Author Topic: Solar to electric Water heating.  (Read 1114 times)

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Offline DJ

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Solar to electric Water heating.
« on: December 18, 2016, 01:57:34 AM »

I would like to make up a controller / converter with an Arduino to take the (excess) output from my solar panels and use it on an old resistance  water heater.

There is -some- detail I have seen of this on another forum although the poster is an expert on such things and just posts some code and expects others to work out the components and circuit involved.  I'm way too much of a beginner for that But if given the circuit, parts and code I could probably nut it out.
I understand the power can be fed direct to a normal 240V element as we use here buy way of PWM to get the panel voltage and the element resistance matched.

I have spent hours today trying to find something like this but apart from what looks like some grossly overpriced commercial components that look very simple, I can't find anything like I want. Plenty of circuits to control a water pump on an an absorption heater but nothing to convert Low voltage Panel output to drive a standard heating element.  And please dear god, no need to tell me that this is inefficient or whatever else, I have read it 50 times already.  It IS efficient when I have power I have no other use for and an old electric HWS sitting round rusting.

I plan to use this initially anyway as a pre heater for the regular HWS.  The solar may only get the temp of this heater to 40-50 o and that feeds into the regular tank making the power I have to pay for from the grid to get it to full temp, a lot less than it would be otherwise and I am still guaranteed a full hot shower. I was also thinking that there is no reason this could not also power an electric stove element. Just needs to be sufficient power as a resistance element is a resistance element no matter how you apply it.  Blow some air across said element and you have a space heater.

If anyone knows of or has info or links to a suitable circuit with all the info that a beginner can build, I would really appreciate the heads up. Right now I don't even seem to be able to come up with the right name of what I am searching for.

Offline frackers

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Re: Solar to electric Water heating.
« Reply #1 on: December 18, 2016, 04:28:01 AM »
The first thing to think about is the voltage you have available from your panels and what the heater requires. Here is an example:

If the heater element in the tank is 220v and you have 8 panels of 250 watts each available, then assuming they are 60 cell panels with a maximum output at about 28 volts then just wire them in series and get a relay that can switch 220v DC (MUST be DC rated) and energise it  via the existing thermostat in the heating element. So that is 2kw at about 220volts and all its cost is the panels and a relay.

This extra relay is required because when contacts are broken on a DC circuit an arc starts up. On AC, the arc dies out due to the voltage changing all the time - on DC it doesn't "quench" and you end up with heating in places you really don't want. The thermostat contacts will be rated for the full power of the element but will be AC rated but they will be fine for the small current that a relay draws.

When the tank is up to temperature, be aware that the open circuit voltage from the panels will be up to 8 x 40 (i.e. 320) volts - another reason for having a suitably rated relay. This voltage will pull down to 220 or below when the contacts close on the tank cooling.

With a smaller solar array  upconverting the voltage a bit (but not too much otherwise you'll 'use up" the current output capacity of the panels and end up with a lower voltage) to balance the load of the heater to the panels output.

With a larger array then its making sure  you don't burn out the heater.
Robin Down Under (or are you Up Over!)

Offline DJ

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Re: Solar to electric Water heating.
« Reply #2 on: December 18, 2016, 08:15:56 AM »

I was thinking of something more like a KW and maybe 150V or less driving the heater.
I am under the highly likely incorrect impression that by using an arduino somehow to create a PWM output, the 240 element can be made to use all the current available at whatever amps it is at regardless of the voltage.

My understanding was that if you have say 1 KW @ 150V from the panels and feed it into the 240 element,  The element would only supply 250 w or whatever heating power. If you modulate or tweak the output from the panel, you could get the full 1Kw even at a much lower voltage.
The commercial unit I have seen on the net looks very small and basic and I thought this was how it was done as was the other thread I saw that only had the arduino code and was missing the rest of the info in the puzzle.

As far as the arcing of the relay/ thermostat contacts, you are saying to set up so the  thermostat switches the relay which is connected to the element?
I have bought some SSR 40A relays recently which are DC to AC. Would they be OK or is something else required? Any Difference with AC and DC being they are solid state and I imagine nothing to arc?

