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By R. Stephen Spinazzola and Rajan Battish
August 2011 -
Data Centers Article Use Policy
The data center industry is booming. Construction is going on all across the country, the Internet is devouring bandwidth, and demand for cloud computing services is skyrocketing. But environmental responsibility and energy efficiency have become mainstays in commercial buildings, and when it comes to energy consumed per square foot, few commercial spaces can match the appetite of data centers.
To compound the problem, energy costs in some high-density locations have risen significantly over the past few years and many regions can expect further increases in the near future especially where deregulation is anticipated or has occurred. For example, in Maryland, utility rates increased from an average of 9 cents per kWh to almost 13 cents per kWh, while Pennsylvania is anticipating increases from about 8 cents to 10 cents. As a result, energy inefficient data centers are attracting the attention of corporations and government agencies, while a variety of other organizations (U.S. Environmental Protection Agency, U.S. Green Building Council, American Society of Heating, Refrigerating and Air-Conditioning Engineers) are revising and creating standards to identify and enhance efficiencies in mission critical facilities.
While inefficiencies in data centers run the gamut, the majority of energy waste can be attributed to utilization equipment, with the second largest losses found in the mechanical system and uninterruptible power supply (UPS). Ultimately, a data center's overall efficiency begins and ends with an effective design for the mechanical and electrical infrastructure.
But before looking with those system, don't forget the basics. Effective design starts with an efficient building envelope. Although it is important to have a tight roof, it is just as essential to have a tight building slab, perimeter, and walls. A tight building envelope will allow for a central humidification system and effective air flow management to the cabinet.
Another basic, but easily ignored, key to efficiency is a lighting control system. These are value-engineered out of many projects, but installation of low-voltage lighting control systems and various lighting switching devices (such as occupancy sensors) can help manage the lighting load. Consideration should be given to high-efficiency lighting with low-harmonic ballasts, such as T5, LED lighting, dimming systems and flexible lighting control.
Given their appetite for power, in a data center it is important to analyze what affects the efficiency of the power supply. First, energy loss. There are several factors behind potential losses in transmission, such as the mechanical systems using inefficient technology and the conversion of power from AC to DC back to AC.
Adjacency of the MEP system to the utilization equipment is critical to minimize first costs and to reduce long-term energy losses. An effective layout will minimize conduit and piping runs and reduce losses.
UPS systems are evolving, with equipment vendors developing more efficient systems by reducing the magnetics due to transformers, rotary UPS modules with flywheels and line interactive systems. In addition to better UPS technology, more efficient power distribution units (PDUs) can be purchased by choosing models that comply with EPACT 2005. The PDUs are typically sized for 250 percent of load capacity for 2N redundancy and derating. Transformers and UPS are less efficient at part load compared to full load; the proper sizing for the data center's need is essential for maximum efficiency.
A factor that can negatively affect energy efficiency is harmonic load. Most data centers produce a large harmonic load due to the nature of the utilization equipment — UPS systems and variable frequency drives. Harmonics and noise on the electrical systems cause equipment to operate under duress. This duress generates heat and impacts the efficiency of the system. Therefore, consideration should be given to harmonic mitigation devices to increase efficiencies. There are myriad devices available ranging from cancellation transformers to filters.
The light load on the facility in the initial year or two can also be responsible for significant losses, with losses on a UPS system alone approaching the critical load the UPS system serves. This is most prevalent in traditional double conversion 2(N+1) systems. Many vendors and designers offer scalable solutions; however, the standard sizing of the equipment (which is often derated based on nominal loading versus standard equipment available and redline rating of the installed equipment) dictates the capacity installed Day One and for future build-out.
Solutions such as a load demand feature and correct sizing of the system for modular, rather than scalable growth should be considered on UPS systems for the initial years of the data center in order to minimize losses. With a modular UPS system, only the minimum number of modules needed have to be installed and operated right away, with the option to add more in the future. For example, consider a building designed for 750 KVA of load. The facility manager has the option of one 750 KVA UPS or three 250 KVA modules. If the initial load is only 500 KVA, it would be possible to install two of the 250 KVA modules until full capacity is needed.
Power plant optimization includes several key elements. When combined, these can produce significant savings in energy usage over the life of a mission critical facility.
Because most servers are rated for up to 240V, utilization of UPS equipment that distributes equipment to the rack directly at 230V is a practical solution. This will remove the requirement for transformation utilizing PDUs to the high power usage equipment. The higher voltage distribution reduces line losses and quantity of distribution equipment. The output of the UPS is routed directly to remote distribution cabinets at the data center raised floor for final cable connection to the racks. Transformation is provided for equipment not utilizing 400/230V system.
Most vendors are migrating toward insulated gate bipolar transistors (IGBTs) at the inverter and rectifiers to increase the quality of the power to the rack and efficiency. However, the energy inefficiencies associated with the magnetics of the iron core of transformers are not addressed. Several vendors have developed or are developing products for commercial use that have UPS efficiency between 95 to 97 percent for conventional double conversion (AC-DC-AC) systems. Traditional direct coupled rotary UPS may also be employed providing the added benefits of not only increasing the electrical efficiency, but also reducing the infrastructure space required.
Another efficiency strategy, which telecom companies have been using for quite some time and with well documented benefits, is DC power at the rack. Although central distribution of DC for large data centers may not be practical, hybrid solutions utilizing AC and DC are very real. Voltage at 480V is distributed to the in-row rectifiers that in turn distribute 48V or 24V to the equipment racks via a bus duct. The energy savings are gained from the lack of multiple transformations and reduced line losses.
Cogeneration (gas-fired generators) can be an effective way to generate power for the electrical systems and utilize absorption chillers for cooling. The application of cogeneration should be carefully analyzed in light of local utility rates, peak demand charges, incentives and initial capital investment.
A simple concept, though one that can be difficult to implement, is load management and trending. Metering the load from the circuit to incoming service can allow the facility manager to identify losses in the systems. Knowing where the losses and consumption of power originate can help identify easily fixable problems. Trending power consumption can also help determine if the cooling systems are oversized for the load.
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