Although AC phase matching is a delicate technical problem, it’s one we’ve solved for over a hundred years. DC presents other engineering challenges that are non-trivial. For example, circuit breakers for AC power are designed to “break” when the AC curve hits zero volts. This eliminates the chance of arcing and makes breakers smaller and cheaper to manufacture. A DC breaker has a chance of arcing and it may be necessary to make them larger, or use exotic gasses with high dielectric values to prevent this from occurring. Either of these increase costs for homeowners.
You're assuming that we won't.
> With solar, it's very possible that ...
How nice the world would be we could all lean on "well because it is possible it must eventually happen."
- Maybe other things will be different too.
- Oh, so everything will be different?!
That's really not how I read that comment.
I could however see a future where cities refuse to subsidize rural homeowners and communities, disconnect from the country-level grids that exist today, let them de-energize and fall into disrepair, and then maintain only a few transmission links over major transportation corridors to connect with other major cities.
With AC, there is no net current anyway - nothing physical is being transmitted any substantive difference. The actual electrons travel on the order of 0.2 microns per 60Hz cycle, and then move back the other way. [1]
In reality, in the absence of fundamental electric components like resistors/capacitors/transformers, all points of a circuit have the same voltage and same current. The transmission lines are just connecting different cities into the same circuit, there's nothing flowing between them. This allows your solar array in the Mojave Desert to power your data center on the Columbia River, but there aren't fewer electrons traveling between them just because you also have a hydro power plant on the Dalles. The load from all devices on the grid is shared across all generation sources.
If you switch to DC, that doesn't work any more. Every amp in requires an amp out. The wiring in your house just got a lot more complicated, not to mention the wiring in the local grid.
Also, running your neighborhood on 85 V (the DC equivalent to 120 V) isn't exactly efficient. Even large ground-mounted transformers only provide power to 10-15 houses at most, and pole-mounted transformers may only service one house. The main power to your neighborhood is 7.2 kV because it's more efficient to to send power at high voltage and low current.
It's not impossible for a residential solar setup to output thousands of volts, but it's not easy and it's pretty constraining for designs. There's also a world of difference between that voltage at the street and that voltage in the house. Things go wrong in the house.
This isn't how it works.
I think it's exactly the opposite. There will frequently be a need to balance them.
With wind and solar in the mix, generation will fluctuate with the weather. In a given area, it could be cloudy one day and sunny the next. Or windy one day but not the next. And consumption won't be correlated with that, so that creates an extra source of mismatches between demand and consumption within each area.
Transmission is one way to solve that. You could also solve it with storage (within every area), but that's probably less efficient and/or more expensive.
High-voltage DC is also extremely dangerous, as it's prone to arcing and electrocution.
But you have complete freedom to choose their frequency, so you can use much cheaper transformers.
Interestingly, HVDC actually becomes a more efficient method of transmission over longer distances. Perhaps it's feasible to generate electricity half a continent away. Maybe tile the Sahara with solar panels and power all of Africa with it.
https://en.m.wikipedia.org/wiki/Xlinks_Morocco%E2%80%93UK_Po...
That's a problem for mechanical switches (were conductors move to make contact or disconnect).
If you use semiconductors to do the switching, it becomes a problem of how fast they switch, how much energy is dissipated during the switch, and how much energy those semiconductors can absorb momentarily (thermal mass).
For small equipment, this is a solved problem. Fast switching FETs are cheap & robust.
For utility-scale, semiconductors are an entirely different ballgame. Big advances have been made over the last decades.
So a HVDC grid might in theory be possible. But in practice, it'll be an engineering tradeoff between HVDC+semiconductors almost everywhere vs. HVAC+more traditional gear like transformers.
And even if a HVDC grid were practical with modern tech, in most places there's existing AC-based grid & power plants. I suspect the "sync AC phases" is an easier problem to solve than "re-do the grid to use HVDC".
But for 'simple' point-to-point connections like an offshore windpark or long international lines, HVDC is sometimes practical (and used, if so).
Over time, more and more components will be built DC (DC long distance cables are already popular, due to being slightly cheaper. DC for electronics is popular due to AC being poorly suited to microprocessors/logic. DC sees wide use in cars. USB-C brings computer peripherals into the DC world).
Eventually, whenever two DC bits of power infrastructure are touching oneanother, someone will notice that removing the DC->AC->DC conversion steps saves money and increases efficiency.
Eventually enough bits of the grid will be DC that AC 'islanding' can occur - whenever every link from A to B is DC, there is nothing to keep the phase locked between place A and place B. Initially that will be solved with software locking means.
