The most common way for cooling an AC superconducting power cable is by means of forced circulation (pumping) of sub-cooled liquid nitrogen. In order for the cable to remain in superconducting state it is extremely important that the cooling media has a homogeneous density and flow in order to dissipate heat from the cable (thermal losses and heat generated by the so called AC losses). Therefore the liquid nitrogen is not allowed to boil and create gas bubbles since these will not take in this heat.
This subcooling of the liquid nitrogen flowing through the cable is achieved by increasing its pressure due to which the saturation temperature is raised. Now the liquid nitrogen can take in the generated heat by temperature increase rather than boiling.
Typically these systems operate in a temperature range of 66-72K at which temperatures a superconductor has it best characteristics. Sub-cooling pressures vary from 3-20 barg.
Stirling Forced Flow System
The above physical phenomenon is used in Stirling Forced Flow Cooling Systems which are specifically designed for this type of use: liquid nitrogen is subcooled (by pressurizing) and forced flowed (pumped) through the power cable in a closed loop by means of a cryogenic pump. The heat of the cable increases the temperature of the liquid while passing through.
This energy is removed from the LN2 flow by either cooling the liquid directly in the cold head of the Stirling Cryogenerator, or by dissipating the heat in a bath of boiling liquid nitrogen re-liquefied by the Stirling Cryogenerators.
The latter option is shown in below schematic with the heat exchanger on the right representing the power cable.
Although cooling of the cable may appear simple, in reality it is a rather complex system. First, an important number of components are not shown in the basic schematic (T & P transmitters, cryogenic valves, control units, safety valves etc).
Additionally a “thermodynamic – economical” equilibrium must be determined. Each component which will be integrated will come at a “thermal” cost: it adds to the total heat in-leak of the system.
Choice of a too small temperature difference over the cable requiring a high flow rate of the LN2 in combination with the small chosen diameters of the piping and cable cryostat, will lead to high pumping losses. This will cause a high heat load for the system, leading in turn to a large cooling requirement.
All engineering choices should therefore be in balance, assuring the total system has an optimal thermodynamic – economical balance, not only the cable itself.
Though the sub-cooled “normal” mode is straight forward, getting into that mode is another challenge. Before the cable can be put into this mode, it will need to be cooled down slowly to cryogenic temperatures to prevent thermal shock. This is called soft-cooldown by use of a cold gas flow. Once near final temperature, the cable is filled with liquid nitrogen and only then get sub-cooled.
Also, at some point in time the cable will need to be emptied and warmed up for maintenance. Both the cool-down and warming stages require valves and control to assure this is possible and done in a controlled way.
The Cryogenerators are generally equipped with capacity control which allows them to reduce their capacity down from 100% to 70 or even 50%. This will allow the system to follow the fluctuation in heat load of the application due to operating changes, without using a heater, saving energy.
Cable cooling system; 6.2 kW; 72K; 90 lpm; 16 bargHTS leaflet Stirling Cryogenics