PV Powered DC Lighting Grid
s that pair well with LEDs (MAGAZINE)DC-power grids offer the potential of even greater energy efficiency when combined with LED-based lighting, and several options for implementing DC grids are either available or coming soon, reports Maury Wright. By Maury Wright
This article was published in the April 2013 issue of LEDs Magazine.
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There is a long-standing debate as to whether we would be better off had Tesla prevailed over Edison, and we had a DC-voltage power grid. Actually the answer isn’t clear cut, but without question a DC grid in a building can increase energy efficiency for many building systems including lighting. Indeed LEDs need DC power, so DC-powered solid-state lighting (SSL) eliminates one lossy power-conversion stage. The technology has become a reality with a proprietary scheme from Redwood Systems that’s widely available and the standardized EMerge Alliance-based products coming onto the market. Moreover, a play from the IT-centric Ethernet networking world may be waiting in the wings.
First, however, let’s step back and consider how the building industry as a whole could benefit from a DC grid – at least internal to the building. Consider the situation even in a home. Nicole Bopp, director of marketing at Nextek Power Systems, said that ultimately 80% of the watts used in the home power DC loads. Most all consumer electronics operate from DC at their core.
In a commercial building, however, the opportunity for savings are more substantial. IT data centers are a huge opportunity because all of the computer equipment could be more efficiently powered by a DC grid. Indeed there has been a movement in the computer industry over the course of the last decade to adopt DC power in the data center. The EMerge Alliance is now shepherding that movement along with supporting DC grids for lighting.
The EMerge Alliance was founded with a broad DC mission originally, but was perhaps most identified with SSL. Brian Patterson of Armstrong World Industries and chair of the EMerge Alliance said that adoption of a DC grid is “probably going to be driven by disruptive opportunities in the playing field.” LEDs provided the alliance just such an opportunity. Most SSL products include an AC/DC power conversion as the first stage of the LED driver electronics and that is ripe for elimination in a DC world.
Patterson in fact said that it’s both energy efficiency and reliability that will ultimately drive a transition to DC in lighting. Simplifying the driver to a DC/DC stage will in many cases eliminate the need for the electrolytic capacitor that is regularly identified as the most likely failure point in an SSL product. Patterson said that the driver can match the long life of the LEDs in a DC-powered scenario. The DC transition would also increase the system-level energy efficiency that’s inherent in LED-based lighting.
It turns out, however, that there are even more reasons for considering a transition to a DC grid for lighting. The EMerge Alliance has adopted a 24V DC bus. Patterson said, “It’s a voltage that can be made very safe and is very near the use voltage of LED lighting.” In fact, electricians are not required to install the DC cable runs.
DC power sources
Two companies currently have power sources that are registered with the alliance – Nextek and Roal Electronics. Nextek calls its products Power Server Modules (PSMs), a sample of which is depicted in Fig. 2. Marketing director Bopp said, “In-conduit AC wiring is required to the PSM,” but the DC cables out of the PSM are treated essentially like computer network cables and can be connected without removing power from the system.
The DC grid can in fact be run on cables that are available from companies such as TE Connectivity or integrated directly into a dropped-ceiling grid frame such as in the DC FlexZone products from Armstrong. The Armstrong products provide connectivity in the suspended-ceiling grid with electrical connections made as you assemble the grid.
Of course, a DC installation does have to account for the total load connected to a power source so there will always be some needed expertise in partitioning a DC system. The Roal Safe Energy Server SES400 has four output channels, each rated at 100W (Fig. 3). Nextek’s PSMs have 16 100W channels.
DC for legacy or SSL
The DC scheme can actually be used with legacy or LED lighting, although you can power far more fixtures in the SSL case. Nextek’s Bopp said that even fluorescent lighting can be operated more efficiently on a DC grid. She said that a typical electronic AC ballast does a conversion to DC and then a second conversion to high-frequency AC. Nextek offers a number of ballasts that can convert fluorescent fixtures to DC operation. Still Bopp said that in the case of typical T5 fluorescents, each PSM channel can only power two fixtures.
With many SSL products operating down in the 10W range, you can in some cases connect 9 to 10 fixtures on one PSM channel. Nextek recommends that you keep cable runs to 30–40 ft, meaning that one PSM can serve a radius as large as 80 ft. With florescent lighting, Bopp said you can typically service 1000 ft2 with one PSM, and in the case of SSL, that number goes to 2000-3000 ft2 because of the more efficient fixtures.
You may wonder why the EMerge Alliance adopted an architecture that still requires AC distribution to all areas of the building. The primary reason is logistics and compatibility with legacy technologies, although there are also some benefits in terms of ease of use with solar systems.
In reality, the EMerge standard does support DC-powered room-level power sources. Indeed, Nextek offers AC-powered PSMs, DC-powered PSMs, and models that accept either type of power. Patterson of the Alliance said the organization attempted to support architectures that range from a direct conversion to DC at the 13.2- or 16-kV pole to hybrid approaches where the DC conversion is done close to a system such as lighting.
In the case of solar, the Nextek product that supports AC and DC inputs comes into play. Bopp said that it can run on a DC input from solar panels during the day and seamlessly transition to AC power as the sun sets. Support for such a solar system is yet another selling point for DC grids. When the output of solar panels has to be inverted to AC, there is yet another efficiency loss.
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