Raise Data Center Temperature and Reduce Emissions

Data centers are projected to draw one-fifth of global power and contribute up to 5% of carbon emissions by 2025. Given that the Asia Pacific data center and hosting services industry is projected to be a USD 32 billion market in three years’ time, Johnson Controls Director, data center vertical market (APAC) Kevin Wee discusses data center energy usage and makes the case for raising the temperature to save consumption, and carbon output.

By Kevin Wee

The issue of cooling and heat management is a complex matter for data centers operators. Industry practices have kept data centers running at temperatures between 20 to 25 degrees Celsius, and at 50% to 60% relative ambient humidity levels. Nearly 40% of the energy used by local data centers goes into cooling IT systems.

Our tropical climate has made cooling data centers, at the prescribed temperature and humidity range, more energy-intensive compared to other locations in the world. Research suggests that the industry would account for 12% of our total annual electricity consumption in a decade’s time due to more data centers setting up in the city-state.

Clearly, we need to strike a balance between pushing for sectoral growth with an aim to achieving nation-wide net-zero emissions.

Renewable energy sources such as solar could be considered, albeit a challenge in our land-scarce country. Building taller data centers could be another option, although concerns about reinforcement of floors to support the concentration of heavy ICT equipment have to be addressed.

Pushing the hot button

It’s time that we challenge conventional norms: Do we need to keep data centers in tropical climates operating at a low of 20°C?

Evidence supports the viability of hotter data centers. Server racks are now better designed to sustain operation at higher temperatures. The US-based industry group for heating and cooling professionals, ASHRAE, has revised its guidelines in favor of warmer server farms. Even Singapore’s IDA has initialed a tropical data center trial in 2016 in which environmental temperatures would be tested to a high of 38°C and ambient humidity at more than 90 percent or more.

However, there are significant resistance at adopting the practice. Opponents include historical inertia stemming from the “if it’s not broken, don’t fix it” mentality. Most data center hardware providers do not warranty their equipment at the proposed higher temperatures. Some older facilities may have legacy cooling systems that are not designed to cope with temperature increases.

Johnson Control’s internal research shows a positive impact a hotter facility has on power savings. Every 1°C increase in chilled water temperature brings about 2-3% savings in power consumption with a typical chiller; and the savings increases to 4-5% using variable speed drive chillers.

In a typical building application, the set point of chilled water temperature is determined by the sensible and latent load of the building. A certain minimum chilled water temperature is required to remove the moisture (latent load). The typical temperature of chilled water supply is set at 6.7°C based on the ratio of sensible and latent load of building.

Two aspects of cooling need to be considered. Firstly, the work done by the chiller compressor is inversely related to the temperature of the chilled water supply. In other words, the work done is reduced as the chilled water temperature is increased. At the same time, the power consumed by a chiller is directly proportional to the work done by compressor, or the “lift” of the compressor. This means that at a higher chilled water temperature, the higher evaporator pressure leads to lower lift for the compressor and consequently, lower power consumption overall.

The chilled water supply temperature in a chiller is determined by the supply air temperature required to meet the pre-set room temperature. Based on the warmer data center scenario, the typical temperature readings could now be as follows:

pre-set room temperature at 26-27°C
pre-set supply air temperature at 20-22°C
pre-set chilled water temperature to air handler units (AHU) at 18-20°C

The biggest change is in the pre-set supply water temperature. It can now be raised from a typical low of 7°C to between 18-20°C — which represents an increase of 11-13°C in the temperature of water supplied to the chiller. This translates to a 40-45% reduction of chiller energy consumption.

As an illustration, a typical water-cooled chiller consumes 0.6KW/RT to supply 7°C chilled water. Following the change, the chiller would now be consuming 0.35 KW/RT to deliver 18°C chilled water supply — thereby achieving a savings of 45%.

Beware of heat traps

No doubt chillers can function more efficiently at a higher chilled water supply temperature. The intent, thus, should be to increase this water temperature to the maximum possible.

However, some issues need to be highlighted. For instance, striking a balance between shaving chiller power consumption and setting off an increase in air handler unit power. The difference between room temperature and supply air temperature reduces as the chilled water supply temperature is increased to beyond 18°C. When this happens, a higher air quantity is required to meet the overall heat load, which leads to an increase in power consumption of the air handler unit.

Safety margin for power failure mode needs to be addressed, too. Since server load is highly concentrated, any disruptions to the power supply and room cooling may result in a rapid increase in the room temperature. To avoid such abnormal fluctuation in the server room temperature, it is a common practice, when designing a data center, to include a safety margin of 1°C to 2°C in room temperature.

Not all chillers are designed to operate at the higher chilled water temperature of 16°C due to issues such as oil return and motor cooling. Even for a chiller that is touted to operate at 20°C leaving chilled water temperature, it is done via the “artificial” increasing of the compressor head (lift) by limiting cooling tower temperature, thereby increasing the chiller power consumption.

Specialized chillers, on the other hand, are designed to operate at temperatures above 20°C, without any limits on the temperature of the cooling water. For instance, Johnson Controls YORK magnetic bearing chillers can even operate in inverted scenario, that is, when the cooling tower temperature is lower than the chilled water supply temperature. Using magnetic levitation technology in its driveline to spin without friction, the chiller offers a quieter, more efficient operation, and is available in variable speed drive.

Based on our experience, any small improvements to the running of data centers will result in major impacts. Running a warmer data center will result in significant energy savings and reduce the carbon footprint of this industry.

And that’s a sustainability goal within our grasp.