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Liquid cooling
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Liquid cooling refers to cooling by means of the convection or circulation of a liquid.
Examples of liquid cooling technologies include:
- Cooling by convection or circulation of coolant, including water cooling
- Liquid cooling and ventilation garments, worn by astronauts
- Liquid metal cooled reactors
- Radiators (engine cooling)
- Cooling towers
Applications
[edit]Computing
[edit]In computing and electronics, liquid cooling involves the technology that uses a special water block to conduct heat away from the processor as well as the chipset.[1] This method can also be used in combination with other traditional cooling methods such as those that use air. The application to microelectronics is either indirect or direct. The former pertains to the category that utilizes cold plate cooling, which uses water as coolant while, in the latter (also referred to as liquid immersion cooling), the surface of the chips comes in contact with the liquid since there is no wall separating the heat source from the coolant.[2] This immersion cooling also offer a higher transfer coefficient, although this depends on the specific coolant used and mode of convective heat transfer.[3] One of the main benefits achieved is the reduction of noise and it is also more efficient.[1] Some of the drawbacks include the risk entailed with the close proximity of liquid to electronics as well as its cost. Liquid cooling systems are more expensive than fan sets, which require fewer components such as reservoir, pump, water blocks, hose, and radiator.[1]
HVAC
[edit]Liquid cooling is also used to remove heat from large buildings by using chillers which transfer the coolant from the evaporator to air handling units, chilled beams and fan coil units inside the building, and to the cooling towers from the condenser if the condenser is liquid-cooled. Some buildings are directly cooled by the cooling towers using plate heat exchangers transferring the heat from the chiller condenser loop to the evaporator loop. Convective heat transfer is used to cool the coolant if the building uses dry or closed-circuit cooling towers.
Liquid cooling garments
[edit]Liquid Cooling Garments (LCG) are used to decrease the wearer’s bodily temperature and keep them comfortable. Generally, an LCG uses a series of coolant-filled tubes and a refrigeration unit and a pump to move the coolant throughout the system. These parts are usually encased inside of a normal garment, usually a vest.[4] Since the coolant is so close to human skin, water tends to be the preferred coolant, as it is both safe for skin contact and effective at transferring heat.[5] Due to their portable and versatile nature, LCGs have been proposed as a solution to overheating in areas where a standard air conditioning system would not be feasible or where the cooling would need to be under protective gear.[6]
References
[edit]- ^ a b c Docter, Quentin; Dulaney, Emmett; Skandier, Toby (2012). CompTIA A+ Complete Deluxe Study Guide Recommended Courseware: Exams 220-801 and 220-802, Second Edition. Hoboken, NJ: John Wiley & Sons. p. 61. ISBN 9781118324066.
- ^ Tong, Ho-Ming; Lai, Yi-Shao; Wong, C. P. (2013). Advanced Flip Chip Packaging. Dordrecht: Springer Science & Business Media. pp. 447. ISBN 9781441957689.
- ^ Yarin, L. P.; Mosyak, A.; Hetsroni, G. (2008). Fluid Flow, Heat Transfer and Boiling in Micro-Channels. Berlin: Springer-Verlag. pp. 13. ISBN 9783540787549.
- ^ Zhang, Mengsheng; Li, Zijun; Xu, Yu; Wang, Junjian (April 2022). "Design and research of liquid cooling garments in thermal environment". International Journal of Refrigeration. 139: 136–147. doi:10.1016/j.ijrefrig.2022.04.014. ISSN 0140-7007.
- ^ Yu, Xu; Zijun, Li; Junjian, Wang; Mengsheng, Zhang; Mintao, Jia; Qiaoli, Wang (5 January 2022). "Man-portable cooling garment with cold liquid circulation based on thermoelectric refrigeration". Applied Thermal Engineering. 200. Bibcode:2022AppTE.20017730X. doi:10.1016/j.applthermaleng.2021.117730 – via Elsevier.
- ^ Zhang, Yingshuai; Yang, Yiyong; Ji, Linhong; Cheng, Jia; Yao, Qi (2023). "Research and application of liquid cooling garment system based on human acupoints and meridians cooling stimulation in the high temperature environment". Thermal Science. 27 (2 Part B): 1687–1697. doi:10.2298/tsci220702163z.
