Exploring liquid cooling in high power applications

23rd April 2015
Nat Bowers

The last few years have seen an increase in high power applications, from railway and EVs to manufacturing and renewable energy. Traditional air-cooling is unsuitable for these applications due to high temperature and low space constraints. Here, Steve Hughes, Managing Director, REO UK, explores the world of liquid cooling and how it can provide significant efficiency gains.

Cataglyphis is a genus of the ant family whose most famous species is C. bicolor, otherwise known as the Sahara Desert ant. Its forte is running on the extremely hot desert sand to forage on the remains of other insects that have died of heat exhaustion. It's able to achieve this mind boggling feat in temperatures exceeding 50°C, making it the most heat tolerant animal on the planet.

It is an unforgiving environment, where even the hardiest of creatures succumb to the harsh midday heat. The ant comes out to forage for ten minutes a day, just as the sun reaches its highest point, and predators as well as competitors scurry to take refuge below the surface. To do this, the ant has some special tricks up its sleeve. The first is speed, weighing only 9.7mg and capable of running at an equivalent human speed of 280mph, it is one of the fastest arthropods on Earth.

Convective cooling certainly helps, but the second advantage is having long legs. Unlike its predator the desert lizard, the ant's long legs raise it 4mm off the surface, just enough to take advantage of the six degrees cooler air temperature.

Finally, the ant's foraging technique is crucial. It navigates by using an inbuilt pedometer, counting every step it takes and simultaneously calculates its position relative to the sun. When food is located, the ant makes a beeline for the nest, pausing only to rest on the cooler stalks of dry vegetation to offload some body heat.

Air vs. liquid cooling

It seems that humans still have a lot to learn from nature. Effective cooling of industrial components has always been a challenge for industries ranging from vehicles and railway engineering to manufacturing and renewable energy production. Traditionally, air-cooling has been used to combat this problem but this is becoming increasingly less effective with the rise, in the last few years, of high power applications.

Air-cooling can be achieved in two ways, natural and forced convection. In natural convection, also called free-air cooling, the ambient temperature difference between the material and its surroundings is used to naturally dissipate waste heat. In forced convection a fan or a pump is used to optimise the circulation of cool air to target and extract waste heat.

As inductive and resistive components are pushed towards higher power specifications in increasingly compact designs, the lack of internal space limits the efficiency of air-cooling in mitigating the higher temperatures.

As a result, liquid cooling, also known as water cooling, is now being used to tackle the problem. A coolant, such as deionised water, glycol or an aqueous solutions or non conductive liquids are pumped through specially designed cooling channels in the heat sink. The liquid and electricity-carrying lines are optimally designed for safe operation.

The higher density and heat capacity of the coolant makes it significantly more efficient compared to air cooling. As an example, if a given application has an assumed power loss of 3200W, in order to achieve a cooling of 5K, 655l/s of air is required or only 0.1557l/s of water. This illustrates the significant advantage of liquid cooling.

This means that liquid cooling can be used with low noise at high power levels. A significant space saving can be made whilst maintaining a very low surface temperature. This is useful in industrial applications where components with low surface temperature are required, such as the wood and textile industries or explosion-protected environments where there is a risk of dust-particle combustion.

To make this possible, the design of liquid cooled components goes through a rigorous process of computer simulation called Finite Element Method (FEM) analysis. This simulates the thermal, structural and fluid dynamics of liquid flowing through the system.

By using FEM, design engineers can optimise the system for the most efficient cooling characteristics as well as calculating pressure, radiation, solid-state temperature and the thermal effect of the system on its environment. Materials for both the resistors and connectors can be changed during this process to ensure that a long component life can be achieved.

In application

Liquid cooling can be used in many applications. REO is building innovative water-cooled resistors for ABB to test converters used in shipping, railway engineering and water power plants. ABB already uses water-cooled systems but this project represented an opportunity for REO to apply knowledge gained in research areas from inductor and resistor construction to entire control boxes.

The load unit is comprised of 15 individual resistor groups each dissipating 3,000kW of power using a coolant distribution system based on the REOhm BWD 330 resistor.

Claiming up to 88% space saving for the BWD 330, the coolant distribution is set up in a Tichelmann Coil configuration, using a dual-pipework fluid transfer method, similar in concept to the electrically equivalent ring main circuit typically found in UK homes.

If one resistor is located nearest to the coolant supply distributor connection on one side, then the return is collected at the furthest distance from the other distributor.

All resistors can be locked individually and there is a needle valve on the feed side, allowing flow rate to be offset to the flow resistances on individual resistor groups. The feed arrangement features a ball valve on each return with which to shut the line.

The low-weight REO BWD 330 braking resistor and its unique cooling system allows a large space saving, high ingress protection to IP 66 and higher power levels up to 60kW, which is not possible with conventional air cooling. The BW D330 is also available in compact form with an integrated braking chopper. The units can also be connected in parallel to provide much larger power capacities if required.

REO offers not only individual components but also complete systems and pre-wired solutions. These include water-cooled load banks for test facility use, as well as complete EMV solutions for water-cooled inverters with increased power for wind, solar, and industrial applications.

A lesson from nature

The humble Sahara desert ant will continue to brave the harsh heat of the midday sun using its unique set of skills. It teaches us that the most effective cooling systems are those that maximise cooling efficiency, optimise space saving and use intelligent processing to adapt to changes quickly. Taking tips from nature may well help us to face the high power and highly adaptive challenges of the future.

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