Energy Savings and Efficiency in Data Centers: Economizer Cooling / by Integrated Design Group

 Digital Realty in Needham, Massachsuetts

Digital Realty in Needham, Massachsuetts

It’s no surprise that cooling methods are one of the most costly and crucial aspects of operating a data center, accounting for approximately 30% of the total energy use of the computer room. As time has gone by, data center designers have been able to invent creative methods of cooling these facilities.


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Renewable energy is among the top emerging technologies being considered by data center owners to help address power and cooling costs, according to a Mortenson survey.

 Any power that goes into the computer equipment comes back out as heat. For years, computer rooms have been cooled through methods such as flooding a raised floor area with chilled air from computer room air handlers (CRAHs) and by various in-row cooling methods in which cold air is introduced, and once warmed, is recycled through an air conditioning unit before it is reintroduced back into the facility. 

This is a costly process, demanding that the air conditioning units are running constantly. Because of the costs associated with these traditional cooling methods, and data center client’s increasing desires for a cheaper, “greener” alternative, many designers are now looking at using economizers for cooling computer rooms. 

Typical Data Center Energy Usage

After operating servers, cooling and HVAC systems share the heaviest load of total data center energy usage.

Even data center owners describe that energy efficiency and cooling are the top most important things to address in regards to how they'd like to change their data centers. In order to institute changes, first we'll have to understand what kinds of options we have available.

There are two specific methods of cooling through the use of economizers, air-side and water side. 

Air-Side Economizer

An air-side economizer brings air from the outside through large ducts and shafts inside the facility for cooling.  It maximizes the cooling system efficiency by utilizing outside air to provide economizer cooling where climate allows.  Because the cooling element is the air outside of the facility, the only cost from utilizing an air-side economizer comes from the fans used to distribute the air, which is far less than manufacturing cool air. Utilizing this method allows for the maximization of the cooling system efficiency by using outdoor air to provide economizer cooling.  This allows the cooling system to operate without the compressor, which uses up the highest portion of energy through traditional cooling methods.

The use of an air-side economizer for cooling in a computer room is theoretically available whenever the outside air (OA) temperature is less than 55D db and within the acceptable enthalpy range.  As opposed to traditional computer rooms, which use CRAHs to allocate the cooled air, the Air-Side Economizer distributes cool air through large Air Handling Units (AHU). The use of an air-side economizer is not necessarily an all or nothing proposition.  This can be used as a supplement to more traditional cooling methods when the OA temperature is above 55F and below the hot aisle temperatures, or during evening hours and certain times of the year when the air outside is at an appropriate temperature to use for cooling the facility.

Water-Side Economizer

Another option for cooling a computer room is the implementation of a Water-Side Economizer. A Water-Side Economizer is a cooling tower condenser water system, which is isolated from the chilled water system by plate and frame heat exchanger, and eliminates the need for compressors.  When the OA conditions permit, the chillers, which work to cool the computer room, are shut off and cooling towers supply water to the system.  These cooling towers are typically larger than the norm, to improve overall economizer operation. 

Starting a chiller tower with very cold condenser water, less that 60F, requires system controls to smoothly transition from economizer to mechanical mode. Heat is eliminated from the chilled water to condenser water at the heat exchanger.  From there, the heat is rejected from this condenser water to outside air at the cooling tower, then after this water has gone back to it’s appropriate temperature, the chilled water supply returns to the air handlers, and the cycle starts a new.  Starting a chiller tower with very cold condenser water, less that 60F, requires system controls to smoothly transition from economizer to mechanical mode.

Drawbacks in Utilizing These Methods

Like all good things, the use of both air and water-side economizers have some setbacks.  When utilizing an air-side economizer, a delicate balance needs to be created in regards to air volume to deter the creation of a “wind tunnel” atmosphere.  In addition, by letting this natural air in from the outside, air-side economizers also allow in humidity and particulates which can be potentially damaging to the equipment in the facility.  To combat these issues, the ducts in which the air enters the facility need to be equipped with filters and moisture control.  Furthermore, air-side economizers demand the installation of large, intricately placed air ducts and mechanical equipment, making it a more appealing option for a new build than a renovation project.

Often times, the process of using economizers for cooling computer rooms is referred to as “Free-Cooling,” this is a misnomer as distribution systems still need to be running to drive the air through the computer room.  However, facilities implementing cooling through the use of economizers do see a significant reduction, over time, in overall computer room energy costs.


The implementation of air-side and/or water-side economizers can directly lead to overall energy savings for the data center owner.  Air-side economizer use is available more hours per year and involves less mechanical equipment to operate.  The water-side economizer is able to be applied in more buildings due to the fact that less duct and air shaft space is required inside the building, thereby demanding less support space in the interior of the facility.