Driving data center PUE, efficiency

06/09/2015


Building envelope and energy use

Buildings leak air. Moisture will pass in and out of the envelope, depending on the integrity of the vapor barrier. This leakage and moisture migration will have a significant impact on indoor temperature and humidity, and must be accounted for in the design process. To address what role the building plays in data center environmental conditions, the following questions must be answered:

  • Does the amount of leakage across the building envelope correlate to indoor humidity levels and energy use?
  • How does the climate where the data center is located affect the indoor temperature and humidity levels? Are certain climates more favorable for using outside air economizer without using humidification to add moisture to the air during the times of the year when outdoor air is dry?
  • Will widening the humidity tolerances required by the computers actually produce worthwhile energy savings?

Building envelope effects

The building envelope is made up of the roof, exterior walls, floors, and underground walls in contact with the earth, windows, and doors. Many data center facilities have minimal amounts of windows and doors, so the remaining elements of roof, walls, and floor are the primary elements for consideration. These elements have different parameters to be considered in the analysis: thermal resistance (insulation), thermal mass (heavy construction, such as concrete versus lightweight steel), air tightness, and moisture permeability.

When a large data center is running at full capacity, the effects of the building envelope on energy use (as a percent of the total) are relatively minimal. However, because many data center facilities routinely operate at partial-load conditions, defining the requirements of the building envelope must be integral to the design process as the percentage of energy use attributable to the building envelope increases.

ASHRAE 90.1 includes specific information on different building envelope alternatives that can be used to meet the minimum energy-performance requirements. In addition, the ASHRAE publication Advanced Energy Design Guide for Small Office Buildings also goes into great detail on the most effective strategies for building-envelope design by climatic zone. Finally, another good source of engineering data is the Chartered Institution of Building Services Engineers (CIBSE) Guide A: Environmental Design 2015.

Building envelope leakage

Building leakage in the forms of outside air infiltration and moisture migration will impact the internal temperature and relative humidity. Based on a number of studies from National Institute of Standards and Technology (NIST), CIBSE, and ASHRAE, building leakage is often underestimated significantly when investigating leakage in building envelope components. For example:

To what extent should the design engineer be concerned about building leakage? It is possible to develop profiles of indoor relative humidity and air change rates by using hourly simulation of a data center facility and varying the parameter of envelope leakage.

Using building-performance simulation for estimating energy use

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. However, using an hourly (and sub-hourly) building energy-use simulation tool will provide the engineer with rich detail to be analyzed that can inform solutions to optimize energy use. For example, using building-performance simulation techniques for data center facilities yields marked differences in indoor relative humidity and air-change rates when comparing different building-envelope leakage rates. Based on project analysis and further research, the following conclusions can be drawn:

  • There is a high correlation between leakage rates and fluctuations in indoor relative humidity. The greater the leakage rates, the greater the fluctuations.
  • There is a high correlation between leakage rates and indoor relative humidity in the winter months. The greater the leakage rates, the lower the indoor relative humidity.
  • There is low correlation between leakage rates and indoor relative humidity in the summer months. The indoor relative humidity levels remain relatively unchanged even at greater leakage rates.
  • There is a high correlation between building leakage rates and air-change rates. The greater the leakage rates, the greater the number of air changes due to infiltration.

Climate, weather, and psychrometric analyses

Climate and weather data is the foundation of all the analyses used to determine data center facility energy use, PUE, economizer strategy, and other energy/climate-related investigations. The data used consists of 8,760 hours (the number of hours in a year) of dry-bulb, dew point, relative humidity, and wet-bulb temperatures.

When performing statistical analysis as a part of the energy-use study, it is important to understand the quantity of hours per year that fall into the different temperature bins. Data visualization techniques are used along with the ASHRAE temperature boundaries. Analyzing the hourly outdoor temperature data, totaling the hours, and assigning them a temperature zone on the graph indicates where the predominant number of hours falls. Along with these analysis techniques, it is important to understand the following qualifications on how to use the weather data:

  • The intended use of the hourly weather data is for building energy simulations. Other usages may be acceptable, but deriving designs for extreme design conditions requires caution.
  • Because the typical months are selected based on their similarity to average long-term conditions, there is a significant possibility that months containing extreme conditions would have been excluded.
  • Comparisons of design temperatures from “typical year” weather files to those shown in ASHRAE Handbook—Fundamentals have shown good agreement at the lower design criteria, i.e., 1%, 2% for cooling, and 99% for heating, but not so at the 0.4% or 99.6% design criteria.
  • ASHRAE Handbook—Fundamentals should be used for determining the appropriate design condition, especially for sizing cooling equipment.

Climate data

The raw data used in climate analysis is contained in an archive of ASHRAE International Weather Files for Energy Calculations 2.0 (IWEC2) weather-data files reported by stations in participating nations and recorded by the National Oceanic and Atmospheric Administration (formerly the National Climatic Data Center) under a World Meteorological Organization agreement. For the selected location, the database contains weather observations from an average of 4 times/day of wind speed and direction, sky cover, visibility, ceiling height, dry-bulb temperature, dew-point temperature, atmospheric pressure, liquid precipitation, and present weather for at least 12 years of record up to 25 years.

Psychrometrics

Psychrometrics uses thermodynamic properties to analyze conditions and processes involving moist air. With this data, other parameters used in thermodynamic analysis are calculated, namely the wet-bulb temperature. The following is an overview of the key thermophysical properties that are necessary to perform an energy-use study:

  • Dry-bulb temperature is that of an air sample as determined by an ordinary thermometer, the thermometer's bulb being dry.
  • Wet-bulb temperature, in practice, is the reading of a thermometer whose sensing bulb is covered with a wet cloth, with its moisture evaporating into a rapid stream of the sample air.Dew-point temperature is that temperature at which a moist air sample at the same pressure would reach water vapor saturation.
  • Relative humidity is the ratio of the mole fraction of water vapor to the mole fraction of saturated moist air at the same temperature and pressure.
  • Humidity ratio (also known as moisture content, mixing ratio, or specific humidity) is the proportion of mass of water vapor per unit mass of dry air at the given conditions (dry-bulb temperature, wet-bulb temperature, dew-point temperature, relative humidity, etc.).
  • Specific enthalpy, also called heat content per unit mass, is the sum of the internal (heat) energy of the moist air in question, including the heat of the air and water vapor within.
  • Specific volume, also called inverse density, is the volume per unit mass of the air sample.


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