Moving my family- I need some thoughts on solar to keep food when grid goes down

inthejungle · August 2016

westbranch- thank you that is a good number to add- What do you think?

If power goes off we wait and don't open the fridge for 2 hours lets say. Then we turn on the fridge for up to 6 hours. This would enable us to have a smaller battery bank almost half the size. This gives us up to 8 hours of backup power for the fridge. Only thing that I wonder about in doing this, is this going to make the fridge work harder to cool it down and use more power in waiting. What do you think?

Dave Angelini · August 2016

You have the luxury of not having to go for more than 12 hours without the grid. Can you get that 500 AH battery? Keep in mind it needs to rated at the 20 hour rate to be meaningful for design. Good luck!

Marc Kurth · August 2016

@inthejunglewhat: Take a long hard look at how long you expect to be there and how many ":no power" days you are designing for. Then balance your "wants" with your budget and whether you will transport the batteries when you leave.

For example, if a given battery is rated to give you 1,000 cycles at a 50% depth of discharge, and that will happen every day - then you will cycle that battery to death in 16 months 2.7 years. If you cycle it every other day, then 32 months, etc. twice that, etc.

So if you decide that you will be there for 5 years, and power failures a daily occurrence - that means 1,825 charge/discharge cycles. I sure as heck wouldn't design for 50% DOD! If power failures occur every other day, 5 years will be under 1,000 cycles so 50% might make sense for that battery.

On the flip side, if you are there 3 years and power failures occur only every three days then it's way under 500 cycles - which coincides with an 80% DOD for many batteries. So, selecting a battery bank for 50% may not be the best decision if you are at an economic break point. (Like buying a lousy inverter to save money for batteries)

NO, I'm not saying to size battery banks at 80% DOD. I am suggesting that there is a balance somewhere. If the 500 cycles and a 3 year life requirement were MY system, I might accept a 60%-70% DOD if going to 50% required a lot more battery money! Remembering that you really expect 6-9 hour power outages, but your 250 amp hours based on 10 hours of compressor run time. (6 hours is 150 AH, and 9 hours is 225 ah)

The batteries that I am most familiar with have a 5%-10% higher amp hour capacity rating when they are appreciably above 77 deg F but also a decreased lifespan. With a good efficient AGM design, you need to return 102% to 110% of the energy that you withdrew.

Another point to consider is that having a battery that will happily accept a very high charge rate can be valuable in this type of application. Think about back-to-back power failures with limited hours for charging. An inverter-charger with a 100-120 amp charger would be advantageous with a 400+ ah bank, with the right batteries. That kind of charge rate will ruin some batteries, so do your homework!

I would never suggest under sizing a battery bank, but what does under sizing really mean? In a hot climate, it is quite possible to lose your batteries from heat, before you have reached their cycle life limit. So, always using 50% DOD isn't always the best economic model!

Anyone here please feel free to correct any errors! This is intended to an open discussion not a lecture. (Yes, I used some over simplification because this about helping new people understand the concepts)

Marc

Edited to fix basic arithmetic error caught by vtmaps. (Thank you)

vtmaps · August 2016

Marc Kurth said:
For example, if a given battery is rated to give you 1,000 cycles at a 50% depth of discharge, and that will happen every day - then you will cycle that battery to death in 16 months. If you cycle it every other day, then 32 months, etc.
<snip>
Anyone here please feel free to correct any errors!

1000 daily cycles is closer to 32 months than 16 months. --vtMaps

Marc Kurth · August 2016

Good point! My error - thank you for pointing that out. Fixed it.

Dave Angelini · August 2016

inthejungle said:

westbranch- thank you that is a good number to add- What do you think?

If power goes off we wait and don't open the fridge for 2 hours lets say. Then we turn on the fridge for up to 6 hours. This would enable us to have a smaller battery bank almost half the size. This gives us up to 8 hours of backup power for the fridge. Only thing that I wonder about in doing this, is this going to make the fridge work harder to cool it down and use more power in waiting. What do you think?

