Comparative study of energy flexibility in model predictive control of single- and variable-speed heat pumps with active thermal energy storage

Buildings present significant potential for demand-side energy flexibility through the advanced control of electrified Heating, Ventilation, and Air-Conditioning (HVAC) systems such as Heat Pump (HP) integrated with active Thermal Energy Storage (TES). Because different types of heat pumps exhibit distinct operational characteristics, the overall energy performance of HP-TES systems under sophisticated control strategies can vary considerably. This study conducts a comparative evaluation of two typical configurations: one employing a single-stage HP (SSHP) and the other with a variable-speed HP (VSHP). Both systems are integrated with active TES and managed by a mixed integer programming-based economic Model Predictive Control (MPC) framework. The results reveal the VSHP-active TES system offered comprehensive benefits, reducing operating costs by up to 63% under an aggressive time-of-use tariff while also decreasing total energy use by 8% compared to the baseline without storage. In contrast, the SSHP-active TES system achieved notable cost savings of around 45% at the expense of an over 10% increase in energy consumption under the same conditions with more pronounced indoor air temperature fluctuations. This study confirms that the effectiveness of an advanced controller is intrinsically linked to the HP's underlying operational flexibility, highlighting that the choice of HP type is critical for achieving cost reductions, energy savings, enhanced energy flexibility, and occupant comfort in grid-interactive efficient buildings.