In April 2017, the government established the Ministerial Conference on Renewable Energy, Hydrogen and Other Related Matters, and at the end of the same year proposed the Basic Hydrogen Strategy as a vision for Japan's future, with a view to 2050 [1]. The Basic Hydrogen Strategy consists of three pillars: "to formulate an action plan (technological development targets) until 2030, based on the premise of social implementation in 2050", "to make hydrogen an option on a par with renewable energies", and "to make the cost of hydrogen comparable to that of conventional energies". In March 2019, Japan's Agency for Natural Resources and Energy revised its "Strategic Roadmap for Hydrogen and Fuel Cells" in response to the formulation of this strategy, and included specific technology development targets, such as future price targets for household fuel cells, targets for the widespread use of fuel cell vehicles, and concrete details of hydrogen power generation [2]. Hydrogen is an extremely clean energy that becomes water after a combustion reaction, but according to the first and second laws of thermodynamics, the energy balance of the entire supply chain, including hydrogen production, compression, liquefaction and transportation, is inferior to that of other energy carriers. In this paper, we review the prerequisites for realising hydrogen in society, and look at the trends in elemental and system technologies and market trends that will make this possible.

Equalisation of Electricity Supply And Demand (Energy Storage)

As long as the first law of thermodynamics (the law of conservation of energy) and the second law of thermodynamics (the law of increasing entropy) hold true, the production of pure hydrogen requires energy to detach hydrogen atoms from other substances (molecules), and the energy produced by the resulting hydrogen can never exceed the energy required to produce it (even more so if we take into account the losses incurred in production, transport and storage). The energy produced by the resulting hydrogen can never be more than the energy required to produce it (even more so if we take into account the losses incurred in its production, transport and storage). Therefore, it is easier to understand that it is not the laws of physics that determine the economics of hydrogen, but the chronological discrepancy between supply and demand: hydrogen is produced when there is a surplus of energy, and hydrogen is used when there is a shortage of energy. In particular, the rapid spread of renewable energies in recent years has sometimes led to increased instability in the power system, and hydrogen production is expected to be a variable load in the power system, reducing the cost of transmission facilities by equalising supply and demand.

Mediation of Carbon Capture and Storage (Carbon Recycling)

On the other hand, the majority of energy consumption is still expected to be provided by fossil fuels, especially in developing countries, and the better use of fossil fuels is required to meet the increasing energy consumption in the medium and long term. In order to meet the increasing energy consumption in the medium and long term, we need to make better use of fossil fuels. Therefore, carbon dioxide released during the combustion of fossil fuels can be extracted, stored or fixed, and combined with hydrogen from renewable energy sources to refine hydrocarbons (especially methane), and hydrogen can be used as a medium to circulate carbon dioxide (carbon recycling). In particular, Japan is planning to start using hydrogen in June 2019. In Japan, in particular, the Ministry of Economy, Trade and Industry (METI) has just published a "Carbon Recycling Technology Roadmap" in June 2019 [3]. Since hydrogen alone, whether compressed or liquefied, has a relatively low energy density and is extremely difficult to handle, carbon recycling, which can make use of existing internal combustion engines and energy infrastructure, can be used as a transitional technology until pure hydrogen can be handled safely, and as a means of converting large amounts of carbon dioxide emitted from thermal power stations into valuable resources. carbon dioxide from thermal power stations can be converted into valuable materials Figure 1.

Figure 1: Carbon recycling technology roadmap of Japan [3].

At the same time, the transport sector, which until now has been difficult to electrify, can be indirectly electrified with hydrogen or hydrocarbons derived from renewable energy sources. The rapid commercialisation of electric vehicles in recent years has left many problems to be solved, such as short driving distances, degradation of lithium-ion batteries and long charging times. Hydrogen and hydrocarbons have the same or better portability than conventional mobility compared to electric vehicles and are ideal for the transport sector if they can be re-energised during fuel generation (CO₂ free hydrogen or carbon recycled fuel). The C₁ chemistry, in which methane is obtained by adding carbon and energy to hydrogen via a catalytic reaction, was technically and historically established in the early 20th century as the Fischer-Tropsch process. The tra/nsport sector accounts for 30% of Japan's energy consumption, the majority of which is supplied by gasoline and diesel oil, so it is expected to reduce dependence on foreign energy sources from a security point of view. In the long term, if technology can be established to safely handle pure hydrogen at high pressure or extremely cold temperatures, completely carbon-free and CO₂-free mobility will become a reality.

