Coolant
A coolant is a substance, typically a fluid, utilized to decrease or regulate the temperature of a system by absorbing heat from components and transferring it to a dissipation site.[1] Effective coolants exhibit high specific heat capacity to store thermal energy, favorable thermal conductivity for efficient heat transfer, and low viscosity to facilitate circulation, alongside chemical stability to minimize degradation under operational conditions.[2] In practice, coolants serve critical roles in preventing thermal runaway in applications ranging from internal combustion engines, where water-glycol mixtures inhibit corrosion and extend boiling points, to nuclear reactors employing specialized liquids or gases for heat extraction from fissile cores.[3] While water remains a baseline coolant due to its superior heat capacity, additives like ethylene glycol address limitations such as freezing in cold environments, though these introduce toxicity concerns necessitating careful handling and disposal to avert environmental contamination.[4]Definition and Principles
Fundamental Role in Heat Management
Coolants serve as intermediary fluids in thermal management systems, absorbing excess heat generated by operational processes—such as combustion in engines, electrical resistance in electronics, or friction in machinery—and transporting it to a dissipation site, thereby maintaining components at functional temperatures to avert degradation or failure. Without effective heat removal, systems experience thermal runaway, where rising temperatures exacerbate heat generation rates, leading to inefficiencies, material stress, or meltdown; for instance, in heavy-duty diesel engines, approximately one-third of total heat energy must be extracted by the coolant to achieve thermal equilibrium.[5] This role is grounded in the second law of thermodynamics, which dictates that heat flows from higher to lower temperature regions, but requires augmentation beyond passive conduction or radiation due to the localized and high-intensity nature of heat sources in engineered systems.[6] The primary mechanism is forced convection, where pumps or fans drive coolant flow to enhance heat transfer coefficients far beyond those of stationary fluids; heat enters the coolant via conduction across a thin boundary layer at hot surfaces, then the bulk fluid's motion distributes it, leveraging the coolant's specific heat capacity to minimize temperature rise per unit energy absorbed (Q = m c ΔT).[6][7] In single-phase systems, sensible heat dominates, while two-phase applications incorporate latent heat from boiling or evaporation for superior density of energy storage and transfer, as seen in electronics cooling where liquids outperform air by orders of magnitude in convective efficacy.[7] Circulation ensures uniform temperature profiles, preventing hotspots that could induce localized boiling, cavitation, or corrosive electrolysis in pumps and passages.[5] Ultimately, coolants enable scalable heat rejection to environmental sinks like radiators or heat exchangers, where convection and radiation further dissipate energy, sustaining steady-state operation across diverse scales from microelectronics to power plants; this engineered intervention is essential because natural dissipation rates, reliant on surface area and ambient gradients, insufficiently match internal heat fluxes in compact, high-power-density designs.[6][7]Key Thermal and Physical Properties
Coolants exhibit a range of thermal and physical properties that determine their efficacy in absorbing, transferring, and dissipating heat while maintaining system integrity. Key thermal properties include thermal conductivity, which quantifies the rate of heat conduction through the fluid (measured in W/m·K), and specific heat capacity, the energy required to raise the temperature of a unit mass by one degree Kelvin (J/kg·K). High values for both enable efficient heat management without excessive temperature gradients or fluid volume changes; for instance, pure water demonstrates superior thermal conductivity of approximately 0.6 W/m·K at 20°C and specific heat capacity of 4184 J/kg·K, outperforming organic alternatives like ethylene glycol. However, glycols are often mixed with water to extend operational ranges, yielding compromises such as reduced thermal conductivity in 50/50 ethylene glycol-water mixtures (around 0.41 W/m·K) but enhanced stability.[8] Physical properties critically influence flow dynamics and phase stability. Viscosity (in mPa·s or cP) governs pumping requirements and convective heat transfer; low viscosity minimizes energy losses, with water at ~1 mPa·s at 20°C far superior to pure ethylene glycol's ~16 mPa·s, though mixtures like 50/50 ethylene glycol-water exhibit ~4-5 mPa·s, increasing pressure drops in narrow channels.[9][10] Density (kg/m³) affects mass flow rates and system weight; water's 998 kg/m³ at 20°C contrasts with ethylene glycol's higher 1113 kg/m³, impacting gravitational and inertial heat transfer modes.[9] Boiling and freezing points define temperature limits: water boils at 100°C and freezes at 0°C under atmospheric pressure, while ethylene glycol raises boiling to ~197°C and depresses freezing in mixtures (e.g., -37°C for 50/50), preventing cavitation or solidification in extreme conditions.[11] For phase-change coolants, latent heat of vaporization (kJ/kg) enables high heat absorption via boiling, with water's 2257 kJ/kg at 100°C providing exceptional capacity, though surface tension and nucleation sites influence practical performance. Trade-offs arise across applications; liquid metals like sodium offer ultra-high thermal conductivity (>70 W/m·K) but pose reactivity risks, while dielectric fluids prioritize electrical insulation over peak thermal metrics.[12]| Property | Pure Water (20°C) | 50/50 Ethylene Glycol-Water | Pure Ethylene Glycol (20°C) |
|---|---|---|---|
| Thermal Conductivity (W/m·K) | 0.598 | 0.41 | 0.249 |
| Specific Heat Capacity (J/kg·K) | 4184 | ~3300 | 2385 |
| Viscosity (mPa·s) | 1.0 | ~4.0 | 16.0 |
| Density (kg/m³) | 998 | ~1060 | 1113 |
| Boiling Point (°C, atm) | 100 | ~106 (elevated under pressure) | 197 |
| Freezing Point (°C) | 0 | -37 | -13 |
Historical Development
Pre-20th Century Origins
Water served as the earliest known liquid coolant, employed for its superior heat absorption and transfer properties in rudimentary mechanical and thermal processes dating back to at least the 18th century. Its high specific heat capacity—approximately 4.184 J/g·°C—enabled effective dissipation of frictional heat in basic metalworking and early engines, though it was prone to freezing and boiling without additives.[13] In machining, craftsmen in the pre-industrial era relied on water sprays or immersion to cool tools and workpieces during cutting operations, preventing overheating and tool wear in operations like lathe turning or forging.[14] The advent of steam engines in the late 18th century marked a pivotal application of water cooling. James Watt's 1769 improvements to the Newcomen engine introduced a separate condenser cooled by circulating water, which condensed exhaust steam more efficiently than air alone, reducing fuel consumption by up to 75% compared to earlier designs.[15] This system relied on water's latent heat of vaporization to absorb thermal energy, maintaining operational temperatures below critical thresholds. By the mid-19th century, water jackets—thin metal casings filled with water surrounding cylinders—became standard in stationary steam engines to prevent overheating from combustion residues and friction.[15] In emerging internal combustion engines toward the late 19th century, water cooling transitioned from steam applications. Karl Benz's 1885 Patent-Motorwagen, the first practical automobile, incorporated a water-cooled single-cylinder engine with an evaporative cooling system using a surface condenser, where boiling water vapor was condensed via air flow over fins.