Crossposted on Politicook.net
This week the suggestion for this topic came from Kossack earicicle. Thanks to her for a good topic. I welcome suggestions for future topics. I will tend to stay with ones in which I have some expertise.
I would also like to increase the authorship of this series. I would very much like for another geek to post one out of four, so if there are any takers, please let me know in the comments. This might turn out to be a whole new website with Kos roots.
Just food for thought. Consider it, please.
Before we get started, have a look at a little gem that I found in a roll of cents I got from my bank last week. This was struck in Philadelphia in 1909, the last year of mintage (replaced in that year by the Lincoln cent). This one grades out to about VF-20, and is worth three or five dollars. I have found many coins worth considerably more, but to find a coin of this age in circulation in this condition is pretty exciting for a collector.
Now, on with the show. Except for nuclear energy, where mass is actually converted to energy in accordance with the famous Einstein equation, E = mc^2, all that we can do is convert one form of energy to another. This is sort of a problem.
We get most of our electricity from heat engines. Some sort fuel is used to heat water to steam (some designs use different working fluids), the energy of the steam is directed to turbines that spin the generators, and electricity comes out of the tubes. Almost half of our power comes from burning coal, with contributes a lot of carbon dioxide to the atmosphere. About 20% or so comes from nuclear fission, good insofar as greenhouse gases go, but still with no long term storage solution for the very dangerous byproducts.
Other heat engines used to generate power are natural gas (fairly clean, but somewhat expensive) and oil (dirtier, and more expensive). Over 90% of our power is generated by either burning fossil fuel or fissioning uranium. Geothermal plants also use heat engines, but with an essentially free, and, insofar as our understanding at present is, a nondestructive source of energy.
The other heat engine uses it solar collection, that is, to focus the heat contained in the energy from the sun to heat a working fluid. Otherwise, the details are the same as conventionally fueled plants.
Another way to generate power is to convert different forms of energy more directly into electricity. Wind, tidal, hydroelectric, and wave generators do that, by converting the kinetic energy of motion directly to electrical energy. Most folks agree that this is an environmentally benign way to generate power, but the energy extracted from those sources is no longer available as it previously was for the environment. I am not completely convinced that soaking up the energy from those sources will have no effect on the natural systems of which they are a part. When you get to the fundamentals, these are actually heat engine derived, with the sun driving the atmosphere and the oceans as huge heat engine. Even hydropower qualifies.
Regardless of how power is generated, it is either used locally or transferred to the grid. The grid is the interconnected set of generating plants and high power transmissions lines that move power from place to place. There are four grids in North America, and they are independent, meaning that those grids cannot be interconnected to provide power to grids with needs from those that have excess, and vice versa. This is because the grids are based on alternating current. More on that later.
One of the problems is that, except under very limited circumstances, it is not economical to store electricity. It has be generated as it is used, and that is a real problem. Batteries to store power are expensive, environmentally destructive, and not very efficient. Thus, the grid.
Because of the essentially zero ability to store electricity, grid operators must constantly balance supply with demand. When demand drops, power plants are directed to lower production to match requirements, and when demand increases, unused capacity is brought on line. When demand outstrips capacity, brownouts or rolling blackouts occur. A brownout is the reduction in voltage to an area of service, and this can cause damage to electronic and other electrical equipment, particularly refrigeration (think heat pumps and air conditioning as well) equipment. A rolling blackout is the intentional complete disruption of power to selected areas for a specific period of time, then cutting power to other areas and restoring it to the blacked out areas.
Rolling blackouts are preferable to brownouts since equipment is not as likely to be damaged with no power as it is with low voltage, but it is not that simple. Whilst most hospitals have backup generating capacity, many other critical areas (such as some nursing homes and most residences where people depend on ventilators, for example) are hit hard by lack of power.
Essential to understanding the way power is transported is the two fundamental types of electrical current: alternating current (AC) and direct current (DC). Direct current is a little easier to understand, so we shall deal with it first.
In DC, a power supply provides a current (a stream of electrons) that is constant (that is, there is no regular fluctuation in the current and voltage waveforms) and at a given level. Batteries and solar panels provide DC, and generators can be designed to provide it as well. Because DC is steady, the electromagnetic field that it produces when moving through a conductor is unchanging. This is important for a couple of reasons, and I will get back to them in a bit.
