The superconducting DC power transmission is one of the most efficient ways of transmission for energy saving, because the loss due to the electric resistance is not exist for the use of the superconductivity. On the other hand, to use the superconductivity, the power is required for the cooling, which becomes the loss of the transmission. Therefore, the heat leak to low temperature parts should be reduced to reduce the power for cooling and to make the superconducting DC power transmission efficient. The cryogenic pipe, which gives thermal insulation, has been used for the transmission line. Since the heat leak depends on the surface temperature of the cryogenic pipe, it varies with environmental conditions, such as the air temperature, the weather and the duration of sunshine, where the cryogenic pipe is installed. The information of the heat leak variation with the surface temperature of the cryogenic pipe is required to design the cooling system.

We measured the heat leak variation of the cryogenic pipe at the surface temperature between 23.7 and 36.4 °C. A schematic cross section of the cryogenic pipe tested is shown in figure 1. It has two inner pipes in a single outer pipe for the circulation of liquid nitrogen. One inner pipe is used to install a cable and another is to return liquid nitrogen, which we called the cable pipe and the return pipe, respectively. Smooth stainless steel pipes, in part bellows pipe, are used for the inner pipes and a smooth carbon steel pipe for the outer pipe. A radiation shield surrounding the cable pipe is adopted to reduce the heat leak to the cable pipe, which is thermally anchored to the return pipe. It is expected that the heat leak to the cable pipe is significantly reduced and, as the result, the temperature rise along the cable is suppressed. The cable pipe and the radiation shield are wrapped with multi-layer insulation to reduce the radiative heat transfer.

A 12 m test pipe was used for the present measurements. After evacuating the space between the inner pipes and the outer pipe, the liquid nitrogen was filled in the inner pipes. The flow rates of the evaporated nitrogen gas were measured, from which the heat leak was estimated. The outer pipe temperature was controlled with heaters wound around the cryogenic pipe.

The results of the heat leak to the return pipe per 1 m of the cryogenic pipe increased from 1.2 to 1.3 W/m for the increase of the outer pipe temperature from 23.7 and 36.4 °C. In contrast to these results, those to the cable pipe were almost constant in the range of the measurements and about 0.04 W/m. By the effect of the radiation shield, the heat leak to the cable pipe was significantly reduced. The details of the experiment will be presented at the symposium.

Keywords: Superconducting power transmission, Cryogenic pipe, Heat leak