[16] Similarly, Nikolaus Otto's four-stroke engines from 1876 onward often featured water jackets to manage cylinder temperatures exceeding 200°C, averting material failure in cast iron components. These systems circulated water via thermosiphon effect—natural convection driven by density differences—without pumps, limiting efficiency but proving reliable for low-speed operations.[16] Early refrigeration prototypes also utilized liquid coolants beyond water. In 1834, Jacob Perkins constructed the first practical vapor-compression machine employing diethyl ether as the working fluid, which absorbed heat during evaporation at low pressures before compression and condensation released it. Ether's low boiling point (34.6°C) facilitated cooling below ambient temperatures, though its flammability posed risks. This laid groundwork for mechanical cooling, distinct from mere heat dissipation in engines.[17] By the 1850s, chemists like Charles-Adolphe Wurtz synthesized ethylene glycol, initially for explosives rather than cooling, highlighting nascent chemical exploration of antifreeze properties without immediate vehicular adoption.[18]20th Century Innovations and Standardization
In the early 1900s, water remained the primary coolant for internal combustion engines, but its freezing at 0°C and boiling at 100°C under atmospheric pressure limited reliability in extreme conditions, necessitating additives for antifreeze and anti-boil properties. Methanol emerged as an early solution due to its low freezing point, but its high volatility, rapid evaporation, and fire hazard prompted searches for alternatives. By 1926, ethylene glycol-based formulations were introduced as superior antifreeze agents, offering a boiling point of approximately 197°C and freezing point depression when mixed with water, enabling effective heat transfer without excessive loss.[19][20] Commercialization accelerated in the 1930s with products like Prestone, which utilized ethylene glycol diluted in water to achieve a 50/50 mixture standard for optimal performance, providing freeze protection to -37°C and boil-over resistance exceeding 107°C in pressurized systems. This innovation addressed corrosion issues inherent in early glycol-water blends by incorporating basic inorganic inhibitors, such as sodium or potassium salts, though initial formulations still required frequent changes due to deposit formation. During World War II, military applications in aircraft and vehicles demanded enhanced stability, leading to refined additive packages that mitigated cavitation and liner pitting in high-output engines.[13][21] Post-1945, standardization efforts formalized coolant specifications through industry bodies. The Society of Automotive Engineers (SAE) and ASTM International developed performance criteria emphasizing thermal efficiency, material compatibility, and longevity, with ethylene glycol-based coolants classified under Inorganic Additive Technology (IAT) using silicates, phosphates, and borates for corrosion control in aluminum and cast iron components. By the 1950s, these green-dyed IAT coolants became the de facto standard for automotive and industrial use, requiring replacement every 2-3 years or 30,000-50,000 miles to prevent additive depletion.[22][13] In parallel, machining and metalworking coolants evolved from straight oils to soluble emulsions and synthetics by mid-century, incorporating glycols and polyalkylene oxides for improved lubricity and heat dissipation in high-speed operations, standardized via ISO viscosity grades to ensure consistency across tools and alloys. Refrigeration systems saw chlorofluorocarbon (CFC) coolants like Freon-12 patented in 1928 and commercialized in 1930, replacing toxic ammonia and sulfur dioxide with non-flammable, low-pressure alternatives that enabled widespread household and industrial adoption.[14][23]Post-1980 Regulatory Shifts
The Montreal Protocol on Substances that Deplete the Ozone Layer, adopted in 1987 and entering into force in 1989, marked a pivotal regulatory response to the discovery of stratospheric ozone depletion linked to chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODS) commonly used as refrigerants in cooling systems.[24] The protocol mandated a phased elimination of CFCs, such as R-12, which had been standard in refrigeration, air conditioning, and automotive systems since the mid-20th century; production and consumption in developed countries ceased by 1996, with developing nations following by 2010.[25] This shift compelled the refrigeration industry to adopt hydrochlorofluorocarbons (HCFCs) like R-22 as transitional alternatives, though HCFCs were later targeted for phaseout due to their residual ozone-depleting potential, with U.S. production banned for new equipment by 2010 and full phaseout by 2020.[26] Compliance drove innovations in hydrofluorocarbon (HFC) refrigerants, such as R-134a for automotive air conditioning, approved under the U.S. Environmental Protection Agency's (EPA) Significant New Alternatives Policy (SNAP) program established in 1994 to evaluate substitutes for ODS.[27] Subsequent regulations addressed the high global warming potential (GWP) of HFCs, which, while ozone-safe, contribute significantly to climate change; for instance, R-134a has a GWP over 1,400 times that of carbon dioxide.[23] The 2016 Kigali Amendment to the Montreal Protocol committed 197 countries to phasing down HFC production and consumption by 80-85% over 30 years, with baselines set from 2020-2022 and initial reductions starting in 2019 for developed nations.[26] In the United States, the American Innovation and Manufacturing (AIM) Act of 2020, enacted via the Consolidated Appropriations Act, authorized the EPA to enforce an 85% HFC reduction by 2036, accelerating transitions to low-GWP alternatives like hydrofluoroolefins (HFOs) such as R-1234yf in new automotive systems since 2017 model year mandates.[28] European Union F-Gas Regulations, updated in 2014 and 2024, impose GWP thresholds—e.g., prohibiting refrigerants with GWP above 150 in new mobile air conditioning from 2017 and banning those above 750 in certain stationary systems by 2025—further standardizing global adoption of natural refrigerants like carbon dioxide (R-744) or hydrocarbons in specialized applications.[29] For non-refrigerant coolants, such as ethylene glycol-based antifreeze in engine systems, post-1980 regulations focused on waste management and toxicity rather than composition overhaul. The EPA's Resource Conservation and Recovery Act (RCRA) hazardous waste rules, effective November 15, 1980, classified spent antifreeze as potentially hazardous due to heavy metal contaminants like lead, requiring proper disposal and recycling to prevent groundwater pollution.[30] Later measures, including state-level bans on ethylene glycol disposal in septic systems and incentives for propylene glycol substitutes (less toxic but costlier), emerged in the 1990s and 2000s, though without the comprehensive phaseouts seen in refrigerants.[31] These shifts prioritized environmental containment over reformulation, contrasting with the refrigerant sector's repeated molecular redesigns driven by international treaties.Applications
Automotive and Transportation Systems
