Data Center Cooling: How to Reduce Chiller Usage and Save Energy
Cooling has a major impact on the energy footprint of data centers. Often times, a large portion of this energy is used in the large chiller units that provide the low temperature coolant to keep data center electronics at safe operational temperatures. But what if there was a way to operate chillers less often? What if they could be eliminated entirely? This post discusses how better cooling can enable lower energy usage in data centers by way of reduced dependency on chiller infrastructure.
To begin, it helps to understand why chillers are even required in the first place. Most data center electronics operate around 80°C to allow for long lifetime and high-performance operation. To keep these electronics cool, the coolant must be well below 80°C to effectively take the heat away. For example, many data centers must chill the coolant down to 25°C in order to sustain the device temperature at or below 80°C.
Therefore, in this example, it is a requirement of the data center to continuously provide coolant at 25°C to sustain device temperatures. How does it do this? In an ideal world, it would simply dump the heat to the surrounding atmosphere using an ambient heat exchanger (e.g. thermosiphons, finned heat exchangers, cooling towers, etc.). This has many advantages: it requires minimal infrastructure, requires low energy input, and avoids environmentally unfriendly materials or refrigerants. However, often times the surrounding air temperature is too high to use an ambient heat exchanger, requiring an alternative cooling technique.
This is where the chiller enters. The chiller employs a refrigeration cycle to compress and expand a refrigerant to achieve sub-ambient temperatures in the coolant. While this is a very useful mechanism, powering the refrigeration cycle requires high energy usage, which calls for a less sustainable, more costly data center. Therefore, the goal is to minimize the amount of time the chiller needs to be used.
Let’s take a look at an example of a daily diurnal cycle for a day in Albuquerque, New Mexico, to illustrate when chillers need to be used:
Figure 1: Example diurnal cycle for a day in Albuquerque, NM. From around 10AM-10PM, the outside air is warmer than the required coolant temperature for keeping electronics at safe operating temperatures. Thus, the energy intensive chiller infrastructure is used for about half of the day.
As can be seen in Figure 1, the chiller needs to be operated for about half of the day. What steps can be taken to reduce the time the chiller is on? The ambient temperature is not under an operator’s control, though there are seasonal variations in many areas that allow more efficient operation during colder times of year. Therefore, the only other option is to raise the coolant temperature.
At first glance, this seems problematic; with all else equal, if the coolant temperature is raised, the device temperature will also increase, thereby causing potentially unsafe device operating temperatures. But, if a more effective device-level cooling mechanism is implemented, the coolant temperature can safely be raised. Let’s take a look at what this may look like:
Figure 2: Example diurnal cycle for a summer day in Albuquerque, NM, with an improved cooling method. With more effective cooling, the coolant can be operated at a higher temperature while still maintaining safe device temperatures. This requires the chiller infrastructure to be used less and therefore saves energy.
With the improved cooling technique, the chiller only needs to be operated for 4-5 hours each day, allowing for significant energy savings. Thus, implementing a cooling technology with the lowest thermal resistance possible is a key consideration for data center operators looking to reduce their PUE and lower operational costs.
Chiller units are used to provide sub-ambient temperature coolant to data centers to ensure safe operation of electronics. Because they require a refrigerant loop to achieve the necessary coolant temperature, they have a high energy usage. By employing more effective cooling techniques, the coolant temperature can be elevated, and chillers can be used less often or even replaced entirely.
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