Analytical function describing the behaviour of a thermocline storage tank: A requirement for annual simulations of solar thermal power plants

Thermocline tank behaviour during dynamic processes of charge/discharge and stand-by periods has been described in dimensionless form by means of the Logistic Cumulative Distribution Function (CDF). The Logistic-CDF parameter, S, has been correlated with both thermocline tank features and operation conditions (i.e. dimensionless velocity, v∗, and time, t∗) with the help of a single-phase one-dimensional numerical model previously developed and validated. For the dynamic processes, it has been found that S becomes independent on v∗ when v∗ > 2500, but it always increases proportionally with v∗t∗ due to the progress of thermocline zone inside the tank. For adiabatic stand-by periods, in which v∗ = 0, S   parameter proportionally increases with t∗ whereas zc∗ varies in order to keep constant the stored energy inside the tank. In terms of thermocline thickness it has been demonstrated that it is proportional to S parameter but its exact value strongly depends on the temperature accuracy that determines thermocline zone limits. This parameterised Logistic-CDF provides a general analytical equation for the outlet temperature of the storage liquid, which allows implementing thermocline storage systems in the annual performance simulations of solar thermal power plants.

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