Figure 1. Diagram of a direct air-side economizer system.


Data centers are the nation’s largest commercial consumers of electric power. Process cooling in these facilities requires more electrical power than the data center equipment because the cooling system must also cool an additional 15 percent to address typical lighting and summer building gains. As a result, data center operators can achieve substantial energy savings by improving the year-round efficiency of data center cooling systems, no matter the location. Fan-power reduction and high-efficiency refrigerants provide much of these savings.

Today many corporations favor siting new data center in cooler climates where maximum energy savings are directly related to the outside temperature. Most, if not all, of these large data centers utilize a central chilled water system, distributed to air handlers, CRAC units, and rack coolers. These tendencies make it important to realize the energy savings possible in large data centers subject to seasonally cold temperatures.

Figure 2. Diagram of water-side economizer system

In the central and northern United States, Canada, and Europe, the outside air temperature is lower than the temperature in the data centers in these areas for significant periods of the year. These low outside air temperatures present the possibility of free-cooling systems that provide cooling without the use of mechanical equipment, thereby saving considerable energy and also extending the equipment life. In these locations winter economizer systems can provide annual energy savings of 30 to 50 percent for cooling.

There are four winter economizer systems: direct air-side economizers, water-side economizers, air-cooled chillers with dry coolers, and packaged air-cooled free-cooling chillers. In all cases, a mechanical refrigeration/chiller system is required for summer operation, but these winter economizer systems all provide data center cooling without refrigeration as the outside temperature falls. The power savings are very impressive.

Direct air-side economizers offer significant savings possibilities and the most free-cooling hours, but these systems come with some complications and risk factors. These systems are as simple as blowing outside air into the data processing center whenever outside temperatures drop below 75 F. What could be simpler? Just turn off the chiller and the air handlers and allow Mother Nature to cool the room (see Figure 1). A 1,000-kilowatt (kW) data center would require approximately 70,000 cubic feet per minute (CFM) of outside air at 30 F, which requires a fan motor of approximately 40 horsepower (hp) (allowing for mixing box, ducting, and filter losses). Typical controls include thermostatically controlled motorized dampers to allow automatic mixing of hot exhaust and cold entering air and VFD control of fan motor(s). Controls that address the need for humidity control, filtration, and security increase the cost and complexity of these systems.

Figure 3. Diagram of air-cooled chiller with dry cooler economizer

Current processor and server designs are less susceptible to humidity issues, but static needs to be eliminated by maintaining the relative humidity (RH) at a reasonable level of 30 to 50 percent. When outside air is heated from 30 F and 40 percent RH to 75 F, RH drops to about 10 percent - unacceptably low in a data center; moisture must be added via humidifiers. Humidifiers require approximately 0.33 kW per pound of steam generated and a constant supply of clean water.

Large volumes of outside air require high-efficiency air filtration because it is relatively dirty and contains contaminants not found in recirculated indoor air. Dust, dirt, pollen, and other contaminants can be filtered from outside air before it can be safely introduced to the data center. Filtration cannot effectively remove smoke, vapors, or fumes, however, so a completely secure area must be established around the air intake to prevent the accidental or intentional introduction of foreign matter. A secured perimeter, a safe distance from the main air intake, is essential for long-term security.

It is possible to utilize direct air-cooling in cooler climates, but operators must guard against disruption of the continuous and uninterrupted supply of city water required for humidifiers and infiltration of airborne contaminants, smoke, fumes, etc., that could disrupt an operation or even precipitate a Halon discharge. (See the whitepaper on airborne pollutants prepared by ASHRAE Technical Committee (TC) 9.9 Mission Critical Facilities, Technology Spaces, and Electronic Equipment.)

These are the main reasons that the direct air economizer system is currently the least-used winter economizer option for large data centers. The operating cost for a 1,000-kW direct air-side winter economizer cooling using a 40-hp fan motor and an estimated 50 kW in humidifier power is approximately 80 kW.

Water-side economizers have been used for many years to reduce winter cooling costs and are probably the most commonly used economizers in large data centers today. These systems employ an indoor water-cooled chiller and an outdoor evaporative cooling tower, with a separate plate heat exchanger, also located indoors. The tower provides recirculated water to the condenser of a chiller, and the chiller provides chilled water to air handlers in the data center. When the outside wet-bulb temperature is low enough, the cold tower water cools the chilled water loop via the plate exchanger, through a system of piping, valves, and controls. A typical tower will be designed for an approach of about 7 F above the wet-bulb temperature and a plate-and-frame heat exchanger for an approach of 10 F or less. Therefore, partial cooling theoretically can commence when the wet-bulb temperature is about 35 F, depending on the equipment design. This allows the chiller compressor to unload or cycle off, thereby saving energy. Water-side economizers require:
  •  A continuous, uninterrupted supply of city make up water for the cooling tower.
  •  A water treatment system for the tower make up water in order to reduce scaling and bacterial growth, with regular attention required for chemicals and testing.
  •  Valuable indoor space for the water-cooled chiller and the plate-and-frame heat exchanger.


