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?

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.

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 . . . .


Steve

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.

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.

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.


Best,

-AC_Hacker

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.

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.

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.

please post the findings of your research.

So, I have ordered 4 TECs (cheaper, worse performance than TEG) 40x40 mm 12706 modules and a voltage converter 1.5V to 12V. Later I'm gonna get two arctic cooling alpine M1 heatsinks with fans (0.36 W) - 80x80 mm.

One heatsink will cool the peltiers on the compressor and the other heatsink will cool condenser coils (I have the passive ones)

The compressor warms up to 45°C (113°F), ambient temp is around 24°C (75° F).

The fan with heatsink arrived (it was from local seller), I just ran it of 6 AA (actually 4AA and 2 AAA) batteries, so 9 volts are enough. I'm guessing it will run at 0.2 W since max is 0.36W. With 2% efficiency of peltiers, I need just 10W of thermal energy from compressor. This seems doable, since it consumes 80-90.

Another update: Voltage regulator just arrived from China and I was able to run the fan from just one AA battery. It's starting to look hella good :-)

I think your concern is valid, and I think that your interest is a good one, despite the analysis (?) that appeared below your original post.

I think that every BTU should be used if possible. But even more importantly, I think that every BTU of 'wasted' energy should be scrutinized and eliminated if possible.

The best approach is to start drastically reducing the waste energy.

If you have a refrigerator that is putting out enough waste BTUs to appreciably heat a house in the winter, and to be a significant cooling problem in the summer, it indicates to me that you are dealing with a very inefficient refrigerator, probably an older one.

Many of the newer refrigerators, 'Energy Star' refrigerators, use far less energy and produce far less wasted heat than conventional units... maybe half of the energy use compared to older units, half of the energy wasted compared to older units.

The next step in the Refrigerator Warrior's attack would be to re-purpose a modest-sized freezer, either chest type or cabinet type, to become a refrigerator. The chest type will ultimately be the most efficient. Some argue that the chest type is less convinent.

I have re-purposed a cabinet type and it is extremely efficient., and convenient to use.

With both types, there will be condensation issues. Chest types have a drain hole in the bottom which is perfect for eliminating condensation. A carefully chosen chest type should have a drain hole, too. I've been using mine for a few years now, and I don't think that condensation is a deal-breaker.

My project is fully explained in the link below. (Please note that I purposely chose an inefficient freezer, to see if even a bad freezer could make a good refrigerator)

http://ecorenovator.org/forum/applia...onversion.html


MEMPHIS91 did a chest type conversion. He has been super good about including photographs and detailed explanations.

http://ecorenovator.org/forum/geothe...eigerator.html

So, my advice is NOT to abandon your project, but to attack the problem at the root, which would be the drastic reduction of wasted energy.

I wish you great success.

Sincerely,

-AC_Hacker

So first test conclusion is just as jeff5may predicted - low dT doesn't produce enough "juice" to run the fan. With hot pot underneath and cold pot sitting on top of peltier plates I got around 1V directly from modules and slightly above 3V from step-up booster. I will continue tests and will probably try more than 4 modules (I guess I can fit 12 on the compressor)

Another test (in the first one I didn't have surface that would be flat enough) shown 2,65V on modules and around 12V on step-up booster, but when I connected the fan, the voltage sharply dropped. I guess the modules are very soft power source. I have found some RC helicopter engine on aliexpress and propellers (which together acts just like fan), so I'll order those and try it.

I finally got around to install TEGs on the fridge compressor and results are very dissapointing. At most I am getting 0,4V without load and between 4 and 5 mA short current. Which is too low to run small electric motor I salvaged from solar toy.

I ran out of thermal paste (surface of compressor isn't flat, so I need something to fill the gaps), adding more should help a little. Also my heatsinks are too small - voltage rises from 170 mV to 320 mV fairly quickly, but than flats out, because heat sinks are "saturated".

I have ordered two more TEC 12706 (there is still room on the compressor) and bigger heat sinks (height 30mm instead of currently used 11mm). That should be enough to run the motor with propeller.

When the motor is running (from a battery) - and cooling the heat sinks - then the voltage rises to 0,7V (with spikes around 0,8V) and short current rises to 10 mA.