Using Grid Back-Feed Capability for Water Heater Dump Load

[SunFarmer]

This is long-ish, please bear with me. Thank you.

Has anyone ever tried using the grid back-feed, or grid-export functionality in an inverter to power a dump load?

First some background, then I’ll explain my idea.

I built an off-grid solar power system using an EG4 6000EX inverter. I now have two months of experience living with the system and I come to realize that I’m leaving a lot of untapped power on the table, even in winter. Many days my 25-kWh battery bank is fully charged my mid-day, then the panels are idle for the rest of the day. Therefore, I’m exploring way to harvest more of the available power.

I’ll probably never be able to sell power to the grid, although wish I could. My local utility requires an electrical permit and passed electrical inspection. This is a DIY system so I have neither. Also, my EG4 6000EX inverter is not certified in the USA for grid-tie.

In month or two, when budget allows, I’m upgrading our domestic water heater to Rheem Performance Platinum Hybrid heat-pump water heater, and of course run it with solar electric. This alone will help me harvest more solar power. Then I want to use my old electric water heater for a dump load.

The inverter has ac input and ac output. The ac output is connected to an off-grid sub panel for critical loads. The ac input is connected to my main panel for backup power via the inverter’s built-in ATS. If I could export power to the grid, the juice would go out via the ac input line.

Here's my idea: I'm thinking of disconnecting the ac input from the main panel and connecting it to the old water heater. Then enabling the forbidden solar energy back-feed to grid setting (program #9). I have a manual method to fail-over to the grid if there is more than 2-3 days of clouds. And as mentioned, I can’t sell power to the electric company, consequently I’m okay with disconnecting the AC input.

I have the inverter output source priority set to Solar/Battery/Utility, where solar first powers the loads, then charges the battery, and whatever is leftover is sent it to the utility. So, I’m thinking that when the battery is fully charged, the inverter should “export” power to the water heater. The switch over to dump load would be handled by the inverter without the need for any external relays or other equipment.

Anybody ever tried this?

I’m first going to experiment with small load connected to the ac input to see if it works.

Thanks in advance for everyone’s sage advice.

 

[400bird]

Seems simpler to use a really or SSR to power on the dump load when you have excess solar. Depending on your inverter, BMS, etc you may already have an available output that could trigger the relay at high battery voltage or SOC.

 

[zanydroid]

You don’t need to use the AC input for that. You can use a PLC or other programmable device to poll the SOC and turn on the dump load with a relay of some sort. The dump load would be connected as a regular branch circuit

Modern HPWH actually have multiple proper interfaces for doing this. You might even be able to instead turn the HPWH into resistive mode and use it as a dump load, if the compressor is not drawing enough. And then when you no longer have a surplus of solar switch it to HP mode.

 

[timselectric]

AC input won't export power unless it sees a grid.

 

[SunFarmer]

AC input won't export power unless it sees a grid.

Timselectric, thanks for the note.

When you say "sees a grid", do you mean that the inverter has to detect a 60Hz sinewave?

If so, I wonder if there is any way to spoof it? Maybe a couple of megaohm resisters to couple the sinewave, but not any current.

 

[SunFarmer]

Seems simpler to use a really or SSR to power on the dump load when you have excess solar. Depending on your inverter, BMS, etc you may already have an available output that could trigger the relay at high battery voltage or SOC.

Does anyone know how to use the dry contacts on an EG4 6000EX inverter to trigger a dump load?

As described in the pathetic manual, the dry contacts seem to be for starting a genset to recharge the battery, the opposite of what I want. According to the manual, with the inverter set to SBU, the contacts will engage when the battery voltage drops to 46V and disengage when voltage reaches 54V. The manual also states that when the battery type is set to "EG4" the values for programs 20 and 21 will been shown as a percentage, but it shows voltage. It's stupid errors like this that make me suspect of how much of the rest of the document is incorrect. But I digress...

Thanks in advance for any input.

 

[zanydroid]

If so, I wonder if there is any way to spoof it? Maybe a couple of megaohm resisters to couple the sinewave, but not any current.

There's clever anti-islanding stuff that will rebel against your attempts at basic spoofing. It will also attempt to push into the grid and monitor things like voltage and response to shifting the current phase a little bit to see if it responds like a huge grid, or a small set of loads. It's a whole cottage industry of research.

If your inverter looks confusing wrt how to configure for that, chances are some other controller would be equally or much easier to set up. I tried to look at some EG4 documentation this weekend and immediately regretted it.

Have you searched for dump load controller in the forum? There are quite a few threads.

You can check, EG, Solar Assistant documentation for EG4 6000EX to see what kind of telemetry is available to an external controller.

 

[solar8484]

Here's my idea: I'm thinking of disconnecting the ac input from the main panel and connecting it to the old water heater. Then enabling the forbidden solar energy back-feed to grid setting (program #9). I have a manual method to fail-over to the grid if there is more than 2-3 days of clouds. And as mentioned, I can’t sell power to the electric company, consequently I’m okay with disconnecting the AC input.

I have the inverter output source priority set to Solar/Battery/Utility, where solar first powers the loads, then charges the battery, and whatever is leftover is sent it to the utility. So, I’m thinking that when the battery is fully charged, the inverter should “export” power to the water heater. The switch over to dump load would be handled by the inverter without the need for any external relays or other equipment.

