The quest for efficient thermal management in electric cars is a critical challenge, directly impacting driving range and passenger comfort. Heat pump systems have emerged as the dominant solution, yet they face a dual challenge: the transition to environmentally friendly refrigerants and the need for high energy efficiency across a wide climatic spectrum. The refrigerant R290 (propane) is considered one of the most promising natural alternatives due to its zero Ozone Depletion Potential (ODP) and a remarkably low Global Warming Potential (GWP) of 3.3. Thermodynamic analyses consistently rank R290 highly for its coefficient of performance (COP) and exergy efficiency in systems designed to replace R134a. However, its flammability necessitates careful engineering, often leading to the adoption of secondary-loop architectures in electric car applications, which enhance safety by isolating the refrigerant from the passenger compartment via a coolant medium.
A significant hurdle for electric car heat pumps is maintaining performance across extreme ambient conditions, from frigid -30°C winters to scorching 50°C summers. During low-temperature operation, the heating capacity and efficiency of a standard heat pump cycle drop precipitously, creating a major bottleneck for electric car adoption in cold climates. To address this, vapor injection (or economizer) cycles are employed. These cycles inject refrigerant vapor at an intermediate pressure into the compression process, effectively increasing the mass flow rate, lowering the compressor discharge temperature, and boosting both heating capacity and COP. On the other hand, for enhancing efficiency, especially in cooling mode, regenerative cycles incorporating an internal heat exchanger (IHX) are utilized. The IHX subcools the liquid refrigerant before expansion while superheating the suction gas, increasing the refrigeration effect. The effectiveness of a regenerative cycle is highly dependent on the refrigerant’s properties, with R290 showing a particularly positive response.
Most existing research focuses on these cycles in isolation. This study proposes an innovative, integrated solution: an R290 injection-regeneration coupled heat pump system specifically for electric cars. The core innovation lies in a single, multifunctional intermediate heat exchanger (IHX) that can operate either as an economizer for the vapor injection cycle or as a regenerator for the regenerative cycle, allowing flexible switching based on the operating mode and ambient conditions. This integrated approach aims to synergistically optimize both heating and cooling performance across the entire required temperature spectrum for electric cars. We present a comprehensive experimental and theoretical analysis of this system’s performance under wide-temperature conditions, detailing the operational advantages of each cycle and providing critical insights into the design and evaluation of the coupled IHX.
System Description and Experimental Methodology
The proposed R290 heat pump system for electric cars is illustrated in the schematic. It comprises a refrigerant circuit and dual secondary coolant loops, a common architecture for safety and modularity in electric car thermal management. The refrigerant circuit includes a vapor-injection scroll compressor, a plate-type condenser, a plate-type evaporator, a main electronic expansion valve (EEV), a secondary EEV for the injection line, and the central component—the multifunctional intermediate heat exchanger (IHX).
When operating in the Vapor Injection (INJ) Cycle mode, typically for heating, high-pressure refrigerant from the compressor rejects heat in the condenser (acting as the indoor heater). The condensed liquid then splits. The main stream flows through the high-pressure side of the IHX (acting as an economizer), is further subcooled, expands through the main EEV, evaporates in the evaporator (acting as the outdoor heat absorber), and returns to the compressor. The secondary stream expands through the injection EEV to an intermediate pressure, enters the low-pressure side of the IHX, where it absorbs heat from the main stream and evaporates. This vapor is then injected into the compressor’s intermediate port, completing the cycle. This process increases the total refrigerant mass flow and reduces the compression work per unit mass.
When operating in the Regenerative (REG) Cycle mode, often for cooling, the system simplifies. The condensed refrigerant from the condenser flows through the high-pressure side of the IHX (now acting as a regenerator), is subcooled, expands through the main EEV, evaporates in the evaporator (now acting as the indoor cooler), then passes through the low-pressure side of the IHX where it is superheated by the high-pressure stream before returning to the compressor. The secondary EEV line is closed. This cycle increases the specific refrigeration effect.
The dual secondary loops (indoor and outdoor) use coolant to exchange heat with the refrigerant via the plate heat exchangers. This isolates the flammable R290 from the passenger cabin in the electric car. Four-way valves facilitate the switching between heating and cooling modes by reversing the roles of the indoor and outdoor coolant loops.
