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 root 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 root t* whereas z(c)* 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. (C) 2013 Elsevier Ltd. All rights reserved.