Review of Electronic Cooling and Thermal Management in Space and Aerospace Applications ^†

Abstract

The continuous miniaturization of electronics, high processing capacity, compact microelectronic devices, and high circuit density contribute to an increasing demand for the efficient cooling of electronics. For aerospace and space applications, where packaging and the optimal use of space, weight, and power are important, adequate and efficient cooling is a limiting factor due to the increased heat flux rates from compact-design electronic units. As a technology enabler, thermal management applications become important with the increasing demand for longer component operation times. This study aims to review the literature and the analysis results of thermal engineering applications on cooling of electronics and thermal management approaches in space and aerospace applications. Many advanced cooling applications with interdisciplinary advancements and their benefits are discussed.

1. Introduction

Aerospace and space applications have advanced remarkably. More countries are involved in aerospace and space research. Aerospace and space systems necessitate high availability, low maintenance, and high performance in harsh environmental conditions. Space and aerospace applications demand packing mission equipment in small volumes to use limited space and power efficiently. For instance, unmanned aerial vehicles (UAVs) served for surveillance and reconnaissance missions initially. Nowadays, they are utilized for a wider range of missions, such as electronic support (ESM), self-protection, and electronic countermeasures (ECM), as well as many other kinds of suppression and destruction mission equipment with an efficient thermal management system of the packed equipment. Radar systems were used in World War II and scanned array radars and the detection possibilities of multiple targets have extended enormously. These improvements were possible due to the latest generation of multifunctional antennas, digitalization, advanced materials, and improvements in the electronics industry. In aerospace applications, the power range of radars and other similar mission equipment continues to increase, while their packaging and weight are becoming smaller. Therefore, advanced cooling techniques are used to improve heat transfer rates for more advanced aerial radar systems with higher power density. For satellite technologies, besides the decreasing cost of launches, the miniaturization of components plays an important role in the exponential increase in countries participating in space activities. Space applications are serving complex missions with the help of thermal engineering applications.

In this literature review, emerging cooling technologies are explained in cooling electronics, where chip-, component-, and board-level-cooling techniques are used. Platforms and system-level thermal management techniques are also addressed along with active and passive thermal management and their elements as an example of a satellite application. The applications of thermal engineering in aerospace applications are not limited to the examples given here.

2. Cooling Electronics

Moore’s law states that the performance of processors per unit volume increases exponentially [1]. Both the International Renewable Energy Development Services and ASML Holding NV predict that process nodes reach 1 nm [2]. New device architectures increase functionality at historical rates. In addition, quantum computing accelerates this miniaturization [2]. Figure 1 from the International Roadmap for Devices and Systems shows the increasing effects of miniaturization and high-power density.

The downsizing of mission systems and miniaturization of new electronic components result in higher power density, higher heat generation density, and localized hotspots. This necessitates heat dissipation of high heat fluxes up to 200 W/cm² [3]. The temperature maxima and the heat fluxes are too high for GaN applications, which on one hand makes challenging operations possible, but it also requires advanced thermal management [4]. The ultra-high heat flux (greater than 1 kW/cm²) and limited heat dissipation capacity of GaN-based devices result in significant performance degradation if the temperatures are not properly managed.

The performance and operation time of semiconductor devices drop with increasing temperatures. The percentage of temperature-related failures in electronics exceeds 55%, according to data supplied by the US Air Force. The operational reliability of semiconductors is directly related to temperature [5]. In fact, as depicted in Figure 2 the failure rate of electronic devices increases exponentially with increasing operating temperature according to MIL-HNBK2178 [6]. Operation time, reliability, and availability are important in aerospace and space applications, which makes solid thermal management even more important.

2.1. Cooling Techniques

Based on heat transfer effectiveness, cooling modes are classified into four general categories: radiation and free convection, forced-air cooling, forced liquid cooling, and liquid evaporation. The classifications are based on heat transfer mechanisms and coolants, as well as their cooling effectiveness. A comparison of the ranges of heat removal rates for the temperature difference between the heat transfer surfaces and the ambient temperature of 80 °C is illustrated in Figure 3 [7].

In addition to the above-mentioned general categories, cooling methods are also classified into air cooling, liquid cooling, and refrigeration cooling based on the coolants used. The electronic devices or chips are cooled by transferring heat to the fluid. This heat dissipation can have phase transitions. This heat transfer is conducted by the forced convection of air and traditional heat fins and heat sinks. As conventional measures are not sufficient for removing heat from advanced devices, new approaches have been developed [8].

