MODELING AND CONTROL OF PCM-BASED THERMAL ENERGY STORAGE INTEGRATED IN AIR DISTRIBUTION SYSTEMS

Abstract

The global transition from fossil-based to renewable energy sources has introduced significant variability in electricity supply, particularly due to the intermittent nature of solar energy. This variability has led to increased reliance on fossil fuel power plants during peak demand periods, exacerbating grid instability and carbon emissions. Buildings, which account for approximately 73% of total electricity consumption in the United States—with HVAC systems alone responsible for 27% and 30% of commercial and residential usage, respectively—present a critical opportunity for demand-side flexibility. Thermal energy storage (TES) systems, particularly those utilizing phase change materials (PCMs), offer a promising solution to mitigate the mismatch between energy supply and demand. This dissertation presents two studies that demonstrate the potential of PCM TES to improve the demand-side flexibility of HVAC systems. Firstly, a novel energy storage solution is proposed by incorporating phase change material (PCM) panels in supply ducts to increase a building’s thermal storage capacity and demand flexibility. During off-peak hours, the system runs at a supply air temperature (SAT) below the PCM solidification point to charge the storage unit with “cooling” energy. During on-peak hours, a higher SAT is utilized so that the stored “cooling” energy can be discharged into the supply-air as a means to reduce the peak air-conditioning power usage. To evaluate the potentials of peak demand reduction and utility cost savings, a numerical model for a PCM panel prototype and its heat exchange with the air flow in the duct was developed and calibrated using experimental data. Whole building energy simulations were conducted in a co-simulation environment that has integrated the developed PCM model, EnergyPlus Department of Energy (DOE) prototypical model for a medium office building and a calibrated model for variable-speed direct-expansion cooling systems. The simulations covered five cities in different U.S. climate zones over a three-month cooling season and used actual time-of-use (TOU) rate schedules offered by the local electric utility companies. The simulation results have shown the PCM storage could reduce the on-peak energy consumption by 23-32% and the seasonal cooling electricity cost by up to 16%, with a simple rule-based control strategy. A simple payback analysis resulted in payback periods from 7.5 to 27 cooling months. Secondly, a model-based predictive control strategy is developed to optimize the operations of phase change material (PCM) ceiling panels coupled with a multi-stage air-source heat pump. A three-stage prototype heat pump unit has been built and tested in the laboratory, with the low and medium stages designed for space heating/cooling and the high compression stage dedicated to charging of the PCM energy storage. To facilitate optimal control of the integrated heat pump system, a mixed-integer linear programming formulation is derived through linearization of the heat pump model and a mixed-integer reformulation of the PCM dynamic governing equations. A predictive control strategy is synthesized based on the resultant control formulation and implemented in a receding horizon scheme that optimizes the PCM charging and the zone temperature schedules simultaneously to leverage both the passive (associated with building construction materials) and active (PCM) storage capacities of a building. The control strategy has been tested along with three benchmarking control scenarios using a co-simulation platform for a prototypical detached house in Atlanta, GA. Test results showed that application of the proposed control strategy to the PCM-integrated heat pump could provide 27.1% electricity cost savings while a fine tuned rule-based control strategy could achieve cost savings of 20.4%, compared to a baseline case without PCM storage, under a time-of-use rate tariff.