Liquid Cooling in Data Centers

Introduction

Data center cooling is one of the IT segments that has been earning a lot of attention lately. Data centers are the lifelines of the organizations or businesses they serve. Data centers process trillions of computer-based transactions every day. It is, therefore, critical for enterprises to understand the high-tech in data center design (ASHRAE, 2006). Key aspects such as cooling, networking and electrical power utilization are very critical for effective and efficient operation of the data centers.

As data centers tend to grow, there is an increasing demand for power. Data centers continue to grow in both size and numbers. Consequently, the data centers generate more heat. The heat density is also increased by the presence of state-of-the-art IT equipment inside the data center equipment. According to Hewlett Packard Laboratories (2009), the processor power dissipation has increased by a factor of ten. With the increase in hardware complexity and power, traditional air-based cooling techniques have become less effective in keeping data centers cool. This limitation has forced enterprises and other organizations to look for alternatives. Some of the alternatives include liquid cooling and hybrid cooling systems (Hill, 2009).

Data center cooling is required to withdraw the excessive heat generated by computer components. These components must be kept within allowable operating temperature limits as defined by the manufactures. Some of the computer components that are susceptible to malfunction or failure due to excessive heat include microprocessors, graphics cards, chipset, and hard disk drives (ASHRAE, 2006).

Types of Cooling

Cooling employs three primary modes of heat transfer. That is, radiation, conduction and convection. Conduction is simply the transfer of heat through solids without any displacement (Novotny, 2010). Radiation is the process of transmitting heat through electromagnetic waves or particles and convection, on the other hand, is the transfer of heat via circulatory currents. There are several types of cooling systems in data centers (ASHRAE, 2006). These include the traditional air-cooling, liquid cooling and the hybrid cooling systems. Historically, liquid cooling systems were considerably employed in high-heat mainframe computers. Despite the decrease in power consumption and heat, air-cooling was extensively used as the standard technology (Hill, 2009). With the advancement in liquid cooling concepts, majority of the cooling system vendors turned to liquid cooling (ASHRAE, 2006). Heat density in air-cooled data centers are managed by spreading the dissipated heat load and under-populating racks. Air-based cooling creates complex, costly, and inefficient infrastructure (Miller, 2013).

 

Figure 1: Heat Flow in a typical fan

Liquid Cooling

Enterprises are increasingly assessing and implementing liquid cooling solutions in the data centers to meet numerous challenges of high-density computing and blade servers. Liquid cooling systems employ liquid-based heat exchangers to provide quiet, uniform and effective cooling. A liquid cooling system consists of three major components (Google, 2013). These are reservoir, pump and the heat exchanger. The heat exchanger absorbs heat from equipment and transmits it to the environment by circulating a single-phase liquid. The pump circulates a liquid coolant in the liquid-to-air heat exchanger. In contrast to air-cooling, a single-phase liquid is utilized as the medium for the heat transfer (Novotny, 2010).

Water is commonly used as a coolant in data centers because it is available and has better specific heat capacity than most coolants. Water has a specific heat capacity of 4.18 KJ/ (kgK). Mercury and air have specific heat capacities of 0.14 and 1.0 respectively (ASHRAE, 2006). This means that water can hold heat 4 times better than air and mercury. Of liquids, water conducts heat the fastest after mercury since mercury is a metal in a liquid state.

How Does the Liquid Cooling Work

Liquid cooling systems typically consist of three basic elements: a coolant reservoir, an integrated pump and a heat exchanger. The heat exchanger is also known as a radiator. These components are linked with the connecting tubes (Miller, 2013). The coolant draws heat from the system component through the tubes of the radiator and emits the heat to the atmosphere through radiation. Conduction takes place between the tubes and the components. Heat transfer through convention takes place within the coolant liquid as it circulates (Miller, 2013).

Types of Liquid Cooling

With the advancement in thermodynamic technologies, various types of liquid cooling have emerged in data centers. The first type is Modular liquid cooling units. They are characterized with fully sealed cabinets mounted at the base of a rack. The system also has cooling modules capable of archiving a cooling capacity of 30KW (Hill, 2009). Modular liquid cooling units also employ variable fans. Water flow is subject to real-time heat load within the server rack.

 

Figure 2: Colorful pipes carry water in and out of the data center. The blue pipes supply cold water and the red pipes return the warm water back to be cooled.

