If you get to -20C a couple of times a year but normally it's -5, it doesn't matter that it's inefficient for 2 or 3 days a year, as it's very efficient for 360 days a year
No, that's about the capacity of the heating system at -20, not efficiency. It does mean that the system will be oversized for the days it doesn't get that cold.
Thanks for putting specific numbers to it. It's really surprisingly efficient, especially at warmer temps (0?) which are more common in most of the US.
COP is still decent at low temps. That is the point. They are 80% efficient at their lowest temp rating. And, at least where we live 99.9% of days will be 100% efficient.
They are more than 100% efficient until you reach stupidly low temperatures; in which case, then you can break out the furnace or whatever. Or put on a blanket.
While it's not the best thing to do, it doesn't matter much as the efficiency of the systems go way way up when the outside temp is near the same as the inside temp.
5 is highly optimistic. Real world average efficiency is closer to 2.5 (lower in colder climates) because there are a lot of variables you have to get exactly right to hit that 5 COP, and they very rarely line up.
Source: I have a Fujitsu 36K BTU cold climate 4-zone system installed two years ago to replace electric baseboard heat. [1]
I was responding specifically to this bit: "Once it hits 0C the efficiency is very low, and by -10C is almost doing nothing at all." I've refuted that. If you want to discuss some more distant goalposts, I won't be able to offer firsthand evidence.
I didn't explain clearly... AC systems don't have a single efficiency number - they have different efficiencies depending on the indoor temperature, the outdoor temperature, and the number of kilowatts you want delivered.
You can generally design any AC system to work efficiently at any specific combination of those variables - but if any variable deviates far from the optimum design point, efficiency will drop.
So the real question is, not "I want an efficient system", but "I want an efficient system when it is 20F outdoors, because that's the temperature most of the year".
Looking at the figure attached to the article, you can see that the only > 100% efficient trial was with ambient 135 degree-C temperature (two other trials at 84 degree-C and 25 degree-C were much less efficient). The extra energy comes from the environment.
Depends. It's not just efficiency, it's cost savings. If someone is running a large enough operation to save millions of dollars a year in cooling by using CFCs (assuming they're actually cheaper), and one is in a, um, lightly regulated industry (like Bitcoin mining) that uses massive power... well, that's a lot of justification. Small savings become big savings at scale.
It's actually exactly the reverse: for a house that's well-insulated, it barely matters how efficient your heating and cooling appliances are, especially if you're considering replacement of working systems.
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