5.

Task 5Development of Hydrogen Fuel Tank System

5.1

Target of Research and Development

The target of research and development for the task 5, “Development of Hydrogen Fuel Tank
System”, in the WE-NET phase II program was “to develop element technologies for hydrogenfuel system of hydrogen fuel cell vehicles taking into account hydrogen supply from thehydrogen refueling station and to verify its facility combining with hydrogen refueling stations”.
To achieve the above target, we developed rapid hydrogen filling method and carried out safety
evaluation for hydrogen absorbing alloy tank in fiscal 2001.
5.2

Results of Research and Development in Fiscal 2001

5.2.1

Development of rapid hydrogen filling method(1) Design and trial manufacture of hydrogen absorbing alloy tanks
Based upon the result of rapid filling test for mini-sized MH (Metal Hydride) tanks performed
in fiscal 2000, we designed the divided chamber type tank and plate-fin type tank of full size andmanufactured experimentally one tank for each type . The effective storage capacity of full sizetanks (defined as difference in hydrogen absorbing capacity between at 1.1 MPa and at 0.1 MPa(atmospheric pressure)) was set for 31.25 Nm3 in this trial manufacture. A rare earth group AB5
alloy was selected as the MH to be filled in full size tank from the viewpoint of its stability andpoisoning tolerance during absorption/desorption cycles. As for the desorption characteristics,
the desorption plateau pressure at 60℃ was set for 0.55 MPa. Based upon the rapid filling test
performed in the preceding fiscal year, the hydrogen absorption condition was determined sothat 80 % of the effective storage capacity for the full size tank, i.e. 25 Nm3, should be filled
within 10 minutes at the coolant flow rate of 125 L/min., coolant temperature of 25℃ andhydrogen supply pressure of 0.8 MPaG.
We designed a new structure for the divided chamber type of tank of full size, which was easy to
manufacture and had the similar advantage in heat conduction performance as the tankstructure manufactured experimentally in the preceding fiscal year. Namely, the average
distance between MH and cold heat source was reduced by arranging a coolant pathway at theintersection of container wall and separators. Heat conduction plates were arranged radially at
the center of sintered pipe filters in the MH filling part for equalizing its temperature
distribution. The thickness of container wall was increased for ensuring the strength during
extrusion process of the manufacture. There was no problem in this new sectional shape,
because the simulation result showed that the hydrogen filling speed for this structure washigher than that for the structure used in preceding fiscal year and higher enough than thetarget. It was found that the temperature distribution for the new sectional shape duringhydrogen filling showed better uniformity than the structure in the preceding fiscal year. We-38-manufactured a small tank (an effective storage capacity of 1.95 Nm3) which is about 1.6 times
larger than the mini-sized MH tank (an effective storage capacity of 1.25 Nm3) manufactured in
the preceding fiscal year. A hydrogen storage capacity of 31.25 Nm3 was attained by arranging 8
small tanks in the upper half and 8 small tanks in the down half in parallel. We contained atank body and pipes for hydrogen and coolant of the full size tank in a box. Joints for pipe ofhydrogen and those for supply and return of coolant were equipped on the same side of the box.
For the plate fin type MH tank, we manufactured a small tank which was about 8.4 times larger
than the mini-sized MH tank (an effective storage capacity of 1.25 Nm3) manufactured in thepreceding fiscal year. A hydrogen storage capacity of 31.25 Nm3 was attained by arranging 3
small tanks in parallel. We contained a tank body and pipes of hydrogen and coolant for thefull size tank in a box, covering tank body and pipes with lagging. We designed that the small
size tanks had the same heat exchange performance per unit alloy quantity as mini-sized MH
tanks. Consequently, the basic structure for the small size tank was the same as the mini-sized
MH tanks, though the number of alloy layers and coolant layers were increased from 4 to 7,
and from 5 to 8. The width and length of the small tank was about 2.85 times larger and 1.81
times longer than the mini-sized tank.
Because there was difference in the effective MH storage capacity between for the divided
chamber type tank and for the plate fin type tank due to difference in their AB5 alloy
composition, MH quantity filled in the former was smaller than that the latter. As the plate fin
type tank had a structure attaching importance to heat conduction performance, it had larger
heat conduction areas for both coolant side and MH side than the divided chamber type tank.
Thus, an overall tank weight and volume for the divided chamber type tank was smaller thanthat for the plate fin type tank.
(2) Evaluation of performance of hydrogen absorbing alloy tanks
We carried out evaluation of the tank performance by means of hydrogen rapid filling tests inorder to verify that the full size tank in this trial manufactured satisfied the basic design
specifications and that the performance of the mini-sized MH tank was secured for the tank. Foran evaluation of the performance of the tank itself, the test was performed without the coupler
to be used in a hydrogen refueling station.
It was showed that the divided chamber type of full size tanks satisfied the design specification,
as it took 7 minutes to fill hydrogen of 25 Nm3 under the above hydrogen absorption conditions.
The performance was found to be similar to that of the mini-sized MH tank, because it took 6.5
minutes for filling hydrogen of 25 Nm3.
The result was obtained that hydrogen of 25 Nm3 was filled in 2.6 minutes for the plate fin
type tank of full size under the above hydrogen absorption conditions. As it took 2.3 minutes forfilling hydrogen of 25 Nm3 for the mini-sized MH tank, it was found that the performance of the-39-

