As the US military shifts from boots on the ground to drones in the sky, there will be an increasing need for computing power on foreign soil, under the sea and in the air. Design objectives will be difficult to achieve with legacy technology. Electronic gear must be deployed by transport plane and require rapid setup once the destination is reached. Systems will need to be hardened to survive extreme temperatures, desert sand, salt air and pollution. Fuel logistics to serve remote locations can be difficult and expensive. Every kilowatt-hour of electricity converted to heat must be dissipated and, ideally, the waste energy should be recycled. For ground installations, it will be useful to consider distributing computing resources around rather than concentrating them at a single location that may be vulnerable to attack. Silent operation also is desirable.

In the LCS server, all electronic components are submerged in the dielectric coolant, which is directed to the hottest components first and then circulated throughout the enclosure to capture the rest of the heat.
The traditional method of cooling electronics by circulating air around and through components achieves none of these objectives. The cooling system comes in pieces, and field assembly takes time. Fans used to circulate massive amounts of air waste energy, take space, make noise and expose computing equipment to corrosion and air pollutants. Mechanical refrigeration systems to maintain humidity levels and cool equipment in hot weather waste even more energy.

Liquid cooling, applied intelligently, can be an ideal solution, but not every approach is suitable for military applications. Cold plates, which circulate water through heat sinks mounted on the processing chips to a heat exchanger in the sever chassis, were developed to move heat away from hot spots, not save energy or isolate electronics from harsh environments. Only 60% of the heat is generated by processors in most servers, so fans are needed to remove heat from the other components. These pumps, fans and heat exchangers in each computer chassis are subject to failure, and when a leak occurs and water touches electronics the result is catastrophic. Other cooling solutions, designed for extremely high power systems, involve refrigerants that remove heat by evaporation, but these two-phase systems add cost and unnecessary complexity.

If the thermal load is less than 100 kilowatts per rack a single-phase cooling system, where all electronic components are submerged in a nonconducting dielectric liquid, is the best alternative. This technology decouples electronic components from the environment, and offers a long list of important benefits for military applications. Electronics are isolated from oxidation and air pollution; there is no noise, vibration or extreme temperature fluctuation; fans and mechanical refrigeration are eliminated; there is no need for humidity control; and heat is recovered in a convenient form for recycling. Because the heat transfer liquid is circulated through the racks and IT devices from a central pumping station, there are no fans, pumps or other moving parts in the chassis.

Total Immersion Technology

A ruggedized 16-server liquid-cooled rack for harsh environments and military applications.
Three companies offer total immersion systems: Green Revolution Cooling, Iceotope and LiquidCool Solutions. Instead of submerging electronics in an air bath, which is current practice, the fluid in the bath is a dielectric liquid that does not conduct electricity or harm electronic components. Properly formulated dielectric fluids are maintenance free and never replaced during the useful life of an IT device. While the technology developed by all three companies that offer total immersion is similar, they differ in scalability, maintainability and cost efficiency. The devil is in the details.

Green Revolution Cooling’s cooling system resembles a rack tipped over on its back, with modified servers inserted vertically into slots in the tank. The tank is filled with a coolant similar to mineral oil, which is circulated through the tank by an external pump. This approach requires a relatively large amount of floor space, limiting scalability in a multi-rack environment. The system is not fully sealed, which could lead to fluid contamination in certain environments. Moreover the liquid-filled tank is heavy and assembly in the field would make its deployment for most military applications expensive, time consuming and logistically challenging.

Iceotope’s version of immersion cooling includes off-the-shelf motherboards mounted inside sealed hot-swappable cartridges that are flooded with a dielectric fluid. There is a secondary circuit with water, pumped from a central station, snaking through a channel inside one wall of the cartridge to take the heat out of the dielectric fluid. This two-circuit approach adds cost, introduces water to the white space and interferes with maintenance.

LiquidCool Solutions currently holds 17 patents surrounding cooling electronics by total immersion in a dielectric fluid. For rack-mounted servers the dielectric fluid is pumped from a central station through a manifold mounted on the rack into sealed IT devices, flooding each chassis and slowly flowing over and around the circuit boards and internal components via directed flow. Within each device, coolant is circulated directly to the components with the highest power density while heat from the remaining components is conveyed by the bulk flow as the coolant is drawn through the unit to a return manifold.

LCS patents also cover the rack system that connects IT devices to the central coolant flow via dripless couplings, which facilitates access for rack management and device maintenance; it is possible to hot swap a rack-mounted IT device in less than two minutes. Once the coolant exits the enclosure, it is circulated outside the datacenter where the heat is captured for commercial reuse or rejected to the atmosphere by a commercially available fluid cooler.

Cutaway view of a ruggedized small form factor computer designed for total immersion cooling.
The adoption of immersion liquid cooling technology has been slowed by a perception that the equipment is expensive and difficult to maintain. Critics, chained by inertia, argue that the technology will be practical only when power densities are too high for air cooling. However the benefits of immersion cooling are compelling enough to convert today regardless of power density.

Over a century ago the world began trading the horse & carriage for a horseless carriage, not to go 30 miles per hour, but to get rid of the horse! The horse ate, wasted space and polluted the environment, and it did not take long for everyone to understand the value proposition of the new technology. Total immersion cooling eliminates fans in IT devices and datacenters. Fans chew up energy, waste space and expose electronics to pollution. They are the root cause of most IT equipment failures, either because the fan itself fails or exposure to air causes electronic components to degrade. So eliminating fans reduces the amount of maintenance required. Eliminating fans is akin to getting rid of the horse!

The Value Proposition

Comparing the value proposition with legacy air systems, or other forms of liquid cooling for that matter, immersion cooling saves energy, saves space, enhances reliability, operates silently, and can be surprisingly easy to maintain in the field. Immersion cooling systems also simplify upgrades because there is enough cooling capacity in the chassis to accommodate future heat loads so only the boards need to be changed. When produced in volume, efficiently designed immersion cooling devices will cost less than air-cooled equipment because they have no moving parts and last longer. Oh, and by the way, immersion cooling systems can dissipate 100 kilowatts per rack.

The above table summarizes the benefits and challenges associated with the universe of cooling solutions for military applications:

As is so often true with innovations that change the world, the military will pave the way as an early adopter of liquid cooling technology to solve problems that legacy systems cannot. Conventional datacenter users will follow soon thereafter as they come to appreciate immersion cooling’s comprehensive value proposition.

This article was written by Herb Zien, CEO, LiquidCool Solutions (Rochester, MN). For more information, Click Here .

Aerospace & Defense Technology Magazine

This article first appeared in the October, 2014 issue of Aerospace & Defense Technology Magazine.

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