Reducing Energy Costs in the Data Center: Developing a Plan of Attack

Against this background, senior-level executives are looking to create IT system efficiencies that will wring costs from data center operation. In fact, energy costs have become a driving factor influencing new data center location and design.

That data center energy costs are getting increased attention was confirmed by a recent survey conducted by the Data Center Users Group. It found that 42% of respondents have either analyzed efficiency or are currently analyzing it. Interestingly, respondents perceived the biggest opportunity for energy efficiency to be cooling equipment, followed by servers.

Is this perception accurate? It’s true that, combined, the information technology and cooling systems account for the lion’s share of energy consumption (87%); however, the IT systems themselves, which use 50% of data center power, offer the greatest opportunity for increasing data center efficiency in most cases. Also every kW reduction in IT load losses is an equivalent reduction in the cooling energy required. The cooling system efficiency is next in line.

A number of strategies can be evaluated for reducing IT system energy consumption, but the most promising is server virtualization—the growing practice of partitioning physical servers so they have multiple virtual environments inside them. Each virtual environment looks and acts like a separate server, without actually being separate. This enables data center professionals to manage workloads so that fewer servers are operating during periods of less-than-peak activity.

According to some studies, average server utilization is 15%. For the sake of example, let’s assume a data center that has a utility bill of more than $5 million annually. By making an improvement in the server utilization rate to 30% through virtualization, more than $2 million will be saved annually, provided the unused servers are turned off. This is a conservative estimate of the savings that can be achieved. Some industry experts claim virtualization can increase server utilization to near 100%.

Beyond virtualization, the server power supplies are another area in which savings can be gained. The average server power supply is 80% efficient. Using a higher efficiency power supply to improve efficiency to 88% can generate an additional 10% savings in energy. (Note that 92% efficient power supplies are available.) Using the same sample data center as above, implementing this power supply improvement would reduce the $5 million annual bill by more than $400,000.

Next to the technology systems themselves, the cooling system consumes the most energy in the data center, accounting for 37% of data center electricity use. Demands on cooling systems have increased substantially in recent years as server densities have risen to unprecedented levels. This change has not only created the need for increased cooling system capacity, but also driven the development of new cooling technologies designed to efficiently cool this new generation of equipment.

When evaluating cooling efficiency, first assess existing cooling systems and how they can be optimized, and then consider additional technologies such as economizers and supplemental cooling systems. Together, these approaches can reduce cooling system energy costs by 30 to 45 percent. Again using our sample data center, a 45% improvement in the cooling system is a savings of more than $800,000.

Increasing Computer Room Air Conditioner Efficiency

Too often the efficiency of computer room air conditioners (CRACs), is evaluated at full speed, full load and the worst design load day. Units rarely operate under these conditions and this may not provide an accurate gauge of efficiency under normal conditions.

There are three technologies that can be deployed to improve CRAC performance at partial loads:

1. Variable capacity compressors such as four-step compressor unloading and Digital Scroll™ compressor technology
2. Variable CFM air flow systems utilizing variable frequency drives or electronically commutated motors on the blower system
3. Advanced unit controls that allow the CRAC units to optimize the operation based on a room solution versus a single unit operation

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% of the energy of a fully loaded system but will deliver 76% capacity because the condenser and evaporator are sized for full load.

Alternatively, Digital Scroll™ compressor technology offers a newer way to more precisely match capacity and power consumption to the desired load and can deliver significantly lower energy consumption compared to standard “fixed-capacity” compressors. Additionally, this technology allows the compressor to never be cycled off. It reduces power consumption linearly as it modulates capacity, resulting in optimum system performance and control.

Using variable speed blower systems to match air flow requirements has been proven to save significant energy. A 10% reduction in the CFM is a 27% reduction in the energy consumption. This is an easy and common addition to a chilled water CRAC system; however, to use it on a compressorized CRAC system requires the additional implementation of a variable capacity compressor to insure proper, reliable operation. The Digital Scroll™ discussed earlier is an excellent choice for this requirement.