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Liquid cooling
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Fundamentals
Heat Transfer Principles
Liquid cooling relies on two primary heat transfer modes: conduction and convection. Conduction occurs through direct molecular interaction within the liquid, governed by Fourier's law, where heat flux is proportional to the temperature gradient and the fluid's thermal conductivity. Convection, the dominant mechanism in flowing liquids, involves bulk fluid motion that carries heat away from the source; it is described by Newton's law of cooling, which states that the rate of convective heat transfer is given by , where is the convective heat transfer coefficient, is the surface area over which heat transfer occurs, and is the temperature difference between the surface and the bulk fluid.[5] This law applies to liquid flow in cooling systems by quantifying how effectively moving fluid removes heat from a hot surface, with depending on fluid properties and flow conditions.[6] The efficiency of liquid heat transfer hinges on key thermophysical properties: thermal conductivity (), specific heat capacity (), and viscosity (). Thermal conductivity measures the fluid's ability to conduct heat internally, with higher values enabling faster heat diffusion through the liquid, particularly important in regions of low flow velocity.[7] Specific heat capacity determines how much heat the fluid can absorb per unit mass per degree of temperature rise, allowing liquids with high to carry substantial thermal loads without excessive temperature increases during circulation.[7] Viscosity influences flow resistance and convective enhancement; lower viscosity facilitates easier pumping and higher flow rates, but excessive reduction can lead to instabilities, while it also affects the boundary layer thickness that impedes heat transfer at solid-liquid interfaces.[7] These properties collectively dictate the overall heat transfer coefficient in liquid systems, balancing conduction within the fluid and convection at boundaries.[8] Flow regime significantly impacts heat transfer rates, determined by the Reynolds number , where is fluid density, is average velocity, is the characteristic dimension (e.g., pipe diameter), and is dynamic viscosity. Laminar flow, prevalent at , features smooth, layered motion with minimal mixing, resulting in lower heat transfer coefficients due to reliance on conduction across thin boundary layers and parabolic velocity profiles.[9] In contrast, turbulent flow at introduces chaotic eddies that disrupt the boundary layer, enhancing mixing and convective heat transfer by factors of 5–10 compared to laminar conditions, though at the cost of higher pressure drops.[9] The transitional regime () exhibits intermittent behavior, but systems are typically designed for fully turbulent flow to maximize efficiency in heat dissipation.[10] A basic liquid cooling loop operates through three sequential stages: heat absorption at the source, fluid circulation, and heat rejection to the ambient. At the heat source, such as a heated component, the liquid absorbs thermal energy via convection, raising its temperature as it flows through channels or jackets in direct or indirect contact.[11] Circulation is driven by pumps to maintain continuous flow, ensuring the warmed liquid is transported away before reaching thermal equilibrium with the source.[11] Finally, heat rejection occurs at a heat exchanger or radiator, where the elevated-temperature fluid transfers energy to a secondary medium (e.g., air or cooler water), restoring the liquid to a lower temperature for recirculation.[11] This closed cycle sustains efficient cooling by leveraging the liquid's capacity to transport and dissipate heat without direct environmental exposure.[11]Common Coolants
Water serves as the baseline coolant in many liquid cooling applications due to its high specific heat capacity of 4.18 J/g·K, which enables efficient absorption and transfer of thermal energy without significant temperature rise. This property, combined with its low cost and ready availability, makes water highly effective for single-phase cooling in systems where electrical conductivity is not a concern, such as industrial heat exchangers.[12] However, pure water is electrically conductive, posing risks of short circuits in electronics cooling, and it can be corrosive to metals like copper and aluminum without additives, potentially leading to system degradation over time. To address water's limitations, glycol-water mixtures are commonly employed, particularly 50/50 ethylene glycol (EG) and water blends, which provide freeze protection down to approximately -37°C while elevating the boiling point to around 107°C for enhanced thermal stability.[13] These mixtures incorporate corrosion inhibitors to mitigate material degradation, reducing the risk of scaling and pitting in cooling loops compared to pure water.[14] Ethylene glycol-based formulations are favored in automotive and HVAC systems for their balanced heat transfer performance, though they exhibit slightly lower specific heat (about 3.5 J/g·K) than pure water, necessitating careful volume considerations. For applications requiring direct contact with electrical components, such as immersion cooling in data centers, dielectric fluids like 3M Novec and Fluorinert series are preferred due to their high dielectric strength, which prevents electrical conduction even under submersion.[15] These perfluorinated or hydrofluoroether-based liquids exhibit low toxicity and chemical inertness, ensuring compatibility with server hardware, while their boiling points (e.g., 34°C for Novec 7000 and 61°C for 7100) facilitate two-phase cooling without risking short circuits.[16][17] However, environmental concerns arise from their global warming potential (GWP), though some Novec formulations, such as Novec 649, have very low GWP (1) compared to older Fluorinert variants (GWP >7,000), prompting a shift toward more sustainable options.[18] Advanced coolants, including nanofluids—suspensions of nanoparticles (e.g., Al₂O₃ or CuO in water or ethylene glycol)—offer enhanced thermal performance, with reported increases in heat transfer coefficients of 10–50% depending on particle concentration (0.1–5 vol%) and flow conditions.[19] These enhancements stem from improved thermal conductivity (up to 52% in some water-in-fluorocarbon emulsions) and better convective heat transfer, though stability and potential clogging must be managed.[20] Phase-change materials, such as refrigerants like R134a or HFO-1234yf in two-phase cooling setups, leverage latent heat during boiling (e.g., 217 kJ/kg for R134a) to achieve higher cooling densities than single-phase fluids, enabling compact designs for high-heat-flux applications.[21][22] Selection of coolants hinges on key criteria: thermal capacity, quantified by specific heat and latent heat for phase-change types, to maximize heat absorption; viscosity, ideally low (e.g., <5 cP at operating temperatures) to minimize pumping power; toxicity, where non-toxic options like propylene glycol are prioritized over ethylene glycol in consumer applications; and material compatibility, ensuring no adverse reactions with seals, tubing, or heat exchangers to prevent leaks or fouling.[12][23] These factors are evaluated against operational demands, such as temperature range and environmental regulations, to optimize overall system efficiency.[24]System Designs
