5.3 Development of Liquid Hydrogen Storage Facilities

5.3.1 R & D goals

As part of the efforts to develop mass storage facility for LH₂, research was conducted to design a storage tank structure combining different storage methods and thermal insulation structures and also to examine and evaluate the feasibility of any combination. As a result, it was found necessary to confirm the performance of each thermal insulation structure under the actual temperature to select a suitable structure for the mass storage of LH₂ in view of the fact that there was a shortage of data on the basic performance of thermal insulation structures applicable to mass storage at the LH₂ temperature ,that is, thermal insulation performance and mechanical strength at low temperatures.

In regard to the "thermal insulation performance", the performance of the large-scale thermal insulation performance test apparatus, that had been fabricated in FY 1997, was checked. In regard to the "mechanical strength of thermal insulation structures", compressive strength of the thermal insulating material at LH₂ temperature was tested by using the relevant test apparatus fabricated under Subtask 6.

The research activities conducted in FY 1998 are summarized below.

5.3.2 Results in FY 1998

5.3.2.1 Thermal Insulation Performance Test

(1) Performance Test of Thermal Insulation Performance Test Apparatus

1. Test on Thermal Characteristics of Heater

   As the test apparatus used the guarded flat plate caloriemeter method, it was necessary to check that the one-dimensional heat flux running through the base plate of the measuring vessel via the specimen results in evaporating the LH₂ inside the vessel while maintaining thermal equilibrium.

   In order to compare the quantity of heat flow through the inspection face of the specimen with the quantity of heat calculated from the evaporated hydrogen gas volume, an electric heater in lieu of the specimen was attached to the bottom face, i.e. the side on which the specimen is installed, of the measuring vessel and was used to heat the inspection face to measure the quantity of evaporated hydrogen gas inside the measuring vessel.

   The test results showed that the quantity of heat calculated from the hydrogen gas evaporation is almost identical to 90 - 95% of the quantity of heat input by the heater (0 - 50 W) and neither the external heat input, i.e. heat input through faces other than the inspection face, to the measuring vessel which would presumably constitute a factor for measurement error nor the condensation of vapor inside the measuring vessel was large.

   In particular, when the level of LH₂ inside the protective vessel to shut off external heat input was lowered, the temperature in the upper section of the measuring vessel was roughly maintained at 20 K. Together with the small temperature gradient of the measuring vessel, the above observation confirms the extremely low level of external heat input by means of solid thermal conduction.

   In addition to the measurement of the degree of vacuum, temperature, pressure and liquid level at the time of the filling of LH₂, the performance of the components of (i) the instrumentation system other than the high temperature plate designed to control the load on the specimen and the temperature at the high temperature side and (ii) the evacuation system was checked in this test.

2. Results of Specimen Performance Tests

   In the case of the thermal insulation test using the type of specimen where a gap was introduced between the base plate of the measuring/protective vessel (face with a temperature of 20 K) and the low temperature face of the specimen, the applicability of the testing apparatus to this test was confirmed in the test on the heat characteristics of the heater. Meanwhile, in the case of the type where the low temperature face of the specimen is cooled by means of contact between the specimen and the base face of the protective vessel, the reduction of thermal contact resistance of this interface in a vacuum will present a stiff challenge.

   A specimen for the performance test as well as the performance check of the high temperature plate to add load to the specimen was fabricated using a porous solid insulating material and the thermal insulation performance test was conducted with vacuum grease applied to the contact face of the specimen to reduce the thermal contact resistance.

   The dimensions of the fabricated specimen were 1.2 m in diameter and 0.2 m in thickness with a weight of 80 kg. Although it had a fine porous silica structure conventionally used as a high temperature insulating material, TiO₂ and other metal oxides were added which block an infrared transmission in order to prevent thermal radiation in the low temperature range.

   Because of the insufficient pretreatment (drying) of the specimen, the water contained in the specimen caused poor vacuum at the early stage of exhaust. After cooling with the water removed, little gas was released and the level of the vacuum at 10^-6 Torr (when LH₂ was filled) was achieved.