Keep in mind that the fridge is cycling right? It is not always on even after power goes off. It is not like it is a warm box being cooled.
How big is the LG and do you have alink to the model you can get?

inthejungle · August 2016

Thank you for all of your help, we have two seasons hot and cool, in the cool power is almost never a problem from the gird. In hot season we can have every other day outages for 1-2 or longer, then sometimes we may go a few weeks and then have an outage. It is hard to tell.

Here is the information on the fridge

LG Model: GN B272SLCL and I will attach the LG spec sheet I found for that compressor.

westbranch · August 2016

I might have missed it but I did not see that model in the tables of the attachment...???

Casey · August 2016

I live off-grid in Mexico and have a 425L/16 cu ft LG smart inverter compressor fridge (GT46BGP). I've been meaning to post a report here on it. I don't know which exact compressor model it has, but mine being bigger I'd assume yours might use less power. We've been using this fridge for almost 2 years now and love it a lot. Our previous fridge was propane, and it sucked.

Typical power draw is 66 watts, but sometimes it draws roughly 100, 130, or 170 if I fill it with warm groceries. Duty cycle seems to be about 50%, similar to most fridges,, although I've never actually timed it per se. The startup is smooth and gentle, taking a few seconds to ramp up to the operating watts.

Defrost cycle is where a lot of power goes, 250 watts for roughly 20 minutes. Unfortunately it does this whenever it feels like it and not during sunshine like I'd prefer. I'd imagine that it's possible for the fridge to defrost several times per day, especially if you open the door a lot, and/or you have very high hunidity.

Over the winter months the fridge was averaging 0.75 kwh per day,. In the summer we're averaging 1.5 kwh per day but here it's 98F/37C by day and not too much cooler at night.

I hope those numbers help you out. I'm pretty sure my fridge would run fine on a quality 500w pure sine inverter, but mines on an Outback 3600w/48v.

softdown · August 2016

Chest freezers are the most efficient because the dense cold air does not fall out when the door is opened. You could get a separate thermometer controller for about $45 (mail order) as memory serves me. This allows the freezer to run at refrigerator temperatures. This has been discussed on the board....i.e.: Convert a chest freezer into a high efficiency refrigerator.

With this high efficiency "refrigerator"....you could operate at 1 degree C and now have a "cold buffer" for a couple hours. Food and supplies last longer as well.

Got to thinking in this direction because I thought your location might be quite warm.

just starting · August 2016

I have a freezer that I converted with a digital temp control it allows me to adjust the temp to what ever I want I worked great as a fridge now it's a freezer again. It was like twenty bucks from eBay .

softdown · August 2016

This is the most suitable option that I have found to date. Wanting a unit that is pre-wired for several reasons.

I found a number for much less. They appear to have a female Euro type power receptacle. Odd considering the units power source is a US type plug.

We discussed this option a year ago. Prices were higher and options were lower. We were also looking at pre-wired units at the time.

https://www.amazon.com/bayite-Pre-Wired-Temperature-Controller-Thermostat/dp/B01KEYDNKK/ref=sr_1_19?ie=UTF8&qid=1471823785&sr=8-19&keywords=pre+wired+temperature+controller

softdown · August 2016

For $25: https://www.amazon.com/WILLHI-WH1436A-110V-Temperature-Controller/dp/B00V4TJR00/ref=cm_rdp_product

jonr · August 2016

How long a refrigerator or freezer can go before an unacceptable temperature drop is highly dependent on the thermal mass in it. So fill all unused space with jugs of water (refrigerator) or water + some antifreeze (freezer).

Would be interesting to know if the defrost on the LG could be put on a timer such that it only occurs during sunlight hours.

Dave Angelini · August 2016

LG knows about the defrost Jonr. The smart phone app they have is suppose to incorporate this on the next factory revision.
Their US 22 cu ft version is really nicely layed out my female customers say. It goes on sale for arround a grand several times a year and no more surges and a high end model with good low energy useage.

Moving my family- I need some thoughts on solar to keep food when grid goes down

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