Liquid cooling is a time-proven approach to maintaining safe operating temperatures of high heat flux electronics. It’s effective, capable of rejecting high heat loads and well understood. Although liquid cooled approaches are active, meaning they consume power and have mechanical components, they are often the best solution when power and temperature requirements demand it. In some cases, the liquid cooling lines need to be routed close to the heat-generating components to maintain thermal goals. This can, however, present a significant risk to the system in the event of a leak. If there were to be a fluid leak in the system, it would result in catastrophic failure to the electronics system.
Risk Mitigation to mitigate fluid leaks
One way to mitigate the fluid leak scenario is by decoupling the ultimate heat sink (liquid) from the heat-generating devices. By adding heat pipes to transfer heat at low thermal resistance, you can increase reliability with a minimal performance impact. The added distance between electronics and coolant could lead to a thermal penalty.
In the example below, ACT designed, developed, and tested a thermal solution that is capable of transferring heat from within an electronics chassis to an external liquid cooling loop. This included the heat pipe heat spreader design as well as the series/parallel flow channel liquid cold plate that acted as the ultimate heat sink. The electronics assembly, referred as a slat, has a fixed thermal resistance that is set by the customer’s electronics design. This slat is mounted to a heat pipe embedded, or HiK™ heat spreader, which transfers the heat through a series of interfaces and aluminum conduction paths to the external liquid cold plates which are mounted on the top and bottom of the electronics chassis.
Each HiK™ heat spreader in this assembly transfers 180W. The power is split between the two cooling loops with each handing 90W of power. This is important to note since heat pipes work both gravity assisted, and against gravity. Since the heat pipes were designed to work in both of orientations, and have a fixed thermal resistance, the electronics were able to maintain symmetric thermal gradients using both the upper and lower HiK™ heat spreaders. There is a combination of 16 HiK™ plates within this system for a total heat load of 2,880W.
Customer Collaboration- What does the design process look like?
ACT began the effort with a detailed thermal analysis that mapped the thermal resistance network from the heat-generating components to the chilled water supply. Several design iterations were required to determine a cold plate design that met both temperature uniformity and pressure drop goals. The final solution was a serpentine tube-in-plate design to ultimately meet necessary thermal requirements. Each serpentine removed heat from four of the HiK™ plates, four serpentines were used in parallel to reject heat from the 16 HiK™ plates within the system as seen in the Figure 1 CAD drawing below.
Following the thermal analysis of the proposed system and the customers’ approval, the ACT team was ready to move forward with manufacturing the prototype. The first step in production was manufacturing the heat pipes, aluminum components and the manifold assembly. Commercial off-the-shelf cold plates were used in lieu of an all-aluminum assembly in order to simplify and expedite the prototype lead time. Following assembly of the prototype, heat loads were simulated using film heaters that matched the heat input footprint provided by the customer and chilled water was supplied using a commercial chiller. An image of the test setup is shown in Figure 2.
Results of the testing in comparison to the thermal predictions are shown in Figure 3. The max junction temperature was predicted to be 118.0°C whereas the assembly measured 118.2°C. The temperature drop highlighted in the red boxes on the left of Figure 3 represents the junction-to-case temperature drop within the electronics slot. This information was provided by the customer and is fixed in the comparison. There are minor differences between the predicted and measured interface materials. The predicted temperature drop of the cold plate was 11.7°C and measured a 14.5°C. One key temperature drop to highlight is for the HiK™ plate. The prediction estimated a 4.1°C temperature drop, whereas the assembly measured only 2.9°C.
It’s not uncommon for large chassis to have such high waste heat that it requires liquid cooling. While this is less reliable than conduction or air-cooled solution, there are methods to increase reliability by separating the liquid flow and potential leak points from the high-value electronics. By using heat pipes to transfer the heat from the source to the liquid-cooled sink, you can vastly reduce thermal resistance in this conduction path and keep the liquid loop a safe distance from the electronics. This hybrid approach can provide both high thermal performance and high reliability when compared to alternative solutions.
