The best cooling solutions!

Heat generated by datacenters is ten times greater than the heat generated by them around 10 years back

A study by Frost & Sullivan’s Gautham Gnanajothi

Datacenter technology has arrived to a point of no return in the recent times. The servers used in them have evolved and have reduced in physical size but have increased in performance levels. The trouble with this fact is that it has considerably increased their power consumption and heat densities. Thus, the heat generated by the servers in datacenters is currently ten times greater than the heat generated by them around 10 years back; as a result, the traditional computer room air conditioning (CRAC) systems have become overloaded. Hence, new strategies and innovative cooling solutions must be implemented to match the high-density equipment. The rack level power density increase has resulted in the rise of thermal management challenges over the past few years. Reliable datacenter operations are disrupted by hot spots created by such high-density equipment.

Emerson's global data center (St Louis MO US) uses the latest energy-efficiency technologies, precision cooling products, and efficiency strategies.

Some of the main datacenter challenges faced in the current scenario are adaptability, scalability, availability, life cycle costs, and maintenance. Flexibility and scalability are the two most important aspects any cooling solution must possess; this, combined with redundant cooling features, will deliver optimum performance. The two main datacenter cooling challenges are airflow challenge and space challenge. These challenges can be overcome with the use of innovative cooling solutions. Some of the cooling techniques used in datacenters are discussed below.

Aisle Containment

Aisle containment strategies have gained immense popularity among data center operators in the past and this trend is expected to continue in the future as well.  With the use of hot aisle and cold aisle containment, energy-efficient best practice in server rooms can be achieved. Usage of hot aisle or cold aisle depends uniquely on the type of application used. Most data centers have a standard hot aisle/cold aisle layout: the aisle containments are the refinements of these layouts. In these layouts each successive aisle is labeled either hot aisle or cold aisle. In the hot aisle, the banks of the server rack exhausts hot air. In a cold aisle, the server racks are aligned in such a way that the equipment inlets face each other in the opposite sides. There is usually a raised floor system known as the “plenum” under which the cool air from the CRAC or the computer room air handler (CRAH) flows to the perforated floor tiles. These floor tiles are located in the cold aisles and facilitate the cool air into the server inlets in front of the racks and exhaust via the hot aisle. By the hot aisle/cold aisle containment, the cool air can be directed closer to the server inlets; thereby increasing the latter’s energy efficiency.

Rows of server racks at the computer center at CERN in Switzerland. (Pic CERN)

However, there are a couple of challenges faced by the aisle containment solution  the first one being “Bypass Air”; this situation arises when the cool air refuses to enter the server. The other one is “recirculation” where the heated exhaust air flows back into the cold aisle through empty space or over the top of the racks. These two conditions are known for creating hot spots in the server rooms. Data center operators use sheets made of plastic, cardboard, and so on to make barriers for the cold aisles so that the hot air does not re-enter the cold aisle.

High-density Supplemental Cooling
Data center densities have increased from 2 to 3 kW per rack to an excess of 30 kW per rack. A different cooling approach needs to be implemented to meet the high-density requirements. This is when supplemental cooling comes into place. It uses two different approaches: “rear door heat exchangers” and “over head heat exchangers”. Rear door heat exchangers come to the rescue of the struggling CRAC by conditioning the hot air and returning it to the room at colder temperature. They require a chilled water source and a connection to a remote chiller unit. The over head heat exchangers, as the name suggests, are suspended above the server rows. They compliment the hot aisle/cold aisle containment by sucking the hot air from the hot aisle exhaust, condition it, and send cool air to the cold aisles. The supplemental cooling reduces the pressure off the CRAC unit.

Liquid Cooling

The Aurora supercomputer from Eurotech, which uses liquid cooling.

With the rise in the number of applications and services that require high-density configurations, liquid cooling is generating a lot of interest among data center operators. As the name suggests, it brings the liquid (either chilled water or refrigirant) closer to the heat source for a more effective cooling. On contrary to a CRAC unit where it is isolated to a corner of the room, liquid cooling solutions are embedded in row of server racks or suspended from the ceiling or installed in a closed relationship with one or more server racks. There are two types of the liquid cooling – “in row liquid cooling” and “in rack liquid cooling”; both of them require chilled water (or refrigirant) and return piping. It is run either overhead or beneath the floor to each individual cooler.

Closed Couple Cooling
Another remedy for the high-density computing would be closed couple cooling where the distant air conditioner is moved closer to the computing load. The latest generation cooling products can be described by the term closed coupled cooling. Although their solutions vary in terms of configuration and capacity, their approach is the same. It brings the heat transfer closest to the source, which is the server rack. By doing so, the inlet air is delivered more precicely and the exhaust air is captured efficiently. There are two configurations in which it operates – “closed loop” and “open loop”. In the open loop configuration, the air stream will tend to interact with the room environment to an extent. However, closed loop configuration is completely independent of the room in which it is installed. It creates a micro climate within the enclosure because the rack and the heat exchanger work exclusively with one another.

The present high-density computing data center has thousands of racks each with multiple computing units. The parts of the computing units include multiple microprocessors – each one dissipates about 250 W of power. The heat dissipation from the racks containing such computing units excedes 10 KW. Assuming that the present data centers have 1,000 racks and more than 30,000 square feet, these would require 10 MW of power for the computing infrasructure. The future datacenters, which would be even bigger with more servers, would have greater power requirements and, hence, more energy-efficient and innovative cooling solutions.

There are a number of cooling solutions available in the market place, however there is not one particular cooling solution which would be suitable for all kinds of data center applications. They are often dependant on many factors like room layout, installation densities and geographic location. On the whole when we compare the different cooling solutions, it can be said that the liquid cooling technique is proving itself as an efficient and effective cooling solution for high density data centers because it brings the cooling liquid closer to the heat source for a more effective cooling. This type of cooling solutions are gaining popularity among the data center operators as the units are embedded in rows of server racks and do not take up floor space, they are also retrofit friendly which means that the data center can stay operational as the units are brought online. Data centers would benefit by using liquid cooling solutions for their high density servers and this would be the best way forward.

Heat generated by the servers in datacenters is currently ten times greater than the heat generated by them around 10 years back

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