However, on the other hand, the concept of a hydrogen society itself is not possible unless the above assumptions, which are hard to imagine given the current social situation, are met, which means that there are high technological and social hurdles to be overcome. On the other hand, the reduction of greenhouse gas emissions is an international commitment under the Paris Agreement, and is not an effort but an obligation to be fulfilled, so enabling technologies are expected to create significant business opportunities for companies. Therefore, this paper reviews the prospects for social implementation of Assumptions A, B, and C, which are being considered in Japan and overseas, and examines the areas in which cryogenics can make a contribution.

Renewable energy, especially solar power, has seen a significant drop in unit costs (equipment costs, not including grid-connected lines) from around 500,000 yen/kW in 2010 to 286,000 yen/kW in 2018, due to a significant fall in panel manufacturing costs resulting from international competition [4].

This unit price is expected to fall further to 100,000 yen/kW by 2030 due to the spread of innovative manufacturing methods (such as perovskite and quantum dot methods). In Japan, the feed-in tariff (FIT) system, which began in 2009, has led to a rush to build mega-solar power plants, and since 2016 there has been a shortage of regulating power (such as pumped storage and gas turbines, which have a fast response time) to keep up with the fluctuations in solar power output, and there have been cases where utility companies have instructed individual solar power plants to curtail output. Since 2016, there has been a shortage of regulating power (e.g., pumped storage and gas turbines with fast output response) to keep up with changes in solar power output, and there have been incidents of utility companies instructing individual solar power plants to curtail output Figure 2.

Figure 2: Cost reduction prospects for photovoltaic systems [6].

The electricity system is an extremely delicate balance, where supply and demand must match within 0.1 second (6 cycles), otherwise voltage and frequency fluctuations can occur. In the past, pumped storage has been used to store surplus electricity, but further expansion of pumped storage has been difficult. In recent years, an on-site energy storage system using lithium-ion batteries (Li-ion) has emerged as an alternative, but lithium itself is a rare element, and recycling technology for Li-ion batteries has not yet been established. However, lithium itself is a rare element, and recycling technology for lithium-ion batteries has not yet been established.

The basic principle of hydrogen production is the electrolysis of water, and the technical hurdles to its production are low. However, since hydrogen has the lowest mass of the elements, it must be compressed or liquefied for storage and use, which requires additional energy. However, it is not necessary to carry out both processes in succession, and it is not impossible to carry out hydrogen compression and liquefaction in succession, while electrolysis can be carried out in large quantities at the same time when surplus electricity is generated (this can be done by designing the more expensive process to have a higher availability).

The electricity market plays an important role in signaling surpluses in the electricity system, especially in Germany, where distribution companies need to make allowances from the market for the amount of electricity they need based on hourly forecasts, such as 24 hours or 30 minutes from now, while power producers (especially coal- and gas-fired power producers) are often forced to sell electricity at negative prices for fear of shortening the life of their facilities due to thermal cycling (negative surprise). Since 2013, power producers (especially coal- and gas-fired power producers) have frequently wholesaled electricity at negative prices without curtailing output for fear of shortening the life of their facilities through thermal cycling (negative prizes). In the case of renewable energies such as solar and wind, when the feed-in tariffs end, there will be no choice but to curtail output when surplus electricity is generated, or to wholesale it to the market at extremely low prices (e.g., 1 yen/kWh). In Europe, arbitrage has already been established as a business, selling electricity from open-cycle gas turbines at momentarily high prices (3 EUR/kWh) and encouraging large consumers to increase output at low prices (negative prices).

In addition to the arbitrageurs described above, the project entity could be used as a business vehicle by the power generation company to compensate for negative prizes (many of the current arbitrageurs are also the trading arm of the power generation company). In areas where generation, transmission and distribution are completely separated, the facilities could be owned by the transmission company, which is responsible for the stability of the electricity system.

