Lighting industry progresses on DC-power grids 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.
This article was published in the April 2013 issue of LEDs Magazine.
View the Table of Contents and download the PDF file of the complete April 2013 issue, or view the E-zine version in your browser.
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.
Given the benefits of DC, it’s a fair question to ask why the EMerge technology has not thus far taken off. About the building industry, Patterson said, “Nobody likes to be first.” But in fairness, there just has not been a sufficient choice of products available in the EMerge case. That’s changed rapidly in the past few months, according to Patterson. Indeed the registered products list is getting quite long (www.emergealliance.org/Products/RegisteredProducts.aspx).
EMerge-compatible products can be recessed or even flush-mounted to a ceiling. For example, the previously-mentioned T-Bar LED linear fixtures from JLC Tech can be flush-mounted to the metal rail of a drop ceiling. Armstrong has demonstrated how even track lighting heads can connect directly to the FlexZone system.
Long term, EMerge technology may become the predominant DC-powered approach, especially considering the pledged support from major lighting companies and the fact that it is an open standard. However, there are other challengers. Redwood Systems, for instance, has been selling its Redwood Building Performance Lighting Platform for several years.
The Redwood platform is far more than a DC power-distribution system. In aggregate, the platform includes application and commissioning software for adaptive control of the lighting system along with a DC-based power-distribution engine that also carries the control commands to fixtures and light- or occupancy-sensor data, and luminaire operational data, from fixtures to the centralized system.
Redwood is working to create a partner network of luminaire makers that support the technology. For example, MaxLite and Lunera have previously announced compatible products. Redwood also supports non-compatible fixtures, including those based on legacy sources, such as fluorescent lamps, via relay-based control boxes.
The Redwood value proposition is ultimately centered on energy efficiency combining inherently-efficient SSL products, controls to dim or turn lights off when they aren’t required, and the more efficient DC distribution. A key advantage for the company is the inherent inclusion of controls. The EMerge Alliance doesn’t explicitly address controls. Nextek, for example, sells wireless controls that are essentially separate from the DC power grid.
|Retrofit costs||Fluorescent||AC LED||PoE LED|
|Fixture install and ballast||$250||$250||$50|
Table 1. Power over Ethernet could lower the cost of controllable lighting
Redwood Systems has completed major projects for a number of high-profile customers. For example, the Volkswagen Electronics Research Laboratory in Belmont, CA, uses Redwood technology, and the lighting is controllable via smartphone apps. The SAP Labs facility in Palo Alto, CA, uses Redwood technology and the company said it delivers $80,000 in operational savings each year.
Power over Ethernet
While the Redwood technology utilizes Cat-5/6 cables (Ethernet cables) to deliver power to fixtures, the Redwood system is proprietary. But there is the potential going forward to use Ethernet products designed for networking to power SSL fixtures. The network industry has developed an extension to Ethernet called Power over Ethernet (PoE) that was designed to allow an Ethernet cable to carry both power and data. The original intent of the technology was powering remote network devices. For example, an Ethernet cable could both power a Wi-Fi access point and provide the data connection between the corporate network and the access point. PoE was envisioned to support such remote devices where ready access to AC wiring wasn’t available.
At the recent Strategies in Light conference, Chris Isaacson, CEO and CTO of NuLEDs, made a presentation on IT networked lighting systems. Isaacson foresees integrating lighting with the IT infrastructure and inherently making every light fixture intelligent.
From a technology perspective, PoE could certainly power SSL products. The first-generation IEEE 802.3af specification makes almost 13W available to an end device – certainly enough to power many SSL fixtures. The more recent 802.3at PoE+ spec makes 25.5W available to the end device, and that’s enough power for most indoor SSL products other than high-bay lighting.
Isaacson sees the situation improving even further. He said the new Universal PoE (UPoE) standard being pushed by network-equipment vendor Cisco will again double the available power. By 2014, a single UPoE Ethernet cable will be able to power two LED troffers with the continuing ramp in LED efficacy, according to Isaacson.
Reliability and PoE availability
Still, there are questions about PoE. Our computer networks are very reliable, but our lights always work so long as the AC power is on. It’s not clear that PoE could deliver the expected level of reliability relative to our experience with AC-powered lighting. When asked about reliability, Isaacson said you could mix lighting connections on multiple switches to assure that a single network switch failure would only impact a part of a building’s lighting.
Moreover, Isaacson discussed the prevalence of Ethernet in the corporate world as part of the justification for the concept. But most installed Ethernet does not include PoE support today. The existing infrastructure isn’t ready for lighting. But Isaacson sees PoE costs dropping and says that as long as the powered ports in a switch are utilized, then the economics will work. He said NuLEDs hope to launch a pilot project this summer with a Fortune 100 company.
DC bears watching
What is increasingly clear, however, is that the lighting world will have a brush with DC power, whether the technology is one we covered here or another system entirely. DC could remain a niche offering, or it could become predominant. SSL product manufacturers and lighting designers would be well advised to play close attention.
Consider research conducted last year by Carnegie Mellon University. The university reported that a DC grid would save $24,000 per year in a 48,000-ft2 building lit by LEDs. Moreover, adding solar power to the equation would increase the savings by $5,000 more. Those numbers are over and above the baseline savings afforded by LED sources.
Patterson of the EMerge Alliance is also predicting big things for DC as this year unfolds. He said, “At Lightfair some companies will announce universal luminaires that can work from AC or DC.” Moreover, he said that many of the registered-compliant products are now truly commercially available and not just being made in pilot quantities.
You will also start to see real case studies based on EMerge Alliance technology in the coming months. For example, a recently opened PNC Bank office in Fort Lauderdale, FL, is among the first net-positive bank branches that can generate more energy than it uses via solar panels. The news stories about the building focused on the solar angle, but an EMerge-compliant DC grid is a major part of the underlying story. We hope to bring you more details on the project and others soon.