Closed-Loop Systems
Closed-loop liquid cooling systems recirculate a cooling fluid within a sealed circuit to absorb, transport, and dissipate heat from a heat source, such as electronic components or industrial machinery, without direct exposure to the environment. These systems enhance efficiency by reusing the fluid, minimizing waste and contamination risks compared to open designs. They are widely used in high-performance computing and precision engineering applications where consistent thermal management is critical.[25] Key components include a pump to drive fluid circulation, a reservoir for air removal and fluid expansion, tubing to connect elements, and a radiator for heat rejection. Centrifugal pumps are commonly employed due to their ability to provide steady flow rates, typically ranging from 1 to 5 gallons per minute (3.8 to 19 liters per minute) in compact systems, depending on the pressure drop and heat load.[26] The reservoir facilitates air bleeding to prevent cavitation and bubbles that could impair heat transfer, often integrated with a fill port for maintenance. Tubing materials vary by application; PETG (polyethylene terephthalate glycol) offers flexibility, transparency, and chemical resistance for custom assemblies, while metal options like copper or aluminum provide rigidity and superior thermal conductivity in high-pressure setups.[27] The radiator, typically a liquid-to-air heat exchanger with copper fins and tubes, dissipates heat via forced convection from attached fans, maintaining fluid temperatures below 50°C in demanding loads exceeding 500 W/cm².[25] In operation, the fluid—often water-based with corrosion inhibitors—absorbs heat from the source via a cold plate, a conductive block with internal channels attached directly to the component. The pump then circulates the warmed fluid through tubing to the radiator, where it releases heat to ambient air before returning cooled to the cold plate, completing the cycle in a continuous loop. This process relies on forced convection for efficient heat transfer, with flow rates optimized to balance pressure losses in tubing and components.[28] Closed-loop systems operate in single-phase or two-phase modes, differing in heat transfer mechanisms. Single-phase loops maintain the fluid as a liquid throughout, transferring heat via sensible changes in temperature, which suits moderate loads with simpler designs. Two-phase loops leverage phase change, where the fluid evaporates at the heat source to absorb latent heat and condenses in the radiator, enabling higher heat fluxes up to several kW per device but requiring pressure management to handle vapor. Thermosiphon systems exemplify passive two-phase operation, using natural convection driven by density differences—no pump is needed—as vapor rises to a condenser and liquid returns via gravity, ideal for vertical configurations in space-constrained environments.[29] A specific implementation in closed-loop systems is direct-to-chip (DTC) cooling, where the fluid interfaces directly with the chip's integrated heat spreader. Single-phase DTC cooling typically uses pumped water or glycol mixtures, which are efficient for loads up to 1000 W but require higher flow rates, larger pumps, and facility water connections, increasing plumbing complexity and leak risks due to conductive fluids. In contrast, two-phase DTC cooling employs dielectric fluids that undergo boiling and condensation, providing 5-10 times better heat transfer efficiency through latent heat absorption, enabling handling of extreme thermal design power (TDP) exceeding 1000 W without high flow rates or pumps, and minimizing leak risks via low operating pressures and non-conductive, waterless fluids that prevent corrosion and electrical shorts.[30][31] Maintenance focuses on preserving system integrity and performance, including periodic leak detection, fluid replenishment, and scaling prevention. Leaks are monitored using pressure sensors or humidity rise tests in enclosed setups, as even minor escapes can lead to corrosion or efficiency loss; helium leak testing ensures rates below 10^{-6} atm·cc/s in critical assemblies. Fluid levels are checked and replenished annually or after 5,000 hours of operation to compensate for minor permeation through tubing, using compatible additives to maintain properties. Scaling, caused by mineral deposits in hard water coolants, is prevented by treated fluids with pH buffers and biocides, alongside filtration to help maintain system performance.[32]Open-Loop Systems
Open-loop liquid cooling systems involve non-recirculating fluid flows where the coolant is used in a single pass or evaporates upon contact with heat-generating components, enabling direct thermal interaction without the need for closed containment. These designs prioritize high heat transfer efficiency through phase change or convective mechanisms, making them suitable for scenarios demanding rapid dissipation of localized heat. Unlike recirculating setups, open-loop configurations consume fluid continuously, often venting vapor or draining liquid, which simplifies system architecture but requires provisions for fluid replenishment.[33] Direct-to-chip and immersion cooling in open-loop formats submerge electronic components, such as processors or circuit boards, directly into non-conductive dielectric liquids like fluorocarbons, allowing heat absorption via convection and potential evaporation. In these systems, the fluid contacts the heat source intimately, with unevaporated liquid drained away and vapor often exhausted to maintain system pressure, achieving uniform temperature distribution across submerged surfaces. This approach excels in handling moderate to high thermal loads by leveraging the fluid's direct proximity to hotspots, though it demands careful selection of electrically insulating coolants to prevent short circuits.