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Seawater Heat-Rejection Loop:
Converting Coastal Heat-Sink Advantage into Sustainable Cooling Capacity
Coastal data centers are often viewed as having a natural advantage in heat rejection, as the ocean represents an enormous and readily available heat sink. When data-hall heat can be rejected to seawater in a stable and well-controlled manner, chiller runtime can be reduced. This shifts a larger share of energy consumption away from compressor work toward higher-efficiency heat exchange and pumping, ultimately lowering overall operating cost.
Seawater, however, is far from an ideal working fluid. Its temperature varies significantly by season, latitude, intake depth, and extreme weather conditions. Seawater intake and discharge are subject to environmental constraints and permitting requirements, and seawater-side scaling and biofouling are unavoidable in real-world operation—leading to gradual performance degradation and increasing pressure drop over time. As a result, whether a seawater-based heat-rejection loop delivers genuine and sustained energy benefits depends on whether it can be engineered for long-term controllability, maintainability, and regulatory compliance.
To prevent seawater-side fouling and corrosion risks from propagating into mission-critical data-hall cooling loops, seawater-based solutions typically adopt a three-loop, layered architecture. The intent is to confine seawater-related risks within the heat-exchanger station boundary while ensuring stable and predictable operation on the data-hall side.
The seawater loop manages intake, pretreatment (screening, filtration, etc.), and discharge, and carries uncertainties associated with suspended solids, biological loading, and corrosion.
The Facility Water System (FWS) transports cooling capacity from the heat-exchanger station to the data-hall boundary and serves as the campus-level distribution and redundancy layer.
The Technology Cooling System (TCS)—served by the CDU—supports precision loads such as racks and cold plates, prioritizing stable supply temperature and flow, controlled coolant quality, and predictable maintenance. With proper segregation, seawater-side fouling and corrosion are prevented from entering mission-critical loops, and facility-side disturbances are isolated from IT-side supply conditions—providing a structural foundation for reliability and serviceability.
Seawater Temperature Defines the Supply-Temperature Boundary
Seawater temperature is strongly influenced by latitude, season, intake depth, and extreme weather events. Engineering design therefore typically adopts the hot-end (worst-case) seawater condition as the sizing and verification basis. From this condition, a temperature budget is established: the achievable facility-water supply temperature downstream of the heat-exchanger station, the temperature rise across distribution to the data-hall boundary, and the TCS supply temperature that the CDU can reliably deliver after heat exchange. This temperature budget directly determines the number of hours during which free cooling is available and the extent to which mechanical cooling is required during hotter periods.
In parallel, seawater intake systems must address ecological considerations—including intake structure design, approach velocity, screening and filtration, and operating strategy—while heat rejection to the ocean requires evaluation of thermal plume behavior, allowable receiving-water temperature rise, and associated permitting requirements. These factors constrain allowable temperature rise, operating modes, and monitoring obligations. They must therefore be incorporated as design inputs from the concept stage; otherwise, a technically viable system may prove difficult to permit or unsustainable in long-term operation.
Once these boundary conditions are defined, overall system efficiency still hinges on a critical conversion step: how effectively the heat-exchanger station translates ocean heat-sink potential into usable facility cooling capacity.
Critical Component:
Plate Heat Exchangers Drive Free-Cooling Utilization and Pumping Energy
Within the layered architecture, the plate heat exchanger at the heat-exchanger station—most commonly a gasketed plate heat exchanger (GPHE)—is the core component that converts the seawater heat sink into usable cooling capacity. Its impact is reflected in two quantifiable and controllable metrics that directly affect energy consumption and operating stability: approach temperature and pressure drop.
Approach temperature governs free-cooling utilization. During hot seasons or in low-latitude, warm-water regions, the available temperature differential is inherently limited. The smaller the GPHE approach temperature, the lower the FWS supply temperature that can be achieved under the same seawater condition. This expands the free-cooling window, delays chiller engagement, and reduces both chiller load and runtime. In seawater applications, approach temperature therefore serves as a quantitative indicator of how efficiently ocean cooling potential is converted into usable cooling.
Pressure drop governs pumping energy and operational stability. Seawater systems typically accumulate pressure losses from strainers, filters, piping, and valves. If GPHE pressure drop is excessive, pumping power can increase to the point that it offsets part of the free-cooling energy benefit. More critically, seawater-side fouling causes pressure drop to increase over time, reducing available flow and heat-transfer capacity. This degradation can lead to supply-temperature excursions, control instability, alarms, and increased maintenance frequency. Pressure drop is therefore not only an energy metric but also a key indicator of controllability and reliability.
Accordingly, in seawater heat-rejection loops, the value of a plate heat exchanger is not defined by day-one performance alone. What matters is whether it can maintain an acceptable approach temperature under worst-case seawater conditions while keeping pressure-drop growth manageable in the presence of inevitable fouling.
Design and Materials:
Enabling Sustainable Lifecycle Performance
Typical seawater-side challenges include corrosion, inorganic scaling, biofilm formation, marine growth (biofouling), and suspended-solids deposition. These mechanisms ultimately converge on the same outcomes: degraded heat transfer, increasing pressure drop, and rising maintenance demand. To keep approach temperature and pressure drop under control over the system lifecycle, materials and design choices must be matched to seawater chemistry and fouling behavior.
Material selection is not a simple matter of choosing the “highest grade.” It must align with chloride concentration, operating temperature, and contamination levels, covering plate material, corrosion-mitigation strategy, and gasket and sealing systems—while remaining compatible with cleaning chemicals. A properly matched material system reduces corrosion-related failures and leakage risk, while robust sealing and mechanical design support repeated open-and-clean maintenance cycles without compromising reliability.
Fabrication quality and channel geometry influence both anti-fouling behavior and recoverability. Plate patterns and channel designs affect not only the heat-transfer and pressure-drop balance, but also particle pass-through, deposition tendency, and cleanability. For seawater duty, engineers typically make an explicit tradeoff between maximum thermal enhancement and plugging resistance or ease of cleaning, incorporating an appropriate fouling allowance at the design stage rather than pushing all degradation into operations.
Cleaning and Maintenance:
Treat Fouling as Planned Work, Not Unplanned Events
Seawater-side fouling cannot be fully eliminated; the engineering objective is to manage it predictably and keep performance recoverable. This is where GPHEs provide practical operational value. They can be opened for periodic cleaning to restore heat-transfer surfaces and, when designed with parallel redundancy, allow maintenance without full system shutdown or with controlled derating.
In practice, operators typically employ a combination of mechanical cleaning (open-and-wash or brushing) and chemical cleaning, with trends in pressure drop and approach temperature serving as maintenance triggers. Cleaning strategy, maintenance access, redundancy philosophy, and service intervals must therefore be designed into the system from the outset, rather than treated as after-the-fact corrective measures.
Conclusion:
Ocean Heat-Sink Potential Ultimately
Depends on Sustainable Plate-Exchanger Performance
The ocean represents an attractive heat sink, but converting that potential into long-term energy efficiency and operational reliability requires that critical constraints be engineered into controllable design parameters. This includes isolating risk through a three-loop architecture; defining operating boundaries based on worst-case seawater temperature and permitting constraints; anchoring energy efficiency and stability to GPHE approach temperature and pressure drop; and ensuring that these metrics remain sustainable through appropriate material selection, channel design, fabrication quality, and a planned cleaning and maintenance strategy.
When these conditions are met, seawater heat rejection becomes not merely a conceptual advantage, but a deliverable, scalable, and durable pathway for data center heat rejection.