The experimental setup was constructed and tested in a psychrometric calorimeter chamber capable of simulating the required wide ambient range. Key components, such as the compressor, heat exchangers, and valves, were carefully selected for the electric car application. The system was instrumented with temperature, pressure, and mass flow sensors at all critical state points. The refrigerant charge was optimized to a plateau value of 320g for the regenerative cycle baseline. Tests were conducted across a matrix of conditions, as summarized in Table 1.
| Operating Mode | Refrigerant Cycle | Ambient Temperature (°C) | Indoor Temperature (°C) |
|---|---|---|---|
| Heating | REG / INJ | -30 | 20 |
| REG / INJ | -20 | ||
| REG / INJ | 0 | ||
| Cooling | REG / INJ | 35 | 27 |
| REG / INJ | 45 | ||
| REG / INJ | 50 |
Performance Analysis and Theoretical Framework
The performance of the electric car heat pump system is evaluated from both the refrigerant (water-side) and air-side perspectives to account for losses in the secondary loops. The heating capacity from the water-side \( Q_w \) (or cooling capacity in cooling mode) is calculated from the coolant loop measurements:
$$ Q_w = \dot{m}_w c_{p,w} (t_{w,out} – t_{w,in}) \times 10^{-3} $$
where \( \dot{m}_w \) is the coolant mass flow rate, \( c_{p,w} \) is the specific heat, and \( t_{w,out} \), \( t_{w,in} \) are the outlet and inlet temperatures of the coolant at the condenser (heating) or evaporator (cooling).
The corresponding capacity from the air-side \( Q_a \) is calculated from the air loop measurements across the indoor heat exchanger (heater or cooler):
$$ Q_a = \rho_{a,in} V_{a,in} (h_{a,out} – h_{a,in}) / 3.6 $$
where \( \rho_{a,in} \) is the inlet air density, \( V_{a,in} \) is the volumetric airflow rate, and \( h \) represents specific enthalpy.
The heat transfer rate of the intermediate heat exchanger (IHX), critical for our analysis, is determined from the refrigerant side:
$$ Q_{IHX} = \dot{m}_r (h_{h,in} – h_{h,out}) \times 10^{-3} $$
where \( \dot{m}_r \) is the refrigerant mass flow rate in the main line, and \( h_{h,in} \), \( h_{h,out} \) are the enthalpies at the inlet and outlet of the IHX’s high-pressure side.
The Coefficient of Performance (COP) for heating or cooling is defined as:
$$ COP = Q / W $$
where \( Q \) is either \( Q_w \) or \( Q_a \), and \( W \) is the total compressor input power. The heat loss \( \Delta Q \) in the secondary loop, an important metric for electric car system design, is simply:
$$ \Delta Q = Q_w – Q_a $$
To deeply analyze the performance of the IHX itself, we employ exergy analysis. The exergy flow rate at any state point \( j \) is given by:
$$ \dot{E}_{x,j} = \dot{m}_j [(h_j – h_0) – T_0 (s_j – s_0)] $$
where \( T_0 \) and the subscript \( 0 \) denote the reference dead state (ambient) conditions. The exergy destruction within the IHX and its exergy efficiency \( \eta_{IHX,ex} \) are calculated as:
$$ \dot{E}_{D,IHX} = \dot{m}_r T_0 [(s_{h,out} – s_{h,in}) + (s_{l,out} – s_{l,in})] $$
$$ \eta_{IHX,ex} = 1 – \frac{\dot{E}_{D,IHX}}{\dot{E}_{in,IHX}} $$
Here, subscripts \( h \) and \( l \) denote the high-pressure and low-pressure sides of the IHX, respectively.
The heat transfer and pressure drop characteristics of the plate-type IHX are also evaluated. The overall heat transfer coefficient \( K \) is:
$$ K = Q_{IHX} / (A \cdot \Delta t_m) $$
where \( A \) is the heat transfer area and \( \Delta t_m \) is the log-mean temperature difference. The friction pressure drop \( \Delta p_f \) on the low-pressure side (most critical for the regenerator function) is estimated by subtracting the inlet/exit loss, acceleration, and gravitational components from the total measured pressure drop. For analysis, we relate it to the kinetic energy per unit volume \( KE/V \):
$$ \frac{KE}{V} = \frac{G^2}{2 \rho_m} $$
$$ \Delta p_f = C \cdot \frac{KE}{V} $$
where \( G \) is the mass flux, \( \rho_m \) is the average two-phase density, and \( C \) is a friction factor coefficient determined from experimental data.
Results and Discussion: Heating and Cooling Performance for the Electric Car
The performance of the R290 system was rigorously tested under the wide range of conditions an electric car might encounter. The results clearly delineate the optimal operating strategy.
Heating Performance (-30°C to 0°C): The vapor injection (INJ) cycle consistently outperformed the regenerative (REG) cycle in heating mode, which is crucial for electric car range in winter. As shown in Table 2, the heating capacity and COP were higher for INJ at all tested ambient temperatures.
| Ambient Temp. (°C) | Cycle | Heating Capacity, Qa (kW) | COPa | Capacity Increase vs. REG | COP Increase vs. REG |
|---|---|---|---|---|---|
| -30 | REG | 1.45 | 1.49 | 44.4% | 7.4% |
| INJ | 2.10 | 1.60 | |||
| -20 | REG | 2.55 | 2.10 | 29.8% | 4.5% |
| INJ | 3.31 | 2.20 | |||
| 0 | REG | 4.21 | 3.32 | 25.9% | 3.9% |
| INJ | 5.30 | 3.45 |
The advantage of the INJ cycle is most pronounced at the lowest temperature (-30°C), where it provided a 44.4% higher heating capacity and a 7.4% higher COP. This is attributed to the increased mass flow rate and reduced pressure ratio across the compressor due to injection, effectively combating the capacity fade typical of standard cycles in extreme cold. The secondary loop heat loss \( \Delta Q \) was present but manageable, ranging from 6% to 13% of the water-side capacity, and was generally lower for the INJ cycle in relative terms.