Offline Pete

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Re: Solar to electric Water heating.
« Reply #3 on: December 18, 2016, 02:54:13 PM »
Hi DJ, just wondering what sort of solar system you have. Is you system a stand alone battery setup or only a grid connect?
If you have a stand alone system there may be a simple way to do what you want.
You could use your regulator to switch the heater on. It would depend on what sort of regulator you have.
When the regulator switches to float you could use that signal going to the LED that indicates that to drive a relay that switches the panels to the water heater.
You could use your arduino for that, just program it to read the input voltage from the LED, and switch a relay on to power the water heater. Of course you would leave the thermostat in the heater so that you don't boil the tank.
There are low voltage elements available, I have seen 12 volt ones, not sure if there are other voltages too.
Otherwise you could use a cheap square wave inverter to drive the element. As it is a purely resistive load it won't mind a square wave driving it.
Hopefully others will come up with more ideas.
Pete

Offline 144VDC

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Re: Solar to electric Water heating.
« Reply #4 on: January 14, 2017, 01:13:35 PM »
Hi DJ,
I have a stand alone (off grid) solar system and use my excess power to heat my domestic water. I have a 7000 watt array and most days 1/4 to 1/2 is excess. I use a pwm shunt regulator, that I built, to control my charge voltage. It shunts the excess power to a 120 gal. hot water tank with three 240v 3500w (10500w) elements. This system keeps us in hot water all the time.
I have a 144v battery bank, so my charge voltage is 174v and float voltage is 160v. Heater elements are just big resistors; the more voltage the more current and therefore the more wattage up to the point they burn out. My point is, any heater can be used at any voltage up to a maximum where the watt density is too high and it burns out. My elements are 4655watts @160v and my pwm duty cycle drops it down from there to whatever is needed to maintain the battery voltage.
I=E/R Not just a good idea, it's the law.

Offline DJ

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Re: Solar to electric Water heating.
« Reply #5 on: January 14, 2017, 08:46:48 PM »

I figured a work around which seems to be doing the job well.

I couldn't work out anything with the arduino so went for a cheap, bulky and effective solution instead.

To upsccale the voltage I have been using a UPS coupled to a pair of batteries. The UPS I already had and is 1000W rated. As the element in the HWS is 3600W, I purchased a PWM controller from fleabay for about $11 I think it was.  This works great as I can vary the load through the PWM so the 3600W element does not overload the 1000W UPS.

The setup is basicaly an off grid system which I am just playing and learning with. I have a Cheap but really effective PWM solar controller which allows me to set battery voltage and load on/ load off voltages as well.  I have this driving a 25A SSR which switches the AC PWM in and out according to battery charge.  When the batterys hit their pre designated float level, the controller turns everything on and the water starts heating. When it drops, the heater switches out.

ATM I am using the setup to drive a small fridge and an Urn for hot water for my  8 or so daily coffee's. It's working brilliantly  for that.  I have instant hot water all day long and the urn having it's own thermostat stops the water boiling away.
The Ac PWM is rated to 4 KW so with a suitable size inverter or UPS, could do a water heater with no problem.

My Learning solar system ( which it is doing well) is only 2 panels for a bit over 500W rating but with this setup I can drive loads well over it's rating.
I'm just using some car batteries atm which I know are not right but I have the controller set so I don't take them under 12V and really only use the flat charge capacity rather than drain them to any appreciable degree.

I am looking to get a lot more used panels and just grid back feeding from there although I think I'll expand my play system to about 1 KW. I'm enjoying playing and experimenting with it and there isn't much of that in a regular home/grid feed system.
I also want to set up at least one of my diesel engines with an induction motor so I can use that for backfeeding and If I can ever find a proper stand alone generator head at a reasonable price I'll do that too for the once every 5 years blackouts we have!  :0)

Offline eraser3000

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Re: Solar to electric Water heating.
« Reply #6 on: January 15, 2017, 07:38:02 AM »
DJ,
I have had a go with this kinda thing for awhile now and have a few questions.

Have you tried actually varying the load with the PWM controller, while load was connected to the UPS?

I have a hard to believing that the UPS is handling the load being pulsed on and off without out tripping out.

If you have, what is the PWM base frequency and which SSR are you using?

Thanks in advance.

Eraser3000

Offline DJ

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Re: Solar to electric Water heating.
« Reply #7 on: January 15, 2017, 11:30:42 AM »
DJ,
I have had a go with this kinda thing for awhile now and have a few questions.

Have you tried actually varying the load with the PWM controller, while load was connected to the UPS?

