Clean Energy Technology

Chemical storage of hydrogen through absorption/desorption, which involves the chemical binding of atomic hydrogen within the structure of a solid material. Hydrogen release from metal hydrides can be achieved in two main ways, mostly via heating (thermolysis) or through reaction with water (hydrolysis). Storage materials should have certain characteristics such as rapid kinetics, good reversibility, high safety, affordable price and high storage capacity at moderate operating temperature and pressure. Metallic based hydrides can compress hydrogen based on their thermodynamic stability by using waste heat. It is expected as metal hydride compressor to supply high pressure hydrogen. Several metallic/metallic-based materials can absorb hydrogen with those characteristics, and hydrides of lightweight elements such as boron and aluminium and some transition metals, such as titanium and zirconium have shown potential for use as hydrogen storage materials. Metallic based hydrides suitable for hydrogen storage are elemental hydrides (e.g. MgH2), interstitial hydrides (e.g. LaNi5, TiFe) and complex metal hydrides (e.g. NaAlH4, LiBH4, NaBH4, Mg(BH4)2). Challenges associated with the use of hydrides are high weight and low hydrogen storage capacity for low-temperature hydrides and slow kinetics and high temperatures for relatively lighter hydrides. Latest research seeks to enhance hydrogen absorption/desorption kinetics at moderate temperatures and high storage capacity by adding catalysts, alloying with other elements and nano-structuring. Metal hydrides are good solutions for stationary use where the weight is not a problem and could also be used in some maritime applications. Furthermore, hydrogen storage in metal hydrides does not pose safety challenges, as it is stored at low pressure, use rather low temperatures and hydrogen is only released from the material when it is heated up.

Salt caverns are artificial cavities in underground salt formations created by the controlled dissolution of rock salt through the injection of water, which returns to the surface as brine and must be disposed of in an appropriate manner. Salt caverns are suitable for the storage of pure hydrogen due to the low cushion gas requirement (typically around 30% of capacity), the high sealing capacity of rock salt and the inert nature of the salt structures, which limits contamination of the stored hydrogen. The geographical availability of salt caverns is limited. Salt cavern storage is considered to be flexible and would allow several cycles of gas injection and withdrawal per year. However, ongoing research on hydrogen storage in salt caverns is still aimed at demonstrating safe operating limits when subjected to rapid cycling with rapid pressure changes. Most knowledge of the effects of cyclic stress regimes on fracturing and fault slip comes from studies of underground natural gas storage, and there is limited data on the effects of hydrogen, which is needed.

Ammonia can be decomposed into nitrogen and hydrogen at a cracking unit. Ammonia cracking at small scale (1-2 ton per day [tpd]) and high temperature (600-900 °C) using inexpensive materials, such as iron, is already commercially available. However, the energy consumption of high-temperature ammonia cracking is around 30% of the energy content of the ammonia and rarely includes hydrogen purification. Ammonia cracking at lower temperatures (~450 °C) would decrease energy consumption, but currently involves the use of precious-metal catalysts, such as ruthenium. Low temperature ammonia cracking without the use, or with a limited use, of precious metals as catalysts is still at low maturity levels. In addition, the technology for separation and purification of hydrogen after ammonia cracking also needs to become less costly and more efficient, e.g. complying with the composition requirements set out for fuel cells (H2>99.97%, NH3<0.1 ppmv, N2

Ammonia is shipped in fully refrigerated, non-pressurised vessels, often designed to carry liquefied petroleum gas (LPG), as it has a lower boiling point [-42 °C] compared to ammonia [-33 °C]. LPG carriers can be used provided there are no parts containing copper or zinc or their alloys in contact with the cargo. While currently ammonia shipments are around 20 million tonnes per annum (Mtpa), and it is a mature technology, research is looking at the use of ammonia as fuel by carrying ships, particularly with separate cargo tanks so that they can carry LPG and ammonia at the same time, adjusting flexibly to demand patterns.

