Liquid hydrogen (LH2 or LH2) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point of 33 K. However, for it to be in a fully liquid state at atmospheric pressure, H2 needs to be cooled to 20.28 K (−252.87 °C; −423.17 °F). One common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form of hydrogen storage. As for any gas, storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. However, the liquid density is very low compared to other common fuels. Once liquefied, it can be maintained as a liquid in pressurized and thermally insulated containers.
As farming comes under increasing pressure to go green, jumping through the many confusing legislative hoops to meet net-zero targets can often feel like rocket science. According to nanomaterials specialist and space station engineer Vivek Koncherry, this may be exactly what the industry needs – particularly when it comes to the future of farm machinery. Based in Manchester, Dr Koncherry heads up several projects at the Graphene Engineering Innovation Centre, and is currently working on creating hydrogen storage systems to be used in habitable space stations destined for life beyond Earth.
Liquid hydrogen is a common liquid rocket fuel for rocketry applications — both NASA and the United States Air Force operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons). In most rocket engines fueled by liquid hydrogen, it first cools the nozzle and other parts before being mixed with the oxidizer — usually liquid oxygen (LOX) — and burned to produce water with traces of ozone and hydrogen peroxide. Practical H2–O2 rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust, which can actually increase specific impulse, despite the incomplete combustion.
Liquid hydrogen can be used as the fuel for an internal combustion engine or fuel cell. Various submarines (Type 212 submarine, Type 214 submarine) and concept hydrogen vehicles have been built using this form of hydrogen (see DeepC, BMW H2R). Due to its similarity, builders can sometimes modify and share equipment with systems designed for liquefied natural gas (LNG). However, because of the lower volumetric energy, the hydrogen volumes needed for combustion are large. Unless direct injection is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses.
Liquid hydrogen is also used to cool neutrons to be used in neutron scattering. Since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum (elastic collision). Finally, superheated liquid hydrogen was used in many bubble chamber experiments. These same hydrogen technologies can be used in farm machinery to replace combustion engines and cut emissions, without the complications that come hand-in-hand with heavy electric vehicle batteries. Making the farming sector the test pilot for his space-bound projects, Dr Koncherry says that hydrogen is one of the most abundant chemical elements in the universe, accounting for around 75% of its overall mass.
Retrofit fuel for tractors
Hydrogen is one of the most promising resources in a carbon-neutral future, but, sourcing and containing it still poses several challenges. Mechanically, the technology for running hydrogen vehicles and machinery already exists, and Dr Koncherry has been working on a system that can be retrofitted to tractors. The setup will include hydrogen fuel cells, tanks, a small battery, and an electric motor to create what is known as a fuel-cell electrical vehicle, or FEV. “There are tractor manufacturers I’ve spoken to that tried hydrogen maybe 10 years ago, and the problem was not to do with making the tractor, or even the associated prices for farmers,” says Dr Koncherry.“The issue was about how the farmer can source the hydrogen to fuel the machine, and this is still the problem that we need to solve today.” The element is derived from three main sources – fossil fuels, gas, and renewables, known as grey, blue, and green hydrogen, respectively. The issue is that producing the grey and blue types carries a worrisome carbon footprint, and the eco-friendly green hydrogen is both hard to come by and hard on the pocket, with costs that will unfortunately be passed to the farmer.
Storage challenges
Further challenges arise in trying to store and contain it, as hydrogen can leak out of ordinary metal storage containers. This causes a logistical nightmare when it comes to infrastructure, as without a means of storing and containing the hydrogen, it has not been considered a viable option. “This is why we use graphene in our storage vessels,” explains Dr Koncherry. “Graphene is a single layer of graphite, that is 200 times stronger than steel, and is the best electrical and heat conductor. In the case of pristine graphene, even hydrogen cannot pass through it.” Engineering these graphene containers has proven an enormous step forward in making hydrogen a more accessible and viable fuel source for both agriculture and the automotive industry.
Invented Hydrogen vehicles
Though infrastructure in the UK is still lacking, the system has been proven in countries such as South Korea and Japan, where sales of hydrogen-powered vehicles have reached the tens of thousands. At the moment, the hydrogen-fuelled movement is being led overseas, where the means for storage, transportation, filling and pumping have already been established, But the UK remains at the forefront of research and development. “We have the cutting-edge technology in the UK to make it a reality, but we need to commercialise it faster. “There are companies working around the world to reduce the cost of green hydrogen, but they are but not quite there yet” said Dr Koncherry. The product of hydrogen combustion in a pure oxygen environment is solely water vapor. However, the high combustion temperatures and present atmospheric nitrogen can result in the breaking of N≡N bonds, forming toxic NOx if no exhaust scrubbing is done. Since water is often considered harmless to the environment, an engine burning it can be considered “zero emissions”. In aviation, however, water vapor emitted in the atmosphere contributes to global warming (to a lesser extent than CO2). Liquid hydrogen also has a much higher specific energy than gasoline, natural gas, or diesel. The density of liquid hydrogen is only 70.85 g/L (at 20 K), a relative density of just 0.07. Although the specific energy is more than twice that of other fuels, this gives it a remarkably low volumetric energy density, many fold lower. Liquid hydrogen requires cryogenic storage technology such as special thermally insulated containers and requires special handling common to all cryogenic fuels. This is similar to, but more severe than liquid oxygen. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away (typically at a rate of 1% per day). It also shares many of the same safety issues as other forms of hydrogen, as well as being cold enough to liquefy, or even solidify atmospheric oxygen, which can be an explosion hazard.
