Simulation and performance analysis of the charging/discharging processes of a condensation thermal accumulator for solar- assisted heat pump system

Driven by China's carbon peaking and carbon neutrality goals and the growing demand for clean heating, solar-assisted heat pump systems have attracted increasing attention because of their ability to integrate solar energy, ambient air heat, and phase-change thermal energy storage. As a key component, the condensation thermal accumulator combines refrigerant condensation, heat storage, and user-side heat release, and its performance directly affects system stability and energy efficiency. In this study, a two-dimensional transient CFD model was developed for a condensation thermal accumulator with an upper paraffin phase change region and a lower water region. The enthalpy-porosity method was used to model paraffin melting and solidification, while the Boussinesq approximation was applied to account for natural convection. Following mesh and time-step sensitivity analyses and comparison with measured mean PCM temperatures, the model was used to study charging and discharging processes and the effects of tube diameter and spacing. Results show that paraffin undergoes sensible heating, latent heat absorption, and a nearly stable stage during the charging process. During the discharging process, natural convection weakens and conduction becomes dominant. Increasing tube diameter from 14 to 20 mm increased stored heat at 8000 s from 24.14 to 27.97 kW·h. Effective discharged heat increased from 20.87 to 24.11 kW·h, while effective discharging efficiency remained approximately 86%. Increasing tube spacing from 38 to 56 mm shortened the time to reach 99% liquid fraction from 8860 to 7610 s. It also increased effective discharging efficiency from 86.24% to 87.94%. These findings provide quantitative guidance for CTA geometry selection under the investigated operating conditions.