Comparison of Sizing Calculations Based on Exergy and Electric Power Production for Molten Salt Thermal Energy Storage Systems

Abstract

High temperature thermal energy storage in liquid molten salts is a cost effective and proven technology for process heat and power plant applications. Examples are the improved use of waste heat from industrial processes, enhancement of the flexibility of power stations and cogeneration, as well as the conversion and storage of fluctuating surplus electricity from renewable energy sources. Some of the main advantages of molten salt technology are its high maturity, low costs for the storage material, high heat transfer rates and operation at ambient pressure levels. The thermocline concept promises further potential for cost reduction by storing hot and cold molten salt inside a single tank, separated due to density stratification. By embedding a low cost solid filler material into the molten salt storage tank, further cost reductions can be achieved. A new test facility to investigate and advance this technology, namely “TESIS:store”, is currently being commissioned at DLR in Cologne. Besides new technological challenges, originating mainly from chemical stability, the sizing of such dynamic systems has taken on greater significance. In most of the studies to the present date, sizing calculations are based on constant boundary conditions, which are derived from the connected energy source (i.e. solar field, industrial process) and energy sink (i.e. power block, industrial process). In reality, these boundary conditions are not constant. For example, during charging, the mass flow of the heat transfer fluid (HTF) is varying throughout the day and the time span of solar irradiation is different over the course of the year. Towards the end of the discharging period, the exit temperature of the storage volume will drop. Consequently, an attached power block will return a decreased HTF temperature back into the storage system. If the return temperature declines too far, the cold zone of the storage volume does so as well. In the subsequent charging cycle, the HTF coming from the storage volume now has a very low temperature. As a result, if the thermal power of the energy source is limited, the charging mass flow has to be reduced. These considerations show that boundary conditions have a significant impact on the operating behavior of such thermocline systems and it is therefore advisable to take them already into account during the sizing calculations. In this work a fast numerical model for sizing calculations, implemented in Matlab®, is presented. With this model, two sizing approaches are used for an extensive parametric study, where variable geometric properties and boundary conditions are applied. The first approach is considered as the classic approach, based on regained exergy after a full storage cycle, whilst for the second approach, the storage model is coupled with a simplified solar field and power block model. In the latter case, produced electric power after a full storage cycle is used for the rating of the storage configurations. The comparison of both approaches show the significance of fluctuating boundary conditions on the cyclic behavior of the storage system and eventually, its sizing results.