Getting to the bottom—and top—of PUE

When developing data center energy-use estimations, energy engineers must account for all sources of energy use in the facility: computers, cooling plants, and other related equipment. Learn how to itemize power usage efficiency (PUE).


Learning objectives

  1. Learn which air economizer strategy is best in a given data center.
  2. Know how to itemize annual energy use of the various data center components. 

A common thread running through many articles about data centers is the idea that approaches to data center energy efficiency are still in the process of a paradigm shift. This shift is moving us away from what we know the most about: designing HVAC systems for office buildings, labs, hospitals, and schools. 

Figure 1: The analysis using Chicago weather data depicts these efficiencies monthly by economizer type. Courtesy: HP Technologies Services

For example, just five years ago, a large portion of legacy data centers were still running supply air temperatures at 55 F—typical of a commercial building. Contrast that to new projects where  data centers will use supply air temperatures at or above 75 F. That is a 20 F increase in supply air temperature—effects cascading down into the entire cooling system. Just this one change has caused a wholesale rethinking of the design and operation of air conditioning systems that are used in data centers. 

Thankfully, there are many smart engineers designing data centers and several industry organizations (such as Uptime Institute, 7x24 Exchange, ASHRAE, and others) dedicated to the planning and design of data center power and cooling systems. Also, many of the manufacturers, arguably the most important link in the chain, now have complete equipment lines dedicated to data centers. All of these continue to provide solid standards, recommendations, and products to assist in the paradigm shift, and look to future data center transformation. 

As computer technology (hardware, networking, storage, and software) evolves at a blazing pace, planning and engineering of power and cooling systems are struggling to keep up. But this should come as no surprise. We see it in our daily life—mobile phones, PCs, notebook computers, and TVs that are rendered obsolete in 9 to 12 months from initial release. Certainly, there are different factors involved in the consumer electronics market, but the core idea is the same as with enterprise-level IT equipment—advances in new technology (manufacturing, materials, software) enable higher performance than the predecessors while using less energy. 

Understanding these constraints, it’s no wonder that power and cooling equipment manufacturers have a difficult time conducting R&D, planning, funding, and manufacturing their next generation of products that will support yet-to-be-developed computer technology. In the end, the power and cooling equipment manufacturers develop products that work well with the latest generation of computer technology, but integrating features that allow the equipment to adapt to future IT equipment design may simply be too cost prohibitive. 

Looking from a different perspective, we see that when a computer manufacturer releases a new generation of servers, the thermal engineers will have likely developed a novel cooling design to make the server run at lower temperatures and to use less fan energy. This is where the data center HVAC engineers and the server thermal engineers need to have a conversation in an attempt to optimize the energy use and efficacy of the servers and the data center cooling system, not just one or the other. An undesirable outcome is to have a high-performance, low-energy server that requires a data center cooling system that is inefficient, too complex, or too cost prohibitive to build. This is where detailed simulation and analysis of data center cooling system energy use come in.

It's the heat and the humidity 

Supply air temperature is the most distinctive feature of a cooling system in a data center. In comfort cooling applications, the primary goal of the HVAC system is to provide enough cooling capacity to satisfy all internal and external loads, ensure that the building occupants feel comfortable (dry bulb temperature and moisture content of the air), and to maintain the appropriate filtration and ventilation rates to safeguard against higher-than-acceptable levels of gaseous and particulate contaminants. Data centers generally need to meet these goals as well, but the electrical equipment loads (when compared to a modern, high-tech commercial office building) are an order of magnitude greater. The good news is that, unlike people, computers don't mind running very hot and are pretty tolerant to a wide range of moisture levels. With this tolerance to high heat and humidity comes tremendous opportunity for energy efficiency opportunities. 

The energy efficiency opportunities come from a combination of reduced compressor horsepower resulting from increased evaporator temperatures (supply air temperatures) and the fact that the compressors will run less often, especially in climates that enable full use of the economization strategy. This is where careful examination of the available cooling system alternatives is necessary; while a certain cooling option might offer a significant reduction in compressor energy, the other components (fans, pumps, etc.) may use more energy when compared to the other options. 

The comparison of the cooling system options must include a full hourly energy simulation of the data center as a whole (as defined by ASHRAE Standard 90.1) with the ability to analyze the cooling systems and subsystems to determine which components consume the largest amounts of energy. The results of the this analysis will provide raw data for the energy professional to make recommendations on the most energy efficient system, and also offer granular data on how each of the subsystems performs under different operational scenarios, such as different supply air temperatures and in different climates.

