5. Task5 Development of Hydrogen Vehicle Systems
5.1 R&D Target
The target of R&D for the hydrogen vehicles in the second
stage is "the development of elemental technology for fuel system
of fuel cell powered vehicle which being expected a hydrogen
supply from a hydrogen refueling station, the technical execution
of running system of hydrogen vehicle which being combined with
a hydrogen refueling system and the estimation of energy efficiency
of the whole system and so on."
Development of the fuel cell powered vehicles is now going on actively worldwide.
Safety analysis of fuel tanks for the vehicles equipped with
hydrogen absorbing alloy tanks, is one of the technological
tasks commonly imposed on each relevant organizations and establishing
technical standards for safety of the tanks will be essential
for approval of the vehicles for the future. Purpose of this
consignment work is to study methodology of the safety analysis
of hydrogen storage tank with metal hydride and to accumulate
basic data that can be utilized for preparation of such technical
standards. Further, as for measuring method of the energy efficiency
of fuel cell powered vehicles, an unified standard has not been
established. It is necessary to establish methods to measure
energy efficiency, i.e., hydrogen fuel consumption rate, based
on reliable testing methods. In this work, measuring method
with high accuracy for fuel consumption rate of fuel cell powered
vehicles as simple and easy as existing methods for gasoline
engine and diesel engine automobiles. In fiscal year of 1999,
following studies were performed.
5.2 R&D Results in Fiscal 1999
5.2.1 Safety Analysis of Hydrogen Absorbing Alloys
As for safety of hydrogen absorbing alloys, judging tests
for dangerous substances based on the existing fire law and
the testing manual recommended by UN (abbreviated as UN method,
below) were applied to evaluate various samples with varying
alloy systems, alloy particle sizes, hydrogen capacity, etc.
As alloy systems, LaNi based alloys that are most frequently
used for this purpose were mainly evaluated and moreover, each
one kind of Ti-based laves phase alloys and BCC alloys were
chosen for evaluation. The judging tests for dangerous substances
resulted in that LaNi based alloys and BCC alloy did not fall
into the category of dangerous substances independently of their
particle sizes and hydrogen capacity. While, among the alloys
evaluated in this study, only the Ti-based laves phase alloy
fell into Category 3 dangerous substances. The alloys that had
not corresponded to the dangerous substances did not catch fire
in small tank rupture test as mentioned in Sec.5.2.2 below and
correlation between result of the rupture test and that of judgment
test based on the existing fire law and UN method could be confirmed.
5.2.2 Impact Rupture Test of (Mini-Scale) Hydrogen Absorbing Alloy Tank
For studying the safety of hydrogen absorbing alloy tanks,
it was considered to be important to grasp behaviors of hydrogen
and the alloys at the time of rupture of the tank caused by
car crush. Accordingly, mini-scale tanks filled with hydrogen
and hydrogen absorbing alloys were prepared and they were subjected
to impact rupture test in order to evaluate behaviors of hydrogen
and the alloys released into the air as a result of rupture
of hydrogen absorbing alloy tank filled with hydrogen caused
by collision or others. As fuel tanks, cylindrical vessels of
30 mm in diameter and 300 mm in length were used and as alloys
to be filled, two kinds of alloys, an alloy with composition
1-2 (Mm(Ce=0.5)Ni5) that did not fall into the dangerous substances
and an alloy with composition 3-1 (Ti0.7Zr0.3Mn0.8CrCu0.2) that
was judged to correspond to Category 1 spontaneous firing substances
by the hydrogen absorbing alloy safety test were selected for
the test. Further, difference of behaviors depending on amount
of occluded hydrogen in the alloy, tank filling rate and number
of test cycles(particle size) were also studied. Though several
kinds of rupture modes such as axial collapse, shear, three
point bending, cantilever bending, etc. could be considered
as possible rupture modes at the time when an impact force was
applied to the tank, however, test of impact three point bending
by drop weight was adopted taking into account of representatively
of rupture, easiness of the test, etc. Impact rupture test under
the condition above-mentioned revealed that:
(1) Ignition of hydrogen released at the time of rupture of
the tank, due to impact energy, friction energy, etc. did not
occur;
(2) In some cases, ignition occurred about 100 ms later than
the time of rupture, where ignition of hydrogen was probably
caused by spontaneous ignition of the alloy. Accordingly, it
is necessary to take the spontaneous ignition of the alloys
into account when a technical standard on fuel leakage at the
time of collision is examined.
