Heat leak variation with the surface temperature of a cryogenic pipe of the superconducting power transmission

Abstract The measurements of the heat leak variation with the surface temperature of a cryogenic pipe of the superconducting power transmission were performed in the range of 23.7 °C–36.4 °C. The cryogenic pipe which was an object of the present study was that used in the project of the superconducting DC power transmission conducted in Ishikari, Japan (Ishikari project). This cryogenic pipe has two inner pipes in one outer pipe, with a radiation shield used to conceal the inner pipe installing the cable from the outer pipe, which is at room temperature, to reduce heat leakage. A test pipe with the length of 12 m was used for the measurements. The liquid nitrogen was filled in the test pipe and the evaporated nitrogen gas rate was measured to obtain the heat leak. The heat leak to the inner pipe installing the cable was almost constant at around 0.04 W/m, whereas the heat leak to the other inner pipe used to return liquid nitrogen for circulation was around 1.3 W/m at surface temperatures ranging from 23.7 °C to 36.4 °C. The latter, with previous measurements, was well fitted by a function considering radiative heat transfer and conductive heat transfer. Portions of the radiative heat transfer and the conductive heat transfer were separated with this function. This information can be used to improve the cryogenic pipe in the future.