In automotive engines, coolant circulates through the engine block and cylinder head to absorb excess heat generated during combustion, transferring it to the radiator for dissipation into the atmosphere.[32] This process maintains optimal operating temperatures, typically around 90-100°C, preventing thermal degradation of components and ensuring efficient combustion.[5] Coolants also inhibit freezing in cold climates by lowering the freezing point of the mixture— a 50/50 ethylene glycol-water blend freezes at approximately -37°C—and raise the boiling point to about 107°C at atmospheric pressure, or higher under system pressurization to 120-130 kPa.[33] The primary component in most automotive coolants is ethylene glycol (EG), mixed with water and additives such as corrosion inhibitors, anti-foam agents, and dyes for identification.[33] EG-based formulations provide superior heat transfer compared to alternatives like propylene glycol (PG), though PG offers lower toxicity at the cost of reduced thermal efficiency and higher viscosity.[34] Inorganic additive technology (IAT) coolants, using silicates and phosphates, were standard until the 1990s, but organic acid technology (OAT) and hybrid OAT (HOAT) types now predominate for extended life and aluminum compatibility in modern engines.[5] Ethylene glycol was first commercialized for automotive use in 1926, replacing earlier methanol-based antifreezes that evaporated quickly.[19] In heavy-duty transportation like trucks and locomotives, coolants must withstand higher loads and extended service intervals, often incorporating supplemental coolant additives (SCAs) such as nitrites for cavitation protection in diesel engines.[35] Truck cooling systems regulate temperatures under severe conditions, preventing overheating in semis and fleet vehicles where engines produce significantly more heat per unit volume.[36] Locomotive coolants, typically EG-based, are discharged as regulated industrial wastewater to avoid environmental contamination.[37] Aircraft engines, by contrast, rarely employ liquid coolants like EG due to weight penalties and reliability demands; piston engines rely on air-cooling fins, while turbines use fuel and oil for heat management.[38]Industrial Machinery and Power Plants
In metalworking machinery, such as lathes, mills, and grinding equipment, coolants primarily function to absorb and dissipate frictional heat at the tool-workpiece interface, reducing temperatures that could otherwise cause thermal distortion, tool softening, or workpiece warping.[39] Water-soluble coolants, including emulsions of mineral oils in water (typically 5-10% concentration), semi-synthetics, and full synthetics, provide effective cooling via water's high specific heat capacity of 4.18 J/g·°C, while also lubricating to minimize wear and flushing debris to prevent recutting.[40] These fluids extend tool life by 20-50% or more in high-speed operations by limiting built-up edge formation and oxidation, improve surface finish through reduced chatter and better chip evacuation, and inhibit corrosion on ferrous and non-ferrous metals via additives like nitrites or phosphates.[41] [42] Straight mineral oils, used neat for low-speed heavy cuts, offer superior lubricity but inferior cooling, making them suitable for applications prioritizing boundary lubrication over heat transfer.[40] For broader industrial machinery, including compressors, hydraulic presses, and diesel engines in manufacturing plants, specialized antifreeze/coolants like extended-life formulations with organic acid inhibitors (OAT) or hybrid OAT (HOAT) protect against cavitation erosion, scale buildup, and overheating under continuous high-load conditions.[43] [44] These ethylene glycol- or propylene glycol-based fluids maintain viscosity stability across -40°C to 120°C, preventing boil-over in pressurized systems and enabling efficient heat rejection through radiators or heat exchangers, which sustains machinery uptime and reduces energy losses from thermal inefficiencies.[45] In conventional thermal power plants employing steam turbines, water acts as the dominant coolant in condenser systems, absorbing latent heat from low-pressure exhaust steam to facilitate phase change back to liquid, with U.S. thermoelectric facilities withdrawing 47.7 trillion gallons in 2021 primarily for once-through or recirculating setups.[46] Recirculating systems, comprising 67% of consumptive water use, rely on cooling towers that evaporate 1-2% of circulated water per cycle to reject heat to the atmosphere, yielding consumption rates of 1,820-4,169 liters per MWh depending on wet-bulb temperatures and design.[47] [48] Inorganic salts or polymers in treated cooling water mitigate biofouling and scaling, ensuring heat transfer coefficients remain above 2,000-5,000 W/m²·K in tube bundles.[49] Nuclear power plants predominantly use pressurized light water as coolant and moderator in light water reactors, which constitute over 90% of operating units worldwide, circulating at 15-16 MPa to maintain subcooled boiling margins and transfer core heat to steam generators.[50] Advanced designs incorporate alternatives for enhanced safety and efficiency: sodium-cooled fast reactors employ molten sodium (boiling point 883°C) for its thermal conductivity of 70-80 W/m·K—over 100 times that of water—allowing passive decay heat removal without pumping.[51] High-temperature gas reactors utilize helium at 7-10 MPa, leveraging its low neutron absorption and high heat capacity under pressure to achieve outlet temperatures exceeding 750°C for hydrogen cogeneration or electricity.[52] Concentrated solar power plants integrate synthetic heat transfer fluids, such as biphenyl/diphenyl oxide mixtures, in parabolic trough or tower systems to capture and transport thermal energy at 300-400°C to heat exchangers, bypassing high-pressure steam limitations and enabling dispatchable output with molten salt storage for nighttime generation.[53] These fluids exhibit thermal stability up to 400°C without decomposition, outperforming water in low-pressure operations but requiring nitrogen blanketing to prevent oxidation.[54]Electronics Cooling and Data Centers
Liquid cooling systems in electronics utilize fluids with superior thermal conductivity compared to air to dissipate heat from high-power components such as processors and power electronics, preventing thermal runaway and extending operational life.[7] These systems often employ closed-loop circuits with water or water-glycol mixtures circulating through cold plates or heat exchangers attached directly to heat-generating surfaces, achieving heat transfer coefficients up to 10,000 W/m²K, far exceeding air cooling's 100 W/m²K.[55] In consumer and industrial electronics, such as servers and telecommunications gear, liquid coolants enable sustained performance under loads exceeding 300W per chip by maintaining junction temperatures below 85°C.[56] Data centers, housing dense clusters of servers, consume substantial energy for cooling, accounting for 30-40% of total facility power in less efficient setups, with hyperscale operations as low as 7%.[57][58] Rising power densities from AI accelerators, reaching 1kW+ per rack, have accelerated adoption of liquid cooling, with market penetration projected to exceed 20% by late 2025 from 10% in 2024.[59] Direct-to-chip cooling, using non-conductive loops of deionized water or propylene glycol solutions, attaches microchannel cold plates to CPUs and GPUs, reducing cooling energy by 20-30% versus air systems while supporting rack densities over 100kW.[60][61] Immersion cooling submerges entire server boards in dielectric fluids, either single-phase (constant liquid state, e.g., mineral oils or engineered hydrocarbons) or two-phase (boiling/condensing, e.g., perfluorocarbons), eliminating fans and enabling uniform heat removal at rates 1,000 times more efficient than air.[62][63] Single-phase immersion operates at 40-60°C, allowing extended free cooling in moderate climates and power usage effectiveness (PUE) below 1.05, while two-phase variants leverage latent heat for densities up to 200kW per rack.[64] Dielectric fluids must exhibit low viscosity (<1 cP), high dielectric strength (>30 kV), and minimal flammability to prevent electrical faults, though fluorinated options face scrutiny for environmental persistence.[65][66] The global data center liquid cooling market, valued at $5.38 billion in 2024, is forecasted to reach $17.77 billion by 2030, driven by AI workloads necessitating these technologies for energy efficiency gains of up to 40% over traditional methods.[67] Challenges include fluid compatibility with materials, leak detection, and maintenance complexity, yet deployments in hyperscale facilities demonstrate reliability, with fluid recirculation systems recycling over 99% of coolant volume annually.[68][69]Specialized High-Temperature and Nuclear Uses
