“Going Green” is applied to construction, vehicles, recycling, or just about any activity or product. If you search for it on Google, you will get more than 200 million hits. In articles last year, we covered establishing an energy-efficiency program and conducting energy audits, both ways to make the facility more green. From the power quality perspective, green is more likely associated with improvements in the bottom line of the financials by keeping productivity high and damage from power quality disturbances low, but there are several ways in which these two concepts can overlap and double the green benefit.
The “War of the Currents” between Westinghouse and Edison in the 1880s has resurfaced in the past few years. Edison championed direct current (DC) power generation and distribution; Westinghouse, with his stack of patents that he purchased from Tesla, promoted alternating current (AC). Though AC has ruled for the past 130 years, DC is proving to be the greener of the two in many applications with today’s loads.
The data center is just one example of how conducting business traditionally is not the best green alternative. Anyone who has worked on any electronic equipment knows that the digital circuitry—from the processors, to the memory, to the components that make it all play together—runs on DC power. While processing power and speeds have increased, DC levels have decreased. Uninterruptible power supplies (UPSs) back up nearly all data center equipment, and UPSs often have batteries, another DC component, as their backup power source. In addition, some of the greener buildings have solar-power generation and/or fuel cells, both DC generators.
But how are these all tied together? The AC power from the utility and/or backup generator is converted to DC by the UPS and then back to AC to feed the power supplies of the servers, modems, routers and other information technology (IT) equipment that, in turn, convert it back to DC to run the digital circuitry within them.
Every conversion introduces an efficiency loss. Conversions can amount to 25–40 percent losses from the power that is supplied to the facility to the power that is actually used for computing.
Though data centers today account for approximately 3 percent of the total electricity consumption in the United States, this is increasing as their numbers and power densities (typically 100 watts per square foot) increase to the point where it takes nearly as much electricity to get the heat out of the building as it does to run the IT equipment. One solution is to supply DC power throughout the building. Not only can the IT equipment and newer lighting run on DC power, but the adjustable speed drives (ASDs) that run the heating, ventilating and air conditioning (HVAC) systems also can run on DC, potentially limiting the losses in their AC-to-DC conversion. Improved efficiencies of 20–40 percent have been reported from initial projects.
This concept can apply to commercial and industrial buildings as well, where there are significant IT equipment loads and ASDs for HVAC and in the manufacturing process. ASDs have a power conversion from AC to DC as the first stage, like a UPS. With improved efficiencies and fewer AC to DC conversions, there is less load current being drawn and, with it, fewer harmonic currents. Fewer harmonic currents also means fewer harmonic losses in transformers and electric motors on the AC portion of the system. Higher efficiency also means less heat from the losses, requiring less power for cooling is needed. Less equipment also means fewer failures that result in productivity interruptions. DC voltage also implies no flicker problems, no frequency problems and no zero-crossing problems. In addition, a DC power source can have a large amount of capacitance on its output, allowing longer ride-through times that can significantly reduce sags and even interruptions.
Though DC systems aren’t about to overtake the industry, it is a good time to start reviewing what the effect is on installation and maintenance. The National Electrical Code already addresses many aspects of DC systems. For example, Article 310—Conductors for General Wiring, doesn’t distinguish between AC and DC when it covers the general requirements for conductors and their type designations, insulations, markings, mechanical strengths, ampacity ratings and uses. Root-mean-square (rms) in an AC system is essentially the DC equivalent when it comes to power. Part VIII of Article 250 specifically covers DC systems, including grounding, bonding and grounding electrodes, while Article 690 and 692 cover photovoltaic and fuel cell systems, respectively. However, AC and DC systems don’t necessarily have interchangeable parts. Some components, such as switches and breakers, may not be rated for DC or may have a lower rating if used in DC circuits.
The future is getting greener, and being prepared to work on DC systems can make it greener for you, too.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.