Consulting-Specifying Engineer - January 2008 - (Page 47) • Does the amount of leakage across the building envelope correlate to indoor humidity levels and energy use? • How does the location (climate) of the data center affect the indoor temperature and humidity levels? Are certain climates more favorable for using an outside air economizer without humidifying the air when outdoor air is dry, or introducing an unnecessary latent load in humid climates? • When using an outside air economizer cycle, how dry will the air in the data center actually get without adding humidification? How much energy can be saved at this extreme? • Will expanding the humidity tolerances required by the data center equipment yield worthwhile energy savings? Performance simulations Typical analysis techniques look at peak demands or steady-state conditions that are just representative “snapshots” of data center performance. These analysis techniques, while very important for certain aspects of data center design such as equipment sizing, do not tell the engineer anything about the dynamics of indoor temperature and humidity, some of the most crucial elements of successful data center operation. For the purposes of demonstrating these interdependencies, a simulation model was developed using EnergyPlus version 2.1, an energy analysis and thermal load simulation program. The simplified model has 20,000 sq ft. of raised floor area at 150 W/sq ft. The systems were modeled as chilled water with centrifugal water-cooled chillers and variable air volume air handling units. The leaving air temperature from the coil is 55 F with a maximum return air temperature of 75 F. The building envelope comprises the roof, exterior walls, floors, and underground walls in contact with the earth, windows, and doors. Many data center facilities have minimal areas of windows and doors, so the roof, walls, and floor are the primary elements for envelope modeling. The parameters to be considered in the analysis of these elements Envelope losses as are: thermal resistance (insuPercent of computer a percent of total equipment running cooling requirements lation), thermal mass (heavy construction such as concrete 20% 8.2% versus light-weight steel), air40% 4.1% tightness, and moisture per60% 2.8% meability. 80% 2.1% When a large data center 100% 1.7% is running at full capacity, Table 1: This example shows how a building’s envelope the affects of the building envelope on energy use (as cooling changes as a percent of total cooling load. a percent of the total) are Source: EYP Mission Critical Facilities relatively minimal. However, because many facilities never ASHRAE Energy Standard 90.1-2007 reach their full build-out potential, or if they has very specific information on different do, it is over an extended period of time, building envelope alternatives that can be defining the insulation and sealing require- used to meet the minimum energy performents of the building envelope need to be mance requirements. Additionally, ASHRAE’s an integral part of the design process. “Advanced Energy Design Guide for Small When analyzed over time as the facility Office Buildings” goes into great detail on the load increases, the envelope losses start out most effective strategies for building enveas a significant component of the overall lope design by climatic zone. Finally, another cooling load, but decrease over time as the good source is the British Chartered Institucomputer load becomes a greater portion tion of Building Services Engineers (CIBSE) of the load, as shown in Table 1. “Guide A on Environmental Design.” 25 ■ Annual chiller energy use Annual electricity use (millions of Btus) 20 15 10 5 0 65 F cold, 75 F hot, no economizer, no outside air 65 F cold, 75 F hot, no economizer 65 F cold, 75 F hot, air economizer 75 F cold, 85 F hot, air economizer 85 F cold, 95 F hot, air economizer Figure 1: Increasing the air temperatures in the cold aisle and incorporating outside air economizer can yield significant energy savings. Source: EYP Mission Critical Facilities Consulting-Specifying Engineer • JANUARY 2008 47
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