Discuss your application with ACT engineers today!
ACT has developed an efficient, compact vapor compression system (VCS) that can handle very high pulsed heat loads including those from high-energy lasers and other non-lethal directed energy weapon systems. Our vapor compression system is unique in that it incorporates thermal energy storage within the system, which in turn enables the system size, weight and power (SWaP) to be substantially reduced. Peak heat loads are rapidly stored in the latent heat of a phase change material (PCM) and dissipated at a slower rate during longer recharge periods. In effect, the thermal energy storage “dampens” the heat load that must be handled by the vapor compression system.
At ACT, particular attention has been given to the design, fabrication and testing of the heat exchanger (HX) that incorporates the phase change material. It has been designed to efficiently transfer large amounts of heat from a liquid coolant into the PCM where it is temporarily stored before being dissipated by the system.
A top-level schematic of a vapor compression system is shown below (left) along with a schematic of a representative pulsed heat load profile (right). The red area represents the transient high peak loads that need to be managed and the blue region shows the much lower heat load that is taken up by a VCS having thermal energy storage.
Figure 1: (Left) A schematic of a vapor compression system with integrated thermal energy storage; (Right) A representative transient thermal energy profile showing that the use of thermal energy storage dampens the heat loads that need to be managed by the vapor compression system.
At present, ACT is developing these systems for both ground and air-based platforms. For more information, contact ACT.
In a recent research study funded by the Office of Naval Research (ONR), Advanced Cooling Technologies, Inc. (ACT) has developed an ocean thermal energy harvesting system using the principles of latent energy storage in phase change materials (PCMs). In the mid- and low-latitude regions of the oceans, there is a significant temperature difference between the warm surface waters, and the water at depths below the thermocline layer. This temperature difference can be harnessed to generate electric power, with the system shuttling between the warm surface water and the colder, deeper water.
ACT’s PCM-based ocean thermal energy harvesting system, shown in Figure 1, consists of the following:
- A warm PCM volume (Tm,warm)
- A cold PCM volume (Tm,cold)
- An array of thermoelectric converters (TECs)
- Embedded thermosyphons in each PCM volume
The PCM-based ocean thermal energy harvesting system then operates as follows: When the vehicle is in the warmer waters near the ocean surface, the system will absorb heat from the ocean, storing it in the latent heat of the PCM by melting the PCM. Additionally, the different melting temperatures of the respective PCM volumes serve to establish and temperature drop across the TECs.
Conversely, when the system is in cold water below the thermocline, the heat stored in the PCM will be released back to the ocean, freezing the PCM, and again generating power as a temperature difference is established across the TECs. A key enabling technology for the ocean thermal energy harvesting system is the use of thermosyphons, which are similar to heat pipes, but rely on gravity for condensed liquid return to the evaporator, to passively control the flow of heat to and from the ocean.
To develop the PCM-based ocean thermal energy harvesting technology, the system was evaluated and optimized at the component level, and a dynamic system model was developed to predict the transient performance of the system. This model was used to perform trade studies and identify an optimum configuration for the PCM heat transfer enhancements within the PCM volumes, as well as other design parameters. The model was also validated through the testing of a “test-cell” prototype, which demonstrated an energy density of roughly 90 J/kg/dive-cycle.
Two additional prototypes were built and tested to demonstrate the feasibility of the technology for different applications. These applications include an on-board power generation system to extend the range of small unmanned underwater vehicles (UUVs). This prototype, designed to be enclosed entirely within the hull of the vehicle is shown in Figure 2. The testing of this prototype, cycled between water temperature of around 2°C to 28°C, demonstrated an energy density of around 30 J/kg/cycle.
The second prototype was designed to demonstrate the potential scalability of the technology using a modular system. This prototype is shown in Figure 3. This prototype was also tested by exposing the system to water temperatures cycled between around 2°C to 28°C. Some test data showing average system temperatures and the resultant power output from the TECs is shown in Figure 4. The prototype demonstrated an energy density of around 70 J/kg/cycle.
