Addressing the dual challenges of global climate change and energy security, battery electric cars have emerged as a pivotal direction for sustainable transportation, characterized by their zero tailpipe emissions and high energy efficiency. The performance of the thermal management system is critical, as it directly impacts not only passenger comfort but also the temperature control of the battery pack, motor, and power electronics, thereby influencing the overall safety, reliability, and energy efficiency of the vehicle. In conventional systems for battery electric cars, fluorinated refrigerants like R134a have been widely adopted due to their excellent thermodynamic properties. However, their high Global Warming Potential (GWP) presents significant environmental concerns. Consequently, the search for efficient and environmentally friendly alternative refrigerants has become a major research focus. The natural refrigerant R290 (propane) has garnered increasing attention from academia and industry due to its low GWP (GWP = 3), zero Ozone Depletion Potential (ODP = 0), and favorable thermodynamic performance, positioning it as a promising candidate for future refrigerant development.
While research on R290 applications in commercial refrigeration exists, its adoption in battery electric cars is still nascent, primarily due to safety concerns related to its flammability. To mitigate these risks, indirect thermal management systems, which employ a secondary coolant loop to separate the flammable refrigerant from the passenger cabin and battery compartments, have been proposed as a safer architecture. Although initial studies have validated the potential of R290 in such indirect systems, significant gaps remain in understanding their cooling performance, especially under high-temperature conditions, the system’s energy efficiency, and the coupled influence of multiple operational parameters. To address these research gaps, this study proposes and constructs an R290 dual-secondary-loop refrigeration system tailored for battery electric cars. The research focuses on investigating the influence mechanisms of indoor and outdoor parameters on system cooling performance and proposes an optimization strategy to address identified performance limitations. Through theoretical analysis and experimental validation, this work aims to provide theoretical and technical support for the practical application of R290 in thermal management systems for battery electric cars.
Theoretical Analysis of Direct vs. Indirect R290 Systems
The core distinction between a direct and an indirect thermal management system for a battery electric car lies in the heat exchange path. In a direct system, the refrigerant (e.g., R290) circulates through all components, including the compressor, condenser, expansion device, and evaporator, where it directly absorbs heat from the cabin air. This architecture offers a relatively short energy transfer path with minimal thermal losses. In contrast, an indirect system introduces an intermediate heat transfer fluid, typically a water-glycol coolant. This system comprises a primary refrigerant loop (R290 cycle) and a secondary coolant loop. In cooling mode, the R290 absorbs heat from the coolant in a chiller (evaporator), and the cooled coolant is then pumped to a cooler (air-cooling coil) to absorb heat from the passenger cabin of the battery electric car. While this design enhances safety by isolating the flammable refrigerant, it introduces additional heat exchange interfaces and a longer thermal path, inevitably leading to increased thermal resistance and potential performance degradation.
A theoretical comparison on a pressure-enthalpy (P-h) diagram reveals the performance implications. For a typical summer cooling condition with a cabin target, consider a direct R290 system operating with an evaporation temperature (Tevap,dir) of 0°C and a condensation temperature (Tcond,dir) of 50°C. Assuming a subcooling of 5 K, a superheat of 5 K, and an isentropic compressor efficiency (ηisen) of 0.7, the key performance indicators can be calculated. For an indirect system, experimental data suggests the evaporating pressure is typically lower, and the condensing pressure is also affected due to the extra temperature difference in the secondary loops. We assume the evaporating pressure in the indirect system is 16% lower and the condensing pressure is 11% lower than in the direct system under comparable heat load conditions for the battery electric car.
The refrigeration effect (cooling capacity per unit mass) and compressor work are central to performance. The specific refrigeration effect, qevap, and the specific compressor work, wcomp, are given by:
$$ q_{evap} = h_1 – h_4 $$
$$ w_{comp} = \frac{h_{2s} – h_1}{\eta_{isen}} $$
where h1 is the enthalpy at compressor suction, h4 is the enthalpy before expansion, and h2s is the isentropic enthalpy after compression. The Coefficient of Performance (COP) is then:
$$ COP = \frac{q_{evap}}{w_{comp}} $$
The pressure ratio, PR, is a critical parameter affecting compressor efficiency and work:
$$ PR = \frac{P_{cond}}{P_{evap}} $$
The following table summarizes the theoretical comparison between the direct and indirect R290 systems for a battery electric car under the stated assumptions.
| Performance Parameter | Direct R290 System | Indirect R290 System | Change |
|---|---|---|---|
| Evaporation Pressure (Pevap) | Higher | 16% Lower (Assumed) | -16% |
| Condensation Pressure (Pcond) | Higher | 11% Lower (Assumed) | -11% |
| Pressure Ratio (PR) | Lower | Higher | +23.8% |
| Specific Refrigeration Effect (qevap) | Higher | Lower | -9.11% |
| Specific Compressor Work (wcomp) | Lower | Higher | +18.24% |
| Coefficient of Performance (COP) | Higher | Lower | -33.5% |
This theoretical analysis clearly indicates that for a battery electric car, the indirect R290 system suffers from significant performance衰减 compared to a direct system under cooling mode. The increased pressure ratio and reduced specific refrigeration effect lead to higher specific work and a substantially lower COP, highlighting a key challenge for indirect architectures that must be addressed through component and system optimization.
