WE-NET Home Page/Summary of Annual Reports 1997

5.2 Development of liquid hydrogen transportation tanker

5.2.1 R&D Goals

Goals of studies in the Phase I are that to make a conceptual design of large-scale liquid hydrogen tanker ships, and to select technical items to be developed necessary for constructing it.

In the New Sunshine Program, the hydrogen energy system is considered one of the most promising candidates for energy systems in the 21st Century. Hydrogen in liquid form (LH₂) is considered to be an effective energy carrying media in the system. The WE-NET study is being conducted to establish a basis for evaluating all technical elements related to the hydrogen energy system.

In Subtask 5: Development of Hydrogen Transportation and Storage Technology, a liquid hydrogen tanker is being developed to transport large volumes of liquid hydrogen efficiently and safely from production sites to places of consumption.

The major research items that have been tackled in the past four years are as follows:

(1) FY1993

Study of the physical property values (nature) of liquid hydrogen
Research of data on material for the tank, thermal insulation, and support materials
Research of current technology of LNG carriers, on-land hydrogen tanks, etc.
Examination of technical problems of liquid hydrogen tankers

(2) FY1994

Examination of specifications required for pilot design
Examination of basic technologies for the hull, tank, tank supports, thermal insulation of the tank, and dome, etc.
Pilot design of a liquid hydrogen tanker with a capacity of 200,000m³

(3) FY1995

Examination of the required performance of the thermal insulation structure and candidate insulation materials
Examination of different thermal insulation structures

(4) FY1996

Examination of technical issues relating to different thermal insulation structures, specifically, to the following items including experimentation plan necessary for resolving the technical problems expected:
Non-vacuum Hold + PUP (Polyurethane Foam): Verification experiment of thermal convection influence
Non-vacuum Hold + Vacuum Panels: Evaluation of the thermal conduction influence of paneling cover
Vacuum Hold + PUP: Measurement of PUP degasification amount
Super Insulation (SI): Verification experiment of vacuum system influence
All insulation method: Verification of the thermal conductivity at the liquid hydrogen temperature level of the thermal insulation material (elemental experiment of thermal insulation)

The objectives of the study in fiscal 1997 are as follows:

To design test pieces of insulation materials in each thermal insulation systems which will be applied for elemental experiment of thermal insulation scheduled in fiscal 1998.
To collect knowledge to examine research items required in the phase II by examining issues of thermal insulation system and methods for resolving these issues through design of test pieces

5.2.2 Results in fiscal year 1997

5.2.2.1 Outline and Objectives of Elemental Experiment of Thermal Insulation

It is scheduled to conduct elemental experiment of thermal insulation in a joint effort with the liquid hydrogen storage subtask team (ST5-3) in the fiscal year of 1998. The objective of the experiment is to verify thermal insulation performance and behavior of each material at the liquid hydrogen temperature. The test equipment is outlined in Fig. 5-2-1.

The inner space is evacuated in order to eliminate the influence of thermal convection and to evaluate only heat input going through the test piece from the heater. Heat generated by the heater contacted at the bottom face of the test piece goes through the test piece, and makes liquid hydrogen evaporate. The mean thermal conductivity of the test piece is calculated by measuring the amount of liquid hydrogen evaporated.

Along with planning of the test, in order to carry out it properly, we researched and reviewed the development experiences of LNG carriers and liquid hydrogen tanks on land.

5.2.2.2 Test Piece Design

With regard to each test piece, we reviewed anticipated problems in conducting elemental experiments of thermal insulation and examined methods for resolving these problems.

(1) Polyurethane Foam (PUP) Panels

We proposed 2 types of PUP panels: with a joint and without a joint. The test piece is outlined in Fig. 5-2-2.

A) Deformation of the test piece
Because the top-side face of the test piece will be exposed to very low temperature and the bottom-side face to an ambient temperature, PUP panels will be warped by the heat stress. That will make a clearance between the panel and the heater, or between the panel and the liquid hydrogen tank. The clearance disturbs heat flux in the PUP panels and causes measurement errors. According to FEM calculation, the deformation will be 10mm at the maximum. We proposed the following countermeasures to minimize the heat deformation:

Fix copperplates to both the top-side and bottom-side faces of the panel and prevent deformation forcibly.
Fix a copperplate to the bottom-side face of the panel and prepare the panel in the shape taking the heat stress deformation into account.
Slit the panel and absorb the heat stress deformation

We conducted FEM analysis of these countermeasures. As a result, we found that the method B can control the deformation of the PUP panel, consquently, a special adhesive effective at the low-temperature is not necessary.

