Numerical study on thermal energy storage performance of phase change material under non-steady-state inlet boundary

• Published in: Applied energy 2011-11, Vol.88 (11), p.4172-4179
• Main Authors: Tao, Y.B., He, Y.L.
• Format: Article
• Language: English
• Subjects: Applied sciences; Boundaries; Energy; Energy. Thermal use of fuels; enthalpy; Enthalpy method; Exact sciences and technology; heat transfer; Inlet temperature; Inlets; mass flow; Mass flow rate; Mathematical models; Melting; Non-steady-state boundary; Phase change; Phase change thermal energy storage; Solar collectors; solar radiation; temperature; Transport and storage of energy
• ISSN: 0306-2619; 1872-9118
• DOI: 10.1016/j.apenergy.2011.04.039

► Based on the unstable solar radiation, a model was established for phase change process under unsteady boundary. ► The PCM melting time decreases with the initial inlet temperature increase under the same average inlet temperature. ► The melting time reduces about 51.9% with the initial inlet temperature increase from 30°C to 90°C. ► The melting time decreases with the initial inlet mass flow rate increase under the same average inlet mass flow rate. ► The melting time reduces about 36.5% with the initial inlet mass flow rate increase from 2.0 × 10 −4 kg/s to 8.0 × 10 −4 kg/s. Due to the solar radiation intensity variation over time, the outlet temperature or mass flow rate of heat transfer fluid (HTF) presents non-steady-state characteristics for solar collector. So, in the phase change thermal energy storage (PCTES) unit which is connected to solar collector, the phase change process occurs under the non-steady-state inlet boundary condition. In present paper, regarding the non-steady-state boundary, based on enthalpy method, a two dimensional physical and mathematical model for a shell-and-tube PCTES unit was established and the simulation code was self-developed. The effects of the non-steady-state inlet condition of HTF on the thermal performance of the PCTES unit were numerically analyzed. The results show that when the average HTF inlet temperature in an hour is fixed at a constant value, the melting time (time required for PCM completely melting) decreases with the increase of initial inlet temperature. When the initial inlet temperature increases from 30 °C to 90 °C, the melting time will decrease from 42.75 min to 20.58 min. However, the total TES capacity in an hour reduces from 338.9 kJ/kg to 211.5 kJ/kg. When the average inlet mass flow rate in an hour is fixed at a constant value, with the initial HTF inlet mass flow rate increasing, the melting time of PCM decreases. The initial inlet mass flow rate increasing from 2.0 × 10 −4 kg/s to 8.0 × 10 −4 kg/s will lead to the melting time decreasing from 37.42 min to 23.75 min and the TES capacity of PCM increasing from 265.8 kJ/kg to 273.8 kJ/kg. Under all the studied cases, the heat flux on the tube surface increases at first, until it reaches a maximum then it decreases over time. And the larger the initial inlet temperature or mass flow rate, the earlier the maximum value appearance and the larger the maximum value.