[34] Spray cooling and jet impingement represent targeted open-loop methods where high-pressure liquid streams or droplets are directed onto specific heat sources, such as microprocessors, to form a thin evaporative film that enhances heat removal. In spray cooling, nozzles atomize the coolant into fine mists that impinge on the surface, promoting two-phase boiling and critical heat fluxes exceeding 1000 W/cm² with water-based fluids due to the large surface area for evaporation. Jet impingement, similarly, employs focused streams for localized cooling, achieving fluxes up to 1000 W/cm² by minimizing thermal boundary layers, with components like precision nozzles ensuring uniform coverage and efficient droplet breakup. These techniques are particularly effective for compact electronics where air cooling falls short.[35][36] Evaporative open-loop systems exploit the phase change from liquid to vapor, utilizing coolants such as water or refrigerants like R134a to absorb substantial thermal energy through latent heat of vaporization. For water at standard boiling conditions, this latent heat measures 2257 kJ/kg, enabling efficient heat extraction without significant temperature rise in the fluid. Key components include evaporators that facilitate boiling at the heat interface and vapor management systems to handle exhaust, making these setups ideal for high-density applications. Refrigerants offer lower boiling points for operation at ambient pressures, broadening applicability in varied environments. The primary advantages of open-loop systems lie in their capacity to manage extreme heat fluxes in demanding scenarios, such as advanced computing chips, where direct contact and evaporation outperform indirect methods by providing superior thermal gradients and minimal pumping penalties. Nozzles and evaporators in these designs optimize fluid delivery and phase change, supporting fluxes far beyond conventional limits while simplifying integration compared to hybrid closed-loop variants that incorporate recirculation for fluid conservation.[36]Applications
Computing and Data Centers
Liquid cooling has evolved significantly in computing and data centers to address escalating thermal demands from high-performance processors. In the 1960s, IBM pioneered water cooling for mainframe systems like the System/360 Model 91 to manage heat loads exceeding air cooling capabilities, using cold plates for indirect cooling of modules.[37] By the 2000s, this shifted to direct-to-chip liquid cooling for CPUs and GPUs, as seen in IBM's Power 575 and 775 systems, where over 96% of rack heat—up to 63 kW—was transferred to water via cold plates on processors, memory, and I/O, eliminating reliance on room air conditioning.[37] In modern AI data centers, liquid cooling integrates via full-chain solutions including cold plates, immersion cooling, and coolant distribution units (CDUs), supporting high-density AI servers from suppliers like NVIDIA and Google. NVIDIA's Blackwell platform, introduced in 2025, integrates direct-to-chip liquid cooling to handle GPU power densities over 1,000 W, boosting water efficiency by more than 300 times compared to air-cooled predecessors and enabling hyperscale AI factories.[38] This direct-to-chip (DTC) cooling comes in single-phase and two-phase variants; single-phase DTC, often using pumped water or glycol mixtures, relies on sensible heat transfer without phase change, making it less efficient for loads exceeding 1,000 W due to the need for high flow rates, extensive plumbing, and facility water integration that increases leak risks and corrosion potential.[30] In contrast, two-phase DTC employs dielectric fluids that undergo boiling and condensation, providing 5-10 times better heat transfer through latent heat absorption, enabling extreme thermal design powers (TDP) without high pumps or flows, operating at low pressures to minimize leaks, and avoiding conductivity or corrosion issues since it is waterless.[30][31] Google has advanced liquid cooling for its AI infrastructure, enabling 1 MW IT racks in hyperscale facilities to meet the demands of high-density TPUs and other accelerators.[39] This adoption is driven by surging compute demands in AI workloads, with penetration rates influencing market scale and implementation in servers and switches; for instance, liquid cooling captured 46% of the data center cooling market by 2024, with projections for further growth in AI-specific applications.[40] Immersion cooling submerges entire server racks in non-conductive dielectric fluids, offering a scalable alternative to air cooling for high-density environments. Single-phase immersion uses stable liquids like mineral oil, where the fluid circulates without phase change to absorb and transfer heat to external exchangers, simplifying maintenance in data centers.[41] Two-phase immersion employs fluorocarbons that boil upon heating—vaporizing to remove heat efficiently before condensing back to liquid—handling up to 200 kW per rack and outperforming single-phase for extreme workloads.[42] These methods reduce overall data center energy use by 30-40% compared to traditional air cooling, primarily by minimizing the 30-40% of power typically consumed by cooling systems.[43] In hyperscale facilities, coolant distribution units (CDUs) integrate liquid cooling by regulating flow from facility water systems to server-level loops, ensuring precise thermal management. CDUs handle flow rates exceeding 1 L/min per cold plate for chips with thermal design powers over 1,000 W, with optimal rates of 2-3 L/min balancing heat removal and system stress while achieving low thermal resistance through advanced cold plate designs; however, two-phase DTC variants require significantly lower flow rates—often one-tenth of single-phase—to manage high TDPs efficiently due to phase-change heat transfer.[44][45][30] This setup supports rack densities above 100 kW, preventing hotspots and enabling reliable operation in AI-driven environments.[44] As of 2025, hybrid air-liquid cooling systems are gaining traction in edge computing to support low-latency AI and IoT applications in decentralized facilities. These hybrids combine direct-to-chip liquid for high-heat components with residual air cooling, improving power usage effectiveness (PUE) by up to 15% and enhancing sustainability through reduced energy and water demands.[46] Such integrations align with carbon-neutral goals, leveraging renewables and efficient coolants to lower operational emissions in modular edge data centers.[46]Automotive Engines
