The EPA posts online the applications they get from each auto manufacturer. Tesla’s application for Model Y has the below description of the HVAC and heat pump system that I haven’t seen previously discussed. There are a few interesting tidbits here for those of us that like to understand how these cars operate.
The two biggest surprises for me are using the battery as a temporary heat sInk and running the compressor to replace the conventional resistance heater in very cold conditions.
08.06.01 Electric cabin heater
The heater unit incorporating a variable speed electric fan is located in the front of the chassis tub with ducting directing the blown air to defrosting, face level and floor level vents in the passenger compartment.
The heater element is of the heat pump, drawing HV electrical energy from the battery pack High Voltage.
Tesla Model Y’s heat pump reduces the energy required by the HVAC system in both heating and cooling scenarios. The energy required to heat the cabin varies by weather and occupant comfort needs, but on-average consumes approximately 10% of the total energy available for driving. However, even moderately cold weather (0°C), consumption can increase to 25% or more. A heat pump consumes a small amount of electrical energy to thermodynamically “upgrade” low-temperature (less useful) thermal energy to higher-temperature (more useful) thermal energy, making it suitable for occupant comfort. That is, for a given electrical power input, a heat pump will return 1 to 5x in useful heating power; an electrical cabin heater provides 1:1 in heating power, and therefore is far less efficient.
Typically, this is accomplished using conventional refrigeration system components, e.g., compressors, valves, heat exchangers and so on, configured or connected together in a specific way. Tesla’s heat pump uses conventional components with unconventional flexibility or cycle configuration. A heat pump must generally have a low-temperature source from which to draw energy. Tesla’s system enables the heat pump source to be either the power-train coolant loop, e.g., low-temperature waste heat produced naturally by the vehicle while driving, ambient air, the battery thermal mass, the cabin thermal mass, or combinations thereof. Another advantage of this architecture is the ability to reject heat into the battery pack via a liquid-cooled condenser for a limited amount of time during cabin cooling scenarios when the temperature of the battery is modest. Therefore, for most startups with AC on, the relatively cool, well- coupled, large thermal mass serves to lower discharge pressure and therefore reduces compressor input power relative to a conventional air-cooled condenser setup.
Modern automotive heat pump systems using an HFC/HFO refrigerant suffer from low heating capacity in extremely cold ambient conditions, e.g., minus 10°C and below. Therefore, these conventional systems retain an expensive high-voltage cabin heater to cover heating deficits whenever the heat pump capacity is insufficient. Tesla’s heat pump system also provides ways to remove a cabin air high voltage PTC heater completely by using the compressor as an electrical heater in specific scenarios. In fact, the electrical power draw capability of the compressor significantly exceeds a typical HV cabin PTC heater capability. This last point is accomplished via Tesla’s unique architecture – the cycle is configured in such a way to provide a controlled environment for the compressor, regardless of ambient conditions, and ultimately unlocks the full electrical input power. Therefore, Tesla’s thermal system can sometimes operate like a heat pump (heat efficiently) and sometimes like an electrical heater when heat pump capacity is not sufficient for comfort – using the same compressor.
Source (Page 9-10): https://iaspub.epa.gov/otaqpub/display_file.jsp?docid=49398&flag=1
Twitter comments on this system design by Elon Musk:
The two biggest surprises for me are using the battery as a temporary heat sInk and running the compressor to replace the conventional resistance heater in very cold conditions.
08.06.01 Electric cabin heater
The heater unit incorporating a variable speed electric fan is located in the front of the chassis tub with ducting directing the blown air to defrosting, face level and floor level vents in the passenger compartment.
The heater element is of the heat pump, drawing HV electrical energy from the battery pack High Voltage.
Tesla Model Y’s heat pump reduces the energy required by the HVAC system in both heating and cooling scenarios. The energy required to heat the cabin varies by weather and occupant comfort needs, but on-average consumes approximately 10% of the total energy available for driving. However, even moderately cold weather (0°C), consumption can increase to 25% or more. A heat pump consumes a small amount of electrical energy to thermodynamically “upgrade” low-temperature (less useful) thermal energy to higher-temperature (more useful) thermal energy, making it suitable for occupant comfort. That is, for a given electrical power input, a heat pump will return 1 to 5x in useful heating power; an electrical cabin heater provides 1:1 in heating power, and therefore is far less efficient.
Typically, this is accomplished using conventional refrigeration system components, e.g., compressors, valves, heat exchangers and so on, configured or connected together in a specific way. Tesla’s heat pump uses conventional components with unconventional flexibility or cycle configuration. A heat pump must generally have a low-temperature source from which to draw energy. Tesla’s system enables the heat pump source to be either the power-train coolant loop, e.g., low-temperature waste heat produced naturally by the vehicle while driving, ambient air, the battery thermal mass, the cabin thermal mass, or combinations thereof. Another advantage of this architecture is the ability to reject heat into the battery pack via a liquid-cooled condenser for a limited amount of time during cabin cooling scenarios when the temperature of the battery is modest. Therefore, for most startups with AC on, the relatively cool, well- coupled, large thermal mass serves to lower discharge pressure and therefore reduces compressor input power relative to a conventional air-cooled condenser setup.
Modern automotive heat pump systems using an HFC/HFO refrigerant suffer from low heating capacity in extremely cold ambient conditions, e.g., minus 10°C and below. Therefore, these conventional systems retain an expensive high-voltage cabin heater to cover heating deficits whenever the heat pump capacity is insufficient. Tesla’s heat pump system also provides ways to remove a cabin air high voltage PTC heater completely by using the compressor as an electrical heater in specific scenarios. In fact, the electrical power draw capability of the compressor significantly exceeds a typical HV cabin PTC heater capability. This last point is accomplished via Tesla’s unique architecture – the cycle is configured in such a way to provide a controlled environment for the compressor, regardless of ambient conditions, and ultimately unlocks the full electrical input power. Therefore, Tesla’s thermal system can sometimes operate like a heat pump (heat efficiently) and sometimes like an electrical heater when heat pump capacity is not sufficient for comfort – using the same compressor.
Twitter comments on this system design by Elon Musk:
- Model Y heat pump is some of the best engineering I’ve seen in a while. Team did next-level work.
- PCB design techniques applied to create a heat exchanger that is physically impossible by normal means. Heat pump also has a local heating loop to spool up fast & extend usable temperature range. Octavalve is pretty special, too. Team did great work. No credit to me.
