I want to power my fridge or freezer with solar and batteries

[cujet]

I live in South Florida in a rural home. I've built my own solar panels and dabble in all things energy related. I'm a capable machinist and fabricator.

However, this is a very specific task. I have a modern fridge and a small chest freezer (5 cu/ft). I'd like to power one or both with solar. Simply because I can. But also so I don't have to worry as much about my food going bad when I'm away if the power goes out. I can afford to purchase the components and I'd like to consider both a "best" setup and a "budget" setup.

I've determined the energy usage on both the fridge and the freezer. The fridge consumes a maximum of 1.37KWH per day. The freezer, much less, at around 250 watt/hours per day.

So, with this in mind, I'd like to purchase some reliable panels, an inverter (or combo charger) and possibly batteries. I do get plenty of 1.5 year old, used 24 volt, 40 AH SLA aircraft  batteries. Obviously, the aircraft batteries are not ideal, and not new. So there will be inefficiencies there that could be overcome by more batteries and panels. Or, I could simply purchase some quality batteries.

I'd like suggestions on what products to choose. There really is a stunning array to pick from.

If possible, I'd like the system to charge the batteries from the grid too. But, maybe that is simply an external charger.

My plan is to simply install the inverter in the garage and run a dedicated 115V line to the fridge, above the current outlet. That way, I can pick between the two. Simple, clean and easy.

Thanks for the input!

[ghurd]

"Best" and "Budget" are worlds appart on this one.

Best goes to a very expensive inverter.  PSW, probably with either 24 or 48V input.  The inverter may cost more than the required solar panels.

Budget is a large 12V inverter from a real company that sells stuff in more places than ebay.
Budget pretty much rules out 24V because of a lack of (US type) inverters that are both decent and affordable.
But that means you can not use the free 24V used batteries.

"There really is a stunning array to pick from"... and there is a stunning array of potential answers.
I think you need to narrow down the options to get any good answers.
G-

[zap]

If you haven't seen this yet, it's a chest freezer converted to a refrigerator and uses 0.1 kWh a day.
http://mtbest.net/chest_fridge.html

[Bruce S]

AND to muddy the water even further 
You could settle on instead of buying a 24Vdc inverter, go with a APC or similar UPS that is 24V batteries internal. the 1500VAC units and above have 2 12V batteries inside.
They have been around long enough you may find them in working condition on ebay.
The internal batteries can then be removed and the aircraft batteries used in conjunction with the panels. Less efficient than inverter, BUT you would have a nice pure sine-wave for the fridge.

Just a thought;
Bruce S
 

[wolfie]

and as you are in florida, you will want to check out sunelec

http://www.sunelec.com/index.php?

their panels are extremely inexpensive, it is the shipping that is high, they are located in miami

[cujet]

Thanks guys!

I have one of those big old APC backup's with dual 12V, 18AH batteries. I removed the batts and set it on top of a Concorde aircraft 40AH, 24V battery. I simply drilled and tapped the Elcon connector pins and used #10 bolts! Works great. But, it's not up to the task of the fridge. Nor do I believe it's pure sine wave.

I will go with 24V as a minimum voltage. I noticed that Zantrex sells pure sine wave inverters that cost far less than they used to.

I'd love some suggestions as to what inverter, what panels and what batteries (as I may use my aircraft batts for other projects).

I suppose the only requirement is that the setup fit neatly and professionally in the corner of my garage.

[cujet]

I suppose where I could really use help is picking the size, number of and brand of panels. And, knowing my power requirements, battery capacity, to prevent killing the batts. Should I go with 3ea, 200 watt Sharp panels? What depth of discharge should I run with the old aircraft batteries. How about with modern "off grid" solar system batteries.

And of course, an inverter suggestion!

I was thinking about 20% depth of discharge, or less.  So that would require about 300AH at 24V. Not particularly difficult. Or will batteries last longer with somewhat more/less discharge?

[ghurd]

The question is a lot like "Ginger or Mary Ann?".

