Author Topic: Wanted, MPPT inverter that will keep running and just lower AC output voltage.  (Read 3973 times)

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Offline PsiTopic starter

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Solar inverter question I'm hoping someone can answer.

I just got 1kW of solar panels (2x 545w 50V panels), but for my application I want the MPPT inverter to keep running even if the input (solar output) cannot handle the load current. In this case the inverter should keep dropping the AC output voltage until it can handle the load. (The load is resistive, so it wont care).

Normally a feature like this would be bad because you want the output to be within the range 220-240VAC or whatever, and the inverter should shut off if it cannot maintain this.   But I want it to keep running down to maybe 80VAC.

Does this have a term or feature name that I can search for to find inverters that can be setup like this?

Thanks.
« Last Edit: June 03, 2023, 06:55:08 am by Psi »
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Offline fourtytwo42

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I don't think there is a product as you describe it.
However if this is a solar water heater project ? there are many solutions depending upon your DIY electronics skills.
What is the rating plate data of your heater (volts, Kw) ?
 

Offline mikeselectricstuff

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One reason this doesn't exist is that if anything is connected to the inverter which has a switchmode PSU, its input current will increase as voltage decreases, so things will get unstable.
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Online tszaboo

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Do you want to run something in an island operation? Like a big water boiler separate from the grid?
 

Offline PsiTopic starter

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Do you want to run something in an island operation? Like a big water boiler separate from the grid?

yes, exactly, i was going to wire it up to a hot water cylinder and see if I can get away with running it from solar alone.
Or at least getting as much solar energy into the water as I can during sunshine hours.
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Offline PsiTopic starter

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What is the rating plate data of your heater (volts, Kw) ?

It doesn't state the kW rating on it sadly, it's 230V though.
It has a model number, (251.180.00) and brand (Rheem) but I cant find any info on it.
It's 180 liters, was made in 1994 and i'm pretty sure only has a single heating element.
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Online tszaboo

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Do you want to run something in an island operation? Like a big water boiler separate from the grid?

yes, exactly, i was going to wire it up to a hot water cylinder and see if I can get away with running it from solar alone.
Or at least getting as much solar energy into the water as I can during sunshine hours.
Electrodacus has a few MPPT controllers for exactly that operation. He is also on this forum, or has a website selling his products.
https://electrodacus.com
 

Offline janoc

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Do you want to run something in an island operation? Like a big water boiler separate from the grid?

yes, exactly, i was going to wire it up to a hot water cylinder and see if I can get away with running it from solar alone.
Or at least getting as much solar energy into the water as I can during sunshine hours.

Wouldn't it be better to use the Sun to heat the water directly by sending it through a solar collector?

The solar panels + conversion + losses in the wiring, etc. can't be more efficient than that. My parents have a small setup exactly like this and it covers their hot water needs the entire summer and most of fall/spring without issues. The requisite setup & plumbing are pretty simple and cheap, certainly much cheaper than solar panels + inverter. It is also possible to build your own easily if you have access to the tubing.
 
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Offline PsiTopic starter

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Do you want to run something in an island operation? Like a big water boiler separate from the grid?

yes, exactly, i was going to wire it up to a hot water cylinder and see if I can get away with running it from solar alone.
Or at least getting as much solar energy into the water as I can during sunshine hours.

Wouldn't it be better to use the Sun to heat the water directly by sending it through a solar collector?

The solar panels + conversion + losses in the wiring, etc. can't be more efficient than that. My parents have a small setup exactly like this and it covers their hot water needs the entire summer and most of fall/spring without issues. The requisite setup & plumbing are pretty simple and cheap, certainly much cheaper than solar panels + inverter. It is also possible to build your own easily if you have access to the tubing.

Probably,  but the choices are
A) Leave the 2 panels in a box until i'm ready to do something serious with them
B) Do something simple with them now so they can be saving a little money

I got an amazing deal on them, or I would have waited until I was ready to do something serious.
But I couldn't resist getting at least a couple of panel now, just to have some ability to generate electricity in an emergency
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Offline janoc

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Probably,  but the choices are
A) Leave the 2 panels in a box until i'm ready to do something serious with them
B) Do something simple with them now so they can be saving a little money

I got an amazing deal on them, or I would have waited until I was ready to do something serious.
But I couldn't resist getting at least a couple of panel now, just to have some ability to generate electricity in an emergency

Use the panels with a regular inverter and tie to the mains to e.g. power a fridge, TV or whatever you may have around the house. You are likely to save more money that way. Here in Germany such small "balcony" setups are getting quite common, even hardware stores sell them. You literally plug the inverter into a normal wall outlet and done.

