Reproduced from the Australian Institute of Energy Journal - Q2 edition
The hydrogen economy
The hydrogen economy is not a new idea, though it has been receiving more attention in the media of late. It is being touted as an energy carrier that can be used to effectively export renewable energy. The concept of using hydrogen as a clean burning fuel has been considered since the 1970s.
As a fuel, hydrogen has the potential to provide energy with zero greenhouse gas (GHG) emissions, for the combustion part of the cycle at least. For transportation, hydrogen fuel cells or hydrogen internal combustion engines have the potential to provide long range transport (up to 600 km on one refuel) with short refuelling cycles compared to alternatives.
Manufacture of hydrogen can occur at industrial scale, using well established processing routes. So given all this, why hasn’t the hydrogen economy taken off yet? And what has changed recently that may make it more attractive this time?
In the past, generating hydrogen at scale has involved steam methane reforming. In this process, natural gas is reacted with steam over a nickel catalyst to generate syngas, which is then purified into hydrogen. The advantages of steam methane reforming are that it is relatively cheap, the technology is well established and large volumes of hydrogen can be produced.
The main disadvantages are that it is still reliant on fossil fuels as a feedstock and generates GHG emissions in the process, both from the combustion of natural gas in the reformer burners and as part of the process chemistry. Currently, the vast majority of hydrogen produced globally is via steam methane reforming. The reforming process is also integral to the production of ammonia.
The other method that has been used to generate hydrogen is electrolysis of water. Industrial electrolysis of water has been used since the late 1800s. It involves passing a current through water, which then decomposes to hydrogen gas (at the cathode) and oxygen gas (at the anode). Modern electrolysis plants are modular in nature so can be expanded to produce the required amount of hydrogen.
The advantages of electrolysis are that it can produce a relatively pure hydrogen stream (the reforming process requires further purification) and that the hydrogen production is zero emissions – though there are likely to be emissions associated with the production of electricity. The main disadvantages are that electrolysis is comparatively expensive compared to steam methane reforming and, although electrolysis is modular and scalable, steam methane reforming can produce larger volumes.
When considering hydrogen as an energy carrier for the export of renewable energy, electrolysis is the technology that is most of interest. Two relatively recent advances in technology make this concept seem more viable. Firstly, advances in electrolyser technology have reduced the unit cost of plant and equipment and improved its ability to handle cycling of on and offline time. New proton exchange membrane technology requires no run up time and no warm up time so it can easily handle intermittent power supply such as from renewable energy.
Secondly, and perhaps more importantly, the cost of renewable energy technology such as solar PV has reduced greatly in recent time. A country like Australia has abundant renewable energy potential, particularly solar PV, but the grid size is limited and the country is too far away from other potential markets to export electricity. The peak generation of electricity from solar PV does not naturally coincide with peak demand as well. Hydrogen could provide a way for excess renewable energy to be exported to the world.
The concept is to use renewable energy to generate hydrogen in electrolysis plants then export this hydrogen to markets where it is required. The technological advancements in electrolysis and in renewable energy technology both work to improve the business case for using hydrogen in this manner.
Another contributing factor to the recent increase in consideration of hydrogen as a low emissions energy source is government policy. The Japanese government in particular has been quite vocal in its support for hydrogen, particularly as a transportation fuel. As such, major Japanese car manufacturers have spent considerable time and effort researching and implementing hydrogen-powered vehicles.
The Tokyo metro government is actively pursuing plans to establish a hydrogen societyin time for the 2020 Tokyo Summer Olympics. This includes installing refuelling infrastructure in the city, 35 stations in total, increasing the number of fuel cell vehicles on the road to 6,000 and having 100 fuel cell buses in operation. This creates a market for hydrogen that countries such as Australia could take advantage of.
Australia and Japan have recently announced a partnership around the export of hydrogen via ship, with liquid hydrogen being touted as one method of export. Kawasaki Heavy Industries have been exploring the possibility of generating hydrogen from gasification of brown coal from the Latrobe Valley in Victoria. This processing route however, although it is possible, would generate relatively large quantities of GHG emissions.
