Hydrogen fuel cell vehicles, like this Honda FCX Clarity, are propelled by an electric motor that runs on electricity generated in a fuel cell, with the only emission being water.
Photo credit: Photo courtesy of American Honda Motor Company
The Mercedes-Benz B-Class F-Cell Hydrogen Electric Drive uses a 136-hp electric motor that generates 214 ft/lbs of torque and has a range of about 240 miles.
Photo credit: Photo courtesy of Daimler
The major components of a typical fuel cell vehicle.
Photo credit: Image courtesy of American Honda Motor Company
The U.S. Army is testing fuel cell vehicles in Hawaii to determine their practicality for military operations.
Hydrogen fuel cell technology has been available for decades. The concept of a fuel cell was developed in England in the 1800s. However, the first workable fuels cells were not produced until the 1950s. Fuel cell technology is being used to power spacecraft, emergency power generators, cell phones, laptops and more. The technology has only been viewed as a viable alternative power source for vehicles over the past few years.
A number of vehicle manufacturers and component suppliers are pursuing hydrogen-powered fuel cell propulsion instead of a standard internal combustion engine alongside other alternative technologies. Among them are Daimler, General Motors, Honda, Hyundai, Nissan and Toyota.
One of the reasons for this interest is that fuel cell vehicles (FCVs) have a significant potential to reduce emissions from the transportation sector. Unlike gasoline- and diesel-powered vehicles, they do not discharge any greenhouse gas (GHG) emissions during vehicle operation.
In a fuel cell, the chemical energy bound in the hydrogen is directly transformed into electrical energy. Power is generated onboard the vehicle through a chemical process using hydrogen fuel and oxygen in the air to yield electricity, heat and water.
What’s more, extracting hydrogen from fossil fuels creates only a tiny amount of pollution.
FCVs could reduce the nation’s petroleum dependence since hydrogen can be derived from domestic sources, such as natural gas, coal and biomass. That would make the U.S. economy less dependent on other countries and less vulnerable to oil price shocks from an increasingly volatile oil market.
Fuel cells are inherently efficient. Fuel cell drivetrains use about 40 to 60 percent of the energy available from hydrogen, compared to internal combustion engines which use only about 20 percent of the energy from gasoline, although this is expected to improve over the long term.
Similar to an electric vehicle (EV), an FCV has quick starts due to high torque from the electric motor and low operating noise.
NUMBER OF CHALLENGES
There are a number of challenges that must be overcome before fuel cell vehicles can be a successful, competitive alternative to traditionally powered vehicles. These challenges include:
Onboard hydrogen storage
Some FCVs can store enough hydrogen to travel as far as gasoline vehicles between fill-ups - about 300 miles - but this must be achievable across different vehicle makes and models, and without compromising customer expectations of space, performance, safety or cost.
FCVs are more energy efficient than conventional cars. Hydrogen contains three times more energy per weight than gasoline does.
However, hydrogen is a very light gas that contains only a third of the energy per volume gasoline does, making it difficult to store enough hydrogen to go as far as a gasoline vehicle on a full tank – at least within size, weight and cost constraints.
Getting hydrogen to consumers
There is currently no national system to deliver hydrogen from production facilities to filling stations like there is for diesel or gasoline. Nor can this extensive system be used to deliver hydrogen.
Consequently, a completely new distribution infrastructure for producing, transporting and dispensing hydrogen will be required to allow mass market penetration of FCVs.
Additionally, more development of low-cost and low-GHG hydrogen production methods will are needed.
FCVs are currently more expensive than conventional vehicles and hybrids. Manufacturers will need to bring down production costs, especially the costs of the fuel cells and hydrogen storage, to compete with conventional technologies.
Fuel cell durability and reliability
Fuel cell systems are not yet as durable or robust as internal combustion engines, especially in some temperature and humidity ranges.
Fuel cell durability in real-world environments is currently about half of what is needed for commercialization. Durability has increased substantially over the past few years from 29,000 miles to 75,000 miles, but experts believe a 150,000-mile expected lifetime is necessary for FCVs to compete with gasoline 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.
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.