Design architecture and performance analysis of automotive heat pump systems

Electric vehicles have never been more popular, but the fear of being stranded due to battery exhaustion remains a key reason for some car buyers to refuse to buy one. A bigger battery is not always the solution as it is directly associated with higher costs and a high impact on weight. Redesign of the most energy-consuming auxiliary equipment is mandatory, with thermal management functions being at the top of the list of redesign requirements. Heat pump solutions are considered one of the best options to save energy and reduce the impact of heating and cooling functions on vehicle range, but automotive applications require careful definition of system functionality to avoid unjustified increase in complexity and unnecessary system oversizing. This article aims to outline heat pump design best practices through a virtual performance comparison of different layout configurations, which have been selected starting from a benchmarking analysis and combined with detailed vehicle segmentation functions. Control strategies, roles, costs and target requirements have been used as drivers for the right solution design, as well as the main constraints for the final solution selection, which cannot be seen as the only winner.

introduce

As vehicle emission regulations become increasingly stringent, the automotive industry is accelerating the development of electric vehicle platforms such as battery electric vehicles (BEVs). Since the available waste heat from the powertrain of these vehicles is very limited, a major challenge for electric vehicles is interior climate control, especially when heating is required. The use of heat pump systems is one of the solutions to increase the driving range of electric vehicles in cold environments. Unlike traditional internal combustion engine-driven vehicles, electric vehicles have much higher energy conversion efficiency, so there is not enough waste heat for interior heating.

A common method of providing heat to electric vehicles is to use a positive temperature coefficient ( PTC ) heater, which converts the electrical energy stored in the battery directly into heat through the Joule effect. Although electric heaters typically have almost 100% first-law efficiency, so that 1 kW of electricity is converted to 1 kW of heat, the energy efficiency as a direct conversion of high-order electrical energy to low-order heat is generally very low for cabin heating applications. For an average electric vehicle, turning on a PTC heater will drain the battery and significantly reduce the driving range. Heat pumps have higher first-law efficiency and can provide the same amount of heat to the vehicle interior with less electrical energy consumption, and can significantly increase the driving range of electric vehicles in cold weather. Several commercial electric vehicle models have already adopted heat pump technology to heat the cabin (see Table 1), including the Nissan Leaf, Renault Zoe, and BMW i3, all of which claim to increase driving range by approximately 20 to 30% in cold weather by using heat pumps.

Table 1 Application of heat pump system in electric vehicle models

Research activities on automotive heat pump systems in the literature date back to 2002. Different prototypes were built and tested by many major automotive suppliers, such as Valeo [3,4], Denso [5,6], Visteon [7], Behr [8,9], Delphi [10], etc. These prototype systems used direct air-refrigerant, indirect air-refrigerant, or hybrid architectures with refrigerants such as R134a, R1234yf, R744, R445a [11], and most recently R290. In these studies, direct architectures were found to be more efficient than indirect systems using the same refrigerant. Indirect systems have the advantage of isolating the refrigerant circuit, enabling compact designs with very low refrigerant charges, while the coolant circuit enables integrated thermal management of the entire vehicle. In terms of refrigerant performance, CO2 was reported to be much better than R134a in heating mode, especially at sub-zero ambient temperatures, while the COP in cooling mode was lower than that of R134a in most cases. R290 has been reported to have a significantly better COP than R1234yf in cooling mode [12] and has therefore been successfully tested in indirect heat pump systems [13], however, current regulations on HVAC safety and design requirements [14] require that only R-1234yf and R-744 be used as refrigerants in components and systems supplied by automotive original equipment manufacturers (OEMs) and the aftermarket (non-OEM). All these research activities have proven that the application of heat pumps in the automotive sector has become a focus of attention.