Figure 4. Diagram of air-cooled chiller with integrated free cooling

The cooling tower water can become too cold for the condensers in the chillers. In addition, the winter operation change-over can threaten the availability of uninterrupted cooling.

Dependence on continuous city water makeup for the cooling towers constitutes the main drawback to this system, because a mains water break would interrupt water supply. Equipment for a water-side winter economizer for a 1,000 kW system includes a 20-hp fan motor in the tower and 50-hp water pump for a total of 52 kW.

An air-cooled chiller with a dry cooler (radiator) used to pre-cool return chilled water makes an efficient economizer system and blends the best features of the direct air-side economizer with those of a water-side economizer. These systems make use of a chiller and the glycol solution used to prevent freezing required in colder climates

Ambient air pre-cools return chilled glycol solution in a separate dry cooler (radiator) whenever the ambient temperature is lower than the return glycol. While there are fewer free cooling hours available compared to direct air-side cooling systems, there are no comparable humidity, filtration, or security issues. Moreover, the system does not require indoor space, makeup water for humidifiers or cooling towers, or associated water treatment. However, the need for outdoor space can become a consideration.

While the free-cooling savings available from the air-cooled chiller with a dry cooler are considerable, the dry coolers require increased fan power during partial free-cooling operation. This system can be automated by using a thermostatically controlled three-way valve to direct the return chilled glycol through the dry cooler in winter or to by-pass the dry coolers in warmer temperatures. The dry-cooler fans are thermostatically controlled to operate only when needed in winter. A practical design for this system provides 100 percent free cooling when the ambient temperature is approximately 20 F below the chilled glycol supply temperature and 50 percent free cooling at an ambient temperature 10 F below the glycol supply temperature. A 1,000 kW data center system will require two 10-fan dry coolers to lower 750 gallons per minute (GPM) of a 40 percent glycol solution from 55 F to 45 F in 25 F ambient. Assuming 2-hp fan motors, only 30 kW would be required, but partial free cooling can be as high as 60 kW when there is simultaneous operation of chiller fans and separate dry cooler fans in winter.

Figure 5. Photo of chillers outside large data center

Air-cooled free cooling chillers are the most recent development in winter economizers and now are frequently used in larger data centers. This design integrates dry coolersinsidea packaged air-cooled chiller, eliminating the footprint of separate dry coolers. Ambient air first passes over the integrated glycol coils, before the condensers. In warmer temperatures, an automatic three-way valve diverts the return glycol solution around the free-cooling coils and the chiller operates in normal mechanical mode. When the ambient temperature is lower than the return chilled glycol temperature, the three-way valve diverts the glycol through the free cooling coil for pre-cooling before it enters the evaporator. This reduces the load on the compressors, which sequentially unload or cycle off to save energy. When the ambient is 20 F below the design supply chilled water temperature, the compressors are switched off and remain in a standby mode while 100 percent of the data center cooling is provided by the free-cooling coils. VFD control reduces fan power for increased savings. The location of the free-cooling coils in front of the condenser coils elevates the entering condenser air temperature in winter and helps to maintain a stable refrigerant head pressure. A custom PLC controls the entire operation sequence. System advantages include:

1) No make-up water required

2) Lower installation cost

3) Increased reliability of a totally packaged system.

The approximate cost for a 1,000-kW free-cooling chiller in winter economizer mode is 18 3-hp fans or 40 kW. This system combines the best features of all the alternative systems without any of the disadvantages. Free-cooling chillers are air-cooled so there is no water required. The building remains sealed without the risk of airborne pollutants and there is no impact on humidity levels in the conditioned space.

Sidebar: Comparing Different Free-Cooling Systems

A detailed engineering evaluation for each installation will provide the most accurate information on which to base a design decision. The preceding information is intended as a guide and is intentionally general in nature. It does not evaluate the number of free-cooling hours available for each system, location, or the utility power cost, which all vary significantly. The power required to operate the alternative winter economizer systems is based on actual systems currently in use or available for this purpose. These estimates show the approximate differences in energy savings for winter free-cooling operation, combined with maintenance, risk and reliability factors for a mission critical application. Chilled water/glycol temperatures are assumed at 45 F entering and 55 F returning to the chiller plant. Savings are increased in all cases with higher chilled water temperatures and/or lower ambient temperatures. Energy savings are of growing importance in the design requirements for many data processing facilities and can be easily justified by rapid payback - especially free winter cooling systems in seasonally cooler climates, with annual energy savings of 30 to 50 percent. Energy savings do not replace the demand for guaranteed data center uptime, but can be achieved as a parallel design requirement, without compromising reliability or security.