Liquid cooling in automotive engines primarily serves to manage the heat generated by internal combustion engines (ICE) and electric powertrains, ensuring optimal operating temperatures for performance, efficiency, and longevity. In ICE vehicles, coolant circulates through water jackets—passages integrated into the engine block and cylinder head surrounding the cylinders—to absorb excess heat from combustion. This heated coolant is then directed via a thermostat-controlled flow, which regulates the temperature by opening or closing to route the fluid either back to the engine or to the radiator for dissipation. The thermostat, typically located in the cylinder head outlet, maintains engine temperatures around 90-100°C to prevent overheating while allowing quick warm-up. A radiator, often fan-assisted, cools the fluid by exchanging heat with ambient air, and a coolant pump (usually belt-driven) ensures continuous circulation throughout the closed loop. Common coolant formulations, such as a 50/50 mixture of ethylene glycol and water, provide corrosion protection, freeze resistance down to -34°C, and boiling protection up to 129°C in pressurized systems.[47][48][49] In electric vehicles (EVs), liquid cooling is integral to battery thermal management systems (BTMS), which regulate lithium-ion (Li-ion) cell temperatures to optimize energy density, charging rates, and safety. These systems employ chillers—heat exchangers connected to the vehicle's air conditioning loop—to actively cool the battery pack, circulating a dielectric fluid or glycol-based coolant through channels or cold plates adjacent to the cells. This prevents thermal runaway, a cascading failure where overheating leads to fires or explosions, by maintaining cells within a target range of 20-40°C during high-load conditions like fast charging or acceleration. Liquid-cooled BTMS outperform air-based alternatives in high-power applications, achieving uniform temperature distribution across modules and supporting sustained performance in demanding environments. For instance, chillers can reduce peak cell temperatures by up to 20°C compared to passive methods, enhancing cycle life and safety. This differs from passive air cooling used in certain earlier EVs like Nissan Leaf models.[50][51][52][53] Hybrid vehicles in 2025 models integrate liquid cooling for both ICE components and EV batteries, often using shared or modular systems for efficiency. A notable example is Tesla's 4680 cylindrical cells, which feature integrated cooling plates embedded in the battery structure to directly contact cell surfaces, minimizing thermal gradients and enabling higher energy densities up to 300 Wh/kg. Tesla's battery cooling system is a liquid-cooled thermal management system that circulates glycol-based coolant through channels in the battery pack to actively manage battery temperature during driving, charging, and in varying ambient conditions. It is integrated with a heat pump for both cooling and heating, optimizing performance, charging speeds up to 250 kW, and long-term battery health by maintaining temperatures around 20-45°C. These tabless designs with steel casings facilitate compact cooling channels that snake or plate between cells, supporting rapid charging at 250 kW while keeping temperatures below 45°C. Such hybrid BTMS reduce overall system weight by 10-15% compared to discrete setups, aiding range extension in vehicles like the updated Model Y.[54][55][56] Automotive liquid cooling systems are typically pressurized to 1.0-1.5 bar via radiator caps, elevating the coolant's boiling point by approximately 20-30°C (e.g., from 100°C for water at atmospheric pressure to 120-130°C) to prevent vapor lock and maintain liquid-phase heat transfer during operation. This pressurization suppresses boiling in hot zones like the cylinder head, but failures such as head gasket breaches can allow combustion gases to enter the coolant, causing pressure spikes, foaming, and overheating. Common failure modes include gasket erosion from localized hot spots or poor circulation, leading to coolant loss, corrosion, or engine damage if unaddressed. Regular maintenance, including pressure testing, mitigates these risks by ensuring seal integrity and fluid quality.[57][58][59]Industrial Processes
In industrial chemical processing, liquid cooling is essential for managing heat in reactors and extruders, where exothermic reactions generate significant thermal loads. Jacketed vessels surround the reaction chamber with a secondary enclosure through which coolants like water or thermal oils circulate, absorbing excess heat to maintain precise temperatures and prevent runaway reactions.[60][61] This setup allows for controlled heat removal, typically using pumps to circulate the fluid at rates that match the reaction's heat release, ensuring product quality and safety in processes such as polymerization or hydrogenation.[62] In extruders, similar jacketed designs employ chilled water loops around the barrel to cool molten materials during shaping, stabilizing viscosity and preventing degradation in applications like plastics compounding. Metalworking operations rely on liquid coolants to dissipate heat and lubricate during machining, significantly extending tool life by reducing thermal wear. Emulsions, which mix water with oils to form stable mixtures, are commonly used as these fluids provide both cooling via evaporation and lubrication to minimize friction at the tool-workpiece interface.[63] Delivery methods include flood cooling, where a high-volume stream submerges the cutting zone to achieve up to 50% temperature reduction compared to dry machining, and minimum quantity lubrication (MQL) via mist, which applies a fine aerosol for targeted cooling with less fluid consumption.[64][65] Flood systems excel in heavy roughing operations by effectively clearing chips and maintaining surface finish, while mist delivery suits precision finishing to avoid over-cooling that could induce thermal cracks.[66] In power generation, liquid cooling protects turbine components in gas and steam plants by preventing scaling and erosion on blades exposed to high-temperature fluids. Demineralized water circulates in closed loops to cool turbine blades internally or via film cooling, removing ionic impurities that could deposit and reduce efficiency by up to 2-3% per percentage point of scaling.[67][68] This approach, often integrated with heat exchangers, maintains blade temperatures below critical thresholds, enhancing turbine longevity in combined-cycle plants where exhaust heat recovery demands precise thermal management. As of 2025, liquid cooling has seen expanded adoption in semiconductor fabrication facilities for extreme ultraviolet (EUV) lithography, where chilled water loops achieve sub-10nm precision by stabilizing optical components. These systems use microchannel modules to circulate ultra-pure chilled water, holding temperatures within ±0.001°C to counteract thermal expansion in scanners processing advanced nodes.[70][71] In high-volume fabs, such as those producing 3nm chips, this cooling enables 15% higher throughput by minimizing overlay errors from heat-induced distortions.[72]HVAC and Building Systems
