Heat Pump Systems for Electric Vehicles

Reversible Vapor Compression Heat Pump

Electric vehicle heat pumps utilize reversible refrigerant cycles to provide both cabin heating and cooling from a single system, maximizing energy efficiency and extending driving range. Unlike conventional resistance heating, heat pumps move thermal energy rather than generate it through electrical resistance.

Fundamental Operating Principle

The heat pump operates on the vapor compression cycle with reversible refrigerant flow controlled by a four-way valve. The coefficient of performance (COP) quantifies efficiency:

$$COP_{heating} = \frac{Q_H}{W_{comp}}$$

Where $Q_H$ is heating capacity (kW) and $W_{comp}$ is compressor work input (kW). Typical EV heat pumps achieve COP of 2.5-4.0 at moderate ambient temperatures, delivering 2.5-4.0 kW of heat per kW of electrical input.

Carnot Efficiency Limit

The theoretical maximum COP for heating mode follows Carnot cycle efficiency:

$$COP_{Carnot} = \frac{T_H}{T_H - T_L}$$

Where $T_H$ is condenser absolute temperature (K) and $T_L$ is evaporator absolute temperature (K). Real systems achieve 40-60% of Carnot efficiency due to irreversibilities.

Reversible Refrigerant Flow Architecture

graph TB
    subgraph "Heating Mode"
    A1[Compressor] -->|High P, High T| B1[Indoor Heat Exchanger<br/>Condenser]
    B1 -->|High P, Medium T| C1[Expansion Valve]
    C1 -->|Low P, Low T| D1[Outdoor Heat Exchanger<br/>Evaporator]
    D1 -->|Low P, Medium T| A1
    end

    subgraph "Cooling Mode"
    A2[Compressor] -->|High P, High T| B2[Outdoor Heat Exchanger<br/>Condenser]
    B2 -->|High P, Medium T| C2[Expansion Valve]
    C2 -->|Low P, Low T| D2[Indoor Heat Exchanger<br/>Evaporator]
    D2 -->|Low P, Medium T| A2
    end

    style A1 fill:#ff6b6b
    style A2 fill:#ff6b6b
    style B1 fill:#ffd93d
    style D2 fill:#6bcf7f

The four-way reversing valve redirects refrigerant flow to switch heat exchanger roles between condenser and evaporator functions. SAE J2765 provides standardized test procedures for EV HVAC performance validation.

CO2 (R-744) Heat Pump Systems

Carbon dioxide refrigerant offers advantages for EV applications due to superior low-temperature performance and environmental properties (GWP=1, ODP=0).

Transcritical CO2 Cycle

CO2 heat pumps operate in transcritical cycle when ambient temperatures exceed critical point (31.1°C, 73.8 bar). Heat rejection occurs above critical pressure:

$$Q_{gc} = \dot{m} \cdot (h_{gc,out} - h_{gc,in})$$

Where $\dot{m}$ is refrigerant mass flow rate, $h_{gc,out}$ and $h_{gc,in}$ are gas cooler outlet and inlet specific enthalpies (kJ/kg).

Cold Weather Advantage

CO2 maintains higher vapor pressure at low temperatures compared to R-134a or R-1234yf:

Higher vapor pressure enables efficient compression and heat pumping at sub-freezing temperatures where conventional refrigerants struggle.

Waste Heat Recovery Integration

Modern EV heat pumps integrate multiple heat sources to maximize COP and minimize battery depletion:

flowchart LR
    A[Battery Coolant<br/>35-45°C] --> E[Heat Exchanger<br/>Network]
    B[Motor/Inverter<br/>Coolant 50-70°C] --> E
    C[Ambient Air<br/>-20 to +10°C] --> E
    D[Cabin Exhaust<br/>20-25°C] --> E
    E --> F[Refrigerant<br/>Evaporator]
    F --> G[Compressor]
    G --> H[Cabin Heat<br/>Exchanger]

Heat Recovery Effectiveness

Heat exchanger effectiveness quantifies waste heat recovery performance:

$$\varepsilon = \frac{Q_{actual}}{Q_{max}} = \frac{\dot{m}c c{p,c}(T_{c,in} - T_{c,out})}{\dot{m}{min} c_p (T{c,in} - T_{r,in})}$$

Where subscripts $c$ and $r$ denote coolant and refrigerant streams. Typical effectiveness ranges 0.6-0.8 for compact brazed plate heat exchangers.

