Total n00b question about microinverters and water heating

[Rednecktek]

So I'll be the first to admit that I don't know jack about microinverters other than they turn the panel's DC straight to AC like a normal inverter, they're just really small.

But I've read many posts about trying to get solar panels to run heating elements and having to match Ohms and panels to exact elements and sometimes it works and sometimes not and it seems to be a HUGE hassle to get everything just right.

Couldn't you throw a used microinverter onto a panel and feed the AC right to the heating element? That's what the elements were designed to take in the first place. Yes, watts would be low and such but wouldn't that eliminate the hassles of getting all the exact right panels to talk at the exact right resistances and such? Then you could just use a bog-standard off-the-shelf element and parallel up a couple panels and walk away for the day, right?

I assume I'm missing something here. Please be gentle.

 

[time2roll]

I thought those micro inverters were designed for grid tie. Could need a power source to activate so to speak. I could be wrong or there could be different models.

What I want to suggest is to stay away from resistance heating if possible. A hybrid compressor water heater will put 2 or 3 times the BTU into the water for the same electric input power.

 

[BentleyJ]

Microinverters are all grid-tie type meaning they do not produce output unless they are connected to a source of utility quality AC. One notable exception is the new Enphase IQ8, these are set up to be grid-tied do have an operating mode where they can form their own microgrid. According to the information I read, you will need a Smart Switch and most likely their Gateway. It could work but going to be expensive and inefficient. I agree with what time2roll said above, a heat pump water heater makes much more sense but would intolerant of voltage fluctuations so then at least a small amount of battery storage would be needed.
Probably better to just go with a direct solar water heater system. If you don't need super hot water maybe an inexpensive black rubberized pool heater is good enough. If hotter water is desired then a concentrator or enclosed tube type is best.

 

[Rednecktek]

I thought those micro inverters were designed for grid tie. Could need a power source to activate so to speak. I could be wrong or there could be different models.

So could a small solar system with a 100w PSW inverter "convince" the microinverters to work by providing that "activation source"?

I just see microinverter units on the 2nd hand market for pretty cheap more often than I see solar panels themselves and was wondering...

Not asking whether it Should be done, only if it Could be done.

No, I have no idea what I'm talking about which is why I asked. :D

 

[svetz]

If you're just looking for heat, you could save the cost of the microinverter and use the DC power off the back of the panel directly.

Not asking whether it Should be done, only if it Could be done.

Nope. Not directly anyway.
Microinverters have anti-islanding, that is they won't produce power unless there is an active grid within acceptable voltage/frequency parameters.

 

[BentleyJ]

So could a small solar system with a 100w PSW inverter "convince" the microinverters to work by providing that "activation source"?

I just see microinverter units on the 2nd hand market for pretty cheap more often than I see solar panels themselves and was wondering...

Not asking whether it Should be done, only if it Could be done.

No, I have no idea what I'm talking about which is why I asked. :D

Theoretically, your proposed system will kind of work BUT it would be highly unstable and most likely be unsatisfactory so I'm going to agree with what svetz said. "Nope, Not directly anyway".
To clarify. A grid-tie inverter can be induced to turn on with a suitable source of clean AC power. However, grid-tie inverters have no internal means of output control. In other words, they always try and produce as much, or little, electricity as possible with whatever solar radiation is available at that instant.
If a fixed load is connected then the system will only work when the load demand is PERFECTLY balanced with the sun intensity, angle, temperature Etc. Under any other conditions the voltage, frequency or current will go out of range (see UL1741 anti-islanding specs.) and the micro-inverters will just shut down if you are lucky. The other scenario is the 100W PSW has a bunch of extra current pushed back through it and gets smoked.
I would seriously caution you from pursuing this line of thought any further unless you are prepared to install a suitable UL1741 hybrid inverter and set up the system properly for AC coupling with the microinverters.

 

[Rednecktek]

I would seriously caution you from pursuing this line of thought any further unless you are prepared to install a suitable UL1741 hybrid inverter and set up the system properly for AC coupling with the microinverters.

Nope, no plans to pursue the idea any more than asking the folks here about it. It just made me curious so I asked.

Yeah, sounds like WWAWAYYY more hassle than it would ever be worth. OK, thanks all!

 

[newbostonconst]

If you're just looking for heat, you could save the cost of the microinverter and use the DC power off the back of the panel directly.


Nope. Not directly anyway.
Microinverters have anti-islanding, that is they won't produce power unless there is an active grid within acceptable voltage/frequency parameters.

An AC heater element running on 48 volt DC puts out about a quarter of the heating energy then its rating....and 24 volts will be half of that.

 

[OzSolar]

An AC heater element running on 48 volt DC puts out about a quarter of the heating energy then its rating....and 24 volts will be half of that.

Here's what Ohm's law is telling me. Cutting the voltage in half results in 4 times less power. Resistance stays the same regardless of voltage, right? Double check me please.

V​	240​	120​	48​	24​
R​	12.8​	12.8​	12.8​	12.8​
Power (watts)​	4,500​	1,125​	180​	45​

I have a mish mash of used solar panels laying around. I also have a few 240v water heater elements.

