Today: AC, Tomorrow: DC?

Johan · Jun 18, 2014

In the last year I've become more and more convinced, as probably many others on this forum, that our global energy future will be heavily dominated by solar. Sure, in some places not but I think in 30-50 years solar will provide the majority of electrical power generation globally. Now, I'm thinking this will very likely make the electric grid far more decentralized, as pointed out by many others of course. In addition with the rise of solar (and other non-sequestered-carbon-releasing forms of energy production) power generation will be more intermittent and hence there will be increased need of energy storage which will probably also be to a large extent decentralized and coupled with generation (think PV on roof with battery in basement).

Our typical energy grids today are based on AC, for well known reasons. With less need for transmitting large amounts of power over large distances in the future, for the reasons mentioned above, won't it make sense that the grid as we know it will move more towards DC? I'm thinking this since solar PV generates DC and batteries are charged with and release DC.

I had a look around my house and started thinking: modern electric lighting (LEDs) all have small AC/DC converters in them (the old incandescent bulbs with glowing wires could run on either AC or DC since with 50 Hz the wire would never cool down enough to make a flicker anyway), everything battery powered from mobile phones to lap-tops to my Tesla is DC charged, my desktop computers PSU converts AC to DC, there is AC to DC conversion going on in my TV, my home alarm system etc. etc. The only application in a home where AC is better is for some things with motors, right? Such as mixers, blenders, vacuum cleaner etc? (please correct me if I'm wrong).

So my point is: do you think that in the future (20-50 years from now) we will have DC in our sockets at home and we could do away with these:

and skip having one or two quite massive AC to DC converters ("chargers") in our cars. Instead we would probably need a few DC to AC converters ("inverters") for a few applicances (but actually many of the motorized appliances in a home probably run DC motors anyway).

Raffy.Roma · Jun 18, 2014

You will always need DC to AC convertion to send electric energy in the grid but yes, for home purposes you will need less DC to AC convertion which is good to avoid energy losses. I don't see the grid to go from AC to DC because the electric energy can travel only in AC mode.

Johan · Jun 18, 2014

Raffy.Roma said:
You will always need DC to AC convertion to send electric energy in the grid but yes, for home purposes you will need less DC to AC convertion which is good to avoid energy losses. I don't see the grid to go from AC to DC because the electric energy can travel only in AC mode.

Uhm, no.

High-voltage direct current - Wikipedia, the free encyclopedia

My point is that if the grid is decentralized to a much greater extent, and we generate in DC, store in DC and use mostly in DC it makes much more sense to just have the grid be DC and then convert to AC in the instances it's needed.

Raffy.Roma · Jun 18, 2014

Johan said:
Uhm, no.

High-voltage direct current - Wikipedia, the free encyclopedia

My point is that if the grid is decentralized to a much greater extent, and we generate in DC, store in DC and use mostly in DC it makes much more sense to just have the grid be DC and then convert to AC in the instances it's needed.

Good point. I thought that the grid was in AC for reasons of propagation of the electrical energy. I didn't know that the same thing could be achieved in DC with high voltages.

Knobby · Jun 18, 2014

The grid is AC because it enables step up to high voltage for distribution and step down for end uses with transformers. Cannot so easily step up and down DC.

green1 · Jun 18, 2014

There are now ways to do the step-up and step-down efficiently with DC, The grid is AC because it pre-dates that ability and the nightmare of converting the grid is just not worth it.

Even now converting AC to different voltages is cheap, converting DC is expensive. For an electrical grid it's actually not a huge deal because they actually make it up in efficiency gains iirc, however for small things at home it is still much cheaper and easier to get 5vdc for your phone from 110vac then it is to get it from 300vdc (and most solar battery setups are not set for 5 or even 12v)

nwdiver · Jun 18, 2014

green1 said:
There are now ways to do the step-up and step-down efficiently with DC, The grid is AC because it pre-dates that ability and the nightmare of converting the grid is just not worth it.

Even now converting AC to different voltages is cheap, converting DC is expensive. For an electrical grid it's actually not a huge deal because they actually make it up in efficiency gains iirc, however for small things at home it is still much cheaper and easier to get 5vdc for your phone from 110vac then it is to get it from 300vdc (and most solar battery setups are not set for 5 or even 12v)

I agree that converting the grid is probably not worth it but due to the drop in the cost of power electronics and the invention of the IGBT Inverters and Rectifiers are now cheaper than transformers!! If we build a colony on mars the only place you'll see AC is powering a synchronous motor.

Doug_G · Jun 18, 2014

Perhaps we'll switch to 400 Hz AC like aircraft use.

TEG · Jun 18, 2014

I assumed AC was useful in preventing corrosion compared to DC.

AudubonB · Jun 18, 2014

Doug_G said:
Perhaps we'll switch to 400 Hz AC like aircraft use.

They do? Wow. Why is that? And why not 360Hz...or 99999Hz?

Chris TX · Jun 18, 2014

TEG said:
I assumed AC was useful in preventing corrosion compared to DC.

This!

DC has the electrons moving in one direction causing anodes and cathodes (corrosion points) to appear in the circuits. Exposed to air or other oxidizers, you're asking for problems. AC eliminates this electrical migration as it just goes back and forth.

FlasherZ · Jun 18, 2014

I don't foresee it, primarily because I still foresee device-by-device conversion. Your electric heating appliances will need high-voltage DC to avoid needing 1" diameter wiring. Computer power supplies already produce 4 different voltages for different uses - high power peripherals, low voltage for the logic circuits, 5v for standard TTL needs, and even -12VDC. Lighting circuits will require intra-device conversion because even a 10W LED bulb will draw 2A at 5V. My basement would require about 60A just to light it, requiring #6 wiring. Use a higher voltage to reduce wire size, and you need on-device DC-DC conversion. As long as you need conversion, you will have problems generating economies of scale enough for a wholesale conversion of the grid.

