Traditionally the IGBT modules were cooled by forced air-cooled heat sinks. Air-cooled heat sinks are still good thermal management solutions for low-power and less temperature-restricting IGBT modules. However, the high-power IGBT modules are exclusively cooled by liquid-cooled heat sinks, also known as cold plates. Learn more about their application in this white paper (PDF, download, no registration needed): Download it here
By Norman Quesnel, Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)
Liquid cooling systems transfer heat up to four times better than an equal mass of air. This allows higher performance cooling to be provided with a smaller system. A liquid cooled cold plate can replace spaceconsuming heat sinks and fans and, while a liquid cold plate requires a pump, heat exchanger, tubing and plates, there are more placement choices for cold plates because they can be outside the airflow. [1]
One-time concerns over costs and leaking cold plates have greatly subsided with improved manufacturing capabilities. Today’s question isn’t “Should we use liquid cooling?” The question is “What kind of liquid should we use to help optimize performance?”
For liquid cold plates, the choice of working fluid is as important as choosing the hardware pieces. The wrong liquid can lead to poor heat transfer, clogging, and even system failure. A proper heat transfer fluid should provide compatibility with system’s metals, high thermal conductivity and specific heat, low viscosity, low freezing point, high flash point, low corrosivity, low toxicity, and thermal stability. [3]
Today, despite many refinements in liquid cold plate designs, coolant options have stayed relatively limited. In many cases, regular water will do, but water-with-additives and other types of fluids are available and more appropriate for certain applications. Here is a look at these coolant choices and where they are best suited.
Basic Cooling Choices
While water provides superior cooling performance in a cold plate, it is not always practical to use because of its low freezing temperature. Additives such as glycol are often needed to change a coolant’s characteristics to better suit a cold plate’s operating environment.
In fact, temperature range requirements are the main consideration for a cold plate fluid. Some fluids freeze at lower temperatures than water, but have lower heat transfer capability. The selected fluid also must be compatible with the cold plate’s internal metals to limit any potential for corrosion.
Table 1 below shows how the most common cold plate fluids match up to the metals in different cold plate designs.
The choices of cold plate coolants will obviously have varied properties. Some of the differences between fluids are less relevant to optimizing cold plate performance, but many properties should be compared. Tables 2 and 3 show the properties of some common coolants.
An excellent review of common cold plate fluids is provided by Lytron, an OEM of cold plates and other cooling devices. The following condenses fluid descriptions taken from Lytron’s literature. [5]
The most commonly used coolants for liquid cooling applications today are:
Water – Water has high heat capacity and thermal conductivity. It is compatible with copper, which is one of the best heat transfer materials to use for your fluid path. Facility water or tap water is likely to contain impurities that can cause corrosion in the liquid cooling loop and/or clog fluid channels. Therefore, using good quality water is recommended in order to minimize corrosion and optimize thermal performance. If you determine that your facility water or tap water contains a larger percentage of minerals, salts, or other impurities, you can either filter the water or you can opt to purchase filtered or deionized water. [5,6]
Deionized Water – The deionization process removes harmful minerals, salts, and other impurities that can cause corrosion or scale formation. Compared to tap water and most fluids, deionized water has a high resistivity. Deionized water is an excellent insulator, and is used in the manufacturing of electrical components where parts must be electrically isolated. However, as water’s resistivity increases, its corrosivity increases as well. When using deionized water in cold plates or heat exchangers, stainless steel tubing is recommended. [5, 7]
Inhibited Glycol and Water Solutions – The two types of glycol most commonly used for liquid cooling applications are ethylene glycol and water (EGW) and propylene glycol and water (PGW) solutions. Ethylene glycol has desirable thermal properties, including a high boiling point, low freezing point, stability over a wide range of temperatures, and high specific heat and thermal conductivity. It also has a low viscosity and, therefore, reduced pumping requirements. Although EGW has more desirable physical properties than PGW, PGW is used in applications where toxicity might be a concern. PGW is generally recognized as safe for use in food or food processing applications, and can also be used in enclosed spaces. [5, 8]
Dielectric Fluid – A dielectric fluid is non-conductive and therefore preferred over water when working with sensitive electronics. Perfluorinated carbons, such as 3M’s dielectric fluid Fluorinert™, are non-flammable, non-explosive, and thermally stable over a wide range of operating temperatures. Although deionized water is also non-conductive, Fluorinert™ is less corrosive than deionized water. However, it has a much lower thermal conductivity and much higher price. PAO is a synthetic hydrocarbon used for its dielectric properties and wide range of operating temperatures. For example, the fire control radars on today’s jet fighters are liquid-cooled using PAO. For testing cold plates and heat exchangers that will use PAO as the heat transfer fluid, PAO-compatible recirculating chillers are available. Like perfluorinated carbons, PAO has much lower thermal conductivity than water. [5, 9]
Conclusion
Water, deionized water, glycol/water solutions, and dielectric fluids such as fluorocarbons and PAO are the heat transfer fluids most commonly used in high performance liquid cooling applications.
