Off grid refrigeration

[Tim C.]

I have been designing a system to provide refrigeration to an off grid cabin. As I calculate various options, I am leaning towards refrigeration without batteries. If I could have a solar panel run a compressor to chill and freeze water, the unit should maintain temperature. I could have panels connected to a buck converter to some capacitors and then an inverter. The inverter starts the compressor and it runs continuously as long as the sun shines.
Could such a design work? I worry about killing the inverter with frequent cycling.

 

[MisterSandals]

I could have panels connected to a buck converter to some capacitors and then an inverter.

I am not sure exactly how the capacitor would work.

Any reason in particular that you do not want to use a battery like everybody else?
In general, it would take a LOT of solar panels to have the backing amps to power a compressor.
Do you know what the surge amps are for the compressor? This will likely be the key to doing some
real math as to what it would take (that and how many watts and at what voltage it runs on too).

 

[Tim C.]

The largest reason for no battery is expense. I am guessing I would need 400 watts of panels and at least 200 ah lion batteries. This would run an apartment size residential fridge. Estimated cost $2000 and up for all of it. An inverter always needs to be on, making the system requirements larger.

The system I am thinking of would be $600. It would power the compressor from a mini fridge with start up around 200 watts. I think the capacitors would cover that.

The other advantage is there would be fewer moving parts. With the correct insulation and water once frozen, it could last a week or more at temp.

 

[Supervstech]

Will made a few videos with lithium batteries, solar controller and ac power supply feeding an inverter all under 700...
Panels of course would raise that price, but Craigslist is great for good used panels around $50/300W

 

[MisterSandals]

Search results for query: batteryless

diysolarforum.com

 

[Hedges]

Starting surge is probably 0.2 seconds long, 5x nameplate of the compressor. (refrigerator label would include defrost heater, so probably not 5x that.
A large capacitor could probably supply the surge. Question is if a SCC would "charge" and regulate voltage on the capacitor. Maybe.
You could use a car battery, just don't power loads after the sun goes down. That would help avoid having voltage draw too low under load, likely with a capacitor if you run fridge when production is low.

PV panels should have multiple angles to spread power out over the day. An acute 60 degree angle like 8:00 AM and 4:00 PM sun would produce relatively constant power over the day.

Inverter could be enabled on a timer. It should be pure sine wave, to be nice to the motor.

Fridge with top freezer should do a good job of riding through the night.
Use brine in bottles, tuned to freeze somewhat warmer than the freezer compartment gets.
(Usually, only fridge has thermostat, freezer gets cold while fridge is being cooled to setpoint.)

 

[BrianAZ]

At the moment I'm running my 7 cubic foot chest freezer with 4 deep cycle fla batteries, not sure what it draws at startup but it's only 80w the rest of the time. I have it on a power strip I turn off mid afternoon, turning it on each morning as soon as I see the panels putting out some amps. It runs for 4hrs or so to catch up, set to its coldest setting. I'm trying to get a second one to make a fridge but for now I am using an ice chest with old milk jugs I swap out to the freezer every few days. I think a small lfp battery would handle the start up no problem. Unfortunately chest freezers have gotten expensive and rare in the last year.

 

[BrianAZ]

What about a tiny lfp battery with high discharge rate? These are rated at 15a continuous and 50a pulse, with 8 of the little guys at 2.99 each in 4s2p?

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[Tim C.]

What about a tiny lfp battery with high discharge rate? These are rated at 15a continuous and 50a pulse, with 8 of the little guys at 2.99 each in 4s2p?

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Hmmmm. This could take the place of the caps. However I would then need a charge controller. Could this be used without a bms?

 

[Hedges]

Hmmmm. This could take the place of the caps. However I would then need a charge controller. Could this be used without a bms?

Nope.
And yes, you will need a charge controller anyway. At least, a voltage regulator.
Get an AGM, big enough to tolerate peak charge rate from PV.
Operate under float conditions, only run fridge when PV can supply needed power. Don't let battery cycle more than 15% DoD. A quality battery should then last 10 years.

 

[Rider]

I have a 3.5cf chest freezer for camping. A 100Ah AGM battery and a 1000/2000w inverter will run it for a minimum of 36 hours (normal cycling, not non-stop) without recharging. Once I connect the 270w solar panel, it will fully recharge even on a cloudy day, so effectively, it will run forever.

Disclaimer: I've only used it during summer months, haven't distance-tested it in the winter.