To supply sufficient cooling to high-heat-producing electronic equipment within small packaging, heat pipes, and conformal liquid blocks with mini-channels are utilized to enhance heat dissipation efficiently. Conventional cooling devices include external air-cooled heat sinks, which are insufficient to cool modern electronic devices and chips with high-power densities. Therefore, such devices need innovative mechanisms and techniques to enhance the heat removal rate, minimize their operating temperature, and maximize their longevity.

Emerging cooling methods are classified into various types such as heat pipes, heat pumps, microchannels, conformal cooling, spray cooling, phase change material (PCM)-based cooling, and thermoelectric cooling. Heat pipes and PCM-based cooling show great potential for the cooling of high-tech electronics and are used frequently.

2.1.1. Heat Pipes

The working principle of the heat pipe, as shown in Figure 4, is based on phase change in the working fluid. Heat pipes are promising for cooling electronic devices such as computers, laptops, telecommunication, and satellite modules [9]. They were introduced by General Motors in 1942 and developed and used by the Los Alamos National Lab and NASA in the 1960s and 1970s.

Due to their extremely high effective thermal conductivity (up to several thousand times higher than even copper rods [10]) and very low effective thermal resistance (typically ranging from 0.05 to 0.4 °C/W [11]), heat pipes are one of the most feasible means of cooling high-heat-flux-generating electronic devices such as CPUs. Nowadays, numerous commercial cooling solutions for laptops, tablets, smartphones, as well as space applications involve heat pipes [12,13].

2.1.2. PCM

PCM absorbs energy at phase transitions to provide useful heat or cooling. PCMs have been studied for various applications including electronic cooling [6,14,15] as they have a high latent heat of fusion, high specific heat, controllable temperature stability, and small volume change during phase change. PCM is used in the efficient passive systems of many defense and aerospace applications, spacecraft and avionic thermal control, weapon systems, missile electronics, surveillance radars, and communication systems, as it can absorb a large quantity of heat without raising the temperature when the heat output occurs during the phase transition. Different designs of PCM for cooling microprocessors were studied by researchers [16,17,18]. The low thermal conductivity of PCM is one of the major limitations that hinder the effective heat transfer capability of the passive system. Several studies optimized thermal conductance with different-shaped (rectangular, circular, elliptical, triangular) pin-fin heat sinks using paraffin wax, n-Eicosane, aluminum, and rubber pad [17,19,20].

2.1.3. Microchannels

Microchannels are compact cooling elements that provide increased heat dissipation rates and reduce temperature gradients across electronic components. Tuckerman and Pease used microchannels in an electronic device to carry the energy from the chip to the environment, using water as a coolant. The heat transfer coefficient of heat dissipation increased as the flow channel diameter decreased [21,22].

Unlike traditional heat sinks that need a large surface area to increase heat dissipation rates, microchannels use small-diameter channels to increase the heat transfer coefficient by forcing the coolant to flow in close contact with the channel walls. Microchannel heat sinks have been optimized by replacing straight channels with different geometries [23], though the improvement in heat transfer performance was accompanied by a high pressure drop. Manifold layers have been introduced to compromise the high pressure drop and nonuniform temperature distributions. Harpole [24] proposed the concept of a manifold microchannel to cope with the flow maldistribution and large pressure drop in the microchannel. Microchannel heat sinks enhanced with manifold structures and jet impingement have great potential [25,26,27]. The microchannel heat sinks are efficiently used for GaN applications for embedded microchannel cooling.

Technologies such as additive manufacturing are used to fabricate monolithic microchannel heat sinks to reduce the pressure drop in the microchannel while enhancing heat transfer performance [28], whereas additive manufacturing enables the fabrication of complex geometries and enhances the permeability of structures. Additive manufacturing enables small fin wall sizes and makes it possible to fabricate more complex channel patterns [29].

2.1.4. Emergent Coolers

The removal of heat using microchannel heat sinks can be enhanced using smart fluids, i.e., nanofluids with effective thermophysical properties such as increased thermal conductivity. Mini-channel cooling systems with liquid coolants are efficient in cooling electronics [30]. The nanofluids are known as colloidal suspensions, where nanoparticles are dispersed in conventional base fluids. This is used to improve the heat transfer capacity of fluid by modifying their thermophysical suspension properties. Figure 5 depicts the thermal conductivity of conventional and innovative cooling fluids.

