Hydrogen storage is a key technology for the development of fuel cell technologies suitable for stationary power, portable power, and transportation.
Hydrogen has the highest energy per mass of any fuel; however, its low ambient temperature density results in a low energy per unit of volume, therefore requiring the development of advanced storage methods that have potential for higher energy density.
Hydrogen can be stored physically either as a gas or as a liquid. Storage of hydrogen as a gas typically requires high-pressure tanks (200–700 bar [3,000–10,000 psi] tank pressure).
Storage of hydrogen as a liquid requires cryogenic temperatures because the boiling point of hydrogen at one atmospheric pressure is −252.8°C.
Hydrogen can also be stored in different solutions or within solids (by absorption).
Compressed Hydrogen in Tanks
Yearly Production: XXXX
Availability: High
Compressed hydrogen is a type of storage where hydrogen gas is kept under pressures to increase the storage density.
Compressed hydrogen in hydrogen tanks at 200 bar (5,000 psi) and 700 bar (10,000 psi) is used for hydrogen tank systems in vehicles, based on type IV carbon-composite technology.
Car manufacturers, such as Honda and Nissan, have been developing this solution. Different types of tanks do exist depending on the operating pressure.
Metallic tanks are used up to 250 bar while composite materials can reach 700 bar.
Liquid Hydrogen in Tanks
Yearly Production: XXXX
Availability: High
Liquid hydrogen is normally stored in on-site storage systems typically consisting of a tank, vaporizer and controls. Systems are selected in accordance to usage rate, pressure and regulations.
The storage of hydrogen in its liquid state allows the energy density of hydrogen to improve with respect to compressed gas tanks. However, for hydrogen to be in a fully liquid state without boiling at atmospheric pressure, it needs to be cooled to -253°C.
The energy requirement for hydrogen liquefaction is actually high, around 30% of the heating value of hydrogen is required for liquefaction. New investments and technological innovations are making the liquefaction process more and more energy efficient. This will lead to prices of hydrogen decreasing in the future. In current plants, the electricity required for H2 liquefaction at -253°C is about 35-43 MJel/kgH2, with potential for future reduction to 25 MJel/kg.
Source: Linde.com
In order to avoid evaporation, the liquid hydrogen is kept at -253°C by means of a super-isolation (principle thermoset) at a low temperature.
The super-isolation stops 3 types of undesirable heat input.
Heat radiation: in order to avoid this, approx. 40 air-dried special foils are placed between the inside and outside tank.
This corresponds to an insulation effect of about 17 meters of polystyrene.
Heat conduction: the aim is to minimize the connecting points between internal and external heaters (heat bridges) and to design them out of a low conductivity.
The connection points are as long as possible in order to reach a maximum temperature reduction on the distance covered.
Convention: in order to avoid heat circulation, a high-vacuum is drawn between the interior and the exterior of the tank.
These three measures prevent the undesired heat input to a large extent. Thus the evaporation of LH2 is largely minimized.
Cryo-compressed storage
Yearly Production: XXXX
Availability: Low
Like liquid storage, cryo-compression uses cold hydrogen (-253°C and slightly above) in order to reach a high energy density.
However, the main difference is that, when the hydrogen warms up due to heat transfer with the environment (“boil off”), the tank is allowed to go to pressures much higher (up to 350 bars versus a couple of bars for liquid storage).
As a consequence, it takes more time before the hydrogen has to vent, and in most driving situations, enough hydrogen is used by the car to keep the pressure well below the venting limit.
Solid State Storage
Yearly Production: 0
Availability: Under Development
Materials-based storage research offers a long-term solution to the challenge of onboard automotive storage, as well as opportunities for stationary and portable power applications, with the potential to significantly reduce the required storage pressure, increase gravimetric and volumetric capacity, and reduce cost.
Metal hydrides (MH) are the most technologically relevant class of hydrogen storage materials because they can be used in a range of applications including neutron moderation, electrochemical cycling, thermal storage, heat pumps, and purification/separation.
Storing H2 in solid metal hydrides (MH) from which it can be readily recovered by heating is an alternative and safe, high-volume efficient storage method.
Conventional MH however suffers from low weight efficiencies and the challenge is to improve them to conform to specifications set by the practical applications.
In response to these challenges, new studies aspire to advance the state of-the-art in three MH classes and develop hydrides based on lightweight, low cost elements with improved H2 storage properties. The final aim is to provide a storage technology that is attractive both economically and environmentally.
Further MH applications have so far been limited by relatively low gravimetric density, poor kinetics for absorption/desorption, expensive materials and/or complicated procedures for activation. Recent research has shown though that these limitations can be overcome.
The aim is to decrease material stability and enhance kinetics down to temperatures at a rate of 100°C targeting again the recent generation of high temperature fuel cells.
Source: ec.europa.eu
Benefits of solid hydrogen storage
- Drastic reduction in safety risk compared to high pressure or cryogenic solutions
- A completely reversible system (storage / retrieval)
- No memory effect, dischargeable at 100 % where power retrieval and energy storage are uncoupled
- Flexible and reactive use (absorbs variations in electrolyzed-produced hydrogen, ideal when it comes to storing intermittent renewable energy)
- 10 years of operation free of intensive maintenance due to a daily storage / retrieval cycle, with no loss of energy over time
- “Floating” storage or retrieval on demand for a hydrogen consumption-intensive network that cannot provide liquid or pressure solutions
- A “plug and play” system
Source www.mcphy.com