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Hydrogen can be stored in three different ways:
- As a gas under high pressures
- In liquid form under cryogenic temperatures
- On the surface of or within solid and liquid materials
Each of these storage techniques has its own requirements and challenges, as shown below:
Compressed Gas
Hydrogen can be compressed and stored in a gaseous form under high pressures. This requires storage tanks to have pressures of 350-700 bar or 5000-10,000 psi.
Cryogenic Liquid Storage
Hydrogen can be stored cryogenically in a liquid form. Low temperatures are required to stop the liquid hydrogen from boiling off back into a gas, which occurs at -252.8°C. Liquid hydrogen has a higher energy density than gaseous hydrogen but getting it down to the required temperatures can be costly. In addition, storage tanks and facilities for cryogenic liquid hydrogen storage must be insulated to prevent evaporation should any heat be carried into the liquid hydrogen due to conduction, convection or radiation. Despite these challenges, liquid hydrogen is in demand for applications requiring high levels of purity and it can be found being used in space travel.
Combined Cold-and-Cryo-Compressed Hydrogen
The storage methods of compression and cryogenic cooling used above can also be combined to create a further development of hydrogen storage. In this instance, the hydrogen is cooled before being compressed. This creates a higher energy density than with compressed hydrogen but, as with cryogenic liquid storage, also requires more energy use to achieve.
The energy used for these different types of hydrogen storage equal 9-12% of the energy made available for compression (from 1 to 350 or 700 bar) and around 30% for liquefaction. The energy use varies depending on the exact method, quantities and external conditions, however work is underway to find more economic methods of storage with a lower required energy input.
Materials-Based Hydrogen Storage
As well as being compressed as a gas or stored as a liquid, hydrogen can be stored using materials. There are three types of hydrogen storage materials; those that use adsorption to store hydrogen on the surface of the material; those that use absorption to store the hydrogen within the material; and hydride storage, which uses a combination of solid materials and liquid.
In adsorption, hydrogen molecules or atoms attach to the surface of the material. In this method, the hydrogen attaches itself to materials with high surface areas, including microporous organometallic framework compounds (metal-organic frameworks (MOFs)), microporous crystalline aluminosilicates (zeolites) or microscopically small carbon nanotubes. Hydrogen adsorption to materials in powder form can achieve high densities of volumetric storage due to the increased surface area for the sorbent.
In absorption, hydrogen is dissociated into hydrogen atoms that are incorporated into the internal solid lattice framework of a material.
Hydride storage, the third of these material storage systems for hydrogen, can use the reaction of hydrogen-containing materials with water or other liquid compounds, like alcohols. This method to store hydrogen, also known as ‘chemical hydrogen storage,’ sees the hydrogen effectively stored in both the material and the liquid.
Metal hydride storage systems work by the hydrogen forming an interstitial compound with elemental metals such as palladium, magnesium, and lanthanum, intermetallic compounds, light metals like aluminium, or some alloys. These metal hydrides adsorb molecular hydrogen onto their surface and then incorporate them in elemental form into the metallic lattice with heat output. They can be released again with heat output and these hydrides can absorb large volumes of gas, with palladium, for example, able to absorb volumes of hydrogen 900 times that of its own.
Hydrogen can also be chemically bound with a liquid organic hydrogen carrier. These chemical compounds have a high capacity for hydrogen absorption and include the carbazole derivative N-ethylcarbazole and toluene.
These material-based methods allow large amounts of hydrogen to be stored by materials of smaller volume, at lower pressure, and in temperatures close to room temperature. Materials based storage can allow for volumetric storage densities greater than those for liquid hydrogen. However, materials-based storage is still in development as the cost of charging and discharging and processing hydrogen is still deemed to be too high as well as time-consuming.
Underground Hydrogen Storage
Salt caverns, exhausted oil and gas fields or aquifers can all provide underground hydrogen storage on an industrial scale. Such underground storage sites have been used for natural gas and crude oil for years, where they were held to balance supply or demand fluctuations or in preparation for a crisis.
Cavern storage is the most expensive of the options, but also the most suitable for hydrogen storage. Operational experience of cavern-based hydrogen storage is currently limited to a few locations in Europe and the USA. The most common of these are depleted underground natural gas stores, which are used as hydrogen reservoirs for surplus renewable energy.
