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Energy & Fuels
Hydrogen is one of the most common elements occurring in nature, and its potential as a source of energy has long attracted the attention of scientists and engineers. In addition to environmental cleanliness, one of the main advantages of hydrogen is its high energy density; this means that small amounts of hydrogen can be used to produce a large energy bulk. In addition, hydrogen can be obtained from several sources, including water, biomass, and various gases, such as methane, which makes it more affordable and cost-effective. The great number of different methods of hydrogen production, storage, and transportation dictate the need to develop approaches to selecting the most promising “hydrogen supply chains” in order to identify the most economically advantageous ways to supply fuel to industrial consumers.
- hydrogen
- storage
- transportation
- compressed hydrogen
- liquid hydrogen
- hydrocarbons
1. Introduction
Today, governments of many countries have updated their energy strategies [1,2] to add new goals to reduce harmful emissions into the environment, primarily carbon dioxide emissions. This trend, first and foremost, is motivated by the problem of global climate change. According to [3], the energy sector makes the greatest contribution to greenhouse gas emissions, accounting for more than 42% of the total emissions.
Hydrogen is one of the most common elements occurring in nature, and its potential as a source of energy has long attracted the attention of scientists and engineers [4]. In recent years, interest in hydrogen technologies has only intensified, and this is not surprising as hydrogen can become a key factor in the transition to more environmentally friendly and more efficient production and transport options, as well as in light of the need to reduce greenhouse gas emissions and shift to more environmentally friendly energy sources [5], since hydrogen combustion does not produce carbon dioxide emissions, but only water.
The use of hydrogen in different areas is already beginning to gain momentum. It can be used as fuel for cars, trains, and aircraft, which will significantly reduce emissions of harmful substances and decrease dependence on oil [8]. Hydrogen can also be of use in electricity generation, for instance, as a source of energy for households and industry [9,10,11,12,13].
Considering policies and measures that governments around the world have already put in place, it is estimated that the hydrogen demand could reach 115 Mt by 2030. In the power sector, the use of hydrogen and ammonia is attracting more attention; announced projects stack up to almost 3.5 GW of potential capacity by 2030 [14]. Benefiting from abundant water and renewable electricity sources, alkaline water electrolysis by using electricity generated from renewable energy sources through electrochemistry is believed to be one of the key techniques for the hydrogen production in future [15,16]. According to [17], although the world market scale of hydrogen economy is merely USD 80 USD in 2015, it will rapidly increase to USD 400 billion by 2030, USD 800 billion by 2040, and USD 1600 billion by 2050.
2. Hydrogen Storage Methods
Hydrogen storage is an important link in hydrogen supply chain, since under normal conditions H2 has a low volumetric energy density and rather high gravimetric energy density.
This dictates the need to reduce the volume of stored hydrogen and increase its weight content in the tank, since a large volume of stored hydrogen requires a big area and weight of the shell. Therefore, there are many storage methods that can be divided into physical and chemical ones (Table 1). Physical methods apply to compressed and liquid hydrogen, while chemical methods are used to hydrogen in the form of ammonia and combined hydrogen in metal hydrides and liquid organic hydrides.
Table 1. Main characteristics of hydrogen storage methods.
