Author Topic: intermittent freezer power - would it harm food?  (Read 10332 times)

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vtpeaknik

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intermittent freezer power - would it harm food?
« on: May 13, 2012, 06:48:50 PM »
I'm seeking to make the most use of my PV power while keeping the batteries mostly full because it's primarily a backup power system and I do have grid power normally.  In that spirit I was wondering if it would be OK to run a chest freezer during the day only (surplus PV power) and not at all at night.  I left it unpowered overnight as an experiment, and it warmed up from 0F to 20F (-18C to -7C).  This was of course far from thawing.  But, would that much temperature swing on a nightly basis have an effect on the freezer contents? E.g., could it cause foods in the freezer to dry out faster or something?

wpowokal

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Re: intermittent freezer power - would it harm food?
« Reply #1 on: May 13, 2012, 07:44:00 PM »
What you are proposing is quite common here where I live as many households only have electricity while their generator is running, however do be careful. The following is taken from http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Food_safety_storage Mr Google showed me that.

Summary

Incorrect storage of food can cause spoilage and food poisoning. High-risk food should be kept at 5 C or below, and above 60 C to avoid the 'temperature danger zone', where bacteria multiply fastest. Do not refreeze food that has been frozen and thawed once. Store raw food separately from cooked food.


Here in the tropics food safety is so important that one can not thaw food by leaving it out, I thaw food in the fridge unless I intend to use or cook it immediately.

Storing food in the fridge

Your fridge temperature should be at 5 C or below. The freezer temperature should be below -15 C. Use a thermometer to check the temperature in your fridge.  also from the same site.


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ChrisOlson

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Re: intermittent freezer power - would it harm food?
« Reply #2 on: May 13, 2012, 10:44:24 PM »
Consider putting your freezer in a place where it is cooler, like the basement, if you live in a hot climate.  It will reduce the power consumption of it.

I think I would consider adding more battery capacity to store the surplus PV power and keeping the freezer powered up 24/7.
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OperaHouse

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Re: intermittent freezer power - would it harm food?
« Reply #3 on: May 14, 2012, 09:08:59 AM »
Having a lot of thermal mass is the secret and starting with a lower temperature.  I run my6 chest fridge this way.  I have to believe this is hard on the compressor since a long run time each morning is not a design criteria.  Something like a UNO to limit on time and progressivly lower temperature through the day would be interesting.

WindriderNM

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Re: intermittent freezer power - would it harm food?
« Reply #4 on: May 14, 2012, 05:18:39 PM »
Some gallon jugs of water could increase the thermal mass. You could put some insulation on the sides and top being careful not to block any ventilation.
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vtpeaknik

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Re: intermittent freezer power - would it harm food?
« Reply #5 on: May 14, 2012, 05:56:46 PM »
The freezer I did the experiment with was rather full.  The ambient temperature was moderate, about 55F (12C).

Be careful about adding insulation to a modern chest freezer: in many models the heat being pumped from the inside to the outside is discharged in the outside walls.  Adding insulation there would make it less efficient, not more.

Talking about adding some sort of timer to turn the compressor on and off, I've been also wondering about doing something to make the freezer and the refrigerator (separate units) run at the same time.  As it is now, simply relying on their thermostats to turn them on and off, they mostly run at different times.  The result is that my inverter is running (rather than being in standby/load search mode) about half the time.  This inverter uses about 25 watts to run itself while not in standby.  Compare that with the average power consumption of the freezer (about 25 watts) and the refrigerator (about 40 watts) and you can see that it's a significant loss of precious PV energy.  The inverter (rated at 2000W continuous, 4000W surge) could easily run both at the same time.  Except that when each of the compressor motors starts up they need a lot more power for a second or two - the inverter can probably handle that, but it's an unnecessary strain on its components to do it on a regular basis.  A timer circuit that, e.g., supplied both with power for 20 minutes of each hour, and each time started one of the devices a few seconds after the other, could help, although I am not sure it's worth the trouble.