It is important to select a heat transfer fluid that is compatible with your fluid path, offers corrosion protection or minimal risk of corrosion, and meets your application’s specific requirements. With the right chemistry, your heat transfer fluid can provide very effective cooling for your liquid cooling loop.
References
[1] https://www.aavid.com/product-group/liquidcoldplates/fluid
[2] http://semi-therm.org/wp-content/uploads/2017/04/How-to-design-liquid-cooled-system.pdf
[3] Mohapatra, Satish C., “An Overview of Liquid Coolants for Electronics Cooling,” ElectronicsCooling, May 2006.
[4] http://www.calce.umd.edu/whats_new/2012/Presentations/David %20Saums%20PPt.pdf
[6] https://www.thereadystore.com/5-gallon-collapsible-water-container
[8] http://www.rhomarwater.com/products/catalog/envirogard-heat-transfer-fluid-antifreeze
[9] http://www.skygeek.com/anderol-royco-602-cooling-fluid.html
Posted in Cold Plates, Liquid Cooling, Qpedia Thermal eMagazine
Qpedia continues its review of technologies developed for electronics cooling applications. We are presenting selected patents that were awarded to developers around the world to address cooling challenges. After reading the series, you will be more aware of both the historic developments and the latest breakthroughs in both product design and applications.
We are specifically focusing on patented technologies to show the breadth of development in thermal management product sectors. Please note that there are many patents within these areas. Limited by article space, we are presenting a small number to offer a representation of the entire field. You are encouraged to do your own patent investigation.
Further, if you have been awarded a patent and would like to have it included in these reviews, please send us your patent number or patent application.
In this issue our spotlight is on spray cooling for electronics thermal management.
There are several US patents in this area of technology, and those presented here are among the more recent. These patents show some of the salient features that are the focus of different inventors.
Full Coverage Spray and Drainage System and Method for Orientation-Independent Removal of High Heat Flux
US 8550372 B2 – Timothy A. Shedd and Adam G. Pautsch
A cooling system and method that significantly improves spray evaporative cooling by using arrays of slot or plane sprays to create coverage of the entire heated surface to be cooled without allowing interaction between plumes that are spraying from the nozzles. The sprays are directed at an angle to the surface to take advantage of the high droplet momentum possessed by the spray to direct a flow of coolant fluid across the surface toward desired draining points, thereby enabling drainage regardless of the orientation of the unit.
The present invention provides a spray cooling system and method that significantly improves spray evaporative cooling by creating a directed momentum flow of cooling fluid across a surface to be cooled. In accordance with the present invention, a spray of cooling fluid is directed directly onto the surface of a work piece to be cooled at an angle with respect to the work piece surface so as to create a flow of cooling fluid in a substantially single direction along the work piece surface. The spray of cooling fluid preferably may be delivered via a plurality of generally fan shaped sprays. The sprays are positioned and aligned to create cooling fluid coverage of the entire heated surface to be cooled without allowing interaction between the spray plumes in a manner that may cause areas of cooling fluid stagnation on the surface.
