I guess we'll see as more details arrive, but I don't think a car with only a heat pump can meet FMVSS requirements. One with two heat pumps might be able to, if they can pump out enough heat in extreme cold, but the need to defrost makes a single heat pump with no backup a non-starter.
-
Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
I guess we'll see as more details arrive, but I don't think a car with only a heat pump can meet FMVSS requirements. One with two heat pumps might be able to, if they can pump out enough heat in extreme cold, but the need to defrost makes a single heat pump with no backup a non-starter.
Mitsubishi's modern heat pumps operate at 100% capacity at 5f/-15c, 62% capacity at -13f.
willow_hiller
Well-Known Member
Is capacity equal to efficiency? Because I thought resistive heat is roughly 100% efficient (1 watt put in yields 1 watt of heat). So this would mean the Mitsubishi heat pump is better than resistive heat down to -15 C, and then less efficient at colder temperatures?
I was confused by this statement too. If the efficiency drops below 100% it's better to just break out the space heaters. If capacity (BTU) drops below max, but it's still >100% efficient it is still better to run the heat pump.
No. Capacity is the rated heat output of the system. The efficiency is a separate number, though that does generally drop as the temperature drops as well.
vjason
Member
Any detailed stats on electricity usage for the sub-freezing temps?
I’d be interested to know the efficiency hit as the temp drops. I know how units take a hit each year they age, wonder if car units would be similar.
My dad used to design units for Lennox so I grew up asking questions/hearing about this stuff. Finding cheap electric heating well below freezing is a cool subject. Clothes dryer cos are testing (I believe) ultrasonic waves, but that is more for separating water from clothes not really heating the air.
I’d be interested to know the efficiency hit as the temp drops. I know how units take a hit each year they age, wonder if car units would be similar.
My dad used to design units for Lennox so I grew up asking questions/hearing about this stuff. Finding cheap electric heating well below freezing is a cool subject. Clothes dryer cos are testing (I believe) ultrasonic waves, but that is more for separating water from clothes not really heating the air.
Mitsubishi has a unit with a 12.5 HSPF (heating season performance factor). An electric resistance heater has a HSPF rating of about 3.4. While I don't know the efficiency at each temp, over the course of whatever the gov. considers a typical heating season, a 12.5 HSPF air-source heat pump would be about 3.6 times more efficient than an electric resistance heater (12.5/3.4=3.6).
“In” before next winter when all the Y complaints roll in due to the heat pump heating issues in sub temps. (Not an engineer but sounds like that’s where this is headed, though I could be wrong and Tesla could have done a Myriad of testing in cold climates properly like they did with icing handles, stuck frameless windows and frozen charge port issues) on the 3. Glad we didn’t have any of that!
Ski
Ski
The efficiency of a heat pump is called the 'Coefficient of Performance' (COP),
Coefficient of performance - Wikipedia
When the heat pump is used for heating (as opposed to cooling, i.e. A/C), then the COP is the heating power delivered, per amount of power consumed. In the case of a Tesla, the heat pump is powered by electricity (gas powered designs also exist), so that's the input power, while the heating delivered comes in the form of heated, circulating fluid. The extra heat that the heat pump can deliver is extracted from the surroundings (i.e. the ambient air), so the colder the surroundings the lower the COP.
Geothermal heat pumps can achieve a COP of 5 (or higher), a vehicle based one less.
For comparison resistance heating has COP=1, i.e. the delivered power equals the input power. This value is also the lower limit for a heat pump. A heat pump could reach a COP close to one, for example by running in an already cold and also closed garage. This would leave basically no heat for the heat pump to extract from the cold air in the closed garage.
In principle, an electric car like a Tesla can use resistance heating without having a dedicated, electrical heating element. Instead, the motors can be run in an deliberately inefficient fashion, causing them to produce heat without (extra) motion. The waste heat can then be transferred via the motors' coolant fluid to where it is needed, via the super bottle and its valves. This is what is happening when the Tesla displays the message 'preconditioning battery for supercharging'.
Tesla has to have some kind of supplemental heating system for brutal cold temps. My guess is they will extract heat from they glycol loop in a similar fashion to how they heat the battery currently....but then how do you heat the battery and the cabin at the same time?
