Wanted, MPPT inverter that will keep running and just lower AC output voltage.

[Psi]

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.

[fourtytwo42]

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) ?

[mikeselectricstuff]

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.

[tszaboo]

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

[Psi]

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.

[Psi]

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.

[tszaboo]

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

[janoc]

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.

[Psi]

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

[janoc]

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.

[mikeselectricstuff]

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

[Psi]

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.

[Psi]

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.

[fourtytwo42]

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 
 

[janoc]

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

[Psi]

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

[Psi]

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

[Ed.Kloonk]

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.

[fourtytwo42]

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.

[fourtytwo42]

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  so not for the faint hearted unless you have deep pockets and are not of a nervous disposition  (loud bangs etc)

[Jeroen3]

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/

[Psi]

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!

[Psi]

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?

[Psi]

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.

[fourtytwo42]

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!!