01Rack density trajectory and the cooling implication

Rack density has been rising through 2022-2028 on a trajectory that no prior generation of data centres anticipated. Typical enterprise racks in 2022 drew 5-15 kW. Modern hyperscaler AI training racks in 2025 draw 60-100 kW. Purpose-built AI training racks announced for 2027-2028 target 400 kW to 1 MW per rack. This trajectory forces both the DC delivery decision (see Part II on the two 800 VDC architectures) and the cooling architecture decision.

02The air-cooling ceiling

Air cooling. Where computer room air handlers move chilled air through the raised floor and across the front of racks. Has been the workhorse data centre cooling architecture for four decades. It works well up to a specific density ceiling and degrades sharply above it. The ceiling is roughly 40-60 kW per rack at PUE below 1.4, and the degradation above that ceiling is not gradual; PUE deteriorates rapidly as the air-handling infrastructure fights to move enough mass flow of air to remove the heat.

The physics is straightforward. Air has low specific heat compared to water or dielectric coolant. Removing 100 kW of heat with air requires roughly 20,000 CFM of air flow at reasonable delta-T. That volume of air requires substantial fan power, and fan power itself dissipates as heat. Above roughly 60 kW per rack, the fan-power-to-cooling-delivered ratio starts to exceed 25 percent, and total facility PUE degrades past 1.5. Above 100 kW per rack, air cooling becomes physically impractical.

03Direct-to-chip liquid cooling: the required architecture above 100 kW

Direct-to-chip liquid cooling (also called cold-plate liquid cooling) routes chilled coolant through cold plates mounted directly on the GPU, HBM, and CPU packages. The coolant absorbs heat at the source and carries it to a rack-level or row-level heat exchanger. The heat exchanger transfers the heat to a facility water loop that ultimately dumps it to ambient via cooling towers, chillers, or heat-reuse systems.

The economics work above roughly 100 kW per rack because the coolant carries orders-of-magnitude more heat per unit volume than air. The engineering complexity is real. Coolant chemistry, leak detection, cold-plate integration with the silicon package, pump sizing, redundancy for pump failure. But each of these is a solved problem individually. What has changed in 2024-2026 is the vendor set that ships integrated solutions.

The alternative is immersion cooling. Submerging the entire server in dielectric fluid. Immersion has better thermal performance and simpler electrical interfaces, but higher operational complexity (service events require draining and cleaning), higher fluid cost, and slower vendor maturation. Direct-to-chip has won the near-term deployment race in hyperscale AI even though immersion may win in the long run for the highest-density workloads.

04Coolant chemistry and DC electrical isolation

Coolant chemistry choice at 800 VDC interacts with the electrical protection design. The primary coolants in use in 2026 are water-glycol mixtures (common in direct-to-chip), engineered dielectric fluids (3M Novec derivatives and successors, in immersion), and specialty fluids from vendors like Solvay and Chemours designed for compatibility with high-voltage DC operations.

The 800 VDC-specific consideration is coolant electrical conductivity. A coolant that becomes conductive under contamination or ion accumulation can create a fault path across the DC bus. Water-glycol mixtures are electrically conductive by design and require dielectric barriers between the coolant loop and the DC electrical system. Dielectric fluids are non-conductive but expensive and require monitoring for degradation. Specialty low-conductivity fluids sit in between on cost and complexity.

Leak detection becomes a new failure mode. A pinhole leak in a direct-to-chip cold plate that would be an inconvenience in a 48 V system can create an arc-fault initiator in an 800 VDC system if the leaked coolant bridges energised conductors. Continuous leak detection sensors, drip pans with conductivity monitoring, and rapid isolation protocols are all Stage-1 design decisions, not Stage-4 monitoring add-ons.

05Vendor consolidation dynamics

The vendor set for combined 800 VDC + liquid cooling has consolidated sharply through 2024-2026 as hyperscaler customers have made clear they want both from one vendor. Three named transactions illustrate the pattern.

Vertiv acquired PurgeRite in 2023 for approximately $1B plus earnout. PurgeRite's mechanical flushing, purging, and filtration services for liquid cooling systems support hyperscale AI deployments. The Vertiv 800 VDC power portfolio (Liebert EXL and eBoost, shipping H2 2026 aligned with NVIDIA specs) plus PurgeRite integration produces a combined 800 VDC + liquid cooling proposition Vertiv can sell as one project. The Vertiv NearZero fluid management service, launched 2025, is the commercial expression.

Eaton acquired Boyd Thermal in 2024 for approximately $9.5B, the largest single thermal-industry M&A deal in the sector. Boyd brings thermal design and manufacturing scale that Eaton's power side lacked. The combined offering is Eaton's answer to Vertiv's Liebert + PurgeRite combination. The DD-series Supplement B (Eaton/Boyd worked example) treats this transaction in full commercial DD detail.