But finally maintaining that anti-islanding tech will be too costly, and all remaining bits of the AC grid will be removed.
But it's gonna take 100 years because grid tech changes slowly, and infrastructure like buried cables can be 70+ years years old.
Then, the house DC voltage is going to be higher than the electronics DC. So you'll need to have a box at every single outlet to convert DC-DC. The appliances are going to need the higher voltage house DC. And the house DC voltage is going to be more dangerous than AC. Also, there are no standards or even proposals for DC electrical system: no voltage and no outlets.
The problem is that replacing DC-AC-DC with DC-DC-DC and there isn't much savings from all those conversions. Would you replace all of your appliances for 1% savings in electrical cost?
However, the AC-AC transformers currently use a lot of Steel+copper. That's expensive. New developments will be pushed towards solid state alternatives which are theoretically cheaper (and exist today, but aren't widely used).
Outlets in your house I suspect will get replaced with super-USB-C. Ie. something which is 5 volts and then negotiates a higher voltage as needed. A future version I bet will support 3 kilowatts for hair dryers, etc. That will be safer. It'll also be pushed by device makers who currently hate the headache of making different versions of electrical devices for every country with different plugs. Fancy houses already have USB outlets in every socket. Cheapo devices like flashlights already use USB power input for worldwide universality.
I could imagine rules might push people to super-USB-C too. Laying AC lines requires highly qualified labour, but plugging in super-USB-C cables into a super-USB-hub can be done by anyone - the safety is in the design, rather than requiring careful installation.
When every outlet in your house is super-USB-C, it won't take much for newly built houses to instead use DC everywhere (maybe even negotiated voltages too - ie. your house only receives 5 volts until any device needs more power, and then it'll ramp up).
In what world is steel and copper more expensive than semiconductors?
Whereas a pound of copper went from $0.89 to $3.91 in the same time.
Transformers of equal wattage cost more today than they did in 1974. Power electronics cost substantially less.
I don't see this trend reversing.
First, the wire thickness needs to be like regular wires to carry enough current. Changing the wiring in the walls is the hardest thing. Second, the voltage needs to be like regular service to carry enough power over regular sized wires. Higher voltages, like 500V, are better since DC loses more energy over distance than AC. Third, the plug needs to be similar size to power plugs to not arc, and DC arcs worse than AC.
Is replacing everything worth the effort to increase efficiency by a little bit? You are optimizing for low-power DC devices at the expense of high-power AC appliances. The only way I can see DC power happening is in isolated community like the Moon or Mars.
5 amps, 2000 volts. 10 kilowatts. Made safe by milliamp precision leakage detection (you won't manage to kill anything with a milliamp of current, and more than a milliamp of current will trigger the protection).
2000 volt insulation made of PVC needs to only be 0.1mm thick (say 0.5mm for a 5x safety factor). That can easily fit in plugs and sockets of the existing size. The leakage detection would also detect current leaked between the power conductors, so dielectric breakdown can be protected against.
The cable would be designed to deliberately cause dielectric breakdown when heated before catching fire, so that in cases of a fatigued conductor or wire draped across a stovetop, a fire is impossible.
This design is only safe if implemented correctly, so each end of the connection will frequently test the other end of the connection to ensure it is adhering to the spec. It would do this by sending an encrypted message saying "In 37 seconds I am about to cause what looks to you like a baby chewing through the insulation". Then, in 37 seconds, that happens, the power gets turned off, and a few microseconds later the encryption key is send over proving that it was a test and the power is turned back on again. Both sides would have capacitors so the user wouldn't notice the power going off for a few microseconds per minute.
The grid frequency is an incredibly useful communication tool that allows any piece of equipment to easily and accurately measure the current health the overall grid, and automatically make adjustments to help balance and improve the health the of the grid (either by increasing or decreasing load/supply). Because the frequency is set by physically large spinning turbines it means it’s also a direct and inseparable measure of total grid health, not something that’s dependent on another system to monitor and communicate grid health.
It’s hard to overstate how much of our electricity grids depend on grid frequency, and one having thousands of systems monitoring and adapting to grid frequency, to remain as robust and stable as they are. In a DC world you don’t get that anymore, and keeping a grid balanced becomes substantially more complex requiring potentially unreliable side-channel communication to allow equipment on the grid to coordinate themselves. Its really hard to beat a system where one of it core fundamental attributes (frequency) needed for power transmission, is also the perfect attribute for distributed coordination of load and supply.
Capacitors do the same for DC. They are also more efficient and reliable.