Liquid cooling plays a central role in heating, ventilation, and air conditioning (HVAC) systems for buildings, particularly through chilled water systems that provide efficient climate control in large commercial and institutional structures. These systems typically feature a central plant equipped with chillers that cool water to temperatures between 42°F and 55°F (5.6°C to 12.8°C), which is then circulated via pumps to air handling units (AHUs) containing cooling coils.[73] The chilled water absorbs heat from the air in the coils, enabling the distribution of cooled air throughout the building while allowing for recirculation to minimize water usage compared to single-pass systems.[74] Chillers, often water-cooled centrifugal types, are prevalent in facilities exceeding 100,000 square feet, accounting for space cooling in approximately 20% of commercial building floor space and 32% of office space.[75] Geothermal and radiant cooling systems leverage liquid loops for enhanced efficiency in heat rejection, particularly in sustainable building designs. Ground-source heat pumps (GSHPs) employ closed-loop configurations where water or a water-glycol mixture—such as propylene glycol to prevent freezing—is circulated through high-density polyethylene pipes buried underground, exchanging heat with the stable subsurface temperature.[76] This setup allows the heat pump to reject excess heat efficiently during cooling mode, achieving lower energy use than air-source alternatives by utilizing the earth's thermal mass. Radiant cooling complements this by distributing chilled water through embedded pipes in ceilings, walls, or floors, where the surfaces absorb radiant heat from occupants and the environment at supply temperatures of 55°F to 60°F (13°C to 15.6°C), promoting even temperature distribution without drafts.[77][78] District cooling networks extend these principles to urban scales, piping chilled water from centralized plants to multiple buildings, thereby reducing individual on-site equipment and mitigating urban heat islands. Singapore's systems, operational since the early 2010s, exemplify this approach; the Marina Bay underground network, for instance, delivers cooling to high-density areas, achieving over 40% energy savings for customers compared to decentralized units.[79] These networks optimize load diversity across buildings, lowering peak demand and enabling shared infrastructure like chillers and thermal storage.[80] Energy efficiency in these liquid-based HVAC systems is quantified by the coefficient of performance (COP), which measures cooling output per unit of energy input and often exceeds that of decentralized direct expansion (DX) vapor-compression units. Water-cooled chillers in chilled water and district systems typically achieve COP values of 5.5 to 6.5, benefiting from efficient heat rejection and part-load operation, while DX rooftop units average COPs of 2.5 to 3.5 due to air-cooled condensers and less optimized scaling.[73] District cooling further amplifies this, with overall system efficiencies up to 5 times higher than standalone vapor-compression equipment through centralized production and reduced transmission losses.[81] By 2025, integration of smart controls—such as AI-driven IoT sensors and predictive analytics—enhances these metrics by dynamically optimizing pump speeds, chiller sequencing, and zonal demands, potentially improving COP by 10-20% in real-time operation.Wearable and Biomedical Devices
Liquid cooling garments (LCGs) represent a key application in wearable thermoregulation, particularly for high-risk professions requiring protection from extreme heat. These garments consist of networks of flexible tubing embedded in clothing, such as vests, through which chilled water or other coolants circulate to absorb and dissipate body heat directly from the skin. Developed by NASA in the 1960s for the Apollo program, LCGs were essential for maintaining astronauts' core body temperatures during spacewalks and extravehicular activities, where metabolic heat generation could exceed 300 W in insulated suits.[82] The system pumps water at approximately 15–20°C through the tubing, leveraging the high specific heat capacity of water (4.18 J/g·K) to transfer heat away from the body without relying on sweat evaporation, which is ineffective in sealed environments.[83] This technology has been adapted for terrestrial use, notably in firefighting, where protective gear traps heat and elevates core temperatures to dangerous levels during prolonged operations. NASA's advanced liquid cooling and ventilation garments, featuring enhanced thermal conductivity materials, have been proposed for integration into firefighters' suits to mitigate heat stress, potentially reducing physiological strain by maintaining skin temperatures below 35°C.[84] A 2023 study introduced a novel liquid cooling strategy embedding microfluidic channels in firefighting ensembles, demonstrating up to 15% reduction in core body temperature rise during simulated fire exposure compared to passive insulation alone.[85] These closed-loop systems, often powered by portable reservoirs and pumps, enhance endurance and safety without compromising mobility. In biomedical applications, liquid cooling plays a critical role in organ preservation during transport, where maintaining hypothermic conditions (0–4°C) is vital to minimize ischemic damage and extend viability windows beyond the traditional 4–6 hours for hearts or lungs. Phase-change slurries, suspensions of microencapsulated phase-change materials in a carrier fluid like water, serve as advanced coolants in transport containers by absorbing latent heat during phase transitions at precise temperatures, providing stable cooling without active refrigeration.[86] Devices like the Paragonix SherpaPak utilize phase-change technology to sustain organ temperatures between 4°C and 8°C, reducing cold-induced injury and improving graft outcomes in clinical trials.[87] Ice slurries, a subset of phase-change slurries, have also been explored for emergency medical cooling, offering high heat transfer rates (up to 300 kJ/kg latent heat) for rapid hypothermia induction in trauma scenarios.