jkirkebo wrote:vegastar wrote:For really cold weathers a heat pump alone would not work. The best would be a heat pump and the current system. Up to 0ºC (32ºF) a heat pump is enough and very efficient, below that it would be necessary a resistive heating element.
Modern heat pumps work well down to at least 0F/-18C. For example the newest Toshibas has a COP of between 2,07 and 2,25 at -15C (5F). So you'd need about 2kW input to get 4.5kW output in 5F weather. A big improvement IMHO. The 5kW resistive element should be retained of course, it can get colder than -40 (C or F) over here some places.
Definitely with blended mode operation of the PiP the need for a heat-pump is less unless your drive is within the 15 mi EV range and you are able to keep speeds and acceleration rate low in cold weather.
There's definitely a bit of complexity to use a heat-pump - no matter what you still need to retain the resistance heater:
1. The condenser tends to ice up so you need to run it in reverse to defrost it periodically - Think of driving through rain in low, but not freezing temps - water will collect on the HVAC condenser and then freeze since you are pulling heat from it. To de-ice the condenser to you need to run the HVAC in AC/mode (pump some heat back out cooling inside air) which means unless you are ready to be blasted by freezing air, you better have backup heat. Driving when it's snowing you'd have the same issue - condenser will quickly clog up requiring it to be de-iced frequently.
2. As mentioned, efficiency drops off at low temps (a lot of the reason is energy used by de-icing activity), so you may want/need resistance heat for backup.
3. Heat pumps typically aren't able to generate as much of a temperature differential and will require more volume of air for high efficiency, so when warming up the car unless you like cold air blowing around you're going to have to run resistance heat at the same time, anyway.
Most of these issues could probably be worked around by automatically blending in resistance heat based on environmental sensors. You'd also want to be able to figure out how to detect when the condenser is iced up accurately so you only have run in de-ice mode as little as possible.