Engineering the Direct Current-powered home -

Engineering the Direct Current-powered home

Recently I had a discussion with some of my colleagues about the benefits of a DC-powered home. I commented that in some ways, my home is becoming more and more DC-powered. Actually, it is kind of a distributed DC power where some rooms have four or more wall adapters, as well as other devices that convert to DC internally like computers and printers.

According to the Environmental Protection Agency (EPA) in the United States, each person has an average of eight adapters1 . Counting what I have at home and the office, I have 10 – although some are duplicates. For example, I have a notebook charger for my home and one for my office.

Over 100 years ago, Edison and Westinghouse were on each side of the DC versus AC debate. At that time the loads were light bulbs, motors, and heaters. All had DC or AC options. AC had the advantage in that the voltage could be changed with a simple transformer. DC had issues like arcing and connector corrosion.

Figure 1 shows how energy is used in an average home1 . Combined, the electronics and “other” categories account for 15 percent of the average home total energy use, according to the Consumer Electronic Association2 . Others like Greg Reed of the Power & Energy Initiative at the University of Pittsburgh suggest the number is closer to 20 percent.

Reed also predicts that by 2020 the number can reach 50 percent. This certainly is possible, if solid-state lighting is heavily adopted. Figure 1 shows that today’s lighting uses 12 percent of the home average energy. It is not likely that we will reach the full 12 percent as adopting solid-state lighting reduces the energy load from lighting.

So what about other household loads? Next in line are appliances. This category includes refrigerators, freezers, clothes washers, dryers, dishwashers, microwave ovens, cook tops, and ovens. Any appliances that use motors can switch to DC brushless motors. This allows DC to power the dominate part of the appliance load. Moreover, the improved efficiency helps lower the total home energy load by as much as 35 percent. 

Appliances such as dryers, dishwashers and some clothes washers have heating elements for drying or heating water to improve cleaning efficiency. These elements can be powered by AC or DC. Converting AC-to-DC for the purpose of powering a heating element is not the most efficient use of energy, so keeping it AC is probably the better idea.

Microwave energy is dominated by the magnetron, which is a DC-powered device. Ovens can be powered by either AC or DC, however, AC power is more efficient. For the range, converting to an induction cooker doubles the efficient use of energy and enables DC power to be used.

The final categories are heating and cooling. If these systems are built from heat pump technology, then compressors and fans used for the condensers, evaporators, and circulating air can be brushless DC motors. This allows DC power to be used to provide higher energy efficiency.

Neither Edison nor Westinghouse, and not even Tesla, could have predicted that someday semiconductors would make AC-to-DC conversion easy and efficient.  And, furthermore, provide the power and modulation to power lights, motors, and other devices with improved energy efficiency.

So what are we waiting for? Well, we have considerable investment in our AC systems. However, more than 25 percent of the world lives off the grid. These homes easily could accept and benefit from a DC power source, such as solar panels.

These households could start out using solid state lighting and simple DC-powered fans. Additionally, DC energy could be stored in inexpensive battery systems. In many ways, homes starting from scratch will have the advantage of not having an AC legacy.

The DC-powered home most likely will need two DC buses. The “other,” electronics, and lighting categories could use a low-power DC bus, for example 12V to 48V. However, the heating, cooling, and appliances categories would benefit from a much higher voltage, for example 380V to 400V. 

So what is it going to take to push the DC home into the mainstream for grid-connected homes? I think it will take a very well-defined benefit. About five years ago a team of us asked several data center managers what it would take for them to change their system power management solution. The response was a 30 percent energy savings. This was certainly a surprise.

We were thinking a 10 to 15 percent reduction would be attractive. Of course, it did depend on the cost to make the changes. The real answer was that they wanted a two-to-three year payback. At this point, we are not there yet. This may be due to our relatively low cost of electric energy. I do not recommend that we increase energy cost just to make a DC-powered home look attractive.

So when do you think we will see the grid-connected DC home? Maybe it is when the smart grid is more distributed, or maybe when the benefit is greater than a 30 percent reduction in energy use – after all the passive methods of reducing energy use are completed. What are your ideas concerning the DC-powered home?

We want to hear from you! By the way, with the holidays coming up, my home is taking one more step to becoming more of a DC-powered home.  We are replacing our 15 year old washer and drier with direct drive models. The washer had taken to imitating a jet engine during the spin cycle. Yes, more DC in the home!

Figure 1. Energy Use Distribution1

For more information about this and other power topics, visit TI’s Power House blog:

  1. Energy Star, Save energy at home
  2. MIT technology reviews: Edison’s revenge – The rise of DC power

Dave Freeman is a Texas Instruments Fellow and Chief Technologist for Power Supply Solutions in the Power Management business unit. Dave has expertise in the areas of battery management ranging from charging to capacity estimation. Dave offices inside Kilby Labs where he manages the energy lab as well as power/energy related research projects. His blog has also been published previously on sister publication EDN. Magazine.

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