Smart thermal energy storage system integrated into the building envelope for photovoltaic surplus management

In the context of increasing residential photovoltaic penetration, the mismatch between electricity generation and demand produces surplus power that can threaten grid stability. To valorize this surplus, a thermal energy storage system integrated within the building envelope is proposed: a radiant coil embedded in the wall mass, supplied by a heat pump powered exclusively by excess photovoltaic energy, and variable-transmittance panels that modulate heat release into the interior. Using a finite-difference model implemented in MATLAB, five typical Italian wall types were characterized, showing thermal time constants between 40 and 80 h and discharge times longer than charging times, ensuring effective energy storage. Dynamic simulations in EnergyPlus, supported by a dedicated energy management system (EMS), validated the solution both in a test room and in a real detached house across four Italian cities with different climates. The integration of a phase change material (PCM) further enhanced storage capacity, while hygrothermal analyses excluded the risk of interstitial condensation. In the simplified model, the system achieved an average efficiency of 67% and reduced thermal energy consumption by approximately 33%, with savings up to 52% depending on photovoltaic system size. In the real building, cooling energy demand decreased by 40–50% and heating demand by 5–15%. This configuration transforms the building envelope into an intelligent thermal reservoir, enhancing PV self-consumption and building energy flexibility.