Zinc-Air Hybrid Buses Get Closer to Market
3 November 2004
Arotech Corporation’s Electric Vehicle Division has successfully run its zinc-air hybrid electric bus prototype with fuel cells using commercially available zinc rather than the proprietary dendritic zinc required to this point. This could boost the prospects for commercializing the zinc-air hybrid bus, which is now in the fourth and final phase of its development program.
The 40-foot, electric-hybrid bus uses Arotech’s zinc-air fuel cells as the primary power source combined with a pack of ultracapacitors recharged through regenerative braking for boosting acceleration. The bus, which uses a General Electric 200 hp liquid-cooled induction motor, has a range of 133 miles on a simulated city-cycle drive.
The Arotech zinc-air fuel cell modules contain 47 individual air-breathing zinc-air cells connected in series. The bus carries three trays of 6 modules each, for a combined on-board energy capacity of 312 kWh.
Each cell consists of a replaceable anodic fuel cassette of zinc particles in an electrolyte solution of potassium hydroxide (KOH). The cassette is flanked on two sides by high-power oxygen reduction cathodes.
Oxygen from the air reacts with the cathode to yield hydroxide ions. These in turn react with the zinc fuel inside the cell, producing zinc oxide and energy. The on-board Zinc-Air fuel cell has an energy density of around 200 Wh/kg and power density of 90 W/kg at 80% depth of discharge—better in energy density than competing types of batteries as shown in the table below, but weaker than most when it comes to specific power output.
Energy density is a measure of how much energy can be stored per unit weight; for a vehicle, this affects range. Power density is a measure of the amount of power released per unit weight; for a vehicle, this affects acceleration. High energy density and high power density deliver long range and great performance, for example. High energy density and low power density provide long range but slower acceleration. Gasoline, by way of comparison, has an energy density of approximately 13,000 Wh/kg (13 kWh/kg).
The United States Advanced Battery Consortium (USABC), established to develop the next generation electric vehicle batteries, has set long-term performance goals for EV batteries of an energy density of 200 Wh/kg and a power density of 400 W/kg. (Members of the USABC include the US Big 3, the Electric Power Research Institute, battery manufacturers, and the DOE.)
To offset the lower power density, Arotech used the hybrid design to boost acceleration through the use of the ultracapacitors.
|Types of HEV Batteries|
|Battery type||Energy density (Wh/kg)||Power density (W/kg)|
|Lead-Acid||25 to 35||75 to 130|
|Advanced Lead-Acid||35 to 42||240 to 412|
|Nickel-Metal Hydride (NiMH)||50 to 80||150 to 250|
|Nickel-Cadmium (NiCd)||35-57||50 to 200|
|Lithium-Ion (Li-Ion)||100 to 150||300|
|Lithium-Polymer||100 to 155||100 to 315|
|Zinc-Air||110 to 200||100|
|USABC long-term goal||200||400|
Instead of recharging depleted zinc-air fuel cells on-board, the operator exchanges spent modules for new ones. The depleted fuel cassettes are electrochemically recharged at a regeneration facility, using off-peak electricity if possible, mechanically recycled and returned to duty.
Arotech is also involved in a German research project to develop a hybrid vehicle using Zinc-Air batteries as the primary energy source. The vehicle design is based on a Daimler Chrysler cargo van and will also use high power Varta batteries and ultracapacitors under development by Dornier GmbH and EPOC AG.
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