As part of an Advanced Research and Development in Naval Integrated Power Systems (IPS) project, work is being done at the EMPF to determine the effectiveness of new thermal management technologies for use in high power naval applications, with the goal of validating these newer technologies using suitable demonstration vehicles. Implementation of these new thermal management methods on various power distribution systems for the DDG-1000 IPS platforms within the Integrated Fight Through Power (IFTP) systems will commence upon demonstrating the effectiveness of the newer technologies against the types currently employed. Three areas of technology are the broad focus of the overall program:

• Fiber Optic Sensors and Networks for Condition Based Maintenance (CBM)
• Wide Band Gap (WBG) High Power Semiconductor Technologies
• Advanced Heat Exchangers

The Cold Plates described here are part of the Advanced Heat Exchanger task, and deals with the new technologies of micro-channel cooling and foamed graphite.

Partnering with the Naval Surface Warfare Center Carderock Division (NSWCCD), in Philadelphia, Pennsylvania, theEMPF will ultimately demonstrate these advanced technologies at NSWCCD’s Land Based Test Site (LBTS).

Advanced heat exchangers are needed to lower the operating junction temperature of the Insulated Gate Bipolar Transistors (IGBTs), as well as other thermally sensitive items used in U.S. Navy power systems. The temperature of these devices must be controlled so that their outputs, both steady state and short duration on-demand, are significantly improved over the existing design. Because of the higher power demands required by IFTP and IPS for the DDG-1000, better thermal interface materials, coolants, and high performance cold plate designs and technologies are proposed to achieve critical higher levels of heat removal.

Various kinds of Thermal Interface Materials (TIMs), Cold Plate designs (e.g. foamed graphite and micro-channel cooling), and coolants were researched and subsequently analyzed using the ALGOR® Thermal Analysis tool in order to narrow down the number of choices to the few that have the highest potential of success for this application.

Existing cold plate technologies utilize copper tubes swaged into an aluminum block.

The newer cold plate designs may use one or more types of new technologies, such as foamed graphite and/or micro-channel cooling technologies. The foamed graphite cold plate is made by starting with a hollow aluminum block, brazing in a block of foamed graphite material, and then machining water channels into the foamed graphite, rather than the existing method that would have copper tubes swaged into a solid aluminum block. High pressure coolant enters the system, and filters through the foamed graphite channel walls toward the outlet fitting, where it will exit the system at a lower pressure, as depicted in Figure 1-1.

Because of the high thermal conductivity of the graphite and the immense surface area of the open cell structure of the foamed material, a very large volume of cooling water is exposed to the hot graphite foam. Since the foam is brazed directly to the aluminum block, and the block is in intimate contact with the hot electronics, this method of heat removal is rather effective.

The original concept for the novel Foamed Graphite Cold Plate was put forward by Material Resources International, which holds patents on the brazing of the foamed graphite to the aluminum to allow the fabrication. After successful Phase I SBIR (Small Business Innovative Research) work, MRI was invited to Phase II SBIR, and is partnering with the EMPF on the IPS application.

Another contender for high thermal management efficiency is the microchannel cooling principle being applied by Mikros Technologies Inc. in another SBIR (Small Business Innovative Research) effort. EMPF will evaluate (side by side with the foamed graphite Cold Plate) a microchannel cooled cold plate based on the Mikros principle. This principle employs a patented Normal Flow Cold Plate (NCP) arrangement that causes vertical (normal) flow of the coolant against the heat source. The normal flow results in a much lower pressure drop for a given flow rate and channel diameter than would occur in a standard parallel flow micro-channel cold plate system. In the standard system, the flow, parallel to the hot surface being cooled, results in long channels of small diameter having very high pressure drop. A schematic diagram of the NCP micro-channel principle appears in Figure 1-2. Because of the short micro-channel matrix that the coolant passes through, the pressure drop is low while the thermal resistance can be made very small.

Both of these vendors have tested their cold plate designs against conventional cold plates that are readily available in the industry. In both instances, the new technology plates have shown lower thermal resistances than the conventional ones. However, the test conditions for flow, pressure drop and thermal loading have been different in each case, making a full comparison. Carefully controlled tests reflective of the U.S. Navy’s intended application for these technologies will be conducted. The EMPF will conduct and test the new technology cold plates against the actual cold plate presently being used. Using flow rates, coolant temperatures, and pressure drops similar to those found in the actual application will be more accurate when validating cold plate performance.

Operating temperatures of IGBT’s and other power electronics devices must be reduced to improve electrical performance for high power systems. Based upon technical discussions with the cold plate manufacturers, either of the Cold Plate concepts discussed in this article can potentially achieve this objective. A Design of Experiments is planned to test both prototypes using identical laboratory set-ups once prototype cold plates designed and built using these advanced concepts are obtained. The most robust, efficient, and cost effective technique will be chosen for this on-going project at the EMPF, with the goal of installation and further testing of the Cold Plate on the actual IPS equipment at the NSWC LBTS in Philadelphia, and ultimately onboard the DDG-1000.