In need of off-grid design guidance

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
12,152
It's possible, but I don't think this exists out of the box. I'm working on something that could be used to do just that, but it's in the early stages of development and definitely not something out-of-the-box. You need something that acts as a supervisor and has an overview of everything and can control individual devices. One possibly easier method would be to put the higher loads on a timer and make sure they don't overlap. That should be easier, using a socket timer for example.

<Is it possible to implement a control strategy to prevent larger loads from turning on if the inverter is highly loaded?>

Some things are easy. Have well pump circuit trip a relay that interrupts A/C thermostat, for instance. Two similar size motors, only one running at a time.

A current transformer on one branch (with resistor, output voltage proportional to current) could be used to connect a lower priority branch.

Max inverter wattage and available battery/PV are two separate things to control.
Battery voltage could be a decent proxy for SoC of lead-acid.
Some inverters and some charge controllers have relays to control loads for that purpose.

Because mine do frequency-shift control of PV production, I could use a frequency measurement to control loads. PV is curtailed from 100% at 61 Hz to 0% at 62 Hz. I could enable loads 0% at 60.5 Hz to 100% at 61 Hz.


Water heater opportunistic heating is a good application. Larger tank, and enable element when surplus power is available. Maybe make continuously variable with a dimmer circuit.
 

cdnorth

New Member
Joined
Jun 29, 2021
Messages
35
<Is it possible to implement a control strategy to prevent larger loads from turning on if the inverter is highly loaded?>

Some things are easy. Have well pump circuit trip a relay that interrupts A/C thermostat, for instance. Two similar size motors, only one running at a time.

A current transformer on one branch (with resistor, output voltage proportional to current) could be used to connect a lower priority branch.

Max inverter wattage and available battery/PV are two separate things to control.
Battery voltage could be a decent proxy for SoC of lead-acid.
Some inverters and some charge controllers have relays to control loads for that purpose.

Because mine do frequency-shift control of PV production, I could use a frequency measurement to control loads. PV is curtailed from 100% at 61 Hz to 0% at 62 Hz. I could enable loads 0% at 60.5 Hz to 100% at 61 Hz.


Water heater opportunistic heating is a good application. Larger tank, and enable element when surplus power is available. Maybe make continuously variable with a dimmer circuit.
Great information, thank you so much! One more question that I keep thinking about. In Arizona, it's mostly sunny, I definitely don't want to tremendously oversize the battery bank (expensive) and obviously cloudy days are what we really size these for (days of autonomy and assuming that all loads run on battery for a whole day when most days some of these can be picked up after battery is charged and it's still sunny out). Does it seem reasonable to instead of adding more batteries, size your battery bank for 1 day of full usage and then have a generator backup to run loads and recharge the batteries? If the generator runs more often than I'd like, then consider adding more batteries. Is this logic faulty?
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
12,152
Yes, generator just gets used more often with smaller battery.
Adding new batteries to an old lead-acid bank isn't recommended, but probably not such a big deal for lithium.

PV is cheap now, so more panels makes running generator less frequent. If you size it for running A/C and don't need that on overcast days, should do well. But if you do need A/C, that requires generator.

My large system has 20kWh of SunXtender AGM, 14kWh usable to disconnect at 70% DoD. Those cost $5000. It is sized to last one night (many inefficient loads, not presently shed at night.) My PV can deliver the watts drained in a little over an hour, but charger is programmed for 0.2C.


Quality FLA (Rolls Surrette) would cost less and last longer, but need more care and feeding.
Commercial lithium costs more. DIY lithium costs less than lead-acid and should last longer.
But lithium batteries for home power are of variable reliability, Caveat Emptor.


 
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