UK researchers are developing a zero-emissions plug-in hybrid London Taxi powered by a large high-energy Zebra battery pack and a small 6 kW hydrogen fuel cell stack that essentially functions as a range extender.
Prof. Nigel Schofield (Manchester University), the project leader, is presenting a paper describing the project this week at the IEEE International Electric Machines and Drives Conference (IEMDC) in Austin, Texas.
The purpose of the project is to explore the potential for using a small fuel cell stack in conjunction with a more robust energy storage system—essentially using a plug-in hybrid architecture to downsize the fuel cell stack. This theoretically can reduce the cost and engineering issues attendant with an application that relies upon a fuel cell for primary power.
Accelerating a 2.5-tonne London taxi from a set of lights requires something like 80kW peak power. If you went to a total fuel cell solution, you would have to have 80–100kW worth of installed fuel cell on the vehicle, and the cost, mass and volume of that would not be commercially competitive, as compared to a hybrid solution.—Prof. Schofield (The Engineer)
The project, called ZESTFUL (Zero Emission Small vehicle with Integrated High Temperature Battery and Fuel Cell), is one of the Foresight Vehicle’s supported projects within the Hybrid, Electric and Alternatively Fueled Vehicle (HEAFV) group.
The Foresight Vehicle Initiative is administered by The Society of Motor Manufacturers and Traders Limited (SMMT).
The prototype uses two 40 kW Zebra sodium nickel chloride (Na/NiCl2) batteries and a 6 kW PEM fuel cell stack (two 3 kW units) produced by MES-DEA in Switzerland. Ninety liters of compressed hydrogen gas at 230 bar (3,336 psi) are stored in carbon composite cylinders. The hybrid uses regenerative braking and plug-in to the grid to recharge the battery pack.
Each battery has its own associated charger which connects to 240V, 50Hz supply for overnight or downtime opportune charging.
The Zebra Z5C battery, with its ceramic electrolyte, has an operating temperature of around 300°C (572ºF)—the battery pack thus needs to be enclosed in a thermally insulated box, and is bulky. Hence, it tends to turn up applied in larger vehicles. ISE Corporation, for example, has used Zebra packs in its hybrid buses.
The team ran simulations testing six case studies: four in pure battery mode (full EV), comparing the Zebra and more common lead-acid batteries (Pb-acid), and two assessing the performance of the combined Zebra and fuel-cell hybrid.
Use of the hybrid architecture doubled the range of the standalone Zebra EV from 120 km (75 miles) to 240 km (149 miles).
The electric cab will start track testing later this year.
This project will provide some insight into the long-term viability of a plug-in architecture, and points the way to perhaps a more tractable way to phase-in the use of fuel cells.