The PCM-based ocean thermal energy harvesting system developed under this ONR program highlights several of ACT’s core technologies and heat transfer expertise, including PCM thermal storage, and passive two-phase heat transfer. Have an application you’d like to discuss? Contact the thermal experts at Advanced Cooling Technologies.
As a leading innovator and manufacturer of new thermal management technologies, ACT is frequently sought out by our military customers to work on thermal challenges for their next generation applications. In this case, ACT worked with Lockheed Martin to help develop the Long Range Anti-Ship Missile (LRASM). LRASM, featuring a variety of highly sophisticated technologies that allow it to execute long-range missions with high accuracy and reliability, is designed to meet the needs of the U.S. Navy and the U.S. Air Force for advanced precision-guided missile systems. It aims to reduce dependence on external platforms and network links in dense electronic-warfare environments.
Our Role Solving Military Thermal Challenges
ACT supported the effort by developing an advanced passive thermal management system to replace the actively pumped cooling system. This advanced thermal management system reduces the number of parts and system complexity, which in turn creates higher system reliability. The passive operation provides critical mission capability by cooling the electronics responsible for dynamic targeting. This advanced design was the result of ACT’s commitment to consistently investing in new and emerging thermal technologies. It once again demonstrated the many benefits of having the right technology at the right time.
November 2016 Launch of GOES-R
The Aerospace industry demands reliable and high quality products from their supplier. ACT has provided critical support to many high profile programs. ACT’s work with ITT/Harris Corporation on the recently launched Advanced Baseline Imager (ABI) aboard NOAA’s GOES-R satellite is one example of those successful collaborations.
The partnership between ACT and ITT/Harris dates back more than a decade to 2006, when ACT first began manufacturing aerospace grade Constant Conductance Heat Pipes (CCHPs). At that time ITT was developing an advanced meteorological sensor system called the ABI for the GOES-R satellite. Each ABI unit utilizes fourteen unique heat pipe geometries to isothermalize mounting structures and transport excess heat from the electronics to the thermal dissipation radiators. ACT worked closely with the ITT team to successfully manufacture and deliver the complex heat pipes for integration at ITT. At the end of that program, ACT received an outstanding supplier award from ITT for our effective support. According to ITT, “ACT was a critical supplier for what will be an important national asset”, referring to the GOES-R satellite. Since then, ACT has delivered similar CCHPs for the GOES-S, T and U satellites.
To date, ACT has produced space qualified CCHPs for over thirty satellites, and has accumulated over 15 million operating hours on orbit.
Advanced Cooling Technologies, Inc. (ACT) designs, develops and fabricates custom hardware for testing and experimental purposes. One unique apparatus was developed for the Air Force to evaluate the thermal behavior of fabrics subject to convective and radiative heat loads as well as open flames. The hardware provides an automated capability in which the user simply inserts test samples, which themselves are instrumented with temperature sensors woven into the fabrics, inputs testing conditions (radiant heat flux, convective heating, combined radiative and convective heating, open flame exposure) and starts the test. Spatiotemporal temperature and flow measurements are then acquired and displayed in real-time. Also important is that the hardware was developed to allow for testing against established test standards, which in this application were ISO 17492, ASTM D 4108-82, ASTM F 1939-99a. With this testing capability, the military can now accelerate the evaluation and deployment of high-performance fabrics and reduce lead times and costs. Other applications of the hardware include its use for the development of personal protective equipment (PPE) for firefighters and first responders, airbag fabric design and development, personal cooling garments and a tool for thermal injury assessment and analysis. A picture of the test hardware developed at ACT is shown in Figure 1.
