Alternatives to traditional chilled-water and direct-expansion systems for data center cooling have become popular in recent years. Data center managers have been able to raise their allowable supply-air operating temperatures and use these new technologies to save energy. Now, a desire to also reduce water use in the data center has led to a new type of free-cooling system: one that uses pumped-refrigerant economization and consumes no water.
Digital Realty, which supports the data center and colocation strategies of more than 1,000 firms around the world, has turned to this technology to save more than 300 million gallons of water annually in its data centers, compared with using chilled-water systems. Specifically, it is installing Liebert DSE free-cooling systems from Emerson Network Power.
“Digital Realty, along with certain major technology companies and other pioneers, is showing that clean energy can power even the largest and most high-performance data centers,” the company said in a recent white paper. “And as more organizations consider moving from traditional to cleaner sources of power, they are also showing that renewable energy can be cost effective.”
In 2012, when Emerson Network Power first proposed its Liebert DSE pumped-refrigerant economization systems to Digital Realty, only air and water coolants were approved in California for data center economization. The California Energy Commission’s Title 24 required that air-side economization systems be capable of carrying 100 percent of the IT head load when outside temperatures were at or below 55°F. Water-side economization systems were required to carry 100 percent of the IT heat load at temperatures of 40°F or lower.
But Digital Realty and Emerson collaborated in 2015 to gain the approval of the California Energy Commission (CEC) for pumped-refrigerant economization as a prescribed option for data center cooling in California. As described in a recent joint case study, this ruling paves the way for companies in that state to more easily deploy the energy- and water-saving data center technology.
“For nine months, Emerson and Digital Realty worked together to explore the energy savings and operational performance of a pumped-refrigerant system and compared it with that of a chilled-water system. The results proved to be promising, and the companies shared them with the California Energy Commission and applied for a formal exception to the air- and water-only rule in place at the time,” said Aaron Binkley, director of sustainability at Digital Realty. “We found that in addition to energy savings, the system delivered another benefit that has since come to be highly valued in California because of the current drought conditions: it saves millions of gallons of water.”
Designed for Efficiency
The Liebert DSE system has several components: a pumped-refrigerant economizer, a highly efficient microchannel condenser and an indoor CRAC unit. It maximizes annual energy savings and provides superior availability without the need for separate economization coils.
When outdoor ambient temperatures are low enough, an integrated refrigerant pump circulates refrigerant in lieu of the compressor to maintain the desired supply-air temperature. The refrigerant pump uses a faction of the energy used by the compressor.
As outdoor ambient temperatures rise, the Liebert DSE system automatically switches on compressors to maintain the desired supply-air temperature. Its thermal controls work to automatically optimize the entire system to provide more free cooling throughout the year. Changeover between compressor mode and refrigerant-economizer mode is almost instantaneous, whereas traditional economizer systems typically operate in economizer mode only when the economizer season is stable—not changing back and forth during the night, for example.
These advanced thermal controls also enhance cooling-unit operation and availability by protecting units from adverse events, with automated routines for unit protection, fast restart, unit cascading and contaminant removal. The unit control communicates alerts; auto-tunes important operating parameters, such as fan speed, compressor utilization and economization; and prevents cooling units from exceeding critical thresholds.
In addition to the water savings, this approach offers other advantages:
- No costly water treatment required
- No outside-air contamination in the data center
- No dampers or louvers to maintain
- Instant, automatic economizer changeover to increase free-cooling hours
The microchannel condenser in the system also plays a part in energy savings, in addition to reducing sound levels and refrigerant charge. Microchannel condensers are smaller, lighter (up to 40 percent) and can require significantly less refrigerant charge (nearly 73 percent) than a traditional fin/tube design.
Because the condenser is equipped with a coil that is only about one inch thick, it can achieve lower pressure drop on the air side, decreasing fan power. Microchannel condenser coils allow for the same heat transfer in a smaller footprint, providing efficiency gains as much as 52 percent relative to traditional fin/tube condensers. Even higher efficiency levels can be realized during low load or lower ambient temperatures, when the fan speed can be reduced, using the condensers’ variable-speed fans. The Liebert DSE can deliver a payback in less than three years compared with traditional air-cooled DX systems.
Binkley and Digital Realty found that the Liebert DSE system delivered tangible results. Compared with water economizers, its pumped-refrigerant economizer reduced time-dependent valuation (TDV) by 8–10 percent, cut electric usage by 16 percent and, since it uses no water, saved around four million gallons of water annually in a 1MW data center.
“Digital Realty’s sustainability policy is aimed at minimizing the use of energy, water and raw materials in our more than 130 data centers. Working with Emerson, we have developed a data center cooling strategy that improves our sustainability and is also economical to scale,” Binkley says.
In addition, Digital Realty and Emerson have found a simple and cost effective to way to site and install the necessary equipment. Emerson adapted a solution it had developed for electrical products, factory mounting the system components on skids so only the skids required installation on the data center roof. This approach satisfied Digital Realty’s strong preference for minimal on-site construction.
“The success of the Liebert DSE system underscores the speed at which the data center market is embracing alternative economization technologies that support their sustainability objectives,” says John Peter Valiulis, vice president of thermal management marketing for Emerson Network Power in North America.
Digital Realty’s success is poised to be replicated by other companies around the country. A recent survey of U.S.-based mechanical engineers and contractors by Emerson Network Power showed that 55 percent expect pumped-refrigerant economization to be the number-one technology, replacing chilled-water systems over the next five years.
Digital Realty has already implemented the DSE solution at its data centers in California, Virginia and Texas, as well as in Australia. The company also says it is exploring additional deployment at its centers in Europe.