The thing about a communication channel is true. But it will become true for AC after almost all of the generation becomes free of rotational inertia too (PV, modern wind, and batteries). And you need side-channel communication to decide what generator will take over what load right now.
Yea.. but their failure mode can sometimes be a dead short. Engineering around this in a power delivery application is a severe hassle.
The fact is that for most applications, there isn't enough technical difference to justify either of the options. It's all path dependency based on random choices made ages ago.
It's a sealed turbine, not a "spinning wheel." It's failure modes, while internally destructive, are often limited to the device itself, and trips are much easier to install and utilize in this path.
Dead short DC failures have a tendency to destroy nearby equipment and start fires as well. You're also going to need a bank of capacitors, so you've multiplied your failure rate for each capacitor required.
> there isn't enough technical difference to justify either of the options
There's a massive amount of difference and AC is obviously justified.
> based on random choices made ages ago.
You don't seem to be aware of the history of the power grid and how we've arrived at the technology we have.
So even in world where all power sources are coupled to the grid via inverters, it’s still possible to use the grid frequency as communication channel.
Side channels that exist physically outside the grid will never be reliable and ubiquitous enough to replace grid frequency, for grid stability you need a feedback loop measured in nanoseconds to avoid scary oscillations in load and supply, that feedback loop needs to be faster than a microcontroller can manage, hence the reason why Grid Forming Inverters are more complex than normal Grid Following Inverters. After all the system you’re trying to monitor and keep stable naturally communicates at the speed of light, so it really isn’t possible to use digital systems to keep it stable. You need some sort of analog inertia (whether that’s spinning rotors, or clever analogue electronics doesn’t really matter) to handle the high frequency changes, and damp them enough for digital electronics to deal with the longer term drift.
Also it’s not just suppliers that coordinate via frequency, it’s also loads. Anyone out there running a large semi-continuous, but interruptible loads (e.g. water pumps, large arc furnaces, heating systems, bulk EV charging etc) can usually get a discount on their energy prices, in exchange for voluntarily disconnecting their load if the grid frequency drops too far, allowing the grid to shed the least sensitive loads first, before it’s starts forcefully disconnecting more sensitive loads.
> Capacitors do the same for DC. They are also more efficient and reliable.
I don’t know who told you that, but capacitors aren’t even vaguely close to reliable compared to a spinning turbine. Not once you consider how many capacitors you would need to store an equivalent amount of energy as a spinning turbine.
Super capacitors top out at about 4 Wh/kg, and can get up to 10 Wh/kg if you’re using hybrid capacitors (which basically a mix of a battery and capacitor). A flywheel energy storage systems are around 100Wh/kg. So at least one order of magnitude more energy per kg. So you need a lot of capacitors to replace the energy storage of a turbine. Once you’ve got that many very expensive capacitors linked up with the needed control electronics, I doubt it’s anywhere near as reliable (or cost effective) as a big spinning chunk of steel.
The problem is that physics also dictates that the interconnection links need to be big enough to handle the power imbalance between the different parts of the networks. So grid management is mostly about balancing production and demand as a whole and in sub-grids.
For some reason HVDC (specifically HVDC, not just DC, most DC devices use low voltages, far below the voltages that would be efficient for transmission), is apparently more power efficient than AC. I’m not sure why, though.
In an AC circuit, a capacitor looks like a resistor, it takes work to fill and empty it every cycle.
Transmission lines over the earth behave not just like wires, but also like capacitors. Higher voltage reduces resistive losses, but in AC they’re penalized by these “fill and empty a capacitor” ones, that DC doesn’t.
> This is because direct current transfers only active power and thus causes lower losses than alternating current, which transfers both active and reactive power.
> Nevertheless, for a long AC overhead transmission line, the current flowing just to charge the line capacitance can be significant, and this reduces the capability of the line to carry useful current to the load at the remote end. Another factor that reduces the useful current-carrying ability of AC lines is the skin effect, which causes a nonuniform distribution of current over the cross-sectional area of the conductor. Transmission line conductors operating with direct current suffer from neither constraint. Therefore, for the same conductor losses (or heating effect), a given conductor can carry more power to the load when operating with HVDC than AC
Basically, while heat losses are the same, the AC system requires extra power to constantly be switching the electron flow whereas in a DC system there’s only active power to move the electrons and the only loss is heat loss. Additionally, for a given conductor, HVDC can be transferring at the peak rated voltage for the wire whereas AC can only transfer that voltage at the peak which means it’s 71% less power (although that’s more a cost savings thing).