PV/T-assisted underfloor heating with phase change material to improve indoor thermal comfort in a test room designed according to nearly zero-energy building principles: experimental and numerical study

Thermal energy storage is required in solar-assisted radiant heating systems because solar heat is mainly available during the daytime, whereas heating demand in highly insulated buildings often extends into the evening. This study experimentally and numerically investigates a roof-mounted photovoltaic/thermal (PV/T)-assisted hydronic underfloor heating system with RT22HC phase change material (PCM) integrated into the floor of a highly insulated test room located in a research building designed according to nearly zero-energy building (nZEB) principles in Elazığ, Türkiye. Field measurements of solar irradiance, indoor and outdoor air temperatures, and hydronic loop temperatures were used to validate a transient three-dimensional computational fluid dynamics (CFD) model based on the enthalpy–porosity method. The validated model was then used to compare the PCM-integrated floor with an identical reference configuration without PCM under the same boundary conditions. The PCM layer delayed part of the solar heat transfer from the charging period to the evening hours, reducing the daily maximum indoor temperature from 28.0 to approximately 26.4 °C (about 1.6 K); at 14:00, the PCM case was 2.6 K cooler than the non-PCM case, and at 23:00 it was 4.7 K warmer. Under the selected October operating conditions, the PCM completed a daily melting-solidification cycle and reached a maximum stored latent energy of 9.94 kWh, corresponding to 1.12 kWh m −2 of heated floor area. The results demonstrate that coupling PV/T collectors with PCM-assisted underfloor heating can smooth indoor temperature fluctuations, extend evening heat availability, and improve the temporal matching between on-site solar heat production and space-heating demand in highly insulated nZEB-oriented buildings.