Quick n Dirty (cheap) Off Grid Freezer Set-up

Jay Peters wrote:Hi Mega Brain,

. Screw the fridge, but I can't live without a freezer...

Sam Stephens wrote:

Jay Peters wrote:Hi Mega Brain,

. Screw the fridge, but I can't live without a freezer...

Back in 2014, I spent 6mos offgrid and very broke in central Texas.

I had a 100w panel, 2 old RV batteries, a cheap,cheap,cheap 1000w MSW inverter and a $30 PWM solar controller.

After messing with different scenarios,  I put a timer on the freezer power cord and set it to turn off daily between 2200 and 0900.

It worked enough to get by.


SS

Hi Jay Peters;

Jay Angler's hubby here.  You have an interesting problem that has a lot of facets to it.  I have read the whole thread (to 22:00 PDT June 28 / 2020) and you have received some very valid advice and also a bit of advice that I am not so comfortable with.

If I was confronted with your problem I would try as much as possible to use thermal storage in some medium (in this case I think brine is the medium of choice) rather than electrical storage in a battery.  A brine thermal storage medium is inexpensive, probably the majority of the mass is readily available on site, and has an infinite cycle life.  A battery is expensive, is heavy and the full mass has to be transported to your site, and has a significantly limited cycle life.

My problem was to freeze 500 pounds of fresh chickens in a short period of time using a domestic type 25 cubic foot chest freezer.  The answer was to fill a large number of 710 mL. pop bottles (we don’t drink much pop, but these things aren't hard to find...) with an 18% brine solution and put them in the freezer a couple of weeks ahead of time and let the freezer freeze them solid.  On chicken day we put a layer of brine bottle into the freezer, then a layer of chickens, then a layer of brine bottles and repeat until all the chickens are in the freezer and any extra space in the freezer is full of brine bottles. (Making alternate layers like this spreads out the cooling power in the freezer and by shortening the path heat has to travel between chickens and brine bottles speeds up the freezing process.) An 18% brine solution freezes at about -14C and the chickens freeze at about -3C to -4C so there is about 10 to 11 Celsius degrees of temperature difference between the brine bottles and the chickens driving the heat from the chicken into the brine bottles. Using this method the freezer can freeze 500 pounds of chickens solid in 24 hours and at the end of that time the brine bottles are completely melted.  They can be removed from the freezer at this point but we just leave them in. The freezer runs a few days to re-freeze the brine bottles and then they act as power failure protection - and are ready to go for the next round of fresh chickens.

Your problem is closely related to my problem, though there are some differences.  The main one is that we were looking for a really hard freeze for long term storage of the chickens.  Efficiency was secondary as we are on grid power.  Also secondary was speed of freezing the brine bottles as we could take weeks to freeze them if need be.  For this reason I chose to set the freezer thermostat to the lowest possible temperature which seems to be about -20C to -21C.  The colder you keep your freezer the larger the temperature difference between inside and outside and thus there will be more heat infiltration into the freezer resulting in higher losses.  You might choose to have your freezer during the brine melting phase of operation at a slightly higher temperature when not powered (say ~ -10C) which will reduce these losses, but also require a lower brine percentage (See the brine table). To get the large heat absorption effect of the phase change of the brine from solid to liquid the freezer has to be able to freeze the brine solid.  You would do this when running the freezer on your generator for a few hours each day and the freezer thermostat should be set as cold as possible to maximize the temperature difference between the inner walls of the freezer (where the evaporator coils are) and the brine bottles so the brine bottles freeze as quickly as possible. Then, when the freezer is not powered the brine slowly melts at -10C (or whatever temperature you select) and holds the freezer temperature (and your food) at that temperature until the brine is completely melted.  The goal would be to have enough brine in your freezer so that all of the brine has not melted before you next run your generator.  (Using gasoline to run a generator that runs a battery charger to charge a battery then then discharges to power an inverter to run a freezer has a lot of energy conversion steps to the cycle and there are losses at every step along the way!  Bad news...)  Rather than layering the brine bottles with your food as we do I would suggest building an inner wall with them inside the freezer where they are as close to the evaporator coils as possible to speed up re-freezing when the freezer is powered by your generator and where they act as a barriers to capture infiltrating heat before it gets to your food when the freezer is unpowered.

A few notes on making the brine bottles:  Sea water is about 3.5% salt by weight and you will need a much higher percentage to make this scheme work.  I will paste in a brine table below that relates salt percentage to freezing temperature.  You have a few of choices to get the correct percentage:

1/  You could start with sea water and evaporate out some of the water to get to the correct salt concentration.
2/ You could start with sea water and add more salt to it to get the correct salt concentration.
3/ You could start with fresh water and add the required amount of salt.