In AC, a power supply provides a current that is constantly changing, usually in a sine wave pattern. Consequently, the voltage associated with the current also fluctuates. Conventional generators are more efficient with AC, but that limitation can be overcome with modern electronic devices. Because of the constantly changing nature of the current, the electromagnetic field that is produces when moving through a conductor is constantly changing. This also is important for a couple of reasons.
Here are key concepts to understand long distance power transmission:
A changing electromagnetic field induces an electric current in nearby conductors. Since DC has a steady field, there is no induction. Since AC has a changing one, there is induction. This has both positive and negative effects.
The inductive effect makes it possible to use transformers to change the voltage of AC, but not for DC. Back in the early days of electrification, Edison built DC plants operating at 100 volts for electrification. 100 VDC works very well for incandescent lights, but is horrible for transmission over more than a few kilometers.
The key to this is the equation describing heat loss in an Ohmic (think copper wire) conductor:
q = i^2R, where
q is heat generated, i is the current in amperes, and R is the resistance of the conductor in ohms.
An extreme example of this is your toaster or range oven, where you can see the elements heat up and glow. They are made of a material with a very high resistance, so the Ohmic heating is large. High power transmission lines are made of carefully purified aluminum or copper (aluminum is not as good a conductor as copper, but is much lighter, so it pays in tower construction to use larger aluminum wires to carry the same amount of current), and the voltage is very high, in some cases approaching half a million volts. Why?
Another of Ohm's law reads:
E = iR, where
E is the electromotive force in Volts, i is the current in amperes, and R is the resistance in ohms. Looking at this equation, and the one before it, you can see that if you keep R constant (and it is in a given length of cable), you can reduce i by increasing E. That is what transformers do.
A transformer is essentially two coils of wire wound around a silicon steel core that can raise of lower electromotive force, with a corresponding reduction or increase in current. The best ones are up to 98% efficient. For instance, if you have a transformer with, say 100 coils on one side and 200 on the other, when you feed 10 volts at one ampere onto the 100 coil side, you get 20 volts at 0.5 ampere out of the other, neglecting losses.
Now think about it. If you have millions of watts of energy to ship a long way, you are better off to increase voltage, thus decreasing amperage as much as possible. Thus, power transmission lines are "stepped up" in voltage as much as possible to reduce the i^2 loss. However, that works only for AC.
DC does not work in a transformer, because its electromagetic field is steady. Only a changing field can induce another one. Therefore, it is not possible to manipulate the voltage and current of DC power streams with transformers. However, it is possible to convert DC into AC, and vice versa.
Converting AC into DC is relatively easy, and that technology has been around a long time. A full wave rectifier takes AC and produces a steady DC current. A half wave rectifier takes AC and produces a pulsed DC current, which is adequate for charging batteries, and some other applications. Pulsed DC has a changing electromagnetic field, by the way.
It is only fairly recently that efficient methods for conversion of DC to AC have been developed. A voltage inverter takes DC and converts it to AC, and then a transformer provides the proper voltage. The better modern designs are pretty efficient. You can even buy them at Sears, and many other places, to run your coffee pot off of your car battery when camping, for instance. One caution: do not try to run delicate electronics off of a cheap inverter. The waveform that the cheap ones produce are fine for light bulbs and coffee pots, but sort of hammer things like TVs. Same goes for home generators. A "clean" sine waveform is important for many applications, and inverters, unless carefully (and expensively) designed and built are not that clean.
Now, let us go back in history to the battle between DC and AC electrical service. For most consumers, lights and heat sources were the main uses for electricity, and DC and AC work equally well for those applications. Edison was a champion of DC, calling AC "death rays". Actually, a 100 VDC shock is also dangerous. George Westinghouse championed AC (aided by research of the brilliant and very quirky Nikola Tesla (who, by the way had worked for and who hated Edison)), and quickly dominated the market. Why?
Let us go back the the i^2R losses. In the late 1800s there was no practical way to convert DC to AC, so DC had to be generated at the source in the voltage to go to the end user. Such low voltages have horrible i^R losses, so generating stations could only be a few kilometers from the end users. This was inefficient, since many, small plants would have been required.
