Design Con 2015

Will Edison’s Direct Current power grid idea catch on?

October 23, 2013

Bernard Cole-October 23, 2013

While we are not about to go back 100 years and reject alternating current as the main means for power distribution, an IEEE committee has announced plans work out on how to make direct current based rechargeable personal electronics and renewable energy systems more compatible with the AC power grid.

According to the call for participation  just sent out to IEEE members, the plan is to get the engineering community to contribute to the development of an IEEE PES Intelligent Grid Coordinating Committee (IGCC) sponsored plan, called "DC in the Home."  They want to find a way to ensure that DC electricity can be safely and conveniently accessed in homes, eliminating the wasteful conversion of AC to DC, and in many cases, DC to AC, prior to entering the home.

The deadline for participation is November 10. You can take part by contacting either Erni Wiebe (isenginc@gmail.com) or Doug Houseman (Doug@enernex.com)..

In my experience it is certainly time for such an evaluation to be made. I am a minimalist user with at most seven devices and systems in my house that that operate via direct current. At the other extreme is Dave Freeman of Texas Instruments, who in “Engineering the DC-powered home,” writes that he has 10 to 12.

But according to EPA statistics that I saw recently the average is currently about eight. But that is a number that is increasing rapidly. The only exceptions are home appliances such as refrigerators, dryers, washers, ovens, the many cooking utensils, and, of course, desktop computers. The laptop operates by direct current, transformed from AC by a converter in the power cord that powers it.

What is most disturbing about this to many engineers who have been behind this effort is not that DC usage is increasing, but that it is so wasteful of energy – exactly the opposite of the trend toward designing for power efficiency.

In its call for participation, the IGCC estimates the losses from conversion of AC to DC for use in powering electric vehicles, consumer electronics, computers, and many other household devices consume DC power are estimated to be between 17 and 35 percent.

But the situation is even worse in the local solar and wind power installations many of us have installed to do our part in reducing global warming and reducing dependence of nonrenewable resources. There are a few solar and wind powered installations in the homes of my neighbors in the small community where I live and work in the high country of northeast Arizona, near the Grand Canyon and the Navajo/Hopi four corners area.

But most of the conversations I have with them are about the problems they have converting the DC that is generated back into some form of AC to power the refrigerators, televisions, and home lighting – and the lack of energy efficiency they have found in that conversion. For every three steps forward they take in energy independence they have to take two steps back for the energy loss from conversion.

Even worse, they tell me, is the energy loss involved using batteries – either charged by DC converted from the power grid’s AC feed or fed directly from their solar/wind installations to store energy for use when things are not going according to plan - which is often.

According to the IGCC, the losses from conversion of rooftop PV to AC is an additional 23 to 28 percent. Most storage devices also operate on DC power, adding to the losses that a home owner would see from installing and operating distributed energy resources.

While the suggested reasons for this are diverse, so are some of the proposed solutions. Even so, based on what I have read so far, I am optimistic that this not result in a repeat of the knock-down drag-out slugfest that typified the war between Thomas Edison with his DC proposals for powering the world and Westinghouse’s based on Nikolai Tesla’s AC ideas at beginning of the Twentieth Century

The IEEE groups involved have been having meetings around the country for the last few months. An interesting resource that I found online is “IEEE Smart Grid Experts Roundup: AC vs. DC Power.”

So far there is agreement that traditional AC power will continue to be the primary mechanism for the majority of transmission and distribution networks. This is largely due to fact that it is far easier to convert AC from one voltage level to another, which is required for moving bulk power across the grid efficiently and reducing losses. AC power is also easier to control/interrupt in switching and fault situations than DC, making the capital investment to build AC infrastructure far less than DC infrastructure.

According to Sam Sciacca of SCS Consulting, where DC holds a tremendous amount of promise is in the home/office/building environment where local generation (wind or photovoltaic) is available. “The conversion of photovoltaic (DC) power to AC, only to have that AC power converted back to DC for many home electrical devices, is an incredible waste of energy,” he writes. “Eliminating this waste, by some estimates, could improve photovoltaic system performance by as much as 25%.”

But he sees two significant problems. First there is the utility interface: in order to connect the DC-capable home to the utility distribution system, a new type of device is needed. Second, most building codes around the world do not cover DC distribution in homes, offices, and commercial buildings, which means that such things as wiring practices, distribution panel ratings, plug/socket conventions, grounding practices, and circuit breaker devices will need revamping to accommodate a DC option.

Carl Imhoff, who manages the Electricity Infrastructure Market Sector within Pacific Northwest National Laboratory’s Energy and Environment Directorate, sees increased synergy between AC and DC systems for low voltage end-use loads, particularly at the building-to-grid interface.

This is an emerging area of interest, he believes, because 40% of U.S. energy consumption occurs in buildings and they offer significant potential for efficiency and delivering ancillary services back to the grid.

“Using locally generated DC power from renewables and distributed generation in DC loads just adds to end use efficiency by avoiding a conversion to AC,” he said. “Buildings with data centers and renewables on the rooftop realize really good synergy to use as much DC power locally. So I see a really nice synergy between AC/DC at that building grid interface.”

The problems faced in achieving a workable solution are momentous, but so are the opportunities that will be available to developers in terms of new applications and new ways to power our lives without sacrificing energy efficiency. It is something that we should all be following closely.

Embedded.com Site Editor Bernard Cole is also editor of the twice-a-week Embedded.com newsletters as well as a partner in the TechRite Associates editorial services consultancy. He welcomes your feedback. Send an email to bccole@acm.org, or call 928-525-9087.

See more articles and columns like this one on Embedded.com. Sign up for the Embedded.com newsletters. Copyright © 2013 UBM--All rights reserved.

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