Eccentricity optimization of a horizontal shell-and-tube latent-heat thermal energy storage unit based on melting and melting-solidifying performance

As for a horizontal single-pass shell-and-tube latent-heat thermal energy storage unit (LTESU), the eccentricity between inner and outer tube is designed to improve the melting and melting-solidifying performances. A fixed-grid numerical method with enthalpy-double-porosities model is proposed to accurately predict the melting or solidifying characteristics of LTESUs with different eccentricities. Some optimal eccentricities are obtained to decrease the total melting or melting-solidifying time. The effects of Rayleigh number on the optimal eccentricities are also discussed. The results show that, when melting process is just concerned, vertically moving down the inner tube from the center of outer tube can obviously decrease the total melting time. However, a greater eccentricity does not always bring a better melting performance. That is to say, there exists an optimal eccentricity for the shortest melting time. It is found that the optimal eccentricity for melting process is linearly dependent on the Rayleigh number. As for a complete heat storage process including charging and discharging process, the eccentric arrangement of inner tube has benefit for decreasing the total melting-solidifying time only when the Rayleigh number ratio of solidifying process to melting process is larger than 2.0. The optimal value of eccentricity for the melting-solidifying process increases sharply with the increase of Rayleigh number ratio when the value of Rayleigh number ratio varies from 2.0 to 3.0, but the growth of optimal eccentricity slows down when Rayleigh number ratio is greater than 3.0.