The industries of tomorrow will need more data centers than are available today. From 5G edge computing, AI, and machine learning to blockchain, cryptocurrency, and high-frequency trading, data traffic will continue to rise with increased internet usage. The popularity of remote work is also driving demand for greater IT infrastructure. According to Aritzon, a market research company, the value of the global data center market is expected to approach $300 billion by 2028 with a compound annual growth rate (CAGR) of greater than 5% from 2022.
In addition to more IT resources, the advanced industries will need data centers that can handle heavier workloads, which will require more power. But, more power generates more heat — the enemy of reliable electronics. As data center capacity is limited by data center cooling, traditional approaches to thermal management may not be enough. Data centers also need solutions that are environmentally sustainable and do not raise significant environmental health and safety (EHS) concerns. Whether it’s for new installations or retrofits, data center designers and operators have important decisions to make.
Traditionally, air cooling has been used to dissipate heat from electronics. Fans and heat sinks are cost-effective, but they cannot meet the large-scale cooling requirements of concentrated high-power computing. Plus, putting many fans in an enclosed environment can create unacceptable levels of noise. Liquid cooling of cold plates with water blocks, pumps, pipes, radiators and reservoirs can cool electronics because of water’s relatively high thermal conductivity. However, the maintenance requirements are considerable — leaks are a concern, and water levels must be maintained because of evaporation.
Immersion cooling offers an alternative approach. Full servers or server components are submerged in a thermally conductive liquid that transfers heat from a coolant to a water circuit. Ideally, this coolant has low electrical conductivity so that it will not interfere with server operations. Otherwise, sensitive electronics could experience electromagnetic interference (EMI). Although de-ionized water can be used with immersion cooling, water is not the best choice for a dielectric fluid because the charged ions that it contains make it a very good conductor of electricity.
For data center designers and operators, it’s important to understand that immersion cooling involves direct contact between the coolant and the server or server components. By contrast, traditional liquid cooling sends a coolant through a sealed loop that is isolated from the heat source. Immersion cooling also offers a choice between single- and two-phase systems. Single-phase cooling is less efficient than two-phase cooling, but the liquid coolant does not change state. Two-phase cooling transforms the coolant from a liquid to a vapor and requires more fluid replenishment because the coolant boils off.
There are other differences between single- and two-phase immersion cooling as well. With single-phase immersion cooling, the coolant carries the heat from the server to a heat exchanger and then returns the coolant to either a dry or adiabatic cooler. Dry coolers do not use water, while adiabatic coolers work by increasing humidity. In a two-phase immersion cooling system, heat from the server causes the coolant to evaporate. After the liquid coolant is transformed into a vapor, it later condenses to a liquid when it contacts a water-cooled condenser. The coolant is then returned to a chiller unit.
For both types of immersion cooling, the benefits include a significantly lower total cost of ownership (TCO) compared to data centers that use air or liquid cooling. According to Mordor Intelligence, a market research firm, data centers that use immersion cooling can achieve TCO savings of up to 30%. Immersion cooling also supports a 91% reduction in water consumption and a 39% reduction in carbon emissions. Mordor Intelligence also reports that immersion cooling requires 85% less physical space and can improve the life span of IT hardware by 20%.
PUE also improves when immersion cooling is used. In basic terms, PUE is the ratio of a data center’s total energy consumption to the energy consumption of its IT equipment. The ideal PUE is 1. According to a cooling technology timeline from BIS Research, the PUE for a computer room air handler (CRAH) or computer room air conditioning (CRAC) unit exceeds 1.8. In-row cooling systems have a PUE greater than 1.4, and rear door heat exchangers (RDHx) have a PUE greater than 1.3. Direct-to-chip cooling has a PUE of less than 1.2, but it still doesn’t beat immersion cooling, coming in at just above 1.
In both single- and two-phase immersion cooling systems, the choice of an immersion fluid is especially important. Traditionally, fluorocarbons, hydrocarbons (synthetic oils), and silicone fluids have been used. Each has advantages, but there are now newer coolants for single-phase immersion cooling systems that provide superior heat dissipation for high-power density electronics. These advanced materials also eliminate EHS concerns and have stable material properties over time. Importantly, these new products support comparisons to existing chemistries and have undergone accelerated aging tests.
Fluorocarbons are the most expensive type of immersion coolant and can be difficult to keep at proper levels because of evaporation. Fluorocarbons also raise EHS concerns because of their high global warming potential (GWP) and ozone depletion potential (ODP). Although they are nonflammable and thermally stable, fluorocarbon immersion coolants are also susceptible to leakage. Among their advantages, fluorocarbons are compatible with silicones and hydrocarbons, both of which are used with server electronics. For example, the epoxy resins used in FR-4 printed circuit boards (PCBs) and board or chip-level adhesives both contain hydrocarbons.
Synthetic oils are compatible with silicones but not with other hydrocarbons. These hydrocarbon-based oils cost less than fluorocarbons and resist leaking. They are also relatively easy to maintain and do not raise significant EHS concerns. Synthetic oils are flammable, however, and they lack thermal stability. By comparison, silicone fluids offer advantages in terms of hydrocarbon compatibility, cost, ease of maintenance and low EHS risk. Although they are nonflammable and thermally stable, silicone fluids are incompatible with the silicone materials that are used in server electronics as adhesives, encapsulants, and thermal gels for heat sinks. That is because silicone fluids cause these other silicone materials to swell and change shape and size.
Today, new immersion coolants that use a hybrid silicone and hydrocarbon chemistry are compatible with both silicones and hydrocarbons. These advanced coolants for single-phase immersion cooling systems are easy to maintain, have a low risk of leakage, and provide superior EHS performance. They are also nonflammable, thermally stable, less expensive than fluorocarbons, and comparable in cost to synthetic oils. These hybrid materials may cost more than pure silicone fluids, but they provide a complete solution for chip and PCB thermal management because of their compatibility with a wide range of materials used in servers and server components. Their stable material properties also provide reliable thermal management performance.
As demand for more data centers and greater data center capacity continues to grow, the designers and operators of these facilities have key choices to make. Immersion cooling has a lower PUE than either air or liquid cooling. Two-phase immersion cooling is more energy-efficient than single-phase cooling, but two-phase systems experience a greater loss of fluids and require more maintenance. There are also EHS issues with commonly used coolants for two-phase systems.
By using hybrid silicone-hydrocarbon coolants for single-phase systems, data centers can eliminate these concerns and realize benefits beyond greater energy efficiency. Today, hybrid oils are suitable for edge data centers that are typically located near population centers and are also scalable to hyperscale data centers. With their demonstrated compatibility with commonly used electronics materials, these silicone-hydrocarbon oils can also support the critically important IT infrastructure that is needed to support the industries of tomorrow.
