A semi-analytical model for evaluating the thermal storage capacity and heat use efficiency of flexible thermal storage heating floor

Highlights: • A semi-analytical model of the thermal storage of the ventilated mortar block was established. • The effect of phase change material on heat use efficiency was analyzed. • The response surface methodology was used to quantify the heat use efficiencies. • The impact of three parameters on heat use efficiency was analyzed. Using the thermal mass of building as the thermal battery is promising to increase energy efficiency and the penetration of renewable energy. However, the thermal storage capacity and energy use efficiency of floor, especially for those that are thermally activated, are still not clear. This study quantifies the amount of heat that can be effectively stored and used of several novel ventilated heating floors, including a concrete floor and a floor with phase change material (PCM). To provide a convenient and reliable simulation tool that can be used in both design and operation stages, a semi-analytical model of the ventilated floor based on the previously built simplified state-space model and the fundamental heat conduction equations was established and validated by experiment. The impact of three parameters on the heat storage capacity and heat use efficiency, including the thickness, the tube diameter, and the velocity was analyzed based on the response surface methodology (RSM), and five models to predict the capacity and efficiencies in different ventilation modes were built. The results show that PCM can increase the heat use efficiency in all ventilation modes compared to common concrete block. RSM results demonstrated that the maximum thermal storage capacity of the ventilated PCM floor is $1113.9\mathrm{kJ}/m^2$ when the thickness is 15 cm and the diameter of the air duct is 2 cm. Besides, thickness is the only factor to increase the heat use efficiency, and inner ventilation mode is more conducive to improving the efficiency.