There are two clear advantages when using hydrogen as a fuel: it can be produced from any energy source and it emits no or few pollutants during use. There is nothing new about the idea, but its large-scale application in the transport sector, particularly in the automotive market, remains hampered by a number of scientific, technological and (especially) economic challenges yet to be overcome. Significant research activities are under way to find efficient and relevant solutions based on battery approaches alone. But the widespread deployment of this solution is envisaged in the long term.
- Hydrogen fuel
- Hydrogen and the internal combustion engine
- Hydrogen and the fuel cell
- The economic and technological challenges
- The onboard storage problem
- The outlook for the hydrogen and fuel cell vehicle
Since very little pure hydrogen exists in the natural state (it is mostly found combined with oxygen in water or with carbon in hydrocarbons), an energy source is required to extract it. The environmental footprint of a particular hydrogen technology depends primarily on the energy source used.
Today, 95% of hydrogen is produced from hydrocarbons (oil, natural gas and coal). However, this process emits CO2, a greenhouse gas. Industrial players are increasingly examining the possibility of producing hydrogen via electrolysis using low-carbon energies. But the obstacle remains the associated production costs, which are considerably higher than those of natural gas reforming, the most widely employed method.
The physicochemical characteristics of hydrogen make it a good candidate for use in a “gasoline”-type spark ignition engine. The principal advantage lies in the environmental footprint: combined with oxygen, hydrogen combustion mainly produces water and heat, only emitting nitrogen oxides (NOx) in the process. However, this solution requires specific adaptations to obtain a high level of efficiency and ultra-low NOx emissions. In particular, there is a need to exploit the different properties of hydrogen, such as its fast-burning capacity in a lean mixture.
The use of hydrogen in an internal combustion engine may benefit from the latest advances in IC engine technology and combination with a hybrid powertrain. Hence using technologies that are more robust and mature than those currently employed for fuel cells, it would be possible to achieve an efficiency in excess of 50%. This could be a transition solution towards a fuel cell since it makes it possible to validate the entire hydrogen production and distribution chain using existing industrial production facilities.
For the long term, car manufacturers are also focusing on fuel cells as power generators for electric vehicles. In doing so, they are seeking to consolidate solutions for battery-powered electric vehicles, currently hampered by limited range and the time required to recharge the batteries. Hydrogen can be used as a fuel for a fuel cell - which produces electricity - to allow the electric motor to operate and propel the vehicle. Hydrogen is one of the best energy vectors for fuel cells today in terms of energy and emissions performances. Their overall efficiency is above 50% over a broad working range, representing a significant improvement compared to existing gasoline IC engines.
Powered by a combination of air and hydrogen, the battery converts the chemical energy from hydrogen into electric energy according to a reverse electrolysis principle. By causing hydrogen to react with oxygen on electrodes (thin membranes covered with a catalyst, platinum), fuel cells can be used to produce electricity, the only emission being steam. The principle dates back to 1839! It has long been used to generate electricity onboard rockets.
Proton exchange membrane fuel cells (PEMFC) are the most suited for use in the transport sector. Car manufacturers are focusing most of their research efforts on this type of cell.
Before the large-scale roll-out of hydrogen cars can be envisaged, researchers are going to have to overcome numerous technical and economic challenges to make the solution (production, distribution and onboard use of hydrogen) efficient and safe throughout the chain.
- Developments and investments will be necessary to transport and store this fuel onboard vehicles and ensure safe operating conditions.
- The use of hydrogen will require the creation of an entirely new production, transport and distribution infrastructure. Production costs and (above all) storage and distribution costs for this gas - volatile and difficult to “contain” - remain extremely high.
- The costs of the hydrogen chain today are far higher than those associated with oil-based fuel. In 2019, it costs between €8 and €9 (excluding taxes) to produce and distribute a kilogram of hydrogen via the electrolysis of water. A target of less than €2 to €3 per kg by 2050 is hoped for but will doubtless be difficult to achieve.
The cost price of the fuel cell, including the onboard pressurized storage tank, is also extremely high: even mass-produced, the target cost for the period 2030-2040 will still be between 3 and 5 times higher than that of a conventional engine. The replacement of materials such as platinum, reserves of which are very limited, will represent another challenge to be overcome.
Hydrogen is a very light gas and in order to obtain the same quantity of energy as a conventional liquid fuel, it has to be compressed considerably to reduce its volume. This represents a problem for both storage and transport.
In vehicles fitted with a fuel cell marketed in 2019, hydrogen is primarily stored in compressed form at a pressure of 700 bars for private vehicles, and 350 bars for heavy trucks and buses.
Private vehicles fitted with a fuel cell, marketed in 2019, have a range of between 500 and 600 kilometers, obtained thanks to a tank containing around 5 kg of hydrogen (30 kg of hydrogen are required for a range of around 300 kilometers for a bus).
With the urgent need to tackle greenhouse gas emissions, hydrogen represents a complementary avenue to vehicle electrification alongside the biofuel approach. But it is an alternative that will require time, technological progress and considerable investments in order to achieve cost levels that are compatible with mass-market roll-out.
Manufacturers are reckoning on a timescale of at least 20 to 30 years for these vehicles to reach a significant market share for private vehicles, with potentially a slightly faster roll-out in the freight (road or otherwise) sector. Until then, and probably beyond, the IC engine will continue to be the dominant powertrain while hybrid and electric vehicles will have started to acquire a significant market share.