Aquifers are similar to natural gas reservoirs in that they are porous sedimentary rock structures, but contain water instead of natural gas. The main requirements for storage are the presence of a reservoir with a dome shape or structural fault to allow the gas to be trapped at the top of the structure, and the presence of a seal over the reservoir consisting of an impermeable formation. All of the challenges identified for depleted gas fields are relevant to aquifers, but aquifers present additional challenges. Unlike depleted gas fields, which are known to be tight because they were originally filled with gas, aquifers may not be tight on all sides and extensive geological investigation is required to determine whether there are pathways for the gas to escape. With the exception of existing aquifer storage and geothermal production sites, aquifers are undeveloped, with no production wells or surface facilities. As aquifers are water-bearing, moving water can cause significant hydrogen trapping during hydrogen injection, resulting in hydrogen loss. Compared to salt caverns and depleted gas reservoirs, aquifers have the advantage of being more widely available.

Lined hard rock caverns are artificial structures consisting of caverns created in metamorphic or igneous rock in geographical areas where salt or depleted fields cannot be exploited. The caverns are lined with a layer of concrete to create smooth walls, which are then lined with steel. Because they are carefully lined, hard rock caverns have no risk of contamination and can be operated at higher pressures than other structures, but steel embrittlement due to hydrogen exposure must be avoided. Hard rock caverns can experience several injection and withdrawal cycles per year, making them well suited for peak load purposes. They require relatively little cushion gas, but are expensive to develop. The construction process is mature, such as rock excavation, but the development of hydrogen-resistant liners and leak-free connections is challenging. Compared to salt caverns or depleted fields, rock caverns are developed at shallower depths (up to several hundred metres) and require shallow basement rock, which is not always available.

Hydrogen liquefaction involves a multi-stage process of compression and cooling to -253 °C, so that it is liquefied and stored in cryogenic tanks, increasing its volumetric density. The process starts with hydrogen compression and an optional (liquid nitrogen) pre-cooling to -193 °C, followed by cryogenic cooling to -243 °C (including heat exchangers and ortho- to para- catalytic conversion) and a final isenthalpic expansion to bring hydrogen to liquid phase at -253 °C and 1 bar. Hydrogen liquefaction is an energy-intensive process, especially for compression. The most recent hydrogen liquefaction plants have an electricity consumption of approximately 10 kWh/kg, equivalent to around 30% of the energy content (LHV) of hydrogen, and while hydrogen liquefaction is considered an established technology, efficiency improvements to values around 6 kWh/kg are expected in larger plants. Electricity costs are only a fraction of the hydrogen liquefaction costs and the capital cost for liquefaction is also expected to decrease with further innovations.

Hydrogen blending is the injection of certain amounts of hydrogen into a natural gas stream using existing natural gas infrastructure. Studies indicate that integrating blended hydrogen into the gas networks is feasible at levels of around 5-10 v% (volumetric share) with relatively minor upgrading, while in distribution networks, with polymer-based pipelines, shares of up to 20% would not require significant changes in the infrastructure, although the gas chromatographs should at least be adapted. While a 20% threshold will require some infrastructure upgrading, such as retrofitting the compressors, it seems to be the technical upper limit above which significant investments may be needed, in particular for some downstream installations and end-use equipment, although higher concentrations could be reached through R&D.