Even though I have sea water available I chose option 3 as it is simpler to get the percentage correct. You just weigh the water, weigh the salt, and mix accordingly.  I used pickling salt as it was the least expensive salt readily available at the grocery store.  Ice melter salt would be cheaper if you can get it - but they don’t sell much of that in the Summer...  Since the pickling salt has large crystals you have to shake it up a few times to encourage it to dissolve and the process takes a few hours.  I also add a couple of drops of food colouring to the brine bottles for identification purposes as I also use pure water ice in pop bottles for other purposes and I want to be able to tell the ice and brine bottles apart.  I mostly use 710 mL bottles as they are a reasonable compromise between volume contained (heat absorption capability) and surface area (heat transfer capability) but you can use other sizes.  In your case, 2L. bottles might work better as you want high heat absorption capability at moderate heat transfer rates.  It would also be easier to build and contain them as an inner wall in your freezer as many 710mL bottles have irregular shapes while the body of most 2L bottles is a simple cylinder.  Do leave a little bit of head space in the bottle to allow for expansion and contraction.

+++++++++++++++++++++++   look at brine chart below   +++++++++++++++++  

      I have a few comments about other aspects of this problem:

1/  Using a large inverter to power a light load is very inefficient due to the parasitic losses in the inverter.  If you absolutely MUST power the freezer from a battery and an inverter for long periods of time (overnight) you might find it wise to buy a smaller inverter just for the freezer.  Chose one that is loaded to ~70% of its capacity and the whole setup will be more efficient - but make sure the inverter can handle the start-up surge of the freezer.  Better to run the freezer off the generator though and use thermal storage as mentioned above.  Fewer energy conversion steps = higher efficiency!

2/  Comments were made about NOT insulating the inside and outside walls of the freezer and the reasons stated are valid as this is where the evaporator and condenser coils are.  The reasons stated do not apply to the lid and floor of the freezer.  Insulating the heck out of them is possibly a good idea. Do be aware that if you do a good job of insulating the floor and lid of the freezer that may bring the outside surface of the floor or lid below the dew point of the surround air which could result in condensation.  This may promote rust and eventually damage the freezer.  The soluton is to insulate in such a way as to prevent ingress of moisture containing air.  Also, make sure the rubber lid gasket is in good shape.  If not, it can be a huge heat leak.

3/  I have used the temperature sensor inside the freezer trick in the past, but the plan was to run the freezer at refrigerator temperatures which are outside the range of the regular freezer thermostat.  This worked well except that the freezer has no way of dealing with the liquid condensation that collects on the walls and in the bottom.  Regular fridges run the evaporator cold enough to freeze out the condensation so it doesn't soak your food - and then you periodically have to either manually or automatically defrost the evaporator.

4/  I looked up the temperature controller suggested by William Bronson and it appears to be a nice 12 volt unit - which is a voltage you have available - but the only rating given in the specifications for the relay contacts is 10 amps at 12 volts DC.  There is no 120 volt AC rating so I would hesitate to use this controller to switch the 120 volt output of the inverter going to the freezer and I doubt that a 10 amp rating would be high enough to use the controller on the 12 volt input side of the inverter.  The internal relay may well have a rating for higher AC voltages but you would have to buy one of these controllers and take it apart and inspect the information on the relay itself to find out.  Keep in mind that AC voltage current rating on switching contacts are frequently higher than the DC voltage current ratings because the AC voltage crosses "0" twice a cycle which helps to extinguish the arc that happens with the contacts open.  With DC there is no help in extinguishing the arc so the current rating has to be lower. If you want to use this controller the safest alternative might be to use an external relay with a 12 volt coil and contacts sized appropriately for the load of the freezer.

5/  As you mentioned, one of things you would like to have this setup accomplish is to slow down the cycling of the freezer so it stays ON longer when it is running and stays OFF longer when it is not running.  There are 2 ways to accomplish this:

A/  Add a lot of thermal mass to the freezer.  The brine bottles will do this very well as they absorb and release large amounts of heat as they  melt and freeze.
B/  Increase the hysteresis of the thermostat controlling the freezer so there is a larger temperature change between when the freezer turns ON and when it turns OFF.  The internal freezer thermostat has a fixed hysteresis so if you want to change the hysteresis you will have to use an alternate thermostat or controller such as the one suggested by William.  Some external temperature controllers may have adjustable hysteresis and some may not.  It appears that the Inkbird controller may have this, though they call it "difference value".

6/  Make sure that whatever you do, that there are at least a few minutes between compressor starts to allow system pressure on both sides of the compressor to equalize.  Starting against a head of pressure is very hard on the compressor.

I hope this helps... Vince