Design architecture and performance analysis of automotive heat pump systems

System complexity

Other important drivers that can influence the design of HP systems are related to the complexities that usually have to be faced during the general arrangement study and system control parameters definition phase.

Vehicle system integration

The complexity of integrating a vehicle heat pump system is related to the following different aspects:

Number of parts: ATW architecture requires more valves and coolant line connections than ATA architecture;

HVAC module modification: Due to the replacement of the standard CBN HTR with ACOND, the ATA architecture required a redesign and new validation of the HVAC module relative to the internal combustion engine;

Sensors and Actuators: A large number of sensors and electric actuators, such as pumps and valves, increase electrical complexity.

All of these aspects typically result not only in higher complexity but also in higher system costs.

Control Definition and Calibration

In high voltage system design, the definition and calibration of control parameters is very important because it has a great impact on the overall effectiveness of the system.

Figure 22 Activation of the electric heater during cabin and battery heating

A good example of this is defining the activation priority between the electric heater and the HP compressor during the cabin and battery heating functions (Figure 22). In this case, in order to minimize the use of the electric heaters (HV HTR and HV PTC), a control strategy based on the following parameters is used in the simulation model :

Control HV HTR power as a function of ΔT between coolant and battery cells. When the battery heating function is activated, the HV HTR operates at maximum power, reaches maximum ΔT as quickly as possible, and then starts regulating power. For the ATW architecture only, when battery heating is coupled with cabin heating , the HV HTR contribution to cabin heating is reduced by the contribution of the high-voltage PTC. In addition, in all heating functions at ambient temperatures below 0°C, the HV HTR is initially set to maximum power in order to heat the coolant to 0°C before the E-CMP switch of the heat pump is turned on.

Control the high-voltage PTC power as the cabin inlet air temperature changes;

The changes of the controlled E-CMP speed with the variables are shown in Table 8:

Table 8 Compressor target parameters.

For hybrid functionality, the target variable depends on the user's priority (cabin or battery). In addition:

Water pump controlled according to required coolant flow

Assume that the electric fan and air conditioning blower are running at maximum power.

The coolant valve shall be controlled appropriately as a function of the operating mode specified in Tables 5 and 6.

Refrigerant EXV controls and ensures correct HP operation (superheat/subcooling calibration).

Cost Analysis

In the previous paragraphs, different heat pump architectures were evaluated based on performance and layout complexity. System cost is one of the most important aspects that must be considered when selecting a solution for a vehicle.

To have a sensitivity to the overall cost deviation relative to the standard solution, the costs of all components involved in the HP architecture were evaluated. The heating function of the reference system is provided by an air PTC heater for cabin heating and a high-pressure coolant heater for battery heating. The battery is always water-cooled via a chiller connected to the A/C system in parallel with the cabin evaporator and a dedicated expansion valve.

In order to give an indication of the deviation in subsystem costs and to highlight the differences between ATW and ATA solutions, the components have been grouped. The results of the analysis are shown in Table 9:

The refrigerant circuits are more expensive for both ATW and ATA due to the increased complexity of the piping and the additional valves required to achieve the heat pump function. ATW requires a water condenser; therefore, ATA refrigerant circuits may be slightly less expensive than ATW.

No significant deviations were found for the HVAC modules. Essentially, the ATW solution uses a retained component thanks to the water condenser (Wcond), while the ATA solution requires an internal condenser (Acond) to replace the standard cabin heater. Despite the similarity in direct costs, the variable costs may differ, taking into account the investment costs: the cabin heater replacement required for the ATA HVAC module requires requalification, which means higher investment costs.

The ECU and sensor are essentially new components and are not included in the standard solution that only requires a refrigerant high pressure sensor.

Table 9 Cost sensitivity of heat pump system

However, there is a significant deviation from the much lower absolute cost relative to other components.

Due to the heat pump function, the high-pressure coolant heater can be smaller relative to the reference solution; therefore, the component cost can be reduced.

In the ATW solution, the coolant loop is much more expensive due to the need for additional lines and valves to connect the water condenser to the cabin and the battery.

Due to the use of the heat pump function, the refrigerant charge is increased, requiring a larger liquid storage tank and additional piping. There is no significant difference in gas charge between ATW and ATA.

Finally, the overall direct cost deviation is lower for the ATA heat pump architecture, but it must be emphasized that a further evaluation of the investment costs must be considered in order to find out the real gap between the two solutions and to compare with the reference solution without HP. Looking at the final system costs, it is interesting to see that the heat pump is not an expensive solution considering the trade-offs of the achievable benefits. This result is even more important when selling in markets with cold or moderate ambient conditions throughout the year.