Realizing a cost-effective zero net energy building through direct current (DC) residential distribution
Increases in global energy costs, coupled with a need to reduce harmful fossil-based emissions, are energizing a worldwide call for clean and efficient energy sources and architectures. Considering the fact that today’s houses and buildings consume 40% of all conventional fossil-based energy generated in the continental United States and Europe, a concept that’s been gaining popularity in recent years is the zero net energy (ZNE) building.
A ZNE building could significantly cut dependence on fossil-based energy and supply the required energy through on-site distributed generation, such as solar, wind, fuel cells, or microturbines. Recent mandates from the state of California that require all new residential constructions be “zero net energy” by 2020 — and all new commercial buildings by 2030 — have added further urgency to the drive for energy self-sufficient buildings.
While conceptually simple, the goal of achieving a ZNE building with a reasonable payback period is challenging due to a myriad of active and passive technologies involved, including: the selection of electrical technologies that consume less energy (high-efficiency appliances, HVAC, and lighting); efficient distribution architecture to cut power losses; portable energy storage for energy buffering; and the integration of renewables, such as solar, wind, and geothermal energy.
The key to realizing a cost-effective ZNE building is to reduce the net energy consumed by the house’s loads. When energy consumption is reduced, a smaller portion of distributed generation and electrical wiring is needed, which directly translates into reduced building costs and a shorter payback period for the owner.
The DC draw
One ZNE-enabling technology is direct current (DC) residential distribution. Through the use of high-efficiency electronics and bus architecture, a DC distribution system reduces the amount of consumed energy and, subsequently, the amount of on-site renewable generation required, improving the cost-effectiveness of a ZNE residence.
Modern conventional houses are fed from alternating current (AC). However, at the same time, many appliances and lighting technologies, such as televisions, computers, and LED light fixtures, are native DC loads, as are electric vehicles, batteries, fuel cells, and renewable sources. These and other appliances are fed from multistage power-conversion equipment that first rectifies the incoming AC into DC. Usually, this is followed by a second DC-to-DC converter stage that converts the rectified DC voltage into a lower regulated voltage as required by the end load (e.g., 12VDC or 5VDC in personal computers). Each of these conversions wastes electricity in the form of heat.
The efficiency of the majority of these power supplies usually varies between 70% and 75%. The average efficiency of all power supplies, as estimated by Lawrence Berkeley National Lab (LBNL), is around 68%. Some high-end power supplies classified by the EPA as “80 Plus” may offer efficiencies greater than 80%, although legacy systems offer much lower efficiency, especially at lower loads. However, in a conventional house, native AC motor-driven loads also exist. In such systems, the rectifier in the first stage is followed by a DC-to-AC inverter that drives the motor.
Renewable resources, such as solar and energy storage elements like batteries, are also inherently DC systems. Multistage power conversions are again needed to integrate them into the conventional AC distribution system. When integrating into an AC system, the DC output from a solar panel is first converted into another DC voltage, followed by an inverter that interfaces into the AC grid. The AC voltage produced by solar and battery inverters must be synchronized to the AC grid before they can be interconnected.
If many of the electric loads are native DC, then why not feed them directly from a DC source, making them more efficient due to the reduced number of power-conversion stages? Although a limited number of DC-to-DC or DC-to-AC conversions would remain, the input rectifier would be eliminated. Some of these DC-to-DC converters have peak efficiencies as high as 98%. The resulting reduction in energy consumption directly translates to reduced costs and volume of renewable resources as well as energy storage required to supply the desired energy.
DC house details
A DC house is a new concept, where the power distribution system is built around DC instead of the conventional AC system. Because the DC house is a miniature grid in itself, comprised of DC loads and sources, some entities call it a “DC nanogrid.” The DC house can be fed from the AC electric utility grid (i.e., grid-parallel operating mode) or can be intentionally disconnected from the grid to function as a self-sustaining ZNE house.
While several DC distribution configurations are possible, one example is presented here (click here to see Fig. 1). A typical DC house could feature two DC voltage buses, a 380VDC for high-power appliances, such as HVAC, washers, and dryers, and a lower 24VDC bus for smaller appliances and lighting. The 24VDC is stepped down from 380VDC and could be distributed throughout the house as a separate power bus (in addition to the 380VDC), or it could be in the form of dedicated power supplies that step down 380VDC to 24VDC for individual applications. A recent study by Darrell J. King and James Brodrick, “Opportunities for Energy Savings? Residential DC Power Bus” featured in the September 2010 edition of the ASHRAE Journal pointed out that in the short term, the latter option would offer a faster cost payback.