Another factor to consider is how well multiple units in a room work together. The data center environment has become more diverse as newer high-density servers are deployed alongside older systems. As a result, without proper coordination between room cooling units, air conditioners may be operating in different modes of temperature and humidity control. For example, a unit on the north side of the room—where the heat density is higher thus return temperature higher—may be sensing low relative humidity conditions and adding humidity, while a unit on the south side of the room—where the heat load is low—is sensing high relative humidity and removing moisture from the air. The actual moisture in the air is actually the same throughout the room and probably needs no action, but because the measurement is a relative measurement (the higher the temperature, the lower the relative humidity), the CRAC units are fighting each other. Advanced control systems can be deployed across all the CRAC units in a room to enable the units to communicate and coordinate their operation, preventing the “fighting mode.”

Using Economizers to Achieve Free Cooling

Economizer systems use outside air, when it is cold enough, to help meet cooling requirements and provide so-called “free cooling” cycles for data centers. When an economizer system is operating, the use of an air conditioning system’s compressor(s) and related electro-mechanical components is reduced or eliminated. In certain geographical locations, economizers can satisfy a large portion of data center cooling requirements.

There are two basic types of economizer systems: air-side economizers and fluid-side economizers. While both have the ultimate goal of free cooling, they operate differently and possess fundamental differences that have a direct impact on the most appropriate economizer choice for a data center environment.

Air-Side Economizers

The air-side economizer admits outside air to cool the indoor environment through the use of a system of sensors, ducts, and dampers. Air-side economizers are available in two types—a “dry air” system and an “adiabatic conditioned” air system. The former is the most common, but its use is restricted to a few geographic locations because of the high cost of energy required to add moisture to the room when the outside humidity is too low. The adiabatic conditioned solution is an economical method for conditioning the air before it comes into the data center, but reliability, mildew concerns and high maintenance requirements have generally made this approach unattractive to most data center operators. Additionally, both types of air-side economizers require air filtration to keep pollen, dust and other external contaminants from entering the data center. This increases maintenance requirements.

Fluid-Side Economizers

A fluid-side economizer system is typically incorporated into a chilled water or glycol-based cooling system, and works in conjunction with a heat rejection loop consisting of either a cooling tower, evaporative cooler or drycooler. CRAC units incorporate a conventional glycol-cooled unit along with a second cooling coil, control valve and temperature monitor. During colder months, the glycol solution returning from the outdoor drycoolers or cooling tower is routed to the second coil, which becomes the primary source of cooling for the room. As long as the “free cooling” fluid is 8°F below the CRAC return temperature, there is some benefit for having the “free cooling” running, as it minimizes the load on the primary cooling method.

Fluid-side economizers are the system of choice for most data center environments because they are not affected by outdoor humidity levels and so are effective in a wider portion of the temperature/humidity band. They also do not add any additional air filtration requirements.

Deploying Supplemental Cooling

Supplemental cooling systems bring cooling closer to the source of heat, reducing the amount of energy required for air movement.

Supplemental cooling is a relatively new approach to data center cooling that was pioneered by Emerson Network Power with its Liebert XD System™. This approach overcomes the cooling capacity limitations of raised floor cooling systems in high heat density applications by using a pumped refrigerant cooling infrastructure that supports cooling modules placed directly above or alongside high-density racks to supplement the air coming up through the floor.

Higher density applications require fluid-based cooling to effectively remove the high concentrations of heat being generated. From an efficiency perspective, refrigerant performs better than water for high-density cooling. The R134 refrigerant used in the Liebert XD system is approximately 700% more effective in moving heat than water, which coincidentally, is 700% more effective than air. In addition, it can be pumped as a liquid but converts to gas when it reaches the air, ensuring that expensive IT equipment is not damaged in the event of a refrigerant leak.

Using this refrigerant with cooling modules mounted as close as possible to the source of heat can reduce cooling system energy costs by 30 percent or more compared to relying exclusively on raised floor cooling. Additionally, it reduces chiller capacity requirements by 20 percent. This increases energy savings and/or also enables additional cooling capacity without adding more chillers.

Conclusion

Once the efficiency and utilization of IT equipment have been addressed, the cooling system represents the next significant opportunity for improving efficiency and reducing energy costs. Room cooling systems can be optimized through new technologies, such as Digital Scroll™ air conditioners and intelligent control systems. Finally, supplemental cooling systems can increase the scalability and efficiency of existing cooling systems to address the challenges posed by increased server density.

To see complete data on the breakdown of energy consumption in data centers, see the white paper Five Strategies for Cutting Data Center Energy Costs through Enhanced Cooling Efficiency available at www.liebert.com.