is my heat pump working?

[wyounger]

August 2014 US-spec BEV with standard heat pump question:

OK, so the first really cold weather has arrived in my part of the world and I'm starting to have doubts about my heat pump. Certainly when the A/C is working hard, the compressor is fairly noisy, and from outside the car it can be heard from the rear left underneath the car, and sometimes even from inside the car with the windows closed. Now that I've run the heat in cold weather, the range has dropped off more than expected (and this is not my first EV), but I never hear the compressor running. I can confirm that the heat exchanger fans are running up front despite the cold conditions (which suggests that it's trying to use the heat pump), and the heating itself is working fine, but I'm starting to wonder if my heat pump is actually working or if it's just using resistance heat.

Is the heat pump heating using the same compressor as AC cooling? Any reason it should be freakishly quiet in heating mode? In my experience with residential heat pumps, they are usually noisier in heating mode than in cooling mode. Is there any good way to confirm heat pump operation/performance, or any known method of heating system operation that would fit the pattern I'm seeing?

By the way, my definition of "cold" is 20-50 degrees F or roughly -5 to 10 degrees C. I park the car in an enclosed garage that doesn't get very cold, charge overnight, pre-condition based on set departure time, drive efficiently (I get an indicated 5.0 miles/kWh when the weather is mild and I'm not on the occasional high-speed highway trip), and left the house this morning with a full battery and an indicated 56 miles of range.

 

[jadnashuanh]

Someone had indicated that the reason the REx doesn't have a heat pump is that it goes in that location on the BEV and there'd be no room (this also would mean that space isn't available for optional storage or extra batteries)...this implies to me that the heat pump and the a/c compressor are not the same unit as is more typical in a non-mobile configuration (those just use a reversing valve). IF that is the case, that may account for the differences. I'm not sure the BEV has resistance heat as a backup...I'll need to research a bit more.

 

[wyounger]

The heat pump will still have a resistance heater; heat pump output is inversely proportional to ambient temp, so at some given temperature the heat pump's output will not be enough, and a supplemental heat source is required to be able to meet the heating demand. Sometimes this fact is lost in translation because many people consider the resistance heater to be part and parcel of a heat pump- they are universally present in central forced air heat pumps in North American residential applications for this reason. Also, heat pumps have to defrost occasionally (usually takes about 5 minutes every 60-120 minutes of operation), and during that time they are back in what is essentially A/C mode, so if you don't want a sudden blast of cold air blowing out for a few minutes, you have to use the supplemental heat source to re-heat that air for comfort purposes.

I imagined the additional componentry taking up space under the frunk on models with the heat pump was a heat exchanger to convey the heat from the heat pump into the water that circulates through the heater core.... which I assume is still present on BEV models with or without the heat pump to avoid the danger of putting high voltage equipment inside the passenger compartment. Given that I have been told that one of the cooling system expansion tanks/caps under the bonnet/hood is for the heat/AC system also suggests that the i3 also works this way. Instead the resistance heater heats water that goes through a little radiator in the ductwork. I know that's how my Volt worked. It's impractical to deliver sufficient heat using a 12 volt system so it has to be high voltage. If they get really fancy, they could even heat the water for the cabin heating while simultaneously cooling the A/C coil for dehumidification. This approach is common throughout the automotive world to provide good humidity control and defrosting in cool conditions. The cabin air is first cooled a little bit more, to remove some moisture, then reheated to provide the correct temperature. If it were set up in a totally traditional way for a stationary-type heat pump, using just two coils and a reversing valve, the cabin air could only be heated or cooled/dehumidified at any given time, not both, since that coil can only have one temperature at any given time.

It might actually be simpler to have a separate heating compressor and cooling compressor to achieve the goal of passing cabin air sequentially through a cold coil and then a hot coil. I know this sounds ridiculously inefficient but at the conditions in which it's necessary (typically around 50 F/10C) it very little additional energy since the goal is only to cool the air a few degrees before heating it, and that's what provides the dehumidification benefit. Under the conditions that are obviously only for heating (cold and dry) or only for cooling (hot and dry or hot and humid) this strategy is not used; then you only heat or only cool.