To supplement the hardware, a detailed finite volume based fabric model was also developed to compute the transient thermal profile within the porous fabric subject to hot gas impingement, radiative heat loads and other thermal input conditions. The model includes the thermal input, flow and thermal transport within and through the fabric (incorporating fabric material properties, permeability, etc.) and coupling to a burn injury model developed by the military to compute the extent of burn injury based on empirical correlations. A separate test apparatus was also developed at ACT for experimental determination of the fabric permeability as a function of temperature and flow conditions. A top-level schematic of the model is shown in Figure 2.
Transient measurements of the temperature and flow speed were acquired and heat transfer rates computed. The detailed model was compared and validated against the experimental results. Moreover, both experiment and model were extended to multi-layer fabrics and the inclusion of water/ vapor transport through the fabric has received recent attention to improving model predictions.
ACT was approached by a customer to develop custom two-phase cold plates for a pumped two-phase (P2P) thermal management system to be used in a high power directed energy weapons application. Under this program, ACT designed, built, and tested two large cold plates to cool and isothermalize a large array of high-power density electronics. To qualify the cold plate design, ACT also designed, built, and delivered a custom test system with a full P2P loop that interfaced with an external chiller.
The two-phase isothermal cooling test system contained a full P2P loop consisting of the custom cold plates, a condenser heat exchanger for heat rejection to an external chiller loop, a reservoir, and a pump. A control system was developed for the P2P loop for maintaining the cold plate inlet temperature and system pressure. This control system included a temperature-actuated flow control valve for inlet temperature control, and heating and cooling of the two-phase reservoir to maintain saturation pressure. The heat load on the cold plates were mimicked by heaters epoxied to the surface in the electronics footprint. The two-phase isothermal cooling test system was fully instrumented to monitor fluid temperature and pressure at multiple locations, cold plate temperature, and coolant flow rates. The final delivered test system, shown below in Figure 3, was equipped with a control panel and data acquisition system and was contained on a movable cart.
The pumped two-phase isothermal cooling test system was used to evaluate the performance of the cold plates, which had strict temperature uniformity (isothermality) requirements. The customer required that the temperature of the cold plates be maintained within ±2°C of a target temperature, and vary by less than 3°C over the entire cold plate. Figure 4 shows the cold plate temperatures along the length of a channel on each plate, which satisfactorily met the customer requirements.
There are a growing number of high power applications across industries that require highly efficient cooling solutions. Some of these products have very high localized heat fluxes, greater than 300 W/cm2, but must maintain a tight temperature range so as not to disrupt sensors or optical devices. Direct die attachment can lead to mechanical stresses at the interface if the coefficient of thermal expansion (C.T.E.) is mismatched between the die and the substrate. The standard method to overcome this is to add an interface material, such as thermal gap pads or thermal pastes, to accommodate the mismatch. Unfortunately the presence of this thermal interface layer increases the thermal resistance and likewise increases the temperature on the device itself.
Vapor Chambers are an important tool in the thermal management toolbox, since they act as flux transformers, spreading the high input heat flux over the entire surface of the vapor chamber. This allows the heat to be removed from the vapor chamber by conventional cooling methods. Most vapor chambers are limited to an input heat flux of about 75 W/cm2, however, ACT has developed a C.T.E matched vapor chamber that allows for direct bonding of the heat source; see Figure 1. This unique device has been demonstrated to dissipate heat fluxes as high as 700 W/cm2 and 2kW overall. The evaporator thermal resistance of vapor chambers with this wick design is only 0.05 K-cm2/W.
The overall envelope structure is aluminum nitride with a direct bond copper exterior. The copper on the inside of the vapor chamber ensures that the well-known water/copper performance is maintained. In areas where the heat source is to be attached, the copper layer is removed exposing Aluminum nitride. Aluminum nitride has a CTE of ~5.5 ppm/⁰C, which is close to many common semiconductor materials. The devices can be directly attached to the vapor chamber, eliminating the need for a thermal interface layer.
