Data-center power glossary

The EPA energy efficiency report

Servers and data centers make up a significant portion of the nation’s energy consumers, and a substantial share of this consumption is due to federal data centers. Therefore, the government is taking steps to analyze and better manage the electricity use of its data centers. In 2007, the Environmental Protection Agency (EPA) published a study examining the energy use of data centers in the United States (EPA Report 109-431). It found that:

•	Server rooms/data centers consumed about 1.5% of all electricity (or about 61 billion kWh) in the U.S. in 2006, at a cost of around $4.5 billion.
•	Federal server rooms/data centers accounted for 10% (or 6 billion kWh) of this electricity at a cost of $450 million annually.
•	Blade servers consumed 68% of the electricity used for IT equipment in data centers in 2006.
•	The amount of energy Blade servers use more than doubled between 2000 and 2006.
•	About 38% of electricity use is due to enterprise-class data centers and those growing most rapidly.

The EPA’s analysis shows that the energy costs in data centers arise not only from energy-hungry servers, but also from the cooling infrastructure necessary to support IT equipment.

Illustration 1: Data Center Power Draws

Data centers today are forced to redefine capacity from terms of space to terms of cooling and power. While efficient blade servers solve issues of space, they raise issues of cooling (by creating intense hot spots) and even the most power-efficient server can strain the electricity bill due to increased cooling demands (see also Energy Efficient Equipment in the Data Center in this issue).

Compared to 2000, the overall use of energy by U.S. servers and data centers more than doubled in 2006. If efficiency standards remain unchanged, the EPA report projects that national energy use by data centers could double again in the next five years. Combined with climbing energy prices, this means that energy needs will claim ever larger shares of data centers’ budgets, leaving less money for other areas such as expansion, new equipment, or general improvements.

Energy efficiency is a budget mandate as much as it is an environmental issue.

If you thought IT acronyms were hard to remember, wait until you sit down with your facilities team to discuss your data center's electric bill. You need to learn a whole new vocabulary when you start talking about lowering the building's energy use.

Here's a crib sheet of a dozen of the most commonly used energy terms and acronyms so you can learn the jargon for going green.

1. AC/DC

Yes, this is the name of Australia's greatest rock band, but it's also a key trend in data-center design. AC stands for alternating current, and DC stands for direct current. Leading-edge data-center designers are looking at power supplies based on DC power -- rather than today's AC power -- because DC power promises to be more energy efficient.

2. Carbon footprint

No relation to Sasquatch, although to corporate executives it can be an equally large and scary beast. A company's carbon footprint is the amount of CO2 emissions its operations produce. In setting goals to reduce their carbon footprint, many companies target their data centers because they consume 25% or more of the electric bill.

3. CFD

It sounds like the acronym for the Chicago Fire Department, but this version stands for computational fluid dynamics. CFD high-performance-computing modeling has been used for a long time in the design of airplanes and weapon systems. Now it's being applied to air flow in data centers for optimal air-conditioning design.

4. Chiller

This isn't what you drink at the beach on a hot day. Rather, it's a machine that uses chilled water to cool and dehumidify air in a data center. Of all the components of a data center's air conditioning system, this is the one that consumes the most amount of electricity -- as much as 33% of a data center's power.

5. Close-coupled cooling

This sounds like a technique that would come in handy on Valentine's Day. In fact, it's a type of data-center air-conditioning system that brings the cooling source as close as possible to the high-density computing systems that generate the most heat. Instead of cooling down the entire room, close-coupled cooling systems located in a rack cool the hot air generated by the servers in just that rack.

6. CRAC

This is not what you sometimes see when a plumber bends over, although it's pronounced the same way. We're talking about a computer-room air-conditioning system. CRAC units monitor a data center's temperature, humidity and air flow. They consume around 10% of a data center's power.

7. DCiE

This acronym has nothing to do with the nation's capital, although its pronunciation is similar. DCiE is the Data Center Infrastructure Efficiency metric (also called DCE for Data Center Efficiency). DCiE is one of two reciprocal metrics embraced by The Green Grid industry consortium; the other is Power Usage Effectiveness (PUE, below).(See "Two ways to measure power consumption.") DCiE shows the power used by a data center's IT equipment as a percentage of the total power going into the data center. A DCiE of 50% means that 50% of the total power used by a data center goes to the IT equipment, and the other 50% goes to power and cooling overhead. The larger the DCiE, the better. 8. kWh

Electric power is sold in units called kilowatt hours, 1 kWh is the amount of energy delivered in one hour at a power level of 1000 watts. This abbreviation for "kilowatt hour" is mostly used in writing rather than conversation.

9. PDU

The acronym PDU stands for power distribution unit, a device that distributes electric power. PDUs function as power strips for a data center and consume around 5% of the power in a typical center.


10. PUE

Not pronounced like the reaction to a bad odor, but one letter at a time. Power Usage Effectiveness is one of two reciprocal metrics embraced by The Green Grid industry consortium; the other is Data Center Infrastructure Efficiency (DCiE, above). PUE is the ratio of the total power going into a data center to the power used by the center's IT equipment. For example, a PUE of 2 means that half of the power used by the data center is going to the IT equipment and the other half is going to the center's power and cooling infrastructure. Experts recommend a PUE of less than 2. The closer a PUE is to 1, the better.

11. RECs

Pronounced like the short version of the word recreation, this acronym means renewable energy certificates or renewable energy credits. RECs are tradable commodities that show that 1 megawatt-hour of electricity was purchased from a renewable source, such as solar, wind, biomass or geothermal. An increasing number of companies are buying RECs to offset the amount of electricity generated from fossil fuels that their data centers consume.

12. UPS

We're not talking about the boys in brown, although the acronym is pronounced the same way. We're talking about uninterruptible power supply, which provides battery backup if a data center's power fails. It's essential that UPS equipment be energy efficient, because it consumes as much as 18% of the power in a typical data center.