Using solar electricity for heating is a very inefficient thing to do. 2 panels are likely not going to be able to supply enough energy to power the water heater in a sensible way anyway.
« Last Edit: June 04, 2023, 10:25:52 am by janoc »
 

Offline mikeselectricstuff

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If its just heating then it may be more efficient overall to just connect the panels direct to a suitably rated heater - any losses in not MPPTing may be outweighed by lack of inverter losses
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Offline PsiTopic starter

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If its just heating then it may be more efficient overall to just connect the panels direct to a suitably rated heater - any losses in not MPPTing may be outweighed by lack of inverter losses

I did ponder trying to replace the heating element in the hot water cylinder with a 48V element. As you can get them, assuming the thread is the same as my one.

Then just wiring both 50V panels in parallel and feeding it in directly as DC.
Obviously still using the tanks thermostats which I assume are probably bimetal temp switches and therefor voltage agnostic.
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Offline PsiTopic starter

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Use the panels with a regular inverter and tie to the mains to e.g. power a fridge, TV or whatever you may have around the house.

That would require a lot of red tape, anything tied to the mains would need inspection.

Thankfully any solar stuff under 120V DC can be installed without inspection here, so i'm all good putting two panels on the roof in series or parallel.
So having the panels is fine, I just have to do something with them that is 100% isolated from the mains.
« Last Edit: June 04, 2023, 11:43:56 am by Psi »
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Offline fourtytwo42

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I did ponder trying to replace the heating element in the hot water cylinder with a 48V element. As you can get them, assuming the thread is the same as my one.

Then just wiring both 50V panels in parallel and feeding it in directly as DC.
Obviously still using the tanks thermostats which I assume are probably bimetal temp switches and therefor voltage agnostic.
Unfortunately that won't work for a number of reasons
1/ You cannot use AC rated switchgear of any sort including thermostats on DC, they will simply arc and catch fire.
2/ You cannot "short out" the panels with a low impedance load that is mismatched to the panels current operating condition so you need a converter of some kind to prevent this happening. This is why I asked you for your heater data, as an alternative measure it's resistance (when disconnected) with a DVM.

Whether you need a buck or boost (or both) converter or a simple PWM/capacitor will depend on your heater impedance vs panel mppv.
You add the thermostat into the converter control circuit so it is not handling high voltage/current DC.

Just to encourage you we have been using a 1Kw array with efficient conversion to provide most of our summer hot water since 2017   :-+
Wait for the rude comments about not washing enough  ::)
 
« Last Edit: June 04, 2023, 12:10:58 pm by fourtytwo42 »
 

Offline janoc

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Use the panels with a regular inverter and tie to the mains to e.g. power a fridge, TV or whatever you may have around the house.

That would require a lot of red tape, anything tied to the mains would need inspection.

Would it? We aren't talking a "permanent" installation. At least here they sell a set with two-three panels and an inverter you are supposed to put on your balcony and simply plug into the wall. It will supply power when there is sun directly into the house mains. The systems are certified and pre-approved, you need only to register them and you need a modern electricity meter that won't "turn backwards" if  you produce more energy than you would use.

Those are obviously small systems, though, few hundred W maybe.

Something like this:
https://www.bauhaus.info/solarstrom-photovoltaik/green-solar-balkonkraftwerk-760/p/30264016
« Last Edit: June 04, 2023, 01:26:04 pm by janoc »
 

Offline PsiTopic starter

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Use the panels with a regular inverter and tie to the mains to e.g. power a fridge, TV or whatever you may have around the house.

That would require a lot of red tape, anything tied to the mains would need inspection.

Would it? We aren't talking a "permanent" installation. At least here they sell a set with two-three panels and an inverter you are supposed to put on your balcony and simply plug into the wall. It will supply power when there is sun directly into the house mains. The systems are certified and pre-approved, you need only to register them and you need a modern electricity meter that won't "turn backwards" if  you produce more energy than you would use.

Those are obviously small systems, though, few hundred W maybe.

