January 19, 2021. By Kolemann Lutz
Hydrogen has a very low volumetric energy density, making it a difficult substance to store and transport. To move large amounts of hydrogen it must be either liquefied or pressurized and delivered as a compressed gas. Therefore, hydrogen is typically transported via pipeline infrastructure, super-insulated cryogenic trucks, or by rail or barge. For fuel cell vehicles, it is dispensed in a fashion similar to gasoline at filling stations.
However, a hydrogen storage material or chemical carrier is not necessarily required to move
large quantities of hydrogen. “You want to transport hydrogen in the densest possible phase,” says Øivind Wilhelmsen, who is researching hydrogen liquefaction at the Norwegian University of Science and Technology. Five boats of 200 bar (2,900 psi) hydrogen gas would amount to the same amount of hydrogen as one boat of liquified hydrogen, reducing energy and cost requirements.
Cryogenic liquid hydrogen (LH2) has a density of 71 kg/m3, which is nearly twice the density of compressed hydrogen in the gaseous state. Hydrogen gas liquefies at –253°C (−423°F) and can be stored at a liquefaction plant in large insulated tanks. While cooling liquefaction is expensive and consumes over 30% of the energy content of the hydrogen, there are many efficiencies to be found by overcoming the quantum effects that come into play at very low temperatures. Current estimates of energy requirements for liquefaction are around 12.5-15.0 kWh/kg of hydrogen compared to 6.0 kWh/kg for compression to 70 MPa (10,152 psi).
Molecular hydrogen occurs in two spin isomer forms, each with identical molecular formulas but different arrangements of atoms. These two forms are often referred to as spin isomers. Parahydrogen occurs when two protons spin opposite, forming a lower, more stable energy state, compared to orthohydrogen, where the two protons spin parallel relative to each other.
Around 75% of dihydrogen is in the higher-energy ortho state at room temperature. As the equilibrium shifts when the temperature is lowered, the dihydrogen is almost 100% parahydrogen at equilibrium. “If you just cool down the room-temperature equilibrium mixture, it is going to convert slowly in the tanks and release a lot of energy, and cause the hydrogen to evaporate very quickly,” Wilhelmsen says. “It is a huge amount of energy you didn’t remove from the hydrogen.”
To speed the ortho-para transition during cooling, catalysts can be used in the heat exchanger, a system used to transfer heat or cool hydrogen steam between two or more fluids, to liquefy the gaseous hydrogen. A significant part of the exergy destruction, or loss of useful work or resources, in the hydrogen liquefaction process is caused by the cryogenic heat exchangers. As ferric oxide catalysts are relatively slow, the researchers found that a nickel oxide (NiO-Si) silica catalyst doubled the catalytic activity compared to the relatively slow ferric (fe III) oxide catalysts and reduced the exergy destruction from the ortho-para hydrogen conversion by 9%.
Wilhelmsen and researchers discovered an alternative approach: a spiral-wound heat exchanger or plate-fin heat exchanger that can liquefy the hydrogen more slowly to cryogenic temperatures, which enables greater time for catalyst performance. When a plate-fin heat exchanger is used to cool the hydrogen from 47.8 K to 29.3 K using hydrogen as refrigerant, researchers found that the two main sources of exergy destruction are the temperature difference between the hot and cold layers and ortho-para hydrogen conversion, which are responsible for 69% and 29% of the exergy destruction respectively.
The most energy and cost efficient known processes use a mix of coolants instead of primarily nitrogen to precool the hydrogen. As cryogenic fluid approaches –253°C, the only coolants that won’t freeze are hydrogen, helium, and neon. One of the challenges is that the fluids behave strangely at cold temperatures due to quantum effects. The particles behave more like a wave and the properties deviate from modern understanding of physics as the equation of state for normal fluids does not apply, which represents a challenge to understand fluid properties and design tailored cryogenic compressor equipment.
After the experiment, the researchers learned that an evaporating Nelium (neon-helium) mixture at the cold-side of the heat exchanger reduced the overall exergy destruction by 7%. A combination of hydrogen at high temperatures and Nelium refrigerants at low temperatures enabled the researchers to reduce the exergy degradation by 35%. Moreover, a combination of improved catalyst and the use of hydrogen and Nelium (Nm) as refrigerants reduced exergy destruction in the cryogenic heat exchangers by 43%.
The liquefaction of hydrogen gas demands around 12 kWh of power for each kilogram of hydrogen, which is about 25% of the energy hydrogen releases in a fuel cell. “Our target is to get below 6 kWh, a reduction of 50% in energy,” Wilhelmsen says.
The storage and transportation of imported liquid hydrogen is key to producing methane CH4 propellant, manufacturing downstream chemicals, fuel cells, water electrolysis, and more to develop self-sustaining settlements on the Moon, Mars, and Venus.
This research will help recover hydrogen mass, fuel, and energy cost savings, while overcoming the quantum effects that come into play at very low temperatures, to design optimal liquid hydrogen storage and production facilities for long term cryogenic storage throughout the Solar system.
Research Paper. ØivindWilhelmsen, et al. Reducing the exergy destruction in the cryogenic heat exchangers of hydrogen liquefaction processes. International Journal of Hydrogen. January, 2018. doi.org/10.1016/j.ijhydene.2018.01.094