Device-Mounted Liquid Cooling systems are designed to work at the device level. The liquid coolant moves through sealed heat plates on the top of a microprocessor. As the dialectic coolant absorbs heat from the device, it vaporizes and then emits the heat into the hot isle. For instance, Rack CDU (Rack Coolant Distribution Unit) transfers the coolant directly to the hottest components inside a server in a data center, thereby saving more than 50 percent in terms of energy and cooling costs (ASHRAE, 2006).

Integrated rack-based liquid cooling systems incorporate a rack-based architecture, which integrates power distribution, uninterruptible power supply unit and cooling distribution unit (CDU). The CDU routes water through coiled aluminum or plastic conduits to cool servers (ASHRAE, 2006).

Figure 3 Plastic curtains in the network room prevent hot air behind the server racks from mixing with colder air in front of the server racks.

Door Units liquid cooling systems are characterized by full-door units that replace the typical server rack door.  These full-door units contain sealed tubes filled with cold water. Lastly, there are Liquid Cooling/Heat Exchangers designed for hot spots (Google, 2013). These systems employ heat exchangers supplied with liquid refrigerant or chilled water. Cool air directed into the hot spots of the data center makes CRAC units in the room work.

When implementing liquid based cooling systems, datacenter managers must consider factors such as condensation, temperature and room humidity (Hill, 2009). Other concerns include the coolant or refrigerant tubing corrosion-related issues. In scenarios when water is used as a coolant, it is recommended that water is distilled or de-ionized (ASHRAE, 2006). The water can be as well treated with reverse osmosis (ASHRAE, 2006). It is also important to take into consideration current and projected heat loads.

Benefits of Liquid Cooling

Liquid cooling systems are much more efficient in transferring heat away from the microprocessor and outside of the system (Novotny, 2010). This is because the heat absorption rates and capacities of liquids such as water and alcohols are better than that of air. The heat retention capacity of water is four times greater than that of air. This allows over-clocking at the ambient temperatures. That is, processors can continue to function optimally despite the massive heat generation because of the potential parallel or multiple threads and processes (Hill, 2009).

The implementation of complex liquid cooling systems can achieve almost double the processor speed (Novotny, 2010). Liquid coolants can draw and transfer heat more efficiently away from almost all parts of a data center, consequently maintaining a sustainable operating environment within the data center. Water-cooling reduces power consumption in data centers significantly (HP, 2009).

The second benefit of liquid cooling is the reduction of noise within the computer (Novotny, 2010). Traditional heat sinks and fan arrangements often generate a lot of noise as the fan blades rotate to circulate cool air over the microprocessor and through the whole computer system. Research indicates that high performance processors require higher fan speeds that generate noise of over 60 decibels (Miller, 2013). When a CPU over-clocks, fans tend to pick up thereby generating more noise. The flow of the coolant within the cooling tubes creates less noise. Two common fans within a liquid cooling system normally run at slower speeds hence minimizing noise (Hill, 2009).

Liquid cooling systems can permit targeted cooling which is difficult to achieve through the normal air cooling method (Novotny, 2010). For example, high-density cabinets or hot spots can be categorically cooled to minimize power consumption. Even though liquid cooling calls for additional computer hardware, it gives a more efficient cooling solution to any data center. Overall, liquid-cooling systems provide a range of benefits for either enterprises or data centers (Hill, 2009).

Disadvantages of Using a Liquid Cooling System

Liquid cooling systems require extra physical computer hardware space and more space in a data center for them to work effectively. Some of the components that need extra space include tubes, fluid reservoir, impeller, fans and power supply units (Novotny, 2010). Liquid cooling also needs some considerable technical background in computer hardware given that it is a new technology. There are various vendors with different standards leading to hardware compatibility issues.  In the event that the cooling systems leaks, data center owners might incur numerous losses (Miller, 2013). The coolant, especially if it is water, might damage the internal components of the system. Due to the complexity of the installations and configurations for liquid cooling systems, they are not recommended for general system use.

Emerging Trends in Data Centers and Liquid Cooling

Two decade ago processors consumed a small amount of power due to relatively less number of transistors packed in a processor and it did not even require multiple fans (Google, 2013). With the advancement in technology the size of the transistor is diminishing and processors are packed with so many transistors to give them the desired processing power (HP, 2009).

