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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Reducing Data Center Cost Using Air Economizer – Intel

Electricity costs play a major role in the detail of the profit, loss or competence of a Data Center. Reducing them to a minimal amount is the key to an efficient and cost-effective site. Apart from the inevitable consumption of power by the server a major part of the electricity is consumed by the cooling systems. If an effectual way of achieving cooling is possible without the use of an air-conditioning system, the expenditure on cooling can be reduced considerably; here is where an Air-Economiser comes into play.

This approach to cooling has always been frowned upon by most in the industry mainly due to the feeble cooling power provided by prior prototypes of conceptually similar systems. The reliability of the servers can be questioned if proper cooling demands are not met; hence a good design towards 100 percent air cooling has never been successfully tested.

The Air-Economizer:

Typically an air-conditioning unit would draw the hot air generated by the working servers, cool it down to the set temperature and recirculate the same. Outside air would be taken at regular intervals to maintain the balance of fresh air in the Data Center. A revision in this process modifies the air-conditioning unit into an air-economizer. Here, the hot air would be expelled to the outside of the Data Center and fresh, cooler air would be drawn in, continuously or at regular intervals based on the heat generated.

The Experiment:

Intel took up a challenge to defy the common assumptions in the industry about the air-cooling technique by conducting a Proof of Concept (PoC) test. The test aimed at cooling servers at 90 degrees Fahrenheit with 100 percent outside air. Potential annual savings by achieving this could be US$2.87 Million for a 10-megawatt (Mw) plant!

The PoC was run using about 900 production Blade servers divided equally by two compartments place side by side. One was provided with warehouse grade Direct Expansion (DX) unit to recirculate the hot air and provide cooling at all times; and the other an air-economizer. The PoC was run in a dry climate over a time period of 10 months.

This is What Happened:

  • The temperature of the air supply in the economizer compartment varied between 64 and a little beyond 92 degrees Fahrenheit. Cheaper air-conditioning units with reduced response rate used in the PoC may have caused this.
  • Humidity varied between 4 to way over 90 per cent.
  • The exterior and interior of the servers were covered with a layer of dust.
  • Total power consumed was 111.78 kilowatts (KW) as compared to 500 KW for air-conditioning in both compartments. This is a 74 percent decrease in energy consumption!
  • The server failure rate in the economizer compartment was 4.46 percent as compared to the 3.38 percent in the main Data Center. The failure in the DX cooling compartment was 2.26 percent which is lesser than that of the main Data Center itself.

The Savings:

If the site of the Data Center can be located in more temperate conditions, the economizer technique can be used for more than 90 percent of the year. If 90 percent of the year a reduced power plan can be achieved and there is 74 percent savings in this plan approximately 67 percent of the total power used annually can be saved; for a 10MW plant this figure would be about US$2.87 million! Capital can be saved on building new Data Centers with fewer air-conditioning units. Potentially about 76 million gallons of water can be saved by applying the air-economizer technique.

The Catch:

Server failures remain consistent with variations with humidity, temperature and inferior air quality; hence air-economizers are suited only for drier and more temperate climates. The availability of such locations can be scarce. As the location of a Data Center is crucial for business and stable networking setting up sites at these locations if too remote is pointless.

The Cherry!

Intel plans to test the economizer technique and check for hardware degradation by using server aging analysis, they also plan on setting up a 1MW demonstration Data Center to conduct further research on this. The cost-effectiveness of the entire setup promises financial gains as well as a greener approach to Data Center construction; clearly, this is technology of the future.

Reference: Intel IT white paper


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