plate fin type tank of full size was similar to that of mini-sized tank.
(3) Rapid filling test in combination with hydrogen refueling stations
We carried out rapid filling tests at the natural gas reforming type hydrogen refueling station in
Osaka with the divided chamber type tank and at the PEM electrolysis type hydrogen refuelingstation in Takamatsu with the plate fin type tank. The hydrogen stored in the MH type storage
unit was filled in the MH tank via a dispenser. After completion of hydrogen filling, thehydrogen filled in the tank was desorbed by warming the tank with heated brine supplied froma desorption unit provided by another WE-NET task. Though the hydrogen supply pressure had
been controlled both in the rapid filling test for a mini-sized MH tank performed in fiscal 2000
and in the evaluation test of the performance for a full size tank, the hydrogen supply flow rate
was controlled in the rapid filling test at the hydrogen refueling station. The fill of hydrogen
increased linearly with time, as the hydrogen flow rate was controlled for constant.
As the result of test for the divided chamber type tank of full size under the following conditions
as coolant temperature of 5℃, coolant flow rate of 125 L/min, hydrogen supply flow rate of 2.83
Nm3/min, it was ascertained that we can fill hydrogen of 25 Nm3 , 80 % of the effective storage
capacity of the tank, within 10 minutes as the target. It took 9.3 minutes for filling hydrogen of
25 Nm3.
As the result of test for the plate fin type tank of full size under the following conditions as
coolant temperature of 5℃, coolant flow rate of 125 L/min, hydrogen supply flow rate of 2.83
Nm3/min, it was ascertained that we can fill hydrogen of 25 Nm3 , 80 % of the effective storage
capacity of the tank, within 10 minutes as the target. It took 9.1 minutes for filling hydrogen of
25 Nm3.
5.2.2

Safety evaluation for hydrogen absorbing alloy tanks(1) Fire-resistance evaluation for hydrogen absorbing alloy tanks
In the fire-resistance evaluation conducted in fiscal 2000, the test was performed for MH
tanks equipped with a spring type relief valve to prevent destruction of the tank caused by riseof pressure. As the pressure had not been entirely released even by opening the relief valve, thetemperature rose under nearly the set pressure for the relief valve. Because the strength of Al
alloy used as the tank material decrease suddenly around 150℃, withstanding pressure of thetank decreased and the tank was destroyed at last. Therefore, in this fiscal year, we learned thesituation in the test for the MH tank with a fusible plug, which released the internal pressure
entirely by melting as temperature increased. We also studied effects of operating temperatureof fusible plug and tank materials upon its destruction. Furthermore, the fire-resistance test
was conducted for a tank unit composed of a number of full size tanks.
As for the operating temperature of the fusible plug, we used a 105℃ type and 177℃type-40-

fusible plug, the former was the same kind as those mounted on valves for high pressure gas
containers in general, and the latter was used for comparison. The test was conducted by fixing
the hydrogen absorbing alloy container on a test jig and by igniting methanol filled in a stainlesssteel vessel, which was installed at the bottom of tank. The quantity for methanol was selected
to be 18 Litter corresponding with about 50 minutes of combustion. The relief pressure of spring
type relief valve was precedently set on 1.4 MPa.
Under the condition where hydrogen quantity had been fully charged (H/M = 1.0), the spring
type relief valve operated in less than 2 minutes. Under the condition where hydrogen quantity
had been nearly empty (H/M = 0.2), it took 3 to 4 minutes till the spring type relief valve
operated. On the other hand, it took a longer time till the fusible plug operated, when thehydrogen storage was high. It was due to that it became more difficult for tank temperature to
rise, as the process of desorption of hydrogen from the alloy was endothermic reaction. The
higher the set temperature of fusible plug, the longer was the time for operation of fusible plug.
Taking a sudden decrease of Al-alloy strength above 150℃ into consideration, we consideredthat an 105℃ fusible plug widely used for high pressure gas applications was appropriate.
From the result of experiment, it was proved that the burst of tank could be prevented when thefusible plug operated and the residual tank pressure reduced down to atmospheric pressure. At
the completion of test, no burst or deformation of the tank was observed.
In case of using SUS as the tank material, no burst occurred even without mounting the fusible
plug in the test.
The fire-resistance test for the tank unit composed of 5 full size tanks was conducted outdoors.
As the safety valve, we used a spring type relief valve and a fusible plug of 105℃ specification.
The test was performed with the tank unit fixed on a test jig and with gasoline filled in astainless steel vessel installed at the bottom of tank. The gasoline quantity was so controlled as
to combust for about 20 minutes that was sufficient time for operating the safety valve. We
conducted the tests twice, changing the number of fuel bats and their locations. In the test No.1,
the flame attacked the fusible plug area directly when the entire hydrogen absorbing alloy tank
was in the flame. In the test No.2, no flame attacked the fusible plug area directly, using two
fuel vats. In the test No.1, the rise of the tank pressure stopped at about 0.8 MPa and then thetank pressure was decreased down to atmospheric pressure. This was due to that the fusible
plug had operated before the pressure reached the set pressure of spring type relief valve. On
the contrary, in the test No.2, the tank pressure reached up to 1.4 MPa and the fusible plug
operated after the spring type relief valve had operated, because we arranged this test so as toprevent the flame attacking the fusible plug.
In both tests, hydrogen emitted through either the spring type relief valve or the fusible plug
was ignited. It was confirmed in the test of the previous fiscal year that as the hydrogen-41-

