Exploring the energy-flexibility potential in the thermal mass of residential buildings

The striving for a sustainable energy system entails an increasing use of renewable energy. Since several renewable energy sources are non-dispatchable (e.g., wind and solar), this development brings challenges in terms of the balance between energy supply and demand. As a result, there is a growing need for demand-side flexibility.The building stock has been highlighted as a promising resource of such flexibility; flexibility can be provided by storing heat in the thermal mass of the buildings and using it for load shifting. The heat-storage potential in Building Thermal Mass (BTM) depends on several building properties, including the insulation level of the building envelope, the heat capacity of building structures and indoor content, and the type of heating system. Regarding heating system type, previous studies have shown that floor-heating systems are beneficial compared to radiator systems since they better activate the thermal mass of the building structures.This work has contributed to a better understanding of the heating system's influence on the energy-flexibility potential of residential buildings using heat storage in BTM. An existing low-energy multi-family house was used as a case building, heated by a Thermally Activated Building System (TABS). The TABS was compared to two other heating systems: an embedded surface system and a radiator system. It was found that the TABS performed best in every studied respect: it had the longest response time, the shortest recovery time, the greatest heat-storage capacity, and the smallest impact on thermal comfort. Furthermore, significant differences in heat-power capacity were observed.Based on the same case building, the economic potential of load shifting using BTM storage was also investigated, including its impact on thermal comfort. The heat supply was provided by a modelled ground-source heat pump, and the heating system was controlled in response to historical day-ahead electricity prices. Also, the dependence of the economic potential on various limits of maximum allowable heat power was evaluated. The results showed that significant electricity cost savings for heat-pump operation were obtained, with a limited impact on thermal comfort. Furthermore, it was found that the temperature fluctuation caused by load shifting was counteracted by the natural temperature fluctuation resulting from varying internal heat gains.Finally, the work includes a review of previous studies on the energy-flexibility potential of residential buildings using BTM storage, focusing on heat-storage potential, influencing factors, and associated benefits and challenges.