Free cooling—using fresh outdoor air to cool a space—can significantly reduce data center energy consumption by lowering your reliance on mechanical air conditioning. But free cooling isn’t without its own risks, despite the potential reward. These six considerations are key to finding success with free cooling for data centers.
1. Operating Temperatures
What temperatures are you trying to maintain in the space? Not surprisingly, the temperature band your data center can tolerate is an important predictor of free cooling success. ASHRAE has increased its recommended temperatures over the years and now advocates for up to 80 degrees F. for air entering each cabinet or rack.
Part of the switch is due to modern equipment that’s more heat-tolerant than legacy equipment, explains Karl Recktenwald, associate and senior mechanical project engineer for Bala Consulting Engineers. However, if you have a legacy data center with older equipment, you’ll want to abide by the previous guidelines.
“The key to realizing savings is setting your room conditions in your data center,” Recktenwald says. “If you can set higher temperatures, you’ll be able to expand the number of free cooling hours you can enjoy.”
2. Regional Effects
Geographic location plays a prominent role in how well you can utilize free cooling in your data center. Data centers in places like Florida won’t be able to use as much outdoor air because outside is already so hot and humid, but data centers in dry, cool Northwestern states reap considerable benefits because the outdoor air is much cooler than the hot data center.
“The key to realizing savings is setting your room conditions in your data center. If you can set higher temperatures, you’ll be able to expand the number of free cooling hours you can enjoy.” - Karl Recktenwald
3. Particulate Matter
Pulling outside air directly into the space requires careful attention to filtration, Recktenwald says. “Particulate in the room will gunk up the servers. It’s definitely not advised,” he adds. “Studies have shown that systems with air-side economizers have higher levels of particulate in the space, so that’s a consideration.”
Look into your surroundings and the types of particulate matter that are common in your area, such as pollen in rural settings or smog in cities. Filters usually specify the size of particulate matter they can catch, so make sure you’re buying filters that can handle whatever your area throws at them.
Are you using hot aisle or cold aisle containment? The way you set up your data center can impact how much free cooling you can use. Hot aisle containment is better suited for free cooling in both air- and water-side economizer applications, Recktenwald says.
The hot air coming out of the server racks is exhausted into the hot aisle designed to contain it. From there, the hot air is exhausted out of the building by an air-side economizer or passed back through the chilled water coil in a water-side system.
“When the goal is free cooling, we definitely believe hot aisle containment is a better strategy,” Recktenwald says. “The trade-off between the two is ride-through,” Recktenwald explains. “In the hot aisle containment option, you have a room full of cool air. If you have a blip in the system and the air conditioning shuts down, but the server keeps running, you have that mass of cold air to pull from for a couple of minutes while things restart. You don’t have that in a cold aisle arrangement.”
Containing the air streams also lets you increase your air delivery temperature because the hot and cool air isn’t mixing, adds Scott Davis, vice president and mechanical department director for Bala Consulting Engineers. Allowing a higher temperature for the incoming air means you can use your free cooling longer than you could if you were restricting your data center to the coldest possible outdoor air.
“We’re picking when the outside air conditions are right and switching things over while monitoring the conditions in the space,” says Recktenwald. “Having controls set up correctly and calibrated is very important.”
Sensor placement is key for success, Davis says. You want to make sure every space in the data center has the cooling it needs, but without overcooling. “Be wary when you have a mix of high-density and low-density spaces,” Davis explains. “Be aware so you don’t have one driving the whole system, but make sure they’re both satisfied so one area doesn’t overheat.”
Monitoring and adjusting humidity is crucial too, especially with evaporative cooling, Davis adds. Know what your humidity limits are in the space and make sure your control suite is keeping an eye on it for you. Buildings with special considerations might have additional control needs; one recent Bala project involved a data center surrounded by cornfields.
“The client was concerned about field fires and wanted to make sure that if he got the tiniest whiff of smoke, we could shut off the economizer,” Davis says. “We used a traditional fire alarm system as a sort of particulate detector. The system can revert to a full recirculation mode under extreme conditions.”
[Related: Should You Implement an Air-Side Economizer for Your Data Center?]
As with anything else in your building, preventive maintenance is critical. Set up your maintenance schedule so you’re changing filters on the manufacturer’s recommended schedule. Build in maintenance tasks that are specific to the type of economizer you have, such as cleaning the cooling tower for direct air-side systems or the condenser coils for water-side systems.
Controls also periodically need recalibration, so ask your manufacturer what they recommend. “If they’re allowed to drift, you’ll lose your ability to maximize your free cooling hours,” Davis says.
Free cooling may not be an option for every data center. Hot and humid outdoor conditions, inadequate filters and sensitive legacy equipment can all throw a wrench into the process. But when it’s implemented correctly, free cooling via a well-maintained economizer can reduce your need for artificial air conditioning. It’s worth taking a look at the technology to see if you can reap the benefits of free cooling.
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