Alternating current (AC) power is ubiquitous in data centers, and it's hard to change the status quo. But a direct current (DC) power demonstration project conducted by the Lawrence Berkely National Laboratory produced some interesting results: a 7% energy savings over top-notch AC technologies.
SearchDataCenter.com recently reported on an experiment in using DC power in a data center at Syracuse University, furthering the practical research into this data center power option. While total DC power infrastructure for data centers isn't quite ready, these investigations are putting the concept on the top of mind for data center professionals concerned about power consumption.
In this face-off, two power professionals from vendor companies debate the merits of DC and AC power for the data center. Rudy Kraus is CEO of Validus DC Systems, the company that is working with Syracuse University on its DC in the data center experiment. Neil Rasmussen is Senior Vice President of Innovation at APC, one of the leading providers of AC power equipment.
Rudy Kraus: DC the way to go
Neil Rasmussen: AC here to stay
The advantages of DC power in the data center
By Rudy Kraus, CEO, Validus DC Systems LLC
Direct current is the native power resident in all power electronics. Every CPU, memory chip, disk drive, etc., consumes direct current power. Alternating current was chosen as a power path based on criteria set 100 years ago, 50 years before power electronics existed.
By leveraging -575/-380/-48 VDC, organizations can achieve numerous benefits over a traditional alternating current design. These include energy efficiency, reliability, a smaller footprint, lower installation and maintenance costs, scalability, easier integration of renewable energy, utility rebates and credits, and safety.
The energy efficiency of DC systems is a measurement of the end-to-end system efficiency, not merely a single component within the system, which is how organizations pay for power. Total energy savings can reach upward of 30% for both mechanical and electrical power savings. Because of this efficiency, DC systems can use various utility rebates and credits available for corporations. In addition, there are EECs and RECs available that further make the choice a solid business decision.
There are fewer power components in a direct current system, making it more reliable than an alternating current (AC) system, because there are fewer pieces to fail. With fewer power conversions, there is also less heat to affect the electronic equipment. A direct current system does not have sine waves or frequency to synchronize across multiple sources, which eliminates multiple points of failure and greatly simplifies the system.
Because a direct current power infrastructure has fewer components, it requires a smaller physical footprint than a traditional alternating current design. In addition, the bulk rectifiers can be sized up to 2.5 MW is a single unit, allowing a denser power factor over AC topologies. This space savings can amount to hundreds of millions of dollars saved over the life of a system. DC systems are designed to accommodate modularity to support a scalable infrastructure growth strategy.
With fewer components, the initial equipment and installation costs are less expensive than AC systems. Likewise, the overall maintenance costs are reduced by up to 50% over an alternating current system.
The majority of renewable energy sources generate direct current power, so there is no need to add multiple conversions to accommodate an alternating current power path. The energy sources can provide their direct current power straight to the power path, eliminating inverters and saving significant amounts of energy.
By utilizing simple power electronics and appropriate DC circuit breaker technology, DC systems can be deployed that are safer than AC systems from 380 to 600 volts. When using 48 VDC power, the system is considered "touch safe" from a code perspective, safer than 110/220 VAC systems.
The world's most reliable platforms already run on direct current. Nuclear submarines, aircraft carriers, data centers (UPS systems are backed up by DC battery strings), manufacturing facilities and telecommunication centers all run direct current. The long-term proven track record of direct current combined with the numerous financial and reliability aspects make it a natural choice for expanding or new data center environments.
Rudy Kraus is the founder and Chief Executive Officer of Validus DC Systems LLC. He brings over 20 years of innovative engineering design and entrepreneurial expertise to Validus. He holds a number of patents and currently has several patents pending for both electrical and mechanical applications.
Why is AC power use in data centers preferable to DC?
There is no real choice between AC and DC power today because over 98% of available IT equipment can run only on AC power. The use of DC power in data centers is very small and has actually declined significantly over the past decade.
Nevertheless, there are a number of proposals for the industry to consider adopting a new DC distribution standard based around 380 V or 500 V, which claim to offer advantages over existing AC systems. Some telecom providers and cloud computing providers plan to build demonstration DC data centers, where they will have IT equipment customized for them to run on DC. Hopefully this will allow the industry to gain experience with DC over the next 10 years.
While the question of DC versus AC is of academic interest, AC is the only practical option for data centers in the foreseeable future. AC is ubiquitous, it is increasingly efficient, it is proven, it works, and it is certainly not going away.
What are the disadvantages of using DC power?
In 2006, a demonstration project reported a 7% efficiency advantage for DC power in the data center when compared with AC. This created a lot of interest in DC, because 7% represents a significant amount of energy. However, since then there have been huge advances in the efficiency of AC power systems for data centers, which, according to reports from The Green Grid, have effectively negated any expected efficiency advantage for DC.
For example, AC power supplies for servers that had 30% energy loss a few years ago are now required to have less than 8% loss to achieve the Energy Star qualification. This is a reduction of over 70% in losses. UPS systems have also recently demonstrated remarkable improvements in efficiency.
Proponents of DC power rightly criticize the inefficiency of historic AC systems, but they ignore the reality that high-efficiency AC systems are already available now. For a new installation, a complete AC power system can be over 96% efficient, which is just as efficient as hypothetical DC designs. Even if DC has no efficiency advantage, there are other analyses that suggest DC might ultimately be more reliable or even less expensive than AC, but these are purely hypothetical until there is significant experience with actual DC data centers.
Why can the potential risks and expenses associated with using DC power in a data center not
be mitigated enough to make DC a truly viable data center power choice?
The risks and expenses of DC today are indeed quite high. In addition to the special safety risks of high-voltage DC with respect to electrical arcs and fire, one of the most difficult issues is the business risk of trying to plan a hybrid data center that provides both AC and DC during any transition period.
There are also major risks associated with the evolving standards relating to DC, which might make early DC designs obsolete or even illegal before the end of their useful life. But these problems are not a fundamental property of DC, they are due to the immaturity of regulations, lack of industry competencies and immaturity of products related to DC.
The key question for DC is why any early adopters would be interested in taking on the large risks and expenses of a DC transition, given that the efficiency advantages are now known to be very small or zero. Practical data center operators interested in efficiency will use high-efficiency AC systems and focus their efforts on improved cooling plants, where substantial inefficiencies still exist.
Neil Rasmussen is Chief Innovation Officer of APC. Neil is currently leading the effort at APC to develop modular scalable data center infrastructure solutions and is the principal architect of APC's InfraStruXure® system.
This was first published in March 2010