There are various studies conducted with nanofluids, which depicts that they have a potential in enhancing the cooling [32,33,34]

• Carbon-based nanoparticles: Carbon nanotubes and graphene are carbon-based nanoparticles employed for cooling in heat sinks. They significantly improve the thermal performance of liquid blocks due to their excellent thermal conductivities. However, they are not stable in aqueous media, because they are naturally hydrophobic and so cannot be dispersed in polar liquids. This hinders the efficient utilization of them. To overcome this challenge, they are functionalized through acid treatment and in this way, become hydrophilic. Nanofluids containing biologically produced graphene nanoplatelets are suggested as appropriate candidates for utilization in electronic cooling [35].
• Oxide nanoparticles: Alumina and titania nanoparticles are oxide nanoparticles applied in cooling electronics. Spherical oxide nanoparticles are employed to synthesize nanofluids in liquid blocks due to their stability, low cost, and thermal conductivity. Turgut et al. [36] investigated the performance of a commercial liquid cooling kit and found out that when using Al₂O₃ nanoparticles suspended in water, the temperature of the contact surface between the heater and the water block was 6.7% lower than that using water only. Abu-Nada et al. evaluated the heat transfer coefficient of Al₂O₃ nanofluids compared to water in a numerical investigation [37]. Bansal et al. evaluated the utilization of nanofluids for spray cooling processes, finding water-based alumina solutions to be promising [38]. The positive results were confirmed by Wang and Xu, who enhanced the heat transfer coefficients and cooling [39].
• Magnetic nanoparticles: Magnetic nanofluids or ferrofluids are the suspensions of ferromagnetic nanoparticles (cobalt, iron, nickel, and their oxides) and non-magnetic base liquid. They have flowabilities of ordinary liquids and magnetic features similar to other magnetic materials simultaneously. This makes it possible to manage the flow, heat exchange, and motion of particles using external magnetic fields. Even if these suspensions can be used, few studies have been carried out on electronic cooling, as the electromagnetic EMI/EMC may be adversely affected.
• Hybrid nanofluids: Hybrid nanofluids are nanofluids that are produced with dispersion, unlike nanoparticles in a composite state or mixture form. The purpose of preparing hybrid nanofluids is to optimize the attributes of different nanoparticles while improving thermal conductivity.

It is important to control the form and size of the nanoparticles. When producing them, employing nanofluids in liquid blocks affects thermal performance considerably. The anisotropic behavior of nanofluids affects pumping power, and pressure loss and energy consumption need to be investigated for the use of nanofluids.

2.2. Design of Electronic Cooling

For electronic cooling, the thermal management design needs to keep the junction temperature less than 125 °C for SiC applications. For space, aerospace, and other military applications, GaN chips are used to tolerate much higher temperatures, up to 250 °C. Conservative design best practice needs a 10% safety zone, and the highest temperature needs to be considered for the worst operation ambient temperature.

The system can be modeled as depicted in Figure 6, including chip level, component level, board level, system level, and environment. Both top-down and bottom-up approaches can be used. A hybrid approach needs to be used, using the capabilities of each level and designing the system according to the application and its limits.

Thermal conditions on the chip and board levels are depicted in Figure 7. Knowing the chips’ maximum junction temperatures, the dominant heat regimes (conduction), and the related variables (like thermal conductivity), the device can be designed.

The PCB architecture can be designed for air or liquid cooling. In both cases, the flow needs to be optimized by CFD computations to satisfy the cooling requirements and minimize the pressure drop. Both air and liquid applications are used in military aerospace applications [40]. Both liquid-cooled cold plate and air-cooled cold wall (using pin fins) applications are reported in Ref. [40]. The cooling technique between the board and the cabinet and/or environment varies according to applications.

According to the constraints, including the utilization area or platform (environment/packaging/power supply/weight and space constraints), the solution must be chosen utilizing conventional and innovative means as well as active and passive thermal management elements.

2.3. Platform Thermal Management

Aerospace and space platforms have extra power density. Satellites have unique challenges such as limited space and limited insulation. Small satellites often possess limited interior volume and valuable payload space. This spatial constraint mandates efficient and compact thermal solutions. Power availability is scarce and allocating power for thermal control systems must be balanced with other mission-critical functions. The heat created by mission electronics needs to be transferred in an efficient way to ease the dissipation. An innovative power-efficient solution becomes essential. Consequently, thermal management strategies are imperative to shield delicate instruments from thermal shock. The challenges of satellites are summarized in

Table 1. Challenges of satellites [41].

