Renewable Energy Questions/Discussion > Solar (heating or electric)
Solar to electric Water heating.
DJ:
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)
eraser3000:
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
DJ:
--- Quote from: eraser3000 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?
--- End quote ---
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.
--- End quote ---
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?
--- End quote ---
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.
lighthunter:
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.
eraser3000:
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.
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