In high-temperature applications beyond conventional systems, molten salts such as fluoride-based mixtures (e.g., FLiBe, composed of lithium fluoride and beryllium fluoride) serve as coolants due to their high thermal stability, with melting points around 450–600°C and boiling points exceeding 1400°C, enabling heat transfer at temperatures up to 1000°C without pressurization.[70] These salts are employed in advanced high-temperature reactors like the Advanced High-Temperature Reactor (AHTR), where they facilitate efficient heat extraction from solid fuel elements while minimizing neutron moderation and corrosion risks through compatible materials.[71] Liquid metals, including sodium and lead-bismuth eutectics, are also utilized for their superior heat transfer coefficients—sodium's thermal conductivity is approximately 70–80 W/m·K at operating temperatures—and ability to operate at 500–800°C, supporting compact designs in space and industrial heat exchangers.[72] [73] In nuclear reactors, specialized coolants enable operation at elevated temperatures for improved thermodynamic efficiency and process heat generation. Sodium-cooled fast reactors (SFRs) employ liquid sodium as the primary coolant, which remains liquid between 98°C and 883°C at atmospheric pressure, allowing core outlet temperatures of 500–550°C and reducing moderation to sustain fast neutron spectra for breeding plutonium-239.[51] Historical prototypes like the Experimental Breeder Reactor-II (operated 1951–1994) demonstrated sodium's efficacy, though it requires inert atmospheres to prevent reactions with air or water.[74] Lead-cooled fast reactors (LFRs) use molten lead or lead-bismuth alloys, with melting points of 327°C and 125°C respectively, achieving outlet temperatures up to 550°C; these offer higher boiling points (>1700°C) and inherent safety via natural circulation but demand corrosion-resistant alloys like T91 steel.[72] Gas-cooled reactors, particularly high-temperature gas-cooled reactors (HTGRs), utilize helium as an inert coolant, enabling core outlet temperatures of 750–950°C for hydrogen production or electricity generation at efficiencies exceeding 50%.[75] Helium's low neutron absorption and high heat capacity (approximately 5.2 J/g·K) minimize activation products, as evidenced in designs like the pebble-bed modular reactor, where it transfers heat via intermediate loops to avoid direct turbine contamination.[76] Molten salt reactors (MSRs) dissolve fissile materials in or use separate molten fluoride/chloride salts as coolant, operating at 600–700°C under low pressure; the Molten Salt Reactor Experiment (1965–1969) at Oak Ridge validated salt's chemical stability and online reprocessing potential, though fluoride salts' corrosivity necessitates Hastelloy-N alloys.[77] [73] These coolants prioritize high-temperature tolerance over aqueous systems' simplicity, trading off material compatibility challenges for enhanced safety margins like negative temperature coefficients.[78]Properties and Selection Criteria
Thermal Conductivity and Capacity
The thermal conductivity of a coolant, a measure of its ability to conduct heat expressed in watts per meter-kelvin (W/m·K), directly influences the efficiency of heat transfer from system components to the fluid, particularly in conduction-dominated regimes or as a factor in convective heat transfer coefficients via the Nusselt number. Higher values enable faster heat extraction, reducing thermal gradients and component temperatures in applications like heat exchangers.[69][79] Specific heat capacity, denoted c_p and measured in joules per kilogram-kelvin (J/kg·K), quantifies the heat absorption per unit mass per degree temperature rise, allowing coolants with elevated c_p to carry larger thermal loads with smaller temperature excursions, which stabilizes system performance and minimizes pumping power needs for a given heat duty. In selection, these properties are balanced against viscosity and phase stability; for instance, pure water offers optimal values—thermal conductivity of 0.598 W/m·K and c_p of 4184 J/kg·K at 20°C—but requires additives or alternatives for extreme temperatures due to freezing at 0°C and boiling at 100°C under atmospheric pressure.[79][80] Common liquid coolants exhibit trade-offs in these properties. Ethylene glycol, widely used in mixtures for antifreeze, has a lower thermal conductivity of 0.258 W/m·K and c_p around 2400 J/kg·K at 20°C compared to water, reducing overall heat transfer efficiency by 20-40% in 50% volumetric mixtures, where effective values drop to approximately 0.40 W/m·K and 3500 J/kg·K. Propylene glycol shows similar reductions, with thermal conductivity near 0.26 W/m·K, prioritizing biocompatibility over thermal performance in food or medical systems. In high-temperature applications, such as molten salts for concentrated solar power, sodium nitrate-potassium nitrate eutectics achieve c_p exceeding 1500 J/kg·K above 250°C but with conductivities below 0.6 W/m·K, suitable for sensible heat storage rather than rapid conduction.[80][9][81]| Coolant Type | Thermal Conductivity (W/m·K at ~20°C) | Specific Heat Capacity (J/kg·K at ~20°C) | Typical Application Notes |
|---|---|---|---|
| Pure Water | 0.598 | 4184 | Baseline for high-efficiency cooling; limited by phase change risks.[80][79] |
| Ethylene Glycol (pure) | 0.258 | ~2400 | Antifreeze base; dilutes performance in mixtures.[80][9] |
| 50% Ethylene Glycol-Water | ~0.40 | ~3500 | Automotive standard; balances thermal and freeze protection.[9][82] |
| Propylene Glycol (pure) | ~0.26 | ~2500 | Non-toxic alternative; slightly inferior to ethylene glycol thermally.[83] |
Corrosion Resistance and Additives
Corrosion in coolant systems arises primarily from electrochemical reactions involving water, dissolved oxygen, and dissimilar metals such as aluminum, cast iron, copper, and solder, leading to galvanic degradation, pitting, and cavitation erosion that compromise system integrity and efficiency.[5][86] These processes accelerate under high temperatures and varying pH levels, with aluminum particularly susceptible to localized attack in glycol-water mixtures.[87] Effective corrosion resistance requires additives that maintain alkalinity, scavenge oxygen, and form protective barriers on metal surfaces without promoting deposits or incompatibility.[88] Corrosion inhibitors, comprising up to 10% of antifreeze formulations, function through anodic or cathodic mechanisms: anodic types like phosphates, borates, molybdates, and nitrites suppress metal dissolution by passivating positively charged sites, while cathodic inhibitors limit oxygen reduction; organic acid technologies (OAT), such as carboxylates, provide targeted adsorption on vulnerable areas rather than uniform films, extending service life in modern aluminum-heavy engines.[5][89] Inorganic inhibitors like silicates offer rapid protection for aluminum via gel-like layers but deplete faster and risk silicate dropout if mixed improperly, whereas phosphates buffer pH effectively for ferrous metals and solder but may form scales in hard water.[88][90] Hybrid formulations combine inorganic and organic inhibitors to balance immediate and long-term protection, reducing the need for frequent replenishment; for instance, silicate-phosphate blends enhance AA6060 aluminum inhibition in ethylene glycol coolants by synergistically minimizing uniform and pitting corrosion rates.[87] Excessive inhibitor concentrations, however, can disrupt coolant stability, promote foaming, or induce reverse corrosion by overwhelming buffering capacity.[91] Compatibility testing, often via ASTM D1384 glassware corrosion evaluations, is essential when selecting additives, as mismatched types—e.g., mixing silicate-based with OAT—can deplete inhibitors, foster sludge, or accelerate metal loss.[92][90]Viscosity, Stability, and Compatibility
