The Carnot limit of heat pump heating from -25C (-13F) to 20C (68F) is over 6.5. That's 650% efficient. There's a lot of heat in our 'cold' air. If you're going to try to claim 'thermal dynamics' [sic] you should at least do the math on it first:
Also, people often don't realize that the temperatures that matter thermodynamically are degrees Kelvin, not Celsius. 30 below 0 is 243K; 20 above (room temperature) is 293K. The coefficient of performance for a Carnot heat pump at those temperatures is 293 / (293 - 243) ~= 5.86; that means that one joule of work will transfer 5.86 joules of heat. This isn't great, but it's less of a difference than people would expect given our subjective experience of how cold -30C is. The coefficient of performance for a heat pump trying to heat 0C ambient temperatures up to room temperature (or alternatively, an air conditioner cooling 40C temperatures to room temperature) is 293 / (293 / 273) = 14.65, so your heat pump in Calgary will be maybe 2.5x less efficient than one in NYC. Drill below the frost line, like you suggest, and you'll get similar efficiencies regardless of where you are.
Heat pumps can heat in cold temperatures, but at decreasing efficiency or COP, which is how much heat you pump from the outside divided by how much heat it took to run the compressor. The theoretical COP is infinite if $\Delta T = 0$, and you can use Carnot to calculate the upper limit otherwise.
Now, the whole point of having a heat pump is to have a COP (much) greater than 1 (an excellent furnace has a COP just below 1). Once the difference in ambientT-outsideT becomes large, you approach COP = 1. The heat pumps manufacture will report the actual COP vs $\Delta T$; that is including losses, vs theoretical.
Now, a heat pump runs (almost certainly) on main power, and the alternative (usually) on natural gas.
It's a simple matter, therefore, to calculate the CO2 and financial cross over point for a heat pump vs. a furnace. I did that calculation when I had one installed in my house (for ductless AC; heating is an added bonus). It's somewhere between about -5 or -10 C, if I recall correctly.
Sure, a heat pump can work at -25C but it'll cost more and emit more CO2. This is because, dumb Joule for dumb Joule, natural gas piped to my home is cheaper and cleaner than the same natural gas piped and burned in a thermal power station [1]
In my small corner of the world this means that the heat pump is usually better to run than my furnace. But I keep my furnace on anyway, basically set on idle because there is no way that a heat pump can compete with a furnace on reliability. I don't want burst water pipes.
In Europe, heat pumps make 100% sense. Even though Europe is significantly north of the US, the winters are mild with few stretches below 0. In contrast, in some parts of the midwest long stretches of -20 C are not impossible. A week at -20 is not unheard of. In this situation Id be loath to have to rely exclusively on heat pumps.
[1] actually coal in my case so I fudged it in favor of the HP. Coal is also the case for many of you as well because coal is typically used as marginal power during the winter since it's easy to store and gas pipelines' pressure drop in the winter due to home demand.
if carnot diagram is drawn, it will show nothing cheap.
try to recall thermal dynamics taught many years ago.
heat pump works well when the exterior temp differ not much from interior temp.
Namely for places like Cal and Tex, still the winters need heating.
if it is freezing temperature, much energy will be spent on the mechanical part. Literally an electric heater but maybe less efficient than an interior heater by your knee. Not very logically coherent. Recall the professor's words.
thermal dynamics is a very chilling subject, basically demysterifying fundamnetally and establish a solid view of engineering. Its experiments were repeated by millions of engineering students, the rules derived still hold firmly on earth so far.
The other way to put it is that your efficiency is proportional to 1/(delta T). In other words if your heat pump is working between two environments with very close temperatures then the heat transfer is very efficient (for example 19 degrees C outside, 21 inside). But if you are trying to heat your house to a livable temperature while it’s -20 outside, your efficiency is terrible.
This is why for example you see heat pumps installed all over the place in homes in the southern parts of the US where you also need little heating all around, while places up north where you do spend quite a bit on heating cannot really take advantage of heat pumps due to winters being too cold.
I'm not an expert on the topic, but I imagine that a heat pump that can handle -20F is also much more energy efficient when the temp is 0F (in comparison to a heat pump that was rated only for -5F, operating at 0F).
> Heat pumps, which heat and cool in one system, are another application in which the motor can save energy, ease installation, and reduce noise.
Assuming the efficiency gains are real, increasing the max COP ("efficiency") of heat pumps beyond their already astounding levels of near 5 (500%) would be a game changer for heating and cooling.
Even at quite cold temperatures (sub freezing), the latest heat pumps achieve a COP of 2.5 (250%), a level at which they beat the efficiency of gas furnaces even when powered by 40% efficient natural gas generated electricity.
Modern cold-climate heat pumps can definitely handle 20F. Yes they're more efficient at higher ambient temperature, but you can get at least 2.5 COP at 20F. (And good ones can continue to outperform resistive heating at much lower temperatures than that.)
We live in Denver and saw -10, -15 this winter. Our heat pump is 100% efficient down to -10, and still reasonably efficient at -25. Our whole house is on it. No gas.
But per Carnot, maintaining a fixed temperature differential against a fixed thermal resistance is cheaper (requires less work) at a higher temperature (1/(1-(Tcold/Thot)) is the carnot-limited "coefficient of performance" for a heat pump).
So theoretically they should be able to maintain the design differential even at higher outdoor temperatures, causing indoor temperatures to raise from ~21°C to 26°C comparing the "still works at 35°C" and "40°C happens regular in $current_year".
This kind of nitpicky debate gets so tiresome. Look, upthread comment implied pretty clearly that a particular heat pump as installed would "work" until zero net energy flow at "-20C". And I thought that was misleading, because that's not the threshold you use to size a heat pump as is doesn't reflect a steady state (at that temperature your home will be inexorably losing heat). And as I happen to have had personal experience in the recent past with measuring exactly this threshold on a newly installed heat pump, I thought people might be interested.
I'm deeply sorry if this offended you for some reason, but I still don't see how you're disagreeing with anything I wrote.
Maybe I'm missing something, but no matter how efficient a heat pump is, doesn't it have a limit on how much heat it can pump as ?T increases?
Even here in western NY we occasionally have to deal with -10 F (-23 C) weather for a week and so that's a pretty big temperature delta if you want a comfortable 68 F (20 C).
Combine that with the fact that gas is still quite a bit cheaper per unit of energy than electricity (in my area) and a heat pump just doesn't make any sense.
The ability to have a single unit heat and cool is definitely nice, but without a geothermal-like ambient temperature to work with, I just don't see this working out, even with all the insulation in the world.
My understanding is that heat pumps operate at less than 100% efficiency at those levels. They don’t start blowing cold air, rather the air isn’t 90 degrees coming out any more.
200-300% efficient is what a very bad heat pump would do under ideal conditions. Good heat pumps have a coefficient of performance above 3 for temperatures above freezing (and above 4 for only mildly chilly outdoor temperatures), and still well above 2 at temperatures around 15°F/-10°C.
My 10 year old apartment has a heat pump rated for a CoP of 2.3 at a temperature of 17°F/-8°C, which is actually below the record low outdoor temperature for this area. This isn't some exotic new equipment or a model designed for cold climates; it's what was cheap and mainstream a decade ago.
1 / (1 - (248.15/293.15))
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