Satellite Property
Low thermal mass
Limited external surface area
Limited volume
Limited power
High power density
Multi-layer insulation edge effects

.

Since the 2010s, smaller satellites have been manufactured at a minimum cost. Swarm concepts have been mentioned for cost-effective Earth-observing missions [42]. To reduce cost and development times, various methods have been used [43]. Satellites face extreme temperature fluctuations when exposed between the direct sunlight and the Earth’s shadow. Thermal management serves to regulate temperature, minimize thermal gradients, efficiently dissipate the heat generated by electronics, keep thermal stresses as low as possible, and expand the lifespan [41]. Temperatures need to be regulated with passive and/or active thermal management technology and design methods or an appropriate combination of each. Satellites must operate in specified temperature ranges to maintain their accuracy and functionality. Maintaining uniform temperatures throughout the satellite prevents differential expansion and affects structural integrity and instrument alignment. Rapid temperature changes, such as those experienced during orbit transitions, cause thermal stress, which leads to mechanical failure or damage to sensitive components. Appropriate thermal management extends the operational life of satellites, ensuring missions for longer durations. Various thermal control designs [44,45,46] as well as thermal health control systems [47,48,49] have been introduced and developed in the last decade.

2.4. Thermal Management

The effective thermal management of satellites involves a combination of passive and active techniques, careful design considerations, and the utilization of specific materials and technologies tailored to the mission’s requirements. These methods ensure that the satellite remains within its operational temperature limits, maximizes mission success, and delivers valuable scientific or technological data back to Earth.

2.4.1. Active Thermal Management

Active thermal management is used with active elements to regulate temperature and offers advantages including efficient heat dissipation, rapid response to dynamic thermal loads, and precise temperature control. Small satellites experience varying thermal loads due to changes in solar radiation, orbital conditions, orientation, and operational modes. Active thermal management systems can quickly respond to these dynamic loads and adjust cooling mechanisms accordingly. This rapid response prevents temperature fluctuation, thermal stress, and related potential failures. Moreover, if the heat needs to be continuously dissipated, active thermal management is indispensable. Active cooling methods, such as fans, pumps, and heat exchangers, enhance heat dissipation by actively moving air or coolants over heat-generating components. This efficient heat removal prevents the accumulation of excessive heat, reducing the risk of overheating and maintaining the satellite’s operational efficiency.

Conventional elements such as fans, pumps, heat exchangers, and control units are utilized in satellites as well as thermoelectric coolers. Louvers or shutters are used on external surfaces to control the amount of heat radiated or absorbed. Louvers, consisting of blades and actuators, open or close based on temperature conditions.

Active Thermal Control Units (TCUs) are used to monitor and regulate the temperature of various components within a satellite. They actively control the operation of cooling systems, such as fans, pumps, or TECs, based on temperature sensors’ feedback. TCUs ensure precise temperature regulation and adapt to changing thermal conditions. They play a vital role in actively managing the thermal environment of the satellite. Thermoelectric coolers (TECs) or Peltier devices transfer thermal energy from its low-temperature side to its high-temperature side by utilizing external electrical energy. TECs are often used in conjunction with heat sinks to dissipate collected heat. They have no moving parts and are high-reliability, lightweight, compact, and easy to operate and regulate. This makes TECs promising for the thermal management of spacecraft subsystems and components. Their successful utilization over a decade has been presented in various studies [50,51,52].

Active thermal management systems may have higher power consumption, weight, and volume demand compared to passive techniques. Additional components are added to the system, which creates new risks for reliability. However, the benefits of precise temperature control and efficient heat dissipation may outweigh these considerations in many satellite applications.

2.4.2. Passive Thermal Management

Passive thermal management techniques do not rely on complex moving parts like fans or pumps, which reduces the risk of mechanical failure and increases reliability. They require little to no power consumption since they do not rely on power input. Passive thermal elements possess reduced complexity and compact design to occupy minimal space, making them appropriate for small satellites or systems with limited physical dimensions. They provide long-term thermal control without the need for frequent maintenance or replacement. Overall, passive thermal management offers a reliable, low-power, compact, low-noise signature, and environmentally friendly solution for managing temperature in various applications, including satellites.

While passive thermal elements are simpler in design than active cooling methods, optimal performance requires careful design considerations. Factors such as material selection, surface area, geometry, and airflow patterns need multi-physics optimization to maximize heat dissipation. Specialized thermal coatings are applied to the satellite’s external surfaces to control heat absorption and emission. These coatings maintain the desired thermal balance [53].

