Freezer to store surplus power

[madlabs]

Hi All,

I live off the grid and just got a chest freezer. In order to reduce night time running I want to set the temperature lower during the day when the batteries are at float and then go back to a normal setting at night. Hard to say how much power that will save, but it will be fun to try. I have a kill-a-watt hooked up to it and I'll keep that logging while I build the new controller. There was a similar thread a while back but as this is a new project I thought I'd give it a new thread.

Firstly, any thoughts as to if this will really save any power at night? As y'all know, the less you discharge your batteries every night the longer they last. So even if it is a modest reduction in run time I am interested. Of course, this needs to be balanced against compressor wear and of course the compressor will be running more in this scenario. How much more is hard to say. Opinions?

The electronic side of it should be pretty easy. My Midnite charge controller has a float output.  I'll use a SS relay to hijack the compressor controls, a uC and a temp. sensor. An optoisolator to the compressor motor to determine if it is running. I'll leave the existing controls in place and run mine in parallel, so if my controller craps out the freezer will stay cold. Not that any project of mine ever would! ;-)

One of the things I recall from the other thread is that the compressor should be allowed time off if it has just turned off. It also seems a run time limit is a good idea too. Here is some pseudo code:

main:
  if batteries at float AND compressor is NOT running AND has not run in last X mins THEN
   turn on compressor
   reset run time counter
   if temperature = lower set point OR compressor has run too long OR batteries <> float THEN
        turn off compressor
        reset off time counter

Anything else I should add to the logic? Suggestions for length of off period and max run time? Any other safeguards?

Jonathan - KK6RPX

[Bruce S]

I don't yet have anything to add to the programming just yet.
However hopefully OperHouse will chime in , he's been tweaking his for better than a year now.

Best
Bruce S

[OperaHouse]

Mine only runs during the day and I keep it close to freezing.  The way mu program works is the fridge only starts near full charge, when the battery is over 13.75V.  That battery only operates the fridge, a secondary battery operates the other house circuits so that is not hard to maintain a minimum voltage.  I just got a new 7CF fridge and the coils are near the bottom.  I now have the problem of the milk freezing.  It is also frozen first thing in the morning so you can go overnight without running the fridge.  I never had this problem before with the GE fridge with the cooling coils on the top section.  Drinks might get a little ice in them by mid day, but it was never frozen in the morning.

A few days ago I did add a small computer fan hanging under the top basket to circulate the air better when running also increased the temperature about a degree and shortened the running time a couple minutes in the program.  That seemed to make no difference at all in the milk freezing.   I have a relay that connects the second battery during the day and will connect the fan to that.  That will be continuous fan running most of the daylight hours.

I haven,t had many problems with food and only daytime running.  Normally keep a large mass of canned drinks (30) to store the cold.  Prior location was on the raised area over the compressor.  New fridge has minimal area there and can barely store 10 cans there.   I may need to run the fan off and on at night with this new chest fridge to get maximum storage time before food spoils.  I never expected this great a difference in chest freezers.  I would think a simple micro program just monitoring battery voltage would be sufficient.  You program needs an inhibit timer to prevent a hot start of the compressor.  Waiting five minutes  from the time the compressor turned off before it can restart is sufficient.

[mab]

I run my chest freezer during the day for the same reason. I don't think there are any issues with running continuosly although there comes a point where there's minimal gain so you may want to turn of when the freezer temp is levelling off.

agree you need a few mins after switch off before turning on again.

with my small chest freezer, I have lined the insides with storage-heater bricks (magnetite) to increase the thermal 'mass' of the freezer. depending on ambient temperatures, overrding the stat through the day can stop the freezer cutting in until, typically, midnite on warm autumn nights, and on cool spring nights with long days it can last right through 'til dawn.

[Mary B]

Mine run 24/7 but shut off the inverters at 25 volts to preserve battery for critical loads. I have them set as cold as they will go. If I have to go a day with no power they will stay cold longer that way.

I just picked up parts for an F&P wind gen. Getting one of those water aerator windmills to mount it on to hide it as a wind gen. Lawn art you know! My way of getting around idiotic zoning laws. It won't be ideal but if I can get a couple hundred watts out of it I will be doing good and it will be a nice added charge for the batteries. I get the most wind during winter when I see the least amount of sun so it will be a good add on.

[dnix71]

You all are talking about a eutectic fridge by another name.

http://www.thebackshed.com/forum/forum_posts.asp?TID=5500  Here is a discussion about freezing water as the eutectic. There isn't enough energy to be harvested from solar alone to do that well.

The easiest, cheapest way to save battery power is lots and lots of insulation around the fridge. The project which converts an unright freezer to a fridge by changing the thermostat is an example of that.

[oscar11]

Last winter in a blizzard we lost power for 3 days. Lots of people put out generators to power their refrigerators, I just put my stuff on the back porch. Then I wanted to build something to measures when it gets cold out and start using outside air for refrigeration. I wanted to save the snow banks in an insulated ice House to use during the summer. Maybe even make an ice pond and go old school and use an ice box.

[bart]

    In my freezer, a upright, r-12, for thermal mass, I use empty 1 liter fruit juice bottles, filled with water and 1/2 of a 26 oz. container of salt. Have gone 2 days without power and had no thawing. Put a bottle of that in water and a thick layer of ice forms around it.

[OperaHouse]

I purchased a number of these electronic thermostats on ebay for $3.60 shipped.  They can be switched from heating to cooling, only a single open set of contacts.  The differential can be set and also time delay for up to 10 minutes I believe.  Just power it off the solar panels.  Normal time delay would work going from day to night.  Evening to morning inhibit could be performed with opto connected to motor and temp sensor (sensor to +5 w/ series resistor) to fake the temperature is a few degrees lower than it actually is.

These chest freezers really have stratified temperature layers.  I'm connecting a fan to run all day by connecting it to solar panel output.

[Mary B]

Yeah I have rigged a fan in mine, doesn't take much and it runs off my main battery bank. Used a little computer fan that draws 1 watt. All it needs to do is slowly move air around.

[madlabs]

Thanks for all the replies!

I wonder which is better, bricks or salt water for thermal mass? I hope to put a bunch of sweet cider in there, but of course that will disappear!

And yes DINX, it does sound like a eutectic fridge. I would say not quite but close.

And while I am going to make a controller to pump the freezer colder during the day, I hear that insualtion would make more difference. So, how much insulation do you think would be needed to make a real difference? 1" of foam? 2" of foam?

Jonathan

[OperaHouse]

Problem is, how do you add insulation?  Most freezers use the outside skin to radiate heat.  My old GE chest freezer had both the cooling coils and the heat radiating coils on the top rim.  That made the insulation really thin there.

[madlabs]

Opera,

Good question. I'll take a better look, but the freezer has a large vent on one end. So hopefully most of the heat dumps there.

However, the compressor is in a little box that intrudes into the freezer space like most of them do. I'd like to insulate that area but it would have to be inside the freezer. The floor should be easy enough, just set the thing on a couple inches of rigid foam. But it gets much more complicated from there.

Jonathan

[dnix71]

I would like to add insulation to my Engel 12v fridge. It sweats a lot on the top, even after removing the foam liner and refoaming it.
It runs way too much in the summer in my hot little apt. I'm not sure where to start, though. There are controls and hot coils hidden under the left side. Maybe a box around everything except the vented part would be worth a try.

I find it strange that the sides don't sweat but the top does.