 

[MichaelK]

Do you REALLY want a cabin with no electricity? You don't want to have lights at night, you don't want to watch TV, you don't want to charge your phone?

For my own off-grid cabin, I have it set up like a regular 120VAC home, with an 18cuft refrigerator, LCD bigscreen TV, a computer, lots of light. All this with an inverter wired into my main electrical panel. It's left on 24/7, 12 months out of the year, and I've never had a problem yet. And when not involved in construction, or other energy intensive projects, I running it all on less that 3kWh per day!

Go to Costco and get four of their 6V golf-cart batteries, about 550$ with tax and core-charges. Wire them in series for a 24V system, then get 3-4 grid-tie panels to keep them charged (220$). An Epever 50A charge controller (220$). Power the cabin with a 1500W Samlex inverter (550$). Select a standard HomeDepot refrigerator with a really good energystar rating. You can have an almost indistinguishable 21st century lifestyle on a little more then about 2kWh per day.

 

[Tim C.]

Do you REALLY want a cabin with no electricity? You don't want to have lights at night, you don't want to watch TV, you don't want to charge your phone?

For my own off-grid cabin, I have it set up like a regular 120VAC home, with an 18cuft refrigerator, LCD bigscreen TV, a computer, lots of light. All this with an inverter wired into my main electrical panel. It's left on 24/7, 12 months out of the year, and I've never had a problem yet. And when not involved in construction, or other energy intensive projects, I running it all on less that 3kWh per day!

Go to Costco and get four of their 6V golf-cart batteries, about 550$ with tax and core-charges. Wire them in series for a 24V system, then get 3-4 grid-tie panels to keep them charged (220$). An Epever 50A charge controller (220$). Power the cabin with a 1500W Samlex inverter (550$). Select a standard HomeDepot refrigerator with a really good energystar rating. You can have an almost indistinguishable 21st century lifestyle on a little more then about 2kWh per day.

This is plan B. I would probably use lithium batteries because the long term costs are lower and they charge better. The location dictates I would need 4 days with low or no sun. This could be as much as 4 kWh. The inverter would need to be one with a sleep mode unless inverter refrigerators become available in the US. i could get away with 800 watt inverter. I estimated 300 to 400 watts of panels necessary. This option does give us a freezer but is much more expensive.

 

[MichaelK]

This is plan B. I would probably use lithium batteries because the long term costs are lower and they charge better. The location dictates I would need 4 days with low or no sun. This could be as much as 4 kWh. The inverter would need to be one with a sleep mode unless inverter refrigerators become available in the US. i could get away with 800 watt inverter. I estimated 300 to 400 watts of panels necessary. This option does give us a freezer but is much more expensive.

Why limit yourself to only 300-400W. My recommendation would be to start at 1000. That would only be about 250$ worth of grid-tie panels. Don't buy 12V panels. They are far too overpriced.

What I'm finding is that in winter, when I am at times away from my cabin, the daily drain is only about 1.2kWh total. That's with my refrigerator on 24/7. An L-16 bank would handle that much drain, and still be at 50% charge after 4 days.

Don't skimp on the inverter. I'd start at 1500W because anything with a motor will have higher starting surges. A 800W inverter may or may not handle the startup surge of a regular refrigerator.

 

[Metalophile]

This is plan B. I would probably use lithium batteries because the long term costs are lower and they charge better. The location dictates I would need 4 days with low or no sun. This could be as much as 4 kWh. The inverter would need to be one with a sleep mode unless inverter refrigerators become available in the US. i could get away with 800 watt inverter. I estimated 300 to 400 watts of panels necessary. This option does give us a freezer but is much more expensive.

I agree with MichaelK that I would go with a larger inverter than 800 watts. I am powering a 9cu ft and a 7 cu ft chest freezers in my garage using 1000 watts of panels, 24v, 200AH LiFePO4 home brew battery, and 2000 watt pure sine inverter. When one freezer's compressor kicks on my battery's BMS records a current draw of ~35A x 25 volts, or about 875 watts. Yes, maybe a 800 watt inverter can start a fridge or freezer if it has surge capacity, but it is a bit dicey, and you have to remember that your inverter-battery combination must be able to start the compressor pretty much every time without fail. Also, you may have other draws on the system - I'm sure you'll want to use the power for other things from time to time. I went with 2000 watts because - although rare there may be times when both of my freezers happen to start their compressors at exactly the same time.