Cooling Performance (35°C to 50°C): In cooling mode, the performance hierarchy reversed, highlighting the adaptive benefit of the coupled system for electric car climate control. While the INJ cycle still delivered a higher raw cooling capacity (18.7% to 23.8% more), the REG cycle achieved a superior COP at all high ambient temperatures, as summarized in Table 3.
| Ambient Temp. (°C) | Cycle | Cooling Capacity, Qa (kW) | COPa | COP Advantage of REG |
|---|---|---|---|---|
| 35 | REG | 2.18 | 2.45 | 9.9% |
| INJ | 2.70 | 2.23 | ||
| 45 | REG | 1.92 | 1.68 | 2.4% |
| INJ | 2.35 | 1.64 | ||
| 50 | REG | 1.70 | 1.44 | 7.3% |
| INJ | 2.02 | 1.34 |
The regenerative cycle’s efficiency gain stems from the effective subcooling provided by the IHX, which increases the specific refrigeration effect without the parasitic power penalty associated with compressing the additional injection mass flow. This makes the REG cycle the preferred choice for efficient cabin cooling in an electric car during hot weather. The secondary loop losses were more significant in cooling mode, ranging from 11% to 15% of capacity.
Analysis of the Multifunctional Intermediate Heat Exchanger (IHX)
The heart of this coupled system for electric cars is the IHX that serves dual purposes. A detailed comparison of its operation in the two modes reveals critical design insights.
The heat transfer duty \( Q_{IHX} \) was consistently and significantly larger when the unit operated as a regenerator (REG mode) compared to an economizer (INJ mode) across all ambient conditions. Concurrently, the exergy efficiency \( \eta_{IHX,ex} \) was consistently lower in REG mode. This indicates greater irreversibilities (and a larger temperature difference) in the regenerative heat transfer process. Therefore, for the design of this coupled IHX in an electric car heat pump, the more demanding REG cycle conditions (higher heat load, lower exergy efficiency) must be the primary sizing criterion to ensure sufficient performance in cooling mode. Compromising on IHX size for the INJ cycle would lead to unacceptable performance degradation in the REG cycle.
Furthermore, the pressure drop on the low-pressure side of the IHX is a crucial parameter, especially in REG mode where it directly increases the compressor suction pressure drop. This reduces suction density and mass flow rate, negatively impacting capacity and efficiency. Our experiments showed that the low-side pressure drop in REG mode could be substantial, on the order of 3 times that of the evaporator itself under some conditions. Minimizing this pressure drop through careful core design is therefore paramount.
Based on our experimental data, we developed correlations to characterize this specific IHX. The overall thermal resistance showed a near-linear relationship with the inverse of refrigerant mass flow rate \( \dot{m}_r \), leading to the fitted relation for the overall heat transfer coefficient \( K \) (in kW/m²·K):
$$ K^{-1} = 3.04 \times 10^{-5} \left( \frac{1}{\dot{m}_r} \right) + 9.98 \times 10^{-5} $$
For the friction pressure drop on the low-pressure side in two-phase flow, the kinetic energy model yielded a fitted coefficient \( C \):
$$ \Delta p_f = 7.42 \times \frac{KE}{V} $$
These relationships provide a practical tool for evaluating and predicting the performance of similar IHX cores in the context of an electric car R290 system.
Conclusion
This study successfully demonstrated and analyzed an innovative R290 injection-regeneration coupled heat pump system designed to meet the all-climate thermal management needs of electric cars. The key findings and implications are:
1. The system’s ability to switch between a vapor injection (INJ) cycle and a regenerative (REG) cycle allows it to optimally adapt to ambient conditions. For heating in cold climates (down to -30°C), essential for electric car winter range, the INJ cycle is superior, providing significantly higher capacity (up to 44%) and better COP. For cooling in hot weather (up to 50°C), the REG cycle is more efficient, offering a higher COP (up to 10% better).
2. The multifunctional intermediate heat exchanger (IHX) is the enabling component. Performance analysis indicates that its design must be driven by the more stringent requirements of the regenerative cycle—namely, a higher heat transfer duty and a need for minimized low-side pressure drop—to ensure effective cooling performance for the electric car. The exergy efficiency is lower in this mode, highlighting an area for potential future optimization.
3. Empirical correlations for the IHX’s heat transfer and pressure drop characteristics were derived from experimental data, providing a valuable methodology for evaluating and designing such components specifically for R290 applications in electric cars.
This work provides a concrete technical pathway for enhancing the efficiency and operational range of electric car heat pump systems using the environmentally friendly refrigerant R290. The coupled cycle strategy effectively addresses the conflicting performance demands of extreme cold and heat, moving closer to a true all-climate thermal management solution for electric cars.