A full coverage spray and drainage system in accordance with the present invention may be implemented in an otherwise conventional spray cooling system including a reservoir of an appropriate cooling fluid (e.g., Fluorinert-72 for the cooling of electronic circuitry, preferably saturated with a non-condensable inert gas, such as nitrogen), a pump for delivering the cooling fluid under pressure from the reservoir to a spray chamber to be sprayed therein from nozzles onto the work piece to be cooled, and appropriate filtering, metering, and control systems. Cooling fluid is returned from the spray chamber to the coolant reservoir via a drainage point or points in the spray chamber.
In accordance with the present invention, the drainage point or points in the spray chamber may be positioned with respect to the coolant spray such that the flow of cooling fluid directed in a substantially single direction along the work piece surface also is directed toward the drainage point or points. Thus, the cooling fluid momentum directs the fluid toward the drainage point, thereby assuring proper drainage of the cooling fluid despite changes in the orientation of the cooling system.
Directly Injected Forced Convection Cooling for Electronics
Electronic circuitry includes a circuit board and at least one component mounted on the circuit board, with the at least one component generating heat while in use. The circuit board includes one or more apertures aligned with one or more respective components, and the electronic circuitry is configured to provide, while in use, a path for coolant fluid to flow through each aperture and past the respective component.
By providing at least one aperture aligned with a component that generates heat in use, improved cooling of the electronic circuitry may be provided, as cooling effects can more efficiently be targeted at those parts of the circuitry that generate or dissipate heat.
Each aperture may be, but is not necessarily positioned at that point or within that region of the circuit board that is a minimum distance from the component or a respective one of the components.
The central axis of each aperture may be, but is not necessarily, perpendicular to the plane of the circuit board and at least one component. Preferably each aperture is arranged such that a straight line extending out of the aperture along the central axis of the aperture would pass through the component with which the aperture is aligned. Preferably each aperture is arranged such that, in use, coolant fluid exits the aperture towards the component with which the aperture is aligned.
The coolant fluid may be liquid or gas. The coolant fluid may be water. The coolant fluid may comprise a dielectric fluid, for example poly-alpha-olefin (PAO), or an inert gas, for instance nitrogen. Preferably the coolant fluid is air. In some circumstances, the coolant fluid may be supplied from a pressurized source, for instance a pressurized gas cylinder.
The position of each aperture may be such that, in use, coolant fluid passing through the aperture approaches the surface of the component with which the aperture is aligned from a perpendicular direction.
Thereby a jet impingement effect may be provided such that, preferably, the coolant fluid breaks through a respective thermal boundary layer next to the or each at least one heat generating component. Such thermal boundary layers are stable layers of air or other fluid which may build up next to the or each component and which exhibit a temperature gradient away from the component. The presence of such thermal boundary layers can reduce convective cooling effects.
Narrow Gap Spray Cooling in a Globally Cooled Enclosure
Electronic circuit boards are arranged as respective parallel pairs defining a narrow gap there between. One or more such pairs of boards are supported within a hermitically sealable housing and cooled by way of spraying an atomized liquid coolant from a plurality of nozzles into each narrow gap. Transfer of heat from the circuit boards results in vaporization of at least some of the atomized liquid within the narrow gap. The housing further serves to guide a circulation of vapors out of each narrow gap, back toward the nozzles, and back into each narrow gap. A heat exchanger exhausts heat from the housing and overall system, wherein vapor is condensed back to liquid phase during contact and heat transfer therewith. Condensed liquid is collected and re-pressurized for delivery back to the nozzles such that a sustained cooling operation is performed.
One embodiment provides for a system, including a first entity and a second entity that are respectively disposed such that they define a narrow gap between them. The system also includes at least one nozzle, wherein the nozzle is configured to spray an atomized liquid so that a flow of the atomized liquid and a vapor is induced through the narrow gap. The system also includes a heat exchanger that is configured to condense some of the vapor to liquid, the condensed vapor defining a condensate. The system further includes a housing configured to guide a circulation of at least some of the vapor, which is flowing out of the narrow gap, away from the heat exchanger and into proximity with the at least one nozzle.