OperaHouse

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Re: intermittent freezer power - would it harm food?
« Reply #6 on: May 15, 2012, 06:57:26 AM »
I ude individual inverters and turn them on.  Had a source for a while of super cheap defective ones. Then other people discovered it and the price is now four times what I used to pay.  The smart use of power is not popular at otherpower and likely never will be.  This is a gearhead site.  omplex scheduling is easy with devices like the UNO and can save lots of money if implemented at the start of a system.  At the backend it is too late to be of value.  Does that vt stand for a state?

vtpeaknik

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Re: intermittent freezer power - would it harm food?
« Reply #7 on: May 15, 2012, 09:04:33 AM »
Yes, a state...  :-)

Did you fix those defective inverters?  What was wrong with them?

Yeah, a complex timer gizmo seems like a significant hassle to build, and a microcontroller could do so much more.  That said, the microcontroller would still need all the cables and relays and such.

wpowokal

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Re: intermittent freezer power - would it harm food?
« Reply #8 on: May 15, 2012, 10:05:28 AM »
It is the starting of refrigeration that is most harmful to the motor (actually most motors but refrigeration is being discuses) It can run all day no problems, any attempt to stop and start it on timers is a waste of energy.

If your freezer does not have an external temperature indication then fit one, then you can observe it and become informed.

The efficient use of renewable energy mostly revolves around load scheduling, for more reasons than having surplus Re available, and sometimes it is prudent to ignore a light for the sake of household harmony, then what would I know I am probably a gearhead. 

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OperaHouse

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Re: intermittent freezer power - would it harm food?
« Reply #9 on: May 15, 2012, 10:15:50 AM »
My camp is in upstate NY.  Hope you don't need to deal with all that cloud cover. Never thought it would take 800W of panels just to run a fridge.  Almost all were easy fixes.  With the 2000/4000W units it usually was one of the pairs of H bridge FET shorted.  I would just wiggle the bad FET till the leads broke.  That gave me areally beefy 1000W inverter.  I was paying about $17 each shipped.  Last time I looked they were going for $60.  Smaller inverters don't have the spare FETs in the output.  I just make them 140V DC out abd run CFL and DVD etc with them.

The starting of compressors is a valid point.  Running them all day is not.  These motors run hotter on inverters.  Reducing the PWM duty cycle is one way to reduce that.  The thermal design of these freezers probably benifits more by running them a set period of time with a cool off.   Normal operation is these comming on twice an hour.  Timed operation leaves more power for other things early in the day.  Once you drop below minimum temperature it is just low priority storage.

MaryAlana

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Re: intermittent freezer power - would it harm food?
« Reply #10 on: May 15, 2012, 02:49:18 PM »
Food quality is going to suffer from the constant wide temp changes. You want to keep around -20f for best long term storage.

wpowokal

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Re: intermittent freezer power - would it harm food?
« Reply #11 on: May 15, 2012, 09:09:34 PM »
These motors run hotter on inverters

OperaHouse what proof can you offer to support that statement?

Yes running most motors off Modified Sine Wave inverters (MSW) is not as kind to them as pure sine wave because of the wave shape and "DC component" within that waveform. However many people have run refrigeration on MSW inverters for years, and with the minimal price difference between low end MSW and low end pure sine wave why would one buy a MSW inverter. And then you say "Reducing the PWM duty cycle is one way to reduce that" it seems to me that this would be bad for fixed load motor, so please enlighten me.

Normal operation is these comming on twice an hour.

We are discussing a chest freezer here, if I had one that came on twice an hour I would be very concerned.

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birdhouse

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Re: intermittent freezer power - would it harm food?
« Reply #12 on: May 15, 2012, 09:33:03 PM »
Quote
These motors run hotter on inverters

are we talking MSW or PSW??  i could see this statement being correct for MSW!

adam

dnix71

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Re: intermittent freezer power - would it harm food?
« Reply #13 on: May 19, 2012, 09:43:48 AM »
There is a specific design of fridge or freezer that runs twice a day and maintains cool, but they are expensive.

Internet search "eutectic freezer." The eutectic is a mix of refrigerant that does not change it's composition when it goes through a phase change. A well known eutectic is 95% ethanol/5% water. Boil it all you want and you still have the same mix.

http://friztech.com/eutectic.htm These are made for commercial use where a power failure would result in product spoiling and the business losing money.

http://www.4x4equip.com.au/showProduct/TS-0010  Just over $2k Au. Made for use off-road and Outback.


