Hydrogen is gaining recognition as a promising clean energy source, offering a path toward reducing carbon emissions. However, effectively utilizing hydrogen presents a significant challenge due to its inherent physical properties. To overcome these hurdles, hydrogen carriers, which are substances or methods designed to store and transport hydrogen more efficiently than its pure gaseous form, are being developed. These carriers are designed to bridge the gap between hydrogen production and its widespread application in various sectors.
Why Hydrogen Requires Carriers
Hydrogen, despite its high energy content by mass, poses considerable difficulties for storage and transport due to its very low volumetric energy density. At standard temperature and pressure, hydrogen gas has an extremely low energy density, approximately 0.01 MJ/L, significantly lower than gasoline’s 34.2 MJ/L. This means large volumes are required to store a meaningful amount of energy. To increase its volumetric density, hydrogen must be either highly compressed or liquefied.
Compressing hydrogen involves pressures ranging from 350 to 700 bar, requiring robust and heavy high-pressure composite tanks. Compression consumes 10-15% of the hydrogen’s energy content. Alternatively, liquefying hydrogen requires cooling it to -253°C (-423°F). Liquefaction consumes 30-40% of the hydrogen’s energy content. Moreover, hydrogen’s small molecular size leads to concerns like material embrittlement and potential leaks through many common materials, making direct handling and transport of pure hydrogen impractical and costly for large-scale energy applications.
Common Forms of Hydrogen Carriers
Various substances serve as hydrogen carriers, each with distinct properties and mechanisms for storing and releasing hydrogen. Compressed hydrogen gas requires specialized high-pressure tanks, often operating at 350 or 700 bar. At 700 bar, its volumetric energy density reaches about 1.33 kWh/L, about 14% of gasoline’s energy density. These large, heavy tanks limit the amount of hydrogen that can be stored, particularly for mobile applications.
Liquid hydrogen (LH2) offers a higher volumetric energy density than compressed gas, allowing more fuel to be stored in a given volume. However, maintaining hydrogen in its liquid state demands cryogenic temperatures of -253°C, necessitating insulated tanks and continuous refrigeration to prevent boil-off. The energy required for liquefaction is a significant drawback.
Ammonia (NH3) is a promising hydrogen carrier, easily liquefied by compression at mild conditions (1 MPa, 25°C). It boasts a high gravimetric hydrogen density (17.8 wt%) and volumetric density (10.7 kg H2/100L). Hydrogen is released through catalytic decomposition around 500°C. While corrosive and toxic, its distinct smell aids in leak detection.
Methanol (CH3OH) is an effective hydrogen carrier as it is a liquid at ambient temperatures and pressures, allowing it to be stored and transported using existing liquid fuel infrastructure. Methanol has a high hydrogen-to-carbon ratio, making it an efficient hydrogen source. Hydrogen can be generated from methanol through a catalytic process called steam reforming at 200-300°C. This on-site hydrogen generation avoids the complexities and costs associated with transporting pure hydrogen.
Liquid Organic Hydrogen Carriers (LOHCs) are organic compounds that chemically absorb and release hydrogen through reversible reactions. In this process, a hydrogen-deficient organic molecule reacts with hydrogen in an exothermic hydrogenation reaction at 30-50 bar and 150-200°C, in the presence of a catalyst. This forms a hydrogen-rich, saturated compound that can be transported under ambient conditions. When hydrogen is needed, the hydrogen-rich LOHC undergoes an endothermic dehydrogenation reaction at higher temperatures, 250-320°C, to release hydrogen and regenerate the carrier for reuse. Examples include toluene/methylcyclohexane and dibenzyltoluene.
The Carrier Cycle and Energy Implications
The utilization of hydrogen carriers involves a systematic cycle that moves hydrogen from its production point to its final consumption. This cycle begins with the “charging” of the carrier, where hydrogen is either physically stored (as in compression or liquefaction) or chemically bound to the carrier molecule. For instance, in LOHCs, hydrogen is chemically added to the organic compound through a hydrogenation reaction. This step often requires energy input, such as the energy needed for hydrogen liquefaction or the heat and pressure for LOHC hydrogenation.
Once charged, the hydrogen carrier is transported, often leveraging existing infrastructure designed for conventional liquid fuels, especially in the case of methanol and LOHCs. This adaptability helps integrate hydrogen into current energy distribution networks. Upon arrival at the consumption site, the carrier is “discharged” to release the hydrogen for use in applications like fuel cells or industrial processes. This discharge step can also be energy-intensive; for example, LOHC dehydrogenation requires elevated temperatures, which can reduce the overall system efficiency if waste heat is not recovered.
The efficiency of the entire carrier cycle is a significant consideration. For LOHCs, the energy efficiency of shipping can range from 60-70% without heat recycling, increasing to 80-90% with effective heat recovery during the dehydrogenation process. While these efficiencies are lower than direct high-pressure hydrogen storage, carriers facilitate long-distance transport and long-term storage. The reusability of chemical carriers like LOHCs also contributes to their economic viability.