There are several emerging trends within this area of computing. Some of the trends are smart and green data center concepts (Novotny, 2010). According to Hewlett Packard Laboratories, the processor power dissipation has increased by a multiple of ten. This translates to higher energy consumption and dissipation of heat. Driven by these observations, it is economical to come up with technologies that mitigate excessive power consumption and massive heat losses within data centers (Miller, 2013). Smart cooling can be achieved through metrology, computer modeling, and intelligent control of air conditioning systems (HP, 2009). Hewlett Packard Laboratories works on low-grade energy and alternative sources of energy to power a data center as a mechanism of going green. One aspect of smart data centers is efficient cooling. Most data centers around the globe are embracing liquid-based cooling solutions to limit the application of mercury within the data center infrastructure. Mercury is poisonous and has several health side effects (Novotny, 2010).

As of 9 April 2013, liquid cooling was the prevailing evolutionary phase of advanced computer cooling systems. Major processor and computer peripheral vendors were innovating water-based cooling system concepts (Novotny, 2010). Researchers in the field of sustainable energy argued that power needed to cool key data centers owned by giants like Google, Oracle and Facebook could be minimized by more than 90 % by implementing liquid cooling technology (Google, 2013). According to a survey by Emerson Network Power, power consumption was highlighted as the primary element that limits data center capacity followed by cooling.

Another new trend in data center cooling venture is the introduction of waterside and airside economizers to facilitate free cooling in situations where external temperature permits. According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), modern data centers have increased their internal ambient temperature permitted levels. This implies that external air could be beneficial for cooling. This can be achieved either through waterside economizers or through direct airside economizers (Novotny, 2010). The increasing high cooling costs are forcing IT giants to try alternative energy-efficient technologies. For example, Facebook’s new data center in Forest City employs free cooling to keep its servers cool. Microsoft’s data center in Dublin relies entirely on free-air cooling approach to cut cooling costs.

One of the projected trends is the introduction of measures to cut operational costs in terms of power consumption in data centers, as well as, reducing the physical space. IT giants like IBM, Microsoft, Cisco and Oracle have embarked on greening their data centers (Google, 2013). This trend is directed at maintaining market share and corporate culture, as well as, to save real money. Green data center is a concept of operating eco-friendly data centers. It involves aspects such as establishing maximum efficiency out of lighting, electrical, mechanical, and computer systems (HP, 2009). The bottom-line of the concept is keeping the environmental impact of data centers to the minimum. Some of the strategies and technologies used to bring together and run a green data center include sustainable landscaping, waste recycling, catalytic converters and alternative energy sources (Hill, 2009).

Conclusion

 Since liquid cooling technology is still not fully developed, it should undergo further refinement to minimize the physical space required during installations and the energy consumed in the cooling process. Research and Development investments should also be directed at this field to come up with sustainable data center cooling technologies. There is also a need to balance the ever-increasing processor speeds with new thermal breakthrough to minimize excessive energy utilization. Liquid cooling is likely to become more acceptable standard of cooling system construction. This is feasible if the hardware vendors lay down common standards that would enhance interoperability or compatibility across various hardware platforms. The liquid cooling systems should also generate minimal noise for them to gain wide usage across the globe. Liquid cooling might be beneficial to people interested in CPU overclocking.

Traditional data center operations have been directed at meeting customer needs. With the increasing growth of cloud computing and data warehousing, efficiency of the centers will outline the performance and productivity. Enterprises must focus on efficiency given that infrastructural costs have surpassed the cost of computer hardware. Efficiency in data centers is achievable through focusing on key data center cooling since it is one of the critical infrastructural costs. Targeted liquid cooling substantially reduces the energy consumed during the cooling process. Data center cooling can be optimized by implementing hybrid-cooling techniques. To summarize, liquid cooling can be implemented modularly hence making it possible to focus on other data center elements as well.

 

 

 

 

 

 

 

 

 

 

 

 

References

ASHRAE. (2006). Liquid cooling guidelines for datacom equipment centers. Atlanta, GA.: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Google. (2013). Water and cooling. Retrieved April 9, 2013 from http://www.google.com/about/datacenters/efficiency/internal/#water-and-cooling

Hill, M. D. (2009). The datacenter as a computer:An Introduction to the design of warehouse-scale machines. Madison: Morgan & Claypool Publishers.

HP. (2009). Smart cooling. Retrieved April 9, 2013 from http://www.hpl.hp.com/research/dca/smart_cooling/

Miller, R. (2013, March 4). The iceotope liquid cooling system in action. Retrieved April 9, 2013 from http://www.datacenterknowledge.com/archives/2013/03/04/iceotope-liquid-cooling-in-action/

Novotny, S. (2010, July 2). The advantages of liquid cooling. Retrieved April 9, 2013 from http://www.datacenterknowledge.com/archives/2010/07/02/the-advantages-of-liquid-cooling/