There are over 3 million data centers in the U.S., accounting for about 2 percent of total U.S. electricity usage (Department of Energy). Cooling systems take approximately 30-40 percent of that electricity, so the impact cooling systems have on controlling energy consumption is striking. With the amount of data centers forecast to grow exponentially to keep up with our increasing reliance on data and cloud technologies, reducing data center energy usage will be essential to achieving our climate goals.
Because of the scale, the 24/7 operation, and other factors, small changes in efficiency improvements in data center cooling have an over-proportional effect on decarbonization and climate change. A sustainable data center is one that actively works to reduce power usage, water consumption, and carbon emissions. Lowering power usage effectiveness (PUE) which measures the ratio of the total power coming into the data center to the power consumed by the information technology equipment (ITE), as well as water usage effectiveness (WUE), carbon usage effectiveness (CUE) and increasing energy reuse effectiveness (ERE), are key to meeting sustainability and net-zero carbon targets.
The adoption of highly efficient chillers, fans, pumps, chilled water hydronic loops, coolant distribution units and other such cooling technologies is key in reducing electrical energy consumption and thereby defining and measuring a sustainable data center.
Increasing Efficiency with Variable Speed Technology
Data centers are running processes 24/7 so uptime and operational efficiency are paramount. Outside of ITE, much of the cooling in a data center is achieved by using rotating equipment and machinery such as refrigerant compressors, fans and pumps. These devices need to be driven by efficient motors connected to variable speed drives (VSD) that match the cooling output to the computing demand while at the same time saving energy by providing the exact power needed by the motors at any moment.
The effect the VSDs have on energy consumption is huge. Chillers with oil-free, magnetic bearing VSD compressors are some of the most efficient pieces of equipment used in data centers because they offer efficient heat transfer and reliable cooling throughout their life cycle without the maintenance requirements and performance degradation over time experienced with other types of compressors.
Reducing Energy Use During Peak Demand
In peak demand periods, strategies to reduce energy consumption in data centers include supplementing energy needs by adding or expanding battery energy storage systems (BESS) to the renewable energy (wind, solar, other) sources already being used, as an offset to carbon emissions. Other options are floating data centers that use the surrounding water for cooling, submerged data centers, data centers in abandoned salt mines, building in cold climate zones and using free or less cooling while allowing the servers to run hotter. Finally, microgrids and fuel cells are popping up in a few locations and small nuclear reactors (SNRs) are on the horizon; all are thought of as primary sources of power with the grid as the backup. Of course, there is “free” cooling when ambient and workload conditions are favorable during peak demand periods.
Cooling Technologies are Evolving
Today’s primary way for cooling data centers is precision air and that will be around for the “low rack power density” data centers for some time. However, with the proliferation of high-performance computing (HPC), artificial intelligence (AI), machine learning (ML), IoT, and other evolving computing technologies, rack power densities are rising, and liquid cooling becomes essential to ensuring reliability and performance.
For example, CRAC (computer room air conditioner)/CRAH (computer room air handler) and row-based containment cooling solutions are ideal up to about 15-20 kW but there is a point, maybe around 20 kW, at which heat removal by air is no longer effective or efficient. At that point, data center operators will start to look at other techniques that are cooling closer to the source of heat and are located very close to the rack doors or even closer than that, as these techniques will be cooling microchips directly by direct-to-chip or immersion liquid cooling. There will be a shift away from direct expansion cooling to liquid cooling using dry-coolers, mini-pumps, and heat exchangers.
As new builds and retrofits of existing data centers increase, the demand for energy-efficient and low-carbon data center cooling solutions will continue to grow.
Finaly, a word about heat reuse. How will data centers ever be able to meet their stated net-zero targets without capturing and reusing waste heat? Recovering heat from data centers with an energy station and providing this heat for industrial or district energy needs is an important step in sustainability. Heat recovery with liquid-to-liquid heat exchangers coupled with optional heat pumps to boost that water temperature, where needed, will provide useful heat to industrial, district or water heating applications.
It is not easy to align all the stakeholders together and make this happen in the United States but there are instances where heat reuse can be applied as a valuable decarbonization tool and is worth considering when designing sustainable data centers.
