Stemming losses: DC vs. AC power in the data center

Delivering DC vs. AC power to a data center can potentially reduce electrical conversion losses and save electricity costs.

There are proponents on both sides of the DC vs. AC power debate. Here are a few answers specifically for data...

centers.

The global power grid distributes electricity in the form of alternating current (AC) instead of direct current (DC). The choice of AC over DC dates back to the 1800s when Thomas Edison first touted the simplicity of DC, while notables like George Westinghouse and Nikola Tesla supported the use of AC. Since AC proved easier to deliver commercially across great distances using thinner -- and far less expensive -- copper wiring, the industry ultimately adopted AC.

However, AC is not necessarily the most efficient means of delivering power, and the use of DC to data center racks and systems has gained a following as power costs force organizations to watch their power budgets. Let's consider several key issues in DC delivery.

What is direct current in the data center?

The problem with AC is loss. AC initially leaves a power plant at very high voltages. As those voltages are carried to cities, towns, neighborhoods and individual buildings, those high voltages are divided down several times using transformers. And even once the AC voltage enters the building at a moderate 600 VAC or 480 VAC, it must be stepped down again to 240 VAC or 120 VAC to feed the rack servers' power supplies, which convert the AC into several DC voltages that power server components such as the processors, memory, disk drives and so on.

The AC-to-DC translation is not perfect, and a certain amount of loss occurs with each conversion. But you're paying for all of the electricity that enters your facility whether it's used or not, so those conversion losses cost the business money. DC proponents suggest that a single conversion from AC to DC would eliminate much of this loss and be far more efficient. The resulting DC would then be distributed to racks and systems throughout the data center, displacing traditional AC power cabling and subsystems.

In the DC vs. AC power debate, what benefits should I look for?

The general benefits are efficiency and cost savings. The concept is straightforward; you save money by eliminating points where power is lost during conversions. Lawrence Berkeley National Laboratory in California performed a demonstration back in 2006 which compared AC and DC power in the data center. The laboratory claimed data centers could save up to 20 percent on power costs using DC distribution.

In addition, power supplies in individual servers or other hardware systems would essentially be removed since power would already arrive at the rack in DC form, which need only be regulated down to lower voltages as needed. This would eliminate the need for redundant power supplies along with their noisy and failure-prone power supply cooling fans.

The actual amount of savings through DC power distribution remains a matter of debate, and later testing performed by other groups -- such as The Green Grid -- questioned the ultimate difference between AC- and DC-powered data centers. For example, The Green Grid's report concludes that there are no significant differences between power distribution approaches -- mainly because no single AC or DC configuration is more efficient under every possible load condition and because servers and electrical distribution equipment are constantly getting more efficient with each new generation.

Still, when savings are realized, the actual benefit will be greatest for the largest data center operators handling multimegawatt installations. Today, giants such as Google and Swiss hosting company green.ch are among the organizations to deploy DC-powered data centers.

What equipment or changes would be needed to support DC power in my data center?

One of the biggest obstacles to DC adoption is that the technology is highly disruptive. It's not simply a matter of switching the electricity from AC to DC. A DC-driven data center would require an entirely different electrical distribution system and wiring to the racks. The electrical distribution would also need to integrate on-site generators so that backup generator power would be converted to DC for the facility.

And the renovations go right to the servers and systems. Existing servers and other hardware systems cannot be retrofit for DC, so an entirely different suite of hardware would be needed. Uninterruptible power supply systems which depend on AC-to-DC conversion to charge internal batteries, and on voltage inverters to convert DC back to AC, would need to be replaced with DC-only units. Organizations will typically wait for a new data center build before deploying DC power.

With rising energy costs and the demands of constant business availability, determining who wins the DC vs. AC power struggle is important. In the end, replacing the traditional AC power distribution scheme with a DC power infrastructure can reduce energy losses by eliminating extra power conversions. When properly deployed, this can save money on the monthly power bill and simplify equipment by removing separate power supplies, lowering its cost and improving reliability.