The viscosity of coolants, a measure of their internal resistance to flow, directly influences pumping requirements, pressure losses, and convective heat transfer efficiency in cooling systems. Low viscosity is generally preferred to minimize energy consumption for fluid circulation and to ensure adequate flow through narrow channels, though excessive thinness can reduce lubricity in components like water pumps. Pure water, often the baseline for comparison, has a dynamic viscosity of approximately 1.5 cP at 40°F (4.4°C), dropping to around 0.3 cP at 120°F (48.9°C), enabling high flow rates with low power input.[10] In contrast, glycol-based mixtures, essential for antifreeze properties, exhibit higher viscosities that rise with ethylene glycol (EG) concentration and fall with temperature; for example, a 50% EG-water solution measures 6.5 cP at 40°F and 1.5 cP at 120°F, while pure EG reaches 48 cP at the lower temperature.[9] These properties are quantified in standards like ASTM D3306 for glycol-based engine coolants, which indirectly inform viscosity through performance requirements, though dedicated test methods such as proposed ASTM WK30376 address non-aqueous variants directly.[93] [94]| EG Concentration (% by volume) | Viscosity at 40°F (4.4°C) (cP) | Viscosity at 120°F (48.9°C) (cP) |
|---|---|---|
| 0 (water) | ~1.5 | ~0.3 |
| 25 | 3.0 | 0.9 |
| 50 | 6.5 | 1.5 |
| 100 (pure EG) | 48.0 | 7.0 |
Types of Coolants
Gaseous Coolants
Gaseous coolants are fluids in the gas phase used primarily for convective heat transfer in applications where liquid coolants are unsuitable due to high temperatures, corrosive environments, radiation exposure, or the need for phase-change avoidance. Unlike liquid or two-phase systems, they operate without boiling or condensation, relying on high flow rates to compensate for generally lower specific heat capacities and heat transfer coefficients, typically on the order of 10-100 W/m²·K compared to thousands for liquids. Their selection prioritizes properties like thermal conductivity, density, and inertness to minimize pumping power and material degradation.[101] Common types include air, helium, carbon dioxide (CO₂), and hydrogen. Air, with a thermal conductivity of about 0.026 W/m·K at standard conditions, is widely used in forced-air systems for electronics cooling, such as CPU fans in computers and servers, where it dissipates heat via turbulence induced by impellers, achieving effective temperatures below 80°C in moderate loads. Helium excels in specialized high-performance roles due to its superior thermal conductivity (0.152 W/m·K at 300 K, roughly six times that of air) and chemical stability, preventing oxidation or reactions in extreme conditions. Hydrogen offers even higher conductivity (0.182 W/m·K) and low viscosity, enabling up to 14 times greater mass flow for the same power input versus air, though its flammability necessitates sealed, pressurized systems. CO₂, historically prominent, provides adequate cooling at moderate pressures but can decompose above 500°C, limiting its use.[101][102][103] In power generation, hydrogen cools rotors and stators in large turbo-generators exceeding 150 MW, reducing windage losses by 90% compared to air and maintaining winding temperatures below 120°C under full load, a practice standard since the 1930s in designs from manufacturers like GE. Gas-cooled nuclear reactors employ helium or CO₂: early British Magnox and Advanced Gas-cooled Reactors (AGR) from the 1950s-1970s used CO₂ at 250-650°C inlet-outlet temperatures for graphite moderation, while modern high-temperature gas-cooled reactors (HTGRs), such as the Xe-100 design, utilize helium for outlet temperatures up to 750°C, enabling hydrogen production or process heat alongside electricity at efficiencies over 50%. Helium's low neutron activation and non-reactivity support Gen IV goals, with prototypes demonstrating core heat removal rates of 5-10 MW/m³. In cryogenics and superconductivity, helium cools MRI magnets to 4 K and particle accelerators, leveraging its boiling point of 4.2 K at atmospheric pressure for near-absolute zero operation without solidification.[102][104][105] Advantages of gaseous coolants include corrosion resistance and compatibility with high-vacuum or radiation-heavy settings, but drawbacks encompass lower volumetric heat capacity (e.g., helium at 5.2 kJ/m³·K versus water's 4,180 kJ/m³·K), requiring larger ducts and fans, and potential safety issues like asphyxiation from inert gases or explosion risks with hydrogen. In machining, compressed air or nitrogen variants enhance tool life by 3-4 times in dry processes via cryogenic effects, reducing friction without residue. Ongoing research focuses on helium for fusion reactors, where its inertness aids divertor cooling at 1000°C fluxes.[101][103][106]Liquid Coolants
Liquid coolants are fluids designed for convective heat transfer in engineering systems, absorbing thermal loads from sources such as engines, electronics, and industrial processes before dissipating them via radiators or heat exchangers. Their efficacy stems from high volumetric heat capacity—often orders of magnitude greater than gases due to liquid densities exceeding 800 kg/m³ and specific heats around 2-4 kJ/kg·K—enabling compact designs with heat transfer coefficients up to 10,000 W/m²·K in forced convection flows.[107][7] This makes them indispensable in applications demanding rapid, efficient cooling, such as internal combustion engines where they manage up to 30% of generated heat, or data center servers handling fluxes over 100 W/cm².[69] Key performance metrics include thermal conductivity (typically 0.1-0.6 W/m·K), viscosity (influencing pumping power, ideally below 10 cP at operating temperatures), and stability across wide ranges, such as -50°C to 150°C for automotive uses. Pure water excels with a specific heat of 4,184 J/kg·K and conductivity of 0.6 W/m·K at 20°C, but its limitations—freezing at 0°C and promoting corrosion without inhibitors—drive the use of formulated mixtures. Additives like corrosion inhibitors (e.g., silicates or phosphates) and biocides extend service life to 5-10 years in closed loops, while pH control (7.5-11) prevents material degradation in copper, aluminum, or steel systems.[108][79] In high-performance scenarios, such as aerospace turbines, coolants must also resist cavitation and foaming, with nucleate boiling thresholds above 100 kW/m² to avoid dry-out.[109] Liquid coolants are categorized by base composition and application demands: water-based solutions dominate low-to-moderate temperature regimes for their unmatched heat transfer; glycol-organic blends provide freeze/boil protection at the cost of 10-15% reduced efficiency; and exotic variants like fluorocarbons or molten salts handle extremes from cryogenic to 600°C. Compatibility testing per standards like ASTM D1384 ensures minimal degradation, as incompatible fluids can increase corrosion rates by factors of 10 or more. Environmental and safety profiles vary, with non-toxic options like propylene glycol preferred over ethylene glycol (LD50 ~4,700 mg/kg vs. 5,600 mg/kg in rats), though all require leak-proof systems to mitigate fire or toxicity risks in enclosed spaces.[110][111] Ongoing research emphasizes biodegradable alternatives to reduce lifecycle impacts, but empirical data confirms water-glycol hybrids retain ~90% of pure water's performance in most real-world loops after accounting for viscosity penalties.[69][109]Water-Based Solutions