Fridges have a notoriously huge start up surge current.
Some guys do OK with a 1250W inverter, some with a 1500W, some with a 1750W.

Some new fridges have fancy control circuits that do not work from MSW, or so I heard.

Many of the physically smaller PSW inverters do not seem able to deal with much surge for very long.

600W of solar in Florida should handle the 1600WH of load per day with no trouble.

The bank should be sized for about 3 days worth of load, without any charging.  That's a generic rule of thumb.

Best to try to keep the batteries above 12.2V / 24.4V / 48.8V.  That's about 'half' charged... or 'half' dead?

Combining 24V 40AH, 'half', and 3 days total capacity, that's about 10 of the used batteries.

Something to consider is the expansion cost, if it is done now.
Once it gets "Big", bigger is not a whole lot more money.
4 panels is not a whole lot more money than 3 in the big picture.
The cost of a larger controller if it is needed is not much different (%), and it may not need to be bigger.
Wire cost difference is not much.
Inverter cost is the same.
Fuses, disconnects, etc, cost the same.
Your used batteries are cheap.
Etc.
Meaning an 800W system may not cost much more than a 600W system, and will be able to provide considerably more power every day.

Do your homework on mounting them.  Get the PVs up in the air with lots of free air circulation under them.
Breaks my heart to see $1000's of $ worth of PVs flat against a roof because they will burn the cells.
G-

[kenl]

Thanks guys!

I have one of those big old APC backup's with dual 12V, 18AH batteries. I removed the batts and set it on top of a Concorde aircraft 40AH, 24V battery. I simply drilled and tapped the Elcon connector pins and used #10 bolts! Works great. But, it's not up to the task of the fridge. Nor do I believe it's pure sine wave.

I will go with 24V as a minimum voltage. I noticed that Zantrex sells pure sine wave inverters that cost far less than they used to.

I'd love some suggestions as to what inverter, what panels and what batteries (as I may use my aircraft batts for other projects).

I suppose the only requirement is that the setup fit neatly and professionally in the corner of my garage.

 If your talking about the new SW inverter I don't think it come in 24vdc

kenny

[don1]

Cujet,
  Just wanted to throw my hat in the ring too.
 How many amp hour are these batts?  Aircraft  batteries are generally not considered deep cycle and can't take to much discharging and charging.  Now I am only farmiler with privet plane type batteries and they in an effort to keep the weight down tend to be short on amp hours or not a lot of run time for your refrigerator.  I would think you would have to have enough battery so you would not take more than about 20%of the top. They can't be cycled like a deep cycle battery. And if your goal is to have a reliable system it might be a good investment to consider the right battries.
  Good luck with your project, Don

[cujet]

Cujet,
  Just wanted to throw my hat in the ring too.
 How many amp hour are these batts?  Aircraft  batteries are generally not considered deep cycle and can't take to much discharging and charging.  Now I am only farmiler with privet plane type batteries and they in an effort to keep the weight down tend to be short on amp hours or not a lot of run time for your refrigerator.  I would think you would have to have enough battery so you would not take more than about 20%of the top. They can't be cycled like a deep cycle battery. And if your goal is to have a reliable system it might be a good investment to consider the right battries.
  Good luck with your project, Don

The aircraft batteries are 42 amp hour ( C1, 1 hour rating ), 24 volt, absorbed glass mat. They are not "deep cycle" rated batteries. However, there have been a number of successful systems using these batteries.

Just an FYI, we are required to perform a capacity check on these batteries every year. If I were to take an average "new" Concorde 42 AH battery, I would see roughly 10% more than the rated capacity. If I were to check it after 1 year, all batteries will "meet" specs. After 2 years, many of them will fail to meet the 42AH rating. After 3 years, they won't meet specs, ever.



With this in mind, I change the batteries at 18-20 months, which is a practical, 100% of rated capacity, service life. This means the batteries, as removed are at better than 90% of the rated capacity. How this would go, over the years is anybodies guess.

So, I'm willing to consider real batteries and size them for long life. It really could be a lot more trouble free.