The integration of hydrogen production, compression and liquefaction as a large-scale load in a system that eliminates the chronological gap between supply and demand by means of price signals is expected to stabilise the electricity market and thus the electricity system.

Fuel cells are the most efficient way of generating electricity from hydrogen when there is a shortage of power, but they require high temperatures and pressures for the catalytic reaction, and are difficult to scale up to over 100MW. In the short term, therefore, the development of hydrogen co-firing technology in natural gas turbines is underway. The use of hydrogen is discussed in the next section. There are other methods of hydrogen storage, such as storage alloys, but for the moment it is difficult to imagine any method that is superior to hydrogen compression and liquefaction when considering scalability due to resource constraints.

As mentioned in the previous section, the economic and technical hurdles to producing hydrogen are relatively low if surpluses from renewable energy sources are utilised, but there are still issues to be overcome with regard to its use, such as the embrittlement of metals caused by high pressure and the difficulty of making fuel cells large enough. The use of fossil fuels is expected to continue as a base load power source (coal-fired power) and a regulating power source (gas-fired power), even though the introduction of renewable energies is expected to increase in the future, and the greenhouse gases produced by these sources must be effectively reduced. The concept of 'carbon recycling' is therefore being considered as a way of bringing together the two issues of hydrogen and fossil fuel use.

As a base-load power source, coal-fired power generation boasts an extremely low unit price of about 4 yen/kWh on average, and coal resources themselves are widely available around the world, giving it a degree of stability not found in other energy types from the perspective of energy security. In Japan, coal-fired power generation accounted for 32.3% of electricity generation as of 2016, and in Germany, which is attracting attention for its nuclear power and decarbonisation policies, 35.4% of electricity generation as of 2018 was generated by lignite power from its own and neighbouring countries, and in Poland, where German companies have a large number of offshore manufacturing facilities, nearly 80% of electricity generation is coal-fired [5]. In Poland, where the offshore manufacturing base of German companies is concentrated, nearly 80% of the power is coal-fired [5]. We can thus see how low-price electricity is driving the competitiveness of advanced manufacturing Figure 3.

Figure 3: Actual power generation by type in Germany (2018) [5].

However, if we look at CO₂ emissions per kWh, coal is almost twice as high at 600-900 gCO₂/kWh compared to natural gas at around 330 gCO₂/kWh. This is due to the chemical differences between natural gas, which is mainly made up of C₁ (methane), and coal, which is made up of CX (carbon compounds), making it impossible in principle to reduce CO₂ emissions alone without reducing electricity generation (although efforts to increase the efficiency of electricity generation per unit of CO₂ emissions are ongoing, with the latest plants However, efforts to increase the efficiency of power generation per unit of CO₂ emissions are constantly being made, with the latest plants achieving an efficiency of 42% for heat engines).

If we look at the exhaust from a coal-fired power plant, most of the exhaust is N₂ and CO_2, since desulphurisation by lime and denitrification by ammonia have already been completed in the furnace. The amine method of CO₂ separation allows the efficient recovery of high purity CO₂ without releasing it into the atmosphere. This CO₂ can be converted into methane by catalytic reactions and pure carbon is being developed worldwide. This form of carbon capture, fixation and reuse is known as carbon recycling. Typical projects include MefCO₂, which is part of the European Commission's Horizon 2020 framework programme [6].

In the project called “MefCO₂”, hydrogen is added to methane to make methanol, thus increasing its universality as a chemical raw material. The so-called C₁ chemistry, in which methane with one carbon atom is added to hydrogen to produce various compounds, has a history of more than 100 years since the invention of the Fischer-Tropsch (FT) method. In the past, hydrogen was generally obtained by reforming natural gas, but by using the hydrogen produced by the surplus electricity mentioned above, it is possible to obtain hydrocarbons (at room temperature and pressure) which are more stable than pure hydrogen (at high pressure and cryogenic temperatures).

