Hydrogen Energy Storage

Hydrogen is stored in compressed form in specially designed light, small and cylindrical tanks due to its low density compared to other gases. High cost and infrastructure are needed to store hydrogen gas at high pressure. These specified gas cylinders cause volume, weight and cost. Technological developments have been made for this storage process, but it seems to lag behind when compared to fossil fuel storage systems, and development studies for storage operations should continue.

The liquefaction process of hydrogen can be performed as cryogenic (process at low temperatures), but there are also problems in this method. It is known that the cost and energy consumption of this process is very high. The safety level of cryogenic tanks could not reach the desired level and the production costs are seen to be too high in our time.

The design, systematic, safety and maintenance conditions of engines using cryogenic hydrogen need to be improved. Hydrogen energy storage; Eight types of methods are widely used: Hydrogen Energy Storage as compressed gas, storage as liquid hydrogen, gel hydrogen energy storage, hydrogen energy storage as metal hydride, hydrogen energy storage in alanates, hydrogen energy storage in carbon nanotubes, hydrogen energy storage with glass microspheres, hydrogen energy storage using NaBH4 in vehicles. These are described below.

Hydrogen Energy Storage as a Compressed Gas

The method used is done at room temperature and tanks with high pressure resistance are used. Storage varies depending on the weight parameter of the tank, hydrogen energy storage can be done between 1% and 7% of the tank weight. There are also tanks made of lightweight materials with higher strength. Under these conditions, they are more expensive because they have more hydrogen energy storage capacity. When using this method, 20% of the energy capacity of the fuel to be stored is spent on storage to store the hydrogen in the tank.

Storage as Liquid Hydrogen

It can be used in cryogenic hydrogen energy storage to reduce tank mass and volume in high pressure gas storage. By storing hydrogen in liquid form, it makes it possible to increase the density significantly at high pressure compared to the gas state, and to work at lower pressures, thus reducing the tank mass. Liquid hydrogen is at a temperature of around -253 ºC and has a density of 71 kg/m3. A mixture of solid and liquid hydrogen can be produced to achieve a greater increase in the density of liquid hydrogen. This mixture is called slush (semi-molten) hydrogen. Hydrogen density for 50% solid, 50% liquid mixture is 80.9 kg/m3.

Gel Hydrogen Energy Storage

The storage method, called gel hydrogen, is realized by the transformation of the substance called gellant into liquid hydrogen. This material may also contain cryogenic materials such as methane (CH4), ethane (C2H6) and silica particles. Gelation methods from other hydrocarbon types such as ethyl alcohol oxide and hexyl alcohol oxide are also in the process of development. Advantages of gel hydrogen production; It causes a decrease in volume and an increase in density.

The amount of increase in this density is around 10%. When liquid hydrogen and gel hydrogen are compared; The boiling rate of gel hydrogen is 2-3 times lower. This low level increases the safety level of hydrogen at the time of storage. The viscosity of gel hydrogen is lower than liquid hydrogen, resulting in a lower pour diameter. Thus, the gel reduces the likelihood of hydrogen spillage, increases the hydrogen permeability, reduces the potential for leakage, reduces motion instabilities, reduces the level of agitation in the storage tank.

Hydrogen Energy Storage as Metal Hydride

The way of storing hydrogen as hydride is also among the methods used. In terms of use of this reaction;

M + (x/2)H2 → MHx (2.1)

is displayed as. Although this reaction is endothermic or exothermic, it changes direction depending on temperature and pressure.

In applications, the desired properties during hydrogen energy storage; high recyclable storage capacity, low setback temperature, resistance to poisoning and, depending on, repeatable filling number properties at the desired level as much as possible. It has been determined that the targeted capacity value of the International Energy Agency (IEA) and the United States Department of Energy is greater than 5-6%, the target return temperature is less than 150 oC, and the targeted service life is more than 1000 fillings.

In the study by Douglas and Derek, it was stated that a hydride that is easy to store and retrieve is not very stable. In the metal hydride storage method, there are mainly magnesium (Mg) based alloys and some intermediate metals.

Examples of intermediate metals AB5 and AB, which can store hydrogen at a pressure of a few bars and at room temperature, are lanthanum pentanicel (LaNi5) and ferrotitanium (FeTi). AB5 is highly resistant to poisoning, the hydrogen energy storage rate of the intermediate metals does not exceed 1% or 2%. Materials with hydrogen energy storage feature completely lose their gas absorbing properties when there is oxygen (O2), water (H2O), carbon monoxide (CO) or carbon dioxide (CO2) in the gas. It is important for the health of the system to keep materials that store hydrogen away from environments containing these gases due to this phenomenon called poisoning.