Schneider Electric acquired Motivair in early 2025 for approximately $1.7B for a majority stake. Motivair's liquid cooling equipment complements Schneider's Galaxy VXL 800 VDC UPS line, replicating the Vertiv/Eaton pattern from the Schneider side.

06Monitoring integration: the next competitive layer

The vendors that dominated the 800 VDC + liquid cooling combined offering through 2024-2026 competed on hardware integration. Being able to sell the customer both power and cooling as a single project. The competitive frontier for 2027-2028 shifts to monitoring integration: bringing thermal and power monitoring into a unified control plane that the operator can manage as one system rather than two.

Currently most facilities run thermal monitoring (temperatures, flow rates, coolant chemistry) on one system and power monitoring (currents, voltages, arc-fault detection) on another. The two systems have limited cross-visibility. A developing thermal fault that would eventually trigger a power event is not visible to the power monitoring until the power event begins. The next-generation control plane merges the two data streams and enables predictive intervention.

The vendor best positioned to deliver this unified monitoring is not obvious. Vertiv, Eaton, and Schneider all have monitoring platforms of varying maturity. Delta and ABB are strong in industrial-style unified monitoring but less penetrated in hyperscale. Software specialists like AVEVA and OSIsoft are potential integrators. The competitive winner will be determined by whichever vendor lands the first named hyperscaler contract for unified thermal + power monitoring at a 100+ MW campus.

07Named cooling technology cases

08Interactions with earlier essays in this series

The cooling architecture choice interacts with prior decisions in this series across three vectors. Arc-fault protection (Part IV): leaked coolant can create arc-fault initiators, and continuous leak detection has to be integrated with arc-fault detection. Grounding topology (Part V): coolant loops that connect multiple racks create shared ground paths that have to be accounted for in the grounding scheme. Battery integration (Part VI): battery thermal management interacts with the facility cooling loop, and battery-side thermal events (rare but consequential) have to be isolated from the compute-side cooling.

The practical takeaway for operators is that cooling cannot be procured as a separate workstream from the electrical protection design. Vendor discussions that treat cooling as an add-on to a power purchase (or vice versa) miss the coordination requirements. The vendors that have consolidated to sell both as one project (Vertiv, Eaton, Schneider) have this coordination inside their engineering; the vendors that supply only one side (CoolIT for cooling, or a power-only vendor) leave the coordination as an operator responsibility.

09Operator procurement checklist

1. Target rack density documented at architecture selection. Above 100 kW per rack, liquid cooling is required and 800 VDC becomes the sensible power choice. Below 60 kW per rack, air cooling and 400 VDC hybrid may be economic.
2. Cooling technology decision integrated with power architecture. Direct-to-chip is the default at 80-400 kW per rack; immersion becomes competitive at 400+ kW; air cooling stays below 60 kW.
3. Coolant chemistry specified against DC electrical compatibility. Water-glycol requires dielectric barriers. Dielectric fluids remove the barrier requirement at premium cost. Specialty low-conductivity fluids are middle path.
4. Leak detection integrated with arc-fault detection (Part IV). Continuous leak sensors, drip-pan conductivity monitoring, rapid isolation protocols specified at Stage 1.
5. Vendor single-vs-multi-vendor decision documented. Vertiv/Eaton/Schneider offer integrated power + cooling; CoolIT/Submer offer cooling-only with vendor-agnostic power interface. Trade-off is integration convenience vs vendor lock-in.
6. Monitoring integration plan. Thermal + power monitoring in one control plane or two? If two, define the cross-system integration protocol at Stage 1.
7. Thermal + power vendor commercial terms consolidated. If procuring both from the same vendor, negotiate as one project with unified commissioning and warranty terms rather than as two separate contracts.
8. Cooling capex projected against the 10-year hold. Direct-to-chip has higher day-one capex than air cooling but lower TCO at high density (see Part VII on retrofit vs greenfield TCO methodology).

10The reframe for anyone specifying now

The 800 VDC transition and the liquid-cooling transition are not two independent decisions. Above 100 kW per rack, choosing one implies the other. Operators that treat cooling as a Stage-4 procurement decision separate from the Stage-1 power architecture decision consistently discover in commissioning that the two workstreams were designed against inconsistent assumptions. The vendor set that has consolidated over 2023-2025 has done so precisely because customers want both from one vendor. The vendors that supply only one side of the transition are increasingly at competitive disadvantage relative to the integrated players.

Part IX moves into power quality and grid interaction. The utility-facing implications of the 800 VDC architecture that most operators discover at interconnection study and that regulators are only beginning to reflect in their standards.