Why DC May Replace AC (2019)

The reason Level 2 charging is current-limited by your car -- even if you had a very high-current AC source available -- is that to take advantage of a high-current AC source your car would have to carry around a bigger, heavier rectifier. Which would decrease your EV's efficiency just by virtue of being big and heavy.

On first thought this is a ridiculous statement: voltage is a measure of potential energy after all. If load is reduced, the voltage just hangs there and less power will be consumed.

But with AC distribution what you have is essentially a large rotating machine. The more power you put into it, the faster it spins. When you connect a generator to the grid, you phase match the AC waveforms, connect it, then start pushing the grid faster to inject power.

So the statement is true. If the load is too low, the frequency starts to go up. But it's also not true.. if each generator independently self limits the frequency, we would be back to the potential energy situation. But power plants want to push power into the grid- this is how they earn money.

So some plants self regulate and some do not, see:

https://www.e-education.psu.edu/ebf483/node/705

Ones that self regulate get to charge a premium for their unused capacity.

Even with DC only, you would be in the same situation. A power plant wants to make money, so it will want to push power into the grid, which for DC means pushing the voltage up.

Distribution implies a network that keeps getting downstepped until it reaches consumer nodes.

You have the same problem with DC. It nothing to do with 'AC' per say. It's a issue of large scale power plants.

> On first thought this is a ridiculous statement: voltage is a measure of potential energy after all. If load is reduced, the voltage just hangs there and less power will be consumed.

Voltage is a measurement of the difference of two electrical potentials. "Potential Energy" is something else entirely.

Voltage is not a measurement of energy.

Voltage works in a similar way that pressure does. Imagine you have two pressure cylinders and one of them is 200 PSI and the other is 220 PSI. If you were using a voltage-style measurement between the two cylinders you'd say that the there is 20 PSI different or 'potential' between them.

That gives you zero information as far as the actual energy potential. A 16 ounce canister at 100 PSI is going to have a lot less energy potential then a 500 gallon tank at 100 PSI, for example.

This is why you can go and get a static electrical shock that involves thousands of volts and your skin doesn't burst into flames.

> But with AC distribution what you have is essentially a large rotating machine.

No with AC, or DC, what you is LITERALLY a large rotating machine. A rotating machine larger then most houses running of of hydro electric, coal, or nuclear power take time to have their energy output adjusted.

They don't work like car motors were you press a button to go "zoom" and another to go "woah".

The generators that can quickly adjust are small ones. Generally natural gas turbines. They are a lot like jet engines. In fact many of them used to be the same type of engines used in jet planes.

And they are much more expensive to operate and less efficient overall, but they are the ones people are moving to because they can keep up with the extremely poor quality electrical output (read: highly unpredictable) you get from solar and wind.

> Even with DC only, you would be in the same situation. A power plant wants to make money, so it will want to push power into the grid, which for DC means pushing the voltage up.

You somehow seem to have "laws of physics' confused with "making a profit".

Of course you are not wrong with the power plants operators wanting to get paid to work for a living and there are plenty of shady things they do that you should be irritated about, but you are barking up the wrong tree here.

For example: the massive scam that is government-subsidized grid-tied residential solar. How that the plant operators have colluded with the regulators to ensure that they have remote control over your inverter's output. Which means that with the hundreds of thousands of solar panel installations that people are proud of and think they can make money from 'selling back to the power plant' are actually operating at a only a tiny fraction potential output.

Which means that home owners that do pay tens of thousands of dollars for these setups are getting burned WHILE accomplishing nothing to help the environment.

And when the grid goes down so will those grid-tied installations. For "safety" reasons, despite the fact that ICE-based generators have had reliable failsafes for generations that automatically prevent any electrical feedback into downed power lines.

--------

The fact of the matter why DC is better then AC for power transmission has to do with inductance.

Every time the wire is subject to electrical current it generates a magnetic field. The higher the current the more powerful the magnetic field. By winding a cable around a iron core you can concentrate that field and make a powerful electro-magnet. The effect is still present in the miles of electrical cable used for power distribution. It's just spread out over a massive area.

With AC that magnetic field needs to be torn down to zero, reversed, and then torn down to zero again 60 times a second. Sure much of that energy is returned to the wire on each field collapse, but it is still something you have to deal with and fight with. It shows up as significant inductance.

With DC you don't have to do that.