cable cooling | Stirling Cryogenics
HTS power cable cooling
The most common way for cable cooling 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 an 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 “bubbles”.
This is achieved by subcooling the liquid nitrogen that flows through the cable by bringing the liquid below its saturation temperature by increasing the pressure. The liquid nitrogen can now absorb the generated heat without boiling. Typically these systems operate in a temperature range of 66-72K (since this is the temperature range where the superconductor has it best characteristics). Sub-cooling pressures vary from 3-20 barg.
Stirling Forced Flow System
The Stirling Forced Flow Cooling systems 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. This energy is dissipated either in a second bath of (boiling) liquid nitrogen on which the Stirling SPC cryogenerators cool away the released energy (1), or directly on the cold head of the Stirling SPC Cryogenerator. Option 1 is shown in below schematic (the heat exchanger on the left represents the power cable).
Although cooling of the cable may appears 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). Since reliability is a major concern most of them require redundancy, what needs to be integrated cryogenically.
Additionally a “thermodynamic – economical” equilibrium must be determined. The number of coolers that can be connected to this system is unlimited in theory. Each component which will be integrated/added will come at a “thermal” cost: it adds to the total heat in-leak of the system.
Last but not least, the temperature difference over the cable allowed by its designer (which determines the flow rate of the LN2), in combination with the diameters of the piping and cable cryostat, can result in pump and friction losses forming an additional heat load for the system.
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 (slowly) has to be cooled down to cryogenic temperatures (to prevent thermal shock), called soft cooldown, filled with liquid nitrogen and then sub-cooled.The available cooling power can be set up modular based on the number of cryogenerators and the other parameters has to be determined during a technical discussion.
The cryogenerators are generally equipped with capacity control which allows them to reduce their capacity down form 100% to 60% . 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.
Stirling offers the cryogenerators in combination with the additional equipment that is needed for this kind of operation. Generally cryostats containing the pressurizing equipment, heat exchanger and liquid nitrogen pump. A valve box to allow the different operation modes and containing the instrumentation. The equipment can be configured upon customer requirements and can be executed redundant (pumps, cryogenerators, instrumentation etc.).
Special precaution is needed with the design of the coldhead, in order to prevent possible freezing of nitrogen. When the plant operates close to the freezing point of nitrogen (i.e <70K), nitrogen could freeze(solidify) on the cold head exchanger as the coldhead itself always operate a couple of degrees below the temperature of the liquid gas. If this happens the coldhead will be blocked and operation will stop.
The coldhead is equipped with so called “antifreeze heaters”, which automatically starts to operate when the application does not consume power and the coldhead approaches the freezing temperature of the cryogenic fluid. Optional solutions and add-ons are available for purging, soft cool down, power generators, control, water cooling, self testing etc.
Stirling Cryogenics' cable projects:
- Albany cable project
- Kepco’s 100 mtr cable project, South Korea, SPC1 & SPC-4RL
- Tratos cable project, Italy, SPC-2FF
- NKT, Denmark, SPC-2 FF
For use in applications as described the following Stirling Cryogenics products may be considered: