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Energy losses – The great DC versus AC debate
(Editor’s Note: This article is not written to disprove any theories about AC transmission lines. We need to be a lot more skeptical about talking DC when the advantages and disadvantages of DC and AC were known about more than a century ago. We talk energy efficiency when most vehicles driven to work are single occupant vehicles. We really do have our knickers in a knot. This article does emphasise DC in the home although common engineering practice would be AC to DC (high voltage) and then back to AC using the advantages of both DC and AC).
Although we are so accustomed to having power at the press of a button many engineers are now rethinking the way in which we get power. There are huge energy losses in the AC transmission line from utility to the home or factory, mostly brought about by the inherent nature of alternating current flow through a conductor. The major concern years back was in the inversion process, converting DC back to AC. Thomas Edison’s first power supplies only carried DC but at the time AC was seen as being superior because of the ease at which voltage levels could be transformed.
Substation Transformer – photo credit Magnus Mansk (Wiki Commons)
Now, fast forwarding to 2015 we find that because there is a lack of energy, because the supply is not meeting demand we start seeing the drawbacks – how to simply feed power back to the grid from the renewable domestic energy source. DC seems like the best answer. Controlling current from source to load using DC is just far simpler than AC. From grid to home or home to grid. However, in all the papers examined the engineering of these DC power lines consists of the source being AC (generator), rectification, transmission, inverter – load. With AC we only have generator, transformer to high voltage, transformer to low voltage – load. Transformers are remarkably resilient, more so than any solid state inverter. Also, what are cost implications? Inverters are dirt cheap when one looks at entry level, low power units but shifting into the mega Watt range one has serious doubts as to the overall price / reliability advantages over straightforward AC transmissions.
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Energy Losses – an off line grid tied system
Looking at modern trends, which will include a grid tied inverter system, the lack of an efficient and economic storage system seems to be the major downfall. Just how much power does the utility company save when all the domestic and industrial voltaic panels are generating maximum power when possibly the load is at the least? Massive amounts, according to analysis. In one industrial area tests proved conclusively that at mid-day, outages were most common. With a grid tied system this means one would lose power in any event to prevent islanding, or backfeed.
With current generation grid-tied systems the big advantage is an economic one, not back-up power. Industrial power still needs huge three phase generators to supply backup power, lead-acid cell accumulators are by no means economic at any level. The problem we still face is not whether the power supplied is AC or DC but storage, even if DC is as efficient as proven. Many grid tied systems do not co-exist with generators, the inverter is shut down when the generator is running. Most auto start generators have no minimum current sense inhibiting start up if there is no load, also a wasteful exercise.
Battery Storage – Nickel Iron and Potassium Hydroxide
Deep cycle (lead-acid) batteries do not have an infinite life-span. In the real world we need to be looking at accumulators being made up of the purchase of individual cells, not one 6 cell unit. Often batteries are recycled only when one cell becomes defective. Many years ago NIFE cells were used in most emergency circuits, they were very durable and lasted upwards of 20 years. Ask any company that service batteries used on ships. Emergency generators still need batteries (not compressed air) to start. NIFE is a life saver. They are more expensive but then again why promote something which is so reliable.
In the home most appliances can be designed to run off DC. Even simpler, the DC voltage is the RMS of the peak rated AC voltage. This would therefore equate to the RMS of sine. Pump motors are designed to only run off AC. This is far more reliable (and cheaper) than using a commutation circuit. Inverters designed to power pump motors from DC are not expensive and if properly rated will last the lifetime of the motor. In high current industry this may prove to be another problem altogether. Three or polyphase supply circuits are very efficient. For the same power draw, three phase conductors are going to be much thinner than a DC connection.
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Volts that Jolt
Many people believe that it’s current that kills, not volts. This is woefully wrong. Ohm’s Law dictates that you will have no current if there is no voltage.
Is DC safer than AC? We know that very little current through the human body will kill. With ground fault interruptor circuits the electricity in your home is much safer than 50 years ago. But still not fail-safe. With DC we do not have the jolting affect of AC in case we do accidentally do touch live, but we can get serious burns. Either form of power is dangerous and will kill if used negligently.
The Cost of non-standardisation
So although we may be embarking on a complete new strategy to get power to the homes and industry we need to look at the cost implications first. It took years for us to have a standard yet we still remain unstandardised. In fact, very much disorganised. All countries should use a common line voltage, e.g. 60Hz and 220V. 50Hz is very inefficient. 110V in the USA may have evolved from T. Edison’s monopoly of DC power but AC at the time just had the upper-hand. 110V uses very thick conductors to power most homes. Just have a look at the myriad plugs, sockets and earthing types available world-wide. 110V is really no safer than 220V.
Before we even think about moving on to the next great thing (which has been available for more than 200 years) let’s talk standardisation, storage and cost implications. The most expensive lesson we can learn is to not remember the past. Both Tesla and Edison were geniuses – maybe, just maybe we should go back to the drawing board. Energy losses within AC circuits was known about a very, very long time ago.
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