If you’ve been paying attention to the electric energy arena in the past couple of years, you have likely been hearing an increasing amount of discussion concerning the hydrogen economy. It seems that every other day, there is a big announcement from another global player jumping into green hydrogen or amplifying an existing commitment. Hydrogen (H2 – the hydrogen atoms like to travel together) is the most abundant element in the universe, and its combustion results in water as a by-product. It can be utilized in sectors of the economy that have been hard to decarbonize, such as steelmaking (on the iron reducing side) and marine transport. It may also serve as a long-term storage medium in the power industry. Therefore, our government and industries are increasingly focusing their efforts on building out the infrastructure necessary to support an Australian hydrogen economy.
In truth, though, many challenges remain to building out a hydrogen economy, including processes related to producing carbon-free hydrogen in the first place. Among the other major barriers to the widespread utilization of hydrogen are our ability to move and store it prior to its end use applications (generally either through direct combustion or fuel cells that convert it to electricity).
How can hydrogen be moved and stored?
Hydrogen can be moved (and stored) in one of the following ways:
- It can be trucked as a pressurized gas (though better containment vessels will be needed to enable shipping of larger volumes at higher pressures).
- It can be transported in specialized and expensive pipelines.
- H2 can be shipped in tankers as a liquid (in cryogenic tanks) – which can increase tanker load by 5 times over pressurized shipments, (but that involves high costs and uses about 35% of the total energy content to liquefy the hydrogen to -253 °C).
Based on the growing drumbeat of the media, cost-effective green hydrogen should be here any day now, and that we will soon have successfully addressed all of the technical challenges related to the production, transportation, storage, and utilization of H2.
Don’t get us wrong.
Few things excite us today as the promise of available hydrogen.
What’s not to like?
It is plentiful (75% of known Universe) and can be used for electricity generation, as fuel or to generate heat. But it has plenty of challenges as well. It is a very small molecule, with a low density of only 0.0899 g/l, and it can only be industrially transported on its own if liquified (at temperatures below -253 °C) or piped at very high pressures (500-700 bar)
The high initial capital costs of new pipeline construction still constitute a major barrier to expanding hydrogen pipeline delivery infrastructure. On top of that there are other factors that need to be taken into account such as:
- The possibility for hydrogen to embrittle the steel and welds used to fabricate the pipelines
- The requirement to control hydrogen permeation and leaks
- The need for lower cost, more reliable, and more durable hydrogen compression technology.
Potential solutions for piping hydrogen include using fibre reinforced polymer (FRP) pipelines for hydrogen distribution. The installation costs for FRP pipelines are about 20% less than that of steel pipelines because the FRP can be obtained in sections that are much longer than steel (up to 0.5 miles) minimizing welding requirements. That may be able to deal with steel embrittlement and (partially) with H2 permeation as it is a very small molecule transported at high pressures.
Equally transporting hydrogen as a liquid require a lot of energy to get to -253 °C, we still need to develop technologies to mitigate the amount of boil-off losses during fuelling and the industrial tanks required to ship vast quantities of hydrogen require similarly massive tank carriers (ships), infrastructure and ports.
But all these challenges represent opportunities. At the end of the day, hydrogen, the lightest element of all, has a fantastic potential as energy carrier for smoothing fluctuating renewable energies or storing the surplus electricity generated from renewable energies.
And let’s not forget one of the supreme advantages of hydrogen: it’s very high energy density. Diesel has an energy density of 45.5 MJ/kg, slightly lower than petrol, which has an energy density of 45.8 MJ/kg. By comparison, hydrogen has an energy density of approximately 120 MJ/kg, almost three times more than diesel or petrol. In electrical terms, the energy density of hydrogen is equal to 33.6 kWh of usable energy per kg, versus diesel which only holds about 12–14 kWh per kg. What this really means is that 1 kg of hydrogen, used in a fuel cell to power an electric motor, contains approximately the same energy as 4.5 litres of diesel.
Chemical energy storage, as hydrogen, has the biggest potential for large-scale energy storage.
This may be achieved simply by storage of compressed hydrogen gas in large stationary tanks or underground cavities, liquid hydrogen, or liquid hydrogen carrier e.g., ammonia and liquid organic hydrogen carriers.
Hydrogen can be moved in liquid organic carriers (such as methylcyclohexane – MCH) that absorb or release hydrogen by means of chemical reactions, and in liquid inorganic carriers (that is, lacking any carbon-hydrogen bonds), such as ammonia (NH3).
As solid-state hydrogen storage materials, interstitial metallic hydrides with appropriate hydrogen uptake and release properties at near room temperature, have been attracting interest as promising candidates for stationary energy storage, in addition to be used as negative electrode materials for nickel metal hydride batteries. Magnesium hydride is also considered as a potential candidate for stationary energy storage due to its advantages of simplicity, abundance, and low cost, even though its dehydrogenation requires temperatures higher than 300°C. Thermal management of such stationary energy system using interstitial hydrides and magnesium hydride may be an important issue to overcome for practical applications. Alternatively, metal hydrides show promising properties in other applications, e.g., thermal energy storage and smart windows.