What Are Coolants?

Coolants are fluids used to absorb, transport, and dissipate heat from systems that generate it as a byproduct of operation. In engineering contexts, a coolant carries thermal energy away from a heat source, such as a power semiconductor, an engine cylinder, or an electrical machine winding, and transfers it to a heat sink or rejection system. The selection of a coolant involves balancing thermal performance, expressed through parameters such as thermal conductivity, specific heat capacity, and viscosity, against compatibility with system materials, safety characteristics including toxicity and flammability, and environmental impact.

Coolants appear in virtually every domain where energy conversion or information processing generates heat that must be managed to protect equipment reliability. The discipline of thermal management has grown significantly as power densities in electronics and electrified vehicles have increased.

Dielectric Coolants

Dielectric coolants are electrically insulating fluids that can be placed in direct contact with live electrical components. Their insulating property is the defining characteristic: conventional water-based coolants conduct electricity and would cause short circuits if applied directly to circuit boards or power modules. Dielectric fluids include fluorocarbons such as FC-72 and FC-87, hydrofluoroethers, silicone oils, and synthetic aromatic and aliphatic hydrocarbons. Fluorocarbon-based coolants are non-combustible, chemically stable, and compatible with most electronic materials, but many carry environmental concerns as persistent greenhouse gases, driving research into next-lower-global-warming-potential replacements. Work on single-phase dielectric fluid cooling of power electronics modules demonstrates how direct immersion in dielectric fluids achieves junction-to-fluid thermal resistances substantially lower than conventional air cooling for automotive inverters. Two-phase dielectric systems exploit the latent heat of vaporization at the component surface, achieving still higher heat transfer coefficients. An overview of liquid coolants for electronics cooling from Electronics Cooling covers the thermal and electrical properties of these fluid families in detail.

Water-Based and Refrigerant Coolants

Water has the highest specific heat capacity and thermal conductivity of any common liquid coolant, making water and water-glycol mixtures the dominant choice where electrical isolation is not required. Deionized water is used in high-performance applications such as laser systems and nuclear reactor cooling circuits, where ionic contamination would cause corrosion or parasitic currents. Ethylene glycol and propylene glycol are added to depress the freezing point and raise the boiling point of water for automotive and outdoor applications. Refrigerants, including hydrofluorocarbons and their low-GWP replacements, serve as working fluids in vapor-compression cooling systems such as air conditioning and chillers; these systems circulate the refrigerant through a compressor, condenser, and evaporator rather than simply pumping a liquid. The thermal properties of these fluids, particularly their latent heats and saturation temperatures, determine the achievable cooling capacity per unit of compressor power. Research published by NREL on power electronics cooling technology addresses how coolant selection interacts with system-level efficiency targets in electrified vehicles.

Applications

Coolants have applications in a wide range of disciplines, including:

• Power electronics cooling, including inverters, converters, and motor drives for electric vehicles
• Data center thermal management, where server racks generate kilowatts of heat per unit
• Nuclear and conventional power generation, where reactor coolants transfer thermal energy to steam turbines
• Space systems thermal control, where spacecraft use fluid loops to reject waste heat to radiators
• Industrial laser and RF transmitter cooling, where precision temperature control is required for output stability
• Transformer and high-voltage switchgear cooling in electrical transmission infrastructure