DC Power Systems: Keep Your Eyes Open

As efficiency becomes a dominant mantra today and in the future, dc is getting another look.

02/07/2014


Although alternating current (ac) has become the dominant medium for the centralized grid and nearly all commercial power applications, direct current (dc) is getting another look for its efficiency. In fact, dc now figures into discussions relating to the grid and to homes, businesses, commercial facilities, and microgrids. High-voltage dc (HVDC) lines are an efficient means to send bulk power over long distances on the grid.

And using dc-generating, renewable energy resources to power dc-consuming facilities – most notably, voracious data centers – promises efficiencies on a scale that makes this application highly cost effective and, therefore, virtually inevitable. Most electronics, such as data servers, run on dc power. Today, we typically take the grid’s ac power and convert it to dc for many electronics-based applications. Solar photovoltaic (PV) cells and wind turbines, however, produce dc power.

Device-integrated inverters convert the power into ac, and then it is rectified for electronic use. By some accounts, the losses in efficiency from inversion and rectification may reach as high as 29%. Using dc power for dc applications also has the attractive benefit of reducing inefficiencies that drive the buildup of heat at the application level. Data centers expend substantial amounts of power and money to shed heat and keep facilities cool for optimal operations.

The use of dc power would eliminate much of that heat and its related costs. Cost savings for solar PV cells and wind turbines are another major benefit because they will no longer require built-in inverters.

To achieve these benefits, a variety of groups are looking at resolving the challenges to employing dc power for dc-consuming applications, particularly for residential and large commercial settings where efficiencies and their derived savings could outweigh costs. For instance, IEEE’s Industry Connections program formed the Indian Low Voltage DC Forum to explore the topic. Meanwhile a working group that draws on both the IEEE Standards Association and Power & Energy Society (PES) formed this past summer to explore IEEE’s Industry Connections program to explore the topic.

Meanwhile, a working group that draws on both the IEEE Standards Association and Power & Energy Society (PES) formed this past summer to explore dc-to-dc systems in the home. 

E-Merge Alliance, an open industry association, is dedicated to developing standards to enable the rapid adoption of dc power distribution in commercial buildings. In fact, late last year, E-Merge announced the first registered products for dc power distribution in data centers, supporting net-zero commercial buildings. All three of these efforts are working to quantify the economic benefits of commercial and residential dc power use. 

As one can imagine, the are many challenges. Some are technical, while others are regulatory and even political. Let’s look at the technical issues from the perspective of the consulting-specifying engineer who may be called upon to design the power system for a new data center or to advise on retrofitted efficiencies for an existing facility. This review is hardly prescriptive, as the technical solutions are only now coming into focus. 

First, there’s the question of whether existing ac-based distribution systems can simultaneously handle DC power or if dedicated dc lines are necessary. There’s also high potential for tying onsite dedicated renewable dc generation directly into a facility. Second, today’s commercial building codes must be updated to include dc power.Third, switching to dc power requires the creation of a different class of equipment for circuit breakers and other protection gear. Fourth, the question of cabling arises: Can we use existing wires to run dc rather than ac. Fifth, what are the appropriate dc voltage levels for various applications? 24 volts? 48? 125?

Finally, should a facility decide to employ dc power created by its own distributed generation sources? What is the backup power scenario? Energy storage? Grid-supplied dc power? Grid-supplied ac power that must be inverted and rectified?
Although these technical issues must still be resolved, the promise of significant efficiencies provides sufficient incentive to overcome such challenges.

The consulting-specifying engineer would do well to monitor developments in this area, including related state and local regulatory discussions. Unless you’re retiring tomorrow, it’s likely that you’ll need to be well versed in the topic as dc rapidly becomes viable in residential and commercial applications. 

Sam Sciacca is an active senior member in the IEEE and the International Electrotechnical Commission (IEC) in the area of utility automation.



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