Perhaps they have a 6kW resistive battery heater in the glycol loop (very cheap unlike Model 3's PTC heater) like the Model S/X have that can be used on very cold days to add heat to the glycol loop (and the cabin via the heat pump). This would only be used on very cold days where both the cabin and battery need heat. On normal winter days between 20-50F the heat pump will be plenty.
This system would be slightly less efficient on short trips in very cold temps (resistive heat to glycol loop with heat pump losses, vs just PTC), but more efficient on long trips once the motor and inverter are producing their own waste heat.
Also, unlike residential heat pumps, a car has the advantage of 60-80 mph air over the evaporator at highway speed which not only provides high volumes of air to work on, but also some mechanical turbulence that will cause a slight temperature rise (drag). The evaporator air intake is low to the ground which is a negative because the air that close to the ground can be very cold. They need a warm air intake from a higher location.
Perhaps they have a 6kW resistive battery heater in the glycol loop (very cheap unlike Model 3's PTC heater) like the Model S/X have that can be used on very cold days to add heat to the glycol loop (and the cabin via the heat pump). This would only be used on very cold days where both the cabin and battery need heat. On normal winter days between 20-50F the heat pump will be plenty.
This system would be slightly less efficient on short trips in very cold temps (resistive heat to glycol loop with heat pump losses, vs just PTC), but more efficient on long trips once the motor and inverter are producing their own waste heat.
Also, unlike residential heat pumps, a car has the advantage of 60-80 mph air over the evaporator at highway speed which not only provides high volumes of air to work on, but also some mechanical turbulence that will cause a slight temperature rise (drag). The evaporator air intake is low to the ground which is a negative because the air that close to the ground can be very cold. They need a warm air intake from a higher location.
Last edited:
I think when they update the model 3 to match the Model y's interior they will include the heat pump. This includes the wireless charger with a fast charge and USB C. This is just speculation from my end as ultimately i believe they want to have cars identical so its one common piece for both vehicles
I'm not an engineer, so I take it with a grain of salt, but the guys in this video state that the heat pump is 300% efficient vs. the 100% of a resistive heater, though the trade-off is that it may heat the cabin more slowly. Does this make sense to those who have more knowledge than I do in this area?.
willow_hiller
Well-Known Member
I'm only familiar with heat pumps as they apply to residential heating and cooling, but they are more than 100% efficient as long as they're above freezing in most cases (they utilize the energy in the environment to provide more heating/cooling than the Wh used to run them). 300% efficiency means that if you put 1 Wh into running the heat pump, it's able to draw 3 Wh worth of heat from the environment into the cabin (even if the environment is colder than the cabin, the heat pump does this by cooling itself below the ambient temperature of the environment). Efficiency drops below freezing because generally the heat pump needs to expend extra energy defrosting itself. Resistive heat is always 100% efficient or less, you get 1 Wh worth of heat for every Wh put in.
Tesla has made the heat pump system fairly complicated, at least according to a patent they filed. In the cabin is both a condenser and evaporator, so yes the single heat pump can both heat and cool at the same time to deal with humidity issues. (@mongo has made several posts with details about the patent in other threads.)
Tesla has designed a number of strategies to deal with extreme cold. The heat pump can scavenge heat from the power train, it can run the heat pump in an inefficient mode to create heat (like how it heats the battery in the Model 3 by running the drive units inefficient), it can run the HVAC blower inefficiently to create extra heat, and finally there are two LV, i.e. 12V, heaters in the cabin that can provide heat. (Though from the patent they are mainly used for when the driver and passenger have different temperature set points.)
So it seems like they have all of their bases covered. Hopefully it proves to be reliable.
Hi all
The patent: US20190070924A1 - Optimal source electric vehicle heat pump with extreme temperature heating capability and efficient thermal preconditioning - Google Patents
Octovalve
Possible design
The patent: US20190070924A1 - Optimal source electric vehicle heat pump with extreme temperature heating capability and efficient thermal preconditioning - Google Patents
Octovalve
Possible design
Yup. I wrote that before any of the patent details came out.
One heat pump can do both if it is running like a dehumidifier, with the same air pushing across both evaporator and condenser - but that isn’t possible for the typical heat pump, where you only have one air to refrigerant interface that you’re swapping between evaporator and condenser.
Of course, Tesla wasn’t satisfied with the typical heat pump. It’ll be interesting to see how the Y pumps perform in real winter weather, but I’m not betting against them.
Similar threads