Experimental System and Methodology
To empirically investigate the cooling performance of the R290 indirect system for battery electric cars, a dedicated test bench was designed and constructed. The system mimics a dual-secondary-loop architecture suitable for a battery electric car, where one coolant loop serves the cabin air cooler and another serves the battery thermal manager (simulated by a heater/chiller unit in this study). The test bench is housed within environmental chambers capable of precisely controlling the temperature, humidity, and airflow for both the indoor (cabin) and outdoor (front-end) conditions.
The R290 primary circuit consists of key components selected based on the operating boundaries and the properties of R290 refrigerant. The secondary coolant loops use a 50% ethylene glycol-water solution. The main components and their specifications are listed below.
| Component | Type | Key Parameters / Description |
|---|---|---|
| Compressor (Comp) | Scroll | Displacement: 34 cc |
| Chiller (Evaporator) | Plate Heat Exchanger | Dimensions: 130 mm × 65 mm × 120 mm |
| Water-Cooled Condenser (WCC) | Plate Heat Exchanger | Dimensions: 130 mm × 65 mm × 125 mm |
| Electronic Expansion Valve (EXV) | Stepper Motor Driven | Orifice Diameter: 3.0 mm |
| Cabin Cooler (Air-Cooling Coil) | Microchannel | Dimensions: 240 mm × 30 mm × 199 mm |
| Low-Temperature Radiator (LTR) | Microchannel | Dimensions: 560 mm × 400 mm × 36 mm |
| Cabin/Battery Coolant Pump | Electronic | Max Flow Rate: 30 L/min |
An extensive sensor network was installed to monitor all critical parameters. The system cooling capacity (Qchiller) was calculated from the coolant side of the chiller using the measured temperature difference and mass flow rate:
$$ Q_{chiller} = \dot{m}_{coolant} \cdot c_{p,coolant} \cdot (T_{coolant,in} – T_{coolant,out})_{chiller} $$
where \(\dot{m}_{coolant}\) is the coolant mass flow rate, \(c_{p,coolant}\) is the specific heat capacity of the 50% glycol-water solution, and \(T_{coolant,in}\) and \(T_{coolant,out}\) are the inlet and outlet temperatures of the coolant at the chiller. The overall system Coefficient of Performance (COPsys) for the battery electric car cooling application is defined as:
$$ COP_{sys} = \frac{Q_{chiller}}{W_{comp}} $$
where \(W_{comp}\) is the total electrical power input to the compressor. The accuracy of the primary measurement instruments is critical for reliable data and is summarized in the following table.
| Measured Parameter | Instrument | Uncertainty |
|---|---|---|
| Refrigerant Temperature | PT100 Sensors | ± 0.1 K |
| Refrigerant Pressure | Strain Gauge Sensors | ± 5 kPa |
| Coolant Temperature | PT100 Sensors | ± 0.2 K |
| Refrigerant Mass Flow | Coriolis Mass Flow Meter | ± 0.15% of reading |
| Coolant Mass Flow | Electromagnetic Flow Meter | ± 0.2% of reading |
| Electrical Power | Power Analyzer | ± 0.2% of reading |
A comprehensive experimental plan was designed to investigate the influence of various parameters on the cooling performance of the R290 system for a battery electric car. The plan consisted of 44 distinct test conditions, which can be grouped into three main categories:
- Indoor-side Parameter Study (Tests 1-16): Investigating the effect of cabin inlet air temperature and airflow rate on system performance at a fixed outdoor condition and compressor speed.
- Outdoor-side Parameter Study (Tests 17-32): Investigating the effect of ambient temperature and frontal air velocity (over the Low-Temperature Radiator) on system performance at a fixed cabin condition and compressor speed.
- Comprehensive Performance Mapping (Tests 33-44): Evaluating system performance under combined variations of indoor/outdoor temperature and compressor speed to simulate realistic high-load scenarios for a battery electric car.
