Air Economizers Fundamentals
An air-economizer is ideal for a situation where cooling can be optimized without using an air conditioning system, thereby significantly reducing the energy expenditures from cooling by more than 60 percent. The goal of an air-side economizer is to cool the data center servers by taking advantage of the cool outside air.
This involves a phenomenon of mechanical cooling, depending on the source, which is estimated to consume around 33% – 40% of a facility’s incoming electricity. The outside air is brought into the building at regular intervals and distributed through a set of dampers and fans, to maintain the balance of natural air inside the data center. The servers, intake the cool air, transfer heat, and eject hot air into the room. The hot air generated by the data servers is sent out through an exhaust. If the outside air is cold, the economizer will mix the inlet and exhaust air, ensuring the resulting air cools down to the set temperature, and is recirculated at an optimal temperature for the working conditions.
The air-side economizer is normally integrated into a central air handling system with ducts for intake and exhaust of air. The setup comprises of filters to reduce the amount of infiltrating particulate matter or contaminants into the data center space. Because the data centers must be cooled 24/7, 365 days per year, the economizers even make sense in hot climates, where they can take advantage of cooler evenings or winter air temperatures.
In dry climate conditions, the controls should include redundant outdoor air humidity sensors to stop economization when the absolute humidity is too low. This will prevent unnecessary large, and expensive, humidification loads on very dry cold days.
Using a cold aisle / hot aisle partition-defined arrangement would considerably increase economizer savings, and in some cases creates a de facto heat exhaust which will save energy even when outside air temperatures are greater than 80 F. In case of small data centers located in mixed-use buildings, some energy savings may be brought about by maximizing the utilization of a house, office or support area system that is equipped with an economizer.
Over-specifying the space temperature and humidity tolerance would result in drastic reduction in economization savings, which at all costs should be avoided. Especially with respect to humidity, actual manufacturer requirements must be followed where 40 to 55 percent of the operating region is often found to be conservative.
The key objective of economizers is for all data center air handlers to have access to 100 percent outside air as well as return air. Apart from annual energy cost savings, there are other benefits such as lower run hours on cooling equipment and improved system redundancy. For good performance of air-side economizers, control systems are very important, and they need to be properly maintained. With proper attention to a few key design issues, economization offers even larger saving benefits to data centers.
An outdoor economizing system is best implemented at the starting stage of schematic design, where any essential architectural accommodations can be made at little or no additional cost. While this is typically easiest with a central air handling system, several Computer Room Air Conditioner (CRAC) manufacturers now coming up with economization packages.
Data centers in very temperate climates with no concern regarding space humidity control could use a standard economizer controls, which operates based only on the dry bulb temperature without considering the humidity factor.
However it is far more common that we encounter variations in humidity. We fix a minimum humidity set point in a data center, which is typically the most critical control parameter influencing the savings from economization. In practice, many large data center facilities have a minimum humidity set point of 30 percent RH without causing any harm to the device. The actual requirements of the installed computer equipment should be evaluated by setting the data center humidity control band, and a minimum humidity higher than the equipment’s minimum requirement.
Of course, nothing is perfect. The air-economizer approach for cooling has mostly been looked down upon because of its comparatively weak cooling power provided by the initial prototypes and conceptual systems. The reliability of the servers are under threat when the proper cooling demands are not fully met, and thus a good data center design will take that into account when considering air-economizers. The issues with server failures, the variations with humidity and temperature, and the inferior air quality are a few other drawbacks of air economizers. These can be countered with proper location choices with drier and more temperate climates. However, the availability of these optimal conditions can be hard to locate, because if the location is remote, then the data center cannot be used at an optimal efficiency.
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