Another embodiment provides for a system, the system comprising a housing configured to selectively open-ably enclose a plurality of electronic circuit boards. The system further includes a plurality of electronic circuit boards supported in the housing, wherein at least some of the electronic circuit boards are arranged to define respective pairs of boards. At least one pair of boards defines a narrow gap there between. The system also includes at least one nozzle associated with each narrow gap, each nozzle being configured to spray an atomized liquid into the narrow gap defined by the associated pair of boards. The housing is also configured to guide a circulation of a vapor exiting each narrow gap into proximity with the at least one nozzle associated with the at least one narrow gap.
Still another embodiment provides an apparatus. The apparatus includes a nozzle configured to spray an atomized liquid in a generally conical distribution pattern. The apparatus further includes a re-shaper that is configured to reform the spray of atomized liquid into a generally planar distribution pattern.
Enhanced Spray Cooling Technique for Wedge Cooling
US 8729752 B2 – Balwinder Singh Birdi, Simon Waddell and William Scherzinger
The present invention relates to apparatus and methods for heat removal and, more particularly, apparatus and methods for spray cooling a wedge of a generator rotor.
In generators, electromagnetic losses occur in the magnetic iron and the copper. These losses result in production of heat which must be removed to maintain overall temperature below that allowable for the copper coating and the insulation used in the construction of the generators. The rotor core, which is made of magnetic iron, can be conduction cooled by flowing fluid through the rotor shaft. However, the removal of heat from copper is better managed if oil is passed through the hollow wedges. Due to lower thermal resistance, the flow of fluid in the vicinity of copper is much more effective in removing heat from the copper and in keeping the overall temperature below the allowable limit. This is done with conduction mode of heat removal.
Since the heat transfer coefficient (HTC) depends upon the velocity of the fluid, the removal of heat is not very efficient, and a very high flow is needed to create a reasonable HTC for conduction cooling. Further, because the rotor is a rotating component, having a large amount of fluid at a radius away from the rotor shaft is not desirable, especially for high powered larger diameter and high-speed machines.
In one aspect of the present invention, a spray cooling manifold comprises a manifold ferrule adapted to circumscribe a shaft of a rotating machine; a manifold pipe having a bend of about 90 degrees having a first end attached to the manifold ferrule and a second, opposite end; a cooling fluid channel running from an inside surface of the manifold ferrule to the second, opposite end of the manifold pipe; and a pipe extending from the second, opposite end of the manifold pipe, the pipe adapted to extend into a wedge of the rotating machine, the pipe having a plurality of holes formed there along.
In another aspect of the present invention, a rotating machine rotor comprises a shaft; a plurality of coils disposed on the shaft; a plurality of wedges disposed between the coils; bands securing the wedges on the rotor; and a manifold comprising a manifold ferrule adapted to circumscribe the shaft; a plurality of manifold pipes, each having a bend of about 90 degrees, each having a first end attached to the manifold ferrule and a second, opposite end attached to a wedge pipe extending into the wedges; a cooling fluid channel running from an inside surface of the manifold ferrule to the wedge pipe; and a plurality of holes disposed along the wedge pipe.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com. To register for Qpedia and to get access to its archives, visit https://www.qats.com/Qpedia-Thermal-eMagazine.
Posted in Jet Impingement, Liquid Cooling, Synthetic Jet
Tagged spray cooling, technology review
By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)
Additive manufacturing technologies have expanded in many directions in recent years with applications ranging across numerous industries and applications, including into the thermal management of electronics. As metal 3-D printing techniques have improved and become commercially viable, engineers are using it to create innovative cooling solutions, particularly heat exchangers.
Why are engineer turning to additive manufacturing?
One reason is that additive manufacturing allows for generous cost savings. Companies can reduce 15-20 existing part numbers and print them as a single component. A single part eliminates inventory, additional inspections, and assemblies that would have been necessary when components were produced individually.
As AdditiveManufacturing.com notes, “Some envision AM (additive manufacturing) as a complement to foundational subtractive manufacturing (removing material like drilling out material) and to a lesser degree forming (like forging). Regardless, AM may offer consumers and professionals alike, the accessibility to create, customize and/or repair product, and in the process, redefine current production technology.” [1]
Developed at the Massachusetts Institute of Technology (MIT), 3-D printing is the most common and well-known form of additive manufacturing. Three-dimensional objects are made by building up multiple layers of material. Thanks to the continued (and rapid) development of the technology and advanced research in materials science, the layers can be composed of metal, plastic, concrete, living tissue or other materials.