[bzrqmy]

Does this system need to use batteries?  Is it time to think about grid tie?  The Enphase microinverter system lets you start small with as little as a couple panels and expand the system as you budget allows. Each panel uses it's own microinverter that can be daisy-chained to each other.  You could accomplish running the fridge/freezer by offestting the energy they use by sending the power produced to the grid, making the grid your battery.  You could probably get into one of these systems for $2000 plus the cost of the contractor to do the connection.  Just a thought.

[Volvo farmer]

Does this system need to use batteries?  Is it time to think about grid tie?  The Enphase microinverter system lets you start small with as little as a couple panels and expand the system as you budget allows. Each panel uses it's own microinverter that can be daisy-chained to each other.  You could accomplish running the fridge/freezer by offestting the energy they use by sending the power produced to the grid, making the grid your battery.  You could probably get into one of these systems for $2000 plus the cost of the contractor to do the connection.  Just a thought.

A few modern 18CF refrigerators have broken the 1KWhr/day barrier. At .20/KWhr (higher than almost anyone really pays in the US) a refrigerator costs about $70/year to run.  At $2000 for an Enphase grid tied system, payback would be on the order of 25 years.

[bzrqmy]

Noone does RE when grid power is available for cost savings.  He stated that he wanted to do it because he could.  I assume it  means that he has the means to spend $$ on this project because he can and to have the bragging rights to say so.  It will take me 25 years to pay back what I have spent on my wind turbine.  But I am the only one I know that produces his own power. 

[zap]

But I am the only one I know that produces his own power. 

Hi, I'm zap and I produce my own power.  Good to meet you.

Say hello to Volvo farmer, he produces his own power.

Others here also produce their own power and might chime in

[TomW]

Glad to meetcha.

Produce 161 KWH last Month.

Not off grid but probably could be if they quit supplying it.

Tom

[Tritium]

Hey Tom I was close to that at 143KwH.

Thurmond

[Madscientist267]

This whole game is all about efficiency. Sure, you can throw a bunch of money at it, and brute-force your way into making it work, but where's the fun at in that? 

I have been tinkering with ideas on uber-efficient cooling for refrigeration purposes, and have really found that the real kick in the head with a Carnot system is the compressor startup surge.
 
Aside from that, I like the idea of that 100Wh/day freezer -> fridge conversion Zap! Gotta admit, that one really surprised me. Lucky me, I just happen to have that thing's little cousin to try the idea out on. Thought about it myself actually, before anyone mentioned it, but didn't know exactly what the gains would be, and hadn't yet bothered to try. Apparently much more than I ever imagined. Didn't I see something like 90 seconds of compressor time per hour? WOOHOO!

The Peltier thing is working very well in the Coleman cooler (2 cu ft), and I thought I was doing very well with that, but the math makes it half as efficient as dude's compressor version, and that thing is more than 4x the size of the Coleman!

It's that damn surge that I can't seem to find a suitable means around... I have a 750W MSW (Schumacher) (1500W "surge") that wants nothing to do with the compressor in my 5 cu footer. Not saying it couldn't be done (for a reasonable compromise), but it's an irritating attribute to say the least!

The LRA on the freezer's label is 12 and some change amps; combine that with the fact that it's MSW, and it aint happening with that inverter. Thats 1500W+ (and that's being 'nice') for a PSW inverter!. Much more for a motor with MSW.

Peltier module cooking along at 9W 24/7? No startup surge, and cools to ~55F (good for soda, beer, whatever... wouldn't use it for perishables). Still working on a reasonably efficient design for these that can reach a deeper cool. Got all kinds of pans in the fire on that one, but some major hurdles in taking it to the next level.

So either way, there are challenges to overcome. Something else to consider: The inverter required to be able to start a compressor will have a higher standby current than the one that could handle keeping it running, which can add rather significantly to your overhead budget. So you also have to compensate for that in the design of your system. Whether it be more panels/battery, or a load sensing inverter (or maybe both).

