Assessing aggregated building thermal mass energy flexibility for the design and operation of local energy communities

Local energy communities (LECs) can enhance renewable energy integration, mitigate grid congestion, and enable coordinated use of distributed energy resources. Existing community-level studies focus on electrical batteries and photovoltaic systems as energy flexibility assets, while the inherent flexibility potential of aggregated building thermal mass remains underexplored. This study presents a simulation-based optimization approach to quantify and evaluate aggregated building thermal mass energy flexibility at the community scale. The approach couples building energy simulations in OpenModelica with a mixed-integer linear programming aggregator model implemented in Python, enabling coordinated day-ahead scheduling of heat pumps, battery storage, and photovoltaic curtailment under multi-criteria objectives. The approach is applied to a case study LEC consisting of seven office buildings in the Netherlands, evaluating economic, environmental, and technical performance indicators. The results show that coordinated aggregator-level control significantly improves LEC performance compared to building-level optimization. Aggregated building thermal mass provided flexibility comparable to a 700 kWh community battery and, during certain periods, exceeded the flexibility contribution of conventional distributed energy resources. Across the design space, a stakeholder-driven and controller-robust design configuration was identified, consisting of a 700 kWh battery and photovoltaic systems covering 70% of the total roof area. This configuration achieved a net present value of up to 1.25 M€, reduced the CO₂ footprint by up to 9.1%, and lowered the one-percent peak by 65 kW across different control strategies. These findings demonstrate that aggregated building thermal mass represents a cost-free flexibility resource that can significantly enhance the economic, environmental, and technical performance of LECs.