In industrial applications, 3-D printing has encouraged creativity. With additive manufacturing, designers can create complex geometric shapes that would not be possible with standard manufacturing processes. For example, shapes with a scooped out or hollow center can be produced as a single piece, without the need to weld or attach individual components together. One-piece shapes can provide extra strength, with few or no weak spots that can be compromised or stressed. [4]
Making 3-D Printed Heat Exchangers
Heat exchangers are integral to thermal management. Any time heat, cool air, or refrigeration are required, a heat exchanger has to be involved to dissipate the heat to the ambient. This can be as simple as a standard heat sink or a complex metal structure used in liquid cooling. It can be as small as a few millimeters or as large as a building. Heat exchange is a multi-billion-dollar industry touching everything from consumer goods to automotive and aerospace engineering.
Compact heat exchangers are typically composed of thin sheets of material that are welded together. The complexity of the designs, particularly the density of the fin field, makes production both challenging and time-consuming, while the material used for the welding process adds to the overall weight of the part. Heat exchangers produced through 3-D printing techniques (such as those pictured below) can be made quicker, lighter, and more efficiently.
In 2016, a Department of Energy-funded consortium of researchers developed a miniaturized air-to-refrigerant heat exchanger that was more compact and energy-efficient than current market designs. CEEE and 3-D Systems teamed to increase the efficiency of a 1 kW heat exchanger by 20 percent while reducing weight and size. The manufacturing cycle for the heat exchanger was reduced from months to weeks. [4]
Using direct metal printing (DMP), manufacturers delivered a 20-percent more efficient heat exchanger and an innovative design. It was produced in weeks not months and with significantly lower weight. The one-part, 3-D-printed heat exchanger required minimal secondary finishing operations.
Ohio-based Fabrisonic uses a hybrid metal 3-D printing process, called Ultrasonic Additive Manufacturing (UAM), to merge layers of metal foil together in a solid-state thanks to high frequency ultrasonic vibrations. [5]
Fabrisonic mounts its hybrid 3-D printing process on traditional CNC equipment – first, an object is built up with 3-D printing, and then smoothed down with CNC machining by milling to the required size and surface. No melting is required, as Fabrisonic’s 6 ft. x 6 ft. x 3 ft. UAM 3-D printer can scrub metal foil and build it up into the final net shape, and then machines down whatever else is needed at the end of the process.
This 3-D printing process was recently given a stamp of approval by NASA after testing at the Jet Propulsion Laboratory (JPL). A report from NASA and Fabrisonic said, “UAM heat exchanger technology developed under NASA JPL funding has been quickly extended to numerous commercial production applications. Channel widths range from 0.020 inch to greater than one inch with parts sized up to four feet in length.” [6]
There are challenges involved, to be sure. In an article from Alex Richardson of Aquicore highlighting research done at the University of Maryland, researchers discuss the problems that 3-D printing still has competing on price against traditional manufacturing techniques and the difficulties involved with physically scaling a technology up.
In the article, Vikrant Aute of the University of Maryland Center for Environmental Energy Engineering noted that his research team was “considering modularization to overcome the latter issue: Instead of making the exchangers bigger, it might be possible to arrange lots of them together to accomplish the same task.” [7]
Research Continues to Improve 3-D Printing Process
While there have been numerous advancements in the technology of metal 3-D printing, research is continuing on campuses and in companies around the world to try and improve the process and make it easier to create increasingly complex heat exchangers.
