Hacker News Reader

Comment by londons_explore | Hacker News Reader

DC will one day be ubiquitous, but I think that day is >50 years away.

Today, DC is common in small 'islands'. At a small scale, you have DC in your phone charger and USB power supplies. At a medium scale, DC is used in high speed electric car charging. At a large scale, DC is used for undersea electricity transmission.

DC has benefits of better making use of available conductors and insulators - for a given mass of copper and plastic, more energy can be transferred from A to B at a given efficiency. Modern DC/DC converters can convert voltages more efficiently and using less metal (ie. cost) than AC transformers.

However... AC is still the standard. And changing power standards is awfully slow, because power cables in the ground can easily have a lifespan of 50+ years, and there is a chicken and egg problem involved with deploying a new standard.

View on HN

> However... AC is still the standard. And changing power standards is awfully slow, because power cables in the ground can easily have a lifespan of 50+ years, and there is a chicken and egg problem involved with deploying a new standard.

The lines can theoretically stay the same, at least in the distribution network - you'd "only" need to exchange the equipment like transformers and switches.

DC has some pretty challenging aspects in implementation, and that not just on the large grid scale:

- changing voltages requires active semiconductors instead of a (relatively) dumb transformer. This has been lessened by technological advances, but it's still more expensive.

- switching DC loads on and off is harder because the voltage never crosses the zero threshold - this is also the reason why relays and circuit breakers are always rated for way lower currents in DC than in AC, and usually have lower cycle ratings as well as there will be an arc that continuously burns.

- the same is also true for ground faults, say a tree branch: the arc isn't "automatically" extinguished once the voltage drops (which happens every 1/50 second)

- it's more difficult to have an actual grid, most current implementations are point-to-point only

- DC introduces the potential for very weird "stray currents" and resulting electrochemical corrosion

- unlike with multi-phase AC, the magnetic forces in a cable that are generated by current flow don't cancel each other out, so that needs to be taken into consideration to verify if cables are suited for DC transmission

Can you (or someone else) write more about the advantages of DC/DC voltage conversion? If I understand correctly this was the main advantage of AC, and the reason why was it chosen.

In the olden days, AC allowed easy voltage conversion with a transformer. The transformer converts the electricity into magnetism in a steel core, and then back into electricity at a different voltage.

This process is quite efficient but requires a lot of steel, since in a 60 Hz AC system, 1/120th of a second of the energy being converted has to be stored as a magnetic field in steel - and steel isn't a particularly good 'store' of magnetic fields...

Modern DC/DC systems actually have similarities! But instead of operating at 60Hz, they tend to operate at more like 1,000,000 Hz. That means far less copper and steel is needed. Unfortunately, 1,000,000 Hz power has a habit of leaking out of cables and becoming radio waves, so we can't send it long distances like that - so we convert it to DC before and afterwards. The conversion to DC is done with electronic switches switched at 1 Mhz or more - usually MOSFETS are used, and one promising but expensive type is a GaN MOSFET. It turns out that the DC->AC, transformer, and AC->DC setup can also be combined and simplified a bit, and we call the result a buck/boost converter.

Overall, buck/boost converters can normally convert DC voltages for less money and at higher efficiencies than their AC transformer counterparts - mostly due to the higher operating frequency allowing use of far less steel and copper, and allowing other engineering tradeoffs be made in the direction of efficiency.

However, neither DC/DC nor transformers have any theoretical cap on efficiency - and with an unlimited budget, you could make either with an almost arbitrarily high efficiency.

Citation needed on DC/DC converters being as efficient as AC transformers. I'd agree they can cost less, but AC transformers are wildly efficient. 99% efficiency is not unheard of. Full load efficiency can be extremely high too.

There are basically no DC/DC converters that hit that efficiency at any load.

Give the benefit of doubt and read the surrounding context more carefully.

Not only did they mention (rephrasing) for unlimited money / resources AC / AC transformers could be far more efficient, but the part you are critiquing is simultaneously comparing COST for SAME efficiency parts (I assume typical) and resulting efficiency for similar COST.

