Control Loads via Solar Panel Voltage

jeffs

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Aug 27, 2021
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I've been reading on here for a while but decided to create an account to post a question I've been thinking about for awhile.

I currently have a very small 24V system with 200W solar and 550wH of batteries. I'm using a Victron 100/20 MPPT and 500VA inverter. I'm building a house and moved in to a small part of it last winter with no electricity. After 5 months I bought and installed the above components and it's amazing how much easier life gets with a little bit of power and pressurized running water!

Anyways, next spring I would like to upgrade (if I can afford it) to a larger system which will run a fridge, kitchen appliances, well pump, etc. I plan to place a large water cistern above my house and pump into it on sunny days. I'm probably going to go with a 1/2hp or 3/4hp 3ph 240V well pump run from a VFD with 120V input because I don't want to set up a split phase system. I only want to run this pump when it's sunny outside. This is mainly a concern in the winter (Western Montana) because I don't get very many days with sun and I don't want to use batteries to pump water. I'm also up against the base of the mountains and lose some morning sun.

A couple options come to mind:

1) Use battery voltage/state of charge to control an input to the VFD to enable the pump. This would work but the pump would keep running after sunset until the battery was sufficiently depleted.

2) Same as 1 but add a timer so it can only run during the middle of the day. This has the same limitations though because if the battery is charged it will still pump even if not sunny.

3) Monitor the solar panel voltage and enable the pump when it is above a certain threshold. I understand that panel voltage varies with charge controller load but based on my observations, it seems that the more intense the sun, the higher the voltage.

4) Same as 3 but buy a tiny solar panel in the 5-10 VOC range and use it as an irradiance measurment device to control the pump circuit.

Has anyone tried this? Is it a dumb idea? Do you have any ideas on how you would actually implement this? I would also need a way to stabilize the output because I would like the pump to stay on for a minimum amount of time even if a cloud passes.

Thanks!
 

efficientPV

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Sep 24, 2019
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I'm going to be working with some OPT101 that give irradiance in a 0-10V range. Even a small garden lamp panel will work. It has to be loaded with a resistor to give a linear range. That will give a probable good time to run a pump, but won't indicate battery state.

Panel voltage is a good indicator of charge condition. Use it as a trip point to an interval on timer for a limited on duration. Once it starts it keeps running till it times out. Prior to the end of the on duration check again to either continue or stop till panel voltage increases again.

I think timed periods is the way to go to protect the battery, I even run a fridge that way so the battery can be minimal. What is percentage of the pump watts to the array watts?
 

jeffs

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Aug 27, 2021
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Yes I'm less interested in battery state than I am sun exposure. I want to pump water whenever the sun is out so I don't run out of water in the winter (though I expect I will have at least several weeks worth of water storage in a 1500 gallon tank). I like the idea of a small solar panel to determine when to run the pump. How would I size the resistor to the panel? Am I just trying to dissipate its rated load?

I probably won't run the fridge in the winter. I'd like to keep my small inverter set up so I can use it for lights and small loads and only switch the large inverter on when I need it. In the winter I might just pipe cold water/glycol from outside through the fridge or use the root cellar I'm planning to build. Question on your timer though, do you have enough thermal mass in your fridge to just run it during the day? That does sound appealing to me.

I'm thinking I will probably have about 2400W solar and I expect the pump to draw around 700W so I can still charge the batteries while it is running.
 

efficientPV

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Sep 24, 2019
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It doesn't have to be a very big panel, load it till the voltage drops in half when the ratings are unknown. like with garden lights If you have a 10W panel it could serve a dual purpose. One of those digital relays draws about 10ma and 25 when the relay kicks in. A 5W 12V panel could supply 250-300ma. A resistor 60-100 ohms would be enough load and it would also power the voltage relay and give a nice operating range of 9-18V and you wouldn't have a vampire load all night.

I just tried this setup and it looks like these controls do not have an under voltage lockout. A very slow rise in voltage can cause the microprocessor to lock up. Some analog boards might work in a self powered system. A 5V relay may be suitable with a zener in series giving just a couple volt range within a 12-16V range. Diodes could change tis by 1/2 volt jumps.

I do run my chest fridge only in the day and store cold in liquids. That keeps it between 32 and 40F. The bottom remains frozen and it is highly stratified. So much that I have to run a 12V computer fan all day. I sense when my battery is over 13.3V, a good indication there is sun. I run a whole house on a car battery and I have hot water, dishwasher with heated dry, a large clothes washer and something to keep me alive at night. So, I can say it is reliable. I just wanted to see if it can be done with proper energy management.
 
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jeffs

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Aug 27, 2021
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That's pretty amazing that you can run all that and just have a car battery in the system. I'll have to experiment with loading a small solar panel but at least it sounds like it's possible to use it as a sun sensor.

I'm probably going to go with a small upright freezer as a fridge just for convenience. Hopefully it doesn't stratify quite as much. Thanks for the ideas!
 

efficientPV

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Sep 24, 2019
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Things will probably change next year. This area has changed, I'm going think about air conditioning next year. I just might buy an electric vehicle and use it as storage. I'm also adding more east panels so I will have to store power. Can't put it all into hot water.
 
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