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Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 10:42pm 09 Feb 2022
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I never said that, I showed you a manufactures spec sheet, where the panels "Maximum Power Voltage and Current" were defined along with the irradiance and temperature affects.
The actual calculated MPPT power value varies about these set values; BUT importantly the voltage hardly varies at all from the manufactures spec, it is this voltage I use for my device reference.
Cheers Mike
bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188
Posted: 03:15am 10 Feb 2022
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Yeh thanks Pete , I'm looking for something that someone actually has working sso I can simply duplicate it . I'm not after ideas or new projects but rather a gone there and done that type person. By "power diverter" I guess you mean the voltage controlled switch or do you have something else in mind?
Agter all a feed directly of the panel DC voltage and current could be done to the 240 volt element and that possibly would work too.I have not tried that . I just want to get something going now.
bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188
Posted: 03:16am 10 Feb 2022
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Ehh? Care to explain that Edited 2022-02-10 13:18 by bob.steel
Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 453
Posted: 03:31am 10 Feb 2022
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Hi Bob, I mean to use the PV load diverter that the article is based on to run an inverter for the HW system. I am retired and at home most of the time, so turnig a switch on when I want to boost hot water works for me. I am only a lowly ex electrician. I have seen far too many clever ideas that just cause massive unreliability issues to want to complicate things too much. There are commercial Hot Water load diverters available , they cost about $1200 here, so are way beyond what I would want to pay for one. I thought that you wanted to make your own, sorry got that mixed up. Pete
bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188
Posted: 09:45am 10 Feb 2022
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No not at all Godoh. Thanks for the suggestions and I would like to make my own but I have tried many and exactly that . They either don't work or blow up with warpspeeds stuff. Hope he will forgive me .
I agree though with the sentiment to keep it simple . It works as a rule on my boat and in all my installs. So you are spot on . I just need to understand where and how Solar Mikes device is going to pick up the switch point and actually connect something . Cheers mate . My good wishes . Edited 2022-02-10 19:46 by bob.steel
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 11:06am 10 Feb 2022
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Will do my best to explain; the PV system I'm putting in now has multiple paralleled sets of 5 series connected PV panels. Each panel has an unloaded max voltage of 39v so giving me a total unloaded value of approx 200v. From the manufactures spec sheet with an irradiance of 1000w/m^2 and 25C the max power output obtained occurs at a voltage of 31v and 9 odd amps, which probably will only ever occur during a frosty morning and a lot of sun, any other time those two voltage and current values will vary according to the manufactures spec for the panels temperature coefficient and irradiance.
The PV charger controller can track this and mostly always arrives at a max output power into the load, the panels mppt voltage spec doesn't change much with temperature or irradiance and will be near to or less than 31v x 5 = 155v most of the time. The current of course and thus power output will fluctuate wildly - but happily not the voltage so much.
So if I set my trip sense point to be say 157 volts, the mppt controller will always keep it to less than that whilst loaded up until the load starts being removed, where the controller no longer uses mppt tracking and allows the total PV voltage to increase. This is where extra loading can be applied to the PV and divert power away from the controller as it doesn't require it and is actually transparent to the controller.
When the PV controller requires more power, to do so it starts pulling the array voltage down to the mppt value, then the external diversion controller starts removing its load, ie. they don't fight each other for power, for the PV controller always wins.
You can test this by measuring your PV voltage fully loaded then on float with little load.
Cheers Mike
bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188
Posted: 10:05pm 12 Feb 2022
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Thanks Pete . I'm unsure of a couple of things you may be able to clear up for me .
1/ An A/C element in a water heater, can that be fed with 300 to 350 volts DC? Would it still work? Its only a load after all and a 8 amp current would be 2800 watts so if I had a 3000 watt element in there it should be OK?
2/ Can I convert the DC to AC efficiently and then feed the A/C to the load. I guess I would drop the voltage to 240 volts and up the amperage through a transformer of some kind.
Apologies Solar Mike . I will not take this too much further and I am watching what you are making closely . Edited 2022-02-13 08:08 by bob.steel
Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 453
Posted: 12:56am 13 Feb 2022
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Hi Bob, the element in the hot water heater is generally just nichrome wire, most are in the form of pyrotenax,( copper tube with powder as insulation). The element won't care whether it is AC or DC. Except that Ohms law will come into play. If you increase the voltage the power consumption will increase. So a 240 volt 3000 watt element would expect to have a resistance of 19.2 ohms and a current of 12.5 amps If you were to put 350 volts DC on it you would end up drawing 18.2 amps, so the power consumption would be 6370 watts. The element would not like it.
