CO2 Refrigerant Systems for Electric Vehicles

R-744 Carbon Dioxide Properties

CO2 (R-744) operates as a natural refrigerant with unique thermodynamic properties that differentiate it from conventional synthetic refrigerants in automotive applications.

Critical Point Characteristics:

The critical temperature of 31.1°C means CO2 operates in a transcritical cycle under typical ambient conditions, fundamentally changing the heat rejection process from condensation to gas cooling.

Transcritical CO2 Cycle Operation

The transcritical cycle operates above the critical pressure during heat rejection, eliminating the isothermal condensation process found in subcritical systems.

graph TD
    A[Compressor Discharge] -->|High Pressure: 8-13 MPa| B[Gas Cooler]
    B -->|Supercritical Gas Cooling| C[High Pressure Receiver]
    C -->|Pressure Reduction| D[Internal Heat Exchanger]
    D -->|Two-Phase Flow| E[Evaporator]
    E -->|Vapor Absorption| F[IHX Low Side]
    F -->|Superheat Addition| G[Compressor Suction]
    G -->|Compression| A

    style B fill:#ff9999
    style E fill:#99ccff
    style D fill:#ffcc99

Pressure-Enthalpy Relationship:

In the supercritical region, the gas cooler outlet temperature varies non-linearly with pressure. The optimum high-side pressure maximizes COP:

$$P_{opt} = 2.778 \cdot T_{gc,out} - 0.0065 \cdot T_{gc,out}^2$$

where $P_{opt}$ is in MPa and $T_{gc,out}$ is the gas cooler outlet temperature in °C (Liao et al., SAE 2000-01-0968).

The transcritical COP is:

$$COP_{cooling} = \frac{h_1 - h_4}{h_2 - h_1}$$

where:

• $h_1$ = compressor suction enthalpy (kJ/kg)
• $h_2$ = compressor discharge enthalpy (kJ/kg)
• $h_4$ = expansion valve inlet enthalpy (kJ/kg)

High Pressure Design Considerations

Operating pressures in CO2 systems significantly exceed conventional automotive HVAC systems, requiring specialized component design.

Typical Operating Pressures:

Tube Wall Thickness Calculation:

For high-pressure tubing, the minimum wall thickness follows:

$$t = \frac{P \cdot D}{2 \cdot \sigma_{allow} \cdot \eta - P}$$

where:

• $t$ = minimum wall thickness (mm)
• $P$ = maximum working pressure (MPa)
• $D$ = outer diameter (mm)
• $\sigma_{allow}$ = allowable stress (MPa)
• $\eta$ = weld joint efficiency (typically 1.0 for seamless)

For copper-nickel alloy tubing rated for 14 MPa with $\sigma_{allow}$ = 200 MPa and 8 mm OD:

$$t = \frac{14 \cdot 8}{2 \cdot 200 \cdot 1.0 - 14} = \frac{112}{386} = 0.29 \text{ mm}$$

Practical wall thickness includes safety factors, typically 1.0-1.5 mm for automotive applications.

Gas Cooler Design vs. Conventional Condenser

The gas cooler replaces the condenser in transcritical CO2 systems, operating fundamentally differently due to supercritical heat rejection.

Heat Transfer Comparison:

The gas cooler heat rejection rate:

$$\dot{Q}{gc} = \dot{m} \cdot (h{comp,out} - h_{gc,out}) = \dot{m} \cdot c_p \cdot \Delta T_{glide}$$

where the specific heat $c_p$ varies significantly in the supercritical region, reaching peak values near the critical point.

Internal Heat Exchanger (IHX) Importance

The IHX is essential in CO2 systems, providing 5-15% COP improvement by subcooling high-side refrigerant while superheating suction gas.

IHX Effectiveness:

$$\varepsilon_{IHX} = \frac{h_{gc,out} - h_{exp,in}}{h_{gc,out} - h_{evap,out}} = \frac{T_{gc,out} - T_{exp,in}}{T_{gc,out} - T_{evap,out}}$$

Typical automotive CO2 IHX effectiveness ranges from 0.6 to 0.8. The capacity gain:

$$\Delta \dot{Q}{evap} = \dot{m} \cdot (h{exp,in,no-IHX} - h_{exp,in,with-IHX})$$

CO2 Heat Pump Mode Efficiency

CO2 systems demonstrate superior heating performance in cold climates compared to R-134a or R-1234yf due to favorable thermophysical properties.