| Storage Method | Mass Ratio of H2 and Shell, % | Storage/Conversion Temperature, °C | Storage Pressure, MPa | Storage Density, kg H2 /m3 | Storage Cost, USD/kg H2 |
|---|---|---|---|---|---|
| Steel pressure vessel (all-metal construction) [62] | up to 1 | 20–40 | 15 | 10.9 | 0.19–0.24 |
| Pressure vessel (mostly metal, composite overwrap) [63] | 5–7 | 20–40 | 35 | 23 | 0.22–0.27 |
| Pressure vessel (all-composite construction) [63] | 10.5–13.8 | 20–40 | 70 | 40 | - |
| Cryogenic tanks for liquefied hydrogen [64,65] | up to 7.1 | -252 | 0.1 | 70.8 | 1.67–2.04 |
| Liquid organic hydrides [66] | up to 7.2 | 20–40/180–280 | 0.1–1 | 70 | - |
| Metal hydrides [67] | 1.5–8 | 20–40/100–300 | 0.1–5 | 110 | - |
| Liquid ammonia [68] | 17 | 25/400–600 | 1 | 107 | 0.91–1.09 |
H2 storage in pressure vessels involves two stages, such as hydrogen compression and hydrogen storage in the cylinder. At the first stage, it is necessary to reduce compression costs. At the second stage, storage pressure should be raised to increase the weight of stored H2, while at the same time, it is necessary to reduce the weight of the cylinder’s shell; therefore, there is a need to use composite materials, which causes an increase in the price of the cylinder.
The main advantages of storing compressed hydrogen in cylinders:
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the technology is well developed and available (no more difficult than storing natural gas);
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ease of operation of the consumer and the absence of energy costs for the issuance of gas;
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the cost is relatively low.
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the disadvantages of gas balloon storage of compressed hydrogen are:
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low volume content (about 7.7 kg/m3 at a pressure of 10 MPa);
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the stored energy density at high pressures (up to 70 MPa) is comparable to liquid hydrogen, but the storage technology at such high pressures has not been fully developed;
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hydrogen compression to high pressures is in itself a complex engineering problem associated with possible gas leaks through seals, as well as with hydrogen corrosion of loaded structural materials;
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hydrogen compression is characterized by rather high energy consumption (10–15% of the calorific value of hydrogen);
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safety concerns (explosive gas under high pressure).
Storage of hydrogen in a liquefied form makes it possible to achieve a greater density than in the case of compressed storage. In a liquefied form, H2 is stored at atmospheric pressure and, therefore, it is possible to have a greater unit volume than in the case of compressed hydrogen. A similar storage method is already used at spaceports and is well-practiced.
The main advantages of liquid hydrogen:
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high bulk density and high content of stored hydrogen (71 kg/m3);
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technologies for hydrogen liquefaction and its storage in a liquid state are well developed.
Significant disadvantages of liquid hydrogen are:
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high energy costs for liquefaction;
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significant losses due to evaporation;
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high costs for thermal insulation;
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the storage method is too expensive and competitive only in special cases.
An alternative way to store hydrogen is its chemical bonding or transformation into another substance. So, recently, methods of transporting and storing hydrogen in the form of a liquid organic hydride, or in the form of ammonia, have been developed [60,61]. The storage of these media is devoid of the disadvantages of storing hydrogen in compressed and liquefied form; however, the main disadvantage of this storage method is the need to convert hydrogen, which significantly increases costs. Thus, storage in the form of ammonia requires the construction of terminals for the production of ammonia, including an air separation plant.
An alternative and promising way of storing hydrogen is its pumping into natural underground cavities (e.g., salt caverns). This technology is widely used to store natural gas and is characterized by minimal investment, though geographical limitations minimize the usability of this storage method.
3. Hydrogen Transportation Methods
The main challenge to the widespread use of hydrogen transportation methods is a significant difference in hydrogen properties when compared with hydrocarbons (primarily boiling point and molecular weight).
Pipelines are one of the promising ways to transport hydrogen to consumers; however, the use of conventional gas pipelines intended for natural gas is not advisable because of the below reasons:
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high potential for embrittlement of hydrogen pipeline steel and welded joints;
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leaks of transported hydrogen through pipeline walls because of diffusion;
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high hydrogen compression costs.
Potential solutions include using fiber reinforced polymer (FRP) pipelines for hydrogen distribution [69].
The use of existing gas pipelines for pumping hydrogen is possible, but only in the case where hydrogen is mixed with natural gas (not more than 20%). This will enable the use of environmentally friendly fuel to reduce natural gas consumption, without the need to create new infrastructure. The restriction regarding fuel fractions is caused by the need to change the design of equipment used by main gas consumers (primarily gas turbine power units) due to the growing combustion temperature and burning velocity [70].
