If all our devices are DC, then why do we still have an AC grid?
During the 1880’s a war was waging between two of the most brilliant scientists in the world and their respective financial backers- Tesla and Westinghouse with Edison and J.P. Morgan. The “War of Currents” as it was later called, was over whether we should use an AC (Alternating Current) or DC (Direct Current) power grid. While Edison was in favour of DC, Tesla thought that an AC grid would be more advantageous. Edison’s argued that DC power was safer at the same voltage level as compared to AC, and he demonstrated it by electrocuting quite a lot live animals(including elephants) using AC power. Tesla however believed that the ability to change voltage levels in AC power using transformers meant that huge amounts of power could be transmitted over long distances which was needed in the case of cities and industrial hubs. Eventually, it was Tesla who won and everywhere in the world, except for a few places, grids are predominantly AC.
Eventually, it was Tesla who won and everywhere in the world, except for a few places, grids are predominantly AC.
Take a look at the different appliances around your home or office or lab or wherever you are reading this now. You will definitely see lights, predominantly fluorescent lamps or LED Lights. Your gadgets, like phones, laptops, eBooks readers and so on, are all power consuming devices. Other appliances that you have like TV’s, refrigerators, microwaves, washing machines, pretty much everything except fans constitute almost 80% of the total power consumption in any building. All these devices have one thing in common, they all use DC power! Everything from the PV panel on your rooftop to the electric car in your garage and the inverters you use to store power are DC loads (load is a term electrical engineers made up to describe anything that consumes power). Datacentres consists of only DC devices, so much so, that there are very few AC powered Datacentres in the world.
All this begs the question, if all our devices are DC, then why do we still have an AC grid? Was Tesla wrong and should Edison have won the “War of Currents”?
Was Tesla wrong and should Edison have won the “War of Currents”?
But everything is fine, our devices are working fine on AC. Or are they? Our devices want DC power, and the way to give them that is by having converter boxes. That is a fine solution, but, if you take your laptop adapter as an example, there is one problem, they get hot. That is wasted energy. And that’s not all, there are other ways that energy is wasted in this conversion and as much as 20% of the input energy is wasted in this AC-DC cycle. Now imagine a solar PV giving power to your laptop or Electric Vehicle (EV). So, DC from PV has to converted to AC to feed it to the grid (which has more losses than AC-DC conversion), then AC is converted to DC to boost the voltage, then DC to AC to send power to the EV, and finally AC to DC to charge the EV battery. The power loss here is huge and the easiest solution would be to just feed the DC power from PV panels to the battery. Moreover, the size of electronics would reduce a lot if we just neglect the AC conversion in between. UPS’s and inverters would weigh half as much and be 1/4th the size if we used just DC power. Chargers would no longer need adapters. And you won’t have to worry about your device burning up if you accidentally plug it into a socket in another country with different power standards.
The power loss here is huge and the easiest solution would be to just feed the DC power from PV panels to the battery.
AC transmission requires three conductors (power lines/wires) at least and in some cases up to 8 are needed. Whereas DC transmission requires just two power lines. AC transmission also has a lot more losses as compared to DC transmission in the form of corona loss, magnetic effects and skin and proximity effects, whereas DC doesn’t. DC transmission also requires less towers to be erected and there is no need for synchronising power in DC grids whereas AC grids have to be synchronised. There is a lot of material savings and land savings in DC transmission. Lastly, DC power is safer “to touch” as compared to AC power at the same voltage level. Accidents in DC power systems have been less fatal than in AC power systems even when they are at lower voltage levels.
The change to a completely DC grid will probably never be made.
So why aren’t we converting to a DC grid then? For one, it’s not that easy, over 100 years of existing infrastructure has to be changed to make the conversion and the costs would far outweigh the benefits. Moreover, recent developments in thyristors and power electronics have reduced losses and reduced the size needed for conversion and there still is no sure-fire way of changing DC voltage levels. Industries too, still need AC power to run their machines and their power consumptions are much more than that of homes and office buildings (predominantly DC). Even the generation of DC power in large scale is still not as efficient as the generation of AC is. DC power is also harder to control than AC power. DC Circuit breakers (which are protective systems in case of faults in transmissions) cost at least ten times as much as AC circuit breakers. Neither fans nor Air Conditioners can run on DC power. Therefore, the change to a completely DC grid will probably never be made.
What we can expect in the near future, though is to have an AC-DC smart grid, where the power sockets in your homes and offices will have both AC and DC power, so you can plug your devices into whichever kind of power you need. We will have smarter infrastructure that can predictably reduce losses using SCADA in both transmission and distribution. We will also have distributed energy resources and micro grids to further reduce faults and transmission losses.
The future is sure to be more safe, efficient, smart and exciting.
Soham Chatterjee 