For example, Australia-based additive manufacturing startup Conflux Technology received significant funding to develop its technology specifically for heat exchange and fluid flow applications. [8] Another example was the University of Wisconsin-Madison, which received a grant from the U.S. Department of Energy (DOE) Advanced Research Projects Agency-Energy (ARPA-E) to build heat exchangers with “internal projections to increase turbulence and facilitate heat transfer. Such intricate shapes are impossible with traditional manufacturing.” [9]
In 2018, U.K.-based Hieta Technologies partnered with British metrology company Renishaw to commercialize its 3-D-printed heat exchangers. Renishaw used its AM250 system to 3-D print walls of the heat exchanger as thin as 150 microns. The samples were heat treated and characterized to confirm that the laser powder bed fusion process was effective. The process took only 80 hours, was 30 percent lighter, and had 30 percent less volume, while still meeting the heat transfer and pressure drop requirements. [10, 11]
Last month, GE Research announced that it was leading a multi-million-dollar program with Oak Ridge National Laboratory (ORNL) and the University of Maryland to develop compact heat exchangers that can withstand temperatures as high as 900°C and pressures as high as 250 bar. This was also based on funding from ARPA-E, as part of its HITEMMP (High-Intensity Thermal Exchanger through Materials and Manufacturing Processes) program. [12]
To build the new heat exchanger, GE engineers are using a novel nickel superalloy that is designed for high temperatures and is crack-resistant. University of Maryland researchers are working with GE to create biological shapes that will make the heat exchanger more efficient and ORNL researchers are providing corrosion resistance expertise to develop the materials for long-term use.
These are just some examples of the many ways that 3-D printing has impacted electronics cooling. Researchers at the Fraunhofer Institute for Laser Technology ILT in Germany have demonstrated the feasibility of 3-D printing copper [13], U.K. researchers 3-D printed “smart materials” for energy storage [14], a researcher at Penn State (soon to be at MIT) is developing methods for creating rough surfaces through additive manufacturing to enhance boiling heat transfer [15], and at Virginia Tech researchers developed a new process for 3-D printing piezoelectric materials [16].
The technology is growing by leaps and bounds each year and is enhancing the options for engineers in the thermal management industry.
References
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com. To register for Qpedia and to get access to its archives, visit
https://www.qats.com/Qpedia-Thermal-eMagazine.
Posted in Heat Exchangers, Liquid Cooling
Tagged 3-D Printing, heat exchangers, liquid cooling
A data center is a large infrastructure used to house large quantities of electronic equipment, such as computer servers, telecommunications equipment, and data storage systems, etc. The data center requires non-interrupted power, communication and internet access to all equipment inside, it also has dedicated environment control system which provides appropriate working conditions for the electrical devices hosted inside.
Traditional data centers use cold air generated by a room air conditioner system (CRAC) to cool the servers installed on the racks. Cooling the electrical devices by cold air generated by an air conditioner is an easy method to implement. However, it is not a very efficient method in terms of power consumption.
The inefficiency of the method can be contributed to several causes: generating and delivering cold air from a chiller to servers is a multiple heat transfer process, such as the mixing of warm and cool air in the room, which reduces the efficiency and power consumption of cooling hardware such as chillers, computer room air conditioners (CRACs), fans, blowers and pumps.
Data center designers and operators have invented many ways to improve the data center’s thermal efficiency, such as optimizing the rack layout and air conditioner location, separating cold aisles and hot aisles, optimizing the configuration of pipes and cables in under-floor plenum, introducing liquid cooling to high-power severs.
While the above methods can improve the data center heat load management, they cannot dramatically reduce the Power Usage Effectiveness (PUE), which is a measure of how efficiently a datacenter uses its power and is defined as the ratio of total datacenter power consumption to the IT equipment power consumption.
An ideal PUE is 1,0. A better way, proposed and used by some new data centers, is directly bringing the outside cold air to the servers. This method can eliminate the computer room air conditioners (CRACs). To achieve this, the data center has to be located in a specific area where cold air can be provided for all four seasons and the servers have to have higher operating environmental temperature.
Another dramatic solution proposed and used by some companies is liquid immersion cooling for entire servers. When compared with traditional liquid cooling techniques, the liquid immersion cooling uses dielectric fluid as a working agent and open bath design. This eliminates the need for hermetic connectors, pressure vessels, seals and clamshells. There are several different liquid immersion cooling methods.
This article will review the active single-phase immersion cooling technology proposed by Green Revolution Cooling (GRC) [1] and a passive two-phase immersion cooling technology proposed by the 3M Company [2].
