Is there a device that will switch solar to mains

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Is there a device that will switch solar to mains

Postby smokingwheels » Sun Jul 30, 2017 6:14 pm

I am in a rental and can get a 1 kw solar system cheap.
I would mount the panels on frames so I could take it with me when I move.

I would only run my pc say 350 watts on solar during the day but I need some sort of black box plug in appliance to switch to mains power if solar is not enough. The change over time needs to be in the order of 10 mS to keep the pc going at all times.
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Re: Is there a device that will switch solar to mains

Postby Gordon-Loomberah » Mon Jul 31, 2017 9:29 am

You can't run a PC from a 1kW system that is not connected to the grid, or is an off-grid system without a battery.
Fast change-over switches exist, but wont work with what you describe.

If you add a battery, a charge controller and a suitable inverter-charger (or inverter + charger) then it can be done.
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Re: Is there a device that will switch solar to mains

Postby smokingwheels » Mon Jul 31, 2017 8:58 pm

Ok Thanks for your comment.
I have an idea will run it by the local sparky.
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Re: Is there a device that will switch solar to mains

Postby Warpspeed » Thu Aug 17, 2017 10:02 am

There is nothing off the shelf you can buy that will do this, but its not impossible, you will have to engineer something yourself.

The basic idea is to rectify the grid power to produce some dc.
Then run that into an inverter to get back to ac mains power.

You can arrange it so that if the voltage coming from solar panels is higher than the rectified dc from the grid, the inverter will draw power from the solar panels placing no load on the grid.

At night it all obviously comes from the grid. In marginal solar conditions you will get as much current as available from solar, so the load is shared. Changeover is a gradual seamless steady process. There is no abrupt switching involved, and no battery is required.
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Re: Is there a device that will switch solar to mains

Postby smokingwheels » Thu Aug 17, 2017 1:04 pm

Thanks for that fantastic idea!

I will look into it, my PC's Idle at ~400 W and if there under high CPU load it rises several hundred watts.
My 60 day power bill is like $500 but it was winter and I am running the A/C unit 24/7.
I have also ordered a Raspberry Pi 3 to see how that works out there 13 Watts max.
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Re: Is there a device that will switch solar to mains

Postby Warpspeed » Thu Aug 17, 2017 1:15 pm

Its a matter of very carefully matching the various components.

Ideally you will need a dc rectifier that outputs a voltage fairly close to the maximum power point voltage of your solar panels. That might typically be about 30v for a 24v rated panel (or multiples of that).
Then just a diode from each power source connecting to your inverter.

The inverter will need to be able to operate between the nominal rectifier dc output voltage, and the maximum open circuit voltage of your solar panels. That is going to be the tricky bit, because you will end up with a very strange inverter input voltage requirement, something like 30v to 40v in this example. If you are going to home brew an inverter, that presents no particular difficulty. But finding a suitable off the shelf commercial inverter may just not be possible.

Assuming you are doing this on the cheap, a home brew inverter is probably the best way to go anyway...

Its not just an abstract idea, it is a proven working system.
The higher the dc link voltage, the more efficient it is likely to be.

The lovely thing about this is, provided the inverter is up to the job, peak inrush loads will come from the grid, but the lower constant loads can be solar powered. So starting up and running a refrigerator (for example) off minimal solar panels becomes quite practical.
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