Cold Temperature Performance

Heat pump COP degrades as outdoor temperature decreases due to reduced temperature lift and evaporator capacity:

Performance values per SAE J2765 steady-state testing protocols.

Capacity Reduction Physics

Evaporator capacity decreases with temperature due to reduced density and enthalpy differential:

$$Q_{evap} = \dot{m} \cdot (h_{evap,out} - h_{evap,in}) = \rho \cdot \dot{V} \cdot \Delta h$$

Lower air temperatures reduce both refrigerant density $\rho$ and enthalpy change $\Delta h$, necessitating higher volumetric flow rates $\dot{V}$ or supplemental heating.

Supplemental Heating Integration

Below balance point temperature (typically -7 to -10°C), heat pump capacity becomes insufficient, requiring supplemental heating:

PTC Heater Backup

Positive temperature coefficient ceramic heaters provide rapid heating response:

$$Q_{PTC} = \frac{V^2}{R(T)} = V \cdot I$$

Where resistance $R(T)$ increases with temperature, providing self-regulating behavior. PTC heaters operate at COP=1.0 but ensure adequate heating at extreme temperatures.

Hybrid Control Strategy

graph TD
    A[Measure Ambient<br/>Temperature] --> B{T > -7°C?}
    B -->|Yes| C[Heat Pump Only<br/>COP 2.0-4.0]
    B -->|No| D{T > -15°C?}
    D -->|Yes| E[Heat Pump + 50% PTC<br/>Effective COP 1.5-2.0]
    D -->|No| F{T > -25°C?}
    F -->|Yes| G[Heat Pump + 100% PTC<br/>Effective COP 1.2-1.5]
    F -->|No| H[PTC Only<br/>COP 1.0]

Optimal control minimizes PTC usage while maintaining cabin comfort per SAE J2765 thermal comfort criteria.

Energy Efficiency vs PTC Heaters

Comparative Energy Consumption

For 3 kW heating load over 1-hour operation:

Heat pump systems reduce HVAC energy consumption by 40-70% compared to PTC heaters at typical winter temperatures.

Range Extension Calculation

For 60 kWh battery with 250 km EPA range:

$$\Delta Range = \frac{(E_{PTC} - E_{HP}) \cdot t_{drive}}{E_{battery}} \cdot Range_{EPA}$$

Operating heat pump vs PTC for 2-hour drive at 0°C:

• Energy savings: $(3.0 - 1.3) \times 2 = 3.4$ kWh
• Range extension: $\frac{3.4}{60} \times 250 = 14.2$ km

Heat pump systems can extend winter driving range by 10-20% compared to resistive heating alone.

System Design Considerations

Key engineering parameters for EV heat pump design:

Compressor Selection: Variable speed scroll or electric compressor with 2,000-8,000 rpm operating range, capacity modulation 20-100%.

Refrigerant Charge: Critical optimization for transcritical CO2 systems; typically 800-1,200 g for passenger vehicles.

Heat Exchanger Sizing: Outdoor coil area 0.8-1.2 m², indoor coil 0.4-0.6 m² with enhanced fin geometry for frost tolerance.

Controls Architecture: Model predictive control (MPC) with 30-second update cycles, integrating battery thermal management and cabin preconditioning.

SAE J2765 and SAE J2953 establish standardized test procedures for thermal performance validation and energy consumption measurement.