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> But heat pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures. This

> But heat pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures. This claim defies thermodynamics (and I expect better from IEEE). Heat pumps are bound to be less efficient as the spread between inside and outside temperature grows. (They might be just as effective or they might be just as efficient as current heat pump tech, but they will not be as efficient at -20°C outside air temp as they are at +10°C.)

fullstop 3y ago
Are they perhaps referring to geothermal heat pumps?
- swechsler 3y ago
  No, they specifically say "air source" heat pumps. And geothermal isn't new.
- solardev 3y ago
  No, they're talking about air source ones (which is what's popular these days): > At about 4 °C, most air-source heat pumps currently on the market start operating at less than their full capacity, and at some point (usually around -15 °C), they can no longer do the job at all. At that point, an auxiliary heat source kicks on, which is less efficient. > But advancements in compressor technology over the last five years have addressed that issue. By controlling the compressor motor’s speed, and improving the timing of when vapor is injected into the compressor, engineers have made heat pumps efficient in colder temperatures.
bryanlarsen 3y ago
" just as efficiently " as what? The way I read it was "just as efficiently as a conventional furnace". IOW, a COP of 1 at bitter cold temperatures.
- sokoloff 3y ago
  the next words are "[as] they do", so I naturally read it as 'those same heat pumps'.
  - bryanlarsen 3y ago
    Yes on rereading it definitely appears phrased the way you interpreted. But as you said that's nonsensical so my brain must have dumped the nonsensical interpretation. :)
unbalancedevh 3y ago
I've found that marketing material takes significant liberties with their claims, with the understanding that the audience isn't actually interested in the gory details, just the general question of "should I buy option A or option B?" I've been asked to critique statements that were flat-out wrong, only to be told yeah, thanks, nobody cares about that.
ajross 3y ago
If you want to get nitpicky: Cycle losses aren't the only source of inefficiency and in fact the end-to-end efficiency curves of most domestic heat pumps tend to be pretty flat through their useful range. Basic thermodynamics, provided a refrigerant that is well matched to the cycle (which is what the article is actually about), isn't really the core design point.
- sokoloff 3y ago
  I've never seen a CoP curve that was not downward sloping with decreasing outside temperature. Do a google image search for "heat pump cop curve" and glance at the results. All of them are significantly downward sloping because the underlying thermodynamic cycle effect is very strong (agree not the only) limit.
  - deleted 3y ago
    [deleted]
atoav 3y ago
My father has a heat pump that works fine in alpine climate (half of the winter snow outside) for more than a decade now. Not sure about the precise english terminology here, but his hear pump works using the deep ground water next to our house. The temperature of that water slightly dips in winter, but it is no more than a degree. It is a very old house that before had a oil heating system and I think the replacement worked quite well.
- sgerenser 3y ago
  That’s a ground-source heat pump (often called geothermal heat pump). These have been around for decades in the U.S. but haven’t been very popular due to the 2-3x installed cost vs. an air source heat pump.
- solardev 3y ago
  This is common in some parts of the world (which is great!), but not in the US. They're quite a bit more expensive, especially compared to modern mini-split air-source heat pumps that can cost just a couple thousand dollars and don't require any digging or duct installations, just a small hole in the wall.
  - atoav 3y ago
    I guess my father was just lucky, because 1m through the wall from our oil cellar we had an old well that could be used, so there was no need for much more digging (except 1m sidways). He added a ton of solar as well, which is a nice combination.
DannyBee 3y ago
this feels pedantic. They are clearly not talking about the theoretical bounds, but instead in practice. The claim can be literally true. It may be they are at x% efficiency in both cases, where x is optimal for bitter cold but not for mild winter.
- sokoloff 3y ago
  > The claim can be literally true. It can be. Would you take an even-money wager that the CoP curve will be flat [or increasing] with decreasing outside temp vs my wager that it will be downward sloping with decreasing outside temp?
  - DannyBee 3y ago
    The claim is true. You've transformed their claim into something else. What they wrote is " But heat pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures." That is about whether they gained the technology to make the finished units as efficient in bitter cold as they are currently efficient in mild winter. Given they were inoperative in bitter cold, and fairly inefficient in mild winter, this is almost certainly trivially true. Here is a real curve: https://hvac-talk.com/vbb/attachment.php?s=f7eb5f83efc8804403081f85ab72d79d&attachmentid=822851&stc=1&d=1615838362 https://hvac-talk.com/vbb/attachment.php?s=f7eb5f83efc880440... If we all mild winter 32F, and bitter cold -10F, they would only have to improve efficiency by 2x at bitter cold to make it as efficient as current heat pumps as mild winter. That is easily possible. As for whether they are strictly flat/decreasing, they are not. Here is more data: https://connect.fujitsugeneral.com/3cfb13a7-a386-4697-aa3e-58187dfd578f https://connect.fujitsugeneral.com/3cfb13a7-a386-4697-aa3e-5... You can see capacity, input power, and efficiency do not have a strictly linear (flat or decreasing) relationship with outdoor temperature. This is an artifact of it being impossible to good at all things at once. It is also obvious to every reader you can not make a "maximally efficient in all" cases heat pump as efficient in bitter cold as in mild winter. So rather than assume that is what they are claiming by adding a lot of implicit things to it, i would just read it in a straightforward manner and when I do, it seems pretty easily true.
cma 3y ago
In Kelvin the percentage difference looks a lot smaller than -20°C to +10°C, more towards a rounding error for an ideal heat pump. So if they can operate at the same percentage efficiency away from ideal, the extra difference from the actual source/sink temperature could be very minimal right?