Green Revolution Cooling has designed a liquid-filled rack to accommodate the traditional servers and developed dielectric mineral oil as the coolant. Figure 1 shows the liquid cooling racks with chiller and an inside view of a CarnotJet cooling rack from GRC. The racks are filled with 250 gallons of dielectric fluid, called GreenDEF™, which is a non-toxic, clear mineral oil with light viscosity.
The servers are installed vertically into slots inside the rack and fully submerged in the liquid coolant. Pumps are used to circulate the cold coolant from the chiller to the rack. The coolant returns to the chiller, after removing heat from the servers. Because of its high heat capacity and thermal conductivity, the GreenDEF™ can cool the servers more efficiently than air.
The server racks are semi-open to the environment and the coolant level is constantly monitored by the system. Figure 2 shows a server motherboard is being submerged in the coolant liquid inside a server rack from GRC.
Intel has conducted a year-long test with immersion cooling equipment from Green Revolution Cooling in New Mexico [3]. They have found that the technology is highly efficient and safe for servers. In their tests, Intel tested two racks of identical servers – one using traditional air cooling and the other immersed in a Green Revolution enclosure. Over the course of a year, the submerged servers had a partial Power Usage Effectiveness (PUE) of 1.02 to 1.03, equaling some of the lowest efficiency ratings reported using that metric.
The 3M Company is also actively engaged in immersion cooling technology and has developed a passive two-phase immersion cooling system for servers. Figure 3 illustrates the concept of the immersion cooling system developed by 3M. In a specially designed server rack, servers are inserted vertically in the rack. The servers are immersed in 3M’s Novec engineered fluid, a non-conductive chemical with a low boiling point.
The elevated temperature of electronic components on the sever boards will cause the Novec engineered fluid to boil. The evaporation of the fluid will remove a large amount of heat from the heated components with small temperature difference. The evaporated fluid travels to the upper portion of the server rack, where it condenses to liquid on the surface of the heat exchanger cooled by the cold water. The condensed liquid flows back to the rack bath, driven by the force of gravity. In 3M’s server rack, the liquid bath is also semi-open to the outside environment.
Because the cooling method is passive, there is no pump needed in the system.
By utilizing the large latent heat of Novec engineered fluid during evaporation and condensation, the coolant can remove heat from servers and dissipate it to water heat exchanger with small a temperature gradient. To enhance the boiling on the component surfaces, 3M invented special coating for electronic chips inside the liquid bath. The boiling enhancement coating (BEC) is a 100 mm thick porous metallic material.
The application of the BEC is illustrated in Figure 4. The coating is directly applied to the integrated heat spreader (IHS) of the chip. Tuma [2] claimed that the coating can produce boiling heat transfer coefficients in excess of 100,000 W/m2-K, at heat fluxes exceeding 300,000 W/m2.
In his paper, Tuma [2] discussed the economic and environmental merits of the passive two-phase immersion cooling technology for cooling data center equipment. He concluded that liquid immersion cooling can dramatically decrease the power consumption for cooling relative to traditional air-cooling methods. It can also simplify facility construction by reducing floor space requirements, eliminating the need for air cooling infrastructure such as plenum, air economizers, elevated ceilings etc.
Green Revolution Cooling and 3M have demonstrated the feasibility and applicability of using immersion cooling technology to cool the servers in data centers. The main advantages of immersion liquid cooling are saving overall cooling energy and maintaining the component temperature low and uniform. However, both immersion liquid cooling technologies require specially designed sever racks. Specially formulated coolants are needed for both cooling technologies, too, and they are not cheap. For the traditional air-cooled data center, the air is free, abundant and easy to deliver.
In both immersion cooling technologies, the servers have to be vertically installed inside the server rack, which will reduce the date center footprint usage efficiency. Because the liquid baths used in immersion cooling are open to the environment, coolant is gradually and inevitably lost to the ambient during long term service.
The environmental impact of the discharge of a large amount of coolant by data centers has to be evaluated, too. The effect of the coolant on the connectors and materials used on the PCB is not also very clear.
Immersion liquid cooling is a very promising technology for cooling high-power servers. But, there are still obstacles that need to be overcome before their large scale application is assured.
References
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.
Posted in Liquid Cooling, Servers
Tagged Data Centers, Immersion Cooling, liquid cooling, Servers