B) Heat influence from the edge part
When we attempt to measure the thermal conductivity accurately, it is necessary to intercept the influence of infiltrating heat from the edge of the panel. To cope with this problem, the distance to the measuring point from the edge of the test piece must be as long as possible, and a radiation shield might be effective. We calculated the width and heat flux infiltrating from the edge, and examined the influence on measurement errors. As a result, we found that we can reduce the measurement error up to 0.1% by making the aspect ratio of the test piece with 200mm in thickness to of 1200mm in diameter together with a radiation shield.

C) Effect of gas in the PUP panels
When we measure thermal insulation performance in vacuum space, it is necessary to clarify the relations between the degree of vacuum and thermal insulation performance.

However, when we attempt to evacuate the PUP material, a foam gas in the cells of the PUP will permeate out, which may degrade the degree of vacuum in the test device. In order to avoid this, it is necessary to evacuate the PUP for a long time enough prior to the test.

D) Evaluation of measurement errors at the joint part
It is supposed that joint will extend to a half distance of the panel in thickness. The joint will be made within the center part of the panel, which will be contacted to the main LH₂ tank in order to avoid heat infiltration form the edge of the panel, because it will be sufficiently great to disturb the accurate measuring, if the joint is extended to the panel edge. The temperature of the PUP surface becomes uniform, if the heater contacts per6ectly to the PUP panel. However, a small vacuum space might exists at bottom face of the test panel, where the joint part will cause a temperature distribution of the panel. Therefore, in order to simulate the actual structure, we suggest that a jointed test piece has a joint on the bottom face, in addition, make space between the test piece and a heater.

(2) Vacuum Panels

We propose two types for vacuum panel insulation: with a joint and without a joint. The test pieces are outlined in Fig. 5-2-3. We pointed out the technical issues listed below to be resolved prior to the test panel preparation. In order to resolve these, we have conducted a vacuum bag test and are conducting a small vacuum panel test.

Selection of the most suitable core material
We prepared vacuum bags made of aluminum laminated film containing core materials of PUF, ceramics and silica beads, and measured their thermal conductivity. In addition to the vacuum bag tests, we prepared small vacuum panels of 280nm in diameter and 50mm in thickness using PUF, ceramics and silica beads as core material, and compared their thermal conductivity. An aging test, a thermal shock test and a thermal deformation test will be conducted in order to determine the most suitable core material for the test piece.
Aging effect
In the preliminary evaluation applying the vacuum bags, silica beads had the best aging durability. More evaluation will be made using the small vacuum panels.
Study for panel production
A practical manufacturing method of the panel will be worked out through the preparation of the test panel.
Measurement of evacuation time
Time required for evacuation will be estimated in preparing small vacuum panels and large vacuum panels to establish practical manufacturing time.
Thermal shock test and deformation test
Small vacuum panels will be soaked in the liquid nitrogen and verify deformation and damages of the panels. The panels will be cooled by the liquid nitrogen and deformation amount will be measured.
Evaluation effects by the joint
One of the major objectives of the experiment is to gauge the effects of a joint by comparing the panel with a joint and one without it. However, it is necessary to examine how to limit experimental errors within permissible range caused by a joint that connects vacuum panels.

(3) Super Insulation (SI)

In term of super insulation, we propose two types: thermal insulation material with joints and without joints. The test piece is outlined in Fig. 5-2-4.

Measuring method / accuracy
The inside of the test equipment is anticipated to be evacuated to the high vacuum at around 10-5 Torr due to cryopump effect. Thermal insulation performance of SI is very high, so that the heat flux through the insulation structure is considered to be relatively small. Consequently, it is difficult to evaluate measurement errors as well as to measure thermal insulation performance itself. Therefore, it is necessary to take accuracy of measuring instruments into consideration and to examine specifications of insulation materials and other factors for accurate measurements.
Heat transfer from the inside of the test equipment
The test piece will be exposed to the heat transfer by radiation from inside of the equipment as well as that from the main heater. Therefore, it is necessary to take measures to limit such heat transfer to the least. As a countermeasure to cope with this problem, we proposed to place the main heater plate as close as possible to the test piece without contacting it and to equip with a shield plate surrounding the edge of the test piece.
Control of degree of vacuum
It is necessary to make experiments by varying degree of vacuum in the test equipment in order to verify influence of degree of vacuum on the thermal insulation performance. We planned to control degree of vacuum (pressure) in the equipment by putting He gas into it. However, we found it causes measuring errors because He gas would generate heat convection. Consequently, we proposed to examine the influence of vacuum degree to thermal insulation performance in the experiments using small model tank test scheduled in the phase III.
Evaluation of effects of joints to the thermal insulation performance
We are planning to evaluate effects of joints by comparing test results of test pieces with joints and without joints. As for the test piece with joints, we plan to prepare in a form in which 10-layer SI sheets are overlaid alternately. It is possible that overlaying condition and degree of contact between SI sheets effect its thermal insulation performance. Further examination including preparing method is necessary.

5.2.3 Research plan for fiscal year 1998

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.