Whether the energy required for the reverse reaction is economically viable depends on a function of the level of the carbon tax, the procurement price of the energy and the price of the raw materials produced (value added). In particular, _MefCO2 strives to reduce costs by using surplus electricity from renewable sources. Carbon taxes are beginning to be introduced in Europe, with the exception of Germany, and can be a powerful incentive to switch from atmospheric emissions to hydrocarbons, as well as an effective policy of exempting the methanol produced from motor fuel tax by granting it an environmental certificate.

There is also the potential to replace the current reliance on amine methods for CO₂ separation with compressor air liquefaction separation using surplus renewable energy. The U.S. Department of Energy's Integrated Gasification Coal Cycle (IGCC) demonstration (Pinon Pine IGCC, now the Frank A. Tracy Power Plant) used a portion of the shaft power from the generating turbine to improve compressor efficiency. When thermal power stations are forced to curtail their output, they could be reallocated from the power generation side to the CO₂ separation side without reducing output, thereby making better use of their facilities without increasing the thermal cycle Figure 4.

Figure 4: Methanol purification process of MefCO₂ [6].

Coal-fired power generation has suffered significant diminution in asset value in the global decarbonisation movement, and is increasingly being excluded from the framework for environmentally responsible investment as a "stranded asset" with a very poor rate of return on electricity generated, based on back-calculated emissions allowances, while it has become cheaper in Europe. It is interesting to note that the investors that are buying up coal-fired power stations are hydro and natural gas-based power producers. This is because the value of these assets will rise significantly if hydrogen generated from renewable energy sources can be used to make cleaner use of existing fossil fuels.

As mentioned in the previous section, if carbon recycling is achieved by using hydrogen generated from renewable energies, it will be possible to store and transport hydrogen in a stable form. Whether it is used as hydrogen or converted into hydrocarbons will depend on how safely the portability of the energy can be technically guaranteed.

Prospects for the Spread of Hydrogen + Fuel Cells

When hydrogen is produced from renewable energy sources by electrolysis, the so-called "economies of scale", i.e. the diminishing marginal cost of mass production in large plants, may hardly apply. This is because the electrolysis of water is itself a very simple chemical reaction with little or no expected yield, so that hydrogen can be obtained anywhere there is a clean water source and a power line. In fact, concepts for on-site hydrogen generators have been presented by car manufacturers. Therefore, the transport of hydrogen by lorry or pipeline is likely to remain very localised in terms of distribution costs (e.g. around steelworks where by-product hydrogen can be obtained). A more detailed cost-benefit analysis will be required in this regard in the future.

However, due to the characteristics of the catalyst in the fuel cell, the hydrogen in fuel cell vehicles and stationary fuel cells, which are currently being used experimentally in society, needs to be raised to extremely high pressures of 70 MPa, and all tanks and dispensers need to be able to cope with such high pressures. Due to the small atomic weight of hydrogen, prolonged exposure of the tank to high pressure will not only allow the hydrogen to escape but also embrittle the metal itself. Nanotechnological surface treatments and film technologies are beginning to solve this problem. The need for on-site pressurisation at hydrogen stations means that it is essential to develop a compressor that is capable of dealing with ultra-high pressure and hydrogen embrittlement, and that is compact enough to be installed at a wide range of stations.

Prospects for the Widespread Use of Carbon Recycled Fuels

In the case of fueling hydrocarbons with CO2 from carbon capture and sequestration and hydrogen from renewable energy sources, it is possible to produce gasoline or paraffin from methane using the Fischer-Tropsch process and to use existing infrastructure. However, it is preferable to use methane (so-called natural gas) in its original form, as it consumes extra energy for methane reforming, and its higher carbon content may produce particulate matter (PM) during combustion. As methane is a gas at ambient temperature and pressure, portability requires a phase change to liquefied natural gas (LNG), which is already in widespread use in Europe as LNG-powered vehicles, especially long-distance cargo tractors from the Netherlands, where Europort is located. Trucks and tractors are highly regarded for their low PM emissions compared to diesel, their quietness due to the Otto cycle and the fueling infrastructure in the European Commission's framework programme named "LNG Blue Corridors".

