Direct current power systems bring micro-grid to the office
When Thomas Edison developed the first practical incandescent lamp in the 1880s, the merits of alternating current (AC) electric power and direct current (DC) power distribution were debated. It was quite a heated battle — more a “war of currents” actually — with AC emerging as the winner. Today, the wide use of compact electronic devices calls for a rethinking of electric power generation and distribution.
Throughout the 20th century, we have grown accustomed to the operating efficiencies of AC power transmission and distribution systems, even though the AC electric power entering a building goes through a series of voltage level transformations to accommodate a variety of electric loads — predominantly lighting and motors. However, since the invention of the transistor and other solid-state components, an increasing number of electrical loads in a building (such as computers, IT equipment, cell phone chargers, and other personal use devices as well as building control sensors, HVAC actuators, signage, security system elements, and A/V systems) are low-voltage DC-based. Energizing each of these devices requires transformations and rectifications from the AC electrical power distribution system, which, at each point, consume electrical energy.
The largest single user of electric power in an average building, lighting is slowly shifting toward solid-state sources — light-emitting diodes (LEDs) and organic LEDs (OLEDs), which are usually served at 12VDC or 24VDC. These luminaires would typically be smaller and lighter than standard ceiling fixtures, thus making it easier to install and change out ceiling fixtures and sensors/controls when a space is reconfigured.
For many years, the workplace has included overhead lighting and a variety of electrical devices that are typically installed and wired for a building’s lifetime rather than occupants’ changing needs. Today, commercial buildings favor flexible furniture layouts, adaptable to each business project, rather than fixed desk locations.
So the question now is, why not layer a DC electric power distribution system over the AC system in a building, avoid all of these power transformations, and satisfy today’s workplace innovations? With that idea in mind, a number of companies — energy service and building product firms as well as many lighting and lighting control vendors (more than 70 as of press time) — created the EMerge Alliance in 2008 (see Group Members). The first open standard developed by the Alliance defines a power system that delivers DC power into the office space for use by a variety of DC-native equipment, thus locating power supplies external to the equipment.
Released in October 2009, the Room Level Power Distribution Platform standard maintains that DC power is ideally suited for integration into the ceiling plane so that light sources, sensors, actuators, and other devices (both within the plenum space and below the ceiling plane) can be arranged and shifted without the need for rewiring. The intent is to make DC power safe, flexible, and easy to use in occupied rooms.
A converter box, called a Power Server Module, installed in the plenum space of a suspended grid ceiling, changes the incoming 208VAC or 240VAC power to 24VDC power. DC power is then distributed to 16 individual, Class 2-rated receptacles, or jacks, on the face panel of the box. Each jack accepts a plug-terminated cable, which would be shorter than 10 m (30 ft), to reduce resistive power losses. These Class 2 power-limited circuits can support a maximum of 100VA.
DC power is also distributed on a set of bus bars that run along the length of the inverted T-bar members, creating an electrified grid. The grid design allows for fixtures and other electric device additions, modifications, and location changes along both sides of the bus bar structure, above and below the suspended ceiling. All cable assemblies installed in the plenum space have flame-resistant insulating material. The polarized cable assembly housings have releasable latching for quick installation and changeout.
The conductors of the Class 2 cabling can also carry communications network traffic, working in ways similar to (or the reverse of) the Power-over-Ethernet (PoE) technology. A challenge with PoE in a structured cabling system is reaching the device locations outside the distance limitations of UTP cabling. According to the TIA/EIA 568 5-A standard for Cat. 5e cable, the maximum length for a cable segment is 100 m or 328 ft. If DC power was available in many locations throughout a building, some of the deployment problems of delivering DC power could be alleviated.
The EMerge system design has provision for a wireless receiver within each lighting fixture. The addressable fixtures could be controlled via a wireless, battery-less switch or an occupancy sensor — or a wireless system that uses batteries.
Keeping pace with publication of the standard, in November 2010 the California Lighting Technology Center at the University of California-Davis evaluated the first group of Registered Products, which are now available for specification.
In an effort to extend DC electric power delivery beyond the scope of the 2009 standard — or beyond a single room in a building — the group has 15 test sites (office buildings and data centers) that use DC power at a voltage higher than the first standard specifies. This has given rise to some interesting concepts being discussed by EMerge Alliance and its industry partners.