Yes, many many times.  I can up the output when it's sunny and the other day I had it running at just 125 watts on the cloudy day which didn't boil the kettle all day to my surprise.  I match the PWM output to the power the panels are producing and go out and adjust it many times a day. I try to set it so as to maintain a constant value on the batteries but sometimes I turn it up so the batteries drain to their very low level drain before the controller kicks out and recharges the battery's so the process cycles. I have put in an amp meter and can gauge by the load gauge on the UPS how much I am putting out.  The other day I had it running at .5A and I have had it up to 5A which is the UPS limit.

Quote
I have a hard to believing that the UPS is handling the load being pulsed on and off without out tripping out.


Mate, you believe what you want. I have been doubted loads of times on things I have done over the years usually by people that think unless something is expensive, complicated and with a shipload of failure points built in, it can't work.  get that every single day on what I have on my YT channel even though 50 other people have built my designs and posted them working and more still have told me about what they have built and done with them.
 
I'm not trying to sell you anything or claim I'm smarter than anyone else, just telling people of whom I know there are some out there like me that don't have the knowledge to build one from an arduino, the cheap and low tech way I found of making it work using off the shelf bits cobbled together.

Quote
If you have, what is the PWM base frequency and which SSR are you using?

No idea on the frequency. Didn't even give it any thought. I bought the thing as a prebuilt unit I just had to wire up off ebay.
Initially I wanted it to control the blowers on my oil burners but I didn't like the way it made them buzz and I wasn't sure if some of them were universal or Capacitor start and didn't want to fry the windings.

What I needed for the heater was a way firstly to ramp up the 24V DC to 240V Ac. Been doing that for years with inverters for my work on location and had some UPS units I was given when the batterys were stuffed which were better because they were pure sine wave where all my cheapie inverters were square.

The next thing was how do I limit that current?  Water heater is 3-6Kw here.  Didn't matter if I didn't give it full tilt, just have to give it what I can which will be the inverter rating and not overloading it.  Logical way, chop the power. What do I have or can get to do that?  Motor speed controller.
Now all I need to do is switch the thing off when the sun goes down or a cloud comes over so I don't drain the batteries and the UPS shuts off which won't restart automatically.

The cheap arse PWM solar controller has that function built in like many of them do so all I need to do is get a relay that has a Low DC side and an AC high side. Initially bought some of the little blue mechanical relays but then saw the SSR's so bought them which arrived first.
Cobbled it all together and I have now made a board up with it all on so it doesn't look a complete dogs breakfast and that was it. Put in a breaker for the panel feed and another one on the battery charge and that was it. UPS feeds direct through some joined car battery cable.  As it's going to pull around 100A at full load, it's unfused and I couldn't find a breaker nor want to muck around wiring up multiple on a bus bar.

 The SSR is one of those white square ones on fleabay. I think I have a 25A unit connected but it could be a 10. I bought a few different capacity's of the same output and wired it up and put it in a junction box so I didn't accidentally fry myself. I have seen it mentioned they should be heat sunk but this one never gets much more than being detectably warm. Maybe because it's being so under driven even if it is a 10A unit. 

Offline lighthunter

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Re: Solar to electric Water heating.
« Reply #8 on: January 17, 2017, 12:18:49 AM »
Hi DJ are you using a pwm similar to this? I think it switches at a terribly annoying 2khz when connected to a motor winding.  ;D.

I tried using a triac based control which varies the voltage of the "switch on" point in the AC cycle.
It worked ok and the 24v pj inverter had no issue with it but my grid tie hated it. Therefore i discontinued using it.

Now what you are doing appears to be a little different. Rather than firing on every half -cycle (at varying time point); you are likely skipping whole cycles and conducting complete cycles alternately. I'm guessing only so anyone who understands pwm driven ssr's in a sine wave application, please confirm or enlighten us.

Neat idea, I might have to try it. Something tells me my grid tie wont like it either. They wrote some very clever analyzing software in it. Almost maddening.
LH

Offline eraser3000

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Re: Solar to electric Water heating.
« Reply #9 on: January 17, 2017, 04:49:03 PM »
lighthunter, What size load are you trying to run? I have found that the load needs to be 1/5 of the inverter's capacity or less. So if you have a 1000 watt inverter, try 200watts per SSR and only fire SSR 2 if SSR 1 is fully on, and SSR 3 if SSR 2 is fully on. Once your GTI is at load it can handle more, so you would then only be bashing it with 200watts at a time.