Something like this:
https://www.bauhaus.info/solarstrom-photovoltaik/green-solar-balkonkraftwerk-760/p/30264016

When you said "and tie to the mains"  I assumed you meant 'grid tie'.
But if you just mean isolated then yeah, can do that and just remove it when selling the house.

The issue powering devices is that you only get 6-8 hours of good solar output. There isn't a lot of stuff in the house that only gets used during that time during the day so you need a battery bank and charger to make that work.
Hot water is one exception because the tanks are so well insulated you can head it up during those 6 hours it stays mostly hot until the next day. No battery needed
« Last Edit: June 04, 2023, 10:10:09 pm by Psi »
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Offline PsiTopic starter

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Unfortunately that won't work for a number of reasons
1/ You cannot use AC rated switchgear of any sort including thermostats on DC, they will simply arc and catch fire.
Excellent point, easy to solve but very important to know :)

You cannot "short out" the panels with a low impedance load that is mismatched to the panels current operating condition so you need a converter of some kind to prevent this happening.

What happens if you do? I was under the impression you just lose efficiency because you're not running them at max power point, not that anything is damaged.

Just to encourage you we have been using a 1Kw array with efficient conversion to provide most of our summer hot water since 2017   :-+
Wait for the rude comments about not washing enough  ::)

How big tank are you running at 1kW successfully?
My tank is 180L but there is only me using it and only for 1 shower a day, so I reckon it would probably work fine at 1kW.


I believe from reading the manual for my cylinder that it's 180L with 3kW element @ 230V.
So that would be 17 ohms when heating at 3kW, ignoring that its resistance would be a little different at lower power levels, two panels in series (100V) would only be 588W.
So a step up inverter is required to use the current element.
Would need around 130V to get the element to work at 1kW.
So either 50V to 130V boost, or 100V to 130V boost depending on if the panels were series or parallel
« Last Edit: June 04, 2023, 10:51:45 pm by Psi »
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Offline Ed.Kloonk

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The problem with heating water this way is it seems alluring when the sun is shining. The are a few things to consider.

A Rheem tank heating element should be fairly easy to swap for a lower rating if so needed for hybrid purposes.

A rainy day won't heat the tank if you calculated only for sunny days every day. A tank should be heated to at least 65 degrees once a day regardless.

Think about acquiring a 120vac inverter and some batteries. Picking the correct sums, your panels can slow-cook the water in the tank over some hours and if need be you can boost in the afternoon fastly with regular 240v. Just do it safely, with isolation and adequate switching, of course.

I once set up a smallish tank in a shed for a hand basin and it's controlled by a arduino with a triac dimmer which 'follows' the output reported by the MPPT and selects the power level accordingly. I used a 1.8kw element and a plain old 12-240v inverter. Works fine, if not ugly as sin. Happy to more provide info if needed.

What you you fellahs think about a (khz-range) pwm circuit applied to the AC, instead of the God-awful triac? Something that could switch resistive loads to 2.5kw/240vac. It's been on my 'roundtuit list for a while.

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Offline fourtytwo42

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What happens if you do? I was under the impression you just lose efficiency because you're not running them at max power point, not that anything is damaged.
Nothing is damaged, you simply loose efficiency (less power) big time, see chart & explanation below.

How big tank are you running at 1kW successfully?
My tank is 180L but there is only me using it and only for 1 shower a day, so I reckon it would probably work fine at 1kW.
My guess would be around 200L (I inherited it) BUT you are not heating the whole tank due to stratification, so the heat builds from the top down, that is with the heater fitted in the normal to the UK top of the tank, of course yours may be different.

I believe from reading the manual for my cylinder that it's 180L with 3kW element @ 230V.
So that would be 17 ohms when heating at 3kW, ignoring that its resistance would be a little different at lower power levels, two panels in series (100V) would only be 588W.
A long time ago I did a simulation of 1Kw worth of panels and a 20 Ohm load (see attached picture). The important thing to note is with a boost only converter the panel voltage cannot rise above the intersection of insolation & the resistive load line (red) limiting power at low insolations. Operating above the red line requires a boost converter whilst operating below it requires a buck, it took a while for me to get my head around that lols

To resolve this issue I actually used a combined boost/buck converter. Depending on your actual panel specification and how much time your array spends at less than half power this may or may not be worthwhile.