XeonPony

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Re: intermittent freezer power - would it harm food?
« Reply #14 on: May 23, 2012, 11:33:28 AM »
Having a lot of thermal mass is the secret and starting with a lower temperature.  I run my6 chest fridge this way.  I have to believe this is hard on the compressor since a long run time each morning is not a design criteria.  Something like a UNO to limit on time and progressivly lower temperature through the day would be interesting.

Actualy it is quite invers, the compressor is at its happiest running at long times rather then short periods, in the refer industry that is the design goal to reduce starts and just keep the thing running.  So long as you haven't exceeded the internal load in btu's the thing will run in perfetc happienes! it is cooled by the suction gass entering the shell!
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OperaHouse

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Re: intermittent freezer power - would it harm food?
« Reply #15 on: May 23, 2012, 02:08:35 PM »
That canned answer has nothing to do with what we are talking about.  If you want to get through a 12 hour period without power, you need to begin with a lower temperature and have a large thermal mass.  Many refrigerators are properly designed today with smaller compressors and are designed to run longer.  I would not consider my name brand 5CF freezer well designed.  The sides become too hot to touch when run for long periods of time and that makes for poor thermal efficiency.  The motor is even hotter.  These just don't have enough thermal radiating surface and are only designed to do long startups a half dozen years in their life.  The compressor is made in Slovokia.  Make a list of ten countries for a compressor to come from, thats not on it.  These things are throw away.  After three years the internal moisture problems are causing corrosion and paint peeling.  I can't imagine keeping this in my kitchen for more than another three years. 

There are a lot of aspects to compressor life and most of them relate to manufacturing.  I actually worked at a Fortune 100 R&D center on a compressor project for the appliance division.  There was one woman in the factory that spun the compressors by hand at the end of the production line.  That was the entire quality control.   We designed a machine that spun the compressor 16 times, divided up each rotation into 16 parts and did a FFT analysis of the sound.  This was breaking edge technology at the time.  We did this with with a dozen sample compressors.   There is a back story to this.  The appliance division also shipped compressors to Korea.  At that time they couldn't do the precision machining.   They complained about compressor failures.  The appliance division attributed this to contamination in the heat exchangers.  For the final test we were shipped 100 compressors diverted from a shipment to Korea.  Almost everyone of these failed the test in our machine.  We opened one up and found rust in it.  The problem then was pretty obvious.  All these compressors were placed in plastic bags and foamed in place in the cardboard box.  The byproduct of the foam reaction was acetic acid.   Certainly these wern't stupid people.  It takes a lot of quality control.  Sometimes changing a factory floor wax can have dramatic effects.  I remember that story from a electron tube manufacturer.  The truth is statistics can show these thing in thousands of units.  Individually nothing you do makes an obvious difference.

Frank S

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Re: intermittent freezer power - would it harm food?
« Reply #16 on: May 25, 2012, 08:25:54 PM »
thawed refrozen food is nothing to mess around with.
  The only time I have even been to a DR in the past 35 years was just last fall while the wife was in the States, when apparently my freezer had warmed enough to allow some all beef sausages to near thaw. and I didn't know it I spent a week living in the bathroom, too week to even get dressed to drive 1 mile to get some fresh fruit and juices. finally after a weak my partner got to worrying about me and sent someone over to check on me . Long & short of it I spent the next 10 days in the hospital , which was a waste of my time when all I needed was a gallon of juice and 1 more day at home I thought.
  The DRs were concerned with my BP & heart rate because they were so low 
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bob g

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Re: intermittent freezer power - would it harm food?
« Reply #17 on: May 25, 2012, 10:17:40 PM »
fwiw

i would do a careful analysis of the load,
what i mean by that is during the night, determine if you were to power it for perhaps a half hour at the mid point, perhaps it will hold the temps acceptably close to the set points you are after.

perhaps it might take two such run times?  or whatever

then you will have the info needed to determine if you can provide for that loads runtime overnight using mains power.

it might well be that it makes economic sense to power it during those times via the mains rather than adding battery capacity which will also require an increase in charging capacity, which means more PV, hydro, wind or whatever.

just saying it might well be that using mains power makes more sense in this case.

