Storage and distribution are the main difficulties to be overcome in developing the onboard applications of hydrogen, notably in the very promising automotive sector. Researchers are making advances, however, notably in solid storage solutions that avoid compression or liquefaction.
The challenge of using hydrogenThe simplest and lightest atom, the most abundant element in the universe. to powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... automobiles is reflected in the following figures: an electric vehicle needs 4.5 kilograms of hydrogen to drive 500 kilometers. At atmospheric pressure, this corresponds to a volume of around 50,000 liters.1/2 The challenge is therefore to manage this volume in all transportation and storage operations, while ensuring maximum safety.
Liquefaction and Compression
LiquefactionConversion of a gas to a liquid. In industrial applications, gas is liquefied by cooling and/or pressurization... is a solution used on an industrial scale, notably in the space and electronics industries, but it requires that the gas be maintained at a temperature of minus 253 °C – something that is difficult in a car, albeit not impossible.
300 grams: the weight of a hydrogen cartridge that can be used for small equipment
Compressed hydrogen has been used in a series of prototypes. Tanks of compressed hydrogen at 350 bar are used to power vehicles such as buses that do not need an extensive driving range and that have plenty of room to accommodate large tanks.
For passenger cars, automobile manufacturers have favored 700-bar tanks. At this pressure, it takes three tanks with a capacity of around 35 liters each to contain 4.5 kilograms of hydrogen, which is enough to deliver a satisfactory driving range, given that a car can travel up to 130 kilometers on one kilogram of hydrogen.
The tanks must, however, be very sturdy, with a liner made of polymer materials to ensure airtightness and a shock-resistant external shell made of carbon fiber, as used in the aviation industry. Carmakers have even conducted stress tests using real bullets. All of this has an impact on weight: it takes a 100-kilogram tank to store six kilograms of hydrogen. And, of course, added weight increases fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant... consumption, which is no small problem.
Solid Storage Solutions
A third possibility – solid storage – has emerged in recent years. Hydrides of titanium, nickel, magnesium and other metals are able to absorb hydrogen and release it later. These materials are used in a powdered form that absorbs the hydrogen rather like a sponge. This approach has three drawbacks, though: the weight of the metallic components, the need for heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... to release the hydrogen and the still-high cost of production. Even so, it is already being used in a number of non-mobile installations, where weight is not a major problem. Research is being conducted to lower the operating temperature and reduce the components' weight.
Miniaturization is a possibility when a large energy capacity is not required. A prototype bicycle, running on a battery and a fuel cellA device that produces electricity by oxidizing a reducing agent (fuel) in one electrode (the anode) and reducing an oxidizing agent in another... , uses small 300-gram cartridges of sodium borohydride that release hydrogen on contact with water.
Bringing Hydrogen Cars to the Market
Strictly speaking, the hydrogen car is a “fuel cell electric vehicle” (FCEV), as opposed to a standard electric car, or “battery electric vehicle” (BEV). Despite these differences, both types of car have an electric motor.
While FCEVs are commercially available, the market for them remains tiny. The Toyota Mirai and the Hyundai ix35 were introduced worldwide in 2015, followed by the Honda Clarity in September 2016. More cautious projects have been undertaken by other carmakers, including BMW, Daimler and Nissan. And a number of custom tests have also taken place. The French postal service, for example, uses small FCEVs in a few regions, while Toyota has been selling fuel cell buses in Tokyo since 2017. In addition, a European project, Volumetriq, aims to develop parts for hydrogen vehicles.
Using a hydrogen car remains difficult, with the following factors to be taken into account:
- Hydrogen filling stations are extremely rare, with only a few hundred currently existing worldwide, notably in Japan (almost 100), California and Germany. At these stations, hydrogen fuel is distributed as a compressed gas. The cost of each station is high due to the need to meet safety standards. This is a drawback, but at a pressure of 700 bar, it takes three minutes to fill a five-kilogram tank, which gives the hydrogen car a clear advantage over electric vehicles, which take much longer to recharge.
- The driving range of a hydrogen car is satisfactory, at 500 to 700 kilometers.
- This type of vehicle’s fuel cell life has been increased to 5,000 hours – with 8,000 a foreseeable possibility – putting it in parallel with a conventional passenger car, which has an average life of 4,000 to 5,000 hours (or 200,000 kilometers at an average speed of 50 kilometers/hour).
- The sticker price of these vehicles is still high, ranging from €50,000 to €80,000.
- Transporting hydrogen to filling stations is tricky. Compressed hydrogen systems are based on expensive compressors and tanks, while liquid hydrogen systems require the use of cryogenics, which poses the challenge of managing the extreme cold chain. However, indepth studies have been carried out by major companies based on the thermos bottle principle, whereby two layers of insulation are separated by a void.
- Advances in solid storage could also allow for a qualitative leap forward in resolving distribution issues.
(1) Ademe (in French only)