fullstop 3y ago

Are they perhaps referring to geothermal heat pumps?

swechsler 3y ago
No, they specifically say "air source" heat pumps. And geothermal isn't new.
solardev 3y ago
No, they're talking about air source ones (which is what's popular these days): > At about 4 °C, most air-source heat pumps currently on the market start operating at less than their full capacity, and at some point (usually around -15 °C), they can no longer do the job at all. At that point, an auxiliary heat source kicks on, which is less efficient. > But advancements in compressor technology over the last five years have addressed that issue. By controlling the compressor motor’s speed, and improving the timing of when vapor is injected into the compressor, engineers have made heat pumps efficient in colder temperatures.

bryanlarsen 3y ago

" just as efficiently " as what? The way I read it was "just as efficiently as a conventional furnace". IOW, a COP of 1 at bitter cold temperatures.

sokoloff 3y ago
the next words are "[as] they do", so I naturally read it as 'those same heat pumps'.
- bryanlarsen 3y ago
  Yes on rereading it definitely appears phrased the way you interpreted. But as you said that's nonsensical so my brain must have dumped the nonsensical interpretation. :)

unbalancedevh 3y ago

I've found that marketing material takes significant liberties with their claims, with the understanding that the audience isn't actually interested in the gory details, just the general question of "should I buy option A or option B?" I've been asked to critique statements that were flat-out wrong, only to be told yeah, thanks, nobody cares about that.

ajross 3y ago

If you want to get nitpicky: Cycle losses aren't the only source of inefficiency and in fact the end-to-end efficiency curves of most domestic heat pumps tend to be pretty flat through their useful range. Basic thermodynamics, provided a refrigerant that is well matched to the cycle (which is what the article is actually about), isn't really the core design point.

sokoloff 3y ago
I've never seen a CoP curve that was not downward sloping with decreasing outside temperature. Do a google image search for "heat pump cop curve" and glance at the results. All of them are significantly downward sloping because the underlying thermodynamic cycle effect is very strong (agree not the only) limit.
- deleted 3y ago
  [deleted]

atoav 3y ago

My father has a heat pump that works fine in alpine climate (half of the winter snow outside) for more than a decade now. Not sure about the precise english terminology here, but his hear pump works using the deep ground water next to our house. The temperature of that water slightly dips in winter, but it is no more than a degree. It is a very old house that before had a oil heating system and I think the replacement worked quite well.

sgerenser 3y ago
That’s a ground-source heat pump (often called geothermal heat pump). These have been around for decades in the U.S. but haven’t been very popular due to the 2-3x installed cost vs. an air source heat pump.
solardev 3y ago
This is common in some parts of the world (which is great!), but not in the US. They're quite a bit more expensive, especially compared to modern mini-split air-source heat pumps that can cost just a couple thousand dollars and don't require any digging or duct installations, just a small hole in the wall.
- atoav 3y ago
  I guess my father was just lucky, because 1m through the wall from our oil cellar we had an old well that could be used, so there was no need for much more digging (except 1m sidways). He added a ton of solar as well, which is a nice combination.

DannyBee 3y ago

this feels pedantic. They are clearly not talking about the theoretical bounds, but instead in practice. The claim can be literally true. It may be they are at x% efficiency in both cases, where x is optimal for bitter cold but not for mild winter.

sokoloff 3y ago
> The claim can be literally true. It can be. Would you take an even-money wager that the CoP curve will be flat [or increasing] with decreasing outside temp vs my wager that it will be downward sloping with decreasing outside temp?
- DannyBee 3y ago
  The claim is true. You've transformed their claim into something else. What they wrote is " But heat pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures." That is about whether they gained the technology to make the finished units as efficient in bitter cold as they are currently efficient in mild winter. Given they were inoperative in bitter cold, and fairly inefficient in mild winter, this is almost certainly trivially true. Here is a real curve: https://hvac-talk.com/vbb/attachment.php?s=f7eb5f83efc8804403081f85ab72d79d&attachmentid=822851&stc=1&d=1615838362 https://hvac-talk.com/vbb/attachment.php?s=f7eb5f83efc880440... If we all mild winter 32F, and bitter cold -10F, they would only have to improve efficiency by 2x at bitter cold to make it as efficient as current heat pumps as mild winter. That is easily possible. As for whether they are strictly flat/decreasing, they are not. Here is more data: https://connect.fujitsugeneral.com/3cfb13a7-a386-4697-aa3e-58187dfd578f https://connect.fujitsugeneral.com/3cfb13a7-a386-4697-aa3e-5... You can see capacity, input power, and efficiency do not have a strictly linear (flat or decreasing) relationship with outdoor temperature. This is an artifact of it being impossible to good at all things at once. It is also obvious to every reader you can not make a "maximally efficient in all" cases heat pump as efficient in bitter cold as in mild winter. So rather than assume that is what they are claiming by adding a lot of implicit things to it, i would just read it in a straightforward manner and when I do, it seems pretty easily true.

cma 3y ago

In Kelvin the percentage difference looks a lot smaller than -20°C to +10°C, more towards a rounding error for an ideal heat pump. So if they can operate at the same percentage efficiency away from ideal, the extra difference from the actual source/sink temperature could be very minimal right?