ABSTRACT

Superconducting power transmission cables have the potential to reduce losses and costs for long distance electricity transmission and provide a low footprint for multi-GW power transfer.

The existing technology hosts the cryogenically cooled superconducting cable in a deep-vacuum, corrugated cryostat that is well-suited for onshore, in-plant and 1 km-applications (typ) but is not adapted to a subsea (externally pressurized) environment and long-distance (>10 km) operation.

This work shows how a rigid pipe-in-pipe cryostat, based on field proven solutions, can provide a robust conduit that meets the thermal, hydraulic and mechanical requirements of a 100-km subsea cryostat.

INTRODUCTION

Electrical superconductivity is the a property that is achieved in certain materials when they are sufficiently cooled. Below a certain temperature they experience a transition towards a state of zero electrical resistance, allowing the transmission of electrical current with zero electrical losses. However, the temperature at which this state appears for practical applications requires cooling to 77K (boiling temperature of Liquid Nitrogen aka LN2) or lower. Controlling thermal losses becomes key, both to reduce costs and maintain temperature.

In the case of power transmission cables, it is achieved by pumping the cooling fluid into a thermally insulated conduit that also hosts the cable. The conduit then has to satisfy the requirements of providing sufficient insulation to maintain the cable within the desired temperature range and have the pressure holding capacity that allows the fluid to be pumped over the required distance. One could of course always add in cooling and pumping stations along the pipeline, but this would defeat the purpose of a robust system, and even more so in an offshore/subsea environment.

The paper will focus on the design and capabilities of the main run of the conduit, taking into account constraints and experience from the offshore arena, in order to show that there is a possible synergy between the existing superconducting cables (developed for an onshore use, especially around high-energy physics instruments and magnetic field generation, to cite but the main industrial applications) and offshorequalified cryogenic pipelines.

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