Retrofitting Heat Pump Control Systems in Residential Buildings with Supervisory Economic MPC

In the U.S., residential heat pumps (HPs) are typically operated with a thermostat equipped with an onboard rule-based control (RBC) strategy to turn them on and off. RBC strategies are reactive feedback control strategies that do not proactively operate HPs in a way that accounts for system dynamics, weather forecasts, and time-varying electricity pricing. This paper proposes a novel approach to retrofit existing HP thermostats with economic model predictive control (EMPC) to optimize electricity costs while maintaining occupant thermal comfort. Specifically, the EMPC, implemented in a supervisory layer above the existing thermostat RBC, determines the temperature setpoint used by the RBC. An RBC model, representing the relationship between the temperature setpoint, indoor air temperature, and HP status, is developed and integrated into the EMPC formulation. The EMPC cost function is augmented to penalize setpoint changes, as frequent setpoint adjustments can be undesirable for residents. A novel soft constraint on the building temperature is developed to penalize temperature deviations from the comfortable range only when the indoor air temperature is less than the lower bound resulting from HP operation (considering space cooling operation). Closed-loop simulations demonstrate that the supervisory EMPC reduces electricity costs compared to a fixed-setpoint strategy.