For a while now I've wanted to set up an experiment where I wire enough panels in series to get up to ~200v, hook them up to element in a bucket of water and see what happens. I'll report back with the the results.

 

[time2roll]

Here's what Ohm's law is telling me. Cutting the voltage in half results in 4 times less power. Resistance stays the same regardless of voltage, right? Double check me please.

V​	240​	120​	48​	24​
R​	12.8​	12.8​	12.8​	12.8​
Power (watts)​	4,500​	1,125​	180​	45​

I have a mish mash of used solar panels laying around. I also have a few 240v water heater elements.

For a while now I've wanted to set up an experiment where I wire enough panels in series to get up to ~200v, hook them up to element in a bucket of water and see what happens. I'll report back with the the results.

Also need the available amps or the voltage from the panels will fold down into the power curve. Need close to 20 amps to fully drive that element at 200+ volts.

 

[OzSolar]

Also need the available amps or the voltage from the panels will fold down into the power curve. Need close to 20 amps to fully drive that element at 200+ volts.

Of course. I'm thinking along the lines of typical old school SHW's which, give or take, need an entire day to fill up the 80G -120G water heater they are using for storage.

Using a 250w 60 cell module for easy math. Vmp of ~30v and 5 in series is ~150v. I theorize that the element would gobble up all 1250 watts for a few hours a day and you'd end up with a far simpler and less expensive solar water heater then the old standard SHW's with heat exchangers, antifreeze, piping to and from your roof and pump.

Ohm's law says that element will draw up to 1760 watts at that voltage. Seems like a nice match to me so I'm going to try it someday when it's not snowing and I'm not covered up with work. I've got a 1200g atmospheric storage tank tied to my wood boiler and radiant in floor heating system. That tank would make a nice place to put that heat.

 

[time2roll]

250w 30v is typical 8.3 amps. For a 12.8 ohm element need about 106 volts to be at 8.3 amps.

At 150v the element will take 11.7 amps and voltage will fold down with the panels possibly producing lower power. A bit self regulating. Should be a fun experiment to add-remove panels and see where max power is found.

 

[svetz]

If you want more heat per volume, they do make low voltage DC heating elements (e.g., 12V, 24V). As long as you're over the panel's voltage it should be safe to go direct. Although as was pointed out current changes via ohm's law and heat output changes via current (Power = I 2 * R).

 

[OzSolar]

250w 30v is typical 8.3 amps. For a 12.8 ohm element need about 106 volts to be at 8.3 amps.

At 150v the element will take 11.7 amps and voltage will fold down with the panels possibly producing lower power. A bit self regulating. Should be a fun experiment to add-remove panels and see where max power is found.

Ahh.... I wasn't seeing the whole picture. It had not occurred to me that the amps would not exceed the Imp of the panel, duh. Thanks for helping me see that!

 

[svetz]

The OP came away with the idea that microinverters can't be used off-grid as was revealed in another thread:

Now see, I had a whole thread about microinverters off-grid and I was told that microinverters required the grid to function and that you can't use them off-grid.

And that's probably because folks like me said:

Nope. Not directly anyway.
Microinverters have anti-islanding, that is they won't produce power unless there is an active grid within acceptable voltage/frequency parameters.

So, just wanted to take a minute to clarify what's what.

For any grid-tied inverter to work in an off-grid situation, something "else" must create a stable microgrid and control the off-grid inverter output. This is why everyone suggested alternatives to hooking microinverters directly to a hot water tank - it takes more gear making it more expensive (esp. when you could use low voltage DC hot-water elements meant for campers for a far lower price).

But, microinverters can work off-grid. You don't see that happening usually because the "something else" is roughly the same cost as the off-grid inverter, so total inverter costs end up being twice as much.

Back in the old days, you were on-grid or off-grid, and got the matching inverter. Through net-metering, grid-tied inverters enjoyed $ from the power company, but when the grid went down they had no battery backup.

To solve that, AC Coupling was invented for microinverters (DC coupling for other systems) - it allowed grid-tied systems to enjoy battery backup at the cost of a new inverter and some rewiring. These systems typically "control" the microinverters by altering their voltage/frequency. For example, if the voltage drops on the microinverter side, they go into anti-islanding mode and stop producing power. Control is vital as grid-tied systems are rather stupid and just try to push out every watt they can, which isn't pretty when you don't have a use for those watts. The most common control for them is via a relay (i.e., on/off); it turns them off when the battery is near full, and on when the battery isn't (this is very simplified, it takes a lot of engineering as there has to be enough battery head-room to absorb any surges when something in the home (e.g., AC) turns off while the microinverters are on).

Similar to AC Coupling, Tesla's Powerwall could flip grid-tied systems on/off, so it became another solution. Then came along the hybrid string inverters that could be on or off-grid.

Relatively new on the market are Enphase's IQ8s microinverters which work in off-grid or on-grid mode and can switch on the fly. They can even provide power off-grid without a battery when the sun is up. The Enphase IQ8s require other components (Envoy which is the "brains" of the system and the Enpower (ATS, NFT, + other stuff), so still not economical to power a hot water tank ;-)