High Conductivity (HiK™) heat sinks can also improve the Size, Weight, and Power (SWAP) compared to standard heat sinks. Placing a discreet heat source on a large metal heat sink will produce large thermal gradients as the heat slowly conducts through the aluminum to the fins. Embedding heat pipes in a HiK™ heat sink can increase the thermal conductivity from around 180 W/m K to 500-1,200 W/m K, providing an opportunity to reduce heat sink plate thickness and fin area. This approach has been proven in a variety of weight/volume sensitive applications including: Ruggedized Electronics, UAVs, Handheld/Portable Devices, LEDs and optical devices.
Embedded heat pipes can improve performance and reduce the mass of forced and natural convection heat sinks. ACT fabricated a HiK™ heat sink and an all-aluminum heat sink with the same performance; see Figure 1. The total heat dissipation is 150W in both cases. The conventional aluminum heat sink is 12 inches (30.5 cm) long, weighs 9.6 lbs. (4.4 kg) and has a base thickness of 0.6 inch (1.5 cm). Introduction of 5 heat pipes, 3 in close proximity to the heat source and another two a little further out for improved spreading reduced the length to 10 inches (25.4 cm), reduced the thickness to 0.28 in (0.7 cm), and reduced the mass to 6.3 lbs. (2.9 kg) for an overall material reduction of over 34%. Thermal images that demonstrate the improvement are shown in Figure 2. The Hi-K heat sink seen on the right maintains the same source temperature, even though the heat sink is shorter, lighter, and thinner. The improvement is directly attributable to the addition of heat pipes which can be seen as red lines in the picture on the right.
Learn more about thermal solutions for power electronics here…
With support from the National Science Foundation, ACT has developed a pumped two-phase cooling system for high heat flux electronic components and laser diodes and is now working on packing the system in a compact, user friendly, stand-alone platform. The two-phase system efficiently handles fluxes on ~ 300-500W/cm2, requires little pumping power, maintains device temperatures below operating limits and provides for a high degree of isothermality over the heated surface. Meeting these requirements is important in many applications including lasers whose emission wavelengths are temperature-dependent.
Unlike single-phase cooling, two-phase systems take advantage of the phase change (latent heat) of the coolant which enables it to handle higher heat fluxes for a given heat load. Two-phase cooling systems can however be more complex and prone to flow and thermal instabilities. As such, techniques to effectively manage instabilities have been developed and characterized. These include the application of engineered microporous coatings on the heated surface(s), which enhance boiling performance by increasing the number of nucleation sites together with a capillary-driven resupply of the coolant to the heated surface, which prevents or postpones dry-out (extends CHF).
A schematic of a pumped two-phase cooling system is shown in Figure 1. Key components include a pump, preheater, surge tank, evaporator (the heat sink), condenser and accumulator. The surge tank and the preheater differentiate this system from a traditional liquid cooling loop. The surge tank consists of vapor and liquid at saturation; by controlling the pressure in the tank, the saturation (boiling) temperature of the working fluid can be controlled. The preheater heats the subcooled liquid exiting the condenser to a temperature close to the saturation temperature before it enters the evaporator. This is important as boiling heat transfer is most efficient at saturation (minimal subcooling).
Figure 1: Schematic of a pumped two-phase cooling system
Also shown in Figure 2 is a representative copper minichannel heat sink coated with a microporous coating.
Figure 2: Minichannel heat sink with porous sintered power coating
The pumped two-phase cooling system shown in Figure1 was fabricated and evaluated. The heat transfer coefficient (HTC) [W/m2K] and the Incipient Wall Superheat [K] were determined as a function of the input heat flux and coolant mass flux [kg/m2s] using refrigerant R134a and others. Representative results for the HTC for coated and uncoated minichannel heat sinks are shown in Figure 3. Clearly, the HTC is higher for the coated heat sinks and the CHF is extended.
Figure 3: Heat Transfer Coefficient (HTC) associated with Minichannel Heat Sinks noting the increase in CHF for coated heat sinks.
