Hydrogen is a promising energy carrier, particularly for decarbonizing transportation. However, storing and transporting it is challenging due to its physical properties. At standard atmospheric temperature and pressure, hydrogen gas has an extremely low volumetric energy density, meaning it occupies a large volume relative to its energy content. To make hydrogen practical for widespread use, it must be significantly compressed or cooled to increase its density.
Why Hydrogen Requires Compression
The need for compression arises from the difference between hydrogen’s gravimetric and volumetric energy densities. Hydrogen has the highest energy content per unit of mass of any fuel, about three times that of gasoline. Despite this high gravimetric density, its low volumetric density means a tank holding the same energy as a tank of gasoline would need to be roughly four times larger, even if the hydrogen were liquid.
This impractical volume requirement for gaseous hydrogen storage is the primary hurdle for mobile and stationary applications. Compression reduces the gas volume, increasing the energy stored within a fixed container size. For instance, compressed hydrogen at 700 bar still has a volumetric energy density significantly lower than gasoline. Therefore, increasing density via compression or liquefaction is essential for achieving usable driving ranges or storing industrial quantities.
Achieving High-Pressure Gaseous Storage
The most common method for increasing hydrogen’s density is mechanically squeezing it into specialized high-pressure vessels, creating Gaseous Hydrogen (GH2). This requires robust compressors capable of handling extreme pressures. For passenger cars, the industry standard for on-board storage is 700 bar, while applications like buses often use 350 bar.
Compressor Types
Several mechanical compressor types are employed to reach these high pressures in refueling stations and production facilities:
- Reciprocating compressors, which use pistons moving back and forth, are a proven method for high-pressure applications.
- Diaphragm compressors utilize a flexible membrane to compress the gas, preferred for high-purity applications because the hydrogen does not contact lubricants.
- Ionic liquid compressors use an ionic fluid instead of a mechanical piston, offering a clean, oil-free process used in stations achieving pressures as high as 900 bar (90 MPa).
Compression is complex due to hydrogen’s low density and the need for specialized materials to prevent leakage. To manage the heat generated during compression, the gas is typically cooled between stages, ensuring the process is more efficient and safe.
Liquefaction for Extreme Density
An alternative to high-pressure compression is cooling hydrogen to its liquid state, resulting in Liquid Hydrogen (LH2). Liquefaction requires dropping the temperature to approximately -253°C (about 20 K) at atmospheric pressure. This cryogenic process yields a density about 1.8 times greater than hydrogen compressed to 700 bar.
The liquefaction process is complicated by the existence of two molecular forms of hydrogen, known as spin isomers: orthohydrogen and parahydrogen. At room temperature, hydrogen is about 75% orthohydrogen, but at the boiling point, the equilibrium shifts to nearly 100% parahydrogen. The spontaneous conversion of the ortho-form to the para-form is an exothermic reaction, meaning it releases heat.
If this conversion occurs after the hydrogen is liquefied and stored, the released heat can cause significant evaporation, or “boil-off.” To prevent these losses, the conversion must be accelerated using a catalyst during the cooling process before the final liquefaction stage. This ensures the liquid hydrogen is stable for storage but adds complexity to the cooling system.
Operational Safety and Energy Requirements
Storing hydrogen in extreme states, whether highly compressed or extremely cold, introduces specific engineering and materials challenges. One significant issue is hydrogen embrittlement, where hydrogen atoms diffuse into the microstructure of certain metals, causing them to become brittle and susceptible to cracking.
Because hydrogen is the smallest molecule, it can easily leak through minuscule openings in seals and materials. This necessitates the use of specialized, compatible components in storage tanks and pipelines.
A significant drawback of both Gaseous Hydrogen (GH2) compression and Liquid Hydrogen (LH2) liquefaction is the substantial energy penalty. Compressing hydrogen to 700 bar consumes energy equivalent to about 5% to 20% of the stored fuel’s energy content. Liquefaction is far more energy-intensive, requiring an input that can be 30% to 40% of the hydrogen’s total energy content. This energy expenditure reduces the overall efficiency of hydrogen as an energy carrier, making efficiency improvements a major focus for ongoing development.