Heat leak of cryogenic pipe for superconducting dc power transmission line (SCDC)

Main loss of the superconducting DC power transmission line (SCDC) comes from the heat leak of the cryogenic pipe because of no internal heat generation in the dc cable. It should be minimized to realize the high performance of SCDC. The heat leak of the cryogenic pipe depends on the ambient temperature, and when it is high, the heat leak is also high. Therefore, we estimated the sensitivity of the heat leak for the ambient temperature at first, and estimated the effect of this for the distance between the cryogenic stations. The major process of the heat leak is the heat transfer through the multi-layer insulation (MLI) and the second process is the thermal conduction of the support leg of the inner pipe in our design. The MLI is made from the multi-layers of the aluminium-coated thin film (ACTF) with the spacer, and the support leg is made of the glass fiber reinforced plastic (GFRP). The aluminium layer reflects the infrared light highly to reduce the heat transfer by radiation, and the spacer can prevent the direct contact with the ACTF. Therefore, the weight of the MLI should be light to minimize the thermal transfer of the spacer. We tested various types of MLI to find the optimum MLI structure experimentally. Usually, the measurement of the heat leak is not easy in an actual cryogenic pipe and we need the long experiment time and a relatively large instrument to evaluate the heat leak. However, since we need to evaluate many MLIs, we developed the different way to estimate the heat leak through the MLI by a small experimental device. To find the optimum MLI, we started to test three samples of the MLI supplied from Kaneka Corp., and measured the temperature of the MLI. Since the heat capacity of MLI is low, we used the 50-micron meters thermocouple (TC) and attached it to measure the temperature of the MLI carefully. The result of the experiment shows that we found that the net type of the spacer is better than the spacer used in Ishikari project, and the heat leak would be half of the Ishikari project. We also measured the thermal conductivity of two test samples of GFRP to reduce the heat conductions of the support leg. The thermal conduction of these two GFRPs are almost 1/2 to 1/4 of the recommended values of the NIST. Because of these efforts to reduce the heat leak of the cryogenic pipe, we can expect the target values of the heat leak is a half of the Ishikari project in the next project, and if we can reach this value, we can extend the distance of the cryogenic stations to ∼100km actually on the earth. The values of this distance is important to transfer the electricity for all over the world like the gas and oil pipeline in the present time.