Minimizing Energy Usage in Data Centers
Evaporative cooling is emerging as a popular option
According to a recent study from BSRIA, data center traffic is expected to grow at a 23 percent compound annual growth rate (CAGR), reaching 8.6 zettabytes by 2018. That means a lot of new data centers will be popping up around the country, particularly in vacant shopping centers and big-box stores, which are often being turned into off-site data storage facilities.
Cooling these data centers is an energy-intensive endeavor, which is why many companies are searching for ways to lower their power usage and operating costs. One way to reduce these costs is to supplement or replace traditional cooling systems — computer room air conditioning (CRAC) units and computer room air-handling (CRAH) systems — with new technologies, such as evaporative cooling, which can provide significant energy savings.
Computer room evaporative cooling (CREC) systems are becoming more popular, because data centers have raised their allowable supply air operating temperature windows, which has allowed them to take advantage of alternatives to traditional cooling methods, explained David Klusas, director of marketing for large cooling systems, Emerson Network Power. “CREC units save operating costs by using outside air and the evaporation of water to provide the desired supply temperature throughout most, if not all, hours of the year.”
Essentially, a CREC system can lower a data center’s power utilization effectiveness (PUE) by shifting some of the electrical consumption of its cooling infrastructure to information technology (IT) power consumption, which leads to a lower PUE, said Jeff St. Clair, product line manager for air economizers, Schneider Electric. “This helps make these data centers more efficient.”
Data center owners also appreciate a CREC system’s other advantages, such as its competitive capital cost, which will often result in an attractive payback period, noted Kris Holla, vice president and chief sales officer, Nortek. “There are plenty of independent studies that claim the upfront installation costs of CREC and CRAC systems are similar, but an important and often forgotten hidden cost is the upstream electrical power installation. Given that the peak power of a CRAH or CRAC system will be much larger for a CREC system, the whole upstream power distribution has to be fused and sized accordingly, which can add significant costs to a CRAH- or CRAC-based solution. With the huge operational cost savings potential, the payback with a CREC almost always occurs within 12 months.”
Another important consideration is that a CREC system is a completely packaged and compact solution that only needs a small water supply/drain and a much smaller-sized power feed, said Holla. “Rapidly deployable on-site with simple end-on or bottom duct connections, a CREC system can be dropped into the construction program at exactly the right time with minimal disruption to the site or other trades. It also features flexible installation options and fast commissioning, and because it’s modular, it can grow with a data center’s needs.”
There are two kinds of evaporative cooling technologies — direct (DEC) and indirect (IDEC) — and while both use water to minimize the need for mechanical cooling, each system is applied very differently and provides different benefits to the end user, said Klusas. “The specific operating requirements and geographic location for the data center will dictate which solution will be most appropriate.”
DEC systems operate by bringing in outside air and mixing it with hot return air from the data center to achieve the desired supply temperature, said Klusas. “When the outdoor air is too warm for the desired supply temperature, a media in the airsteam is wetted to bring the air temperature down toward wet bulb temperature. When even the wetted media does not meet the required supply temperature, mechanical cooling is used to trim the final few degrees to achieve the desired set point.”
DEC systems are highly efficient, carry a relatively low first cost, require only limited use of water, and can utilize a lower total electrical infrastructure (e.g. generators), said Klusas. “But because the system introduces humidity directly into the airstream, a wide supply air operating window is required to maximize unit performance. There are also challenges with the overall control of an ever-changing system, and redundancy needs to be considered, as the system must have outside air at all times [unless a full mechanical system is in place].”
In addition, the introduction of outside air with a DEC system increases the risk of contamination, so keeping air pollutants out of the data center becomes a larger concern. “This is typically done with extra filtration of the outside air prior to entering the data center space, which can result in additional costs,” said St. Clair. “Also adding to the cost of a DEC system is the need for backup DX [direct expansion] cooling. It is not uncommon for DEC systems to have 100 percent backup DX cooling, especially if the data center owner is risk-averse.”
An IDEC system keeps the outside airstream completely separate from the data center airstream and only introduces water to the outside airstream, explained Klusas. “Both airstreams pass through a heat exchanger, which transfers heat from the data center air to the outdoor air, but contamination and humidity are not introduced into the data center airstream. These systems are highly efficient and can maintain tight temperature and humidity set points.”
Because an IDEC system uses a heat exchanger to transfer heat from the data center to the outdoor air, it can run continuously, lessening the amount of backup DX cooling needed, said St. Clair. “In most cases, the amount of DX required is just for trim cooling when ambient wet bulb temperatures can’t support the required supply air temperature to the data center space. This lessens the amount of DX needed, which helps reduce costs. But because the evaporative cooling process is dependent on the availability of water, the design should account for either a backup water source or full redundancy of the cooling capacity with DX.”
Due to the fact that IDEC does not impact humidity or introduce contaminants to the space, Holla believes it has advantages over DEC. “However, it is possible to use DEC and IDEC in one design. For example, IDEC can be used as the main source of cooling for the space, and the DEC part can be used to control the humidity inside the data center, especially in winter, when humidification might be required. In winter, the DEC system can be used as the primary cooling method, which will offer more energy savings but will require more filtration.”
A CREC system can be used in just about any climate, but its annual efficiency and hours of economization will vary greatly based on the climate and allowable operating temperature and humidity requirements for the data center, said Klusas. “Supplemental cooling may be required to meet the desired set point throughout the year, and high outdoor humidity does limit the effectiveness of the evaporative process.”
CREC systems are mostly being utilized in new construction, but they can also be paired with existing cooling infrastructure to reduce energy costs. “For example, if the site has existing DX systems, an evaporative system can be added to provide free cooling,” said Klusas. “In the event mechanical cooling is required, the existing units can be used to provide those few hours where the evaporative infrastructure cannot provide 100 percent of the cooling.”
A hybrid system that incorporates CRECs and CRACs can be a good idea, said St. Clair, because with this configuration, it is possible to leverage the ambient dry and wet bulb temperatures to save power consumption and still effectively cool a data center space. “In this case, it is necessary to weigh the amount of economization available and the costs associated with maximizing the CREC [containment] and the capital expenditure to deploy to see if an acceptable ROI [return on investment] is worth the expense.”
As data center owners are quickly learning, the ROI for a CREC system can be very attractive, which is why sales of these units are expected to be strong for the foreseeable future.
Publication date: 7/4/2016