Towards More Effective Power Distribution at Data Centers

The second meeting in the PG&E series at the Critical Facilities Roundtable was about power distribution at data centers and was held at Cupertino Electric in San Jose.

The first speaker was Spas Lazarov of GE Energy Service, who spoke about points of power loss in data centers.

Spas Lazarov

The power use ratio in a data center varies from one data source to another, but Spas showed the ratio in the following picture.

Power use ratio in a data center

In his graph, I think the cooling ratio is a little too small, but it is within a reasonable range. The two items that consume the most power are IT and cooling. The power required for power distribution is 10–20%, small enough not to be very noticeable. There has been a lot of discussion about conserving power for IT (refreshing hardware and virtualization) and cooling (hot and cold aisle, containment, and air economizer). Although some work has been done, efforts to curb power consumption for power distribution have been minimum. Once power enters a data center, it changes its voltage and form a few times before it hits IT equipment. The following is a picture that shows the normal course of power distribution in a data center.

In a normal data center, UPSs are placed to prepare for power loss or poor quality power.

There are several UPS configurations, but Spas talked about double conversion mode and high efficiency mode. In the double conversion model, power, which entered the data center from the grid, always goes through the UPS. This is to ensure that IT equipment always receives good quality power in terms of frequency and voltage. Power from the power grid is AC. When that AC enters a UPS, AC is transformed via a rectifier to DC power for the battery inside the UPS. Then DC power is taken out of the battery and converted to AC power via an inverter. Two conversions introduce power losses, with an additional loss at the battery. If instead the UPS is operated under the high efficiency model, which bypasses the UPS in a normal situation and taps the UPS battery only in the case of an emergency, power loss is minimized, as shown in the next picture. In the figure, PUE increases because more power is fed to IT equipment rather than wasted in power distribution.

Spas pointed out that power quality in the US and most developed countries is good enough not to worry about. He gave us the example of a 50,000 square foot data center with a 5 MW load and a power price of $0.01 per kilowatt hour saving $3M annually by means of the high efficiency model. He also said that if all UPSs were operated under this model, $3B could be saved globally.

Virtualization: Optimized Power and Cooling to Maximize Benefits

This is great, but what if the supply power is cut off from the grid? Can your servers survive the power interruption? Spas showed a diagram, below, with some details about the way power components are put together inside a server.

Normally, a server contains a capacitor. As in the power grid, which has capacitor banks at strategic locations like substations and distribution networks, this capacitor inside the server maintains power for the server in case of power loss from outside. As shown in the picture above, a typical capacitor can maintain power for 20 ms in case of power loss. When you check your server’s specification, that information should be described on it.

Spas also showed us that operating UPSs in the high efficiency model does fit within the ITI (CBEMA) Curve set by the Information Technology Industry Council. ITI is the high tech sector’s advocacy organization in Washington, D.C., and in foreign capitals around the world. It deals with such issues as corporate tax reform, trade, telecommunications, cybersecurity, energy efficiency, workforce and STEM initiatives, regulatory compliance, accessibility, and environmental sustainability.

The ITI (CBEMA) Curve and application note describe an AC input voltage envelope that typically can be tolerated (no interruption in function) by most information technology equipment.

As I dive into smart grid and power engineering, I revisit the issues in electrical and mechanical functions in data centers. Although I came from the IT sector, I think I am beginning to understand the facilities perspective a little bit better.

Zen Kishimoto

About Zen Kishimoto

Seasoned research and technology executive with various functional expertise, including roles in analyst, writer, CTO, VP Engineering, general management, sales, and marketing in diverse high-tech and cleantech industry segments, including software, mobile embedded systems, Web technologies, and networking. Current focus and expertise are in the area of the IT application to energy, such as smart grid, green IT, building/data center energy efficiency, and cloud computing.

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