Liquid cooling

Definition

Liquid cooling is a thermal management method in which a liquid coolant circulates through channels, jackets, cold plates, or immersion baths to absorb heat from components and reject it to the environment or a secondary loop via a heat exchanger (e.g., radiator, liquid-to-liquid plate exchanger, or chiller). Most systems are closed-loop, sealed to control contamination, evaporation, and aeration.

How it works (mechanism)

  • Heat is conducted from the source (e.g., semiconductor junction, battery cell, motor stator) into a cooled interface such as a cold plate or jacket.
  • The coolant convects heat away as it flows through internal passages.
  • Heat is rejected to air or another fluid through a heat exchanger; fans or blowers often provide forced airflow across radiators.
  • Variants include direct liquid cooling (coolant in direct contact with heat-generating surfaces), immersion cooling (components submerged in a dielectric liquid), and two-phase liquid cooling (boiling within the cold plate to leverage latent heat, with condensation and recirculation).

System elements and architectures

  • Coolant loop(s): single or multiple loops; series or parallel branch manifolds for component-specific flow.
  • Pump(s): typically brushless DC centrifugal pumps; may be magnetically coupled to avoid dynamic shaft seals.
  • Thermal interfaces: cold plates, jackets, heat spreaders, and thermal interface materials (TIMs).
  • Heat rejection: radiators (air-cooled), liquid-to-liquid plate heat exchangers, dry coolers, chillers, or heat pumps for temperature control and heat recovery.
  • Balance-of-plant: reservoir/expansion tank and degasser, filters, deionizers (when needed), sensors (temperature, pressure, flow), valves (on/off or proportional), and fans.
  • Plumbing: elastomer hoses, rigid polymer or metal lines, quick-connects, clamps, and fittings.

Coolants

  • Water-based: deionized water or water–glycol mixtures (ethylene or propylene glycol) with corrosion inhibitors, anti-foaming agents, and biocides; widely used due to high heat capacity and low viscosity.
  • Dielectric fluids: synthetic hydrocarbons (e.g., PAO), silicone oils, esters, and fluorinated fluids for direct-contact or immersion cooling where electrical isolation is required; chosen for compatibility, stability, and safety.
  • Selection factors: thermal properties (specific heat, thermal conductivity), viscosity (pumping power), freezing/boiling points, compatibility with metals and elastomers, toxicity and environmental impact, flammability, and cost.

Control and operation

  • Thermostats or electronically controlled valves route flow based on temperature targets.
  • Variable-speed pumps and fans maintain desired temperatures while minimizing energy use and noise.
  • Model-based or coordinated control (e.g., with a vehicle, battery, or facility management system) supports multi-loop designs, precooling/preheating, and heat recovery.

Performance metrics

  • Thermal resistance and junction-to-ambient/junction-to-coolant temperature rise.
  • Heat transfer coefficient and allowable heat flux.
  • Coolant flow rate, pressure drop, pump head, and pumping power.
  • Maximum junction or cell temperature, temperature uniformity (ΔT across components), and transient thermal capacity.
  • Reliability indicators (leak rate, coolant life, maintenance intervals).

Applications and relevance

  • Electric vehicles (EVs): battery thermal management (supporting high charge/discharge rates, fast charging, temperature uniformity, cold-weather operation), power electronics (inverters, OBCs, DC/DC converters), and electric machines (stator jackets, rotor/shaft or winding cooling). Integrated thermal circuits and heat pumps enable cabin conditioning, battery preconditioning, and waste-heat recovery for efficiency and range.
  • Data centers and computing: direct-to-chip cold plates and immersion cooling increase rack density, reduce fan power, and enable warm-water cooling and heat reuse.
  • Industrial and energy: drives and inverters, high-power LEDs, lasers and welders, additive manufacturing, medical imaging, telecom base stations, renewable energy inverters, and energy storage systems.
  • Conventional automotive and heavy-duty: engine cooling (radiators, pumps, thermostats) and hybrid powertrains.
  • Aerospace and defense: avionics, radars, and high-power mission systems.

Advantages

  • Higher heat removal capability and more uniform component temperatures than air cooling.
  • Compact packaging and lower acoustic noise for a given heat load.
  • Decouples hot spots from ambient conditions and enables heat transport over distance and heat reuse.

Limitations and risks

  • Greater complexity, cost, and mass than air systems.
  • Leak, contamination, and corrosion risks; coolant aging and maintenance requirements.
  • Pump energy consumption and potential cavitation/erosion if poorly designed.
  • Material compatibility and galvanic corrosion management are critical; freeze/boil protection needed.

Materials and manufacturing (typical)

  • Cold plates and jackets: aluminum alloys for weight and conductivity; copper where higher conductivity or compactness is essential; stainless steel or polymer composites for chemical compatibility. Channel designs include serpentine, parallel microchannels, pin/fin arrays, and jet impingement.
  • Fabrication methods: extrusion with machining; vacuum brazed or diffusion-bonded plate-fin/microchannel structures; CNC machining with cover-plate welding (laser/TIG), friction stir welding, die casting, and additive manufacturing for conformal or topology-optimized channels.
  • Heat exchangers: aluminum tube-and-fin or plate-fin radiators; brazed or gasketed plate heat exchangers for liquid-to-liquid duties; polymer end tanks where appropriate.
  • Pumps/valves: cast aluminum or polymer housings; BLDC centrifugal pumps; proportional or on/off valves in brass, aluminum, or engineered polymers.
  • Plumbing and interfaces: EPDM/FKM/silicone hoses, PA12/PA610 rigid lines, stainless/aluminum hard lines, quick connects; TIMs (greases, pads, gap fillers).
  • Sensing and diagnostics: NTC thermistors or PT1000 RTDs, pressure and differential pressure sensors, flow meters, and leak-detection provisions.

Design and maintenance considerations

  • Corrosion and fouling: select compatible materials; use inhibitors; control pH and conductivity; mitigate stray current corrosion.
  • Cavitation and aeration: ensure adequate net positive suction head (NPSH), proper reservoir/degassing, and smooth flow paths.
  • Serviceability: provide fill/drain/bleed points, filtration, and accessible components; monitor coolant condition and replace per schedule.
  • Safety and compliance: overtemperature protection, pressure relief, electrical isolation where needed, leak detection, and appropriate handling/disposal of coolants; consider toxicity and global warming potential of specialty fluids.
  • Environmental and end-of-life: coolant recyclability, minimal fluid volume, and material choices that support recycling.

Related terms and variants

Water cooling; hydronic cooling; liquid-cooled system; coolant loop; cold plate cooling; direct liquid cooling (DLC); immersion cooling (single-phase or two-phase); two-phase liquid cooling; jet-impingement cooling; radiator; heat exchanger; thermal management system (TMS); battery thermal management system (BTMS); chiller; heat pump.

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