Cost-optimal thermal storage retrofits for low-income housing in Tabriz

Low-income residential buildings in Tabriz, Iran, experience significant winter heating demand, peak-load stress, inadequate thermal comfort, and increasing household energy costs. Thermal storage retrofits offer potential improvements in heating flexibility and indoor comfort; however, their cost-optimal performance in cold-climate vulnerable housing has not been sufficiently quantified. Unlike most prior digital-twin or retrofit-optimization studies that focus primarily on energy-cost trade-offs, operational control, or single-building performance, this study explicitly couples calibrated archetype-level digital twins with thermal-storage sizing, household energy-burden constraints, and uncertainty-based robustness screening for low-income cold-climate housing. To address this research gap, a calibrated, physics-informed digital-twin framework was developed to identify cost-optimal thermal storage retrofit packages for low-income housing in Tabriz. This framework integrates measured utility data, indoor temperature monitoring, EnergyPlus simulation, thermal storage modeling, NSGA-II multi-objective optimization, and Monte Carlo uncertainty analysis across representative residential archetypes. Model reliability was validated using CVRMSE, NMBE, and indoor-temperature RMSE criteria. The selected balanced retrofit package, which includes moderate envelope insulation, low-emissivity glazing, a 300 L thermal storage tank, and smart control, reduces heating demand by 33.9%, peak heating load by 33.4%, thermal discomfort hours by 54.8%, and operational CO2 emissions by 33.9%. Additionally, it lowers the average household energy burden to 10.7%, achieves a TOPSIS score of 0.842, and attains a robustness score of 89.1%. These results indicate that medium-depth thermal storage retrofits can outperform deeper packages when cost, comfort, carbon, and uncertainty are considered collectively. The proposed framework enables archetype-specific subsidy design and supports equitable clean-energy retrofit planning for cold-climate low-income housing.