Gas Grid Hydrogen Storage
As an alternative for underground cavern storage, surplus hydrogen can be fed into the public natural gas network to create hydrogen enriched natural gas (HENG).
Hydrogen-enriched town gas or coke-oven gas, with a hydrogen content over of 50% volume, was distributed to homes in Germany, the USA and Britain via gas pipelines into the 20 Century. The infrastructure used at the time still exists, although it was later modified to carry natural gas.
While it is generally accepted that gas with 10% hydrogen content could be introduced into the existing natural gas system without causing a negative impact on end users or pipeline infrastructure, a number of critical components have been deemed unsuitable for use at these levels of hydrogen concentration.
Despite this drawback, it is felt that large quantities of hydrogen gas could be stored in this manner by using much of the existing natural gas networks in industrial nations and then directly converted back into electricity via hydrogen fuel cells.
Hydrogen storage is important if it is to form part of the future renewable energy mix. With international efforts to reduce emissions and the use of carbon based fuels, hydrogen fuel cells could help create a greener solution to our power generation needs, including powering anything from small electronic devices to vehicles, aircraft and even whole buildings.
Another advantage of hydrogen as an energy source is that it can be obtained by electrolysis from electricity produced from surplus renewables, at the same time allowing hydrogen to fulfil a corresponding energy demand. Alternatively, hydrogen can be stored in large quantities for extended periods of time. Unlike with batteries, this energy is not lost over time and can therefore be produced and stored on an industrial scale as part of a green energy mix. This stored hydrogen can then be retrieved as a back-up energy supply when needed.
Hydrogen can also be used as a complementary fuel source alongside batteries in the transport sector. The hydrogen system provides the bulk of the energy storage and a small capacity battery will act as a buffer to provide regenerative braking, meet any sudden increased power demands and increase the lifetime of hydrogen fuel cells by reacting to load changes. This complementary fuel method is already in use for some commercially-available vehicles, such as the Honda FCX Clarity hydrogen car. Of course, hydrogen fuel cells have already been used safely for decades to provide clean power for forklifts that need to operate cleanly in indoor environments.
Hydrogen storage is important if it is to be part of our future clean energy solutions, yet more research and infrastructure improvements are required in order for hydrogen to realise its full potential. As such, the United States’ Hydrogen and Fuel Cell Technologies Office (HFTO) focuses on applied research, development and innovation to advance hydrogen use for transportation and diverse applications. Meanwhile, the United States Department of Energy (DOE) supports research and development of a range of technologies to produce hydrogen economically and in environmentally friendly ways.
Hydrogen is difficult to store due to its low volumetric energy density. It is the lightest of and simplest of all elements, being lighter than helium, and so is easily lost into the atmosphere.
Another challenge is the very low boiling point of liquid hydrogen (−252.8°C), which means that it needs to be kept cryogenically stored at low temperatures. Storing hydrogen as a gas also has its challenges as it typically requires the use of high pressure tanks (350-700 bar or 5000-10,000 psi).
As mentioned above, another solution for hydrogen storage is through adsorption or absorption, although with these storage techniques, further steps are then required to release the hydrogen once again.
All fuels have a level of danger associated with them based on three factors; ignition source, oxidant, and the presence of the fuel itself. Using the correct engineering controls can limit the dangers of any given fuel type, including hydrogen.
In fact, hydrogen has a number of properties that make it safer than many other commonly used fuel types. It is non-toxic, for example, and because it is lighter than air, it dissipates quickly into the atmosphere when released. This is important as it means that the fuel will dissipate into the air in the event of an accident, rather than remaining in place to potentially catch fire, as is the case with batteries or petroleum, for example.
However, there are still hazards related to hydrogen that mean additional engineering controls need to be put in place to ensure its safe use. With a lower ignition energy than petrol or natural gas, hydrogen has a wide range of flammable concentrations in the air meaning that ventilation and leak detection are important for hydrogen systems. Special flame detector are also required as hydrogen burns with a near-invisible flame. Material selection for hydrogen systems is also important as some metals become brittle when exposed to hydrogen.
Hydrogen requires staff training in how it should be safely handled, while systems should be tested for leaks and other potential problems, ensuring it is produced, stored and dispensed safely. Of course, for all of these measures, we have already seen hydrogen used in a wide variety of common applications, which shows that we can improve safety and build confidence in hydrogen as a safe, clean and renewable fuel for the future.
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