Direct Current Power Distribution in a Data Center

When I started this blog about two years ago, there was a lot of discussion of direct current (DC) power distribution at data centers. It is very easy to see why DC is better than alternative current (AC) power distribution. AC power enters your data center from your utility. It then goes to a set of UPS that take in AC but convert the power to DC. This is because the power goes through a set of batteries (which only takes DC). Then, at the other side of the UPS, it is converted back to AC. Then the power is distributed to IT equipment via PDUs. This AC power is then converted again by the IT equipment for its internal use. Each conversion loses some percentage of power.

 If the AC power entering the data center is converted only once to DC power and distributed to IT equipment that takes DC as input, there would be no conversion loss. Unfortunately, a few years ago, Green Grid put out a white paper comparing the power distribution of DC and AC and concluding that there were very few differences between the two configurations. After reading it, I moved my focus away from power distribution. Now I am back on this subject because some friends who started a company in that area and Keizo Hoshijima of NTT Facilities pitched me the merits of DC distribution recently.

 I cannot forget another DC power distribution advocate, Dennis Symanski of EPRI. I met Dennis when he was a panelist in my Nordic Green panel session.


 Dennis Symanski

 At the conference, he gave a presentation on DC power distribution. Cooling is known to consume about 30–60% of power in a data center, but we seem to be getting a handle on that. Once cooling and other culprits are under control, then we can pay attention to power distribution to further cut power consumption.

 Recently, I had a chance to visit Dennis at EPRI to chat about the current status of DC power distribution at data centers. The following is an edited summary of our conversation.

Q: What do you do at EPRI, and what is your background?

A: My focus is to make any computer equipment energy efficient (EE). Prior to coming to EPRI, I spent 18 years at Sun working on international standards and regulations. EPRI is funded by utilities but is an independent nonprofit research organization. It strives to "do good for society,” and it is quite refreshing for me when I do not have to pay attention to revenues and stock prices.

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Energy Impact of Increased Server Inlet Temperature
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Q: So what’s new in the DC power distribution area?

A: In 2006, we conducted a set of experiments on the use of DC power distribution at Sun (currently Oracle), along with LBNL, Ecos Consulting, and other vendors. Our conclusion was that DC power distribution increases data center EE and reliability. Since the experimentation, we formed an organization called DC Power Partners to further our efforts. The members include LBNL, EPRI, EMC, IBM, HP, Oracle (former Sun side), Intel, and even APC. (ZK; APC had a different opinion of DC power distribution before.) We have a once-a-month teleconference to discuss new technologies and installations to share information. In addition to the U.S. companies, some European and Japanese companies call in.

Q: You only cover technology aspects at your DC Power Partners?

A: EMerge Alliance works on DC lighting, led by Armstrong Ceiling for 24 VDC. DC Power Partners is joining the alliance. They can take care of sales and marketing, trade shows, and web promotion.

Q: Are there any major changes in the DC power distribution area?

A: There are no drastic changes. We are currently working to make sure the components that comprise DC power distribution comply with UL and/or FCC regulations. For example, connectors that are used for DC power distribution.

Q: I have no issue with the technology. What about the market? Which region of the world is more receptive to this technology?

A: In general, the telecom industry is keen on picking this up. Companies like AT&T, Verizon, and NTT. In Europe, the European Telecommunications Standards Institute (ETSI) has prepared a specification for 380 VDC. There is at least one CO in each city and dozens in large cities, so there are 10′s of thousands of COs in the U.S.

Q: But IT-centric data centers and COs are different, and the IT data centers may not be so enthusiastic about DC power distribution.

A: They used to be quite different, but, these days, they are becoming increasingly similar as they share the same kind of IT equipment. Look at AT&T, which needs to process IP traffic coming from iPhones and iPads.

Q: A couple of years ago, I read a white paper from Green Grid (GG) that said there were few differences between AC and DC power distribution. What is your take on that?

A: GG members do not include any direct current facilities equipment suppliers. Several of the GG IT equipment suppliers are researching direct current power supplies for their equipment.

Q: What do server vendors need to do to support DC power distribution?

A: All they have to do is to swap the AC power supply with the DC one. The direct current power supplies have a smaller component count, are more efficient and as a result, research demos may show them to be more reliable. It is not hard to create a data center solely with DC power distribution. IBM created such a data center at Syracuse University.

Q: Is there any data center with a solely DC power distribution system in the San Francisco Bay area?

A: Several are going in parallel. But one in the Bay Area will be announced in November.

Q: Right now, generated power is carried via the AC transmission system. If DC power enters a data center, we do not need to convert power at all. Can you transmit power via DC?

A: In the days of Edison vs. Westinghouse, we could not transmit DC power at 5,000 V or higher. These days the high DC voltage is transmitted over a long distance. A hydropower plant in northern Quebec transmits generated DC power to New England over several hundred miles, in addition to much-closer Toronto and Montreal. (ZK: High-voltage direct current is a technology for transmitting high voltage DC over thousands of miles)

Q: Is there anything else I should know about what EPRI is doing in the DC area?

A: We talked about the power supply for IT equipment. Those power supplies were very inefficient. The conversion rate was only 65% on the average. 80 Plus is an organization that promotes increasing the conversion rate to at least 80%. EPA adopted their specification, and EPRI is running a test lab for 80 Plus.

I got very useful information talking with Dennis. EPRI covers a large area of energy and its efficiency. I have not heard much about DC power distribution recently, but that does not mean efforts on its behalf were terminated. On the contrary, they are going very strong, including DC for home and power transmission. I will report on this subject from time to time in this blog.

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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