Fuel Cell Vehicles

The benefits and challenges of these zero-emission vehicles


Safety and public acceptance

For fuel cell technology to be embraced, concerns about the dependability and safety of FCVs will have to be overcome.

One safety concern is the pressurized storage of hydrogen onboard vehicles. Another is that hydrogen gas is odorless, colorless and tasteless, and thus unable to be detected by human senses. Unlike natural gas, hydrogen cannot be odorized to aid human detection.

HOW THEY WORK

FCVs resemble normal gasoline- or diesel-powered vehicles from the outside. Similar to EVs, they use electricity to power a motor that propels the vehicle. But unlike EVs, which are powered by a battery, FCVs use electricity produced from onboard fuel cells to power the vehicle.

An FCV includes four major components: fuel cell stack, hydrogen storage tank, electric motor, and power control unit and battery.

Here is how officials at the Center for Climate and Energy Solutions (C2ES), an independent, non-partisan, non-profit organization working to advance strong policy and action to address the twin challenges of energy and climate change (www.c2es.org), explained these components:

Fuel cell stack

The fuel cell is an electrochemical device that produces electricity using hydrogen and oxygen. Basically, a fuel cell uses a catalyst to split hydrogen into protons and electrons. The electrons then travel through an external circuit – thus creating an electric current, and the hydrogen ions and electrons react with oxygen to create water.

To obtain enough electricity to power a vehicle, individual fuel cells are combined in series to make a fuel cell stack. The amount of power generated by a fuel cell is determined by several factors, including fuel cell type, size, operating temperature and pressure at which the gases are supplied to the cell.

The most common type of fuel cell used in FCVs is the polymer electrolyte membrane (PEM) fuel cells, also called Proton Exchange Membrane. These use hydrogen fuel and oxygen from the air to produce electricity.

Hydrogen storage tank

Instead of a gasoline or diesel tank, an FCV has a hydrogen storage tank.

The hydrogen gas must be compressed at extremely high pressure at 5,000 psi to 10,000 psi to store enough fuel to obtain adequate driving range. In comparison, compressed natural gas (CNG) vehicles use high-pressure tanks at only 3,000 psi to 3,600 psi.

FCVs can also be powered by a secondary fuel - such as methanol, ethanol or natural gas - which is converted into hydrogen onboard the vehicle. Vehicles powered through a secondary fuel emit some air pollutants during operation due to the conversion process.

Electric motor and power control unit

The power control unit governs flow of electricity in the vehicle. By drawing power from either the battery or the fuel cell stack, this unit delivers electric power to the motor, which then uses the electricity to propel the vehicle.

Battery

Like a hybrid electric vehicle (HEV), fuel cell vehicles also have a battery that stores electricity generated from regenerative braking, increasing the overall efficiency of the vehicle.

Regenerative braking slows a vehicle by converting its kinetic energy into stored energy in a battery, which can later be used to power the electric motor.

The size and type of the batteries, similar to those in HEVs, will depend on the degree of hybridization of the vehicle. That is, how much of the power to propel the vehicle comes from the battery and how much comes from the fuel cell stack.

The development of any new technology often exhibits a “chicken-and-egg” problem, say C2ES officials. Vehicle manufacturers are unwilling to produce vehicles unless there is a guaranteed supply of hydrogen, while hydrogen producers will not supply fuel unless there is a demand for it.

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