Reversible Heat Pump

Open Model

This example shows a refrigeration cycle that can operate in a heat pump mode for heating and in an air conditioning mode for cooling. The refrigerant is R-410A. The system consists of a compressor, an outdoor heat exchanger, an electronic expansion valve (EXV), an indoor heat exchanger, and an accumulator. A 4-way directional valve separates the compressor and accumulator from the rest of the system to control the refrigerant flow direction.

In the heat pump mode, the hot refrigerant first passes through the indoor heat exchanger, which acts as a condenser, to transfer heat to the house. The EXV then expands the refrigerant to reduce its temperature. Finally, the cold refrigerant passes through the outdoor heat exchanger, which acts as an evaporator, to absorb heat from the environment. The component sizing is based on the heat pump mode operating conditions because it is expected to be more demanding than the air conditioning mode operating conditions.

In the air conditioning mode, the hot refrigerant first passes through the outdoor heat exchanger, which acts as a condenser, to reject heat to the environment. The EXV then expands the refrigerant to reduce its temperature. Finally, the cold refrigerant passes through the indoor heat exchanger, which acts as an evaporator, to transfer heat from the house to the refrigerant. Because the heat exchangers are sized for the heat pump mode operating conditions, they are likely over-sized for the air conditioning mode operating conditions.

The thermal load in this model is the house, which is represented as a volume of air in the moist air domain. A thermal network models heat transfer between the external environment and the air in the house via the walls, roof, and windows. Additionally, occupants and appliances generate heat inside the house. A fan circulates air between the house and the indoor heat exchanger.

The simulation starts with a cold external environment at -10 degC and a relatively cold house at 16 degC. After 3 hours, the external temperature gradually rises to a hot external environment at 35 degC. The house temperature setpoint is 21 degC. The system starts in heat pump mode and then switches to air conditioning mode at around t = 2.31e4 seconds or 6.4 hours.

Model

External Environment Subsystem

House Subsystem

House Thermal Network Subsystem

Control Subsystem

HVAC Mode Subsystem

The controller uses a relay to switch to heating mode when the house temperature is colder than the setpoint by 1 degC and to cooling mode when the house temperature is hotter than the setpoint by 1 degC.

EXV Control Subsystem

The simplified open-loop control for the electronic expansion valve sets an opening fraction of 70% in heating mode and 40% in cooling mode.

Compressor Control Subsystem

A PI controller controls the variable-speed compressor to maintain the desired house setpoint temperature. The controller output is limited by a high-pressure cutoff and a low-pressure cutoff to ensure safe operating conditions. The controller resets whenever the system switches modes.

Simulation Results from Scopes

The scope shows temperatures, rates of heat transfer, and control signals in the model. The compressor is initially at 100% because the house starts in a cold state at 16 degC. The compressor ramps down once the house reaches the setpoint temperature, eventually shutting down when the environment is warm enough. After switching to air conditioning mode, the compressor ramps up to bring the house temperature back to the setpoint temperature before settling down again.

Simulation Results from Simscape Logging

This plot shows the heat transfer in the indoor and outdoor heat exchangers as well as the power consumed by the compressor and fans. The coefficient of performance (COP) is the ratio of the indoor heat exchanger heat transfer to the total power consumed. The COP of this system is around 2 or less, which is low, because the system has not been optimized. For example, you can optimize the electronic expansion valve control to ensure that there is subcooling under a wide range of operating conditions.

This plot shows the high and low pressures in the refrigeration cycle and the corresponding saturation temperatures. During heat pump mode, the directional valve directs refrigerant to the indoor heat exchanger first, so the electronic expansion valve flow rate and the compressor flow rate have opposite signs. During air conditioning mode, the direction valve switches the flow direction so that the electronic expansion valve flow rate and the compressor flow rate have the same sign.