Do I need to turn on AC for heat?

I don't understand all of the thermodynamics (it's been a while since my college course in that, and I don't work in that field), but the general idea of a vapor injection heat pump is that when the refrigerant is compressed, not all of the gas is liquefied, and the vapor separator cycles the vapor back into the compressor, while the liquid refrigerant goes to the outside heat exchanger, which in heat pump mode, acts as the evaporator. This increases the efficiency of the heat pump at low temperatures, and lets it work down to 14F.

 

In a generic heat pump(thing that moves heat around) the compressor pressurizes the refrigerant, which causes it to heat up. It flows to the condenser, there it sheds heat to the environment. Next comes the expansion valve. After passing through that, the pressure the refrigerant was under drops, so it expands. As it does, it absorbs heat from the environment at the evaporator, before heading back to the the compressor.

In a system with vapor injection, the refrigerant line splits after the condenser and there is a heat exchanger(called an economizer). The main refrigerant line goes through the economizer before going to the expansion valve and evaporator. The second line is the injection one. It passes through its own expansion valve before hitting the economizer. So the economizer is acting as an evaporator for the injection refrigerant. The refrigerant picks up heat, but not from the environment, but from the main refrigerant. Then it is injected into compressor.

With the main refrigerant being extra chilled by the injection refrigerant, it can pick up more heat at the evaporator, as the greater the heat difference, the faster heat moves. The compressor also works more efficiently, but I admit to not understanding exactly how.
https://climate.emerson.com/documents/enhanced-vapour-injection-for-zh-copeland-scroll-compressors-en-gb-4215484.pdf

And it turns out there are top types of vapor injection. What I described above is vapor injection with economizer. It seems to be commonly referred to as enhanced vapor injection. The other, which the Prime uses, is vapor injection with flash tank. Not much detail here, but page 5 has side by side schematics of the two. https://www.energy.gov/sites/prod/files/2014/10/f18/emt47_shen_042314.pdf

In the simple heat pump description, the flash tank design adds an expansion valve and flash tank(also called a phase/liquid-gas separator) after the condenser. The refrigerant doesn't completely expand after passing through that expansion valve. In the flash tank, the liquid portion is separated from the gas, and continues to the evaporator. The gas portion is diverted directly to the compressor, where it is sprayed on the scroll, cooling it. This lets the compressor spin at faster speeds, increasing the pressure it puts the refrigerant under.
Keep Warm This Winter: INVERTER Technology for Any Climate

In the Prime, vapor injection is only happening while in heat mode, and I think that is so for other dual heating/cooling heat pumps. Moving excess heat out is easier than trying to find it in a heat poor environment.

Vapor injected heat pumps are more efficient, but the big advantage is in them being able to heat at lower ambient temperatures. For an EV this means the heat pump can be used to lower temperatures before turning on range robbing resistant heaters or an engine.

 

The key point with all this heat pump discussion is that, with a heat pump, one is not creating heat (like from burning fuel or electrical resistance), but simply moving it from one place to another, and this process is much more efficient than creating heat. The vapor injection system improves the efficiency of the heat pump, and allows it to work at a lower temperature.

Even more weird is that one can use fire in a cooling system. Refrigerators in RVs use burning propane (or electric heating element if commercial power is available) to heat the refrigerant (ammonia) and drive the cooling cycle, with no moving parts. But that is a topic for another forum.

 

What about primes that are not ADVANCED models? What happens with the AC and hearing in the lower models?

 

All primes have the humidity sensor. It's not clear what it does exactly.

I have a Plus, and the other day I was driving with the heat off to maximize range. It was about 33 degrees outside, and I noticed the windshield start to fog. a few seconds later,I hear a motor whir, and when I looked down it had switched the the airflow direction from floor to floor and windshield. That is weird since the fan was completely off, and because even if it was on, I rarely use auto mode.

The humidity sensor doesn't seem to be used in conjunction with the forward camera. I've had the windshield get quite foggy or icy before I get a warning that the forward collision system is disabled. And since the camera is not heated, it's often much foggier on that section of the windshield from the outside than anywhere else.

 

All 2020 prius prime models have a heat pump.

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