« Last Edit: June 05, 2023, 09:38:17 am by fourtytwo42 »
 

Offline fourtytwo42

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What you you fellahs think about a (khz-range) pwm circuit applied to the AC, instead of the God-awful triac? Something that could switch resistive loads to 2.5kw/240vac. It's been on my 'roundtuit list for a while.
It's a good idea Ed and I had considered it too especially in combination with an off the shelf inverter but then the cost & complexity of such a device approaches that of direct conversion from the panels so I saw no advantage. Depends on your overall system topology of course.

Slightly drifting off topic my PV converter performs as both a GTI and direct conversion water heater, I had considered simplifying it or using a COTS GTI in combination with a high voltage AC immersion controller as you suggest. Some of my experiences with the GTI unfolder (attached to the grid) that has used both HVmosfets and IGBT's is somewhat off-putting as it took me a long time to clamp the grid transients sufficiently for it to be reliable  :-/O so not for the faint hearted unless you have deep pockets and are not of a nervous disposition  (loud bangs etc) :-DD
 

Offline Jeroen3

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I'd find a suitable microinverter, have the panels operate in grid mode. This way they do as you ask.
Then get one of those small controllers, or make one yourself, that just chops the boiler heater the same amount as the exported power.
Eg: https://www.gpceurope.com/product/gpcontrol-smart-boiler-controller/

Or buy this, if it's even available yet: https://www.nexol-ag.net/
 

Offline PsiTopic starter

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A long time ago I did a simulation of 1Kw worth of panels and a 20 Ohm load (see attached picture). The important thing to note is with a boost only converter the panel voltage cannot rise above the intersection of insolation & the resistive load line (red) limiting power at low insolations. Operating above the red line requires a boost converter whilst operating below it requires a buck, it took a while for me to get my head around that lols

Ah yes, of course.  When solar power is reduced and you want to use that power you have to run the heating element at a lower voltage and you reach the point where the panel output is higher than the voltage you must drive the load with to draw the power you have available.   Yeah, bit hard to get your head around until you actually have a think about it.

Thanks!
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Offline PsiTopic starter

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I'd find a suitable microinverter, have the panels operate in grid mode. This way they do as you ask.
Then get one of those small controllers, or make one yourself, that just chops the boiler heater the same amount as the exported power.
Eg: https://www.gpceurope.com/product/gpcontrol-smart-boiler-controller/

Just so I understand you correctly,  you're suggesting a micro-inverter that's setup for grid-tie but in a grid offline state 24/7.
Then using that controller to command the micro-inverter to drive the heating element instead (since its in grid offline state) in a way where it switches to driving a resistive load (water heater) and will maintain max solar power into the resistive load with a very wide voltage range?
« Last Edit: June 05, 2023, 11:46:09 am by Psi »
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Offline PsiTopic starter

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Question,  Are there any off the shelf solar inverters that have open-source firmware on the MCU?
Either someone has written new firmware for them or hacked or anything.
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Offline fourtytwo42

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Question,  Are there any off the shelf solar inverters that have open-source firmware on the MCU?
Either someone has written new firmware for them or hacked or anything.
In a word NO! BUT there are plenty of cheap INVERTERS out there. It depends what you want to do.
The solar heater diverters people talk about are designed to divert surplus export power from fairly large GTI installations to a heater, not much use to you unless you buy a GTI for your panels. Incidentally GTI's are NOT ALLOWED to operate without a live grid attached for safety reasons.
A small INVERTER will simply throw an overload if you try plugging a 3Kw heater into it and a large inverter will simply shut down when a large load overloads the PV panels.
So in summery I can see no economical COTS solution to your requirement, DIY electronics or bust I am afraid!!
« Last Edit: June 05, 2023, 12:41:20 pm by fourtytwo42 »
 
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Offline PsiTopic starter

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Yeah, probably need to run panels in parallel to get ~48V @ 22A and then DIY a boost converter to get that to like 60-130V to drive the current 3kW element at 1kW at 130V down to whatever min power 60V is.

