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Old 10-26-15, 10:23 PM   #1
pete c
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Default using waste heat from fridge

In the winter it helps warm the house, so no worries there, but in the summer it heats an already hot house. Has anyone every tried putting together something that would use this heat, possibly to preheat water for a water heater. I suppose if you had a hybrid water heater, you could channel the warm air from the fridge to the hybrid. But for other types how do you do it?

I had an idea of a system that uses water to do it.

For starters, make a manifold that would enclose the fridge's condenser coil. Run a line to the storage tank near the water heater and back. Install a low flow pump in this line. Insulate all of this. Now run the water heater feed through a copper coil in this tank.

I see one issue with this system. Let's say you go away for a weekend. You use no hot water, but the fridge is still running. Eventually that tank is going to get too hot to accept anymore heat. I guess you could have another coil outside the tank which you could pump water through to radiate enough of the heat.

The question is can this be done cheaply enough to be practical?

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Old 10-27-15, 08:09 AM   #2
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Very well could be a good idea.. I suppose if you did something similar to Memphis with is reversing valve hack to heat water and air. You could have it default to heat the water and when the water was at its max temp you could have it switch to heat the air.
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Old 10-27-15, 08:33 AM   #3
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Pete

Great idea but the numbers simply won't support it. Here is why . . .

First, the specific heat of water is ~ 1000 times that of air. By removing a small amount of temperature from air, it will minimally heat up water.

In other words you can heat up a given mass of water by 1 degree, but you have to move 1 degree of heat from 1000 times that mass of air. The volume of air in a fridge is small and increasingly constrained from temperature changes by better and better insulation.

The ability of a home 'fridge to overheat almost any amount of water to boiling over a weekend is numerically impossible. Well, perhaps an ounce - maybe not even that.

Your points are well taken, the kitchen operated, food preservation heat pump does always give off heat and in the summer it does contribute to the homes heat load. That said, most desktop computers and the monitor give off even more heat!

The difference between heat and temperature is not intuitive. I did not truly understand the concept of specific heat until I literally felt the input and exit water lines from a water to air geothermal heat pump. The temperature difference across the water pipes was maybe 5 degrees F, but the unit was blowing out 120F hot air. The rise in air temp was some 50 degrees F with a hardly perceptible decrease in output water temp. That was a HUGE "a ha" moment where the specific heat of air vs water finally sank in.

Capturing heat requires a substantial amount of "stuff". And the cost of that equipment is not trivial. But in applications where you can move a lot of heat, like a geothermal heat pump, you can heat/cool the house with it. I know, as I do it.

My perspective is that using the home fridge to heat water is not a practical solution on a mass scale. Fun to do, great to play with intellectually, but difficult to put into widespread production.

But I REALLY like the idea . . . .


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Old 10-27-15, 07:11 PM   #4
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Thanks for the education Steve. It does make sense. I guess about the only thing that might be worth doing would be venting the fridge waste heat outside during cool weather. Unfortunately, my fridge is on an interior wall, so it would not be practical.

If it was on an outside wall, it might also be practical to circulate air directly from the outside to the fridge inside when outside temps were cold enough. But this is probably not practical as it creates leaks which would lower efficiency the rest of the year. I guess the best think to do is to superinsulate it.

Actually, that might be something I can try given my new kitchen cabinets. My fridge is an older model that will be replaced soon. I want to get one of those fancy new ones with the french door fridge and bottom pullout slider freezers. I had the cabinet opening made for a 36" wide big one. My old fridge is 30 inch wide. So, I have 3.5 inches of clearance each side and 5 inches on top. If I put fiberglass batts on either side, it will increase insulation and should create a very nice chimney effect for the rear mounted condenser coil.
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Old 10-28-15, 01:49 AM   #5
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Quote:
Originally Posted by pete c View Post
it might also be practical to circulate air directly from the outside to the fridge inside when outside temps were cold enough
If the circulating air cooled your fridge then the temperature of that air would rise. So you would be bringing cold outside air into your home (where your fridge is), heating it, then sending it outside. That cannot be a sensible thing to do - unless you particularly want to heat up the great outdoors.
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Old 10-28-15, 03:41 PM   #6
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Originally Posted by pete c View Post
In the winter it [A FRIDGE] helps warm the house, so no worries there, but in the summer it [A FRIDGE] heats an already hot house.
Seems to me that you should not have that much heat being dumped into your home.

You didn't say anything about it, but do you have several large commercial refrigerators operating?

How large is your family?

Do you have several teen-age kids who like to eat and party all the time?

Before you go rogue over capturing all that wasted heat, I think you should consider that:
  1. Your refrigerator might be extremely inefficient.
  2. Your refrigerator might be way too large.
  3. You could benefit from having an 'all refrigerator' in the kitchen and a separate freezer located outside the house envelope.
  4. You could also build a "freezerator", and reduce your refrigerator power usage by a minimum of 50%. My guess is your power reduction would be much larger.

I'm very much in favor of salvaging wasted heat, but I think you should focus on reducing the energy wastage at the source, than trying to remove wasted heat later. Otherwise, you will be building more technology on top of bad technology.


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Old 10-30-15, 01:27 PM   #7
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Get a Kill A Watt and find out how much electricity your refrigerator is actually using. If it's using enough electricity to noticeably heat your house, then the most cost effective solution is to buy a new refrigerator.

A house that's hot in the summer is almost always poorly insulated and air sealed.
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Old 08-11-16, 02:15 AM   #8
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Wouldn't it be possible to use waste heat from fridge with seebeck TEG to run fan on compressor to cool it?

I'll try to do something like that, but right now we have small baby and it takes a lot of my time
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Old 08-13-16, 12:17 PM   #9
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Originally Posted by JimiQ View Post
Wouldn't it be possible to use waste heat from fridge with seebeck TEG to run fan on compressor to cool it?

I'll try to do something like that, but right now we have small baby and it takes a lot of my time
One problem with that idea: delta T.

To get much power flow out of a seebeck pile, there has to be a massive temperature differential OR a mmmassssive amount of surface area in the pile. With a sterno flame, the delta T is enough for some types of generators. Solar can barely get there with high-density (concentrator or evacuated-tube type) collectors. A vapor compression system would be operating at low COP to generate useful power. Even with a mmmmassssive TEG pile, you would trade COP for generated power.

Good idea, bad application. Slap the thing on a buck stove like a stirling motor-heat powered fan.

Best you can do with a fridge is wrap a worm around the compressor shell and plumb that worm into your kitchen faucet. No moving parts, nearly instant hot water, no need to hack the fridge. Larger worm = less cold-water sandwich between depletion of worm and arrival of heated water. It ain't gonna boil or scald you.
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Old 08-13-16, 02:27 PM   #10
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Quote:
Originally Posted by jeff5may View Post
One problem with that idea: delta T.
snip...
Oh, thank you for information. I've found a few applications of seebeck/peltier on youtube where they're using body heat to either move a fan or light a led and it works. So I hoped it would work for delta T of compressor and air (I'm guessing 20-30 Kelvins). I'll try it anyway, but maybe later this year or early next one.

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