This was last published in December 2012

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DC or AC power in the data center?
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DC power in data centers certainly improves
electrical efficiency
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I think hardware availability is a big part of the initial challenge.
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DC is fully regulated and can reduce spikes that happen in AC.
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Devil's advocate here, datacentres should prioritize uptime. That means in a real sense using well known methodology. Google and green.ch might be touting the 'greener datacentre' but the actual cost at the end of the day for needing to find new and reasonably untested redundancy solutions (or plain multisite redundancy) is just not a suitable solution for a company which has any value in continuity and disaster recovery.
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We have started this conversion already. All blade chasi distribute DC internally. Scale out and you get whole datacentre running on DC.
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AC would be lot less a hazard to personnel
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There will always be a conversion. It's just a matter of where.
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DC is smoke and mirrors
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Interesting article. But does it really worth the change? Are there any studies with specific figures, costs and savings within 1, 2, 5 years after the investment?
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DC-DC regulation is usually more lossy, so not sure this makes a lot of sense
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Every % drop in DC power & cooling costs translates to millions of dollars to the bottomline.
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Since the input is transformed to DC anyway, it seems that DC is a better choice reducing the losses involved.
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For a small DC, the cost of converting all the AC equipment to support DC would be more than the savings becuase of conversion losses. Also you cannot use off the shelf servers in future. You have to order DC PS based system, which is not easy to get if you do not have a bulk order.
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AC Supply to till the premises is a more viable & efficient transmission & distribution mechanism.
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dc is mostly used in electronics field works.
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The problem with DC distribution is the amount of Copper required for the pannels.The distribution switches will need to be of very high rating from 100 Amp Circuit breaker it will need to handle 1000Amps and it would not be possible to run a Datacentre unless allvendors supply DC equipments
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now there are some pilot projects arising,especially in telecom industry as the equipment needs DC powerin datacenter you will have to customize theservers etc.longway to go for a DC application in data center.
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If DC is more efficient than naturally it is the preferred choice however lots has to happen to make that a reality for smaller shops.
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becouse of after the conversion from ac dc on the power supply. every enformation on the data are communicat on dc mood.
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Agreed for the DC Power sidtribution for more power consumbtion stabilization & energy savings.
Also to avoid any loads to subject any fluctuations
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I think DC is more energy efficient, so it is only a matter of time until all the necessary building blocks will be in place for widespread usage.
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Dc is better
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DC distribution makes sense for loads (servers and switches in this case) which are inherently DC. More and more loads, from consumer electronics to EVs are DC by nature so it would seem obvious that the distribution of DC power would be more in tune with the loads. As to the conversion losses, the modern PE (Power Electronics) is comparable to the AC transformer losses so reducing the number of conversions, being AC-conversions or DC-conversions, would lower the overall losses.
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It´s more effective than AC, less noise.
Finally all equipment are fed from DC through their power suppliers.
The matter is select an apropriate voltage.
On operations is more reliable DC rectifiers than UPS´s
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DC offers a great strategy to truly simplify data center design while creating opportunities to save energy, floor space, construction and maintenance costs while enabling the use of more efficient technology equipment.
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Because the Data Center Industry is not ready and rebuilding an existing Data Center is too costly. Maybe a solution for 'Data Center in a Container' where the DC comes and goes as a building bloc.
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Responsible engineers and ITC specialists understand the conversion of commercial power from ac to DC can waste up to 25% of the energy consumed in a data center. This includes the power to cool the unnecessary power conversion generated heat.
A) Take a 2MW Data center, all AC powered:
1) 400VAC into TRUE On Line UPS: 480 rectified 95% efficient ,inverted back to 400VAC 92% efficient.
2) Computer AC switching power supplies 93% efficient.

B) The same data center DC powered
1) 400VAC Rectified 95% efficient.
2) Computer DC voltage divider power supplies 98% efficient.

comparing A to B all else being the same efficiency (distribution system) .
System A wastes (5% + 8% + 7%) @ 2MW. 100KW+ 152KW+ 122KW= 374KW

System B wastes (5% + 2%) @2MW.
100KW + 38KW =138 KW

236KW is only the conversion losses then add the cooling another 236KW and don't forget you will need to have additional power losses for the inefficient cooling.
In conclusion when considering an upgrade or building new data centers the DC powered data center is by far more efficient and capital costs may be less if implemented correctly. To retrofit a Data center may not be economical.
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less losses
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The preponderance of distribution investment today is AC. Conversion to DC will require equipment and staff training investments that have yet to be quantified, so an ROI remains unclear. Google can afford the innovation risk, but it will be a few years (3 - 5) before most DC operators are ready to take a DC plunge.
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Most of the devices run using AC Power..
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DC is more efficient in managing and a single conversion from AC to DC is better than conversion by each and every device in the data centre (each server/storage/switch component).
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Only really effective for a brand new extremely large project - not practical for heterogeneous data centers.
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It will take years, even decades, but DC will eventually win.
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Is grid dependence the real limiter against on-site DC? I have heard about a guy who has high, over fuel cell, energy efficiency generation tech. He just doesn't know where go to connect with data center power infrastructure decision makers.
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DC is Best
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Solar roof panels :)
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On the 50 K altitude vision, it looks really challenging and disruptive... with the right approach we can minimize the risk to zero and benefit great advantages. The simple view that data centers equipment needs to be completely changed is not entirely correct. On the other hand, replacing older technology can result in higher capacities, less maintenance costs that could result in positive ROI
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