A Microcab urban car. Click to enlarge.

UK-based Microcab Industries Ltd. has ordered 10 Serenus high-temperature (HT) 3kW PEM fuel cell systems from Denmark-based Serenergy A/S for use in Microcab’s next generation of demonstration fuel cell hybrid vehicles.

Microcab employs lightweight construction techniques and fuel cell hybrid powertrains with electric drive in vehicles for light transport operations in urban and suburban areas. Initially Serenergy will supply a system module comprising the fuel cell, its control system, and power-conditioning circuitry to supply the hybrid battery and electric drive. Microcab and Serenergy intend to work closely together to develop this and future automotive applications of the HT fuel cell systems.

Low-temperature PEM fuel cells (LTPEM) operate at a membrane temperature of approximately 80° C. If the temperature greatly exceeds this value, fuel-cell performance breaks down and the fuel cell can be irreparably damaged. Accordingly low temperature fuel cell systems require sophisticated and expensive cooling systems. Furthermore, in LT systems the supply of hydrogen gas and air must be continuously humidified; humidification systems also add additional weight and cost. (Earlier post.)

Serenus HTPEM fuel cells have a higher internal operating temperature of 150 °C or more, enabling them to operate over a wider range of environmental temperatures, and to use less pure hydrogen fuel. The high temperature exhaust also facilitates the use of otherwise wasted thermal energy for heating of the vehicle interior, thus increasing overall system efficiency. Other benefits of the Serenergy HTPEM technology include:

  • No humidifier. There is no need for a humidifier due to the composition of the Membrane Electrode Assembly (MEA) which results in a simpler, cheaper and more reliable fuel cell system.
  • No compressor. There is no liquid water in a HTPEM MEA and therefore there is no need for a compressor. This further reduces system complexity, cost and noise level while greatly increasing fuel cell system efficiency.
  • No radiator. Due to the high operating temperature, heat can easily be removed from the fuel cell system. A simple, small fan can be used for cooling instead. The parasitic loss from the fan reaches a maximum of 3.5%.
  • Patented bipolar plates. The flow-field patent ensures minimal temperature difference over the bipolar plate (< 15 °C). This relatively homogeneous temperature distribution results in prolonged lifetime, higher efficiency, greater power density and an easily controllable system. Moreover, it significantly reduces cathode/cooling pressure loss, directly reducing parasitic power consumption from the blowers.
  • High system efficiency. Partly due to the low parasitic losses, Serenergy says it has achieved fuel cell system efficiencies of up to 57%. Most fuel cell systems have an energy usage of approximately 10-20% of the output, just to achieve operational status. The best LTPEM systems have parasitic losses of 6–8%, but parasitic loss exceeding 10% are common. Serenergy’s parasitic losses are < 4%.
  • Low system cost. The systems are built using a very simple internal architecture reducing the Balance Of Plants (BOP) cost to approximately 25% of the fuel cell stack cost. This is achieved by eliminating a number of traditionally used components such as humidifiers, pumps, compressors and radiators. The fuel cell itself only requires a single air-fan, similar to those used to cool CPUs in computers.
  • Easier to control. Since HTPEM fuel cells can be operated over a broader temperature operating window compared to low temperature fuel cells, they are easier to control.
  • Easier to utilize the heat. The waste heat from the HTPEM system is of high quality, and is concentrated in a usable directional stream. In Serenergy’s case more than 90% of the waste-heat leaves the fuel cell system through the exhaust-pipe and is therefore easy to utilize.

Due to the higher operating temperatures, hydrogen with a higher CO concentration can be used. This makes it possible to directly use hydrogen reformate, originating from inexpensive, and easy to handle energy-carriers such as methanol, ethanol, diesel etc. The HTPEM fuel cell can tolerate up to 3% (30,000 ppm) CO and up to 20 ppm of sulfur without permanent degradation.

In comparison, low temperature PEM cells normally can tolerate less than 30ppm CO and less than 1 ppm of sulfur. Because of the high operating temperature, a PrOx (preferential oxidation) reactor is normally not necessary. PrOx reactors are expensive, bulky and significantly lower the system efficiency. The result is that very simple, lightweight and inexpensive reformers can be used to produce hydrogen from a broad range of energy-carriers.

The waste-heat from the fuel cell can also be employed to evaporate liquid-energy-carriers which boil at a temperature lower than the operating temperature of the fuel cell. Ensuring tight integration between endo- and exothermal processes greatly increases overall system efficiency.

Microcab and its associates will initially manufacture 8 vehicles to the new design, which will be supplied to Coventry University for participation in a 12-month trial as part of the Coventry and Birmingham Low Emissions Demonstrator project.

The Coventry and Birmingham Low Emissions Demonstrator (CABLED) consortium, supported by the UK Technology Strategy Board and Advantage West Midlands, and led by global engineering consultancy Arup, is undertaking a 12-month demonstration of 110 ultra low carbon vehicles and associated infrastructure.

Microcab, assisted by additional funding through Coventry University from the UK Department of Energy & Climate Change, is providing the only hydrogen fuel cell vehicles within the demonstration fleet. CABLED is the largest of eight regional consortia in the Technology Strategy Board’s £25-million (US$37.4-million) UK-wide demonstrator trial of more than 340 ultra low carbon vehicles.