1. Introduction
For the generation of electricity from renewable sources, such as wind and photovoltaics, two technical topics need to be addressed: the time shifted production and the consumption, which can be diurnal or seasonal [
1]. This issue will be part of seasonal storages, Power-to-X and multi energy grid scenarios. The availability of renewables in secluded regions demands electrical energy to be transported over potentially thousands of kilometres. Future, long-distance electrical grids will use high voltage direct current (HVDC) transmission to minimise the energy losses as discussed by May et al. [
2] and Nguyen and Saha [
3]. Nguyen and Saha discuss the benefits of using HVDC, particularly at the power transfer level and as the interconnection length increases [
3]. Usually, however, the production of renewable energies at remote locations, such as wind farms in the North Sea or in Northern China, is performed using alternating current (AC) and therefore thyristors and high voltages are required for the alternating current/direct current ACDC conversion [
4]. On top of this, transformers are required for the high voltage, and the working of thyristor devices leads to a high heat generation rate and if the heat cannot be dissipated then the thyristor will accelerate ageing. As discussed by Bhandari et al. [
5], the thermal management of thyristors is of the essence.
Oftentimes the cooling of sensitive components in power electronics is performed with air [
6,
7,
8]; however, in cases that have higher heat loads to dissipate and smaller sized geometries, the cooling is generally done by liquids [
9]. The intrinsic compactness, the reduced noise and the low power consumption has made liquid cooling the most suitable solution in a large number of cases [
10]. Sparrow et al. [
11] extensively studied the thermal management of electronic equipment using thermal fluids with cold plates. The authors found that a proper design of a compact cold plate is achieved by solving the combined problem of fluid flow, convective heat transfer and wall heat conduction [
11]. The interaction of these three problems is solved in regards to the number of sequential fins in a periodic structure showing the heat transfer benefits of a periodically deviated flow [
11]. The presence of fins and pore geometries, although beneficial for improving the cooling performances, present the draw back of being difficult and costly to manufacture (machining and brazing) and structural issues when the cold plate is clamped at high force in an array [
11,
12].
New cooling fluids, with enhanced heat densities, have also been proposed, but they might present difficulties on the design of a real implementation [
12]. The Slatt-Buckley 500 kV line used a water/glycol mixture to cool the thyristor valves and the 500 kV line used in Fengtun, Northeast China Grid Co. Ltd., used a pure water cooling system with ethylene glycol mixed in to improve the cooling efficiency of the controller [
13]. Whilst deionised water offers good heat dissipation and electrical insulation, its cooling ability relies on sensible heat storage, and thus large quantities of water are needed for effective cooling. Recently, the direction of two-phase fluids using the liquid-vapour phase change has been looked into for the cooling of electronics [
14]. In the report by Darin et al. [
14], different thermal management fluids for use in electrical component cooling are compared. Amongst the best are ethylene glycol, propylene glycol and water. Whilst water can exhibit the best cooling ability, the evaporation and boiling of water can render it unviable for many applications. An alternative class of heat transfer fluids, which have gained recent traction in cooling systems and use phase change materials incorporated into a base fluid, exploit the latent heat of phase change and thus offer high cooling capacities around a narrow temperature range [
15,
16,
17,
18]. These fluids are called phase change slurries (or phase change dispersions (PCD)) and to the author’s knowledge have not previously been used in the cooling of HVDC components. Despite this, over recent years, PCD have been extensively studied in terms of their heat transfer performance [
19,
20,
21,
22], rheological performance [
23,
24], thermophysical properties [
25,
26].
In this work, an investigation will be performed into the heat transfer and rheological performance of a PCD for application into the cooling of thyristor converter valves. The temperature level of the system is determined by the re-cooling system and therefore by the local climate conditions and in this experimental investigation, the application for operation in Southern China has been chosen. In Southern China, the summers are warm, and free cooling with ambient air, or with hybrid cooling systems, is performed at about 40
C. The temperature level of the converter valves is approximately 10 K higher than this, at approximately 50
C. The aim of this paper is to propose a large increment of the heat transfer in the cooling of a sensitive component of HVDC converters such as the thyristor, by proposing a new cooling fluid that can be implemented without any structural change of the cooling system. This cooling fluid consists of a PCD, which is discussed in [
27]. In this investigation, the PCD’s heat transfer and rheological behaviour is compared to water, which is the current industrial standard for cooling such components, at different mass flow rates and electrical heating inputs.
5. Conclusions
Overall, the test setup was deemed appropriate for testing and evaluating the effects of PCD in thyristor cooling. This was demonstrated through validation and comparison with water testing using energy balances. The results highlight that the use of the PCD in the system reduced the temperature increase in comparison to water by 50%. This was found to be due to the increased apparent heat capacity of the phase change dispersion compared to water, as at certain mass flow rates it reached almost twice that of water. Additionally, global heat transfer coefficients of up to 6100 W m K, were achieved for mass flow rates of 8 kg min. Furthermore, it is interesting to note that the global heat transfer coefficients were the same, or higher than water at the same mass flow rates and electrical heating input. Additionally, it was found that the global heat transfer coefficients of water were dependent on the electrical heat input applied at the wall, but this was not the case for the phase change dispersion, which appeared to be independent of the wall electrical heat input. However, due to an increased viscosity of the PCD compared to water, higher pressure drops were found for the PCD than for water. This higher viscosity of the PCD could possibly lead to excessive heating of the HVDC thyristor, due to higher pumping power requirements. Additionally, further investigation should be carried out in determining whether the PCD is Newtonian or non-Newtonian. Whilst still at the initial testing phase, it is acknowledged that future testing would involve verification of stress-testing and longer term cycling to demonstrate the feasibility of introducing PCD into HVDC cooling technologies. Furthermore, numerical testing is ongoing to obtain models of pressure drop and rheology of the PCD in the system. Overall, the investigation highlights the application of cooling of electrical components with phase change dispersions in a real application, which to the author’s knowledge has not previously been done.