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Forum Index : Solar : Fridges and freezers
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yahoo2 Guru  Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
One of the first builds that really made me think hard about how energy is used was the Mt Best chest fridge  100wh per day seemed impossible and yet when I thought hard about how it was done I could see the problem wasn't defying physics rather the problem was the limited way I was looking at refrigeration. At one time this was the most famous fridge in the renewable energy community. in the years since I have seen beer temp-rites ale brewing fridges cheese caves wine humidors fermentation rooms coldrooms air-conditioned and insulated houses all using some of the principles I learned from this original demonstration. So the question is, Can we tweak it and do the original idea better? Given that it is possible to do a standard fridge in the 15-25 wh range and freezers in the 25-50 wh range, is it worth it? I usually ignore direct solar usage because there is rarely a lack of energy during the day. We are looking at 240 watts of battery overnight for a fridge and 420 watts for a smallish chest freezer. Round it up to 700 watts overnight from a battery bank for the two. the other objective is good temperature control to keep food in better condition for longer. I have noticed in temperature probing my own fridge that there is a large temperature gradient inside the fridge and the upper half has wild swings from 0.5C to 5.0C. I think a 100wh per day upright fridge that keeps the temps in a narrow range is entirely doable with the right donor freezer to modify. I'm confused, no wait... maybe I'm not... |
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| hotwater Senior Member  Joined: 29/08/2017 Location: United StatesPosts: 120 |
I've been living with a chest fridge in the summer so long I've worn the first one out. The numbers I've see posted I give a stink eye to. Sure it might be possible if it is never opened up. My first fridge was a GE 5CF model with the coils evidently near the top. I didn't see a serious stratification problem with that one. The second was a 7CF Haier. That had the cooling coils lower and I saw greater temp stratification than you quote. I had to install a computer fan running all the time to get it any where near acceptable. Mine runs only daylight hours and only needs about 5AH of battery storage to help in the mornings at low charge levels. The fridge is kept just above freezing and temp is maintained by massive number of drinks. What can I say, I've been doing this 5 months a year for at least six years. Granted it is summer, but it has quite successful. I have a rather complicated micro program that operates the fridge for a fixed time period and then checks battery voltage to determine if operation continues. Time delays prevent hot starts of the compressor. On delay prevents startup from just opening door for a moment. Moisture buildup is a real problem. I run a drip pot just under the drain. This year I have been replacing all the floor joists that are rotted. Didn't notice that the fridge tipped away from the drain hole. At end of season I had over an inch on ice on the bottom. If you ask me, that guy is so bogus. He belongs in the free energy category. 2 minutes running each hour! I'm at about 20. It takes more than two minutes before the coils get up to pressure and it even starts to cool. Of course most people have no idea how to measure things. |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
The manufacturers make more energy efficient fridges but don't bring them into Australia. Panasonic make a full-size side by side fridge freezer with ice maker etc, uses less than 1kwh per day but you can't buy it here. I know this is not 0.1 KWH per day but it is a big improvement on fridges that are available currently. Home Ice Cream have brine tanks in the top of them that they cool down to some incredibly low temperature so their trucks can go around selling ice cream in the streets with no refrigeration running. Once cooled down they can go 16 hours from memory and still be lower than -20 degrees. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
[quote]If you ask me, that guy is so bogus. He belongs in the free energy category. 2 minutes running each hour! I'm at about 20. It takes more than two minutes before the coils get up to pressure and it even starts to cool.[/quote] Have to agree. I did a complete energy audit of all my household appliances about two years ago, and decided my 25+ year old ancient refrigerator just had to go. So I replaced it with a (second hand) current model frost free smallish upright, that used less than half the power of what it replaced. At the time I carried out some monitoring that was at about this time of year late winter/springtime. Consumption 728Wh, running power 140W "on" duty cycle about 19%. The internal temperature fluctuated rather more than I expected about 8 degrees peak variation in both the fridge and in the freezer section. That was measured overnight without any door openings. It was recorded with a thermocouple meter that has a peak hold feature. Slight extra running power beyond the 140W compressor power I assume is accounted for by startup surges and extra much higher defrosting power. All pretty average performance I would expect. Barely adequate and hardly outstanding. There are various controls to alter the proportion of fan forced airflow between the freezer and refrigerator sections. Knobs with cryptic labels such as "fast ice" and "humidity" have a major effect on how well the whole thing works. The automatic defrost is pretty interesting. It appears not to work from an internal time clock but from a thermistor temperature sensor clipped to the evaporator. It cycles on and off erratically typically for about ten minutes every 24 hours. I have had problems with the evaporator solidly icing up if the various air controls are maladjusted so became interested in how the defrost system actually works. I am monitoring the power to the 375W defrost heating element through a current transformer and a digital counter. When defrost is running the counter simply counts up mains cycles. Typically to about 30,000 which is 600 seconds of defrost time. For several days it defrosted in the afternoons starting at no particular time. Then suddenly it has started defrosting overnight instead of during the afternoons. It seems to work in some odd way from how the evaporator temperature cycles and how that varies with ice accumulation. This automatic defrost is probably important, not only because of the much higher power of the defrost element than the compressor, but its heating energy consumed to do negative cooling work. I have never owned a chest freezer, and have no idea of how any automatic defrost system works on those. But its probably a feature that is at least as important as the thermal insulation, and something that might be "tuned" for overall optimum performance. Either too much, or too little defrost power is going to impact efficiency. When converting a chest freezer for a higher internal running temperature, defrost system "tuning" may possibly be an overlooked detail that may be worth looking into ? Cheers, Tony. |
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| hotwater Senior Member Joined: 29/08/2017 Location: United StatesPosts: 120 |
What fond memories. I read that same article too when I started out building mine. Certainly no one would lie to you on the internet just to sell you a controller. None of the small chest freezers have defrost since they don't pass aur over a cooling coil. Ice houses kept ice for months till summer, it is time to rediscover straw. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Frost free fridges have a timer that runs when the compressor is running when a defrost is triggered it runs continuously for the 7-12 minutes depending on the model. There is temperature regulation on the defrost, it should turn off at around +5 degrees. Also there is a thermal fuse in case it gets stuck on for some reason. Without the defrost the evaporator fills up with ice in about a week and then can't circulate air and the fridge gets warm. There are lots of fridges thrown out because of a failed defrost timer or element, both of these are easy fixes. A basic chest freezer has no defrost you just get an ice build up on the inside where the evaporator coil sits. If you are running one of these as a fridge it would never ice up as any ice build up while running would quickly melt as soon as the compressor cycles off. So fitting a drain to the bottom would be a very good idea. Ice houses are no go here, would be an extremely rare event to see a frost. I run a standard side by side fridge freezer and a chest freezer, no problem if you have sufficient batteries. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Thank you Madness, very helpful. Its sometimes a bit difficult to figure out the exact operating mode of something when there is more than one sensor disappearing into a software mystery box. Cheers, Tony. |
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