Data Centers Cooling
Data center industry is rapidly growing, with ever greater focus on faster connections and increasing uptime. Cloud data center traffic will grow every year, and the need for greater online storage will drive server capacities higher and higher.
The worldwide energy consumption of data centers increased nearly 56 % between 2005 and 2010, and reached 237 terawatt hours (TWh) in 2010, accounting for about 1.3% of the world’s electricity usage . Cooling systems (primarily air conditioners) in data centers account for a large part of this energy consumption: in 2009, about 40% of the energy consumed by data centers was for cooling purposes [2,3].
A common cooling method for data centers is the use of computer room air-conditioners (CRAC), these units have several configurations:
- Aisle Coolers:
which supply cold air to the IT equipment racks through a raised floor. The air flows across the IT equipment and then removes dissipated heat from the back of the rack. In order to avoid mixing hot and cold air, and thus reduce the cooling efficiency, the typical practice is to arrange alternating rack rows of “hot aisles” and “cold aisles.” Since hot air is lighter than cold air, the hot exhaust air from the IT equipment rises and recirculates into the CRAC, where it is cooled and supplied into the racks again.
- In-rack coolers:
In-Rack is the most precise cooling available, as the rack and the air conditioner operate in a closed relationship with one another. Cold air has no choice but to pass through the servers; hot air has no choice but to pass through the heat exchanger. The airflow paths are small, requiring less fan energy. In addition, the exhaust air is captured at its hottest point, maximizing the temperature difference on the cooling coil.
Highly Efficient Cooling Systems:
Cooling systems consumes roughly 40 % of the overall energy consumption in data centers, so there are a lot of opportunities to reduce the power by using energy saving cooling systems. 
Data centers usually have a tendency to overcool to prevent equipment downtime and maintain an operating environment of about 20 °C and 50 % RH. There are some “smart” or “adaptive” cooling solutions that allow for dynamic modification of the data center cooling air flow and temperature set points based on heat load monitoring throughout the data center. These methods save excess energy consumption due to overcooling and also prevent the formation of hot spots. 
- Free Air Cooling (FAC):
Free air cooling (FAC) is one of the simple and most promising methods to reduce the energy consumption for cooling. FAC uses air outside data centers to cool equipment directly (under prescribed temperature and humidity levels). When the outside air is cooler than the return air, an airside economizer exhausts the hot return air and replaces it with cooler, filtered outside air, essentially “opening the windows” to cool the data center equipment.
To solve the indoor air quality problem of the aforementioned system, heat exchangers can be added between the indoor and outdoor airs. Therein, rotating wheels are widely used. The following schematic gives the principle of rotating wheel heat exchangers where the wheel keeps rotating at a speed of 10-12 RPM and the airs flows into different paths to avoid mixing. Then the indoor air flows back to the data center for space cooling after heat exchange whereas the heated outdoor air is exhausted. So in order to guarantee a reliable refrigeration system, electrical cooling equipment is often integrated with the rotating wheel heat exchanger.
FAC has been investigated by companies including Intel, Google, Microsoft, and Vodafone.
Intel conducted a 10-month test to evaluate the impact of using only outside air via FAC to cool a high-density data center in New Mexico 2007. The center had 900 heavily utilized production servers. In this test, the system provided 100 % air exchange with a temperature variation in the supply air from 18 °C to more than 32 °C, no humidity control (4–90 % RH), and minimal air filtration. The results showed that about $2.87 million (a 67 % savings in total energy costs) was saved by the new cooling method. 
- Water Free Cooling (WFC):
the main difference between water-based free cooling system and traditional air conditioning system is that a heat exchanger is installed in parallel with electrical chiller to make full use of free cooling capacities using cooling tower or underground water to exchange heat with the water used to cool the data center. In other words, according to climatic conditions (especially the wet bulb temperature), the whole system can work under three different modes:
(a) when the outdoor temperature is low (winter), the cooling water can be used to produce chilled water directly through the heat exchanger and the chiller can be turned off, so that the system works under “free cooling” mode;
(b) when the outdoor temperature is high (summer), the chiller is activated instead while the cooling tower is only used to handle the condensation heat, so that the system works under “electrical cooling” mode;
(c) when the outdoor temperature is moderate (spring and fall), the chiller and heat exchanger work together in parallel, so the system works under “free cooling + electrical cooling” mode. Therefore, the working conditions of the water-based free cooling system are greatly impacted by the ambient temperature variation. 
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