THE Earth’s supply of crude oil will run out. This, coupled with climate change and the need for a cleaner environment, is a compelling factor that has forced automakers to consider alternative energy solutions if they plan to survive beyond the next 50 years and continue to supply us with a means to personal mobility.
To understand the concept of powering a vehicle using the smallest molecule – hydrogen – I recently travelled to Germany to study the Toyota Mirai, a new production vehicle that represents the latest in hydrogen fuel-cell vehicle (FCV) technology (to find out how the vehicle drives, flip back to page 40).
The fuel
Hydrogen (H2) is the lightest known gas and is colourless, odourless and non-poisonous. However, it is not found in its pure form on Earth and has to be produced. One production method is to use fossil fuels through a process called steam reforming, but this does not make sense from an environmental or energy efficiency point of view. A better option is to produce H2 from biomass or excess energy that are available at industrial plants.
According to Toyota, the most appropriate and efficient production method is the electrolysis of water, where the bond between the hydrogen and oxygen molecules is broken – but this requires energy. The input energy of this process is electricity, preferably generated from renewable sources such as wind or solar power. Excess electricity during off-peak hours can also be used for “greener” H2 production.
What this essentially means is that H2 should not be viewed as an energy source, but rather an energy carrier. Thanks to the law of physics pertaining to the conservation of energy, H2 production requires more energy than can be retrieved from the gas as a fuel. Storing and transporting H2 is tricky, as the small molecules tend to escape from any container over time. It also accelerates the fatigue and embrittlement of metal pipes.
Fuel stack operation
Chemical process:
2H2 + O2 –> 2H2O + energy
The smallest unit of a fuel stack is called a cell and consists of positive and negative electrodes, as well as two separators (see graphic on page 112). Supplying H2 to the negative electrode and oxygen to the positive electrode generates electricity. A catalyst on the negative electrode causes the electrons from hydrogen to be released. The hydrogen ions then move through the membranes to the positive electrode where they join with the available oxygen to form water – the only by-product during the process.
As the individual cells are tiny and generally only have a small output of around one volt, hundreds of cells need to be joined in series to increase the overall voltage of the fuel stack and power the vehicle. In the case of the Mirai, its fuel stack currently has the highest power-per-volume-density in the market – 3,1 kW/litre. The 370 cells produce 650 V and up to 114 kW peak (nett) power. The stack is therefore able to supply the electric motor continuously at peak power. The power output is controlled by varying the H2 and oxygen supply to the fuel cell. In terms of delivering power, response time from the fuel stack is better than that of an internal-combustion engine.
The electric motor
The Mirai has a permanent magnet, synchronous motor delivering 113 kW and 335 N.m (it’s shared with the Lexus RX450h). The motor is connected to the front wheels through a single-speed reduction gearbox (3,48 to 1 ratio) and via a differential. Reverse gear is achieved by changing the direction of rotation – a process similar to the one used by a pure electric vehicle. As electric motors deliver almost maximum torque at zero rotational speed, no clutch is needed and the drivetrain coupling is fixed.
The hydrogen tanks
For packaging reasons, the Mirai has two cylindrical storage tanks. In order to store enough H2 in gas form to achieve the claimed 550 km NEDC range, it has to be compressed to 700 bar. Toyota has developed its own tanks that have been manufactured from carbon-fibre and lined with plastic to achieve the desired result.
The tanks are rated to with-stand pressure up to 1 575 bar as a safety precaution. To prove that the tanks would not explode when punctured, they were shot at with high-velocity weapons. Unfortunately, all this safety carries a mass penalty and the total storage system weighs 88 kg but can carry only 5 kg of H2.
The battery
Interestingly, the Mirai is actually a hybrid vehicle because it carries a 1,6 kWh, nickel-metal hydride battery sourced from the Camry hybrid sold abroad. This small battery helps power the vehicle when acceleration is required and can also store excess energy produced by the fuel stack, or via regenerative braking. The battery and fuel stack are connected in parallel, enabling either to supply power to the electric motor as commanded by the energy control module.
Is the technology safe?
H2 got a bad rap in 1937 – 36 people died when the hydrogen-filled German airship Hindenburg went up in flames during an attempted landing. However, technology has moved on since then and the Mirai meets and exceeds all relevant safety regulations.
Apart from the normal crash-test procedures, Toyota also conducted a further test where a stationary Mirai was rear-ended by another vehicle that was travelling at about
80 km/h. The test data showed that the hydrogen system’s main supply valve was shut off milliseconds after the vehicle hit the rear bumper of the Mirai and just before major deformation took place. The hydrogen tanks were removed from the wreckage and showed no signs of structural damage. According to Toyota, its hydrogen vehicle is as safe as its fossil-fuelled vehicles.
The future
The fact that Toyota, and other carmakers, invest heavily in hydrogen-powered fuel-cell vehicles is proof that the technology has a chance to succeed and establish hydrogen as an alternative fuel. The biggest concern surrounds the production, storage and transportation of hydrogen. On our visit to Hamburg, we were shown that most of these challenges can be overcome, but this requires major capital outlay from governments and private equities. Toyota predicts a slow uptake of the technology and a sales forecast of only 50 to 100 Mirais next year. We are, therefore, still some way off a hydrogen-powered society, but at least the signs look promising that our
beloved private transport will still exist in the next 50 years.