[88] For implantable devices, microfluidic liquid cooling enables precise thermal management in biocompatible formats, addressing heat dissipation from electronics in confined biological spaces. In brain-computer interfaces (BCIs), emerging microfluidic systems circulate biocompatible fluids like saline through sub-millimeter channels to remove heat from high-density electrode arrays, preventing tissue damage from localized temperatures above 42°C.[89] Similarly, for pacemakers and other cardiac implants, microfluidic cooling integrates with leadless designs to dissipate power-generated heat (typically 0.5–2 W), using perfluorocarbon fluids for their low viscosity and inertness.[90] A 2022 bioresorbable implant demonstrated evaporative microfluidic cooling to numb nerves for pain relief, achieving 10–15°C reductions at the implant site without systemic effects, highlighting potential for long-term implantable thermoregulation.[91] By 2025, wearable liquid cooling has advanced into smart suits for athletes, incorporating micro-pumps and sensors for dynamic thermoregulation independent of ambient conditions or sweat production. These suits, often featuring embedded tubing and IoT-enabled controls, circulate chilled fluids to lower core temperatures during endurance events, with studies showing performance improvements of up to 10% in time-to-exhaustion tests under heat stress.[92] For instance, microfluidic networks inspired by natural sweat glands, as in the SweatMD garment, enhance evaporative cooling efficiency, potentially boosting endurance by redirecting body fluids through textile channels.[93] Commercial liquid cooling vests, adapted from NASA designs, provide athletes with 2–4 hours of active cooling, reducing perceived exertion and dehydration risk in marathons or triathlons.[94]Advantages and Limitations
Performance Benefits
Liquid cooling offers superior thermal performance compared to air cooling primarily due to the significantly higher volumetric heat capacity of liquids, which allows them to absorb and transport approximately 3,500 times more heat per unit volume than air.[95] This enhanced capacity enables more efficient heat removal in constrained spaces, facilitating compact system designs that maintain optimal operating temperatures without the bulk required for air-based systems.[96] By providing uniform temperature distribution across components, liquid cooling effectively mitigates hotspots that can throttle performance in high-heat environments. In overclocking scenarios, such as those in immersion-cooled data centers, this results in performance gains of up to 20%, allowing for higher clock speeds and increased virtual machine density without thermal limitations.[97] In data center applications, liquid cooling yields substantial energy savings by reducing the power required for cooling infrastructure, with reports indicating up to 40% lower energy consumption for cooling compared to traditional air systems. This efficiency stems from minimized fan usage and improved heat rejection, contributing to overall lower operational costs. Additionally, liquid cooling operates more quietly than air cooling, achieving noise reductions of up to 55% in data center environments by eliminating the need for high-speed fans.[98] The stable temperatures provided by liquid systems also enhance component reliability, extending operational life through reduced thermal stress and cycling.[99]Challenges and Risks
One of the primary operational risks in liquid cooling systems is leakage, which can arise from corrosion of system components or failure of fittings and seals. Corrosion, often exacerbated by incompatible materials or contaminants in the coolant, can degrade pipes, cold plates, and connectors over time, increasing the likelihood of breaches that allow fluid to escape. Such leaks pose significant safety hazards in electronics applications, where escaped coolant—particularly if conductive—can lead to short circuits, component damage, or widespread system contamination. For instance, in data centers, even minor leaks from direct liquid cooling setups have been shown to cause hardware failures by seeping into server electronics.[100][101][102] Maintenance demands for liquid cooling systems are more intensive than those for air-based alternatives, requiring regular fluid inspections, pump servicing, and mitigation of biological growth. Pumps, critical for coolant circulation, typically have a lifespan of 5 to 7 years under normal operating conditions, after which wear from continuous use can lead to reduced flow rates or complete failure, necessitating replacement. In water-based systems, biofouling presents an additional challenge, as microorganisms like bacteria and algae form biofilms on internal surfaces, reducing heat transfer efficiency and promoting further corrosion or blockages. Effective management involves periodic chemical treatments or filtration to control this growth, with cooling systems often needing biocide applications every few months to maintain performance.[103][104][105][106] Cost barriers further complicate adoption of liquid cooling, with initial setup expenses often 2 to 3 times higher than air cooling due to specialized components like pumps, reservoirs, and heat exchangers. This premium is compounded by the need for skilled expertise during installation, as improper assembly can amplify risks like leaks or inefficiencies, requiring certified technicians familiar with fluid dynamics and system integration. Ongoing operational costs, including fluid replacements and maintenance, add to the economic hurdles, particularly for retrofitting existing infrastructure.[107][108] Environmental concerns associated with liquid cooling include the proper disposal of dielectric fluids and the high water consumption in certain configurations, especially amid tightening 2025 regulations. Dielectric coolants, while non-conductive, must be handled as hazardous waste upon degradation to prevent soil or water contamination, with disposal processes regulated under frameworks like California's AB1459, which addresses storage and release of such fluids in closed-loop systems. Water-based systems exacerbate issues in water-scarce regions, where evaporative cooling towers can consume millions of gallons annually per data center, drawing scrutiny from U.S. regulatory bodies focused on sustainable resource use. Additionally, the EPA's 2025 phase-down of high-GWP hydrofluorocarbons in cooling technologies indirectly impacts fluid selection, pushing for eco-friendly alternatives to minimize long-term environmental footprints.[109][110][111][112]Historical Development
