Analysis of the experimental behaviour of a 100 kWth latent heat storage system for direct steam generation in solar thermal power plants

A latent heat thermal storage prototype was tested under real working conditions with steam produced by a parabolic-trough collector test facility at the Plataforma Solar de Almería. The prototype contained KNO3/NaNO3 eutectic mixture as phase change material (PCM) and expanded graphite fins arranged in a “sandwich configuration” for improving thermal conductivity. In this paper, experimental data such as steam quality, PCM temperature distribution, stored/delivered energy and thermal power have been analyzed for a selected day. A mismatch between steam quality results and the corresponding PCM temperature/time curves has been observed. Furthermore, it has been noted that stored/delivered energy and the resulting thermal power are 40 kWthh and 50 kWth, respectively, and hence, lower than the expected from design parameters. The reasons for these deviations seem to be deficient thermal insulation at the top of the prototype, use of working conditions other than design, and also thermal inertia introduced by excess PCM mass. In this paper, we also demonstrate the applicability of the quasi static model for describing the general performance of a latent thermal energy storage module with a sandwich configuration. In our particular case, the model fits the experimental data quite well when 8 W/mK is taken as the storage medium thermal conductivity. However, for a more accurate description, a sensible heat exchange term should be introduced in the model.

Research Highlights
► Storage module with PCM/EG in sandwich configuration tested under real working conditions.
► Thermal power and stored/delivered energy lower than designed.
► Strong thermal inertia due to PCM mass excess.
► Module performance fairly described by the “quasi static” model.
► Effective PCM/EG conductivity about 8 W/mK.