Hydrogen is most commonly stored under compression in pressurized steel or carbon composite cylinders. However, the low volumetric density of hydrogen offers the economic advantage of being compressed into greater densities and thus requiring lower storage capacities.
As a result, the use of liquefaction and the exploration of other chemical carriers such as ammonia have increased in popularity for the storage, and transportation, of hydrogen. Table 1 presents an overview of some of the available storage technologies for hydrogen as detailed by Bruce, et al. (2018).
Storing hydrogen in its gaseous state is achieved through compression inside pressurized tanks that have mechanical devices to control the pressure. Steel tanks can typically store hydrogen at pressures up to 200 bar, while composite tanks can do so up to 800 bar. This form of hydrogen storage is well established, however, may not prove economical for hydrogen on a large scale due to the low volumetric densities of the gas.
Line packing is the process of storing compressed gas within a pipeline network by altering the pipeline pressure. The hydrogen gas can be stored inside a pipeline for days, at a large scale, and then can be distributed when deemed necessary such as during periods of peak demands.
Hydrogen can also be compressed into underground salt caverns through the injection of hydrogen into salt rock, typically intended for long-term storage. However, this storage method is limited by the availability of salt caverns (Bruce, et al., 2018).
Hydrogen can be liquified through the use of a multi-stage process of compression and cooling which is then stored in cryogenic tanks at temperatures of -253 ℃ (Bruce, et al., 2018). Liquified hydrogen has a much greater volumetric density, and therefore provides greater economic advantages in storage capacities.
However, it should be noted that the process of liquefaction typically consumes around 30% of the hydrogen’s energy content. Furthermore, losses of hydrogen are experienced through evaporation, or “boil-off”, and the process itself is expensive (EERE, 2021).
Hydrogen can be converted into ammonia by combining nitrogen and hydrogen using the Haber Bosch Process. The use of green hydrogen creates green ammonia which can be used as a climate-friendly energy carrier, mineral fertilizer, and fuel. Ammonia is much less energy-intensive to liquify than hydrogen and is, therefore, simpler to store and transport (McMahon, 2020).
Ammonia produces zero carbon emissions during combustion and is therefore considered an alternative for long-haul shipping fuel.
At present, 2% of the world’s greenhouse gas emissions originate from shipping – 80% of which is from long-haul shipping. The use of green ammonia as a marine fuel can therefore contribute to the achievement of a net-zero future. Ammonia is also commonly used as fertilizer (Statkraft, 2021; Zumdahl, 2020).
In cases where the ammonia is intended to be converted back into hydrogen, additional conversion costs will arise. This is achieved using electrochemical cells with a proton-conducting membrane and an ammonia-splitting catalyst and is expected to consume around 8kWh per kg of hydrogen (McMahon, 2020; Bruce, et al., 2018).
Hydrogen Transport and Distribution
Hydrogen is commonly transported by either or a combination of trucks, rail, shipping, and pipelines. Each method presents suitability towards the different storage technologies and travel distances, as shown in Table 2.
Due to the international scale of energy markets, the main transportation methods to be considered are shipping and pipelines, however, trucks and rail could also be considered at the macro-scale for the shorter travel distances between electrolysis plants and pipelines and/or shipping docks.
Trucks and Rail
Trucks and rail can be used to transport tanks of hydrogen in its gaseous and liquid form as well as ammonia to create a virtual pipeline for short distances that is already commercially available and easy to set up.
It is less common for trucks and rail to be used to transport ammonia as the use of ammonia tends to be cost-effective at higher scales due to the energy requirements for hydrogen-ammonia conversion.
Shipping is used for long-distance transportation and can thus be utilised for can be considered for the transportation of hydrogen. Typically, shipping has high operation costs and is, therefore, an unlikely carrier for gaseous hydrogen due to its low energy density.
To be more cost-effective, liquid hydrogen or ammonia is normally transported in ships due to their higher energy densities. It should be noted that shipping ammonia has greater costs due to the reconversion losses. It is therefore suggested that ammonia is better suited to decarbonize sectors where it can be used as a direct fuel or feedstock without the need for reconversion (Wang, et al., 2021).
Pipelines are a suitable transport for simultaneous distribution to multiple points or intercity transmission of hydrogen in its gaseous state. The use of existing pipelines plays a major role in Europe’s transition into hydrogen due to the well-established natural gas pipeline connections that can be repurposed for hydrogen.
This can take two forms: fully repurposed natural gas pipelines to carry 100% hydrogen; or hydrogen blending in which concentrations of hydrogen are injected into the natural gas.
The repurposing of pipelines can only go as far as the capacities of the existing pipelines as well as being limited to their locations. As such, the construction of new pipelines that are intended for 100% hydrogen should also be considered when the demand for hydrogen is expected to exceed the hydrogen carrying capacities of existing pipelines.
Hydrogen blending is the process of injecting hydrogen into existing natural gas pipelines at specific concentrations to reduce the carbon content within the distributed fuel, thus achieving a level of decarbonization. While concentrations of 25% have been achieved (Ibeh, et al., 2007), a hydrogen concentration of 20% is deemed more suitable due to the limitation of current end-use appliance compatibility with higher concentrations of hydrogen (Hydrogen Europe, 2020).
At the end of the pipeline, the hydrogen can be extracted from the natural gas mixture through various methods such as pressure swing absorption (PSA), membrane separation, and electrochemical hydrogen separation, however, these add additional costs.
For example, PSA extraction costs of a 20% hydrogen blend can range from EUR 1.77-6.56/kg depending on the desired recovery rate ranging from 100-1000 kg/day (Melaina, et al., 2013).
The growing use of electrolysis has opened up at least 42 emerging occupations, such as hydrogen lab technician and hydrogen plant operations manager, with more to be expected as the industries mature (Bezdek, 2019).
The 2x40GW Green Hydrogen Initiative is expected to produce 130,000-170,000 jobs in manufacturing and maintenance (Wijk & Chatzimarkakis, 2020). Based on this, electrolysis plants can have an expected job creation rate of around 1,875 jobs/GW.
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