Learnings

Outcomes

Four different cooling approaches have been considered in the project:

1. Liquid Nitrogen Cooling

2. Chilled Inlet Air

3. Water Pipes

4. Thermosiphons

From the investigations conducted the first two approaches are not considered viable options.

Experimental testing demonstrated considerable reductions in the mechanical properties of cable insulation when subject to liquid nitrogen temperatures. There are also issues with significant ice formation if adequate thermal insulation is not utilized which would impact the structural integrity of cable supports and potentially the tunnel itself. Experimental investigations into chilling the inlet air demonstrated that minor moisture ingress could lead to significant delamination in the outer layer of the cable due to the radial temperature gradients across the cable.

Using water pipes for cooling emerged as a highly effective solution. This approach does not require precooling the inlet air, as evidenced by the COMSOL modelling with a starting air temperature of 35°C and 12.5°C for the water, achieving effective heat dissipation with a 22.5 K temperature difference. The water-pipe model successfully maintained target ratings (2350 A) without inducing significant temperature changes in the cables.

Thermosiphons appear to be a promising technology where water pipes cannot be placed in close proximity to cables, such as joint bays and tunnel shafts. Their application along the entire cable length is less practical due to factors such as mechanical complexity and installation challenges. Simulation based investigations demonstrated that 5 thermosiphons placed around the cable surface were capable of transferring nearly all of the heat from the cable leading to a considerable increase in ratings (11.2%).

Lessons Learnt

A general lesson from the project would be that the provision of suitable data, for example cooling system configurations, was difficult to obtain, in the case of this project necessitating a site visit. For future projects it could be beneficial to make significant allowances for missing or incomplete data when planning the project.