Despite all these negative properties of magnesium, development activities in the field of hydrogen energy storage have recently been given importance. The reason for this importance is its high capacity. Mechanical grinding; It is the most common method applied to improve kinetics. This grinding process can be done pure or metal [such as Vanadium (V), Titanium (Ti), Nickel (Ni), Copper (Cu), Iron (Fe)], metal oxide [Copper oxide (CuO), Aluminum oxide (Al2O3). ), vanadium oxide (V2O5)], intermetallic (such as LaNi5, FeTi) additions, and the additive grinding process results better.

Hydrogen Energy Storage in Alanates

One of the recent studies on hydrogen energy storage methods is the use of complex hydride structures containing aluminum (Al) and boron (B). Boron-containing complex hydrides are used in liquid conditions.

As with powder-based storage in metal hydrides, hydrogen energy storage in alanates will also be on this basis. Studies have focused more on aluminum hydride, and there are also complex alanate studies such as sodium alanate (Na2LiAlH6). The removal of hydrogen from sodium alanate, which has 7.4% hydrogen energy storage by weight, is carried out in several stages and is shown in Table 2.2.

The reaction takes place at 185 oC, which is the low temperature standard, and the recycling capacity remains at 3.7% at this temperature. In the last step, the sodium hydride compound is separated, the reaction is processed in the high temperature region and is considered unusable in practicality. As a result of this reaction, the capacity of sodium alanates remains at the level of 5.55% by weight.

Table 2.2. Hydrogen reflux reactions of NaAlH4

13NaAlH4→ Na3AlH6 + 2Al  + 3H23,7185
2Na3AlH6→ 3NaH + Al + 3/2H21,85260
3NaH → Na + 1/2H21,85>425

Hydrogen Energy Storage in Carbon Nanotubes

Tubular smooth carbon structures with a size of two nanometers (nm) are called carbon nanotubes. The tubes have hydrogen energy storage features within themselves. Hydrogen storage rate is high and storage systematic is similar to metal hydrides. Theoretically, they have a hydrogen energy storage capacity of between 4% and 65% by weight. The development process of this very new technology continues. If the desired developments are achieved as a result of these technological developments, there are predictions that hydrogen energy storage methods will be the lightest and most useful way.

Hydrogen Energy Storage with Glass Microspheres

Hollow glass spheres are used in this storage method. Storage is done by incorporating the heated spheres into a high-pressure hydrogen medium. Figure 2.5 shows the micro glass sphere and its structure. Glass spheres are at a temperature of approximately 300 oC and are filled at a pressure of 350-700 bar.  With the cooling of the spheres, the hydrogen is trapped in the spheres. The sphere temperature is increased to release the hydrogen.

In this storage method, safety is paramount, resistance to deterioration is high, but there is a low volumetric density, high pressure is required for filling, very slow leakage from the spheres at room temperature, there is a possibility of breakage during refilling, and PEM fuel used in vehicles and will be discussed in the following topics. operating above the operating temperature of the battery.

Hydrogen Energy Storage Using NaBH4 in Vehicles

The boron-based storage method of hydrogen to be sent to fuel cells used in vehicles is important because it is both reliable and 73% of the world’s boron reserves are located in our country. Sodium boron hydride is used in the boron-based storage method and it has the ability to maintain its stability up to 500 oC. The use phase of sodium boro hydride in vehicles is made as a liquid and it transforms into sodium metaborate (NaBO2) according to the equation below and releases the hydrogen in it. As a result of this exothermic reaction, approximately 300 kJ of heat is released.

NaBH4(s) + H2O → 4H2 + NaBO2 (5.1)

Due to the high reaction rate of the NaBH4 molecule, a suitable catalyst should be used for hydrogen gas control and activation.

The most important advantage of using NaBH4 for the storage process is that the temperature required to liberate the hydrogen is room temperature and the reaction can be easily controlled if the chosen catalyst is suitable. NaBH4 in solution is a very safe system for storage. It is not dangerous when exposed to flame directly. However, if a suitable catalyst is included in the solution, it releases H2.

The production of the hydride, the selection of the appropriate catalyst and the conversion of NaBO2 formed as a result of the reaction to NaBH4 are the priority issues in studies on the storage method with boron.

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