Cryogenic design of liquid-nitrogen circulation system for long-length HTS cables with altitude variation

Liquid-nitrogen (LN2) circulation systems with altitude variation are investigated and designed for application to practical long-length HTS cables. This study is motivated by the KEPCO's immediate plan to install new HTS cables in existing utility tunnels including inclined and vertical sections. The distribution of pressure and temperature along the LN2 circulation loop is examined for various geographic conditions, taking into account the gravitational effect of altitude variation. The cryogenic cooling requirements are defined in terms of the pressure and temperature of LN2, and a design process is graphically demonstrated on phase diagram. It is concluded that the LN2 flow rate along the cable with altitude variation should be carefully determined with the constraints on pressure and temperature, and the proposed graphical method is useful in the design.