If your solar panels could only output 8 amps max then most likely it would be fine as the voltage of the panels would be pulled down quite a bit. Really experimenting would be a good way to go. The more you load the panels the more the voltage will fall so I expect that the system would work fine. The big thing is that the thermostat would not like DC switching, it would tend to arc and burn the contacts. If you put a separate thermostat on the heater and used it to turn the panels off via a dc rated relay then it would work.
Most high frequency inverters have a DC to DC converter in them to boost the battery voltage to 350 volts before it goes into the output section to be chopped and converted to AC. So you could just get hold of an inverter then bypass the DC converter section and feed your panels straight into the output section. It would be easier if you could get hold of a circuit diagram of the inverter, but basically the large electrolytic capacitors are there to supply surges on the DC side. Remember that the peak voltage of a 240 volt sinewave is 339 volts so you need that level of DC to get a 240 volt sinewave out. Hope my brain is working correctly there. Pete
bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188
Posted: 09:26am 14 Feb 2022
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Thanks Pete . I'm working on that
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 10:21am 14 Feb 2022
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Bob, have a look at an older High Freq. style quasi- sine converter around 2kw odd, you could pickup a 2nd hand one for next to nothing; they have a boost converter that steps up the input DC from 12/24/48V etc to around 350 volt mark, that is converted to a very rough multi-step 50 Hz sine output 220-240 AC.
Quite possibly you could "Modify" one and bypass the DC up converter part, there will be some big internal 400V electros, feed your 350vdc to them, hopefully you will get an AC output, depends on what control system its using.
The AC can connect direct to your element via a standard AC spec relay.
Cheers Mike
bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188
Posted: 07:23pm 14 Feb 2022
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If I had but your knowledge Mike . I have 2 second hand inverters and one new one sitting on my shed floor. I picked them up with purchases of solar cells on Gumtree. I am not experienced enough at getting in to use those so they have sat so for 3 or 4 years now. I'll photo them and start a new thread and maybe some kind soul might lead me through it by the nose.
In Australia all this second hand stuff cannot be used to feed back to the grid. The Electricity suppliers, who are Government controlled and bled, won't allow it so they go for a song and a quick dance routine around here.Suppliers pay only 6 cents a KWh and charge 26 cents for what they supply you. There really is no support for Private Solar in Australia. Not like the guys in USA get.
How is it in NZ? Edited 2022-02-15 05:32 by bob.steel
Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 453
Posted: 08:47pm 14 Feb 2022
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Hi Bob, the regulations in Oz are designed to keep line workers alive. If a solar system could feed into the house while the grid were down then, any workers who turned the grid off to work on the lines could be fried by backfeeding of a solar inverter. If you are going to feed your hot water from solar, the safest way is to use a Double Pole changeover switch. That turns the hot water system on for solar and off from the grid and vice versa. There are systems allowed in Oz where a generator or standby system can power the house when the grid is down, but there has to be a double pole changeover switch on the switchboard to prevent backfeeding. It should be pretty easy to cut the track that feeds the 350 volts to the big capacitors and power the inverter from your panels. There may be some low voltage circuitry that needs to be fed but that is where a circuit diagram would help. Pete
bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188
Posted: 03:13am 15 Feb 2022
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Well I know what you are referring to as far as anti islanding is concerned and all approved inverters in Australia must have it built in or they will not be approved .
Makes obvious sense too of course
My private Solar is not connected to the grid Pete
This Ive got to look at and understand yet. I know what you mean but finding it in any inverter I choose to use here is the question
flyingfishfinger Senior Member Joined: 12/09/2020 Location: United StatesPosts: 110
Posted: 10:23pm 21 Feb 2023
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This is really neat. Did you complete one & try it?
I was just designing a diversion load to FOLLOW the MPPT at 48V by switching (at some duty cycle) between a load and the batteries in float mode.
That would not be very efficient, but I couldn't figure out how to do it seamlessly after the MPPTs. Before, on the other hand didn't even occur to me, how silly.
I have a 4500W (I think) 240V water heater that I want to use, but I'd still have to swap the heating elements to 120V ones - my panels near MPPT output more or less 120V so I'd only be able to sink 1200W if I used the original elements.
If you have a BOM as well, I would definitely make one. Edited 2023-02-22 12:30 by flyingfishfinger
flyingfishfinger Senior Member Joined: 12/09/2020 Location: United StatesPosts: 110
Posted: 04:55pm 01 Mar 2023
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Hm, looks like this project / topic is dead huh?
I will try designing my own based on the schematic / pictures in the first post, I hope nobody minds!
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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