Can you use the heat from fridge/freezer to warm shower water.

[Dave_S]

Sorry, this question is way out of my comfort zone and is probably not as cleaver as it sounds in my head

If the fridge and freezer can lower air temp in the 225lt box by 35°C all day, surely it can raise the temperature of 40lt of water by 15°C.

Has this been tried ?

[LakeSuperior]

You should name it a heat pump...

[Jammer]

It can be done but you need an extra heat exchanger which takes up valuable space. Refrigerant pressures are typically too high for the more compact flat plate heat exchangers so you're stuck with the bulkier shell and tube ones. Could it be done? Sure... is it done? Never seen it on a boat.

[requiem]

Don't underestimate the capacity of water to absorb heat!

Spitballing here... the weight of the 225 liters of air in the fridge/freezer would be about 0.28 kg. It's heat capacity is about 700 Joules/kg/°C. That means you're pulling out at least 700*0.28*35 = 6.9 kJ of energy to cool the air.

In contrast, you have about 40 kg of water, which has a heat capacity of 4,184 Joules/kg/°C. To raise that 15° would require adding 4184*40*15 = 2,510.4 kJ of energy.

This assumes perfect insulation for both, of course: you cool the air and it stays cool; you heat the water and it stays hot. In reality you'd need to extract more heat from the air over time as well as pumping more heat into the water tank. It also assumes the cooling process is perfectly efficient, which it won't be.

[Uncle Bob]

Ozefridge have a water cooling option on their marine fridges, I believe that it is designed to connect to the fresh water tank to assist the fridge cooling, so yes it does exist. A call to them may well prove enlightening.

[RaymondR]

Originally Posted by Uncle Bob

Ozefridge have a water cooling option on their marine fridges, I believe that it is designed to connect to the fresh water tank to assist the fridge cooling, so yes it does exist. A call to them may well prove enlightening.

Ozefridge use a single condenser to achieve both air and water cooling of the hot refrigerant vapour. They achieve this by running the water cooling circuit flow through the fins of the air cooling condenser in parallel to the refrigerant cooling tubes. They recommend using the fresh water from the vessels pot water tanks rather than sea water.

Using an Ozefridge AW systm one could fairly readily get an experimental result by recording the water inlet and outlet temperatures whilst flowing the return fresh water into a container and timing the fill rate. If you put the pail on a bathroom scale and got the result in pounds the conversion to BTU's would be simplified.

[donradcliffe]

Originally Posted by requiem

Don't underestimate the capacity of water to absorb heat!

Spitballing here... the weight of the 225 liters of air in the fridge/freezer would be about 0.28 kg. It's heat capacity is about 700 Joules/kg/°C. That means you're pulling out at least 700*0.28*35 = 6.9 kJ of energy to cool the air.

In contrast, you have about 40 kg of water, which has a heat capacity of 4,184 Joules/kg/°C. To raise that 15° would require adding 4184*40*15 = 2,510.4 kJ of energy.

This assumes perfect insulation for both, of course: you cool the air and it stays cool; you heat the water and it stays hot. In reality you'd need to extract more heat from the air over time as well as pumping more heat into the water tank. It also assumes the cooling process is perfectly efficient, which it won't be.

Thank you for injecting a dose of reality.

[GordMay]

IIRC:
One gallon of water requires 8.33 BTU to raise the temperature 1ºF.
BTU OUTPUT = GPM x Temperature Rise (°F) x 8.33 Lbs/Gallon x 60 Minutes

Eg:
10 GPM [or 5 GPM] x 20°F x 8.33 x 60 = 99,960 BTU/Hr [or 49,980 BTU/Hr] to to raise the water temperature just 20°F [11°C].

➥ https://www.advantageengineering.com...tageFYI288.php

A typical marine refrigerator compressors [Danfoss BD 35/50] produce on the order of 500 BTU/Hr of heat transfer, or about 1% of that required, to raise 1 gal of water 20°F [much less than you specify @ 59°F], at a flow rate of 5 GPM.

requiem’s ‘spitball’ calc [#4] is probably more accurate - IDK, I was never a pro'.
However, both indicate the concept is impractical.

The average North American household has a water flow rate capability* of 6 to 12 GPM.
* To service individual fixtures:
Kitchen faucet: 2-3 GPM
Shower: 1.5-3 GPM
Dishwasher: 2-4 GPM
Washing machine: 3-5 GPM

[CaptTom]

Originally Posted by requiem

...Spitballing here... the weight of the 225 liters of air in the fridge/freezer would be about 0.28 kg. It's heat capacity is about 700 Joules/kg/°C...

