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Old 07-09-17, 02:11 PM   #1
SentinelAeon
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Default Fridge heat produced

Hello,

I have a very simple question regarding a fridge. To simplify let us just assume a few things to remove variables:

- fridge uses an average 250W for 24/7, all goes into heat
- it moves 250W x 4 = 1000W amount of heat

If we have 2 identical kitchens where 1 has the named fridge inside and the other has no fridge, heater of what Wattage would we have to put into empty kitchen in order for both kitchens to have the same temperature ?

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Old 07-09-17, 04:06 PM   #2
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Easy, 250 watts.
The heat the fridge is trying to reject is coming from the kitchen. It takes 250 watts to reject that heat back where it came from.
Now say if you took hot water bottles from your car and stuck them in the freezer then it would make it hotter than just the 250 watts.
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Old 07-09-17, 08:58 PM   #3
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Thank you for your reply, that makes sense. If i might, i would use this topic for another question or two, if someone knows the answer.

Let us compare 2 identical fridges in 2 identical kitchens, both fridges are cycling (they turn on when temperature in fridge rises and turn off when temperature is low enough). The only difference is that 1 of the fridges has a big fan blowing onto the coils and compressor behind him. Which of the following would in this situation be true:

1) fridge with additional cooling will finish its cooling cycle faster and will therefor turn off faster and in turn use less energy (let us for this case ignore the energy used and heat produced by the fan)

2) noise level of the fridge will be lower due to lower temperature of the compressor
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Old 07-09-17, 09:18 PM   #4
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..well not necessarily the way this works. I've let me house drop down into the 40s in the winter when nobody would be home for awhile and the fridge will use about 10 watts less. 130 watts instead of 140 watts but it will also run much longer cycles too and then spend much longer periods of time with the fridge off. It's counterintuitive but repeatable. In the end it uses less energy because it's rejecting less total heat entering the box from the room. I've even unplugged the fan to allow the condenser coil to get warmer but it still runs a cycle roughly the same amount of time but the wattage increases as the exterior coil heats up.

Compressor noise doesn't really change with the condenser temp, it's very noticeably louder when the evap temperature is hotter, such as when a defrost cycle just finished and the compressor is just powering up.
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Old 07-09-17, 09:36 PM   #5
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Thank you for all the answers,

Btw, before knowing that fridge only produces about 250W, i was thinking wouldnt it be nice to get all that heat the fridge produces and just remove it from kitchen, make my place a little cooler. Sadly if it was 1250W it would be worth it, now with 250W it isn't. But i might as well share the ideas with u, again, when i thought it would be worth it:

- first one would be to use heatpipes (i have tons) to transfer heat from compresor to a copper plate and put big tank of water on it. I water my window shelf plants almost everyday and i thought it wouldnt hurt to transfer some of the heat to water so that when i water the plants i remove some of the heat from kitchen.

- second one was, my kitchen sink is right next to fridge and i noticed a use it very often. It is mostly cold water that sadly goes to waste. And i thought, why not attach a copper pipe to fridge coils/compressor and connect it with my sink so that when i use water to wash my hands, that cold water cools down the compressor and coils and then leaves the house. Sadly with 250W it simply isn't worth it.

Both ideas were obviously mean't for summer. Now the only other thing with that 250W of fridge heat i can do is use it as a heater for joghurt machine
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Old 07-10-17, 02:18 AM   #6
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Now you're comparing heat transfer to electrical power draw. The two are not equal.

In the original question, static situation in the room, the heat pumped from the box eventually leaks back into the box. Same thing that happens when the box is opened, only slower due to air sealing and insulation. Regardless of the performance of the refrigeration circuit, the net power usage will depend on the thermostat setting (colder setting equals more runtime) and the thermal leakage (less insulation or opening the box equals more runtime). This is the fundamental set of conditions leveraged by the "freezerator": freezers generally have more insulation and have the door on top, so the heat leakage is lower even when the door is opened (cold box air can't pour out the bottom). Running a box freezer at refrigerator temperature reduces runtime by lessening the temperature gradient between the inside and the outside. Since less heat is pumped out, less can leak in before the gradient disappears.

