Was Edison Right?

Back in the 1880s, the so-called ‘War of Currents’ was raging, with Thomas Edison supporting DC transmission of electrical power, opposed by George Westinghouse and various European companies, who favoured AC. At the beginning of the War of Currents, DC was standard in homes in the US, with Edison holding all the patents for DC transmission. Heavy copper wires were used for transmission but the load still had to be within a mile or so of the generator. The development of the closed-core shunt connected transformer in Hungary in 1884 gave AC the boost it needed to win the war. Stepping AC voltages up and down became easy, so transmission across many miles at high voltage with minimal losses was now possible.

Fast forward to today. The increasing electronics content in homes, businesses and cars rely on semiconductor devices, all of which require a DC power supply. AC-DC converters provide this power supply, but not without losses. The other part of the problem is that environmental and financial driving forces are increasing the percentage of power generated off-grid, typically by PV solar panels or wind turbines, which produce DC power. Currently, power from solar panels is converted from DC to AC by an inverter, fed into the mains, then converted from AC back to DC by a transformer in an electronic device’s power supply. There are substantial energy losses associated with both conversions.

A better way?

So, was Edison right? Is there a case for DC power distribution? Well, the laws of physics can’t be changed, so DC still isn’t suitable for long distance transmission. However, a movement in the US is proposing a hybrid solution, with AC used for long distance transmission and DC used in smaller power grids at the building level, called ‘micro grids’. These micro grids may suit lighting ballasts, for example. The idea is to avoid having a lossy AC-DC converter on every single lamp in an office block, instead converting from AC to DC in bulk in a more efficient way and supplying all the lighting from that. An added bonus is the option to connect the micro grids directly to energy sources such as solar panels that produce DC anyway, doing away with the inverter. The resulting systems use less electronics and are less complex, adding to reliability.

The idea is gaining credibility. A new industry group, called the EMerge Alliance, has been set up to promote the use of these DC micro grids. The Alliance has also published a standard, called Occupied Space, for the use of such micro grids, as shown in Figure 1.

Figure 1. An example of a DC micro grid in a commercial office

“Efforts to create a Smart Grid need smarter buildings that use more adaptable power infrastructures that can minimise conversion losses and make measuring and controlling power easier,” says Alliance Chairman Brian Patterson. “This helps to reduce the overall load on our energy resources and make the use of alternate clean energy generation more likely… we believe that ongoing and increasing demand for improved reliability and energy efficiency across all areas of commercial buildings provides the need for our broad platform.”

Wiring losses were the main reason for DC’s demise in the 1880s, so the EMerge Alliance is advocating 24V DC microgrids used over distances that won’t materially affect efficiency. The length of the low voltage cables that connect power supplies to distribution busses are limited by the Occupied Space standard.

“AC-DC power supplies will be dispersed throughout a space in order to purposely keep their output cable lengths short,” Patterson says. “This means that individual supply components will likely be designed to handle loads less than 100 Watts.”

Distribution wiring is maintained at 12 AWG, the same as traditional AC circuits, but because this wiring is only carrying 24V, it doesn’t generally require metal jacketing, metal junction boxes, ground wires or other protection means, keeping the cost of installation low. Use of existing wiring may even be possible, in some situations. Installations fall under Class 2 of the US’s National Electrical Code (limited to 100 Volt-Amps), so can deliver up to 4.1A of current at 24V and still provide acceptable protection from electrical shock and fire initiation.

Designing real applications

The design of such DC microgrids should be relatively straightforward, according to Paul Savage, CEO of Nextek Power Systems.

“The first thing to know is that it’s not an all-or-nothing proposition,” he says. “A load like lighting or vehicle charging can be done without impacting the design considerations for conventional AC power delivery. For Class 2 systems, it’s a snap, because the user-facing power is below the shock and startle hazard.”

Nextek makes power server modules (PSMs) which are at the heart of a DC micro grid system, converting 240V AC power to 24V DC. The company also makes an interface module to add power from solar PV cells to the DC bus, that works in combination with the PSM. Savage says that Nextek can also supply an IPv6 wireless control node, called the Skyrouter, that controls the PSMs in a mesh network.

As far as end products go, Nextek manufactures DC-compatible ceiling fans under its Fanworks brand. Compatible lighting products are available from Philips, Osram, Acuity, Cooper and others. LED lighting is particularly well suited to the DC micro grid because it is driven electronically, but electronic fluorescent lighting can be modified to work with DC as well. Luminaires with DC input ballasts or LED drivers use the same lamps, have the same output lumens and wattage per fixture, and have the same lighting performance characteristics as AC-input devices. The efficiency gain for LED lighting installations can be 10-15%, according to the EMerge Alliance.

The future?

DC micro grids may soon be coming to homes as well as commercial buildings. Nextek is working with NextEnergy and Champion Homes on a demonstration called the NextHome, a DC connected house. NextHome will showcase companies and allow them to test, demonstrate and commercialise their technologies in a real world residential home. The project should be up and running by the end of the year.

“I think it’s a global, inexorable trend that will bring benefits to the rich and the poor,” says Savage, referring to the company’s STAR trailers: mobile solar-powered electricity generators for rural areas of developing countries that are completely off-grid. Of course, the trailers take DC power generated by the solar panels and directly power homes and small businesses with it – no conversion to AC required. Ultimately, he says, anything with a semiconductor in it will benefit.

Meanwhile, the EMerge Alliance’s vision is firmly focussed on DC micro grids throughout commercial buildings, including the occupied space, data centres, outdoor applications and building services, according to Chairman Brian Patterson.

“We see great opportunities with lighting and computing technology as well as electric vehicle charging and larger building loads such as HVAC, motor loads and high bay/industrial applications which are often DC based and are ideally suited to micro grids,” Patterson says.

Top image courtesy of ningmilo at FreeDigitalPhotos.net