   In regard to the thermal insulation characteristics, insufficient thermal contact with the base face of the protective vessel (20 K) only produced a minimum temperature of approximately 70 K at the low temperature face. With the uneven temperature distribution, a high temperature region was appeared inside the specimen, failing to meet the test criterion.

   There were presumably several factors causing this failure, including the flatness of the specimen, loading mechanism pressing the specimen down onto the base face of the LH₂ vessel, flatness of the base plate of the LH₂ vessel, material used to reduce the thermal contact and strength of the high temperature plate. After careful examination of these factors, the loading mechanism was improved and the flatness of the base plate of the LH₂ vessel was adjusted using a block gauge.

   In regard to the temperature control of the high temperature plate, the use of a high temperature plate heater (60 W) successfully maintained the entire face of the high temperature plate (an area of 1.1 m² ) at 290 K from the 250 K recorded for the high temperature face of the specimen.

3. Time Required by Specimen to Achieve Thermally Stable State

   One important requirement for the thermal insulation performance test is estimation prior to the test of the time required by the specimen to reach the thermally equilibrium state with the quantity of LH₂ vapor inside the measuring vessel becoming constant.

   For the present test, the heat flow inside the specimen was assumed to be governed by the solid thermal conduction and non-equilibrium heat calculation was conducted with a fine porous silica structure and hard polyurethane foam, etc. with a different thermal diffusivity, i.e. thermal conductivity divided by density-specific heat, to estimate the time in question. The estimated time for the fine porous silica structure was approximately 10 days for it to achieve a thermally equilibrium state from the initial temperature of 300 K.

(2) Design of Specimen for Thermal Insulation Performance Test

As membrane-type storage requires support for the membrane which lacks pressure resistance, the thermal insulation structure must combine a thermal insulation section and supporting section at every part. In FY 1997, a specimen with the same structure as the actual equipment was examined to measure the performance of the entire thermal insulation structure. In FY 1998, however, a specimen of which the purpose was to measure the performance of the glass-fiber composite(FRP) support unit as the main structure was examined and evaluated in terms of the thermal stress, thermal deformation and heat gain in the testing apparatus at the time of a low temperature load so that the specimen could be made smaller.

5.3.2.2 Strength Test of Thermal Insulation Structure

(1) Compressive Strength Test of Thermal Insulation Structure Material

The low temperature strength testing apparatus fabricated under Subtask 6 (development of cryogenic materials) was used as the testing apparatus. Methods to prevent the scattering of the broken pieces of the specimen (to prevent clogging of the pipes) and to fix the specimen during the filling of liquid nitrogen were devised and their effectiveness was confirmed through the test.

It was also confirmed that the type of specimen breakage was collapse at the loading face.

(2) Strength Test Results of Solid Thermal Insulation Material

The test in FY 1998 was conducted with 90 kg/m³ class PUF, one of several types of PUF with a different density planned for use, to be used for the general sections of the tank's bottom. The PUF used in the test was made by the CO₂ forming method in view of the future suspension of the fluorocarbon substitute forming method.

The test was conducted at three temperature levels, i.e. ambient temperature, LN₂ temperature and LH₂ temperature, to establish the respective compressive strength of PUF. The test results showed that there is no significant difference between the LN₂ temperature and the LH₂ temperature for the compressive strength of the PUF tested. It was also shown that the compressive strength at low temperatures is approximately double the corresponding strength at the ambient temperature. These results indicate that the design of PUF using its compressive strength at the ambient temperature as in the case of LNG storage tanks promises the sufficient safety of PUF applied for the bottom of the LH₂ tanks.

5.3.3 Research Plan for FY of 1999

After the modification, one or two test piece(s) of thermal insulation structures out of those designed in this fiscal year will be prepared and measured its thermal conductivity, and checked its insulation performance at 20K.