But of course the goal of the refrigerator is to cool the contents of the fridge. The air is just the medium used to transfer the heat from the contents to the cooling plates/fins.

In my boat, the area above the fridge can get quite warm. A shallow pan/tank of water in there would indeed warm up over time. Nothing I'd want to use for showering, but a bit warmer than what's in the fresh water tank.

Still, I think your calculations show how the two sides of the equation (heat out from the fridge, heat in to the water) don't match up well.

[Tedd]

Originally Posted by CaptTom

But of course the goal of the refrigerator is to cool the contents of the fridge.

Well, yes and no. If you continuously refill the fridge with warm stuff, then yes. But that's not how people normally use one. Normally, the stuff you put in is already close to fridge temperature when you put it in. (Except maybe for a few things like canned drinks.) So, you're really only cooling air that comes in each time you open it, plus removing heat that comes in through the insulated walls when the fridge is closed.

That's why the old trope about wasting energy by leaving the fridge door open is an exaggerated concern. Unless you leave it open an absurdly long time, you're at worst having to cool a few ounces of room-temperature air.

[JBP]

Originally Posted by requiem

Don't underestimate the capacity of water to absorb heat!

Spitballing here... the weight of the 225 liters of air in the fridge/freezer would be about 0.28 kg. It's heat capacity is about 700 Joules/kg/°C. That means you're pulling out at least 700*0.28*35 = 6.9 kJ of energy to cool the air.

In contrast, you have about 40 kg of water, which has a heat capacity of 4,184 Joules/kg/°C. To raise that 15° would require adding 4184*40*15 = 2,510.4 kJ of energy.

This assumes perfect insulation for both, of course: you cool the air and it stays cool; you heat the water and it stays hot. In reality you'd need to extract more heat from the air over time as well as pumping more heat into the water tank. It also assumes the cooling process is perfectly efficient, which it won't be.

That's a bit extreme for an experimental hack. You don't need 40kg of water. I can easily take a quick shower with 4kg (1 gallon). As people said, you are not cooling the air of the fridge, you are cooling the contents. Also a delta-T of 15° C (27F) is not necessary. A quite comfortable, if not hot, shower can be had with water of 35°C (95F), less than 10°C/15F delta-T if it's 26C/80F ambient.

[nmccubbin]

The starting point for the calculation should be the rate of heat leaking into the fridge/freezer, because that is what the refrigeration equipment must put out
A typical well built system leaks about 200 BTU/ hour
A minimal shower is about a gallon, say 8 lbs water, so it could be heated about 50F in an hour
Seems to be enough brute heat, BUT the performance of the fridge would drop with the warm cooling water, severely reducing the real benefit.
At the least, a relatively large, custom, heat exchanger.
Before spending money on hardware, have an engineer with experience in marine fridges do a proper calculation.
It is a lot easier and simpler to heat the water with an engine heatex.

[Cheechako]

Originally Posted by nmccubbin

The starting point for the calculation should be the rate of heat leaking into the fridge/freezer, because that is what the refrigeration equipment must put out
A typical well built system leaks about 200 BTU/ hour
A minimal shower is about a gallon, say 8 lbs water, so it could be heated about 50F in an hour
Seems to be enough brute heat, BUT the performance of the fridge would drop with the warm cooling water, severely reducing the real benefit.
At the least, a relatively large, custom, heat exchanger.
Before spending money on hardware, have an engineer with experience in marine fridges do a proper calculation.
It is a lot easier and simpler to heat the water with an engine heatex.

Interestingly, the under settee refridge unit on my 47 footer had both water cooling and air cooling. The air fan seldom came on. I would think that would work fine for gradually heating some shower water in an insulated tank.

[requiem]

Originally Posted by CaptTom

But of course the goal of the refrigerator is to cool the contents of the fridge. The air is just the medium used to transfer the heat from the contents to the cooling plates/fins.

Good point. I imagine if the fridge was largely filled with water you'd pull a great deal more heat out of it.

[Cheechako]

Originally Posted by Dave_S

Sorry, this question is way out of my comfort zone and is probably not as cleaver as it sounds in my head

If the fridge and freezer can lower air temp in the 225lt box by 35°C all day, surely it can raise the temperature of 40lt of water by 15°C.

Has this been tried ?

If you have a water cooled refrigerator then surely you can rout the discharge water through a small water tank with a coil in it, then out. (like a 5-6 gallon water heater) Not sure how hot it will stay or get but probably warm enough to help. I was surprised once when on the hard how quickly 5 gallons of water heated up trying to use a water cooled refrigerator via a big bucket.!