Multi-layer Insulation (MLI) blankets consist of multiple layers of reflective material separated by low-conductivity spacers. MLI is used to insulate sensitive components from extreme temperature fluctuations and reduce heat transfer through radiation. The reflective layers reflect solar radiation, while the spacers minimize conductive heat transfer. Heat pipes, as mentioned earlier, are passive devices that use phase-change principles and capillary action to transport heat. They are used for passive thermal management in small satellites [54,55]. Shukla showed various examples of heat pipes for aerospace and satellite applications [56]. Their utilization of nanofluids was presented by Mashei [57]. More advanced heat pipe techniques include nitrogen-charged cryogenic grooved heat pipes [58] and the utilization of different fluids of ammonia and methanol [13].

PCMs undergo a phase change (e.g., solid to liquid) while absorbing or releasing a significant amount of latent heat. They can be integrated into the satellite’s structure or components to provide passive thermal buffering [59]. Their various applications have been investigated, developed, and used since then [60,61,62].

Passive techniques involve controlling the satellite’s albedo (reflectivity) to influence the amount of solar radiation absorbed. This can be achieved through the choice of surface materials and coatings. Radiators are considered passive components when they are not deployable. These fixed radiators allow heat to be radiated away from the satellite’s surface into space. Passive radiators are integrated into the satellite’s structure on the outer surface. The satellite’s structural components serve as passive thermal masses that stabilize temperature fluctuations by absorbing and releasing heat over time. Passive thermal management techniques are generally less adaptable to dynamic thermal loads or changing environmental conditions. They have slower heat dissipation rates than active cooling methods. They operate under specific parameters and may struggle to handle sudden increases in heat generation or varying ambient temperatures. Passive thermal management techniques may not effectively address localized temperature spikes. If heat-generating components are concentrated in specific areas, passive techniques struggle to distribute and dissipate the heat evenly. Still, due to the above-mentioned advantages, passive systems are preferable to be utilized as much as possible.

2.4.3. Best Practice for Small Satellite Thermal Management

Small satellites, named CubeSats, aim to pack more equipment into a smaller volume. This necessitates a mass-efficient design and increases thermal density. Overheated electronic boards and mission equipment cannot be replaced or serviced; therefore, the thermal solution must be robust and appropriate for the mission’s lifespan. For CubeSats, due to limited power and budget, the passive techniques are preferred, as long as they are possible. It is crucial to optimize the passive thermal design concurrently with thermal, mechanical, and electrical input to deliver low-power, low-mass, small-volume, and reduced-power consumption versions of existing technologies.

In most space systems, all heat must be rejected via radiation along radiators on the outer surface. Therefore, the heat needs to be transferred to the radiators in an efficient way to ease dissipation. Both copper–water heat pipes and ammonia heat pipes are typically installed for board- and system-level heat transfer. A commonly used technology is PCM, which balances short-duration and duty cycle applications. PCMs’ thermal storage provides a thermal buffer in duty cycle applications with high peak power. CubeSats experiencing transient power operation utilize PCM to meet thermal performance objectives while maintaining a low-complexity design that is optimized for mass and volume.

For duty cycle-operating components, storing thermal energy in small volumes is a desirable design option. The best practice is to utilize an intelligent combination of active and advanced passive elements such as oscillating heat pipes, miniature cryocoolers, micro louvers, and specific heat straps [63].

After the conceptual design phase and selection of appropriate thermal technology elements, detailed thermal analysis must be conducted to keep all components within the specific temperature range. Accurate yet cost-efficient numerical modeling of the system supported by tests is crucial to ensure reliability while avoiding over-design at the same time. Jurkowski et al. conducted sensitivity analysis (SA) and uncertainty quantification (UQ) to verify the thermal design of a nanosatellite data processing unit (DPU) [64].

Bonnici 2019 developed thermal finite element modeling for the thermal design of small satellites to calculate the transient heat load history on the pico-satellite’s external surfaces due to environmental sources [65]. A thermal management system, consisting of a rollout deployable radiator, PCMs, and integrated heat pipes for high-power CubeSats was offered and proven to be feasible to be utilized in CubeSats [66].

3. Conclusions

A review of traditional and innovative cooling and thermal management techniques was conducted for cooling electronics in the aerospace and space industries. There are many methods to transfer and dissipate the created heat, each having advantages and drawbacks. Therefore, it is important to choose the right methods and elements for each case. The best practice needs to be chosen to provide thermal solutions.