Data-center liquid cooling has two broad families
As air cooling approaches its limits for high-density AI servers, liquid cooling, which moves heat with liquid, is spreading. Liquid cooling is not one method. It is broadly divided into direct liquid cooling(DLC) and immersion cooling. Both can carry more heat than air, but their mechanisms, ease of application, and maintenance practices differ.
What is direct liquid cooling(DLC / direct-to-chip)?
Direct liquid cooling places a cold plate tightly against high-heat components such as CPUs and GPUs, then removes heat using liquid passed through the plate. The whole server is not immersed in liquid. The defining feature is that only hot components are cooled locally, and many configurations still combine air cooling for memory and lower-heat components. Because it can preserve much of the existing rack and server structure, the adoption hurdle is relatively low.
Commercialization is also progressing. CoolIT's rack CDU(coolant distribution unit) CHx200 is specified to handle 200kW of heat load in a 4U chassis and cool up to 200 servers, with warm-water cooling support and N+1 redundant pumps and power supplies (CoolIT Systems). Rack-scale liquid-cooled designs such as NVIDIA's GB200 NVL72 are also built on this kind of direct-liquid-cooling premise (NVIDIA).
Direct liquid cooling evolves: two-phase direct-to-chip(2P DLC)
The DLC described above is mainly single-phase, meaning the liquid does not boil inside the cold plate. In fact, single-phase liquid cooling is estimated to hold about 55% market share in 2026 and is expected to remain mainstream for the next five to ten years. However, AI accelerator TDP(thermal design power) increased 2.5x from 2020 to 2024, revealing areas that single-phase systems may struggle to cool. This is where two-phase direct-to-chip(2P DLC/P2P), which removes heat by boiling refrigerant through phase change inside the cold plate, is drawing attention.
OCP(Open Compute Project) published a white paper on P2P(Pumped two-Phase) refrigerant DLC on February 14, 2025, advancing the standardization discussion. The performance described in the paper is high. P2P cold plates show heat removal of up to 2,200W per socket and about 250W/cm2, and processors above 2kW and servers above 10kW in 1OU have already been demonstrated. The cold plate outlet vapor quality is targeted around 70%, and CDU configurations at 400kW, 180kW, and 100kW are shown. Refrigerants such as R-515B, an A1 nonflammable refrigerant with GWP 293, are presented against the backdrop of HFC restrictions under the Kigali Amendment.
Commercialization has also begun. AEWIN says its two-phase direct liquid cooling(2P DLC) achieves more than 2kW of heat removal per chip and first showed it publicly at SC25 in November 2025.
Still, two-phase DTC is not a cure-all. Cold-plate heat-transfer coefficients exceed single-phase systems, but initial cost and the complexity of managing phase-change refrigerants are barriers. A realistic transition path is a two-layer structure: single-phase remains mainstream for now, while two-phase DTC is adopted first for the highest-heat components.
What is immersion cooling? single-phase and two-phase
Immersion cooling directly immerses electronic equipment in a dielectric(non-conductive) liquid. Because components and liquid touch directly, heat transfer can be efficient. Immersion cooling is further divided into two types.
- Single-phase: the liquid does not boil or vaporize; heat is moved through circulation and a heat exchanger.
- Two-phase: component heat boils the liquid into vapor, and a condenser returns it to liquid. Because it uses phase change, it is generally described as providing higher heat transfer than single-phase systems.
Academic research has reported that immersion cooling can reduce energy consumption by roughly 50% and footprint by roughly two-thirds compared with air cooling, while also noting challenges such as maintainability and the handling of IT equipment (arXiv). In implementation, the tradeoff is that high cooling capability comes with operating and maintenance practices that differ substantially from air cooling.
Direct liquid cooling(DLC)
Locally cool high-heat components with a cold plate. Easier to apply to existing racks, with a lower adoption hurdle. Low-heat components may still use air cooling.
Immersion(single-phase)
Immerse the whole system in a dielectric liquid. Provides high cooling capability, while cabling and maintenance practices differ from air cooling.
Immersion(two-phase)
Uses phase change for still higher heat transfer. Refrigerant, condenser design, and failure-mode management are key points.
Selection criteria
Judge by heat-removal capacity, use of existing facilities, maintainability, and ecosystem maturity, including OCP standardization.
Summary
DLC can be summarized as "locally cooling hot components while preserving existing configurations," while immersion cooling can be summarized as "immersing the whole system to gain high cooling capability, with different operations and maintenance." The choice depends on rack power density, existing facilities, maintenance organization, and standardization maturity. Cooling-method selection also affects assumptions for power supplies and power-device design, so the details are covered in the related article.