The incipient wall superheat is also shown in Figure 4 as a function of input heat flux on coated and uncoated heat sinks. Again, the microporous coating enhances the thermal performance of the heat sinks as evident by lower values of incipient wall superheat; in other words, a heat-generating device mounted on a two phase heat sink with microporous coating will be maintained at a lower temperature compared to one that is mounted on an identical uncoated heat sink.
Figure 4: Incipient wall superheat decreases with the application of microporous coatings
In addition to improving the heat transfer, the thin copper coatings help suppress two-phase flow instabilities. Figure 5 shows flow instabilities with a bare copper cold plate, where the vapor flow occasionally reverses and enters the input plenum. In contrast, instabilities are suppressed in the cold plate with a thin porous coating, see Figure 6.
Figure 5. High heat fluxes with uncoated mini- and micro-channel systems can have unstable flow. Note the intermittent vapor flow back into the inlet plenum.
Figure 6. The maximum heat flux in two-phase systems can be increased by adding a thin porous layer. An additional benefit is that the flow becomes more stable.
In short, pumped two phase cooling systems have been developed. Flow and thermal stability issues were well managed with the use of porous coatings, which increase the heat transfer coefficient and extend the CHF. Pumping power required is minimal and the application of the coating does not increase the pressure drop in a measurable way as the coating thickness is very small compared to the channel dimensions. Additional work on flow boiling heat transfer in minichannel heat sinks is ongoing with a focus on maximizing performance and understanding the parameters (i.e., the coating properties – thickness for a given particle size, etc.) that affect performance for specific applications.
For more information on Pumped Two Phase Cooling, contact The Thermal Experts at ACT.
Directed energy weapons (DEWS) are being developed to compliment conventional weaponry and offer advantages that include: faster speeds, longer ranges, and higher precision at less cost. Some systems such as the Active Denial System involve microwave emission while other systems are laser-based. Common to these high-energy systems is the need to effectively handle transient, high heat loads with minimal Size, Weight, Power and Cost (SWaP-C) dependent on platform.
Thermal Management of DEWS
Thermal management is a critical component in DEWS. Further, it is often impractical (due to large SWaP) to efficiently dissipate the high heat loads in real-time in mobile and lightweight airborne platforms. The SWaP of the cooling system can however be substantially reduced with a cooling system having thermal energy storage, which is especially attractive for systems with a short duty cycle. ACT has developed several custom cooling systems with thermal storage for DEWS that were 40-60% lighter than the systems otherwise needed to continuously handle the peak heat load. The ultimate heat sink in these systems was ambient air, which could be hot depending on local conditions.
How ACT’s System Works
Our system is based on a compact vapor compression system (VCS) integrated with a thermal storage unit. The thermal energy generated is stored at a high rate when the DEWS is on and dissipated to a heat sink (typically ambient air) during the remainder of the duty cycle. For thermal storage, Phase Change Materials (PCMs) and metal hydrides have been used. In effect, the thermal energy storage unit stores the heat and effectively “dampens” the heat load such that the time-averaged heat load, rather than peak heat load, needs to be handled by the cooling system. This enables the SWaP of the vapor compression system to be reduced compared to that otherwise needed to handle very high peak heat loads.
Figure 1: (Left) A schematic of a vapor compression system with integrated thermal energy storage; (Right) A representative transient thermal energy profile showing that the use of thermal energy storage dampens the heat loads that need to be managed by the vapor compression system.
Systems Developed at ACT
ACT has developed several cooling systems for DEWS including work done for MDA, the Navy Vehicle Stopper program, and the Navy Ground-Based Air Defense (GBAD) system (featured below). The GBAD program, for example, has been a 5-year development effort that employs an advanced laser (30kW nominal power), lightweight beam director, on-board battery power storage and ACT’s DEWS cooling system, all packaged on a tactical vehicle platform.
ACT has also developed advanced, compact, large nearly-isothermal heat exchangers that have been integrated with DEWS developed by military primes. Some of these systems were proof-of-concept while others are being validated through field testing.