)

Gas turbine?

Electric motor with fuel cell system?

Internal combustion engine

Fig.: 2: From natural gas to hydrogen

References

[1] Ledjeff, K. (ed.): "Brennstoffzellen, Entwicklung - Technologie - Anwendung". C.F. Müller Verlag, Heidelberg, 1995. ISBN 3-7880-7514-7.

[2] Szyszka, A; Schimpf, G.; Tachtler, J.: "Bisherige Erfahrungen mit einer 6,5 kW elektrisch alkalischen und einer 80 kW elektrisch phosphorsauren Brennstoßzellenanlage im SWB Projekt in Neunburg vorm Wald (Stand Dezember 1991)". VDI Berichte No. 912, pp. 147-161. VDI Verlag, Düsseldorf, 1992.

[3] Straßer,K.; Löhberg, R.: "Present PEM FC Technology and ist Potential Role within the SWB Program". First European FC Group Workshop, 26.- 27. Juni 1991, Regensburg, Germany.

[4] Braess, H.-H.; Strobl, W.: "Hydrogen as a Fuel for the Road Transport of the Future: possibilities and prerequisites". Proceedings of the 11th World Hydrogen Energy Conference. Stuttgart, 23-28 June 1996, pp. 1373-1404.

[5] Dams, R.; Adams, K.; Bourne, C.; Smith, J.: "Critical Components for Fuel Cell Hybrid Power Trains". Autotech 1995 Conference, Birmingham, UK,October 1995.

[6] Langen, P.: "Pathway to NGV Markets: an automaker's view". Proceedings of the 2nd Annual European NGV Conference and Exhibition, Basle, 8-10 May 1996.

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