Charging characteristics of finned thermal energy storage tube under variable rotation

Solar photovoltaic-thermal (PVT) systems effectively offset the drawbacks of intermittent solar and low photovoltaic conversion efficiency. Thermal energy storage (TES) tanks of PVT systems with high charging efficiency and consistent thermal safety might achieve efficient utilization of solar energy for building. A new variable rotational strategy has been proposed to optimize the charging characteristics for TES tubes, taking into consideration the non-uniformity of melting. A series of simulations based on the volume-averaged model are conducted to investigate the thermal energy storage property of TES tubes under variable rotary mechanism. Qualitative and quantitative comparisons are made between variable rotation (ω = 1.5–0.5, 1.5–1.0, 1.5–2.0 rad·s⁻¹), constant rotation (ω = 1.5 rad·s⁻¹), and stationary systems. The focus of the comparison is melting efficiency, temperature distribution, and natural convection. The results indicate that rotation effectively shortens charging time, with a 57.62% and 15.73% reduction with variable rotary mechanisms of 1.5–1.0 rad·s⁻¹ when compared with stationary and constant rotating tubes. Meanwhile, in the final moment, the greatest medium-temperature (55–65 °C) proportion of 90.58% and less low-temperature (25–55 °C) and high-temperature (65–70 °C) paraffin occupation of 4.67% and 4.75% could be obtained, reflecting the completed latent heat storage and stable thermal safety. The optimal variable rotation achieves improvements of 47.84% and 106.73% in time-integral Grashof number (Gr) and heat storage rate, compared with traditional stationary tubes.