Is DC’s Place in the Home?
- Written by Doug Houseman
Given changes in how people are using electricity, is an AC-only electrical system still the right option for a residence of the future? Conversion losses already are growing with wide use of consumer electronics, and they will get bigger still as photovoltaic and car-charging systems evolve. This is why experts are starting to seriously consider the prospect of hybrid AC/DC residential electrical systems.
According to the Energy Information Agency (EIA), the fastest growing portion of residential electricity use is consumer electronics and small appliances. In 1993, the EIA did not even bother to measure the consumption in either category; eight years later it counted over a dozen types of devices that fit in this category. By 2013, when a group of IEEE members audited their houses to get a snapshot of what they had, the list of categories expanded to over 50 small appliances and consumer electronics devices.
These devices primarily run on DC power. Even with improvements in power supplies, many of these devices have a conversion efficiency of no better than 80 percent and some low-end devices have efficiencies as low as 65 percent in converting power. Such devices now account for between 15 and 30 percent of a residence’s consumption, depending on demographics, country and weather zone.
In terms of electricity used, in 2012 the average U.S. home consumed 11,252 Kilowatt-hours (kWh). Assuming the average home used 20 percent of electricity for these devices, that translates into 2,250 kWh consumed by each residence. With an average efficiency of power conversion of 75 percent, that means 562 kWh were lost in power conversion in an average home.
If this were the only loss from power conversion, it might be ignored, but this is not the case. On the production side of the equation, residential photovoltaic systems are coming into wider use, producing DC power that also involves significant losses. The smallest PV system typically installed has a capacity of about 1 kilowatt (KW) and produces 5250 kWh annually.
According to the National Renewable Energy Lab’s (NREL) PVWatts tool, the losses associated with converting DC to AC in a typical system come to 23 percent, or 241 kWh. The average size installed is 5 KW, so the annual conversion loss amounts to 1,200 kWh for the average system.
Then there are electric vehicles, a third major DC element. According to GM, the Chevy Volt needs to have 10.4 kWh fed into the battery for a full charge because of losses and battery conditioning, doing that actually requires 12.9 kWh of electricity. Assuming the Volt is driven the 35-miles-a-day national average, which is roughly the number of miles the car gets per charge, it will consume 4,700 kWh of electricity per year, of which 912 kWh is lost in conversion and charging the batteries.
So, if current trends continue, more renewables, electric vehicles and consumer electronics will be installed, leading to growing conversion losses. Today, a home with photovoltaic and electric vehicles will see conversion losses of 2,674 kWh annually on a consumption of 15,952 kWh, or 16 percent. This means more electricity is lost within the home than in delivering that home’s power across the distribution and transmission grids.
While there are still few homes with photovoltaic and electric vehicles, the question is, should conversion of DC to AC and AC to DC continue in the future? Should a resident be able to plug a DC-based device into a DC outlet and an AC-based device into an AC outlet? Or should we continue to make all the conversions? In 2004, CEATI published one of the first reports that asked the question of what it would take to mix AC and DC on the same distribution system. Over the last seven years some efforts have been made in the laboratory to understand what it would take to mix, either on the distribution system or just within the home, AC and DC power. Additionally the impact of different outlets for AC and DC power has been discussed. This is not a trivial problem; safety of the user, breakers, switches, reactive power and other issues all need to be sorted out. In short, it is a big change that will take major research to sort out. IEEE will start a formal effort this spring to look at the challenges of DC power in the home and potentially mixing AC and DC there.
A small group of IEEE members working with the IEEE Power & Energy Society (PES) and IEEE Standards Association (SA) will launch a working group from the IEEE PES General Meeting. This working group may be the core for the potential revolution. Right now no one knows whether it will be cost effective to mix AC and DC in a household, how fast EV use will grow—recent history is not very encouraging— or how fast renewables will be installed. Another uncertainty concerns penetration of battery storage, which may be installed to back up building-based renewable energy systems.
What is completely unknown at this point is what the impact of this will be on the implementation of smart grid and the overall design of distribution systems in the future. Is this an important part of the future, a flash in the pan, a specialized system for high-end consumers, or completely nuts?
With 132,000,000 households in the United States, losses from conversion are more than 70 terawatt hours (billion kWh) annually for small appliances and consumer electronics. These losses equate to 700 trillion BTUs of primary energy that is lost with these conversions in the United States alone. As the rest of the world catches up with the US in using electricity for entertainment (there are over 1.4 billion more households in the world), what is the right thing to do? Without research and engineering, we don’t have the basis to make a good decision. The ball is clearly in our court to decide. Won’t you join the effort?