Or, might be simpler to just get a cheap generic 48V solar inverter to 110 or 230V and then rectify the output and PWM that into the element with some caps to reduce avg current loading the inverter sees to what the solar system can supply. Might be a bit tricky to get tuned right though. Has to respond faster than the inverter can so it doesn't shut down from min input volts error.
And pwm freq would have to be fast to avoid needing huge caps.
« Last Edit: June 05, 2023, 01:20:45 pm by Psi »
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Offline fourtytwo42

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You might find this forum https://diysolarforum.com/forums/solar-water-heating.60/ helpful except most of them live in the desert and have never heard of clouds and short winter days  ::)
Lots of ideas though and mostly off-grid isolated systems.

Have you ever heard of or used LTspice ? very useful tool for modelling solar panels and trying out circuits without actually building them!
Also try this site https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#DR for insolation information in your backyard (seems to cover NZ).
« Last Edit: June 05, 2023, 02:22:04 pm by fourtytwo42 »
 

Offline marekm

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Another solution to consider - switch the PV panels in series or in parallel to better match the resistive load, depending on the amount of sunlight.  Less efficient but no EMI issues, use MOSFETs as DC switches without arcing.  If you can have more than one heater element - these could be switched too.
 

Offline PsiTopic starter

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Another solution to consider - switch the PV panels in series or in parallel to better match the resistive load, depending on the amount of sunlight.  Less efficient but no EMI issues, use MOSFETs as DC switches without arcing.  If you can have more than one heater element - these could be switched too.

Yeah, at the cost of a few contactors its probably a good idea,  just need to be DC rated.
Or could use semiconductors I guess.   Maybe cheap AC contactors and single N-Mosfet in series so contactor changeover is done while current is zero.
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Offline Ed.Kloonk

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Yeah, probably need to run panels in parallel to get ~48V @ 22A and then DIY a boost converter to get that to like 60-130V to drive the current 3kW element at 1kW at 130V down to whatever min power 60V is.


I don't know if I would run a water tank at >50vdc. The themostats just isn't rated for that. Apply a leakage breaker at least.
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Offline Ed.Kloonk

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How about this:


from:
https://www.instructables.com/safe-and-simple-AC-PWM-Dimmer-for-arduino-Raspberr/

I've yet to get around to seeing what it looks like in spice with a beefier fet, say 25A or more.
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Offline Seekonk

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I've been doing that for years with water heaters.  I use a fixed power point voltage so I can use it in parallel with MPPT charge controllers for diverting excess power. It can temperature correct to follow the natural power point or just adjust seasonably. Simple and efficient. It pulses the heater element at no faster than a 50Hz rate with an arc interrupt off period should the ON pulse last longer than 10ms.  This allows standard mechanical thermostats to be used without burning contacts. I heat three separate tanks and a priority for each tank can be set. Graph is a half hour of diversion in passing clouds.

 
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Offline PsiTopic starter

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Yeah, probably need to run panels in parallel to get ~48V @ 22A and then DIY a boost converter to get that to like 60-130V to drive the current 3kW element at 1kW at 130V down to whatever min power 60V is.


I don't know if I would run a water tank at >50vdc. The themostats just isn't rated for that. Apply a leakage breaker at least.

Other than the contact current ratings, why would the thermostat care if the water is being heated by 50V or by 220V?  I assume its just a mechanical thermal switch triggered by water temp?
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Offline PsiTopic starter

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I've been doing that for years with water heaters.  I use a fixed power point voltage so I can use it in parallel with MPPT charge controllers for diverting excess power. It can temperature correct to follow the natural power point or just adjust seasonably.

Not too sure I understand that correctly.
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Offline marekm

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Other than the contact current ratings, why would the thermostat care if the water is being heated by 50V or by 220V?  I assume its just a mechanical thermal switch triggered by water temp?

The switch is rated for AC which extinguishes the arc at zero crossing, when interrupting high power DC the arc will burn for much longer, could catch fire or weld the contacts closed, overheat and "rapid unscheduled disassembly".
 
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Offline Jeroen3

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I'd find a suitable microinverter, have the panels operate in grid mode. This way they do as you ask.
Then get one of those small controllers, or make one yourself, that just chops the boiler heater the same amount as the exported power.
Eg: https://www.gpceurope.com/product/gpcontrol-smart-boiler-controller/

Just so I understand you correctly,  you're suggesting a micro-inverter that's setup for grid-tie but in a grid offline state 24/7.
Then using that controller to command the micro-inverter to drive the heating element instead (since its in grid offline state) in a way where it switches to driving a resistive load (water heater) and will maintain max solar power into the resistive load with a very wide voltage range?
No, what I mean is that you use the micro grid tie inverter as designed. Exporting to grid.
Then you wire a switch or controller with a CT or power transducer into your export point and have it turn on the boiler heater when the power is negative. The same way Dave's myenergi charger works on eco mode.
This way the panel will operate at mpp regardless, that's what the micro is for. And even if your boiler is full you can use the excess yourself or export for the negligable amount of money it gives you.

