Power to the Home: AC or DC?

What are the relative benefits of AC power systems — which are found in almost all residential housing and most commercial buildings — versus an alternative DC power distribution system, which has not yet been adequately defined? How that system would be defined, and who would define it, are questions that must be answered before household DC power distribution could even consider moving beyond a hobby type of implementation. The only commercially built-in DC power systems of which I am aware are in travel trailers and some motor homes, which are considered mobile structures governed by different regulations and requirements. The permanent structures with DC power distribution systems are primarily those of the “off grid” folks, not the type of individuals choosing to adopt some standard developed for the industry’s benefit.

So just what are the technical considerations standing in the way of using DC power to feed all of our many electronic devices that currently depend on 120-VAC power supplies? LED lighting and entertainment systems come to mind first, followed by computer systems and then appliances, and then climate control systems. Security systems would be lumped with lighting, and communications systems with computers.

Both LED and fluorescent lighting systems require current-limiting controls, as a minimum, because of the negative resistance characteristics of the lights and the constant voltage properties of diodes. For many years, while a simple reactive impedance ballast has been adequate for fluorescent lights operating on AC, any type of DC operation has demanded a considerably more complex and expensive current-limiting system. LED series strings mostly seem to be the choice, which is primarily because they are the simplest way to ensure that the current is equal in all of the devices. However, it also requires different voltages that depend primarily on the number of LEDs in the string. Efficient control of the current demands a switch-mode controller, and the additional components and design effort to add an AC rectifier are a small part of the task.  Likewise, for a modern switching-type of fluorescent lighting ballast, having the higher DC voltage provided by the rectified AC lines makes the components smaller and the currents lower, both reducing the cost. Therefore, in these instances, the DC supply voltage would need to be higher than one would guess without knowing how they work. The immediate and obvious disadvantage would be the need to keep track of the supply polarity. Controlling the intensity of the light is marginally simpler in an AC system, although advances in DC inverter ballasts could change that.

Entertainment systems, computers, communications systems, and security systems mostly have switch-mode power supplies to provide the multiple voltages, often regulated to specific voltages, that their circuits require. If the input power were DC instead of AC, that requirement would not change. One more consideration is that a much lower input supply voltage, given that the powered circuit’s requirements were not changed, would demand higher currents in the power supply circuits, increasing both electrical stress and resistive losses. It is not likely that the required power filtering to reduce the effects of external electrical noise would be less complex, but the current ratings would probably be increased because of the lower voltage requiring more current to deliver the same amount of power.

Big appliances, such as refrigerators, washers, and dryers, use motors, which until recently were all induction motors running on AC direct from the mains. Some applications that needed different speeds used multi-speed induction motors, a simple but somewhat less efficient method than a good variable speed drive system. But AC-powered washers and dryers with induction motor drives all include additional speed reduction hardware to provide the much lower speeds needed for the processes involved. Refrigerators, which have a sealed-unit compressor, utilize on/off control to hold a specific temperature set-point. Dishwashers would benefit because the brushless DC motor for the DC-powered version would allow more convenient speed control for the different wash cycles. But the advantage over AC-powered systems would be slight.

A major change is in process as brushless DC motor direct drives are taking over in all of the applications.  Washing machines in particular have changed fundamentally, at least the higher-end ones with horizontal axis drums, in which the drum reverses rotation several times a minute. The constant reversing would be hard on an induction motor because of the frequent starting, but the brushless motors have no problems with this mode of operation. The driver circuits for a brushless DC motor utilize DC power, which currently is just rectified AC power that does not need much regulation or filtering. So these appliances could work well on DC power, but the supply voltage would need to be fairly high, probably near the peak value of the present AC supply voltage. In the special case of electric dryers, the ones that use resistive heating to provide the hot air for drying, DC heating would be slightly more effective than AC-powered heating, BUT the phase control for setting the temperature on AC-powered systems would need to be replaced by PWM controllers for a DC-supplied system. That could raise the cost and possibly generate more electrical noise.

Climate control systems, including air conditioners and heat pumps, consist of two or three blocks: one to heat the air, one to cool the air, and a common block to move the air. Other kinds of systems may utilize water as an intermediate medium for moving heat between locations. In the past, all of the blocks used induction motors, which demanded a reasonable quality of AC power. In many current systems, the induction motors are replaced by brushless DC motors, allowing improved efficiency through speed control.  But all of the motors in these systems require significant amounts of power, and, therefore, they would need a fairly high DC supply voltage to the BLDC control driver module.

What conclusion can we draw? It is clear that many devices do indeed run on DC internally, even those that use a fair amount of power. But it is also clear that there are nearly as many different DC voltages used as there are devices to use it. This means that any whole-house DC system would need to provide those different voltages to the proper locations, because neighborhood distribution of multiple DC voltages would be both complex and inefficient, primarily because of the voltage drops. A single-voltage-to-the-house DC supply would still need to have a means to provide the correct voltage at each device, except that it would be utilizing a switch-mode regulator instead of a transformer. While some switch-mode power supplies are well designed and last a long time, many use the least expensive components that will usually last until the warranty expires. Those assemblies probably do cost quite a bit less than a transformer of slightly better quality. They may or may not be as efficient, and they undoubtedly will produce more electrical noise.  So we can say that in a lot of instances the price might be a bit less, but the cost of ownership would be similar, or even a bit higher.

Although DC for those long-distance, high-power transmission lines does make a lot of sense, AC power is more rational in our homes. And for local distribution, the improved efficiency of DC does not make sense, given the number of transformers that would need to be replaced by step-down converters. If we want a more efficient local distribution system, we need better transformers. While the idea of DC everywhere sounds good, it does not make sense for our lifestyle. For those who disagree, consider how many transformers have failed for you in the past 20 years versus the number of switching power supplies that have failed. In my case, that is two transformers and about 23 switching power supplies.

William Ketel designed industrial test systems, primarily for the auto industry, for 30 years. He holds a BSEE from Lawrence Institute of Technology and an amateur extra-class license, and has a wonderful grandson. Currently, William is retired except for the occasional distractions of his consulting business, called #28 Engineering. He also enjoys riding his mountain bike everywhere except on mountains.