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May 2011 -
Power & Communication
Cutting-edge technology delivering DC electricity has the potential to increase energy efficiency in commercial facilities — particularly in data centers — in the years ahead.
The EMerge Alliance, an industry coalition that encourages the use of DC- powered microgrids, has written standards for "touch safe" DC electricity in occupied spaces and is currently working on a standard for electric vehicle fast charging and use as electricity storage. In addition to energy efficiency gains, the use of a DC microgrid within a building could make space more flexible.
Electricity from the utility grid enters a building as AC power and has to go through a rectifier for any device that runs on DC power. Many devices used in buildings run on DC, and the process of converting AC to DC results in power losses. For example, a cell phone rectifier gets warm as it wastes energy converting AC at 120 volts and 15 amps to the 3 volts of DC and the milliamps needed to charge the phone.
Every time electricity is rectified (from AC to DC), transformed (from AC to AC), or converted (from DC to DC), a power loss occurs.
What's more, says Brian Patterson, chairman of the EMerge Alliance, and general manager, business development, for Armstrong Building Products, Armstrong World Industries, most alternate sources of electricity generation are either natively DC — photovoltaic panels and fuel cells, for example — or need to go through a DC circuit in order to produce 60 Hz of AC.
Electricity storage is also best done in DC (batteries, ultra-capacitors) or in a form of energy that needs to go through a DC circuit (flywheels, compressed air, thermoelectric electricity) to be useful, according to Patterson.
"We are not arguing AC versus DC, because you absolutely need both," says Patterson. Instead, he looks for hybrid AC-DC systems. "We need to balance our bias toward AC and make appropriate use of DC to save energy," he says.
A common misperception is that it's not possible to transmit DC power at higher voltages or that it is not possible to step it down. But, according to Patterson, engineers had figured out how to convert DC to lower voltages by the mid-20th century. Nonetheless, it is still AC electricity that enters our buildings and homes through the power lines, even though most electronic devices are using DC power.
"Semi-conductors began to evolve in the 1940s and 1950s and have become the predominant means of using power, and about 80 percent of power used in commercial buildings must go through some form of power electronics so it can be converted to DC," says Patterson, quoting from studies conducted by the Center of Power Electronics Systems at Virginia Tech.
There are power losses even with devices that use AC power. Devices such as variable speed motors or fluorescent lights may use AC, but not at 60 Hz. "You must put AC through a DC stage and then reconvert it back to AC on the other side," says Patterson. "Therefore, 60 Hz AC electricity must be converted by devices called rectifiers, to DC, to become useful." Whether the rectifiers are external or internal to the device, the rectifier consumes 5 to 50 percent of the power it converts.
Thanks to modern power electronics technology, voltages can be changed in DC power using DC to DC converters, particularly in the voltage range used in buildings, which is less than 600 volts.
Paul Savage, CEO of Nextek, a founding member of EMerge Alliance, says that when energy prices were low, there was little emphasis on efficiency, despite the fact that it was a good idea. "Only in 2007 things started to perk up because of the sustained rise in energy prices and the focus on efficiency as an energy policy plank," he says.
Native DC-powered data processing and power management equipment make up about 60 percent of the power load in a modern data center. The rest is air conditioning, which can be a mix of natively AC and DC equipment, says Patterson. The cooling system is only about 60 percent efficient when removing heat from the system; for every watt of electricity wasted in the form of heat, 1.7 watts of cooling must be used.
"As much as half of your capital expense in data center infrastructure is for cooling," says Patterson. "DC is more efficient and does not waste as much power because you don't produce as much heat if you are not converting it back and forth to AC." He estimates that if 15 percent of the energy used in computer servers and uninterruptible power supplies, or UPS, could be saved, then up to 24 percent could be saved in overall electricity.
One way to get that savings is to use a DC power source for a DC device. Power comes into the data center as AC and is then converted to DC with a rectifier housed in the UPS power management unit. Some of that DC power maintains the charge on "back-up" battery banks attached to the UPS unit. Then the rectifier output is converted back to AC power, which is then fed to the power distribution unit, where it
is generally stepped down to a lower voltage. The AC power then goes to the server racks, where it is converted back to DC and further stepped down to the very low voltage used by the server.
Large wattage loads in data centers need 380 volts of DC to keep currents reasonable, says Dennis Symanski, senior project manager of the Electrical Power Research Institute (EPRI) and chairman of the 380-volt DC standard committee at EMerge. "So the first thing the power supply does is create DC at about 380 volts, and then it steps it down to lower voltages."