Throughout all tests, the coolant flow rates for both the high-temperature (condenser) and low-temperature (chiller) sides were maintained constant at 25 L/min, and the initial relative humidity was controlled at 40% ± 2% to ensure comparability.
Analysis of Cooling Performance and Parameter Influence
Influence of Indoor-side (Cabin) Parameters
The indoor-side conditions directly affect the heat load on the secondary coolant loop and consequently the heat absorption rate at the chiller (evaporator). Experimental results show that both the cooling capacity (Qchiller) and system COP increase with rising cabin inlet air temperature and airflow rate. However, the mechanisms and magnitudes differ. Higher cabin air temperature increases the coolant return temperature to the chiller, elevating the evaporation temperature and pressure. This is expressed by the log-mean temperature difference (LMTD) for the chiller:
$$ LMTD_{chiller} = \frac{(T_{coolant,in} – T_{evap}) – (T_{coolant,out} – T_{evap})}{\ln\left(\frac{T_{coolant,in} – T_{evap}}{T_{coolant,out} – T_{evap}}\right)} $$
A higher \(T_{coolant,in}\) increases the LMTD, enhancing heat transfer and raising Pevap. Similarly, increased airflow across the cabin cooler improves its heat transfer coefficient, lowering the coolant temperature returning to the chiller, which also benefits the evaporation pressure by reducing the required temperature difference. The following data illustrates the trend: at a fixed compressor speed of 4000 rpm and an outdoor temperature of 43°C, increasing the cabin air temperature from 28°C to 43°C (in 5°C steps) raised the average evaporation pressure by 5.2% to 5.9% per step, leading to a 6-8% increase in cooling capacity per step. The impact on COP was positive but slightly lower, at 3.2-4.3% per step. The effect of increasing airflow showed diminishing returns; while boosting airflow from 250 m³/h to 550 m³/h increased cooling capacity, the incremental gain decreased with each step. This is because the dominant thermal resistance eventually shifts from the air-side to the refrigerant-side or coolant-side within the heat exchangers of the battery electric car system.
Influence of Outdoor-side (Ambient) Parameters
The outdoor-side conditions govern the heat rejection process at the condenser via the Low-Temperature Radiator (LTR). The results demonstrate a more pronounced effect on system efficiency (COP) than on cooling capacity. The condensation pressure (Pcond) is highly sensitive to the ambient temperature (Tamb) and the air velocity (vair) over the LTR. The heat rejection rate can be modeled as:
$$ Q_{cond} = U_{LTR} \cdot A_{LTR} \cdot LMTD_{LTR} \approx \dot{m}_{air} \cdot c_{p,air} \cdot (T_{air,out} – T_{amb}) $$
where \(U_{LTR}\) is the overall heat transfer coefficient and \(\dot{m}_{air}\) is the air mass flow rate proportional to vair. A higher Tamb reduces the LMTDLTR, requiring a higher condensing temperature (and pressure) to reject the same amount of heat. Conversely, a higher vair increases \(\dot{m}_{air}\) and improves ULTR, facilitating heat rejection at a lower condensing pressure. Experiments show that for a battery electric car under a fixed cabin load and compressor speed, every 5°C increase in ambient temperature (from 30°C to 45°C) caused an average COP reduction of 9.3-10.0%, associated with a 10-12% rise in condensation pressure. The impact of air velocity was significant only at lower ranges; increasing vair from 2 m/s to 5 m/s reduced Pcond and improved COP, but the effect diminished as velocity increased. Notably, the cooling capacity was relatively insensitive to outdoor changes, decreasing by a maximum of only 300 W (from ~4.0 kW to ~3.7 kW) under the most severe condition (45°C, 2 m/s). This is because the evaporation pressure, which primarily governs capacity, remained relatively stable as the compressor work increased to overcome the higher pressure ratio (\(PR = P_{cond}/P_{evap}\)), limiting the net refrigerant mass flow rate increase.
Performance Under High-Temperature, High-Load Scenarios
Mapping the system performance across combined high indoor/outdoor temperatures and varying compressor speeds reveals a critical limitation for battery electric car applications. The data indicates that even at the maximum tested compressor speed of 7000 rpm, the system struggled to achieve high cooling outputs under high ambient heat loads. For instance, at an indoor/outdoor temperature of 45°C, the cooling capacity plateaued around 5.2 kW. Increasing the compressor speed from 3000 to 7000 rpm raised capacity, but with drastically diminishing returns and at the cost of efficiency. The COP consistently decreased with both higher temperature and higher speed. Analysis shows that the enthalpy-based specific cooling effect (\( \Delta h_{evap} = h_1 – h_4 \)) decreased under high condensing pressures, while the increase in refrigerant mass flow rate (\( \dot{m}_{ref} \)) was insufficient to compensate, leading to the observed plateau in total capacity \( Q_{chiller} = \dot{m}_{ref} \cdot \Delta h_{evap} \). This underscores a fundamental performance bottleneck in the indirect architecture for extreme cooling demands in a battery electric car, necessitating component-level optimization.
