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Somewhere within all that terminology is the realization among those in HVAC and refrigeration that all that hardware and software needs to operate in a narrow temperature range. Today, an added issue at data centers is cooling efficiency. As noted by the white paper, “Five Strategies for Cutting Data Center Energy Costs Through Enhanced Cooling Efficiency” (from Emerson Network Power):
“As electricity prices and IT power consumption continue to rise, IT-related energy costs are getting increased scrutiny. Cooling accounts for approximately 38 percent of electricity usage within a well-defined data center and, in many cases, represent a significant opportunity to reduce IT energy costs.”
In the five strategies, special attention is drawn to compressors, including concepts such as “four-step compressor unloading” and use of digital scroll compressors.
FIVE STRATEGIESFor the record, here are the five strategies:
1. Proper sealing of the data center environment. A vapor seal plays a critical role in controlling relative humidity, reducing unnecessary humidification and dehumidification.
2. Optimizing airflow. Rack arrangement, computer room air conditioner placement, and cable management all impact the amount of energy expended to move air within the critical facility.
3. Using economizers where appropriate. Economizers allow outside air to be used to support data center cooling during colder months, creating opportunities for energy-free cooling.
4. Increasing cooling system efficiency. New technologies, such as variable-capacity systems and improved controls, are driving increased efficiency of room air conditioning systems.
5. Bringing cooling closer to the source of heat. Supplemental cooling systems bring cooling closer to the source of heat, reducing the amount of energy required for air movement.
The white paper noted, “Together, these methods can reduce cooling system energy costs by 30 to 45 percent and generate significant, recurring savings. Coupled with emerging technologies such as higher-efficiency processors and new chip-based cooling technologies, these measures can keep energy costs in line as server densities and the price of energy continue to rise.”
The entire paper can be found at Emerson Network Power’s Liebert Website (www.liebert.com). This article will focus on point four: compressor technology.
INCREASING EFFICIENCYThe paper said three factors are critical to optimizing the efficiency of computer room air conditioning (CRAC) units:
• How efficiently the units operate at partial load.
• How efficiently the units are at removing sensible heat as compared to latent heat.
• How well multiple units work together.
The report said, “Data centers are designed with some level of cooling system redundancy. Plus, the actual capacity of a direct expansion or air-cooled CRAC unit increases as the outdoor ambient temperature decreases below the peak design condition (typically 95°F). This means equipment is operating at less than 100 percent load all of the time, creating the opportunity to design systems to operate more efficiently during normal operating conditions. Because operating conditions aren’t stable, this requires some method of varying capacity based on operating conditions.”
COMPRESSOR CONSIDERATIONSThe report noted that there are several approaches to providing variable capacity in a direct expansion CRAC unit. It said the two most common are four-step compressor unloading, and digital scroll compressor technology.
“The concept of four-step compressor unloading works by shutting off the flow of refrigerant to some of the cylinders within the system, thereby, minimizing the need to cycle compressors on and off to control capacity. Because unloading essentially changes the compressor operating point, it enables the cooling system to operate more efficiently at lower capacities.
“For example, a system operating with two compressors “unloaded” will consume approximately 50 percent of the energy of a fully loaded system but will deliver 76 percent capacity because the condenser and evaporator are sized for full load.”
Digital scroll compressor technology was described as “a newer way to precisely match capacity and power consumption to the desired load and can deliver significantly lower energy consumption compared to standard ‘fixed-capacity’ compressors.
“Digital scroll technology allows the compressor to rarely be cycled off. It reduces power consumption linearly as it modulates capacity, resulting in optimum system performance and control.”
The research for the white paper was based on technology being applied to a data center through an exclusive agreement between Emerson Climate Technologies, which developed the Copeland Digital Scroll™ compressor, and Emerson Network Power, which developed the Liebert DS precision cooling system and Liebert XDF high-heat density rack enclosure.
Publication date: 03/31/2008