The DC house would feature one or more renewable sources, such as solar, that would tie directly into the 380VDC bus, as would an energy-storage system, such as a portable battery. The energy storage device would provide back-up power to the house in case of any electric utility power interruption and would also support the loads when the DC house is intentionally operating in an island mode (self-sufficient, standalone zero-energy operation). Because a DC source is being tied into a DC bus, no synchronization is required as in AC systems. All the lighting would be DC-based LEDs. Many of the commercial LED lamps operate from 24VDC. Electric vehicles already contain a battery, which can be charged more efficiently from a DC source that already exists in the DC house.
The DC house today will be powered by a conventional distribution transformer, which will transform the electric utility medium voltage of 13.8kVAC to 240/480VAC, followed by a rectifier that converts the 240/480VAC to 380VDC. On the other hand, the DC house of the future could be powered by smart, highly efficient solid-state (all power electronic) transformers, which are presently under development by several vendors. These solid-state transformers will convert the medium voltage of 13.8kVAC directly to 380VDC.
Because it is designed to be powered in full by local distributed generation sources while disconnected from the grid (island mode), a DC house is inherently a ZNE structure. As a result, no retrofits or modifications are necessary to reach an energy-neutral state, as in the case of a conventional AC house. Due to the higher efficiency of the appliances and power converters used in a DC house, the actual energy consumed by the house loads is less than that of a conventional AC house. This means that less energy needs to be produced by the integrated distributed sources, and the distributed sources can be downsized relative to the requirements of an AC house. Lower-power distributed resources translate into reduced capital costs. In effect, using DC houses reduces the payback period and could significantly improve the feasibility of widespread implementation of ZNE houses. In addition, to achieve ZNE architecture, passive design techniques could be used to supplement the system, as has been envisioned in standard AC-fed houses.
Further increasing efficiency, multiple DC houses can be interconnected in a DC microgrid (click here to see Fig. 2) so that energy can be shared from one house to the other, if needed — a highly efficient and simple DC-interconnected world. In this configuration, multiple houses can be supported by a common distributed generation source (instead of each house having its own distributed generation). This configuration is nothing but an extension of the DC distribution concept to a small microgrid. As in the case of a stand-alone DC house that was discussed earlier, the DC microgrid would possess independent controls and be able to seamlessly transition from grid-parallel to island operation.
Although you may think this all sounds like a myth, don’t dismiss the DC house quite yet. The concept has been put into practice in certain applications. For example, data centers and telecom networks are already converting to DC. In fact, several DC data centers have been installed or planned for in Japan and Europe. In the United States, EPRI (in partnership with the LBNL and computer OEMs) conducted the first successful demonstration of a DC data center in one of Sun Network’s facilities in California in 2006. More recently, further demonstrations of DC data centers have been conducted at Duke Energy, Charlotte, N.C., and the University of California at San Diego.
The concept is now being extended to commercial buildings and office spaces as well. DC dwellings have also been constructed in Japan and Taiwan. As a result, several vendors are already developing products or offering services aimed at the DC distribution market.
Breaking new ground
What about standards, codes, and personnel safety? An open industry association called the E-Merge Alliance is already addressing this and other issues, including ground-fault protection and arc flash. The
E-Merge Alliance is developing standards that could lead to the rapid adoption of DC power distribution in commercial buildings. This group includes several industry vendors and services, electric utility representation through EPRI, and product safety testing and certification organizations like UL.
The first standard to be released by E-Merge is “24VDC for Commercial Spaces.” Used safely in the telecom industry for a long time, safe practices for 48VDC systems have been developed for these systems and can be adopted in DC houses. In DC data centers, the industry has agreed upon 380VDC as a standard voltage, organized as a split ±190VDC system, to ensure safety. A similar configuration could be adopted in residences. In addition, OEMs are developing connectors that would allow safe disconnection and connection to DC receptacles.
The DC house can be a net winner in terms of higher overall efficiency and reduced energy consumption, offering higher reliability than comparable AC houses due to the energy-storage support for backup power, easier ability to tie in renewables and storage, easier interconnection to a grid (due to no need for synchronization, as in AC systems), and reduced cost due to the need for smaller renewables.
Preliminary studies (such as “DC Microgrids: Benefits and Barriers” by Paul Savage, Robert R. Nordhaus, and Sean P. Jamieson, http://www.reilproject.org/05-DC-Microgrids%20(1).pdf) have indicated a diverse range of benefits from DC in residential structures, one of which estimates that DC in residences can save a total of 121 TWh of energy annually, which corresponds to a reduction in net national load by up to 2.98%. Similarly, another study (“Opportunities for Energy Savings? Residential DC Power Bus” cited earlier in the article) pointed out that average annual savings per house could be in the order of 300 to 350kWh, resulting in a national savings of more than $4 billion in energy usage. With higher efficiencies and simpler integration, the DC house is a step closer to realizing a net zero energy building.
Rajagopalan is a senior project engineer/scientist and Fortenbery is program manager with EPRI, Knoxville, Tenn. They can be reached at firstname.lastname@example.org and email@example.com.