Plenty of people use immersion water heater as diversion load for excess solar power but your idea won't work safely because it doesn't address situations like over-temperature condition in the water heater that requires shutoff/curtailment of excess solar power production. Your water heater can be viewed as a thermal energy battery with finite capacity to store energy similar to to your electric energy battery.

 

[timselectric]

When you say "sees a grid", do you mean that the inverter has to detect a 60Hz sinewave?

It looks for correct voltage and frequency.

If so, I wonder if there is any way to spoof it? Maybe a couple of megaohm resisters to couple the sinewave, but not any current.

I wouldn't even attempt that. Too risky.

Does anyone know how to use the dry contacts on an EG4 6000EX inverter to trigger a dump load?

They can be set to activate and deactivate at specific SOC's.
This is the best way to do it.

 

[Texican]

what you are wanting to do, is to power the dump water heater from your off grid systems unused potential.
do you need it automated ?
if not, you could just wire the heater to your critical panel and flip on the breaker whenever you want

if you want automation, you may want to start using Home Assistant, I would think that some type of controller/switch must already exist that folks are using automations on
if you can get the dry contacts working based on SOC, that may not give you all the control you want

some folks have changed the heater element to lower power ones and I think just feeding 120v power is also sometimes used to lower the power sent to a dump load so it can run more frequently using excess power

things to consider

 

[myles]

You don’t need to use the AC input for that. You can use a PLC or other programmable device to poll the SOC and turn on the dump load with a relay of some sort. The dump load would be connected as a regular branch circuit

Modern HPWH actually have multiple proper interfaces for doing this. You might even be able to instead turn the HPWH into resistive mode and use it as a dump load, if the compressor is not drawing enough. And then when you no longer have a surplus of solar switch it to HP mode.

This is going to be one of the more robust ways of doing it, PLC's are used everywhere commercially for this type of automation.

Does anyone have a direct link to a plc that he could use helping him out? My PLC knowledge is lacking, just my understanding of them is somewhat coherent.

Also I don't think you mentioned your battery and bms setup, that might help us figure out if your bms has any switching capabilities to close contacts on a relay that would divert power to a subpanel with your "Smart loads". i've done exactly this with the sol ark, but it has that capability built in.

 

[Texican]

to make a dump load smart it needs more info than battery SOC
how will it know what the "potential" pv can provide ? and the answer is always changing
you dont want to be draining the battery when a cloud goes by, or buying from the grid when a cloud goes by

hence the need for a brain of some kind with a way to know both "potential" pv and also current loads
using items like solar assistant. home assistant. iotawatt/emporia monitors make creating a brain possible
decisions need data

 

[myles]

to make a dump load smart it needs more info than battery SOC
how will it know what the "potential" pv can provide ? and the answer is always changing
you dont want to be draining the battery when a cloud goes by, or buying from the grid when a cloud goes by

hence the need for a brain of some kind with a way to know both "potential" pv and also current loads
using items like solar assistant. home assistant. iotawatt/emporia monitors make creating a brain possible
decisions need data

What I ended up doing on the sol ark is using voltage to trigger the smart load not SOC %. So example, If you have your bulk charge set to 55 volts, and your float set to 53.9 volts, you can say to the smart load do not come on until voltage is above 54.5 volts, and then turn off when voltage hits 53.6 volts, this gives you a window where the load can come on only when powered on solar, as soon as the large loads starts on the battery it will shut off. I am sure the sol ark is doing more than just that with the smart load but I do not know all the inner workings of how it is activated.

I suggest a larger window to help cover the cloud cover issue.

 

[David in nh]

What I ended up doing on the sol ark is using voltage to trigger the smart load not SOC %. So example, If you have your bulk charge set to 55 volts, and your float set to 53.9 volts, you can say to the smart load do not come on until voltage is above 54.5 volts, and then turn off when voltage hits 53.6 volts, this gives you a window where the load can come on only when powered on solar, as soon as the large loads starts on the battery it will shut off. I am sure the sol ark is doing more than just that with the smart load but I do not know all the inner workings of how it is activated.

I suggest a larger window to help cover the cloud cover issue.

this is today the red line shows' smart load kicking to heat water in this case about 5000 watts this is on solark 12k

 

[Texican]

not familar with reading the sol ark graphs.
did turning on the dump load, ever cause the batts to start draining or cause the grid power to be used ?

 

[myles]

not familar with reading the sol ark graphs.
did turning on the dump load, ever cause the batts to start draining or cause the grid power to be used ?

you can see the purple is the battery, red is load, green is PV, the light blue line is the soc %. it maybe caused the battery to start draining for a few minutes, then solar kicked in again, then battery again till the cutoff presumably.

 

[David in nh]

you can see the purple is the battery, red is load, green is PV, the light blue line is the soc %. it maybe caused the battery to start draining for a few minutes, then solar kicked in again, then battery again till the cutoff presumably.

just took this snap shot water heater just shut off

 

[Texican]

why did the heater turn off ?
cause it reached max temp ?
or cause batts were draining pretty good looks like, even though it looks like good pv power at the time ?

 

[myles]

why did the heater turn off ?
cause it reached max temp ?
or cause batts were draining pretty good looks like, even though it looks like good pv power at the time ?

If batt is going down that means charging. Looks like water heater hit temp cutoff

 

[David in nh]

in this case it kicks around 54.5 volts and must have to 5000 watts of solar to start

yes the tank is now hot , it takes about a hour to go from basement temp to hot , nobody is home so no usage for the moment