Liquid Hydrogen Storage

Hydrogen is in its liquid state (LH2) at a temperature as low as 20 K. It requires about 30 % of its energy for cooling and liquefaction. LH2 must be stored in a superinsulated tank, called "dewar" or "cryostat", and even then, depending on the insulation quality and the surface-to-volume ratio, a certain fraction is an unavoidable boiloff to keep the rest cold. Safety is another concern. Energy can potentially be recovered when the cryogen is vaporized. The theoretical maximum is 10 % of the liquefaction energy. There are many opportunities of recovery; its realization is part of downstream engineering consideration

[61].

6.1.2.1. Basic Design of a Cryogenic Tank

A storage tank to hold a cryogenic liquid requires an effective insulation since temperature difference is the driving force for heat to enter the tank. It usually means a second vessel around the fluid container [13]. A supporting structure and interconnections between the two vessels have to assure that the inner shell can freely undergo contraction / expansion, also for numerous temperature cycles (refills) that may occur during its lifetime.

An even more complicated structural design is necessary for large vessels. Typical materials used for a cryogenic tank are carbon steel for the outer shell and stainless steel or aluminum for the inner vessel. Tubing from the inner vessel to the outside is generally made from stainless steel with vacuum-sealed transition joints on the cold side [51].

A multilayer insulation consisting of a reflective foil fixed on the outside of the inner vessel is usually chosen to minimize the transport of radiation heat With increasing number of radiation shields, however, additional heat transfer is introduced due to conduction via physical contact An optimal number is between about 60 and 100 layers [51]. The whole remainder of the intermediate space, in larger vessels about 1 m in thickness, serves as a vacuum jacket to avoid heat transport by convection or residual gas conduction. A typical pressure value in this space is < 0.013 Pa. Appropriate getter materials help maintain the vacuum over longer times.

An alternative which is used for large capacity tanks, is to fill the annular space with perlite powder of reflective particles. Tiny spaces in between avoid heat convection resulting in a less demanding vacuum of < 1.3 Pa. On the other hand, a shifting of the perlite powder during contraction of the inner vessel, a phenomenon called "perlite compaction" needs to be prevented in order to avoid breakage of the supporting structure [13]. A further decrease in heat conduction compared with the powder can be achieved by using hollow glass microspheres of boron silicate which have diameters in the range of

15 - 150 fim and a very low weight [51].

In large containers, the boiloff rate as a result of heat leakage can be significantly reduced by cooling the insulation with cold venting gas. For this purpose, metallic shields are installed with the insulation. In smaller containers, also liquid nitrogen cooling is being successfully used for boiloff reduction, but they are not as handy as the vapor-cooled container because of their weight and periodic refilling [51]. Multilayer super-insulation tanks exist at sizes of 300 m³ with around 100 radiation shields. Storage and transport containers for LH2 have been developed practically for all sizes. The autonomy of the system, i.e., the time with no loss of the product, is typically 30 days.

Further improvement is foreseen with the introduction of high-temperature supracon-ducting materials which allow the contact-free placement of the inner vessel in the outer con tamer ("magnetic levitation") [33].

6.1.2.2. Passenger Car Tanks

The schematic of an LH^ (or LNG) tank for a passenger car is shown in Fig. 6-1.

The tank materials typically used are austenitic steel and cold-tough aluminum alloys. A minimum wall thickness of 2 mm is prescribed in Germany. The tanks are checked upon fabrication in leakage and pressure tests [50]. Present cylindrical double-walled storage tanks have a capacity of up to 150 1. The 30 mm thick insulation consists of 70 layers of aluminum foil and glass fibers allowing a boiloff loss of not more than 1.5 %/d. The total weight is 50 - 60 kg. Maximum operation pressure is 0.6 MPa.

An extensive experimental study as a part of the Euro-Quebec project has been conducted by BMW to investigate the safety of LH2 storage tanks for passenger cars [50]. In particular, vibration and acceleration testing, air flooding of the vacuum, vessel deformation and perforation, and fire testing have been made to check the stability of the supporting structure and the performance in conceivable traffic accidents (see Fig. 6-2).

Burst tests revealed a safety factor for the LH2 tank walls of more than 14. The results have been used for design improvement. Preparation of the tanks with a safety burst

solenoid shut off solenoid control valve superinsulation

and

mechanically optimized suspension

safety vent line

vacuum valve/

burst disc

liquid level gauge

for pressure regulation

Fig. 6-1: Schematic of LH2 tank for passenger cars, from [Messer Griesheim]

f Safety tests with liquid hydrogen storage vessels

——— t I

Acceleration Vibration Flooding of vacuum

Deformation Perforation Firebrand

Fig. 6-2: BMW safety testing on LH2 (or LNG) tanks for passenger cars, from [42]

device, a circular notch, prevented the catastrophic failure and rather dampened flow and pressure relief, but its reliability over the whole lifetime has yet to be proven. Fire tests meeting IAEA requirements have exposed the tank to a temperature of 900 °C. Safety valves opened after 4 minutes. The LH2 was completely vaporized after 10 more minutes before the inner aluminum tank started to melt while the outer tank hardly exhibited any changes. The tanks have demonstrated good-natured behavior during depressurization via the safety valve. Neither test resulted in LH2 pool formation.

6.1.2.3. UJ2 Model Container for Maritime Transportation

Within the frame of the Euro-Quebec project various technical concepts for the transportation and distribution of liquid hydrogen have been developed. A 61 m3 model container has been constructed in a joint effort between the Federal Institute of Materials Research and Testing (BAM), Berlin, responsible for infrastructure and tank operation, and the Germanischer Lloyd (GL), Hamburg, taking care of the scientific program including the evaluation of the test results. Its purpose is to demonstrate the safe storage and transport of large quantities of LH2 [69]. It serves as a model simulating a future tank for maritime transport and storage. The cylindrical double-walled tank has a diameter of approx. 5 m and a length of 9 m with a designed system pressure of 0.5 MPa. The insulation between inner and outer tank consists of 30 layers of aluminum foil at the inside plus another 0.5 m vacuum space. For instrumentation, 67 temperature and pressure sensors are deployed to examine the thermal behavior of the tank.

