Winter fridge assist - using jugs of ice

[Piwoslaw]

Originally Posted by RobertSmalls

If you do the math, I think you'll find moving jugs of liquid water moves far too little energy to make much of a dent in your heating bills. The fact that the water changes phase when it's cold outside is what allows you to transport a meaningful amount of heat in the jugs.

You're right about the dent in the bills. But how would I go about making that dent bigger (just a thought experiment)?

I'd start with a dedicated freezer unit. Set the temperature to -20°C and put in a 1 liter jug of water at 15°C. When the temperature of water drops to 0°C, 15 kilocalories of heat will be released. Freezing the water releases 80 kcal, then lowering the temp to -20°C another 20(?) kcal, totalling 115 kcal = 0.13 kWh.
What is the COP of a freezer in a room-temperature (18°C) environment? I'm guessing around 2, so the freezer would use 65 Wh to cool 15°C water down to -20°C.

That's the basic math, now we start splitting hairs Raising the freezer's temperature to something like -5°C would slightly raise its efficiency, say the COP is now 2.5, and let's bring the incoming water's temperature down to 5°C. Let's assume the freezer can freeze 5kg per 24h, so cooling 5 liters of water from +5°C to -5°C releases 5*(5+80+5)=5*90=450 kcal = 0.523 kWh of heat into the house, using 523/2.5=209 Wh of electrical energy.

Next question: How much energy is needed to heat a house when the outdoor temperature is +5°C? I'll have to check my gas usage and get back to this.

Originally Posted by RobertSmalls

I've thought about placing my fridge's condenser outside the thermal envelope of the house. However, that would deprive me of the heat generated by the fridge, and the heat that leaks from the kitchen into the fridge. My condenser would spend a fair amount of energy making icicles.

Would it be possible to make an enclosure with ductwork which can be seasonally rerouted inside/outside?

[Piwoslaw]

OK, I checked that when the average outside temperature is around 5°C (ie ~10 heating degree day average for 15.5°C HDD), then we use 4.5-7.5 cubic meters of natural gas per day. That would be around 5.5 m3 after subtracting 0.5 m3 for cooking and hot water. For nat gas 5.5 m3 = 59.6 kWh (1 m3 = 10.83 kWh). The efficiency of our central heating boiler was ~90% when new, now it's probably around 85%, so the house needs 59.6 * 0.85 = 50.6 kWh of heat per day, on average of course.
From the previous post I know that freezing 5 liters of water per day would give me 0.523 kWh per day, so only about 1% of our heating needs.

So yes, the dent would be more like a scratch, and barely noticeable.

This might allow the central heat to stay off during the day if the outdoor temperature was 8°-10°C and I used 10 liters of water, but that would probably still be less than 10% of our needs. On the other hand, if the house had better insulation and needed much less heat, then this might be a fun experiment.

[RobertSmalls]

I mean you'll need to move a lot more water if it's not changing phase. Namely, you'll want to pump a working fluid through plumbing to move it through heat exchangers in your fridge and outdoors. However, isn't your particular, high efficiency fridge limited to <100W capacity? That's mighty small for the compressor on an ASHP.

[Piwoslaw]

Well, yeah. The fridge's compressor uses 60-75 watts when on, so even with a COP of 2.5 it'll heat the room like two 200W lightbulbs. Might work for a passive house if on for 24h per day.

But at least I have it calculated now, and I know it's not worth the hassle. It's also a starting point to estimating the global energy loss if used as Daox originally proposed.