AC transformers have fixed thermal power density, but load power scales with mass to the 4/3rd power (i.e., scales with linear size by the 4th power, while volume and mass and loss only scale to the 3rd power).

And no, 99% isn't hard for a resonant switched capacitor converter. They just happen to be restricted to integer voltage ratios. (With sometimes a few percent regulation around this ratio without substantial efficiency loss.)

Changing properties of electricity (ie current to voltage or vice versa) requires a trip through the magnetic field and that means varying the current through a conductor. With AC that's easy as it's already changing and so you can trivially (passively) convert between voltages at the same frequency with the efficiency cost of heating the conductors of the coils used in the transformer.

With DC it's harder because you don't have the time changing nature necessary for the magnetic field so you have to turn the DC on and off which requires a switch. Nowadays we have very fast switches (transistors) that allow us to tune a circuit to the power required and temporary energy storage (capacitors and inductors) available. Ignoring (or shielding) the RF interference that's created with fast switching we have systems that can efficiently convert between one DC voltage and another.

I'm not so sure we'll have DC to the home for supply, a zero-crossing is helpful to keep circuit breakers small and reduce damage in brief, accidental contact (eg broken insulation on a lamp etc).

- AC motor are very easy to build and are very, very durable. - AC / AC voltage transformation is very easy and very effective.

- Switching AC is much more easy than DC.

- The grid we have today was not made for small small cellphone chargers. It was made for light and motors.

One big advantage is that you don't have a wave you have to sync with (Which is why DC is used for connection between grids). One of the harder parts of AC grid management is that every generator on the grid has a timing component to make sure it's producing power in the same waveform as the grid. It's not enough to produce AC at 60hz, if that 60hz is misaligned then you generator turns into a load on the grid.

It also means that if 2 grids aren't in sync, they can't connect (even if they are both 60hz) without some expensive equipment to sync the waveform. A 60hz grid cannot connect with a 50hz grid without a DC phase.

There will never be a switch to DC in the home. The advantage is small, and it would involve changing everything. Would have to replace every outlet and switch, every appliance, every light socket and light bulbs. The only way that would happen is if building in new place, like on Mars.

There aren't any standards for DC in the home, no plugs, and no voltage. The voltage would be pretty high, 480V is likely, that would have to step down for USB and lights. It is telling that boats and RVs, which use 12V and 48V, just have inverters and AC plugs.

The only place where DC in home makes sense is between battery backup and solar panels. That way can have one, large, efficient inverter instead of inverters on the batteries and each panel. There isn't much difference between AC inverter and DC power converter. Although, I think will still need DC converters between solar panel, which varies, battery, and inverter/DC wiring.

Lights and TVs need a fraction of the power than they did 30 years ago.

That means they can run off USB C even though it's only speced to 180 W.

Converting an electric geyser to run from USB C should even be practical: Especially if it's a bachelor who typically only use it for showering.

https://en.m.wikipedia.org/wiki/USB_hardware#USB_Power_Deliv...

Maybe. It sort of depends on how renewables shake out.

I could see DC gaining in popularity because you don't have to invest the hardware to convert renewables to AC for transmission. But at the same time, IDK, The AC grid works just fine so I have a hard time envisioning replacing the whole grid with a DC grid. Just doesn't seem like there'd be enough gains.

I do think intergrid connections will be more common. Micro/macro grids might also be more popular. Perhaps we start seeing subdivisions with their own grid/battery backups to improve reliability and allow the overall grid to disconnect them temporarily under load?

> Today, DC is common in small 'islands'.

I'll take those 'islands' to include examples you mentioned.

Outside such specific cases, advantages of AC usually outweigh advantages of DC. Or: DC dis advantages outweigh those of AC. Especially safety related.

In short: AC will stay for utillity scale & in-home power distribution. Regardless of history.

But within eg. a solar farm, or a vehicle, yes DC may be more practical. And thus... used there.

Reply on news.ycombinator.com