Cold Weather Heating COP:

graph LR
    A[-20°C] --> B[-10°C]
    B --> C[0°C]
    C --> D[10°C]

    A -.->|COP: 1.8| E[CO2 Performance]
    B -.->|COP: 2.2| E
    C -.->|COP: 2.8| E
    D -.->|COP: 3.2| E

    A -->|COP: 1.2| F[R-1234yf Performance]
    B -->|COP: 1.6| F
    C -->|COP: 2.1| F
    D -->|COP: 2.5| F

    style E fill:#90EE90
    style F fill:#FFB6C1

Heating COP at -10°C Ambient:

$$COP_{heating} = \frac{h_2 - h_3}{h_2 - h_1} = \frac{Q_{heating}}{W_{comp}}$$

For a typical EV CO2 heat pump at -10°C ambient, 35°C cabin supply:

• Compressor discharge: 90-100°C
• Heating capacity: 3.5-5.0 kW
• COP: 2.0-2.4
• Range impact: 15-20% vs. PTC heater

Optimum High-Side Pressure Control

Transcritical CO2 systems require active high-side pressure control to maximize efficiency across varying ambient conditions.

Control Strategies:

The optimum pressure correlation (Kim et al., SAE 2004-01-0450):

$$P_{opt} = a + b \cdot T_{gc,out} + c \cdot T_{evap,out}$$

Typical coefficients for automotive applications:

• $a$ = 0.8-1.2 MPa
• $b$ = 0.26-0.32 MPa/°C
• $c$ = 0.02-0.05 MPa/°C

Environmental Benefits and Regulatory Compliance

CO2 provides significant environmental advantages over synthetic refrigerants, driving adoption in European and Asian automotive markets.

Environmental Comparison:

The total equivalent warming impact (TEWI) includes direct and indirect emissions:

$$TEWI = GWP \cdot L_{annual} \cdot n + GWP \cdot m \cdot (1-\alpha_{recovery}) + n \cdot E_{annual} \cdot \beta$$

where:

• $L_{annual}$ = annual leakage rate (kg/year)
• $n$ = system lifetime (years)
• $m$ = refrigerant charge (kg)
• $\alpha_{recovery}$ = end-of-life recovery rate
• $E_{annual}$ = annual energy consumption (kWh)
• $\beta$ = CO2 emission factor (kg CO2/kWh)

Safety Advantages

Despite high operating pressures, CO2 offers safety benefits as a natural, non-toxic, non-flammable refrigerant (ASHRAE 34 Class A1).

Safety Characteristics:

• No flammability risk (unlike R-290, R-152a)
• Non-toxic at concentrations below 3% (30,000 ppm)
• Odorant not required (detectable by humans at 3-5%)
• High-pressure components tested to 2-3× working pressure
• Minimal environmental release impact (natural atmospheric constituent)
• Compatible with standard lubricants (POE oils)

Cabin Concentration After Full Charge Release:

For a typical passenger vehicle interior volume of 3.5 m³ and 1.0 kg CO2 charge:

$$C_{CO2} = \frac{m_{refrigerant}}{V_{cabin} \cdot \rho_{air}} \times 10^6$$

$$C_{CO2} = \frac{1000 \text{ g}}{3.5 \text{ m}^3 \cdot 1.2 \text{ kg/m}^3} \times 10^6 = 238,000 \text{ ppm} = 23.8%$$

This exceeds safe limits (5% TWA), but rapid ventilation with open windows reduces concentration to safe levels within 30-60 seconds. Modern systems include leak detection and automatic ventilation activation.

CO2 System Component Specifications

Compressor Requirements:

• Type: Swash plate, scroll, or electric scroll
• Displacement: 15-40 cc/rev
• Maximum speed: 8000-12,000 RPM
• Discharge pressure rating: 14 MPa minimum
• Efficiency: Isentropic efficiency 0.65-0.75
• Lubrication: PAG or POE oil, 120-200 cc charge

Gas Cooler Specifications:

• Core construction: Aluminum microchannel
• Tube diameter: 1.0-2.0 mm hydraulic diameter
• Frontal area: 0.3-0.5 m²
• Air-side face velocity: 2.5-4.0 m/s
• Heat rejection: 5-12 kW at design conditions
• Pressure drop limit: 0.2-0.4 MPa refrigerant side

Evaporator Design:

• Type: Plate-fin or microchannel
• Capacity: 3-6 kW cooling
• Evaporation temperature: -5 to 10°C
• Superheat control: 5-8°C at compressor suction
• Frosting protection: Defrost cycle integration
• Air-side effectiveness: 0.6-0.75

These specifications enable CO2 systems to deliver equivalent or superior climate control performance to conventional refrigerants while providing significant environmental benefits and cold-weather heating advantages critical for electric vehicle range preservation.

References:

• SAE J2765: Procedure for Measuring System COP of a Mobile Air Conditioning System on a Test Bench
• SAE J2842: R-744 (CO2) Recovery/Recycling/Recharging Equipment
• ASHRAE Standard 34: Designation and Safety Classification of Refrigerants