Water-based coolants consist primarily of water, often deionized or purified to minimize conductivity and impurities, and are employed in applications requiring high heat transfer efficiency due to water's specific heat capacity of 4.184 J/g·K and thermal conductivity of approximately 0.6 W/m·K at ambient temperatures. These properties allow water to absorb and transport substantial thermal loads with low pumping energy demands, making it suitable for once-through or recirculating systems in power plants, where it cools turbine exhaust steam by rejecting heat to the atmosphere via cooling towers or rivers.[112] In thermoelectric power generation, water-based systems account for the majority of cooling in the United States, with recirculating wet cooling reducing freshwater withdrawal compared to once-through methods, though still consuming significant volumes for evaporation.[113] To mitigate corrosion—a primary drawback stemming from water's reactivity with metals like steel and aluminum—formulations incorporate inhibitors such as phosphates, azoles, or organic compounds, which form protective films on surfaces and reduce degradation rates by orders of magnitude in controlled tests.[114] [115] Deionized water is preferred in electronics and data center liquid cooling loops to prevent electrolytic corrosion and short circuits, achieving corrosion rates below 0.1 mm/year on compatible alloys when properly maintained.[69] In industrial machinery, such as metalworking, water-based emulsions provide superior cooling for high-speed operations but lack lubricity, necessitating separate lubricants and risking bacterial growth if biocides are absent.[116] [117] Limitations include a freezing point of 0°C and boiling point of 100°C at atmospheric pressure, restricting use in extreme temperatures without pressurization or additives, and potential for scaling from minerals if untreated tap water is used.[118] Environmental concerns arise from thermal pollution in discharge waters, prompting regulatory shifts toward closed-loop systems that recycle up to 95% of the coolant volume in modern facilities.[119] Despite these challenges, water's abundance and cost-effectiveness—typically under $0.01 per liter for treated variants—ensure its dominance in large-scale applications like nuclear reactors, where it also serves as a neutron moderator.[120]Glycol and Organic Mixtures
Glycol-based coolants primarily consist of ethylene glycol (EG) or propylene glycol (PG) diluted with water, typically at concentrations of 30-50% by volume, along with corrosion inhibitors, dyes, and stabilizers to prevent freezing in cold conditions while elevating the boiling point for elevated-temperature operations.[34][9] These mixtures are widely employed in automotive engines, HVAC systems, and industrial heat exchangers to maintain operational temperatures between -37°C and 149°C depending on the blend ratio.[121] EG exhibits higher thermal conductivity (approximately 0.25 W/m·K at 50% concentration) and lower viscosity than PG, enabling better heat transfer efficiency and reduced pumping energy requirements, though both glycols lower the mixture's specific heat capacity by 15-20% compared to pure water.[122][69] PG, while offering marginally higher specific heat, is preferred in applications requiring lower toxicity, such as food processing or closed-loop systems accessible to humans, due to its lower acute oral toxicity (LD50 >20 g/kg versus EG's 4.7 g/kg in rats).[123][124] A key limitation of glycol mixtures is their propensity for corrosion without additives, as they can degrade aluminum, copper, and solder in cooling systems unless buffered to a pH of 7.5-11 and supplemented with inhibitors like phosphates or azoles.[125] Organic acid technology (OAT) formulations represent an advancement in glycol-based systems, utilizing carboxylate salts and organic inhibitors instead of inorganic compounds like silicates or nitrites, which reduces silicate gel formation and extends coolant life to 150,000-250,000 miles or 5-10 years in heavy-duty applications.[21][126] OAT coolants, often EG- or PG-based, provide slower but more persistent corrosion protection through adsorption on metal surfaces, minimizing electrochemical reactions in modern aluminum-intensive engines, though they may offer less immediate passivation for cast iron compared to inorganic additive technology (IAT) predecessors.[127][128] Hybrid OAT (HOAT) mixtures blend organic acids with limited inorganic silicates or phosphates, achieving compatibility across diverse metals while mitigating the rapid depletion seen in pure IAT systems; for instance, HOAT formulations maintain efficacy for 100,000 miles in mixed-fleet operations.[126][129] These organic-enhanced glycols also exhibit improved stability against cavitation erosion in high-load scenarios, such as diesel engines, but require precise formulation to avoid reduced heat rejection rates—up to 10% lower than water alone—necessitating larger radiators or fans in some designs.[32][130] Environmental considerations favor PG-OAT over EG due to faster biodegradability, though EG remains dominant (over 90% market share) for its superior performance in extreme conditions.[131][132]Molten Salts and Eutectic Metals
Molten salts serve as high-temperature coolants in advanced nuclear reactors and concentrated solar power systems due to their liquid state over wide temperature ranges, low vapor pressure, and compatibility with fissile materials. Common compositions include fluoride salts such as FLiBe (2LiF-BeF₂, or 66.7 mol% LiF and 33.3 mol% BeF₂), which has a melting point of approximately 459°C and a boiling point of 1430°C, enabling operation at atmospheric pressure without the risks of high-pressure systems.[133][134] These salts exhibit high volumetric heat capacity, low neutron absorption cross-sections, and Newtonian fluid behavior with viscosity decreasing exponentially with temperature per the Arrhenius equation, facilitating efficient heat transfer in molten salt reactors (MSRs) and thermal storage.[135][136] In MSRs, they function as both coolant and fuel carrier, dissolving uranium or thorium fluorides while maintaining chemical stability up to 700°C.[77] In solar applications, nitrate-based molten salts (e.g., 60% NaNO₃–40% KNO₃, known as solar salt) operate between 250–565°C for heat storage and transfer, retaining thermal energy for dispatchable power generation.[137] Advantages include enhanced safety from inherent low pressure and high heat capacity, allowing higher operating temperatures (up to 700°C) than water coolants for improved thermodynamic efficiency.[138] However, challenges involve corrosion of structural materials like Hastelloy-N or graphite, necessitating additives or coatings, and potential tritium production in nuclear contexts from lithium-6 impurities.[139] Eutectic metals, particularly liquid metal alloys, provide coolants for fast-spectrum nuclear reactors, leveraging high thermal conductivity and boiling points exceeding 1600°C to enable compact, high-power-density designs. Lead-bismuth eutectic (LBE, ~44.5% Pb–55.5% Bi) melts at 125°C and remains liquid up to 1670°C, offering neutron transparency and natural circulation potential in lead-cooled fast reactors (LFRs).[72] Sodium, while not strictly eutectic in pure form, is used in sodium-cooled fast reactors (SFRs) with operating temperatures of 400–550°C, but its reactivity with air and water poses fire risks, as demonstrated in incidents like the 1995 Monju reactor leak in Japan.[140] Pure lead coolants in LFRs avoid bismuth's alpha-emitting polonium-210 production (yielding ~10¹¹ Bq/kg/year), but require higher pumping power due to density (10.5 g/cm³) and viscosity.[141] Key advantages of these eutectics include inherent safety from high boiling points preventing boil-off accidents and compatibility with fast neutron fluxes for breeding plutonium-239 from U-238, supporting closed fuel cycles.[142] Drawbacks encompass corrosion (e.g., LBE's dissolution of steel at >400°C, mitigated by oxide layers), high system mass from density, and seismic demands, though LFRs demonstrate passive decay heat removal via conduction.[143] Development traces to Soviet Alfa-class submarines using LBE in the 1960s–1980s, with modern Gen-IV designs like Russia's BREST-OD-300 targeting deployment by 2026.[72][144]Cryogenic and Specialized Fluids
