I recently had a conversation with a friend, who is interested in third world economic development, "leapfrogging" technologies, and energy, about AC vs. DC for electrical power transmission and distribution. He asked me for my further thoughts on the matter, so I will put them here, along with some caveats about what things I know that I don't know. I invite readers who know more about the topic to leave corrections and additions in the comments to this post.
A bit of history
The debate about whether to transmit power as AC or DC raged in the early part of the 20th century with Thomas Edison advocating for DC transmission of electricity, and Tesla advocating for AC. At the time, AC was adequate for most household applications (many of which involved the use of electrical current to produce heat, or to produce lighting through heat, and thus didn't care about the direction in which current travelled at any given time) and was vastly easier to efficiently step up or down in voltage, allowing for efficient transmission of power over long distances. One exception was that it was easier to build a variable-speed electric motors to run on DC (it is trivial to build single-speed AC motors, especially if one has 3-phase AC power, which is generally what is transmitted over longer distances, and what is supplied to industrial facilities). Older elevator technologies (circa early 1900s) tended to use DC motors, and tended to be installed in dense urban cores, where a large number of electricity customers could be served without the need for long-distance transmission. For these reasons, small DC power grids existed in many of America's large cities for decades after AC otherwise won what was known as the "war of the currents". See, for instance
- http://www.coned.com/newsroom/news/pr20071115.asp
- http://spectrum.ieee.org/energy/the-smarter-grid/san-franciscos-secret-dc-grid
- http://cityroom.blogs.nytimes.com/2007/11/14/off-goes-the-power-current-started-by-thomas-edison/?_php=true&_type=blogs&_r=0
To make a long story short, one generally wants low voltages, and the capability of producing high currents, at the place where the electrical energy is used. But it is far more efficient to transmit electrical power at high voltage (and comparatively low current). AC is fairly easy to step up and down in voltage using transformers. It has not, historically, been easy to do the same thing with DC power.
Advantages of DC in the modern world
One drawback to AC power stems form the fact that modern power grids are extremely interconnected. Placing two or more AC power sources on the same network requires that the sources be synchronized. This requires some form of dynamic control. A simple thought experiment should reveal why two out-of-phase AC sources wired to one another effectively create a short. Dealing with these synchronization issues has led to some fascinating control theory papers, but I could understand why practical engineers might want to dispense with these problems altogether and transmit electrical power via DC current. I don't know how much more complicated AC synchronization problems become when individual consumers are allowed to produce their own power and feed it back into the grid.
As an extra piece of terminology (and one which might be very important to understanding other discussions of the electrical grid), power engineers divide the grid, conceptually, into two distinct sorts of networks.
- Transmission networks carry high-voltage electricity over long distances and interconnect different power plants on the grid. It is my impression that AC synchronization problems generally occur in the transmission network portion of the power grid.
- Distribution networks carry lower-voltage electricity from the transmission networks to end users, either industrial, commercial or home consumers. It is my impression that there is generally only one logical path for power to take between any two points on the distribution network (with the note that a "path" in this case might be 2, 3 or even 4 wires, depending on the form of the AC current).
Further, modern appliances are quite different from those of the days of Edison and Tesla. Computers and other digital equipment generally require DC power sources (the "power supply" of your desktop computer contains a transformer, a voltage rectifier to convert AC to DC and a voltage regulator to maintains the voltage of the supply at a steady level, while your laptop likely has the same equipment mostly embedded in its power chord). LED lighting works with DC current (any LED light that you can screw in to your conventional lighting fixtures must come with, at bare minimum, a voltage rectifier to convert the AC of your light fixture into DC for the LEDs). Today's TVs, being digital appliances, require DC power internally.
DC is of particular appeal to off-grid power systems for a variety of reasons. For one thing, solar cells naturally produce DC current. Most conventional electrical power generation (coal, oil, nuclear and the sorts of large solar installations that use mirrors to heat water) at some point heats water to turn a turbine which turns a generator. It is fairly easy to design such a system to produce 3-phrase AC power. I am not sure how difficult or easy it is to make such a system generate DC without using a rectifier. Equally important, perhaps even more important, to off-grid systems is that DC is required to charge backup batteries, and is the natural output of chemical batteries. An off-grid power system using solar for energy generation and batteries for storage will naturally want to be DC. I note here that one should be wary about using off-grid solutions with battery backup to leapfrog economic development in the developing world : the most economical rechargeable battery solution at this point is still lead-acid batteries, which come with a host of problems. I will try to see if I can dig up resources on it. One might want to ask whether solar generation with battery backup becomes more feasible when all of one's appliances become much lower power (lighting could be LED-driven, heating is less of an issue in parts of the developing world, computing and communications are becoming more efficient everyday. I still wouldn't want to run my washing machine off of batteries though). Battery technology is also rapidly improving, driven by consumer demand for things like smartphones and laptops with long battery life.
A revolution in transmission technology
One of the main advantages of AC current for electrical distribution, as mentioned above, is the ease of stepping voltages up and down, to allow transmission to occur at very high voltages, while giving end-users safe and convenient low-voltage electrical energy with high current capacity. Safety aside, giving high-voltage to end users would be infeasible for a variety of basic electrical reasons. Common materials, such as air, behave differently under high voltage and would need extra considerations.
But, because of the problem of AC generator synchronization, utilities have found it to be desirable to have their large high-voltage interconnects run on DC power, which is much easier to synchronize and coordinate.
Thankfully the technology to convert high-voltage AC (HVAC) to high-voltage DC (HVDC) and to step DC voltages up and down have improved radically during the semiconductor revolution, as technologies originally designed for lower-power applications have found their way into the world of power electronics. A good summary of the state of things is provided by Wikipedia : see http://en.wikipedia.org/wiki/High-voltage_direct_current. Photographs of some of these new pieces of equipment are spectacular in their scale and design, see
http://en.wikipedia.org/wiki/File:Pole_2_Thyristor_Valve.jpg
Summary
Advantages of AC current
- Ease of stepping up/down voltage (for efficiency in transmission)
- Ease of making a single-speed motor (for instance, your coffee grinder)
- AC is the natural output of the sort of electrical generator I would design were I to design a generator
- Adequate for heating applications
Advantages of DC current
- Avoids the AC synchronization problem
- Good for variable-speed motors (anything from the motor on the Honda insight, to the stepper motor in your hard drive, to wheelchair motors, and I think even washing machine motors).
- What batteries want to be charged with
- What batteries output
- What solar cells output
- What computers and digital electronics want to work with
Technologies to watch if one is interested in these issues
- High-voltage rectification
- High-voltage DC - DC step-up / step-down
- Battery technology
- Socio-economic situations that might produce micro-grids
- Technology that allows households to accomplish basic tasks using less power (I suspect there is little to no room for improvement in this area for things like electric stoves and electric heaters, but quite a bit of recent progress for communications, computing, lighting and entertainment. I am curious as to basic things like "can one make a significantly lower-power automatic washing machine")
One thing I have not given much thought to is which form of power is easiest for people with little electrical knowledge to effectively deploy in micro-grids, and what sorts of technologies could change this.