But if the data center could be supplied with 380 volts of DC electricity at the outset — instead of having to convert the 480 volts of AC coming into the building — that would save energy and take away about one-third of the power supply's physical size.
To that end, EPRI created a simplified DC UPS that creates 380 volts of DC that charges the batteries and then is fed directly into the server power supplies. "You get rid of three conversions at once," says Symanski, "and save a lot of power. The simplified power feed is also less expensive."
EPRI has done demonstrations at various sites, including one at Duke Energy in Charlotte, N.C., which showed a 15 percent energy savings, not including cooling energy.
New data centers would purchase a 380 volt DC UPS instead of a 208 volt AC UPS. In some existing data centers, it would be possible to do a retrofit, says Symanski. "Some people might want to do a retrofit in stages, and some might want to do a hybrid and keep adding on the DC side," he says.
The use of DC could save 3 to 10 percent of server energy in a data center, according to Patterson. For every watt saved at the server, an additional 1 to 1.8 watts could be saved in power management and in cooling. Thus, Patterson says, a DC data center could see anywhere from 7 to 27 percent in energy savings, while the physical footprint of the data center could be reduced by a third.
Another way to generate savings with DC power is by using low voltage plug-and-play power buses that can be easily relocated. Until recently, facility managers hardwired devices like occupancy sensors to optimize energy use. But if space configuration is changed, hardwired controls no longer work properly, and it becomes necessary to re-do the wiring or simply abandon the device. "Low voltage DC allows easier relocation and wiring," says Patterson. "Instead of running power to each device, it is run to the grid that holds devices."
The microgrid is powered by 24-volt DC current, which is "touch safe" according to the electrical code — meaning that a person can touch it without receiving a shock. Being under 30 volts also eliminates the startle hazard. Twenty-four volts are good for wattage applications of 100 watts or below, says Symanski.
One company currently makes a ceiling system with a DC microgrid. Other companies are working on the same concept for furniture or walls or baseboard. "If you want to move a light or a sensor, you simply pull it out and reconnect it through the microgrid and still enjoy your original efficiencies," says Patterson. "Any place I put a light fixture in the grid, it can connect into the power that's in the grid. The same applies to sensors or anything else. You can run a fan or an actuator that opens and closes air conditioning louvers, or a motor that runs a Venetian blind or window shades, or you can run a projector."
The concept of a DC microgrid has also driven innovation in lighting and similar products. For example, lighting ballasts are available that do not include a rectifier section to change AC to DC power. The ballasts allow the conversion of a building lighting system to DC, where DC circuits are driving the DC ballasts.
Increasing the use of DC power to create energy efficiencies is still in its beginning stages. Basic power parameters for DC microgrid power distribution should be set at EMerge's 24-volt DC standard for occupied spaces and limited to Class 2 current, which is touch-safe at 100-volt amps. Distribution voltage in a building will be at a higher DC voltage, perhaps at the same 380-volt DC level used in data centers, and that will require a different standard, on which EMerge is currently working, along with a standard that allows fast direct charging of electric vehicles.
Although interest in DC power is growing, sources say that a lot of education still needs to be done. "People think this is a 100-year-old argument, so what new thing could there be," Savage says. "There is nothing that can't be done more efficiently. In the U.S., less than 1 percent of data centers are using the most advanced DC technology. There is a long way to go, not just in the U.S., but internationally. EMerge has a road map that includes promoting DC standards around the world, and we've had multinational corporations as part of the conversation for several years. We do intend to create a global standard."
The mission of the EMerge Alliance is to develop standards to spur the use of DC power distribution in commercial buildings. The first EMerge Alliance standard — an open power distribution platform for the use of safe low voltage DC power in commercial interiors — was approved in October 2009. The Alliance has established a third-party registration and evaluation program for labeling products based on its first completed standard, and registered products are available today. The Alliance is working with the California Lighting Technology Center to evaluate product compliance with the standard.
The Alliance has grown from five members when it was founded in 2008 to 75 members that represent industry, government and academic stakeholder interests. The focus of the Alliance has expanded from the "occupied space" of commercial buildings and now includes data and telecom centers and hybrid and electric vehicles. They will eventually have standards that cover all building services and outdoor uses of power as well. Currently there are dozens of data centers running on a proprietary version of a DC power topology, and several new ones running on draft versions of the EMerge standard. The Alliance has also set up more than a dozen interior beta sites, some of which have been running for almost three years.
For more information about the EMerge Alliance, visit www.EMergeAlliance.org.
Making the Case for DC Power