Liquid helium, with a boiling point of 4.2 K (-269°C), serves as a primary cryogenic coolant for achieving superconductivity in materials, enabling applications such as magnetic resonance imaging (MRI) machines and particle accelerators like those at CERN.[145][146] Its superfluid phase below 2.17 K exhibits zero viscosity and enhanced heat transfer, allowing efficient cooling of superconducting magnets without mechanical pumps in some designs.[147] However, helium's scarcity and high cost—driven by global supply constraints—limit its use to high-value scientific and medical contexts, with consumption exceeding 100,000 cubic meters annually for MRI systems alone as of 2020 data.[148] Liquid nitrogen, boiling at 77 K (-196°C), functions as a versatile cryogenic coolant in industrial processes requiring rapid freezing or low-temperature maintenance, including food preservation, metal cryomilling, and semiconductor manufacturing.[149][150] It provides high specific heat absorption during phase change, enabling cryomilling of tough materials into ultrafine powders with reduced contamination, as demonstrated in pharmaceutical and aerospace component production where particle sizes below 10 micrometers are achieved.[151] Annual global production surpasses 30 million tons, primarily for these applications, though its asphyxiation risk necessitates stringent handling protocols in enclosed spaces.[152] Specialized cryogenic fluids, such as liquid hydrogen (boiling point 20 K) and neon, address niche engineering needs like rocket propulsion cooling and infrared detector operation, where their low density and high thermal conductivity outperform nitrogen in vacuum environments.[153][154] For instance, liquid hydrogen cools turbopump bearings in engines like the Space Shuttle main engines, preventing thermal failure during cryogenic fuel handling at temperatures near 20 K. These fluids demand insulated storage systems like Dewar flasks to minimize boil-off, with losses typically under 1% per day in well-designed setups, but their flammability—hydrogen's wide explosive range of 4-75% in air—imposes safety engineering costs exceeding those of inert alternatives.[155][156]Two-Phase and Phase-Change Systems
Two-phase cooling systems exploit the latent heat of vaporization to transfer thermal energy more efficiently than single-phase liquid or gas coolants, as the phase change from liquid to vapor absorbs heat at nearly constant temperature, enabling high heat fluxes without proportional temperature gradients. The working fluid, often a dielectric liquid like perfluorocarbons or refrigerants such as R134a, evaporates upon contact with heated surfaces, creating vapor that migrates to a cooler condenser section where it releases heat and condenses back to liquid, driven by capillary action, gravity, or pressure differences. This cycle yields effective heat transfer coefficients up to 10,000 W/m²·K, compared to 100-1,000 W/m²·K for single-phase convection.[157][158] Heat pipes represent a passive embodiment of two-phase technology, comprising an evacuated, wick-lined tube partially filled with a working fluid like water, ammonia, or methanol, tailored to operating temperatures from cryogenic to 1,000°C. Invented in the 1940s but refined for NASA spacecraft in the 1960s, heat pipes achieve effective thermal conductivities of 10,000 to 100,000 W/m·K—orders of magnitude above copper's 400 W/m·K—allowing heat transport over meters with minimal temperature drop, as demonstrated in satellite thermal control systems handling 100-500 W loads. Limitations include orientation dependence in wickless thermosiphons and reduced performance against gravity, where axial heat flux can drop by 50-90% without capillary pumping.[159][160] Two-phase immersion cooling submerges components directly in boiling fluids, such as engineered fluorinated liquids with boiling points of 30-60°C, facilitating rack-level power densities exceeding 100 kW in data centers, where traditional air cooling caps at 20-40 kW. This method leverages nucleate boiling for heat transfer rates up to 10^5 W/m², reducing coolant flow rates by 90% versus single-phase immersion and enabling energy savings of 30-95% over air-cooled systems, per simulations and prototypes tested in high-performance computing environments. Fluid selection prioritizes low global warming potential alternatives to hydrofluoroolefins, amid concerns over material compatibility and vapor management to prevent dry-out.[161][162][163] Phase-change materials (PCMs) augment coolant systems by storing latent heat during solid-liquid transitions, typically 150-250 kJ/kg for organic PCMs like paraffins, which melt at engineered temperatures (e.g., 20-60°C) to buffer peak loads in electronics or building HVAC without active circulation. Integrated into heat sinks or slurry coolants, PCMs extend cooling duration by 2-5 times during transients, as shown in evaluations for airborne power electronics where they maintained junction temperatures below 100°C under 500 W/cm² fluxes. Drawbacks include low thermal conductivity (0.1-0.5 W/m·K), often mitigated by encapsulation or nanoparticle doping, and cycling stability degradation over 1,000-10,000 cycles due to phase segregation. Inorganic salt hydrates offer higher latent heats (200-300 kJ/kg) but risk supercooling and corrosion.[164][165][166] In power electronics and electric vehicles, two-phase loop systems—variants of heat pipes with separate evaporator and condenser—dissipate 10-50 kW/m² while minimizing weight and volume by 50% over single-phase loops, as validated in U.S. Department of Energy prototypes from 2012 onward. Emerging adaptive designs, such as flexible heat pipes reported in 2025, conform to irregular geometries for aerospace, maintaining efficiency across orientations via dynamic wick structures. Overall, these systems excel in high-density applications but demand precise fluid properties to avoid instabilities like slug flow, which can halve heat transfer rates.[165][167][168]Emerging Nanofluid and Solid-State Variants
Nanofluids represent an advanced class of coolants engineered by dispersing nanoscale particles, typically metals, oxides, or carbon-based materials such as TiO₂, Al₂O₃, or graphene, into conventional base fluids like water or ethylene glycol mixtures. These suspensions enhance thermal conductivity by 10-40% compared to base fluids, enabling superior heat transfer rates in applications including automotive radiators, electronics cooling, and fuel cell thermal management.[169] [170] For instance, a 2024 study demonstrated that adding 0.6% TiO₂ nanoparticles to engine coolants improved convective heat transfer coefficients by up to 40.8% in internal combustion engines, potentially allowing for smaller radiator sizes and reduced pumping power requirements.[170] Similarly, hybrid nanofluids combining multiple nanoparticle types have shown promise in vehicle cooling systems, with experimental data indicating 20-30% gains in heat dissipation efficiency under high-load conditions.[171] Despite benefits, nanofluid stability remains a challenge, as agglomeration can degrade performance over time, though ultrasonic synthesis and surfactant stabilization mitigate this in recent formulations.