Solar power and DC

[fustbariclation]

The most expensive part of fitting solar power isn't the panels, it's the inverter.

If it was possible to do away with the inverter, solar power would be affordable to many, many more people.

Wouldn't it make sense to convert the mains to DC?

The only device that I know of that really needs AC at 50Hz is a wall clock that uses the mains frequency as a reference - and there can't be many of those left.

Computers, 'fridges, washing machines, even air-conditioners all have their own rectifiers to turn the mains AC back to DC.

Wouldn't it be more sensible to remove these inverters and connect them directly to mains DC? Then you could connect the solar panels directly to the mains, as well as the battery, and do away with all the expense of the inverters?

If you still wanted some power from the grid, you could fit a rectifier.

What practicality am I missing?

 

[GregLocock]

Not too sure where you get your costs, for instance a 300W inverter is 700 bucks or less

http://www.aliexpress.com/item/3000...-24V-48V-to-AC-110V-220V-230V/1549427240.html

whereas 3 kW of panels is about 3000 dollars, or more.

So, given your initial premise is wrong, can we rescue anything from the wreckage? Some of those devices do have AC to DC rectification in them, but the high power devices in your home do not. Your fridge has a 250W AC motor. Your kettle could not care less whether it is AC or DC.

But the problem is each device with an AC to DC converter needs different DC voltages. So you'll need a bunch of DC to DC converters, which may be cheaper than AC to DC, but they aren't hugely different in cost.

Cheers

Greg Locock


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[IRstuff]

I think you fundamentally have the same issues with DC that you had with AC. In order to reduce line losses, the AC distribution voltages start VERY high, ditto DC. So, depending on where the solar generation is, you still need to do DC-DC conversion. BUT, and a HUGE but, efficient DC-DC REQUIRES both DC-AC conversion, then, and AC-DC conversion. The end result is that you need to have about twice has much hardware dumping large amounts of current, resulting in double the parasitic loss.

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[zdas04]

Let's restart the War of the Currents]/link] from the 1880's again. Both Edison and Westinghouse threw tens of millions of dollars at the first one, and Westinghouse won because low-voltage transmission is so expensive and DC-DC transformers didn't exist then (and aren't that effective today). Being able to get into the kV range to transmit power has allowed the mega-generating stations that make modern life possible (at the time of the first War transmission of DC power more than a mile was considered impractical, think of a power plant in every 3.1 square miles of L.A., New York, or London).

David Simpson, PE
[url=www.muleshoe-eng.com]MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[TheTick]

Hollywood film production uses a lot of DC. Not sure why.

 

[fegenbush]

IRstuff explained it very well. The debate was settled in the 1880's as to whether the distribution network for electricity would be AC or DC, and the transformer was the key to Tesla's (and Westinghouse) victory over Edison.

 

[GTTofAK]

Stability is also a problem with HVDC. Specifically voltage stability when you are looking at quasi steady state.

 

[MintJulep]

What practicality am I missing?

If you are "on the grid" then you are missing that there is gazillions of dollars of power generation, power conversion and power transmission infrastructure existing for AC. Chances of starting over with DC are exactly zero.

If you are "off the grid", as could be inferred by your statement "If you still wanted some power from the grid..." then what you are missing is that everything that uses electricity is designed to plug into the above mentioned AC infrastructure.

 

[beej67]

If you're off the grid, just buy everything for your house at the RV store or West Marine and you're good. Cruising Sailboats are essentially fully equipped houses all on DC12V, including things like refrigeration, and if you get your power systems properly designed and maintained (wind/solar) and keep your usage low, you can live on the hook for months purely off of renewable power.

You'll need one inverter for whenever you decide to run your microwave or TV, and you'll need a different kind of power supply for your laptop. Might also want to use a sun shower.



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[cranky108]

I think what was missed first was that the distribution transformer outside your house only works on AC. But if you want to use DC in your house, go ahead, you just can't export it to the power company.

Everyone else has agreed to play by the rules of AC power, so why can't you? Are you special, or is your connection to the grid so much better that we have to redesign the grid it for you?

Solar power inverters are costly because they do more than just invert the power to AC. They are also protective devices, and they match the existing phase angles, and monitor for conditions that the inverter automatically disconnects from the grid.

Have you taken into account that many things in your house do convert the AC to DC but that most use a different DC voltage. Also the converter can be used to control the speed of the motor by chopping the wave to control the voltage. And using AC lowers the cost and size of circuit breakers, and light switches in your house.

 

[EdStainless]

The exception to long range transmission in AC is the Pacific DC Inter-tie.
It runs 600 miles or so at +500kV (1,000,000V across the poles).

While this transmission is very efficient the only reason that this is practical is that you are converting to AC at one location.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[djs]

The other issue with DC, especially at voltages greater than 12VDC is that it is really hard to interrupt the current flow. DC supports and sustains arcs much more readily than AC. In AC the current goes through zero twice each cycle, whereas DC does not. Switches and relays for DC voltages of 100+ volts are large and expensive.

 

[JohnRBaker]

But something like 440 AC can jump-out and grab you.

I once put the palm of my hand across two phases of a three-phase 440 volt circuit when I tried to shut off a machine from an open electrical panel. I was in a bakery in Canada and I wasn't familiar with the particular brand of breaker that was being used and didn't realize that the poles of the circuit were as exposed as they were. I was used to breakers where you just turned the shaft that extended out that engaged in the handle in the door of the panel when it was closed. But this was totally different and I reached for the wrong thing and came too close to those 440 lines. Anyway, I was wearing neoprene-soled work-shoes (no nails) and my other hand was not touching anything so the current just went from one pole across to the other, but it did leave a nice burn and knocked me on my butt.

John R. Baker, P.E.
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[cranky108]

What would it have done if it were 440 V DC? DC has the issue of not letting go (no zero crossings).

There again, if we were to use DC in our homes, what voltage would it be? Perhaps 230 V DC so I can run my dryer and stove? Or 120 V DC so I can run my vacuum, and drill?