DJ you mentioned you are using one of the "white square ones on fleabay". Those are usually only good for the half cycle firing at max. Usually your PWM controller output goes high then the SSR sees the signal and there is a delay after the delay SSR waits for zero cross point on sine wave and SSR goes fully closed. If the PWM controller then goes low SSR has another delay and then waits for zero cross point to "open up again".  Usually that means out of your 120 half sine points per second and the added delays in there (10-30 ms) you are only getting about 40 points you will realistically close or open on. With Random SSRs you get alot more control and little delay but they can cause some noise issues.

Offline lighthunter

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Re: Solar to electric Water heating.
« Reply #10 on: January 17, 2017, 08:06:53 PM »
lighthunter, What size load are you trying to run?

It wws either 3000 or 2000w element, probably the 3. I didnt use ssr though.
I dont own one as of yet. I was using the triac fire angle delay circuit which produces nasty switching harmonics every half cycle. Interesting ideas you have. I will need to play with it again
and see if i can get it to behave better. I do have my house loads sequenced as you suggest but was trying to adjust triac dimmer at a reasonable point and cycle it on and off with plc. It was the on off moments the gti didnt like. You got me thinkin now.

LH

Offline DJ

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Re: Solar to electric Water heating.
« Reply #11 on: January 21, 2017, 04:16:02 AM »

DJ you mentioned you are using one of the "white square ones on fleabay".

This is what I am using. I did not look up the technical details.  Don't know, don't care, not interested.   ;D
It's working just fine and that's what I WAS interested in.
I think the one I may be using is only 10A if that makes any difference. I bought 4 of them in different sizes and I think the one I put in the box was a 10 as thats our full 240V std socket amperage anyway and far more than the UPS will hold.

As I said, just got the off the shelf bits and pieces and cobbled them together and they did exactly what I wanted. I'm afraid I'm pretty electronically ignorant. I want to learn about it but it seems so overwhelming. While I can buy off the shelf boards and controllers and get them to do what I want, I'll remain happy.
That said, I just opened another 2 Arduino Nano's I ordered.
Wondering If I'll ever use the things now.  :0)


Offline DJ

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Re: Solar to electric Water heating.
« Reply #12 on: February 03, 2017, 06:31:48 AM »

I had a score on fleabay on the weekend and got myself a good brand 2000W 24V pure sine wave inverter with remote control Sell said it was never used and I totally believe it. Price paid, Just under $125. Price it is still being sold in the store for, $1525.00  Bargain.

I hooked it into my test system to replace the UPS I have been using.  With the doubt cast here I was a Little apprehensive about putting it on the system with the PWM but I did it anyway and works without any problem what so ever.
I can boil the kettle at 1800W ( I purposefully allow some margin) and the thing is quite happy as long is the sun is shining to stop the batteries dropping too fast and the low voltage protection cutting in.

Clearly its not a fluke and the setup is creditable. All I did was unplug the UPS, Plug in the inverter, fire it up and it worked exactly the same.

For anyone wanting to drive a resistive load from panels over their rating and with no electronics fabrication knowledge, this is an easy and low cost way to do it.

Offline eraser3000

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Re: Solar to electric Water heating.
« Reply #13 on: February 07, 2017, 07:18:18 PM »
That is great DJ, I'm glad it worked for you. The reason I asked is just that others including me have had issues with several  brands of inverters, that just aren't happy with massive loads being turned on and off rapidly. 
Which inverter model did you just pick up that is working really well?
Again, glad its working in your application, that is the overall goal with all this technology, meeting our needs.
Cheers

Offline DJ

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Re: Solar to electric Water heating.
« Reply #14 on: February 08, 2017, 04:53:07 AM »

This is the one I got.
https://www.jaycar.com.au/2000-watt-24vdc-to-230vac-pure-sine-wave-inverter/p/MI5712

Been running it about a week now and it's working perfect. No issues with the PWM at all. The remote for the inverter has a display in Volts, watts and amps so I can align that with what the solar controller tells me the panels are producing and go a bit under so I can pump the max power into the Kettle. I got a little multi display meter off ebay so I'll wire that up into a box so U can use it to see the loads things are drawing or producing for a bit more info.

Maybe I should buy some more of the PWM controllers as they work and others don't seem to?
I'm thinking that it might be the pwm's that have caused problems others have seen rather than the inverter? I have a couple more inverters up the back, Might drag them out and see how they do. If they are OK, would suggest it is more the compatibility of the PWM than the inverters driving them.