Performance Optimization Strategy and Results
The analysis pinpointed the low-side pressure (evaporation pressure) as a critical limiting factor for the cooling capacity of the indirect R290 system in a battery electric car. The chiller (evaporator) is the key component dictating this pressure level through its heat transfer effectiveness. Therefore, optimizing the chiller’s design to reduce its thermal resistance and improve the temperature approach between the coolant and the evaporating R290 was identified as the primary strategy.
The optimization focused on enhancing the internal geometry and refrigerant circuitry of the plate-type chiller to improve the heat transfer coefficient (Uchiller) and reduce the pressure drop on the refrigerant side. An improved chiller prototype was fabricated and tested. First, its standalone performance was evaluated on a component calorimeter, demonstrating significant gains. The following table compares the performance of the original and optimized chiller under three different coolant inlet temperatures, which represent different operating points for the battery electric car system.
| Coolant Inlet Temp. [°C] | Coolant Flow [L/min] | Original Chiller Capacity [kW] | Optimized Chiller Capacity [kW] | Performance Increase |
|---|---|---|---|---|
| 9.7 | 25.0 | 5.20 | 6.41 | 23.3% |
| 4.4 | 25.0 | 5.93 | 7.22 | 21.8% |
| 34.6 | 25.0 | 9.61 | 11.99 | 24.8% |
The optimized chiller was then integrated into the full R290 indirect thermal management system test bench for the battery electric car. Comparative tests were conducted under the previously challenging high-temperature condition of 45°C ambient and cabin temperature. The results, plotted against compressor speed, show a remarkable improvement. The optimized system achieved a significantly higher cooling capacity across all speeds. At 7000 rpm, the capacity increased by 20.1%, reaching 5.76 kW. More importantly, the system efficiency (COP) also improved substantially, by 11.7% to 18.9% across the speed range, with the highest COP reaching 2.2 at 3000 rpm.
The underlying thermodynamic improvement is clearly visualized on a pressure-enthalpy diagram comparison for the 7000 rpm, 45°C case. The optimization successfully raised the evaporation pressure by 14.3%. The condensing pressure also saw a minor increase of 3.5%, attributable to the higher heat rejection load from the increased capacity. Crucially, while the specific refrigeration effect (\(\Delta h_{evap}\)) remained largely unchanged, the refrigerant mass flow rate (\( \dot{m}_{ref} \)) increased by approximately 21% due to the higher suction density and improved compressor volumetric efficiency at the higher suction pressure. Furthermore, the specific compressor work (\(w_{comp}\)) decreased by 10.3% because of the reduced pressure ratio. These combined effects—higher mass flow and lower specific work—explain the simultaneous gain in both cooling capacity and COP, effectively mitigating the primary weakness of the indirect R290 system for battery electric car applications.
Conclusion
This study provides a comprehensive investigation into the refrigeration performance of an R290-based dual-secondary-loop thermal management system designed for battery electric cars. Through theoretical analysis and systematic experimentation, the influence mechanisms of key operational parameters were elucidated. It was found that indoor-side (cabin) parameters primarily govern the low-side pressure and have a dominant effect on the system cooling capacity, while outdoor-side (ambient) parameters dictate the condensing pressure and exert a more pronounced influence on the system Coefficient of Performance (COP). The experimental mapping under high-temperature, high-load scenarios revealed a performance bottleneck in the baseline indirect architecture, where increasing compressor speed yielded diminishing returns in cooling capacity at a severe efficiency penalty.
To address this limitation, a component-level optimization strategy focused on the chiller (evaporator) was proposed and implemented. The results demonstrated the effectiveness of this approach: the optimized chiller enhanced the system’s low-side pressure, which in turn increased the refrigerant mass flow rate and reduced the specific compressor work. In a demanding 45°C ambient condition, the optimized system for a battery electric car achieved a 20.1% increase in cooling capacity and a simultaneous improvement in COP of up to 18.9%, effectively breaking the performance plateau. This research clarifies the key performance drivers and presents a viable optimization pathway for R290 indirect systems, contributing valuable theoretical foundations and design support for developing efficient, environmentally friendly thermal management solutions in battery electric cars. Future work may explore further system integration optimizations, dynamic control strategies, and investigations into combined cooling and heat pump modes for year-round efficiency.