Another plus to the Peltiers is that there are no conversion losses (ok, much less, ie no inverter) associated with using them; the Coleman chews on a paltry 5V @ 1.8A. A suitably sized buck converter makes gravy out of that!

FWIW

Steve

[DamonHD]

bzrqmy: I produce my own electricity too, about twice as much as the house uses in a year.  But as everyone has been banging on about, I mainly manage that because I reduced our consumption by 6x!

See here:

http://www.earth.org.uk/saving-electricity.html#meter

Generated 284kWh last month, used 158kWh.

With (more) storage of various forms we could be off grid, including heating and refrigeration.  We're carbon-neutral as-is.

Rgds

Damon

[zap]

...brute-force your way into making it work, ...

It's that damn surge that I can't seem to find a suitable means around... I have a 750W MSW (Schumacher) (1500W "surge") that wants nothing to do with the compressor in my 5 cu footer. Not saying it couldn't be done (for a reasonable compromise), but it's an irritating attribute to say the least!

Steve

Time for the "Ultimate Soft Start"?
It's probably vaporware and may always be that.
It would have to play nicely with a MSW inverter but with the many talented brains that hang around here...?

[Harold in CR]

 Couldn't one of the brainy folks here, explain why a motor start capacitor would not provide the surge needed to start the compressor ?? I don't recall any type capacitor on my fridge compressor??

[Madscientist267]

That's not what a start cap does.

Briefly looked into that as a concept however, in another thread - http://fieldlines.com/board/index.php/topic,145266.0.html

By normal design, a start cap provides a phase shift to a secondary winding (usually heavier) in the motor to help give it more torque when starting.

That energy however isn't stored in the cap, it comes from the AC line.

Back to square one on that. 

Steve

[dnix71]

MadScientist is right. When the appliance starts both windings are energized, but the heavier start windings are not pulling in sync with the run windings because of the inductance differences between the two windings. The start cap is sized so both windings are in phase pulling together.

A different approach might be to set a sequence relay to only energize the start windings for 1 or 2 seconds and then switch to run. A start cap would still be useful in that it would help keep the voltage and current phases closer together to minimize the apparent power required, which is what is tripping your inverter.

[Harold in CR]

 OK. It probably won't work. The reason I asked, is, there are folks using caps for quick bursts of energy, for battery packs.

 Also, you say (start) caps don't store energy. I test motor caps by charging them, and then shorting them. Get a decent spark if they are good.  I'm just not a motor guy. 

[Madscientist267]

Yep caps in AC land is a bit of a different animal than for DC.

Can't fault you on the thought process tho, went down that road myself, and it might be doable, just can't seem to figure on a practical method to implement it. 

Steve

[cujet]

Well, the project has been started. Sorry about the delay, but spring time is busy at work.

I now have 720 watts of solar panels (4ea. 180W). And a good number of healthy 42AH, 24V AGM lead acid batteries.

I went to purchase a PSW inverter yesterday from Sunelec in Miami and they did not have it in stock. What I would really like is a 24 or 48V inverter/charger, PSW (for motor reliability) and the ability to charge batteries from the grid/generator (if/when necessary). I'd like the fridge and chest freezer to run on this setup 100% of the time.

Suggestions please!

[TomW]

I do pretty much the same thing with 820 Watts of solar, a couple turbines and 1350 AH of 24 volt battery.

I use an Outback FX2524 Pure SineWave inverter with charger built in.

Not sure pure solar will do it as it depends where you live and stuff like that.

Actually I run a few items 24/7/365 off my setup.

Satellite Receiver

A couple cordless phones and answering machine.

My internet wireless radio and router gear.

Laptop chargers, printer and desk light.

chest freezer.

Full size refrigerator.

Other items as possible based on production

It does it most of the time with just RE but the outback is set up to protect the batteries by grid charging them if they get low. At the same point it powers loads from the grid. Does this all automatically.

If you go with Outback, buy the Mate up front. It allows you to easily  monitor and customise the system configuration.

Just what I do.

