Thermal Analysis and Consequences of Thermosyphon Installations on Cable (TACTICS)

Started

Other Power and Storage Technologies(Electricity transmission and distribution)

Applied Research and Development

ENGINEERING AND TECHNOLOGY (Electrical and Electronic Engineering)

Not Cross-cutting

Project Contact
National Grid Electricity Transmission

Network Innovation Allowance

Ofgem

01 November 2024

31 May 2027

ENA months

£674,000

Power

London

Network Innovation Allowance

Project Contact, National Grid Electricity Transmission

The ampacity of cable circuits is often limited by specifichot spots at critical locations. These can include joint bays, or shafts in the case of cable tunnels. This project aims to investigate the potential of thermosyphon systems, an established technology in gas pipelines and circuit breakers, to improve heat transfer in cable systems, potentially increasing the rating of cables circuits.Thermosyphon systems are widely used on gas pipelines to keep ground frozen around buried supports to prevent mechanical damage that could arise due to natural thermal gradients. Their use in high voltage installations is largely focused on generator circuit breakers to improve heat dissipation. An advantage of thermosyphons is that they are a passive technology, with no operating noise or power requirements. This project will aim to look at using thermosyphons in cable systems, which has received very little research attention. Focus will be given to the installation requirements of thermosyphons, and the development of simulation tools to assess their potential impact, validating models against a laboratory test bed.Thermosyphons have been employed to stabilise foundations since the 1960s, with the largest installation of ~120,000 units employed on the Trans-Alaska Pipeline in 1977. Thermosyphons have also seen application in cooling turbine rotor blades and solar heating systems. Thermosyphons transfer heat from a source to a coolant within a pipe. This coolant is a fluid with a low latent heat of vaporisation and consequently it evaporates. This vapour rises until it reaches a cooler region where it condenses and falls as a liquid on the surface back to the source. A key advantage of thermosyphons are that they are passive systems, requiring no external power supply and producing no audible noise.A recently completed Network Innovation Allowance projectCable Alternative Cooling Technologies for Underground Systems, CACTUS, simulated the potential impact of installation thermosyphon systems within a cable tunnel shaft. The results indicated that significant heat dissipation was possible, with thermosyphon pipes installed on the cable surface transferring the majority of the heat produced by the cable. This would indicate a significant reduction in mutual heating between circuits, and a potential uprating of existing equipment.The simulation results from CACTUS motivate the need for further work, including experimental investigations which can be used to validate the simulation methodology. The intention is to determine whether thermosyphons are a viable technology to cool localised hot spots in cable systems, such as those in tunnel shafts or joint bays. The work will be divided into 6 work streams focused upon the achievement of objectives, with each producing a clear set of deliverables:WS1. State of the art reviewWS2. Experimental assessment of heat dissipation from thermosyphon systemsWS3. Simulation of heat transfer within thermosyphon systemsWS4. Assessment of Thermosyphon Fluid PropertiesWS5. Recommendations to National GridWork Stream 1: State of the art reviewThere is a requirement to determine prior literature on thermosyphon thermal modelling and the identification of suitable fluids for HV applications. The University of Southampton has previously conducted work in this space, but it is from 2019 and was focused on circuit breaker applications. There is therefore a need to analyse more recent literature, and to appropriately considerhistorical work on thermosyphon applications in gas pipelines, which are a reasonable thermal analogue for cable systems.Fluids will be analysed considering their environmental properties including ozone depletion, global warming potential and flammability. Other important properties include their operating temperature range, latent heat of evaporation and boiling point. As the thermosyphons will be placed outside of the cable sheath the dielectric properties of the fluid are less critical, provided that it is sufficiently insulating to prevent induced heating.Work Stream 2: Experimental assessment of heat dissipation from thermosyphon systemsTo demonstrate the potential cooling benefit of thermosyphon systems it is necessary to design and build a suitable test-bed in the laboratory. A number of temperature measurements will need to be made in order to demonstrate any potential cooling, but it is also important that they do not impact the fluid dynamics within the thermosyphon. At present it is intended to also use a high-speed camera to record the experiment. This intended to monitor the liquid film size on the external surfaces, and potentially the formation and motion of any bubbles within the system.It is important that the experimental testing considers the need to orientate the heat source with respect to the thermosyphon in order to provide insight into systems where cables may be vertical (e.g. tunnel shafts/risers) and horizontal (e.g. buried). Consideration of thermal performance over a temperature range should also be undertaken. It is intended to consider up to three different fluids over a range of conditions.Work Stream 3: Simulation of heat transfer within thermosyphon systemsBased on a review of prior literature in WS1, and experience gained as part of the CACTUS project, a thermal model of a thermosyphon will be constructed. The model will be validated against the experimental arrangement developed in WS2, but it is also intended to develop afirst pass model to inform experiment design before validation.It is envisaged that a two phase CFD model will be coupled to a thermal model within FEA software. In addition to the experimental arrangement, it is intended to develop models to demonstrate the impact of thermosyphons across a range of cable systems including cables in shafts, and traditional buried circuits. A further potential scenario is in joint bays. The University of Southampton has a range of extant models of National Grid cable circuits which can be altered to incorporate thermosyphons.Work Stream 4: Assessment of Thermosyphon Fluid PropertiesGiven the length of cable systems design life it is imperative to confirm whether thermosyphon fluids are sufficiently stable such that their cooling performance is not impacted. A series of thermal ageing experiments and characterisation techniques will be undertaken to determine whether thermal properties (e.g. boiling point) or electrical properties, in particular conductivity, may degrade over time. This will also allow for a comparison of measured values against those of the supplied data sheet.Work Stream 5: Recommendations to National GridThe final work stream will combine findings from across the project into a report outlining recommendations to National Grid including an assessment of the practical implications of thermosyphons. Using data from simulations it is intended to provide estimates of capital expenditure (excluding installation costs) that may be required to construct thermosyphon systems.It is intended to collaborate with NGET staff in this workstream to ensure that the recommendations provided can be utilised by NGET swiftly, including the identification of any remaining unknowns or risks. The objective of this project is to develop and optimise a cooling system using thermosyphons for cable tunnels to enhance heat dissipation and increase the ampacity of installed cables. The aspects of this project are as follows:Model heat transfer in thermosyphons based on existing literature to inform simulations.Identify readily available candidate fluids for cable systems to conduct experiments.Determine achievable heat dissipation in a laboratory test bed as an indicator of cable system performance.Investigate how thermosyphon thermal performance depends on cable location and temperature range.Assess the accuracy of heat transfer models for thermosyphons.Analyse the impact of thermosyphons on cable ampacity.Evaluate potential degradation of thermosyphon fluids with age.Compare measured fluid material properties with datasheet values.Explore the retrofitting of thermosyphon systems in existing cable circuits.Assess implications for maintenance and health and safety during thermosyphon installation.Analyse the cost-benefit ratio of installing thermosyphons.

This project aims to enhance cable circuit ampacity by investigating the potential of thermosyphon systems for improved heat transfer. Through a literature review, suitable thermosyphon working fluids and thermal modelling methodologies will be identified. An experimental apparatus will be designed to assess heat transfer in cable systems using thermosyphons, and simulations will validate the design. The project will also examine the impact of thermal aging on thermosyphon fluids and compare measured values with datasheet information. Recommendations will be provided to National Grid, including practical considerations and potential challenges for thermosyphon system installation.

02/04/25