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Author Topic: Solar export controller, DIY suggestions.  (Read 1280 times)

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Fellow members.

My modest 2 KW solar system has just passed its second year of operation with an output of 2.75 MW hours. Most of which has been given away to the national grid. Don't get me wrong, I've already saved so much that another 2 years will see the system paid for.

I've been watching the fuel prices steadily rising lately and considered the purchase of an export controller to heat my water rather than use my oil fired boiler/furnace, particularly during the summer months.

These things are ridiculously expensive for what they do, I found one on eBay today at a starting price of £130.00 I placed a bid and found others were already at £200.00+!!

Can we put our heads together and design a simple unit that can made for a fraction of the cost? I'm pretty sure both other members and guests could benefit.

Cheers Graham.


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Dear Graham

I've only a small understanding of what an export controller is supposed to do. Maybe a definition would be good and how you intend to use this feature to heat water.

As I understand it, this is a device that caps the amount of power put onto the grid due to regulations . But how do you use the excess? Seems like an excess power transfer device would be needed that redirects the excess into the heaters of your electric boiler. But that too must be limited to prevent overtemp condition.

Is there an output on the export controller that sheds the excess into a side load?

Regards



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Dear ION.

The device I looked at this morning was quite clever. It can be programmed to switch on up to three resistive heating elements when there's a surplus or export condition.

What I'm looking for is a single load ( 1 KW ) immersion heater element to be activated once my system produces over a kilowatt.

Here's a link to the device that was on sale earlier today.

https://www.immersun.co.uk/wp-content/uploads/2014/08/Instruction-Manual-v1.1.pdf

Cheers Graham.


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Dear ION.

The device I looked at this morning was quite clever. It can be programmed to switch on up to three resistive heating elements when there's a surplus or export condition.

What I'm looking for is a single load ( 1 KW ) immersion heater element to be activated once my system produces over a kilowatt.

Here's a link to the device that was on sale earlier today.

https://www.immersun.co.uk/wp-content/uploads/2014/08/Instruction-Manual-v1.1.pdf

Cheers Graham.

Graham
Im a little lost here==maybe our grid tie systems work different over here?

In Australia,where you have grid tie solar systems,the home draws the power it needs from the solar system via the grid tie inverter,and the grid tie inverter sends the excess back to the grid.

If we have electric water heaters with grid tie systems,then a timer is fitted to the water heater,so as it only comes on during the day. (They do have an over ride switch should you need to switch it on at night.)
So the solar system powers the electric water heater-along with the rest of the home,and the excess is automatically sent to the grid.
Should your home need more power during the day than the solar system can deliver,then the grid tie inverter draws the needed excess from the grid.

If your system works the same,then all you need to do is wire up the heating element,that has a thermostatically controlled switch,to your homes wiring,and the grid tie inverter takes care of the rest.


Brad


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Dear ION.

The device I looked at this morning was quite clever. It can be programmed to switch on up to three resistive heating elements when there's a surplus or export condition.

What I'm looking for is a single load ( 1 KW ) immersion heater element to be activated once my system produces over a kilowatt.

Here's a link to the device that was on sale earlier today.

https://www.immersun.co.uk/wp-content/uploads/2014/08/Instruction-Manual-v1.1.pdf

Cheers Graham.

Dear Graham

I have read the manual and believe I understand it's operation.

Question Is that 1kW transfer over and above what you are back feeding the grid or just kicks in when your system reaches over 1kW, even if you are using domestically the 1kW?.

It would not work to use a simple current controlled switch, as this would put the full 1kW heater load in all at once, whereas you need a proportioning system so that only the overrage is sent to the heater. This can be done by monitoring the current threshold and direction as I assume you do not want this transferring during a cloudy day when you are using (drawing from the grid) over 1kW and proportionally sending the overrage to the heater be it 1 Watts or 1k Watts. So direction control is a "must have".

Since you are feeding the overrage into a resistive heater, you can probably get away with phase control using a thyristor (Triac as in light dimmer) that has a filtered feed and setpoint adjustment from the current transformer. You don't need a pure sine wave inverter for this type of resistive heater load, only for sensitive inductive loads, as that is where most of the money is spent in the unit described in the pdf. The direction control is still a problem we need solved.

Do I understand the problem yet? or am I still confused?

Regards


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Hi Brad.

Well, I really don't know. My grid tie inverter is sat firmly onto the grid via a permanent Live and Neutral. When the sun shines, not very often, we get some energy. This energy if in excess of what's being used by the home gets exported, or effectively wasted.

So that neatly brings me to ION ....

As we're connected to the grid, solidly there's no need for a soft start.  A CT fitted to the grid tie inverter that could determine whether 1 KW or not was being developed and then activate a relay, for arguments sake, would suffice.

My system although rated at 2000 Watts output rarely develops more than 1.8 but as I posted earlier has now passed through 2.75 MWH !!

I hope the above makes sense?

Cheers Graham.


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Well if your sure a full load  of 1kW connected wouldn't cause a problem with the rest of your domestic usage, you could try something as simple as
a current transformer  driving an AC power relay that kicks the load in over I kW.  For adjustment you could shunt away some of the current from the CT with a rheostat in parallel with the relay to act as trip point adjustment. You could also try putting the rheostat in series with the relay.

Alternately you could full wave rectify and filter the output of the CT and drive  a burden resistor then a more sensitive DC relay in pilot duty mode to a much larger load handling relay. Also  there are solid state power relay modules that operate on low voltage and low current.

You have too many choices.

Regards



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Dear ION.

Your last post certainly brings to mind the " KISS " acronym!!   O0

So....

Let's make it slightly more complicated, shall we?

My solar panels are mounted upon the roof of my workshop with the grid tied inverter inside. A hefty cable connects the workshop to the house and main distribution box.

Would there be a way of determining energy direction? Obviously on many occasions the workshop will be taking up to 3 to 4 KW to drive my heavy machinery and would naturally " kick in " the immersion heater. Would I need 2 CT's and a comparator?

Cheers Graham.



---------------------------
Nanny state ? Left at the gate !! :)
   
Group: Elite
Hero Member
******

Posts: 3227
It's turtles all the way down
Dear ION.

Your last post certainly brings to mind the " KISS " acronym!!   O0

So....

Let's make it slightly more complicated, shall we?

My solar panels are mounted upon the roof of my workshop with the grid tied inverter inside. A hefty cable connects the workshop to the house and main distribution box.

Would there be a way of determining energy direction? Obviously on many occasions the workshop will be taking up to 3 to 4 KW to drive my heavy machinery and would naturally " kick in " the immersion heater. Would I need 2 CT's and a comparator?

Cheers Graham.

Dear Graham

If you only intend to use solar power to feed the heater, then you only need to monitor your solar output to the domestic or to the grid and set the trip point, no need to worry about direction as you will only monitor the solar output.

Yes it gets more complex if you want direction control, and yes, 2 CT's and burden resistors connected in series out of phase might work.

Only first cup of coffee so subject to sluggish engine errors.

Regards


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"Secrecy, secret societies and secret groups have always been repugnant to a free and open society"......John F Kennedy
   
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