It is very likely that such a project would require carbon capture and storage (CCS) to be incorporated, at the very least from a social license standpoint. This would add significant additional cost. Including CCS in the project may result in it being more cost effective to use renewable energy and electrolysis to supply the required hydrogen.
Recently in Australia, the Gas Vision 2050 report was released which looks at a potential future of natural gas and hydrogen and where they may be used in a low carbon world. In these scenarios, hydrogen is used extensively through the economy as a replacement for natural gas in reticulated networks and in fuel cell applications for residential and light vehicles.
In the Gas Vision 2050 report, natural gas is used as the feedstock for large scale centralised production of hydrogen – this would again require the use of CCS in a net zero emissions world.
Barriers to overcome
Reducing the cost of producing hydrogen from renewable energy sources reduces the impact of one barrier to entry for a hydrogen-based economy. There are other potential barriers that will require addressing however.
When considering the safety implications of increased hydrogen use through the economy, it is easy to think about the potential of fire and explosion as the key safety related considerations, particularly given historical incidents involving hydrogen. The reality of hydrogen as a potentially explosive substance is a little different however.
Hydrogen gas is very low density. Although it does have a relatively wide range of explosive limits (lower explosive limit of 4% in air and upper explosive limit of 75% in air), the low density means that it disperses very quickly. A hydrogen leak in a well ventilated area is unlikely to achieve a concentration above the lower explosive limit.
There is a potential issue however in that hydrogen gas has a very low ignition energy, the lowest of all fuels. This means that the energy required to start a fire is low and fires can be caused by build up of static electricity, from instrumentation, mobile phones and other ignition sources. Engineering controls can be put in place and intrinsically safe equipment can be specified, but a large scale education campaign may also be required if members of the public are expected to be able to refuel vehicles at public refuelling stations.
Storage and transport
The low density of hydrogen may be an advantage when reducing the risk of explosion but it does make storage of hydrogen slightly more complex. Hydrogen itself has a good energy density per unit of mass but its low density means that hydrogen’s energy density per unit of volume is much lower than that of liquid hydrocarbons.
The very low density of hydrogen also means that it takes a large volume to store a very low mass. At atmospheric pressure and 20°C, the density of hydrogen is 0.08 kg/m3 (compared to 1.2 kg/m3 for air). To store large masses of hydrogen, it is necessary to compress it to high pressures, which enables more reasonable volumes of storage.
As higher storage pressures are used, to minimise storage volumes, storage tanks may need to be made of more exotic materials such as carbon fibre wrapped cylinders. This adds additional cost and there will be a trade off between storage volume and material.
Potential issues associated with transport are related to storage issues. For pipeline transport, there is a similar trade off between transport pressure, materials of construction and mass transported. If existing natural gas pipe networks are to be used, as proposed in the Gas Vision 2050 document, this provides a pressure limit and therefore a maximum capacity. For example, the Dampier to Bunbury natural gas pipeline operates at 85 bar, which corresponds to a density of approximately 40 kg/m3, compared to a density of natural gas of over 65 kg/m3 at this pressure.
In addition to the potential issues associated with low density, hydrogen is also a very small molecule and may have a tendency to leak through pipework fittings and flanges. Care has to be taken with maintenance of leaks to ensure losses of hydrogen are minimised in pipeline networks.
An idea that is under consideration for large scale transport of hydrogen is the transport of liquid hydrogen. Kawasaki Heavy Industries are working on plans for a liquid hydrogen vessel to transport liquid hydrogen from Victoria to Japan. It is however, quite energy intensive to liquefy hydrogen, and the materials of construction may be tricky.
Liquid hydrogen is transported at approximately 20 K (-253°C), which presents an engineering challenge. Current hydrogen liquefier design, using a neon/helium mixture as the refrigerant, can achieve a specific power in the order of 12 kWh/kg of hydrogen. To put this in perspective, a hydrogen liquefier with a production rate of 10 tonnes per day of hydrogen will have a continuous power draw of 5 MW. The liquid hydrogen carrier pictured has a capacity of approximately 175 tonnes.