The tanks of LNG trucks have a thermos structure for insulating and storing LNG, and can maintain -120°C for a long time because they use vaporised gas as fuel. On the other hand, as the LNG level is lowered during gas supply, it is necessary to either inject LNG with a force exceeding the vapour pressure or to release the pressure in the tank and adjust the temperature while injecting the LNG, which requires advanced compressor adjustment technology (pressure and temperature) and standardisation for widespread use of dispensers.

When considering energy portability, distribution costs should not be forgotten. The advantage of hydrogen plus fuel cells is that hydrogen itself requires little distribution network, and can be generated and stored on-site with water and electricity, and then simply filled into fuel cell vehicles with compressors. Carbon-recycled fuels, on the other hand, require a new supply chain, including LNG lorries and storage tanks, and the question is how to pay for this. Here we present some examples of how car manufacturers are developing technologies to solve each of these distribution problems Figure 5.

Figure 5: LNG lorry and pressure regulator [7].

The Smart Hydrogen Station SHS, which Honda unveiled in conjunction with its Insight fuel cell vehicle, has an ISO half-container footprint (just the equivalent of a car park) and can be used as a hydrogen station anywhere water and electricity are available. The concept is to increase the number of stations in the early years, so that they can be replaced by larger stations and the SHS itself can be moved to another location. The system is designed to produce hydrogen directly at 35 MPa by electrolysis under high pressure (differential pressure system), and is compact and quiet by eliminating the need for a compressor to fill the tanks Table 1.

However, as fuel cell vehicles are expected to be loaded with tanks at pressures of 70 MPa during their widespread use, the next key to technological development will be whether it is possible to achieve ultra-high pressures using the principle of this system Figure 6.

Figure 6: Installation of a smart hydrogen station [8].

Table 1: Main features of the smart hydrogen station [8].

German car manufacturer Audi is piloting a "Power 2 Gas" plant that converts surplus renewable energy into methane, a concept that it claims indirectly offsets the greenhouse gas emissions of its car fleet. Specifically, the methane produced by the Power 2 Gas plant is injected into a gas pipeline and the equivalent amount of natural gas is obtained near the gas station.

Table 2: Main specifications of the Power 2 Gas plant [8].

So far, we have introduced the outline of the hydrogen society being considered in Japan and other countries, and have summarised the various conditions that must be met in order to achieve it, not only from a physical point of view, but also mainly from an economic and social point of view. The key to realising a hydrogen society lies in three points: whether the surplus electricity generated by renewable energies can be cheaply procured on the electricity market; whether carbon recycling, which enables the sustainable use of fossil fuels, can be achieved; and whether stable forms of energy portability can be technically realised.

In this context, compressors are expected to play an important role, especially in terms of power loading, air separation, high pressure generation and liquefaction cooling. Compressors will play an extremely important role in overcoming the difficulty of handling hydrogen. The active participation of physics and chemistry community in international projects and in the European Framework for Technological Development is expected in order to set up a concrete image of social implementation and technological development targets for each function and to bring new products to market as quickly as possible.

1. Ministerial Conference on Renewable Energy, Hydrogen and Other Related Matters. Basic Hydrogen Strategy.
2. Japan Hydrogen & Fuel Cell Strategy Council, Strategic Roadmap for Hydrogen and Fuel Cells: An Industry-Academia-Government Action Plan for Realizing a Hydrogen Society.
3. Carbon Recycling Office, Agency for Natural Resources and Energy, Ministry of Economy, Trade and Industry Carbon Recycling Technology Roadmap.
4. Japan Science and Technology Agency, Center for Low Carbon Society Strategies (JST-LCS), Photovoltaic Systems (Vol. 5 Cost Reduction Technology Assessment of Crystalline Silicon Solar Cells and Perovskite Solar Cells Based on Quantitative Scenarios). Tokyo. 2019.
5. Kerstine A, Yannick H, Julian W. Germany's energy consumption and power mix in charts" Clean Energy Wire, Berlin. 2019.
6. i-deals / et al: "Project Overview of MefCO₂" Madrid Photo by the author (AGA Stockholm, Sweden). 2018.
7. Honda "Smart Hydrogen Station".
8. Goetze T. Die AUDI-e-gas-Anlage in Werlte ein P2G-Projekt am Standort einer Biogasanlage. Hannover. 2017.