[172] Emerging research extends nanofluids to specialized sectors, such as polymer electrolyte membrane fuel cells, where they outperform traditional coolants by improving temperature uniformity and reducing hot spots, with numerical models projecting up to 25% enhancement in overall thermal management efficacy as of 2025.[173] In unmanned aerial vehicle radiators, nanofluid integration has been experimentally validated to lower operating temperatures by 5-10°C, supporting compact designs for aerospace applications.[174] These developments stem from first-principles improvements in Brownian motion-driven particle-fluid interactions, which boost effective thermal diffusivity without proportionally increasing viscosity in optimized low-concentration dispersions (0.1-1% by volume).[175] Solid-state coolant variants shift away from fluid-based systems toward materials that exploit intrinsic properties like caloric effects or phase transitions for heat absorption and rejection, eliminating refrigerants and mechanical components. Thermoelectric solid-state coolers, utilizing Peltier effect in nano-engineered thin-film semiconductors, achieved efficiencies twice that of bulk devices in 2025 prototypes, enabling silent, vibration-free operation for electronics and data centers.[176] Elastocaloric systems, employing shape-memory alloys that cool upon stress release, emerged as a refrigerant-free alternative in Slovenian research announced in May 2025, demonstrating cooling capacities comparable to vapor-compression cycles with 20-30% lower energy use in prototype refrigerators.[177] These technologies leverage causal mechanisms such as lattice entropy changes under applied fields (electro- or magneto-caloric variants), offering scalability for building and vehicle cooling without global warming potential from leaked fluids.[178] Solid-to-solid phase-change materials (PCMs), a subset of solid-state variants, undergo reversible crystalline transitions to store latent heat, providing stable thermal buffering without leakage risks inherent to liquid PCMs. Recent advancements in Ni-Mn-Ti alloys, reported in 2023 and refined through 2025, yield ultrahigh latent heats exceeding 200 kJ/kg at temperatures above 500°C, suitable for high-temperature industrial cooling and aerospace heat sinks.[179] Integration with thermoelectric modules enhances hybrid systems, where PCMs extend cooling duration by absorbing transient heat loads, as validated in prototypes achieving 15-20% efficiency gains over fluid-only setups.[180] While commercialization lags due to material costs and cycling durability—typically limited to 1,000-10,000 cycles—these variants prioritize reliability in hermetic environments, contrasting fluid systems' evaporation and corrosion issues.[181]Safety, Health, and Environmental Impacts
Toxicity and Handling Risks
Liquid coolants, particularly those containing ethylene glycol used in automotive antifreeze and heating systems, pose significant ingestion risks due to their sweet taste, which attracts pets and children; even small amounts—such as one teaspoon for cats or one to two tablespoons for dogs—can lead to acute poisoning characterized by central nervous system depression, metabolic acidosis, cardiopulmonary effects, and renal failure in humans and animals.[182][183][184] Propylene glycol, a less toxic alternative employed in some food-grade and environmentally sensitive applications, exhibits lower mammalian toxicity, with poisoning occurring rarely and primarily from massive overexposure, though it remains an irritant to skin and eyes upon contact.[185][186] Gaseous refrigerants, such as certain hydrofluorocarbons or ammonia, carry inhalation hazards including toxicity at low concentrations, asphyxiation from oxygen displacement, and potential flammability, necessitating confined-space monitoring and leak detection during handling.[187][188] Handling risks for glycol-based coolants include skin and eye irritation from direct contact, as well as low but present flammability when exposed to open flames or high heat, though they are classified as combustible rather than highly flammable liquids under standard storage conditions.[189][190] Some formulations exhibit corrosiveness toward metals, requiring compatible materials and inhibitors to prevent system degradation.[191] For molten salt coolants in high-temperature applications like advanced reactors, primary hazards stem from thermal burns, corrosiveness at elevated temperatures (often exceeding 500°C), and toxicity from fluoride components, demanding specialized protective equipment and inert atmospheres to mitigate reactions.[192][193] Cryogenic coolants, such as liquid nitrogen or helium used in specialized cooling systems, present severe cold-related risks including frostbite, tissue freezing upon contact, and asphyxiation in enclosed spaces due to vapor expansion displacing oxygen, with safety protocols emphasizing insulated handling, ventilation, and pressure relief to avoid explosions from rapid boiling.[194][195] General handling precautions across coolant types include using personal protective equipment like gloves and goggles, storing in well-ventilated areas away from ignition sources, and immediate spill containment to prevent environmental release or secondary exposure.[187][196]Ozone Depletion and Global Warming Potential
Certain coolants, particularly chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) used as refrigerants, possess ozone depletion potential (ODP) due to their release of chlorine atoms in the stratosphere, which catalytically destroy ozone molecules through chain reactions. ODP is quantified relative to CFC-11, assigned a value of 1; for instance, CFC-12 (R-12) has an ODP of 1.0, while HCFC-22 (R-22) has an ODP of 0.055.[197] These substances contributed to the Antarctic ozone hole, with peak depletion observed in the 1990s, where springtime ozone levels dropped by up to 60% over the continent. The Montreal Protocol, signed in 1987 and entering force in 1989, mandated global phase-out of CFCs by 1996 in developed nations and HCFCs by 2030, resulting in atmospheric concentrations of key ODS declining by over 99% since peak levels around 1993.[24] Hydrofluorocarbons (HFCs), developed as ODS replacements with zero ODP, exhibit high global warming potential (GWP), a metric comparing radiative forcing over 100 years relative to CO2 (GWP=1).[198] Common HFC refrigerants like R-134a have a GWP of 1,430, and R-410A (a blend) has 2,088, making their emissions equivalent to thousands of times more CO2 by mass; HFC leaks from cooling systems accounted for about 2% of total anthropogenic greenhouse gas emissions in 2019, projected to rise without intervention.[199] Non-fluorinated coolants, such as ammonia (R-717, GWP=0) or carbon dioxide (R-744, GWP=1), show negligible contributions to either ODP or GWP, though their adoption is limited by toxicity or efficiency constraints in certain applications.[197] The Kigali Amendment to the Montreal Protocol, adopted in 2016 and ratified by over 140 parties including the U.S. via the 2020 AIM Act, initiated HFC phase-down starting 2019, targeting an 85% reduction by 2036 in developed economies to avert 0.3–0.5°C of warming by 2100.[198] This has spurred transitions to low-GWP hydrofluoroolefins (HFOs), like R-1234yf (GWP=4), though lifecycle emissions must account for higher energy use in some systems.[199] Liquid coolants like ethylene glycol, prevalent in automotive and industrial applications, have effectively zero ODP and GWP, as they biodegrade rapidly without stratospheric persistence or infrared absorption potency.[197]| Refrigerant | Type | ODP | 100-Year GWP | Common Use |
|---|---|---|---|---|
| R-12 (CFC-12) | CFC | 1.0 | 10,900 | Historical refrigeration/AC |
| R-22 (HCFC-22) | HCFC | 0.055 | 1,810 | Air conditioning |
| R-134a | HFC | 0 | 1,430 | Automotive AC, refrigeration |
| R-410A | HFC blend | 0 | 2,088 | Residential AC |
| R-1234yf | HFO | 0 | <1 | New automotive AC |
| R-744 (CO2) | Natural | 0 | 1 | Emerging refrigeration |