Simulation of thermocline storage for solar thermal power plants: From dimensionless results to prototypes and real-size tanks

A single-phase one-dimensional model called CIEMAT1D1SF has been developed for characterizing the behaviour of thermocline tanks with an effective storage medium formed by either a liquid or a liquid and a packed-bed. Despite its simplicity, this model has been validated with experimental data and the results of tank performance are similar to those obtained by other authors using more complex simulation models. In order to obtain general results the thermal equation has been nondimensionalized and the resulting expression only depends on the parameter called dimensionless velocity, v∗. It has been observed that thermocline thickness decreases as v∗ increases attaining a minimum value when v∗ ⩾ 2350 while tank efficiency increases with v∗ up to a maximum of about 87% also for v∗ ⩾ 2350. From these results the design equation for building thermocline storage tanks with maximum theoretical efficiency has been established. Since this design equation depends on tank dimensions and thermal power, small thermocline tanks and hence prototypes are not expected to behave in the same way as large or real-size tanks. Therefore maximum efficiency guideline plots for thermocline tanks with different storage media have been presented for various temperature intervals. In these plots thermal power has proven to be the critical design parameter because the larger the power the higher the degree of freedom for choosing tank dimensions and hence storage capacity and charging/discharging time. Therefore, we strongly recommend the use of these guideline plots in the design process of thermocline prototypes.