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Part 2: Free Air, Water-Cooled Servers Increase Data Center Energy Efficiency
By James McEnteggart
February 2012 -
Leading companies are coming around to the use of new HVAC technologies and operating procedures — air-side economization, evaporative cooling, and operating the data center under a wider range of temperatures.
An air-side economizer intakes outside air into the building when it is easier to cool than the air being returned from the conditioned space and distributes it to the space; exhaust air from the servers is vented outside. Under certain weather conditions, the economizer may mix intake and exhaust air to meet the temperature and humidity requirements of the computer equipment.
Evaporative cooling uses non-refrigerated water to reduce indoor air temperature to the desirable range. Commonly referred to as swamp coolers, evaporative coolers utilize water in direct contact with the air being conditioned. Either the water is sprayed as a fine mist or a wetted medium is used to increase the rate of water evaporation into the air. As the water evaporates, it absorbs heat energy from the air, lowering the temperature of the air as the relative humidity of the air increases.
These systems are very energy efficient as no mechanical cooling is employed. However, the systems do require dry air to work effectively, which limits full application to specific climates. Even the most conservative organizations, such as financial institutions, are beginning to use these types of systems, especially because ASHRAE has broadened the operating-temperature recommendations for data centers. ASHRAE's Technical Committee 9.9 recommendations allow dry-bulb operating temperatures between 64.4 degrees F (18 degrees C) and 80.6 degrees F (27 degrees C), with humidity controlled to keep dew points below 59.0 degrees F (15 degrees C) or 60 percent RH, whichever is lower. This has given even the most reluctant owners a green light to consider these options.
Airside economizers and evaporative cooling systems are difficult to implement in existing data centers because they typically require large HVAC ductwork and a location close to the exterior of the building. In new facilities, these systems increase the capital cost of the facility (i.e., larger building volume), HVAC equipment and ductwork. However, over the course of the lifetime of the facility, these systems significantly reduce operating costs when used in the appropriate climate, ideally, locations with consistent moderate temperatures and low humidity. Even under ideal conditions, the owner of a high-density data center that relies on outside air for cooling must minimize risks associated with environmental events, such as a forest fire generating smoke, and HVAC equipment failures.
Just as water is an effective heat-exchange medium in evaporative cooling systems, it can also be circulated throughout the data center to cool the IT equipment at the cabinet level. In fact, water cooling is far more energy efficient than air cooling. A gallon of water can absorb the same energy per degree of temperature change as 500 cubic feet of air. This yields significant operational savings in typical applications because the circulation of air to remove heat will require 10 times the amount of energy than would be required to move the water to transport the same amount of heat.
However, it is more expensive to install water piping than ductwork. An engineer can provide cost comparisons to provide the owner with the financial insight to make a sound decision when constructing a new facility. It is not usually a feasible retrofit for an existing data center.
Rear-door heat exchangers and integral water cooling are options in existing air-cooled data centers to reduce the energy use and cost associated with cooling. They put the water-cooling power of heat exchangers where they are really needed: on the server racks.
Rear-door heat exchangers are mounted on the back of each server rack. Sealed coils within the heat exchanger circulate chilled water supplied from below the raised floor. Hot air exhausted from the server passes over the coils, transferring the heat to the water and cooling the exhaust air to room temperature before it re-enters the room. The heated water is returned to the chiller plant, where the heat is exhausted from the building. Owners can achieve significant operational savings using these devices. To protect the systems during loss of utility power, many facilities put the pumps for the systems on a dedicated uninterruptible power supply (UPS) system.
Owners have been cautious in adopting this approach due to the risk of leaks. The heat exchanger is equipped with baffles that prevent water spraying into the computer in the rare event of a leak. However, water could still leak onto the floor.
Another alternative is integral cooling, a sort of a "mini AC unit" between the cabinets. This close-coupled system takes the hot air discharged from the servers, cools it immediately and then blows it back to the inlet of the server. The system contains the water within the AC unit itself. The installation can also be designed to drain to a piping system under the floor, and it can incorporate leak detectors.
Part 1: PUE is One of Many Metrics for Data Center Energy Performance
Part 3: UPS Energy Saver Mode, High Voltage DC Power Cut Data Center Energy Use
Part 4: In Data Centers, Simplicity of Design, Commissioning Saves Energy