The aim of this paper is to give an overview of the design of an automotive heat pump system, taking into account the main drivers that may influence the final architecture: objectives, system performance and complexity, and cost. First, the main functional objectives typically required for an electric vehicle thermal management system are described, followed by an overview of the main automotive heat pump architectures. Two different heat pump architectures are proposed as solutions, and both systems are virtually evaluated and compared to highlight advantages and disadvantages. Finally, complexity aspects and cost sensitivity are described. In summary, no single solution can be identified as the absolute best heat pump system, and design constraints must be considered as driving factors, which will influence the choice of heat pump system architecture.

Electric vehicle thermal management system goals

First of all, the design of a vehicle thermal management system (TMS) must take into account the vehicle's purpose (passenger car, sports car, light commercial vehicle, etc.) and the geographical area in which the vehicle is used. Some features, such as more powerful or additional electric heaters, can be considered "optional" for extreme cold weather regions. In addition, the vehicle's HVAC system must be designed to operate safely in all weather conditions, ensure human thermal comfort, and keep all glass areas free of ice or fog, but an electric vehicle's TMS has other important additional thermal requirements related to the operation of the powertrain's electrical components. In particular, in order to operate within the ideal temperature range, the electric vehicle's high-voltage batteries often need to be cooled or heated. All of these aspects strongly influence the definition of the so-called "functional targets" that the system must meet.

Considering the generic EV, Table 2 summarizes the main functional goals defined for TMS and their impact on customers.

Table 2 TMS: possible functional targets for electric vehicles

All functions that require cooling or heating of the vehicle cabin have a direct impact on customer comfort and safety, so specific targets for cabin preheating and cooling (temperature levels to be achieved within short time intervals) must be carefully defined.

Of all the functions, only those where the vehicle battery must be cooled or heated generally have an indirect impact on the customer, as the battery performance may affect the vehicle's range or battery charging time. As fast or ultra-fast charging capabilities become more common, specific functions related to both cabin and battery thermal conditioning may be required. For example, when the customer wants to stay in the vehicle during a fast charging operation.

In the second case, the functional objectives of the vehicle TMS can be implemented in different ways, taking into account different vehicle segments or can be excluded for cost reasons. Considering the electric vehicles on the market (such as Hyundai Kona, Jaguar I-Pace, etc.), the inclusion of heat pump systems in the TMS functions is more applicable to high-end vehicles, as shown in Table 3. More functions mean higher system complexity and higher costs in terms of system control (additional valves, pipes, sensors , etc.). For these reasons, usually low-end electric vehicles do not include waste heat recovery, cabin heating and battery cooling as well as dehumidification at low ambient temperatures (e.g. T<0℃). Cabin and battery heating can be present, but with limited efficiency (electric heater).

Table 3 TMS: functional objectives vs. vehicle segmentation (V = essential, X = non-essential, TBE = to be evaluated)

Automotive heat pump architecture

The described functionality can be realized by different heat pump architectures.

Indirect heat pump

The structure of the indirect heat pump is based on the principle of coolant flow reversal. Its characteristic is that the refrigerant cycle always works in the same way, and at the same time, the heat power is transferred by the water-cooled heat exchanger, hot water and cold water in all seasons, which is very useful for cabin comfort or battery thermal regulation.

Figure 1 Indirect heat pump system architecture based on CRU architecture for battery and cabin thermal conditioning in BEVs

As mentioned earlier, indirect systems benefit from a very low refrigerant charge compact refrigeration unit (CRU), while the coolant circulation allows for comprehensive thermal management of the entire vehicle (e.g., heat recovery from the powertrain and electronics ).

Figure 2 Compact Refrigeration Unit (CRU) components: plate heat exchanger, compressor and expansion valve. The compact location of the components allows for short pipe lengths.

This architecture is not currently used in any passenger car on the market due to efficiency issues (which can be overcome by using alternative refrigerants such as R290), high complexity (associated with coolant valve control optimization) and additional costs (HVAC redesign requires the use of air coolers instead of standard evaporators). However, automotive suppliers and OEMs are investigating this topic, as demonstrated by the OPTE MUS [13,15] and UTEMPRA [16] publicly funded projects in the EU and the US, respectively.