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Bruce S

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Re: intermittent freezer power - would it harm food?
« Reply #18 on: May 30, 2012, 03:10:53 PM »
vt;
I have a nice easy to work with 5CF freezer and just because I too was curious, I did a quick little test.
The freezer is a chest type, kept about 2/3rds full. Starting temp is about 25F, food kept colder below freezing will keep longer. I put a multi-set timer on it and had it off the full 8 hours of darkness.
Room temp at the time was mid 80s. Morning temp using digital instant probe was up to 30F. I left it this way for about a week. Each time during the day time it would stay in the 25F range and each morning it would be up around the 30F mark. Only thing I saw was 10x more frost around the top than I normally do.
I saw now difference in the food stability.
FYI: My wife is a Chef and IF there was any possibility I would have messed up ANY food including frozen chocolate desserts, I'd been told right away.
Hope this helps;
Bruce S
PS> After I completed this little "test" I had to clean the frost off which was a pain, but once done, I plugged it back in to normal and no abnormal frost since.
 
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MaryAlana

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Re: intermittent freezer power - would it harm food?
« Reply #19 on: May 30, 2012, 04:16:52 PM »
25f is really warm for a freezer.

Bruce S

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Re: intermittent freezer power - would it harm food?
« Reply #20 on: May 30, 2012, 05:13:07 PM »
Sorry should've been more explicit but work keeps getting in the way. 8)
This one we use for monthly food and chilling veggies to go into long term freezer, the bigger one is sitting at zero (0F), that holds the annual stuff (1/2 side cow 1/2 side pig 12 chicks, and I certainly wasn't going to let $$$$ go to waste on the off chance.  ;)
At 25F it's still below freezing yet won't take two solid days of fridge time to thaw.
We don't open thaw our food, its a fridge that's kept at no higher than 38F.
Thanks for reminding me!! :)
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vtpeaknik

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Re: intermittent freezer power - would it harm food?
« Reply #21 on: May 30, 2012, 06:31:05 PM »
Most of the "coolth" is in the latent heat of the phase change to solid, rather than the temperature.  Thus if you put a 0F chunk of food in the frig to thaw, or a 25F one, I don't expect the time to fully thaw it would be all that different.

I also suspect that the reason your freezer didn't go above 30F during the power-off periods is because a slight thawing of some foods would suck the incoming warmth and prevent further temperature rise.  Items that are not distilled water, but rather have some dissolved salts in them, i.e., most foods, freeze (and thaw) at a temperature somewhat lower than 32F (0C).

A way to test my theory is to lower your thermostat to, e.g., 20F instead of 25F, and see if an 8-hour power-off would still cause a 5F rise to 25F, or would it still go up to about 30F.  In the case of my freezer the temp rise was 20F and I assume both of these are standard freezers with similar insulation?

The temp gradient matters too, you had an initial diff of 55F between the inside and outside of the freezer, and a final diff of 50F, I had only slightly more than that to start with, and less than that at the end, so that does not explain my 4x larger temp rise.



Bruce S

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Re: intermittent freezer power - would it harm food?
« Reply #22 on: May 31, 2012, 10:53:36 AM »
I may give that a try just for curiosity sake:-).
They probably are similar in design. a standard off the shelf 5CF chest type freezer we picked up from a guy who bought a bigger one.
Once my wife has finished the wedding cakes and its safe to give it a try I'll let you know.
The 4x temp change is an unknown, could be the level of items in your freezer?
For us, most of the food is NOT store bought. I buy piggie from a Centennial farm, super lean and totally tasty, chicks to same place, cow is from hometown and raised by a cousin, NO injected salt water in any of them  ;). Also there was no ice crystal on the packages , but only on the underside of the lid. Missouri is a humid area during the summer . I do not have any extra insulation on the freezer due to the sides getting warm while the compressor is running .
Hope this helps
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vtpeaknik

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Re: intermittent freezer power - would it harm food?
« Reply #23 on: May 31, 2012, 04:17:41 PM »
My freezer is rather full, so that does not explain it.

The humidity that turned into frost could have come from inside (partial thaw) or outside (air exchange).  During the time your freezer collected that extra frost, was it opened many times?

When I mentioned salt I did not mean added salt, I meant that the natural contents of both meats and veggies is not distilled water, it includes a moderate amount of dissolved stuff.

For example, the first writeup I found via Google says that the freezing point of "meat" is -1.7 to -2.2 C (around 28F).  That same writeup also talks about latent heat, ice crystal formation in the near-freezing range, etc. - even "Migratory recrystallization".  Complicated!