desorption reaction was endothermic, it took a long time for hydrogen to discharge fromhydrogen absorbing alloy, while gas was lost in a short time in case of high pressure gas storage.
Also in the test for this time, the combustion continued till the end of test, even though theflame size became smaller. Attention must be paid to that the desorption of hydrogen continued
corresponding to the rise in temperature of the hydrogen absorbing alloy, as long as the firecontinues. We verified that hydrogen absorbing alloy tank itself showed no deformation ofcontainer after completion of the test and it was ascertained that we could prevent an abnormalrise in pressure of the tank, deformation and burst of the container by using a spring type relief
valve and a fusible plug together.
(2) Deformation test for mini-scaled tank
As various factors have effects on the deformation of hydrogen absorbing alloy tank due torepetition of absorption and desorption cycle, it is important to clarify a causal relationshipbetween the deformation and various factors. Therefore, we paid attention to the structural
factor, and tried to clarify degree of effects of various factors on deformation using mini-scaled
tanks, in this fiscal year.
From the viewpoint of weight saving, we adopted aluminum alloy as the material of tanks.
Two kinds of filter structure were used. One is a plate filter which is cheap and easy to
manufacture, and the other is a tube filter used for the full size tank. In order to investigate theeffects of honeycomb structure adopted for the full size tank promoting heat transfer andseparation plates equalizing filling rates of alloy in the tank, we manufactured various kinds oftanks combining these structure with the above filters. Further, we changed wall thicknesses oftank with the tube filter structure according to three levels of 2, 3 and 4 mm in order toinvestigate the influence to tank strength. As alloy material, we adopted AB5 (Mm(Ce = 0.5) Ni5)
and BCC (Ti20Zr5V35Cr40) having volume expansion rate on hydrogen absorption of 20 % and
40 %. Regarding that stress occurred in MH tank by expansion of alloy, we manufactured AB5tanks with filling rates of 40, 45 and 50 %, and BCC tanks with those of 35, 40 and 45 %(considering its higher expansion rate), as limit values obtained from our experience.
The strain increased as alloy filling rate increased. At the filling rate of 50 %, the maximum
strain of 1354

(strain in scale of 10-6) was generated at the cover side of the tank with wall

thickness of 2 mm. The strain tends to be reduced in general, though there was some variations
in data, as wall thickness increased, consequently, the strength of tank improved. Strain wasespecially large for the case of the specification of wall thickness of 2 mm and high filling rates.
All data on BBC resulted in much smaller strain value compared with those on AB5. It isconsidered that this was resulted from the difference of bulk density.
For plate filter, it was found that the repeated absorption/desorption induced the advance of
deformation at the bottom part. Then, it was proved that the tank with the structure having-42-

separation plates and honeycomb combined with plate filter had smaller strain than the tankwithout it, and no advance of strain was observed at some measuring points. For tanks with theplate filters, however, cracks occurred at 70 cycles for both case of wall thickness of 2 mm and 3mm. But, no crack was found in tanks with tube filters.
5.3
5.3.1

Future Subject in Research and Development
Development of rapid filling method

Following the work in this fiscal year, rapid filling tests will be conducted under various
different supply conditions of hydrogen and coolant, in the hydrogen refueling station. In the
rapid filling tests conducted in this fiscal year, we filled an MH tank from empty to 80% of theeffective storage capacity. As there remains some hydrogen in MH tanks when actual filling, it isnecessary to investigate the influence of residual quantity on filling time.
5.3.2

Safety evaluation of hydrogen absorption alloy tank

Based upon the result obtained in this fiscal year, the following evaluations will be made:(1) Fire resistance test for hydrogen absorbing alloy tank
Verification of sealing property of spring type relief valve to hydrogen and investigation of thelocation of safety valve for tank unit of practical scale for real vehicle.
(2) Evaluation of deformation in tank by hydrogen absorption/desorption
The estimation technology for stress occurred by alloy expansion will be required for thedesign of strength of tank.

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