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Forum Index : Electronics : Alternative for switching my HWS

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nickskethisniks
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Joined: 17/10/2017
Location: Belgium
Posts: 458
Posted: 12:45pm 26 Feb 2021
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I see the term HWS is used here for hot water system?

Ok, so I have a roof with 24 panels, configuration is 2S12P because of the shading it's a low voltage system, mppt/battery. 6160Wp


Then I have 2 grid tied systems, 1 is also connected 2p5s. 2800W also terribly shaded, it's on a shed. With a low voltage gti.

Then a temporary 6s system 1680wp with a gti.

At the moment my hot water element (3000w 240v) is controlled with a contactor and Arduino and kick's in at about 54,5V. Works great but that way I create micro cycles in my battery. At least in the morning or during clouded days. I was thing about it the other day how I could reduce that. I think the only was is to build an inverter that would regulate at my battery voltage and solar current going in to the battery. So the amplitude is regulated in a way that the resistor is drawing less current when there is not enough sun.

I don't like to chop the waveform of the AC because it is a ozz inverter that won't like that.

Are there other ways around it, I would like to know? On a winter day like last week it is contributing to the floor heating, in summer it's not needed because of the thermal panels. Although a few days without sun is not good for the hws.
 
mab1
Senior Member

Joined: 10/02/2015
Location: United Kingdom
Posts: 209
Posted: 01:32am 27 Feb 2021
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  Quote   think the only was is to build an inverter that would regulate at my battery voltage and solar current going in to the battery.


I did  this:- I made a MSW 'push-pull' type inverter based on an SG3525 IC that regulates it's input voltage to 27.6v and connected to a standard 240v immersion via two 100v 80A FETs and a 18-0-18:120v 660VA transformer - 'cos that was the transformer i had at the time. I run it at ~83Hz partly to compensate for driving an 18v trans at 28v, and partly to avoid having two inverters both drawing from the battery at 50Hz and getting any strange beat frequency effects.

It maxes out at about 1000W due to the trans output voltage ~175v - I need to get a bigger trans sometime...

If the stat disconnects under load it blows the fets so I re-wired the stat to shut down the SG3525 and activate another dumpload once the water is hot.
 
mab1
Senior Member

Joined: 10/02/2015
Location: United Kingdom
Posts: 209
Posted: 09:32pm 27 Feb 2021
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The only other ways I can think of are:

Low voltage immersion and PWM controller

pick up an old cheap high frequency 'dual conversion' type inverter - they quite often fail on the H bridge A.C. sine generator, and still generate 350-400v d.c. just fine. Then adapt the high voltage d.c. regulator cct to regulate input voltage.

There are off-the-shelf diversion controllers that use PWM of the A.C. rather than triac control that will divert the excess generation from your GTI's to your 240v immersion, but the PWM ones tend not to be cheap, and will only divert the GTI connected PV.

get a 3*1kw immersion instead of the standard 3kw which will allow a smoother on/off control.
 
nickskethisniks
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Joined: 17/10/2017
Location: Belgium
Posts: 458
Posted: 08:59am 28 Feb 2021
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Thank you for the tips.

I opened my heating element to see the inside. But it only had 1 230V element and a thermostat. I was hoping for 3, 1kw elements or 400V connection.

Buying a new element would be a solution to do a simple 3 state regulation. But buying the same quality element would cost me between 2-300eur. So for that price I could build a sine wave inverter, I do have all the parts. Or your suggestion about a simple square wave inverter. It's actually a matter to find sparetime. For a long time I'm planning to build a spare inverter, but never got to it. I think I could adapt poida's code to accomplish what I need. And control it by sending serial commands for adjusting the output voltage.
 
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