Early Uses
The earliest precedents for liquid cooling trace back to ancient civilizations that harnessed water for temperature regulation in arid environments. In ancient Persia, around 1000 BCE, qanats—underground aqueducts—facilitated evaporative cooling by channeling groundwater to the surface, where it evaporated to lower ambient temperatures in homes and storage areas.[113] These systems, often integrated with windcatchers (bâdgirs), directed breezes over water surfaces to enhance cooling effects, providing passive climate control in desert regions like Yazd. By 400 BCE, Persians advanced this with yakhchals, domed evaporative coolers that stored ice year-round through water evaporation in insulated pits, demonstrating early mastery of liquid-based refrigeration for food preservation.[114] In the Roman Empire, liquid cooling emerged in industrial contexts through aqueducts that supplied water not only for hydraulic power but also for moderating heat in milling operations. Constructed from the 4th century BCE onward, these elevated channels delivered consistent water flows to complexes like the Barbegal mills near Arles, France, around 120–130 CE, where cascading water wheels generated power.[115] This integration of water circulation for both mechanical drive and thermal management represented a foundational application of liquid systems in early industry. The 19th century marked a pivotal shift toward engineered liquid cooling in mechanical systems, particularly with steam engines. In the 1770s, James Watt refined the Newcomen engine by introducing a separate condenser, patented in 1769, which used cold water to condense exhaust steam outside the main cylinder, dramatically improving efficiency by up to two-thirds and reducing fuel consumption.[116] This water-cooled condenser maintained the cylinder's heat while efficiently rejecting waste heat, laying the groundwork for widespread industrial adoption of liquid cooling in power generation. Early automotive applications further propelled liquid cooling into mobile machinery. In 1886, Karl Benz incorporated the first practical water radiator into his Patent-Motorwagen, the inaugural gasoline-powered automobile, to manage heat from its single-cylinder internal combustion engine.[117] The radiator circulated water through engine passages and an air-cooled core, preventing overheating during operation and enabling sustained performance at speeds up to 8 mph. By the 1910s, industrial-scale liquid cooling matured with the advent of dedicated cooling towers for power plants. Pioneered in the Netherlands around 1918, hyperboloid natural-draft towers near Heerlen used evaporative water flow to dissipate heat from steam condensers in electricity generation, optimizing large-scale operations amid growing energy demands.[118] These structures, which sprayed hot water into rising air currents for efficient cooling, became essential for early 20th-century coal-fired facilities.Modern Innovations
In the realm of computing, a pivotal milestone occurred in 1985 with the introduction of the Cray-2 supercomputer, which employed immersion cooling using Fluorinert, a non-conductive perfluorocarbon liquid, to manage the heat from its densely packed circuit boards and achieve peak performance of 1.9 gigaflops.[119] This approach marked a significant advancement over air cooling by directly submerging components in the coolant, circulated under pressure to external heat exchangers, enabling higher transistor densities without thermal throttling. By the 2010s, the industry shifted toward direct-to-chip liquid cooling, where cold plates or microchannels contact processor dies directly, addressing escalating power densities in data centers that reached 15-20 kW per rack and reducing thermal resistance by up to 50% compared to indirect methods.[120] This transition was driven by hyperscale operators like Google and Microsoft, integrating it into hybrid systems for CPUs and GPUs to support AI workloads. Advancements in heat exchanger design since 2000 have further enhanced liquid cooling efficiency, particularly through microchannel and pin-fin configurations that promote turbulent flow and increased surface area for heat transfer. Microchannel heat sinks, with channels typically under 1 mm wide, combined with pin-fin arrays, have demonstrated thermal performance improvements of approximately 50% over traditional plate-fin designs by optimizing pressure drop and Nusselt number in single-phase flows.[121] These innovations, often fabricated via etching or additive manufacturing, allow for compact integration in electronics, dissipating over 500 W/cm² in high-heat-flux applications like laser diodes.[122] The 2020s have witnessed breakthroughs in intelligent and eco-friendly liquid cooling, notably AI-driven predictive systems in data centers that use machine learning to forecast thermal loads and dynamically adjust coolant flow rates, achieving up to 30% energy savings over reactive controls.[123] For instance, algorithms analyze real-time data from sensors to preemptively optimize pump speeds and valve positions, integrating with building management systems for holistic efficiency. Complementing this, sustainable bio-based coolants, derived from renewable sources like vegetable oils or recycled glycols, have emerged to replace petroleum-based fluids, offering comparable thermal conductivity while reducing environmental persistence and toxicity in case of leaks.[124] Companies like Arteco have commercialized these for direct-to-chip applications, certified under mass balance standards to ensure biodegradability.[125] Looking ahead, capillary-driven self-cooling systems represent a promising trend, leveraging porous wicks or micropillars to passively transport coolant via surface tension without pumps, enabling coefficient of performance exceeding 100 in thin-film evaporation for chip-level cooling.[126] These passive mechanisms are particularly suited for edge computing devices, minimizing energy overhead. Additionally, integration with renewable energy sources, such as solar-thermal loops, has led to 2025 prototypes of zero-water liquid cooling systems that recirculate dielectric fluids in closed loops, eliminating evaporative losses and aligning with sustainability goals in arid regions.[127] Microsoft's pilots, for example, combine these with adiabatic cooling to achieve net-zero water use in AI data centers by 2026.[128]References
- https://ntrs.[nasa](/page/NASA).gov/api/citations/19790014256/downloads/19790014256.pdf