Ashrae Journal - October 2008 - 35

While row-based designs addressed the issue of proper heat removal and cold air supply, they also brought with them inherent energy-efficiency advantages. The first of these was a reduction in fan power requirement to move the air. by IT equipment further compounding energy consumed by fans. To address the air delivery and heat removal challenges of CRAH and CAHU systems, row-based cooling systems have begun to appear in many data center designs (Figure 3). To address the separation of cooling units and heat loads, rowbased designs place the air-conditioning units in the row of rack enclosures. Incorporating a hot/cold aisle design, heat is removed from the hot aisle as it is dispelled from the IT equipment. The hot air is then cooled and discharged to the cold aisle. While row-based designs addressed the issue of proper heat removal and cold air supply, they also brought with them inherent energy-efficiency advantages. The first of these was a reduction in fan power requirement to move the air. Close coupling to the heat load allows for a much shorter air delivery and heat removal path. This represents a shift in the mindset of data center air distribution from cold air supply to heat removal. Removal of heat from the hot aisle before it has a chance to mix with surrounding air in the room makes the remaining areas in the room a large volume of supply air. With this in mind, the length scale for air delivery in row-based systems is only a few feet (varies with number of racks and air-conditioning units). In most CAHU and CRAH implementations it is necessary to maintain a fixed fan speed to deliver the necessary pressure for uniform airflow through delivery vents. In close-coupled designs, such as row-based, the static pressure requirement is significantly less, with only the cooling unit resistance to overcome. Without the requirement for constant pressure, rowbased designs allow for variable air volume to scale back fan speed with heat load demand. This feature boosts the energy efficiency through part-load operation with increasing gains at lower loads as shown in Figure 4. Eliminating mixing of hot and cold airstreams produces another energy benefit resulting from much warmer return air temperatures to the cooling units. Some advantages to warmer air return temperatures are: • An increase in cooling capacity per unit that reduces the overall cooling footprint. The warmer return air temperatures provide a higher temperature differential to the cooling coil over rooftop and perimeter systems, and, therefore, more heat removal. • More effective capture of hot air enables a much warmer October 2008 supply temperature (no need to overcool the air to compensate for mixing). • Limited or no condensate removal, reducing makeup humidification requirement. Several row-based configurations are available in the market, which use varying placement of the cooling unit in the row and different methods of heat rejection. While these approaches to row-based cooling can be compared for the best energy efficiency, the real energy gain is with the row-based architecture over distributed air delivery systems like perimeter CRAH and CAHU systems. The following comparison of these different architectures illustrates the energy-efficiency advantage of the row-based architecture. Data Center Cooling Architecture Efficiency Comparison Let’s compare three cooling architectures for the cooling of a mission critical information technology space. The key metric for this comparison shall be power consumed by the cooling infrastructure versus power dissipated by information technology equipment. This comparison attempts to understand and account for all power consumed across the entire length scale of thermal transport (IT rack exhaust to outdoor ambient). Efficiency Metric = Cooling Power [kWh ] IT Power [kWh ] (1) The general format of the metric equation, from above, yields the ratio of cooling power to IT power. Proper understanding of this metric reveals that the lower the value, the more energy efficient the cooling architecture. Symbols and Constants Used = = = = = = = H = g =  Q NetSensible = CpAir CpWater ρAir ρWater η ηPump  V Specific Heat Air, 1.022 kJ/kg · °C Specific Heat Water, 4.188 kJ/kg · °C Density Air, 1.173 kg/m3 Density Water, 999.7 kg/m3 Fan or Pump Efficiency Pump Efficiency, 0.65 Volumetric Flow Rate (m3) Head Loss (m) Gravitational Acceleration 9.81 m/2 Net Air Handler Cooling Power (kJ · sec) ASHRAE Journal 35

Ashrae Journal - October 2008

Table of Contents for the Digital Edition of Ashrae Journal - October 2008

Ashrae Journal - October 2008

Contents

Commentary
Industry News
Letters
ASHRAE HQ Dedication Booklet
ASHRAE Rebuilds HQ to Reflect Mission
Feature Articles
ASHRAE's Living Lab
Meetings and Shows
Sizing Pipe Using Life-Cycle Costs
Feature Articles
Energy-Efficient Data Centers: A Close-Coupled Row Solution
The Future of 90.1
Ventilation Standard for Health Care Facilities
Grosvenor Humidity Chart
ASHRAE Research Report
ASHRAE Honor Roll
Building Sciences
Special Products
Emerging Technologies
Product Showplace
Classified Advertisting
Advertising Index
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Contents