Cooling accounts for around 15% of the energy used in buildings in the US and contributes heavily to greenhouse-gas emissions. Worldwide, energy consumption related to cooling is expected to surpass that used for heating by 2070.
Cooling challenges are only exacerbated by the harsh environments in which increasing numbers of electronic subsystems – and especially power electronics – are deployed. Classic examples of such challenging environments are subject not only to significant changes in temperature but also to shock and vibration – include automotive and other forms of transportation; industrial equipment; military vehicles; and power generation and networks.
The problem of heat generation in military systems and commercial systems is becoming increasingly complex with ongoing trend of miniaturized electronics in spaces that are continually becoming smaller. Recently South Korean giant Samsung suffered $3 billion in lost income from to its move to scrap the fire-prone Galaxy Note 7 phone. One of the main causes of catching fire appears to continuous increase in energy density of Lithium-ion battery units driven by increased user requirements including full HD, large processing requirements of multi-core CPUs and increasing desire to produce sleeker design.
Increased densities, of course, mean increased heat – and increased heat means greater chances of unreliable operation and premature component failure. Additionally, despite the high efficiency of power electronics, the enormous amounts of electrical power transmitted in many applications mean very large heat loads must be managed.
Further, Military systems must operate under conditions far more demanding than most civilian applications, from radar transmit-receive modules functioning in oppressive desert or tropical heat, to satellite and aerospace systems that deal with both electronics generated heat and intense ambient cold. Automotive and other forms of transportation are subjected to shock and vibration. Even in this harsh environment military systems are expected to have mission-critical reliability.
Thermal engineers have developed a number of creative approaches to deal with these challenges, but all have their drawbacks. Traditionally, designers have favored passive heat transfer devices like heat sinks or heat pipes, generally made from aluminum or copper. These thermal devices offer the advantage of having no moving parts to fail, reducing maintenance needs to a minimum. But aluminum has limited conductivity, which has a greater impact as electronics become more powerful, and copper is relatively heavy (three times as heavy as aluminum).
GE in conjunction with DARPA (the US Defense Advanced Research Projects Agency) have developed technologies such as Thermal Management Technology Bridge, Nano Thermal Interface, Thermal Ground Plane and Dual Cool Jets.
Thermal devices for today’s military system must be small and compact, due to stringent weight and space/volume constraints, as in fighter aircraft, where every ounce and each cubic inch can impact performance. SWaP-constrained systems are, by definition, difficult to cool with restricted areas for air to move around and fans to drive air around simply add weight, power and are a mechanical source of potential unreliability.
Thermal solutions must also be able to reject heat efficiently, even where packaging volumes are limited and cannot be expanded, as in satellite applications. Maintenance needs must be kept to a minimum because repairs may be difficult in the field or at sea (e.g., submarine electronics cooling systems), or even impossible in certain aerospace applications such as satellites, according to Aavid Thermacore.
New weapons and systems like electromagnetic (EM) launchers, high power microwaves and laser systems use stored electrical energy, rather than explosives, to attack or destroy the target. They exhibit pulsed operation and require electrical energy at the gigawatt power level. The thermal cooling solutions are also required in these applications.
New metamaterial enhances natural cooling without power input
A team at the University of Colorado Boulder (CU-Boulder) in the US developed a new metamaterial film out of glass microspheres, polymer and silver, that provides cooling without needing a power input.
Radiative cooling is the natural process through which objects shed heat in the form of infrared radiation. All materials at room temperature emit infrared at wavelengths of 5–15 μm. However, the process is not typically very efficient because it is counteracted by external influences that heat the object, such as sunlight and air currents. Air, meanwhile, absorbs and emits very little radiation with wavelengths 8–13 μm. The Earth cools itself at night by emitting infrared through this “atmospheric window” and into space.
While night-time radiative cooling materials, including a pigment paint, have been successfully developed, a daytime version has proved challenging. The problem is that the materials absorb sunlight, which quickly exceeds the cooling power and instead heats the surface. So the challenge for the CU-Boulder researchers was to create a material that both reflects sunlight and also allows infrared emission.
They created a thin, flexible material with two layers; a sheet of polymer polymethyl pentene containing randomly dispersed silicon-dioxide (SiO2) glass microspheres 8 μm in diameter and a 200 nm-thick silver coating. The combination of the two layers is only 50 μm thick. The polymer-microsphere film is transparent to the whole solar spectrum but radiates infrared. The broad collective resonance among the microspheres ensures the film is highly emissive of infrared within the atmospheric range of 8–13 μm. This property therefore enhances the naturally occurring radiative cooling. Meanwhile, sunlight travels through the metamaterial and is reflected back by the silver coating, which prevents any solar heating.
Researchers also developed low cost production method for the material. “The key innovation of this work is to produce the designed material at scale using the roll-to-roll process,” explains Yang. The researchers used a roll-to-roll extruder to distribute the microspheres in the polymer and a roll-to-roll sputtering machine to apply the silver coating. This means they are able to produce large amounts of the material in mere minutes. “When produced at scale, we estimate that the material cost is only $0.50 per m2 (yes, 50 cents per square metre), since it can be produced at 100 square metres per minute,” adds Yang.
Field tests in Boulder, Colorado and Cave Creek, Arizona, revealed that the film’s average cooling power was more than 110 W/m2 over 72 hours. Even in the midday Sun, its average was 93 W/m2. This is roughly equivalent to the electricity generated by a typical solar panel of the same area.
The glass-polymer sheet has many potential cooling applications. By applying it to a solar-panel’s surface, the film could not only cool the panel but also recover an additional one or two per cent of solar efficiency, because overheating hampers the ability to convert solar energy. “That makes a big difference at scale,” says Xiaobo Yin, another researcher on the project.
