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Old 07-10-17, 02:18 AM   #6
jeff5may
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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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