Data centers are massive consumers of energy. In 2014, data centers in the US consumed 70 billion kilowatts of power. A large part of this power consumption is used for cooling, meaning inefficiencies in the cooling system can hurt the bottom lines of data centers.
What's Cooler than Being Cool? Ice Cold.
Initially, when businesses started to establish data centers, cooling was simple. Facility managers could just add air conditioners to the computer rooms to control the temperature. More servers meant adding more air conditioners. Soon, managers realized that there were major inefficiencies in the cooling process. With larger facilities and higher server densities, pumping cold air into the computer rooms wasn’t enough. Facility managers discovered that they could optimize air circulation and equipment placement for better cooling.
The hot and cold aisle configuration is the result of data center optimization. The servers are organized to let the front sides face each other in the cold aisles. The backs of the servers also face each other and the server fans can push the hot air into those aisles. This configuration results in alternate aisles of hot and cold throughout the server room.
Hot and cold aisles make it easier to efficiently cool the environment. Cold air is pumped into the cold aisles through the perforated tiles of the elevated floors. On the hot aisles, the hot air coming out of the servers is pulled by the air conditioners or air handlers.
Various Types of Air Cooling
Modern data centers use multiple types of cooling systems. Computer Room Air Conditioner (CRAC) is the classical tool used for data center cooling. CRAC is similar to the air conditioners used in our homes. These air conditioners use refrigerant-filled coils. Hot air is cooled by passing it over the coils. Generally, CRAC systems can only modulate on and off. More recent units might have more granular control over the airflow.
Nowadays more data centers are using Computer Room Air Handler (CRAH) units. CRAH units work more like the cooling systems used in high-rise buildings. Instead of mechanical refrigeration for heat removal, CRAH units use fans, cooling coils and water-chiller system. Some CRAH units even use outside air temperature. Of course, in that case, the data centers need to be in a region where the outside environment is relatively cold. Because CRAH systems are dependent on the surrounding environment temperature, they use less energy than CRAC's mechanical refrigeration systems.
Data centers are using other techniques in conjunction with CRAC and CRAH. Rear door cooling systems are mounted on the back of the racks and in-row cooling systems are located in between rows of the racks. These systems provide cooling closer to the source of the heat. With these systems, managers have to worry less about unpredictable air circulation patterns. Also, with CRAC and CRAH systems, if more server racks are needed, the whole hot and cold aisle design requires recalibration. But with in-row and rear door cooling systems, managers don't have to worry about the whole system when adding a new server rack column.
Today’s data centers can combine CRAC, CRAH, in-row and rear-door cooling systems to minimize their energy consumption significantly. Also, data centers are responsible for 2% of the global green house gas emissions. So adopting better cooling methods is a positive and environment-friendly step.