Exploring the optimal thermal mass to investigate the potential of a novel low-energy house concept

In conventional buildings thermal mass is a permanent building characteristic depending on the building design. However, none of the permanent thermal mass concepts are optimal in all operational conditions. We propose a concept that combines the benefits of buildings with low and high thermal mass by applying hybrid adaptable thermal storage (HATS) systems and materials to a lightweight building. The HATS concept increases building performance and the robustness to changing user behavior, seasonal variations and future climate changes. In this paper the potential of the novel HATS concept is investigated by determining the sensitivity of the optimal thermal mass of a building to the change of seasons and to changing occupancy patterns. The optimal thermal mass is defined as the quantity of the thermal mass that provides the best building performance (based on a trade-off between the building performance indicators). Building performance simulation and multi-objective optimization techniques are used to define the optimal thermal mass of a case study in the Netherlands. Simulation results show that the optimal quantity of the thermal mass is sensitive to the change of seasons and occupancy patterns. This implies that the building performance will benefit from implementing HATS. Furthermore, the results show that using HATS reduces the heating energy demand of the case study with 26% and reduces weighted over- and underheating hours with 85%.