Abstract
Thermal energy storage systems help to couple thermal energy generation and process demand in cogeneration facilities. One single deposit with two design temperatures and one main temperature step in sensible thermal energy storage define the thermocline systems. Performance of one high size real thermocline thermal energy storage system is analysed. Starting from temperature and mass flow rate data registered by the plant control system, one advanced thermodynamic analysis is performed. The quality of heat storage is analysed in terms of evaluation of the stratification in the thermocline zone. The temperature data registered at 21 positions is extended by displacement analysis generating detailed profiles. Fraction of recoverable heat, thermocline width, stratification indices based on energy and exergy analysis, and mean temperature gradients in the thermocline region are calculated. These parameters are monitored under real operation conditions of the plant. The calculated parameters are studied to check their distribution and correlation. First and Second Law indices show parallel behaviour and two values are found that delimit situations of high and low values of mean temperature gradients. It was observed that buoyancy generates uniform forced movement with the right water temperature entering the diffusers, but good control strategies are essential to avoid mixing. The system demonstrated great stability in this use.
Highlights
Technologies for thermal energy storage (TES) allow mismatching between thermal energy sources and demands, so they have special value in systems in which energy availability and demand do not coincide in size or time
The study of the thermocline thermal energy storage systems has been proposed starting from real data acquired from the control system of one thermal storage facility
The tank is situated between one combined heat and power unit and one industrial drying process, matching thermal energy availability and process demand
Summary
Technologies for thermal energy storage (TES) allow mismatching between thermal energy sources and demands, so they have special value in systems in which energy availability and demand do not coincide in size or time. The thermal energy can be stored by modifying the internal energy of accumulated materials. This change can be made as the latent heat in a phase change [1,2] or by temperature variation as sensible heat [3,4]. When the sensible heat is stored in a single tank, the system takes advantage of the stratification. The hot fluid rests stable in the upper layers of the tank while the denser cold fluid stays in its lower part. If the hotter and colder temperatures are approximately constant, there is a temperature step in height between cold and hot zones called thermocline
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