Nanofluids next generation thermal management solution of Electronics, Transformer, Space and Nuclear systems, military vehicles and submarines, power electronics and directed-energy weapons
Nanofluid is a new type of heat transfer fluid, having nanoparticles (1–100 nm) which are evenly distributed in the base fluid. These uniformly distributed nanoparticles are generally metal or metal oxides which have a great enhancing effect on the thermal conductivity of the nanofluid, thus increasing conduction and convection coefficients and allowing for higher heat transfer.
The cooling applications of nanofluids include Crystal Silicon Mirror Cooling, Electronics cooling, Vehicle cooling, Transformer cooling, Space and Nuclear systems cooling, Defense applications and so on.
Annealed pyrolytic graphite, or APG
Annealed pyrolytic graphite, or APG, encapsulated within a structural shell made from traditional materials such as aluminum, copper, beryllium, ceramics or composites is another promising approach. Encapsulated APG was first used operationally in high-flying DoD aircraft, where its lightweight characteristics earned it early acceptance in applications where each pound saved could be transformed into another pound of fuel or additional avionics. The low mass of encapsulated APG based solutions is still a key factor in reliable cooling solutions for remote electronics and navigational avionics, writes Mark J. Montesano, VP of Engineering and Technology, Thermacore, k Technology Division.
Encapsulated APG offers additional thermal advantages that go beyond light weight, and apply to many military systems. The most fundamental advantage is high conductivity at low mass. Encapsulated APG material offers three times the conductivity (k) of copper with a mass less than aluminum. This results in a significant improvement in conductivity for any encapsulant paired with APG.
Encapsulated APG’s high conductivity, combined with its low mass density, results in a material system with outstanding performance per pound, or specific conductivity (W/m•K/g/cm3). The specific conductivity of encapsulated APG materials range from 4- to 10- times that of traditional thermal management materials. For example a copper encapsulated APG heatsink with an 80% APG volume fraction would have approximately eight times the specific conductivity of copper alone.
Another benefit of encapsulated APG is that the coefficient of thermal expansion (CTE) offered by this solution can be tailored to specific application needs by altering the choice or configuration of the encapsulant. CTE can also be matched to a specific application, allowing dissipation of dramatically increased heat fluxes by permitting direct attachment, thereby minimizing thermal resistance.
By combining the high thermal conductivity of APG with an easily-tailored CTE encapsulation material, engineers can create solutions for high-powered military electronics while keeping weight and footprint under control. Designers can choose the encapsulant that most closely matches the CTE of electronic materials such as silicon and gallium arsenide, allowing the direct attachment of these devices and providing the thermal benefits of both APG and the encapsulation material.
Encapsulated APG also offers simple integration into current and planned systems. Because the APG is hermetically sealed within the encapsulating material, it is compatible with standard finishing and processing manufacturing steps as well as with the encapsulation materials themselves. These encapsulated APG solutions, with no moving parts, give thermal engineers greater design flexibility, more durability, and less maintenance concerns.
All of these advantages make encapsulated APG an ideal material for military applications, enabling the technology that satisfies today’s needs for high-density packing requirements in a limited space. Thermal technologies based on encapsulated APG have proven to perform well under demanding temperature, stress, load, vibration and other conditions as protection for avionics, target acquisition, imaging and other systems in mission-critical applications onboard fighter aircraft (such as the F-16, F-22 and F-35 Joint Strike Fighter) and helicopters. Encapsulated APG-based solutions help sensitive electronics continue to function in temperatures down to -70° C and in 9g load conditions.
The properties of encapsulated APG, at work in so many military applications today, are also opening up new possibilities for the future. One example is flexible thermal links for aircraft, integrating APG with a flexible heat pipe to cool target acquisition sensors while isolating them from the aircraft’s vibration. The flexibility of APG and the strength of the encapsulation material combine to provide a thermal solution with mission-critical reliability, writes Mark J. Montesano, VP of Engineering and Technology, Thermacore, k Technology Division.
Compressible Soft-PGS thermal interface material (TIM)
Panasonic Automotive & Industrial Systems Europe in Munich is introducing the compressible Soft-PGS thermal interface material (TIM) for electronics cooling and thermal-management in extremely thin spaces — particularly for power electronics devices.
Soft-PGS enhances the thermal coupling between heat-producing devices (heat sources) and heat-dissipation devices (heat sinks). The Soft-PGS is a 200-micron thick graphite sheet designed as a thermal interface material for insulated-gate bipolar transistor (IGBT) modules.
As Soft-PGS can be compressed by 40 percent it is a solution for reducing thermal resistance between a heat sink and an IGBT module. The 200-micron thick Soft-PGS sheet is easy to install, and has far lower labor and installation costs than thermal grease or phase change material, Panasonic officials say.
Soft-PGS guarantees thermo stability to 400 degrees Celsius and high reliability against intense heat cycles from -55 C to 150 C. Its thermal conductivity is guaranteed at 400 Watts per meter-Kelvin for X-Y direction and at 30 Watts per meter-Kelvin in Z direction.
Dana recognized for Power Electronics cooling technology
At the convention in 2015 and 2016, Dana was named Best Battery Solution Provider for its research and contributions to thermal-management technologies for hybrid-electric vehicles and battery-electric vehicles.
Dana’s IGBT cooling technology addresses the high-performance thermal requirements of today’s electric and hybrid vehicles. The precision-manufactured aluminum cooling plates feature superior flatness, a critical requirement for reducing thermal resistance to prevent IGBT chips from overheating and possible failure. Dana’s IGBT cooling technology is exceptionally clean and flux-free, which ensures low coolant conductivity levels by minimizing contamination.
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