On the other hand, AC could be transformed to any voltage desired for transmission, reducing i^2R losses, and then back again to whatever voltage that the end user requires. At the time this was the much better approach. But that was then.
AC has a disadvantage insofar as transmission goes. Since it has a constantly changing electromagnetic field, it induces currents in any conductors nearby, as stated earlier. This is great in transformers, but not so great when passing by a steel transmission tower, where there is energy dissipation by inducing currents in the towers that is shunted to earth ground, sapping energy. Here is a nifty experiment: if you live near large, high voltage lines, walk under them whilst you carry a fluorescent light tube. It most probably will light up, because of the current induced from the transmission lines.
High voltage DC power distribution is realistic today. Transmission lines carrying power at hundreds of thousands of volts would operate with greater efficiency than the existing AC ones do, because of the elimination of induction. In addition, they would be safer to service. Even with the power cut in half of the AC lines, the norm for servicing them, induction from the other lines can generate potentials of several thousand volts in the "dead" lines. This simply does not occur with DC.
I realize that this has been sort of a rambling post, but I think that it is important to look into the history of why we have the legacy high voltage AC distribution system. I do not think that we will ever get away from AC, because it is so easy to manipulate, but HVDC distribution is the wave of the future since we can convert DC to AC efficiently now. But for over 80 years we could not, and we now have to deal with those legacy systems.
So now we see what we have, and what we can develop. But there is one more advantage to DC transmission: it does not require any careful balancing except for voltage. In AC transmission systems, phase and frequency have to be carefully balanced, in addition with voltage, to get it to flow in the right direction. If any of these are off, destructive interference occurs and energy is lost. This is a very, very difficult task. That is why the four grids in North America can not "talk" to each other: they are out of phase with respect to one another.
With DC, it only has to be gated properly, if it be at the right voltage, to allow it to flow into the DC grid and be transferred. Phase and frequency matching are not required, since DC has neither of those properties. This would allow the four grids to interconnect, by rectifying AC to DC, transferring the power, and then inverting it to match the phase of the other grid. This has the potential to help balance loads across all four grids.
The real advantage is for HVDC to transmit power for long distances, say from wind farms in the wind belt to other parts of the country, regardless of which grid is involved. At present, it appears that it is more economical to use HVDC for distances over a couple of hundred kilometers (because of the cost of rectifiers and inverters) when AC power is the original source, and even shorter distances when DC is the original source because there is no rectifying step.
AC generators are in general more efficient than DC generators (dynamos), and full wave rectifiers are not that expensive and are reliable. Another disadvantage with dynamos is that they produce pulsed DC, which has few of the advantages of steady DC and many of the disadvantages of AC. Except for photovoltaic, most electricity will continue to be produced as AC and rectified before heading to the grid.
Well, this is a very basic primer to they way we transmit power from one place to another, why we got where we are, and a look to the future. As always, any questions, comments, corrections, or any other science related topic are welcome.
UPDATE: I solicit ideas for future topics, so please do not be bashful.
Second update: I will be posting guidelines for guest hosts for Pique the Geek next time. Accuracy is paramount, but everyone makes a mistake now and then. I sure do. Respect for the readers and commenters is important, even it they are rude. And I would like at minimum of at least an hour to respond after the first comment comes to you. I have been here for almost 3.5 hours. Dedication is important. Finally, no bull. If you do not know, either look it up or say so. I will be more specific next time. I look forward to other contributors.
Last update: I have been at this for 3.5 hours now, and I think that I will go and try to find another gem in my unsearched rolls of coins for a little while. For everyone who read without commenting, please become more involved. For everyone that commented, thank you. For those of who that challenged me, many thanks, because that keeps me honest and constantly checking my facts.
Finally, for the one of you who wants to write, bring it on! Kos does not run this site alone, and I can not run a quality series every week alone, either.
Thanks to all for all of the tips and recommendations.
Oh, as always, I will check for any comments that came in to late for me to respond to tonight and do so tomorrow before the clock runs out on us. My intention is to recognize everyone who contributes here.
Warmest regards,
Doc