Power generation resources, such as solar photovoltaic (PV) arrays, wind turbines, and fuel cells produce DC power. This energy can be stored in batteries, flywheels, and ultra-capacitors. Taking advantage of this equipment, a DC power micro-grid “island” — occasionally operating independently of the bulk power grid — could improve the reliability and security of the electric power system. A micro-grid is a small electric power “network” that could encompass a single office building, a small community, or an academic campus. It could also operate isolated from the utility grid in an emergency (brownout or blackout), relying on its DC energy storage capability. Within a localized micro-grid, solid-state switching devices can quickly interrupt faults, achieving improved reliability and power quality.
By eliminating the need for multiple conversions in a building, energy losses could be slashed up to 35%. Less waste heat and a less complicated conversion system could also potentially translate into lower maintenance requirements, longer-lived system components, and lower operating costs.
Currently, the best application of DC power is in data centers and telecommunications facilities. The U.S. Environmental Protection Agency’s (EPA’s) 2007 Report to Congress on “Server and Data Center Energy Efficiency” states that centers in the United States could save up to $4 billion in electrical costs annually by using more energy-efficient equipment and better operational practices.
Typically, a data center takes 480VAC power from the grid and converts it to DC power to charge up a battery-based uninterruptible power supply (UPS). The DC power is then converted back to AC and transformed to 208VAC for distribution, only to be rectified back to 380VDC at the first stage of each server’s power supply. In total, up to six or more power conversions can be required. For every watt of power used to process data, about 0.9W is needed to support conversion. In addition, it takes about 0.6W to 1.0W to cool the power conversion equipment.
Along with energy savings, other potential benefits of DC power distribution in a data center include improved power quality, reduced cooling needs, high equipment densities, reduced heat-related failures, improved reliability, and a more efficient integration of on-site renewable energy generation sources. Besides reduced voltage drop, other advantages include eliminating the need to balance phases or to synchronize multiple sources and the simplification of wiring by reducing the number of breakers.
The U.S. EPA’s Energy Star program and similar initiatives to drive up the efficiencies of AC power supplies should narrow DC’s advantage over time. However, industry analysts predict that even when compared with premium high-efficiency AC systems, DC distribution in a data center will use 7% less power. Perhaps the biggest savings might be a reduction in square foot requirements. A DC data center takes up 25% to 40% less space that an AC counterpart — largely because computer equipment can connect directly to backup batteries.
In cooperation with other industry associations and standards-making bodies, the EMerge Alliance started working on a data center DC power standard in 2009. The selection of 380VDC as the distribution voltage is based on the agreement of engineering data from a number of organizations in the United States, Europe, and Japan. The consensus on technical matters was seen, for example, at the first annual DC Building Power Japan (DCBPJ) Conference, held recently in Tokyo, which included a number of firsts, including: a comprehensive review of DC power architectures for homes, data centers, and micro-grids; a joint meeting of the Electric Power Research Institute (EPRI) and the Japan DC-Power Industrial Partners; the approval of a provisional standard for 380VDC data center, and the first public tours of high-voltage DC-powered data centers. Japanese conglomerates like Panasonic and Sharp are making large investments in systems that could bring DC to commercial buildings or homes, with solar PV panels directly powering appliances or electric cars.
At the end of the DCBPJ meeting, the EPRI DC Power Partners, a group of industry power experts, and the Japan DC Power Industrial Partners approved a provisional standard patterned after a similar document by the European Telecommunications Standards Institute (ETSI). Concurrently, the EMerge Alliance is working with ETSI to achieve a harmonized DC power standard, and the group made a similar effort with EPRI at the 2010 Green Building Power Forum (GBPF).
Although telephone central offices have been using 48VDC systems to serve circuit switching equipment for years, very few regulations for (higher voltage) commercial DC power distribution exist. For example, there are no arc flash regulations for commercial DC in North America, which allows for a variety of interpretations regarding clearances, access requirements, etc. New equipment is already being developed in some areas, however, and firms are offering 380VDC fans, servers/server power supplies, and UPS units for a total DC distribution system.
Sidebar: Grid Power
Although high-voltage direct current (HVDC) is now a viable means of long-distance power transmission — used in about 100 applications worldwide — the idea of a wholesale change in the infrastructure from an AC power grid to a DC power grid is not being advocated. That would be impractical and expensive, requiring a completely new set of government regulations.
Sidebar: Group Members
The governing members of the EMerge Alliance include: Armstrong World Industries, Johnson Controls, Nextek Power Systems, Osram Sylvania, Philips, and WAVE. The first liaison members include: BACnet International, EnOcean Alliance, and ZigBee Alliance. Acuity Brands Lighting, Cooper Industries, Emerson Network Power, GE, Hubbell, Ideal Industries, Tyco Electronics, Universal Electric Corp., WattStopper/Legrand, and Crestron Electronics are also participating members of this group.