How do fuel-cell vehicles work?

The principles of fuel cell vehicles are the same, whether the vehicle is Honda's FCX Clarity, Toyota's experimental FCHV or Benz's F-Cell.

Air and hydrogen are combined to form water and this chemical process generates an electrical charge. The electrical power is stored in a battery for later use or diverted directly to a motor that converts the electricity to motive power. While the technology relies on precious metals and is costly to develop and build, it's actually a simple means of generating zero-greenhouse-emissions power onboard a car.

There are few moving parts and the electric motor that propels the car produces plenty of torque from low revs and adequate power at high revs -- higher than most internal combustion engines are capable of achieving.

During a visit to Honda's Erlensee technical training centre in Germany, the Carsales Network met with the company's Senior Engineer for Advanced Technology Research, Thomas Brachmann (pictured at the wheel of the company's advanced fuel cell vehicle, the FCX Clarity). Brachmann outlined for us the pros, cons and complexities of developing -- and commercialising -- the technology for consumers.

Pull over in Mittagong and hook up the car's battery to a quick recharging unit; take the family for a quick mid-morning bite and return an hour later to carry on your journey. The quick charger can top up the battery about 80 per cent -- enough to get you as far as Goulburn before needing another top-up. According to Brachmann, once they're warm from constant running, the batteries take significantly longer to recharge, perhaps as long as an hour.

What is a ten-hour trip in a diesel-engined car could end up taking 15 hours in a BEV with hour-long recharge delays for every hundred to 140km travelled. To refuel Honda's FCX Clarity with a load of hydrogen at 300bar pressure takes three to four minutes and the car has a range of around 400km or better. Two fuel stops along the way for about the same running time as for the internal-combustion car -- and the fuel stops are incidental to lunch and toilet stops you would have to make anyway.

Fair enough, no one would ever undertake such a long journey in a battery-electric vehicle with current battery technology, but with the appropriate refuelling infrastructure along the Hume Highway -- hydrogen highways already exist in other parts of the world -- fuel cell vehicles would be an entirely practical solution to the demands of long-distance touring in this country.

"Why is it so difficult for battery-electric vehicles to... achieve certain goals?" asks Brachmann rhetorically. "The difficulty is simply electro-chemical. For battery-electric vehicles it's energy density which is of importance. That means range. To store lots of energy is difficult..."

"Comparing [the energy density of batteries] with the fuel cell system... which means tank plus stack -- those two components, in terms of weight, compared with battery weight, gives us a factor of 3.8 better.

"We can also put it the other way around... we have to at least add a minimum of 400kg of batteries, in order to come close to the [fuel cell's energy density].

"Therefore, we believe that for larger cars, long distance driving, the fuel cell is the only alternative."

It's not such a consideration in Australia, but fuel cells are proving to be more reliable than batteries in cold climates -- in temperatures down to -30 degrees Celsius.

EV builders are frequently talking about how electric vehicles plugged into the national power grid could supply power to the grid in reverse, during hours of peak demand, but this could leave the car with insufficient battery charge. Furthermore, said Brachmann, the constant charge-discharge-recharge cycle could degrade car batteries faster than anticipated.

"The acceleration and flexibility on autobahn driving has been improved," said Brachmann.

Honda has also worked out a way to compare the Clarity's fuel consumption that parallels the NEDC standard for petrol and diesel-powered cars.

"Fuel economy in the New European Driving Cycle is 117km per kilogram, but that doesn't give you anything -- so we converted this kilogram of hydrogen to something comparable to diesel economy...

"If you take this one kilogram we use, this is very much similar to 2.8L/100km of diesel."

That figure is less than half the 6.4L/100km fuel use of a comparable diesel-engined vehicle. Brachmann confirmed that the basis for this comparison was megajoules per unit of weight.

Well to wheel emissions
Manufacturers can mount a case for battery-electric vehicles in a state like Tasmania, where power generation is from clean, hydro-electric schemes. Water spilling from a dam drives generators to produce electricity. There's no burning of non-renewable brown coal, as is the case in Victoria.

In Tassie, electric cars are arguably true zero-emissions vehicles. In Victoria, the CO2 produced by burning coal to power an electric vehicle is roughly equivalent to the amount of CO2 produced by Toyota's Prius.

The example Brachmann provides is Honda's EV-Plus experimental battery-electric vehicle, which, powered by electricity supplied from California's natural-gas-fired power stations, produces broadly the same CO2 as the CNG-fuelled Civic GX -- a car that's powered by a conventional internal-combustion engine.

"Unfortunately, the powerplants in California are running on natural gas, and their pollution was much, much higher than the exhaust gas or tailpipe emissions of this car [the Civic GX] using natural gas," Brachmann explained.

And unlike diesels, fuel cell vehicles only discharge water from their exhaust pipes. There are no diesel particulates or CO2 emissions operating a fuel cell vehicle.

It's the PEM that is the key component in the electrolytic process, melding hydrogen with air to form water and generating an electric charge at the same time. The humidifier keeps the PEM moist for a long service life. Without the humidifier, the PEM would dry out and need replacing in a short space of time.

"The operating range of the fuel cell stack itself has been improved from -20 degrees C to -30 degrees C, up to 95 degrees C, from previously 80 degrees C. That's why I said we would like to have high-temperature membranes, because the higher the temperature tolerance of the stack, the less cooling performance we need."

Storing the hydrogen becomes a design challenge in any fuel cell vehicle. Toyota overcame this by opting for four tanks under the rear of the (front-wheel-drive only) FCHV, based on the Kluger/Highlander SUV. By being a sedan rather than an SUV, the Honda Clarity adds packaging constraints to the question of a tidy hydrogen storage installation. Honda has been able to reduce the development and production cost of the FCX Clarity by using just one tank instead of the four in the FCHV or the two in Honda's own FCX prototype.

"On our previous [prototype FCX], we had two tanks because... we had to integrate those two tanks in the previous EV-Plus body," said Brachmann.

"For the new dedicated body of the FCX Clarity, we have more freedom... This now has 171 litres of volume, with about 4.1kg of hydrogen at 350bar."

Brachmann explained that halving the number of valves and other storage tank parts, Honda has also reduced the cost of manufacturing. That cost will reduce further as economies of scale impact on the price of parts, but it's a Catch-22 situation. The car has to go into production and Honda must procure the parts by the thousands before they'll see any significant decrease in price. It can't go into production if it won't sell and it won't sell if it's priced too high. It won't come down in price if the cost of its parts are too expensive... And so it continues...

That need to reduce cost has been balanced by Honda against packaging efficiency. Two tanks would have left more boot space in the FCX Clarity. There is one further advantage to the single-tank model -- in the event of a crash, all valves close immediately to prevent the hydrogen leaking from the tank and fuelling a fire, although Brachmann believes hydrogen is a fundamentally safer gas than LPG, as another example of a gas used in automotive applications. Hydrogen is lighter than air and dissipates quickly in the open, whereas the gas that fuels Australia's taxi fleet is heavier than air and wells up inside cars.

"[It's] the same for natural gas, which is also lighter than air, so we have an advantage, compared to LPG... which is heavier..."

LPG-fuelled cars are forbidden from entering multi-storey car parks in the Netherlands and Belgium, according to Brachmann, for the very reason that such cars pose a theoretical risk.

So the hydrogen tank onboard the Clarity won't make it a 'Hondaberg'.

Read also our drive impressions of the FCX Clarity.

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