Depleted natural gas reservoirs are underground geological structures that naturally contained hydrocarbons and, once depleted, can be used to store gas. Depleted reservoirs consist of porous, permeable sedimentary rocks located underneath an impermeable cap rock and sealed on all sides by impermeable rocks. Depending on the reservoir size and allowable pressures (in some cases a few hundred bars), it may be possible to store up to several billion cubic meters of gas. The injection and withdrawal rates of porous structures are limited by the permeability of the rock, being generally more adequate for balancing seasonal fluctuations, due to their large storage capacity, and less for short-term variations. The proportion of cushion gas in pore storages is typically 50-60% of their total gas capacity, higher when compared to salt caverns. Hydrogen's higher compressibility factor, diffusivity, and lower viscosity should be further evaluated, as it may be more difficult to contain than natural gas. Hydrogen is also more reactive than natural gas, and in the presence of sulphate-reducing bacteria reacts with sulphate-containing minerals to produce hydrogen sulphide, a contaminant, also leading to hydrogen losses. It also reacts with CO2 and carbon-containing minerals in the presence of methanogenic bacteria to produce methane. Therefore, further validation and testing, both in the laboratory and in a real subsurface environment, is required to verify and quantify seal and reservoir integrity, dynamic flow processes (important for fast-cycling injection and withdrawal performance), hydrogen recoverability, geochemical reactivity and potential hydrogen consumption and conversion by microorganisms. Advantages to depleted gas fields as hydrogen storage are that they are larger in volume than salt caverns, and their geology is already well understood from being in operation for natural gas. Compared to the development of new salt caverns, they already have a well infrastructure for natural gas, some of which can be potentially retrofitted or repurposed for hydrogen.

Salt caverns are artificial cavities in underground salt formations created by the controlled dissolution of rock salt through the injection of water, which returns to the surface as brine and must be disposed of in an appropriate manner. Salt caverns are suitable for the storage of pure hydrogen due to the low cushion gas requirement (typically around 30% of capacity), the high sealing capacity of rock salt and the inert nature of the salt structures, which limits contamination of the stored hydrogen. The geographical availability of salt caverns is limited. Salt cavern storage is considered to be flexible and would allow several cycles of gas injection and withdrawal per year. Experience to date has shown that hydrogen can be effectively stored in salt caverns under low-frequency cyclic loading conditions. Ongoing research into hydrogen storage in salt caverns is aimed at demonstrating the feasibility of re-using caverns that have been used for natural gas and oil storage, particularly the risks of contamination and loss of stored hydrogen due to microbial activity.

Liquid organic hydrogen carriers (LOHCs) are organic molecules that can store hydrogen through a catalytic exothermic hydrogenation reaction at a certain pressure and mild temperature to produce a hydrogen-rich molecule, releasing heat. Subsequently, this hydrogen-rich molecule will be dehydrogenated in an endothermic catalytic reaction, which requires high temperature and mild pressure to produce the original organic molecule and hydrogen. Although there is some degradation during the dehydrogenation process, the original organic molecule is reused in the following hydrogenation stages. LOHCs must allow reasonably high hydrogen storage capacity (>5.5 wt%), and should be safe to handle (non-toxic, non-flammable, non-explosive), abundant and cheap. Some LOHCs are cycloalkanes, N-substituted heterocycles, 1,2-BN-heterocycles, liquid inorganic hydrides, and methanol and formic acid. Currently, however, the hydrogenation and dehydrogenation processes require energy, corresponding to around 35-40% of the energy content of the stored hydrogen, because, among other things, dehydrogenation temperatures are high. Research seeks to improve the overall efficiency of using LOHCs by looking for improved catalysts that enable dehydrogenation at lower temperatures (

Hydrogen deblending extracts pure hydrogen for dedicated uses as well as reasonably hydrogen-free natural gas from blended hydrogen and natural gas streams. Deblending allows for the extraction of pure hydrogen for dedicated uses (e.g. hydrogen fuel cells, feedstock) as well as reasonably hydrogen-free natural gas for gas quality-sensitive consumers (e.g. some industrial applications, feedstock, compressed natural gas refuelling stations). Deblending involves the separation of hydrogen from the methane-rich gas stream through different technologies or combinations among them, including gas permeation (e.g. polymer membrane, palladium membrane, carbon membrane, metal membranes, glass/ceramic membranes), pressure swing adsorption (PSA or membrane-PSA) or cryogenic separation, with different degrees of selectivity and efficiency. Although gas separation technologies have been used in the industry for decades, the technology has not yet been used on a large scale, such as in a distribution network.