Tests were conducted, the first with an LH2 tank of such a size, to investigate the thermodynamic behavior of the liquid hydrogen in the tank. One important goal was to examine the pressure rise in the closed tank. Three test series were conducted [52, 69]:

• Open vaporization at atmospheric pressure to determine the energy input into the tank from the boiloff quantities and provide a homogeneous temperature distribution for subsequent tests

• Closure of tank and measurements of pressure increase rates simulating conditions of a maritime transport

• Intentional pressure increase by means of a vaporizer device to investigate the energy input by hydrogen preheated in a heat exchanger

The boiloff rate was measured to be approx. 1.3 %/d (about 0.7 m3/d) for the filled open tank, while it was decreasing to 0.6 %/d at filling levels below 40 %. The decrease of the boiloff rate is attributed to the stronger heat uptake of the steel at higher temperatures which occur when in contact with the gas phase. In the self-pressurization tests, mass transfer from liquid to gas was found to have a dominant influence. The tests with the model tank on the BAM test site near Berlin have been terminated in the meantime, the tank is now being offered to third parties for further experimentation.

The fluiddynamic processes in a tank are very complex and are currently subject of investigation. A calculation model has been developed to enable the simulation of all conceivable states of the tank. The heat transfer from outside into the inner tank was measured to be 200 W whereas a value of 97 W was predicted [69].

6.1.2.4. Large-Scale Stationary Storage Tanks

For a large-scale liquid hydrogen storage, tanks of spherical shape are used to minimize boiloff losses. Two spherical dewars with a capacity of 190 m³ of LH2 plus 10 % ullage each were components at the Nuclear Rocket Development Station, Nevada, during the ROVER project starting in 1955 (see section 9.5.1.). The insulation consisted of a vacuum jacket and a 0.9 m perlite powder layer allowing an estimated boiloff rate of

0.3 %/d [11]. Later a 1900 m³ LHa dewar was in use with an evacuated perlite insulation and a boiloff rate down to 0.08 %/d.

The largest LH2 tank constructed so far is the NASA 3407 m3 vacuum perlite-insulated spherical storage tank at the Kennedy Space Center, Florida, used in the US space shuttle program. The outer sphere is made from carbon steel with an inside diameter of 21.34 m and the inner sphere is made from austenitic stainless steel with an inside diameter of 18.75 m; the ullage is about 10 %. The tank has a boiloff rate of 0.03 % or approx. 800 1 per day [51]. For comparison purposes: the LH2 storage tank within the External Tank (47 m height, 8.4 m diameter) of the US Space Shuttle has a volume of almost 1600 m3.

The largest stationary LH2 storage in Japan is at the Tanegashima space center in two spherical double-walled tanks with an outer diameter of 12.6 m and a capacity of 540 m3 each.

The Baikal- 1 nuclear rocket test facility in Semipalatinsk, Kazakhstan, is reported to have constructed in the 1970s three large underground tanks for LH2 storage with an estimated 18 m in diameter. In 1982, one of the tanks developed a leak and became unusable [53],

6.1.2.5. Future Storage Tank Designs

A concept of a composite LH2 container has been developed in Canada where the outer shell is fabricated from fiber reinforced plastics. The tank volume is 60 m3. Such vessels of which a first set has been manufactured already, are to be used for the transportation of smaller amounts of LH2 onboard conventional container ships, railway carriers, or as a storage medium near power or refueling stations [34, 35].

The LH2 storage container considered for the Euro-Quebec barge carrier is designed as a cylindrical pressure vessel type with a volume similar to that of the NASA tank.

Geometric data are a length of 22 m outside and 18.7 m inside, a diameter of 14.7 m outside and 14.0 m inside. The total weight is 1000 tons including 213 tons of LH2- To keep the vessel at cryogenic temperatures, 5 % of the LH2 contents is left in the vessel on its way back to Canada. The tank must guarantee a stand-alone period of more than 25 days. The containers are designed to serve both as a transportation vessel and as an onshore storage vessel in order to avoid the undesired losses of LH² if it were to be pumped from one into another container. This concept eases at the same time the operation of the ships [18, 35].

Improved concepts of insulations have been developed for future large-size storage tanks, e.g. a spherical tank with inner / outer diameters of 36 m and 40 m, respectively, which means a volume of about 24,400 m3 or six times the NASA tank volume. The characteristic feature of the new insulation design is the partition of the interspace volume of approx. 9000 m3 into many (some 104) small insulation boxes which can be arranged as a combination of vacuum and foam layers on the inner tank wall. The insulation character is maintained even if a single box has lost its vacuum. Target boiloff is 0.1 % per day. Experimental and theoretical investigations have been conducted for

design optimization, in particular, the design of the supporting structure which is subjected to dynamic loads during maritime transportation. The new insulation concept was found to exhibit a thermal conductivity which is on the average one order of magnitude lower than that of conventional foam insulations [40].

Tank specifications for a large-scale LH2 storage tank as projected within the WE-NET project, are a storage capacity of 50,000 m³ with a target boiloff rate of 0.1 % per day. Various insulation concepts are currently being under investigation [32].

Also within the WE-NET project, a metal hydride/LH2 hybrid storage system is proposed, where a hydrogen storage alloy serves as a buffer for a liquid hydrogen storage for the sake of ensuring a stable supply plus it absorbs flash hydrogen and boiloff losses [47].