Other question: Is using (pwm) DC in electric heaters a problem with galvanic corrosion?
 

Offline PsiTopic starter

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Other question: Is using (pwm) DC in electric heaters a problem with galvanic corrosion?

Fair question. 

but I think the element is electrically isolated from the water
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Offline Seekonk

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There are a number of commercial products available, epically in Europe for stand alone systems. I think a using PV only to heat domestic water is a little goofy since the idea is for it turn off. There are a lot of other places that energy can be used.  But panels are cheap.  Search ACTii and a number of other companies in Poland. My laptop died and I lost the list of products.  Here is one using my design in Lithuania.   It looks like for adults only, not for people who sit at the kiddie table of solar.



     AC7391 video

https://create.arduino.cc/projecthub/stevetearle/loadmaster-xp-a-smart-pv-mppt-solar-hot-water-controller-4a813f

« Last Edit: June 06, 2023, 02:21:19 pm by Seekonk »
 

Offline f4eru

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Wouldn't it be better to use the Sun to heat the water directly by sending it through a solar collector?
The solar panels + conversion + losses in the wiring, etc. can't be more efficient than that.
From the energy efficiency, yes.
From economic efficiency, it does not make sense to invest in thermal solar.

I have thermal solar on my roof, at the time of install it made economic sense at my location, but today, not any more.
PV is widely in front, because it costs the same, but you get not only hot water, but also excess electrical energy for the same install cost.
This is especially true if using a thermodynamic water heater, but even with resistive, it makes plenty of economic sense.

Offline janoc

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From the energy efficiency, yes.
From economic efficiency, it does not make sense to invest in thermal solar.

I have thermal solar on my roof, at the time of install it made economic sense at my location, but today, not any more.
PV is widely in front, because it costs the same, but you get not only hot water, but also excess electrical energy for the same install cost.
This is especially true if using a thermodynamic water heater, but even with resistive, it makes plenty of economic sense.

Not sure what kind of thermal solar are you talking about but the small water heating setups are literally just a lattice of water pipes painted black and connected to a reservoir. Plus you want a pressure release valve.
 
Good luck finding a PV setup + inverter + installation costs for the price of a bunch of scrap piping and some valves.  :-//

Heck, you could even get a small portable system - they are commonly sold to heat pools ...

The bigger sets that heat enough water for 4-5 people household (~300 liters or so) cost about 3000€ (see e.g.: https://www.solar-eshop.sk/c/solarni-termika/solarni-sety/ ) if you want to buy an off-the-shelf system. A PV system with comparable power output is not going to be cheaper for sure.

« Last Edit: June 06, 2023, 04:52:47 pm by janoc »
 

Offline Seekonk

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I'll put my PV water heater against your scrap pipes any day. $20 for electronics, #300 for a KW of panels, and a free water heater someone gave me because the plastic cold water down tube broke off. And mine only operates on excess solar normally wasted with 70% of my panels shaded at any one time. It would be absolutely impossible to heat with solar thermal at my location.

This is a 1KW control made from a $10 ebay inverter board.
« Last Edit: June 06, 2023, 11:21:01 pm by Seekonk »
 

Offline PsiTopic starter

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Yeah, I've been thinking about the best approach for my application.

Best plan for me is to buy a generic 1.5kW 48V to 230V inverter.
That way i can actually power 230V stuff in an emergency during the day.

Then for water heater thing, just rectify it and PWM into the current 3kW element at like 1khz and smooth out draw with some caps.  Max duty is going to be around 50% to max out panels but that's fine.

Saves me having to do much work other than making something to PWM the element with feedback to keep panel voltage at max power voltage. (and low current loop through thermostat).

