Some friends just had their AC and furnace replaced with a heat pump. The system is rated to provide heat when the outside temperature is as low as -4F. Certainly not as efficient as providing heat when it’s 40F outside, but it’s similar to what happens when running the AC when it’s 80F or 110F outside.
Much of the heat pump’s ability to move heat from a very cold outside to inside is dependent on the properties of the coolant that is used to transport the heat.
I was over there last week when it was about 5F outside, and the heat pump was keeping the house at 68F without using the auxiliary heater. -4F is probably fine for our area (Colorado front range). It occasionally gets colder, but running the auxiliary heater a handful of nights every few years isn’t enough to wipe out the savings from the efficient heat the rest of the time.
To bring it back to the topic of the thread. My EV uses resistive heat, not a heat pump. Two drives from home to the same place 15 miles away, one by me with the heater off, and one by my wife with the heater on.
My drive
Wife’s drive
Outside temp
34.1F
31.3F
Inside temp
55F
68F
Wh/mile
212
306
Avg MPH
35
38
Cost
$0.26
$0.37
So a real, measurable difference, but also a difference that doesn’t matter for this type of trip.
Tl;dr:
Automotive heat pumps still give a COP better than unity at -20C outside and 20C inside. But unity means no better than a resistive heater. If there is waste heat from the motive components things will improve, but in the limiting case of a car stranded on the road, they still work.
Clearly the COP improves significantly as the outside temperature increases. Given they also cool the cabin in summer and can operate to manage battery temperatures as well there are many reasons they are used.
My geothermal heat pump has four vertical well/pipe loops just under 200’ deep. In Missouri you need a permit from Dept. of Natural Resources to go deeper than 200’.
I’m on 5 acres and had room for horizontal trenches, but it would have disturbed more of the landscape.
NHTSA is issuing a recall (PDF) of certain Teslas with heat pumps to correct a heating problem. The explanation of the error:
the Electronic Expansion Valve (EXV) may experience controller communication interruptions, after which the EXV driver may attempt to realign the EXV rotor by rotating it in a small increment toward the open direction. When heat pump vehicles delivered with firmware release 2021.44 through 2021.44.30.6 experience such interruptions, the vehicle software does not close the valve, and the accumulation of interruptions and subsequent realignments over extended periods where the vehicle is awake may result in an unintended valve opening event. This, in turn, may trap refrigerant inside the evaporator and may deplete the refrigerant from the active components in the system. The depletion may result in fail-safe compressor stoppage, and cause loss of cabin heating, particularly in temperatures –10 C or colder.
For the third (I think) time in a few weeks Tesla has a recall which is really just a software update. An over the air software update is easier than bringing hundreds of thousands of cars into service centers to replace a faulty valve, but also shows they need some more rigorous quality control around their software updates.
It appears I’m not the only Canadian asking this question. A recent survey by KPMG found that while there is growing interest amongst Canadians in buying an EV, one factor creating caution is concern about how EVs handle the cold:
Whether it’s a refrigerator, or an air conditioner, or something similar mounted in reverse, a heat pump simply pumps calories “uphill”. using the heat of evaporation of a fluid, it takes heat from the colder side and pumps it into the warmer side.
Like any analogous pumping uphill, this works best when the heat difference is less. Pumping heat from -4F to 68F (-20C to 20C) requires more energy than pumping heat from 32F to 68F (0C to 20C) for example. In most common scenarios, pumping calories uphill is more efficient than creating them from electricity. It may even be more cost effective than burning carbon fuels like natural gas.
I presume one of the advantages of the heat pump in newer Tesla automobiles is that it is both the heater and the air conditioner, simply by redirecting the hot and cold flows.
There was a Tesla bug which apparently cause loss of cabin heat in cold weather. From what I read, this was attributed to a software flaw which did not properly fully close a valve in the pump with repeated use - more likely to occur in colder weather. The error accumulated until the valve stayed open. Apparently an over-the-air software update can fix this.
Interestingly, this does not tell me anything about whether or not EV’s are reliable in cold, harsh weather.
It does tell me that it is very possible that there has been a very successful disinformation campaign that is spreading the worry that EV’s are not reliable in cold, harsh weather. I have seen a lot of disinformation about EV’s on a wide variety of fronts that are trying to tell us that EV’s are not good. And most of this disinformation is complete and utter bullshit, without a grain of truth.
EV’s DO lose range in cold weather. This is a fact. Does this mean they are not “reliable enough”? I don’t think so.