Thermal-mass-driven energy flexibility in office buildings: a simulation-based assessment of building design parameters and setpoint control strategies

The increasing penetration of renewable energy sources has heightened the need for demand-side energy flexibility in buildings, particularly in heating and cooling systems, which account for a substantial share of building energy use. This study investigates the potential of utilising building thermal mass in combination with flexible temperature setpoint control to enhance energy flexibility in office buildings. A comprehensive simulation-based methodology was developed using EnergyPlus and a Python automation framework to systematically assess the influence of building design characteristics on flexibility performance. The analysis considered variations in envelope performance, HVAC system type, construction thermal mass, building geometry, window-to-wall ratio, and internal gains. For each building configuration, 810 predictive rule-based control strategies were evaluated using half-hourly electricity price and grid carbon intensity data for London in 2024, and the optimal strategy was identified by minimising annual HVAC energy cost. The results demonstrate that energy flexibility potential is highly dependent on building characteristics. In particular, the interaction between HVAC system type and construction thermal mass was found to be a dominant factor, with VRF, fan coil, and radiant systems consistently outperforming all-air VAV systems. Flexible setpoint control achieved reductions in annual HVAC energy costs of up to 15% and decreased average peak power demand by approximately 43%, alongside associated reductions in carbon emissions. These findings highlight the importance of considering multiple building characteristics simultaneously when assessing energy flexibility and underscore the need to integrate thermal-mass-based flexibility strategies into early-stage building design and future smart building performance frameworks.