No need to change water cylinder element or anything.
and inverter is off the shelf
and pwm + filter for 1kw shouldnt be that hard.
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Offline NiHaoMike

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You don't actually need AC, just a non continuous current for the thermostats to be happy. How about a boost stage and then a fixed 100Hz or so PWM to interrupt the current? The boost stage operates at whatever duty cycle it needs to in order to stay at MPP, unless the output voltage is already at the 300V or so limit.
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Offline PsiTopic starter

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You don't actually need AC, just a non continuous current for the thermostats to be happy. How about a boost stage and then a fixed 100Hz or so PWM to interrupt the current? The boost stage operates at whatever duty cycle it needs to in order to stay at MPP, unless the output voltage is already at the 300V or so limit.

DIY DCDC boost stage would work well, but would then need to spend time actually designing, building and testing one.   I kinda want to get a proper 230V inverter anyway, just so i have the option to run things from the panels

So it's easier to just use a off-the-shelf inverter, rectify then filtered PWM so it can drive the 3kW element with 1kW and not freak out the inverter.  But that creates the need to separate out the connection to the thermostat so it doesn't have to switch full DC load. But it needs a MCU anyway to do the PWM so pretty easy to add an extra input. Probably do more than one temp cutout just for safety. 

I guess it could be inverter and the two stage pwm, first pwm does mpp and runs at like 1khz,  2nd pwm runs at 50hz with no filtering. Then the thermostat would be happy and wouldnt have to do any rewiring of the cylinder.
« Last Edit: June 07, 2023, 04:16:49 am by Psi »
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Offline Ed.Kloonk

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Then just use the triac method. I cheated and used that common arduino ebay light dimmer module with zero cross detection and substituted the 8A triac with the BTA41-600B Triac 600V 40A remoted on a giant bit of aluminium. Snubber builtin is iffy but circuit worked with 4.8kw element, 2000w inverter (that is really max 1500w   >:( ).

Just looked the 1.8kw element was fed about 4kwh @1kw today. The inverter can and does power computers and whatnot up to 400w sometimes as well so the waveform isn't -too- bad. Apparently. :-\

 :-+
« Last Edit: June 07, 2023, 07:20:21 am by Ed.Kloonk »
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Offline PsiTopic starter

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You're saying to just use a MCU controlled Triac dimmer on the 230V AC inverter output to power the 3kW heating element at 1khw and under for doing MPPT?

Hm.. yeah, might need an inverter with a higher surge current, but that's ok. They are usually a lot more than 1kW anyway.

Definitely quite a bit easier to wire up
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Offline Ed.Kloonk

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The software for the dimmer is already in the arduino lib. You call set_power(x) percent and there is a 100 int array that maps the power percent to timer wait-off time after the zero cross. So yeah, at 50% it turns on right at nearly the peak of the wave. After so long now, I still haven't done the sums on how well the supplied snubber matches the inverter impedance versus the grid it was designed for. Also to account for some inches of wire the triac is now away from the snubber. Does that even matter?

That's why I said a PWM would be a bit kinder but more complicated. I just set it up to see if it could be done not to be elegant.

The early iteration with the 4.8kw element and a rather small AGM battery, yes it often crashed when feeding 1.5kw and the shade blew over the panels. Until you get the algo tightened up with the proper power setting, your battery should be able to withstand the load until your controller decides how to respond.

Polishing the home-made algo that decides on the power setting is the tricky bit. Depending on how you detect the instantaneous output of the solar panels, the mood of the mppt and the charge level of the battery, hold your tongue right.

You'll get a good power result if you rely on the battery to pick up the slack and then recharge, but you prolly shorten the life of AGM batteries by doing severely that over many days and months.

I should point out that depending on the MPPT controller, ramping power up and down can drive it nuts. You have to decide whether it prefers a slow gradual ramp (up or down) or a stepped ramp (up or down). That's why, I think, the project suggested above included their own MPPT as well, as a closed loop. Bells and buckles.

Now, I'd probably use a Pi instead of the Arduino simply because of the ability to surf on in from anywhere to tweak the algorithm performance across four weather seasons and if you add more panels. Though my Arduino has Ethernet, you can't really change the code so easily, obviously.
« Last Edit: June 07, 2023, 11:02:24 am by Ed.Kloonk »
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Offline Ed.Kloonk

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I forgot to mention my algorithm listens to the serial output of the MPPT and specifically the MPPT state flag. Ramps up slowly with a j-curve and drops about 25% power when the MPPT flag raises. But of course, YMMV.
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