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To reduce the emission of carbondyoxide (CO2) of combustion engines, liquefied natural gas (LNG) is used as an alternative fuel. LNG is transported via truck, ship or railway for long distances. Double walled stainless steel tanks are used for transportation, which are heavy and expensive. The vacuum insulation between the two walled structure ensures that the LNG stays liquid over the transportation time (boiling point of LNG: -162 ◦C). This causes a high temperature difference between the transported good and the ambient air.
A simplified tank construction is used to reduce the weight and price of the tank. Instead of stainless steel, glass fiber reinforced plastic (GFRP) is used. The design is changed to a single walled construction with a solid insulation material outside on the GFRP structure.
Goal of this work is the characterization of a suitable insulation material and configuration as well as the analysis of the mechanical properties of GFRP under cryogenic conditions. Several experiments are carried out. Numerical models of these experiments can then be used for parameter studies.
The use of glass-fiber reinforced plastic (GRP) can reduce the weight of tanks significantly. By replacing steel with GRP in tanks for gases (propane, etc.) a weight reduction of up to 50 % was reached. In this project not only the material should be optimized, but also the design. Previous tanks consist of a double-walled structure with an insulation layer between the two shells (e.g. vacuum). Goal of this project is to realize a single-walled construction of GRP with an insulation layer on the outside.
To determine the temperature dependent material values, two different experiments are performed: In the first experiment, temperature dependent material properties of liquid nitrogen found in literature research are validated in a simple setup. The level of liquid nitrogen in a small jar is measured over the experiment time. Numerical simulation shows the change of nitrogen level with sufficient precision. In the second experiment, a liquid nitrogen is applied on one side of a GRP plate. Temperature is measured with thermocouples on top and bottom of the GRP plate, as well as in the middle of the plate. By use of numerical simulation, temperature dependent thermal conductivity is determined.
In the third experiment, a test stand is designed to examine different insulation materials. In this test stand, the insulation material can easily be changed. A numerical simulation, in which the determined material data is used, is performed as well for this test stand.
The experiments show, that GRP can be used in cryogenic environments. Multiphase simulations are a suitable tool to describe the energy absorption of thermal energy due to thermal phase change. Results on different insulation materials will follow.
This paper describes the testing methods used to determine the thermal properties of insulation materials and mechanical properties of materials used for the load-bearing structure for pressure tanks (up to 4 bar, relative) and cryogenic liquids (LNG, −166 °C to -157 °C at atmospheric pressure). Goal is to design a transportation tank that does not exceed 4 bars (relative) within 10 h, starting at atmospheric pressure. PUR-foam is a suitable material for the insulation. A 12,5 l small scale tank prototype reached 4 bar (relative) within 87 minutes, which is, regarding the influence of the size, a satisfying result. The mechanical properties change significantly at cryogenic temperatures. The bending modulus is similar at first, but decreases at a certain point by appr. 50 %. However, the maximum stress is much higher and could not be reached within this testing setup.
To reduce the emission of CO2, liquified natural gas (LNG) is used as fuel. As the pipeline network is not developed all around the globe, LNG needs to be transported via ship or truck. Double-walled tanks made of steel with a vacuum insulation are currently used to keep LNG at cryogenic temperatures (-162 °C; 111 K). The double-walled construction makes the tanks heavy and expensive. Furthermore, there are some restriction to carry out in-service inspection using a double-wall design. Main topics: Lightweight design, superior thermal properties of GRP compared to steel Aim of this project is to develop a single-walled tank made of glass-fiber reinforced plastic (GRP) and an insulator, so that the tank pressure will not exceed 5 bar within a certain time, relative, as a result of the rising fluid temperature. First, the thermal and mechanical properties of GRP and the insulator at cryogenic temperatures must be determined. Liquid nitrogen (-196 °C; 77 K) is used for all experiments at cryogenic temperatures for safety reasons. Mechanical properties are analyzed by performing 3-point bending tests on cooled specimen. The tests show, that there are now significant changes on the mechanical properties of GRP, so that this material can be used in a cryogenic environment. To examine the thermal conductivity of GRP, a test rig is designed, in which one side of a GRP-laminate plate is cooled down while the other side is at room temperature at the beginning. Temperature is measured on both sides of the plate as well as inside the laminate. The temperature curves are then implemented in a ANSYS simulation to calculate thermal material properties. The experiments show, that the thermal conductivity of GRP is much lower than the one of steel, but still not low enough to design a single walled tank without an additional insulation. Therefore, a closed GRP pipe with insolation inside is immersed in liquid nitrogen for a defined time. Sensors record the surface temperatures inside and outside the specimen, as well as between insulation and GRP. With the data gained in this experiment, another ANSYS model is done. For correct simulation of the heat transfer between insulation and liquid nitrogen (or LNG later), a fluid simulation is necessary, which simulates the phase change from liquid to gaseous nitrogen. After validation of the model, a parameter study in the material properties of the insulation is performed, until a satisfying setup is achieved.
Präsentation im Rahmen eines BAM-internen Kolloquiums über den aktuellen Stand der Forschung im ZIM-Vorhaben "Tieftemperatzr-Transporteinheit aus Faserverbundkunststoffen"
This presentation describes the testing methods used to determine the thermal properties of insulation materials and mechanical properties of materials used for the load-bearing structure for pressure tanks (up to 4 bar) and cryogenic liquids (LNG, -162 °C at atmospheric pressure). Goal is to design a transportation tank that does not exceed 4 bars (relative) within 10 h, starting at atmospheric pressure. PUR-foam is a suitable material for the insulation. A 12,5 l small scale tank prototype reached 4 bar (relative) within 87 minutes, which is, regarding the influence of the size, a satisfying result. The mechanical properties change significantly at cryogenic temperatures. The bending modulus is similar at first, but decreases at a certain point by 2/3. However, the maximum stress is much higher and could not be reached within this testing setup.