Economization techniques 

A critical component of any energy-efficient cooling system is the economizer. An economizer is simply a combination of operational sequences and equipment hardware that is intended to reduce energy use of an HVAC system by taking advantage of the positive psychrometric attributes of the outdoor air. Because different economizers rely on different psychrometric conditions, each one will have distinct performance characteristics. Depending on the economizer type and control strategy, the economizer will operate in three distinct modes: 100% off, partial operation, and 100% on. 

The partial operation mode will operate at a specified range of temperatures and humidities. Depending on the climate, partial economization could be in effect a large percentage of the year; it is important to account for these hours in determining the efficacy of the economizer solution. Calculating partial economization is done by adding up the hourly cooling load in tons (ton-hours)  in the period of hours being analyzed (8760 hours total). This sum becomes the numerator. The denominator is the sum of the hourly cooling load in tons-hours (simply 8760 x cooling load). The resulting percentage is essentially the amount of time the economizer can be used. 

The analysis using Chicago weather data depicts these efficiencies monthly by economizer type (Figure 1). When analyzing a climate that is south of the equator (Figure 2), the data will show the greatest savings during the “summer” months in the northern hemisphere. Another way to look at the efficacy of the economizer is the supply air temperature it can produce with no mechanical cooling. Depending on the climate type, some economizers can be used nearly 100% of the time with little or no mechanical cooling. These are examples of data visualization techniques that are useful to gain a quick understanding of the potential energy reduction.

Figure 2: When analyzing a climate that is south of the equator, such as in Sao Paolo, Brazil, the data will show the greatest savings during the “summer” months in the northern hemisphere. Courtesy: HP Technologies Services

Economization strategies 

Because the economizer will be a major driver in the energy efficiency of the overall system, it is useful to group cooling systems by economization technique and then by types of components used in the system, as shown in Figure 3. (Note: this analysis is intended to compare the energy use characteristics of the alternatives; no judgment on the operational efficacy of the systems is implied.) 

Figure 3: It is useful to group cooling systems by economization technique and then by types of components used in the system. Note: this analysis is intended to compare the energy use characteristics of the alternatives; no judgment on the operational ef

Direct air—When conditions allow, air is taken directly from outdoors and mixing data center return air with the outdoor air. Out-of-range moisture levels of the outdoor air will limit full use of the economizers. Adiabatic cooling can be added to extend the use of the economizer. At higher outdoor temperatures, outside air volume can be modulated to maintain the lowest return air possible with compressorized cooling handling the balance of the cooling requirement. 

Indirect air—Heat from data center return air is transferred to the outdoor air using a heat exchanger (heat wheel, heat pipe, etc.). When the outside air is cold enough, the return air can reject 100% of the heat to the outdoors. At higher outdoor temperatures, the system will maintain the lowest return air temperature possible with compressorized cooling handling the balance of the cooling requirement. Adiabatic cooling to reduce the temperature of the outdoor air can be used to extend the use of the economizer. The inherent efficiency losses of the air-to-air heat exchangers will reduce the usefulness of the outdoor air temperature. 

Direct/indirect water—Water cooled directly using outside air is usually accomplished by open cooling towers that dissipate heat from the water into the air. This water can then be used to cool the evaporator of a packaged water chiller, cool the compressors in a self-contained computer room unit, or to cool computers directly. The water typically is run through a water-to-water heat exchanger to avoid fouling of the secondary cooling equipment. The temperature of the water that can be produced is dependent on the moisture level of the outdoor air, and at cold outdoor air temperatures, additional equipment may be required to avoid freezing in the cooling towers. 

Indirect water—Typically an air-cooled chiller is used to generate chilled water for air handling units (AHU), water-cooled IT racks, or for water-cooled computers in the data center. Economization is achieved by using a chiller-integrated free cooling coil or by a separate closed-circuit cooling tower. Because the heat transfer between the outdoor air and the water is completely sensible, the moisture content of the outdoor air has no impact on the water temperature that is produced using the economization technique. An adiabatic process, such as water sprays added to the condenser coils, can be added to lower the outdoor air temperature; in this case the moisture level of the outdoor air becomes a factor in the temperature of the water.

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