The United States has an extensive network of more than 1600 miles of dedicated hydrogen pipeline [71]. Hydrogen produced through clean pathways can be injected into natural gas pipelines, and the resulting blends can be used to generate heat and power with lower emissions than using natural gas alone.
Europe is taking the lead globally with pipelines planned on and offshore. The recently announced H2Med Barcelona–Marseille subsea hydrogen pipeline is budgeted to cost around USD 2.1 billion for a stretch of 450 km, and it was recently announced that it will be extended to Germany too [72].
Another way to transport compressed hydrogen is ground and sea transport. To date, it involves using vehicles with a maximum capacity of up to 1 ton of hydrogen [73].
A low density of hydrogen is the cause of high costs of compressed hydrogen transportation. This problem can be solved by hydrogen transportation in a liquefied form. Due to the higher density of liquified hydrogen, it becomes possible to transport more fuel. An obvious disadvantage of this method is the need to provide low temperatures during transportation, which requires significant energy costs.
Transportation of liquid hydrogen is carried out by tank trucks with a capacity of 25 m3 and 45 m3. Hydrogen liquefaction is a highly energy-consuming process and, therefore, it is expensive, but transportation costs for liquid hydrogen are minimal and are roughly the same as the cost of delivery through pipelines.
A distinctive feature is that hydrogen is liquefied at a temperature of −253 °C, and special cryogenic tanks are necessary for its storage to minimize hydrogen losses. To this end, there are studies of materials, and aluminum tanks and containers of synthetic materials can be used as advanced technology.
In 2021, Kawasaki Heavy Industries obtained approval in principle from Nippon Kaiji Kyokai for a large, 160,000 m3 liquefied hydrogen carrier [74]. The carrier is designed to transport cryogenic liquefied hydrogen, cooled down to a temperature of –253 °C and reduced to one eight-hundredth its initial volume, by sea in large amounts on each voyage, helping to reduce hydrogen supply costs.
Railway transport for transportation of liquid hydrogen has rather a restricted application because of the limited railway network. In cryogenic railway tanks, hydrogen loss is about the same as in the case of tank trucks. With single chilldowns in tank trucks, up to 15% of hydrogen is lost, while losses associated with bad thermal insulation are 0.5% per day of the transported hydrogen amount.
There are also options to transport hydrogen using carriers, which can be hydrogen chemical compounds, such as ammonia or hydrocarbons. They go into chemical reactions to produce hydrogen. For example, an alternative option for transporting hydrogen is its conversion to ammonia or another form, such as a liquid organic hydrogen carrier (LOHC), to transport hydrogen in usual ways without significant costs to maintain the physical form and with minimal leaks. At a normal temperature, ammonia is liquefied at a pressure of 1.0 MPa, and it can be transported by pipes and stored in a liquid form (ammonia’s liquefaction temperature is −33 °C). Hydrogen is produced from ammonia through catalytic decomposition, with 5.65 kg of ammonia needed to produce 1 kg of hydrogen. However, a key drawback of this transportation method is the need to create ammonia/liquid organic hydrogen carrier plants with further decomposition at consumers.
The choice of a hydrogen delivery option is primarily based on transportation costs. They depend on many factors from the amount of transported fuel to the distance between the manufacturer and the consumer. They affect both the capital cost of means of transportation (i.e., pipelines or ground transport) and operating costs (i.e., electricity costs for pumping hydrogen or truck fuel). Figure 1 presents tips on how to choose the most cost-efficient methods of hydrogen transportation over small and long distances, depending on the transported amount.
Figure 1. Recommendations for choosing an economically viable method of transporting hydrogen, depending on the distance and consumption.
This entry is adapted from the peer-reviewed paper 10.3390/en16145482
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