Each new generation of server silicon is pushing traditional data center air cooling closer to its operational limits. In 2025, the thermal design power (TDP) of top-bin CPUs reached 500 W, and server chip product roadmaps indicate further escalation in pursuit of higher performance. To handle these high-powered chips, more IT organizations are considering direct liquid cooling (DLC) for their servers. However, large-scale deployment of DLC with supporting facility water infrastructure can be costly and complex to operate, and is still hindered by a lack of standards (see DLC shows promise, but challenges persist).

In these circumstances, an alternative approach has emerged: air-cooled servers with internal DLC systems. Referred to by vendors as either air-assisted liquid cooling (AALC) or liquid-assisted air cooling (LAAC), these systems do not require coolant distribution units or facility water infrastructure for heat rejection. This means that they can be deployed in smaller, piecemeal installations.

Uptime Intelligence considers AALC a broader subset of DLC — defined by the use of coolant to remove heat from components within the IT chassis — that includes options for multiple servers (see Air-assisted DLC: an overlooked option). This report discusses designs that use a coolant loop — typically water in commercially available products — that fit entirely within a single server chassis.

Such systems enable IT system engineers and operators to cool top-bin processor silicon in dense form factors — such as 1U rack-mount servers or blades — without relying on extreme-performance heat sinks or elaborate airflow designs. Given enough total air cooling capacity, self-contained AALC requires no disruptive changes to the data hall or new maintenance tasks for facility personnel.

Deploying these systems in existing space will not expand cooling capacity the way full DLC installations with supporting infrastructure can. However, selecting individual 1U or 2U servers with AALC can either reduce IT fan power consumption or enable operators to support roughly 20% greater TDP than they otherwise could — with minimal operational overhead. According to the server makers offering this type of cooling solution, such as Dell and HPE, the premium for self-contained AALC can pay for itself in as little as two years when used to improve power efficiency.

Does simplicity matter?

Many of today’s commercial cold plate and immersion cooling systems originated and matured in high-performance computing facilities for research and academic institutions. However, another group has been experimenting with liquid cooling for more than a decade: video game enthusiasts. Some have equipped their PCs with self-contained AALC systems to improve CPU and GPU performance, as well as reduce fan noise. More recently, to manage the rising heat output of modern server CPUs, IT vendors have started to offer similar systems.

The engineering is simple: fluid tubing connects one or more cold plates to a radiator and pump. The pumps circulate warmed coolant from the cold plates through the radiator, while server fans draw cooling air through the chassis and across the radiator (see Figure 1). Because water is a more efficient heat transfer medium than air, it can remove heat from the processor at a greater rate — even at a lower case temperature.

Figure 1 Closed-loop liquid cooling within the server

The coolant used in commercially shipping products is usually PG25, a mixture of 75% water and 25% propylene glycol. This formulation has been widely adopted in both DLC and facility water systems for decades, so its chemistry and material compatibility are well understood.

As with larger DLC systems, alternative cooling approaches can use a phase change to remove IT heat. Some designs use commercial two-phase dielectric coolants, and an experimental alternative uses a sealed system containing a small volume of pure water under partial vacuum. This lowers the boiling point of water, effectively turning it into a two-phase coolant.

Self-contained AALC designs with multiple cold plates usually have redundant pumps — one on each cold plate in the same loop — and can continue operating if one pump fails. Because AALC systems for a single server chassis contain a smaller volume of coolant than larger liquid cooling systems, any leak is less likely to spill into systems below. Cold plates are typically equipped with leak detection sensors.

Closed-loop liquid cooling is best applied in 1U servers, where space constraints prevent the use of sufficiently large heat sinks. In internal testing by HPE, the pumps and fans of an AALC system in a 1U server consumed around 40% less power than the server fans in an air-cooled equivalent. This may amount to as much as a 5% to 8% reduction in total server power consumption under full load. The benefits of switching to AALC are smaller for 2U servers, which can mount larger heat sinks and use bigger, more efficient fan motors.

However, radiator size, airflow limitations and temperature-sensitive components mean that self-contained AALC is not on par with larger DLC systems, therefore making it more suitable as a transitory measure. Additionally, these systems are not currently available for GPU servers.

Advantages of AALC within the server:

• Higher cooling capacity (up to 20%) than air cooling in the same form factor and for the same energy input, offers more even heat distribution and faster thermal response than heat sinks.
• Requires no changes to white space or gray space.
• Components are widely available.
• Can operate without maintenance for the lifetime of the server, with low risk of failure.
• Does not require space outside the rack, unlike “sidecars” or rear-mounted radiators.

Drawbacks of AALC within the server:

• Closed-loop server cooling systems use several complex components that cost more than a heat sink.
• Offers less IT cooling capacity than other liquid cooling approaches: systems available outside of high-performance computing and AI-specific deployments will typically support up to 1.2 kW of load per 1U server.
• Self-contained systems generally consume more energy than larger DLC systems for server fan power, a parasitic component of IT energy consumption.
• No control of coolant loop temperatures; control of flow rate through pumps may be available in some designs.
• Radiator and pumps limit space savings within the server chassis.

Outlook

For some organizations, AALC offers the opportunity to maximize the value of existing investments in air cooling infrastructure. For others, it may serve as a measured step on the path toward DLC adoption.

This form of cooling is likely to be especially valuable for operators of legacy facilities that have sufficient air cooling infrastructure to support some high-powered servers but would otherwise suffer from hot spots. Selecting AALC over air cooling may also reduce server fan power enough to allow operators to squeeze another server into a rack.

Much of AALC’s appeal is its potential for efficient use of fan power and its compatibility with existing facility cooling capabilities. Expanding beyond this to increase a facility’s cooling capacity is a different matter, requiring larger, more expensive DLC systems supported by additional heat transport and rejection equipment. In comparison, server-sized AALC systems represent a much smaller cost increase over heat sinks.

Future technical development may address some of AALC’s limitations, although progress and funding will largely depend on the commercial interest in servers with self-contained AALC. In conversations with Uptime Intelligence, IT vendors have diverging views of the role of self-contained AALC in their server portfolios, suggesting that the market’s direction remains uncertain. Nonetheless, there is some interesting investment in the field. For example, Belgian startup Calyos has developed passive closed-loop cooling systems that operate without pumps, instead moving coolant via capillary action. The company is working on a rack-scale prototype that could eventually see deployment in data centers.