TomW

[Madscientist267]

And just to append what Tom said, and make it clear, the conclusion I ultimately came to was just as others around here have found...

Make sure the inverter is big enough to handle the starting loads. Compressor surge is several times what the running current is, and most inverters are just not up to the task.

Nothing more irritating than tripping an inverter and everything goes black just because it wasn't sized right. 

Steve

[Justoine Knight]

Why not miss out the inverter and power the refrigeration directly from 24VDC?

http://www.danfoss.com/BusinessAreas/RefrigerationAndAirConditioning/DanfossCompressorsDatasheets/DS_Direct_Current_Compressors_R134a_12-24V.htm

[Madscientist267]

That's all fine and good, but a retrofit is outside the scope of a good percentage of the DIY department when it comes to refrigeration because of the refrigerant.

There are a few options, but unless one has the freon license and what not, replacement isn't really viable.

There are ready made "whole unit" DC refrigeration systems available, but they are generally either more expensive than their "grid" counterparts, or performance suffers (read "BTU per unit of currency").

Peltier is another alternative, but the same applies. They are about half as efficient as a Carnot system (by my findings, in my experimentation within the BTU range we are talking, YMMV). The efficiency is dependent upon how "hard" the modules are driven.

Inverter powered refrigeration probably falls roughly in line (in terms of bang per buck) with the above, as there are more losses before the  (relatively) more efficient compressor, resulting in a wash. If the design was solely to run refrigeration and nothing else, this method would probably be the bigger waste of money. But since generating 120/240 is generally part of the mix anyway, a larger inverter isn't really a waste, since the "extra" capacity can be used for bigger loads at a later date as needed.

For someone like me, where only 60W of panel is available, I can't justify the cost of a 2kW inverter to do nothing more than start a compressor. Even my 750 is overkill for most of my loads.

Where reliable refrigeration is required and the only source available is RE, its best to fork out the bucks on what will work (and keep working). If you don't have a need for 120/240, by all means, skip the middle man. If you have other appliances that need grid level juice, don't bother with the extra cost of a DC system. In the long run, it probably won't be worth it.

Can't remember where now, but something else like this came up in a thread, with a chest freezer that had been converted to a fridge, and uses all of 100WH (no typo) per DAY. It involved a custom thermostat, and didn't actually involve any modifications to the freezer itself. This was the path I was on before I too ran across the dreaded startup surge a compressor brings to the table...

Steve

[wdyasq]

Just to throw a real wrench in the works ......

A few years back, I paid to have a board designed to 'soft start' compressors used in refrigerators, and freezers. It actually started refrigerators and freezers with MSW inverters rated at 500W reliable and allowed them to run with small solar systems and batteries. Yes, I am talking about the CHEAP inverters.

The problem is there is not enough demand to build the device. To put it simply, the folks who need the tools are too flipping cheap to purchase them.

Ron

[DanG]

ALL the Danfoss 12/24V DC models have there own electronic controller module - its an inverter with variable low-voltage cut-offs, locked rotor and low compressor RPM check, and variable compressor RPM modes to best tune the unit into their environment, ie: tropical ice box versus Minnesota refrigerator.

[ruddycrazy]

Well after spending over 2K on a gas fridge only to find it won't work in higher temps and the dirty gas stuffed the gas regulator. I got my mate to custom build me a 550 litre fridge/freezer that runs on 24 volts DC. It worked well for over a year then the compressor went short circuit. Considering the size of the fridge it did a good job considering the compressor was over 15 years old. Via insurance I got a new compressor but the fridge was too big so my mate put it in a near new Westinghouse Signature 390 litre fridge/freezer and it's been running sweet ever since. For power I'm using my old 6 volt 110ah batteries I got from the scrap yard 7 years ago and I have a 8 amp smart charger connected. Everytime I look at the charger it's on float so both the fridge and this 64 bit puter I made to run on 24 volts DC doesn't draw much power. For people on RE a DC compressor is the way to go in my opinion.

Cheers Bryan