In the second situation, the heat flow is considered for alternate uses. Nearly all residential refrigerators have a Coefficient of Performance (COP) of at least 2.5 or better. So for your 250 Watts of electricity, the refrigeration circuit is moving 625 Watts or more of raw heat. With an American-sized refrigerator, the heat rejected by the refrigeration circuit is sufficient to keep a small hot water tank under a nearby sink warm enough that it supplies instant hot water. If the main water heater is close enough, the tank under the sink will not run out of heated water before the hot water arrives at the under-sink tank. This has been done before with success by members here: look up "heat pump water heater" or "HPWH" in the google custom search bar at the top of the page. These Ecorenovator people are hardcore about making crazy impossible stuff that works.

For reference, I am using some USA-centric general figures. A typical refrigerator uses around 2 KWH of electricity a day, so with a COP of 2.5, we have 5 KWH of heat being generated. This is enough energy to heat at least 30 liters of water by 30 degrees C, counting losses, every day. Euro-style refrigerators are generally smaller, so unless yours is being opened more often than normal, the power usage (and heat recovery)will be smaller as well.
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Old 07-10-17, 07:48 AM   #7
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Thank for your answer. So after this, i am back to thinking that it would be worth to remove this heat from apartment in order to cool down the kitchen. Or as you said, connect it with heat pump.
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Old 07-10-17, 08:10 AM   #8
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Default Buy a new Fridge

Quote:
Originally Posted by SentinelAeon View Post
Thank for your answer. So after this, i am back to thinking that it would be worth to remove this heat from apartment in order to cool down the kitchen. Or as you said, connect it with heat pump.
I don't know how large your fridge is, but modern fridge freezers use so little energy I doubt it worth the effort. If it really bothers you running an air-conditioner to cool down the kitchen would cost a few cents a day.

We have a Bosch KGE36BW41 which is plenty large for 2 people (300Liters) and due to it's A+++ rating only uses 150kWh a year. I measured our usage to be around 120kWh, so that would only be something like 0.3 kWh per day. Half the year it's beneficial heat, the other half not.

I would focus on replacing incandescents to LED, shutting down energy hungry devices like servers & TV's and just keeping the heat out with sunshades or insulation.
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Old 07-10-17, 10:26 AM   #9
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Ator: I agree, still it was a question that was really bothering me so i am happy i got the answers

I have another, non related question. See image below. We have to identical boxes, 50x50x50 cm, we have 1 intake fan 12x12cm on 2000rpm, we have 100W heating element that is turning on/off every 30 minutes (working half the time), we have 200g aluminum heatsink on that heating element. We add 1l of water in closed plastic bottle, room temperature, into 1 of the boxes.
Am i right in assuming that the second box, with added water, will be cooler ? The way i see it: since we add 1l of water, the heater will have a lot more mass to heat in that 30 minutes. Therefor the temperature of the box after 30 minutes will be lower then the first box with no water. And this holds not only after first cycle but after 10, 100, 1000 cycles, because in that 30 minutes the heater is off, that fan has enough time to cool down the bottle to room temperature or at least close. Granted plastic bottle is a very bad heat conductor, but just as fan will have trouble cooling the water through the plastic, the same way heater will have trouble heating it.

Am i right in assuming this ? Or will both boxes have same temperature after 10, 100 or 1000 cycles ?

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Old 07-11-17, 10:26 PM   #10
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Doh, in your illustration, the mass of water is upstream from the heat source. It is not heated by the element, so it truly is insignificant. Both units would function the same.Try again.

Your question has one simple answer and lots of considering and pondering to it. Yes, the heat store would tend to level the outlet temperature. A better comparison would have the heating element placed inside the heat store. The difference between the two is known as a reactive component. Whether or not the additional reactive component is useful depends on the application.

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