I still believe we'll see mechanical generation - hydro, for example - which is much easier to produce as AC. Even with regulation, I don't believe you'd be able to just tell the power plant operators who just built new coal or even NG-fired plants that their multi-million dollar investments are now worthless and they must shut them down. Because they produce AC more easily than DC, this goes against a DC grid conversion.

Then you have simple momentum -- I have hundreds of plug-in devices around my home. You'll have to design different sockets so that you don't blow up AC devices in a DC-provisioned home and vice-versa. This means adapters - both rectifiers to use DC devices in an AC home, and inverters for AC devices in a DC home.

The only thing that DC as a distribution grid has going for it is that PV and variable-speed wind turbines must use it in production. I think the only way you reach DC as a distribution mechanism is if solar+storage becomes the greater majority (75%+) of generation, and even then it will likely take 2 decades of adoption. Undoubtedly, someone will make a comparison to the digital TV switchover -- the difference here is that I have hundreds of plug-in devices in my home, and only a couple of TV's.

And, of course, it would all have to make economic sense. At this point, it's much less expensive to create AC from the solar PV we have out there than the other direction.

nwdiver · Jun 18, 2014

Chris TX said:
This!

DC has the electrons moving in one direction causing anodes and cathodes (corrosion points) to appear in the circuits. Exposed to air or other oxidizers, you're asking for problems. AC eliminates this electrical migration as it just goes back and forth.

I think the PV industry has mostly solved that... not sure how... but PID (Potential Induced Degradation) used to be a huge headache... now most panels are immune to it.

Mookuh · Jun 19, 2014

A professor at my university (who is also an avid proponent of EVs) keeps preaching the same message: a DC grid would be more efficient with our current technology. He envisions us going to a full DC grid eventually.. The amount of power you can transfer with a given diameter of cable is also higher for DC, apparently. However, the cost in switching would probably be prohibitive at the moment.

freds · Jun 19, 2014

AudubonB said:
They do? Wow. Why is that? And why not 360Hz...or 99999Hz?

Running at 400Hz allows the use of smaller transformers. Less energy has to be stored in the transformer core per cycle, so the core can be smaller. A smaller core means a lighter transformer, and reducing weight is a good thing in an aircraft.

http://www.academia.edu/4768401/Why_we_use_400Hz_Power_Supply_in_Aircraft

FlasherZ · Jun 19, 2014

The problem with these types of opinions is that they've been made way too simplistic. The argument says "with CFL and LED bulbs, most of our loads are now DC, so DC is better to use!" and "With solar PV, we now have the more distributed grid, which is what Edison said would work better!" which neglect to consider all the different DC voltage levels required, the conversions that are required to get us there and the cost of those conversions, and the load requirements for electric heat / heat pumps / stoves / etc.

As for diameter of cable and amount of power, that's just not true for any non-negligible amount of power. Power is volts (RMS if AC) times amps. At a given voltage, the same size wire is needed to carry the same amount of amps. Ohm's law and Joule's heating law don't really change for AC vs. DC except in corner technicalities that have no bearing on real-world usage.

Johan · Jun 19, 2014

As the OP here I'd just like to point out that this was just a thought that stuck me thinking about Photovoltaics, decentralized grids and all my iPhone chargers and AC-DC converters lying around the house. I don't have much more than general knowledge on these subjects (my work is as an MD). However as always on TMC a lot of people with great insight contribute and I learn a lot of great info!

TEG · Jun 19, 2014

8 Main Disadvantages of HVDC Transmission | EEP
...
...
The high-frequency constituents found in direct current transmission systems can cause radio noise in communications lines that are situated near the HVDC transmission line.
...
The flow of current through the Earth in monopole systems can cause the electro-corrosion of underground metal installations, mainly pipelines.

This shows some safety benefits to using DC though:
Koshcheev_paper_final1.pdf

- - - Updated - - -

nwdiver said:
I think the PV industry has mostly solved that... not sure how... but PID (Potential Induced Degradation) used to be a huge headache... now most panels are immune to it.

As you said 'most'... Buyer beware...
galvanic-corrosion-considerations-for-pv-arrays
One little design mistake...

nwdiver · Jun 19, 2014

TEG said:

Well, there's no solution to an improperly installed system... galvanic corrosion and PID are not related. I was responding to the concern that having a consistent polarity causes ION migration. Galvanic corrosion requires dissimilar metals to be in contact with each other and is well understood. PID was a mystery for years that has only recently been solved and mitigated.

http://solarenergy.advanced-energy.com/upload/File/White_Papers/ENG-PID-270-01 web.pdf

AC and DC each have their advantages but by FAR the largest advantage to AC, the ability to easily change voltage, has largely gone away as DC has caught up in that area. When we decided to make the national grid AC ~99% of the components in a modern DC-DC converter had not been invented. If you want a dramatic demonstration of the difference compare an older Grid-tie inverter to a newer "transformer-less" inverter. The weight dropped by 50% when they got rid of the transformer.

Robert.Boston · Jun 20, 2014

Given the huge imbedded capital in our existing AC transmission and distribution grid, it seems most likely that the first step would be in-home centralized DC. Still, as @FlasherZ points out, different devices use different voltage; how many parallel DC systems would you want in the house? Manufacturers would also need to standardize plugs, etc.

Today: AC, Tomorrow: DC?

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