The safe transport and storage of large volumes of hydrogen do indeed present a challenge. It is not insurmountable but may take time. There could be alternatives available that may make large scale transport of hydrogen easier. These generally involve using the hydrogen produced in electrolysis to manufacture another chemical that isn’t faced with the same challenges.
The most likely candidate in the short term is using the hydrogen to manufacture ammonia. The ammonia manufacturing process is well established via the Haber-Bosch processing route. Liquid ammonia transport is also very common with established transportation procedures and shipping routes. The issue then becomes how to use the ammonia at the destination. It is possible to crack ammonia back into nitrogen and hydrogen and then use the hydrogen.
The traditional cracking process requires high temperatures and the use of catalysts. Recently, CSIRO announced that they are working on a thin metal membrane that selectively removes hydrogen from a dissociated ammonia-nitrogen-hydrogen mixture. This has the ability, combined with the high temperature cracking process, to provide pure hydrogen from ammonia at the point of use.
It is also possible to burn ammonia directly in internal combustion engines and potentially even in gas turbines. This combustion will be free of carbon emissions though may generate oxides of nitrogen, which present a separate environmental risk. There are also safety issues that would need to be overcome. Ammonia is less flammable than hydrocarbons such as petrol but it is a toxic chemical.
In addition to ammonia, hydrogen could potentially be used to manufacture other storage media. Metal hydrides show promise as a way to absorb and then desorb hydrogen gas. The hydrogen laden metal hydrides could be used to transport hydrogen to destination markets. High temperatures are then applied to desorb the hydrogen from the metal.
The use of metal hydrides in this way is still in the research stage. A lot of work is required to be able to do this at an industrial scale. Another avenue being explored is methanation. That is, reacting hydrogen and carbon dioxide to form methane, which can then be liquefied in a traditional LNG process. This process, however, requires a stream of carbon dioxide as a feedstock. It also requires high temperatures to achieve reaction and there is still the issue of GHG emissions that are created when burning the product methane at destination.
The question now is whether the hydrogen economy will take off. As discussed, there are technical barriers, mostly to do with the very low volumetric energy density of hydrogen and the difficulties this then poses with regard to storage and transport, but these challenges are certainly not insurmountable. There is also the question of whether alternative technologies such as battery storage and electric vehicles will dominate the market before hydrogen has a chance to take hold.
There are many advantages to using hydrogen and the long range of hydrogen fuel cell vehicles with short refuelling times do provide an edge over electric vehicles. Using hydrogen as energy storage could also fill a gap in very large scale storage that would be expensive to achieve with battery technology. The actions of governments such as Japan’s provide some certainty for the hydrogen market going forward.
Australia is well placed to take advantage of its abundant potential for renewable energy and be in a position to export this to the world using hydrogen or ammonia as an energy storage medium. Given our generally carbon intensive power grid though, it might be better to go through a process of decarbonising this and using the renewable energy directly before pursuing the export hydrogen market.
 Hyundai ix35 fuel cell vehicle specifications: www.hyundai.com/worldwide/en/eco/ix35-fuelcell/highlights
 Siemens Silyzer product specifications: www.industry.siemens.com/topics/global/en/pem-electrolyzer/silyzer/discovering-pem-technology/Pages/comparison-of-pem.aspx
 Tokyo Aims to Realize Hydrogen Society by 2020, Government of Japan: www.japan.go.jp/tomodachi/2016/spring2016/tokyo_realize_hydrogen_by_2020.html
 Gas Vision 2050, Australian Pipelines and Gas Association: www.apga.org.au/wp-content/uploads/2017/03/Gas-Vision-2050.pdf
 Increasing hydrogen liquefaction in Europe – Strategic Energy Technologies Information System, August 2015 https://setis.ec.europa.eu/setis-reports/setis-magazine/fuel-cells-and-hydrogen/increasing-hydrogen-liquefaction-europe
 Membrane to fill gas in hydrogen export market, 3 May 2017: www.csiro.au/en/News/News-releases/2017/Membrane-for-hydrogen-fuel-cells