(3) Spontaneous ignition at the time of failure of a tank is correlated
with judgment test for dangerous substance based on the fire
law.
5.2.3 Deformation Study on Hydrogen Absorbing Alloy Tank
Crystal lattice of hydrogen absorbing alloys expand and
contract in accordance with absorption and desorption of hydrogen
and this makes particle finer. If hydrogen absorbing alloy showing
such behavior is stored in a tank, the storage tank will be
pressurized due to volume expansion caused by pulverization.
It is possible that this causes deformation of the tank and
at the worst, rupture of the tank. This phenomenon is supposed
to be different from deformation simply caused by increased
internal pressure as seen on a high pressure tank. Accordingly,
it is important to grasp deformation behavior of a tank due
to repeated absorption and desorption of hydrogen for the purpose
of establishing technical standards and designing.
Therefore, in fiscal year 1999, behavior of deformation and aspect of rupture
were investigated. Aluminum tanks having a withstanding pressure
3 MPa were filled with hydrogen absorbing alloy. Then, storage
and release of hydrogen were repeated 1000 cycles under pressures
from atmospheric pressure to a pressure of 1MPa. As a result,
the tank showed relatively large deformation due to grain fining
within earlier 10 cycles, however, the tank showed no further
deformation afterward. No cracks nor other defects were observed
even after 1000 cycles of hydrogen absorption and desorption.
5.2.4 Measuring Method of Fuel Consumption Rate of Hydrogen Fuel Cell Powered
Vehicles
As a measuring method of fuel consumption rate, method of
adding up of continuously measured hydrogen flows on the fuel
supply side is possible. However, this method requires to put
a hydrogen flow rate measuring system between hydrogen store
system and fuel cell system, which also requires remodeling
of the vehicle system. Depending on situation, this remodeling
might hinder the normal operation of the vehicle. Moreover,
it is a problem how to reflect the energy balance between battery
and others during running on the fuel consumption measurement
for the fuel cell powered vehicles as they are often driven
by hybrid power system including battery and others.
Taking these situations into account, simple and easy methods with high precision
to measure fuel consumption rate suited for use for hydrogen
fuel cell vehicles were studied. Some candidate methods such
as flow rate measuring method, full tank method, electric current
method, hydrogen balance method and oxygen balance method were
examined for their applicability, precision of measurement,
etc. It was found that electric current method was the most
practical because of its supposed simplicity and high precision.
5.3 Future Plan and Issues
(1) Development of a rapid filling method of hydrogen in
a hydrogen absorbing alloy tank on a car
For the establishment of a rapid filling method of hydrogen, the most important
subject is the improvement of heat exchanging abilities of hydrogen
absorbing alloy and its reservoir tank. For quantitatively understanding
the heat exchanging abilities, the following shall be studied.
<1>A quantitative confirmation of a thermal conductivity of hydrogen
absorbing alloy and a heat transfer coefficient in hydrogen
absorbing alloy powder.
<2>An influence to temperature distribution in tank between different
hydrogen absorbing quantity or between different pressures in
hydrogen filling.
(2) Study for measuring method of fuel consumption
Since we received a proposal in 1999 that electric current method is practically
excellent, therefore the propriety of the method will be verified
by a generator with Solid Polymer Fuel Cells.
(3) Safety estimation for hydrogen absorbing alloy and hydrogen absorbing alloy
tank
A hydrogen absorbing alloy tank will be produced and with the tank an impact
test, a fireproof test in fire and a rupture test will be carried
out, and its safety performances in troubles will be certified
by the test results.
