Dynamic model reduction of a thermocline storage integrated in a micro-scale solar power plant

Concentrated solar power plants (CSPs) are one of the growing technologies that
will help increase the share of renewable energy in the world’s electricity production.
Coupling them with a storage tank allows for the storage of excess energy during
sunny periods to be reused during the day, hence improving the plant’s capacity factor
and reducing the cost of electricity. Thermocline storage tanks are a very good
compromise between cost and efficiency constraints, compared with other storage
technologies. Nevertheless, powerful dynamic simulation tools are needed to model
efficiently the transients linked to the intermittency of the solar source. The aim of
the proposed thesis is to contribute to the development of such tools.
This paper first compares existing physical deterministic models of a thermocline
storage tank and a parabolic trough solar field to reduced models over four reference
days. The deterministic models give accurate results with high simulation times,
whereas the reduces models are fast, but loose some precision in the results. Some
flaws of the simplified tank model are detected, and a third model of storage system
is designed. Based on the study of numerous charging and discharging processes, the
law that characterizes the evolution of the thermocline is computed and integrated
in the new model. This model is then validated over the same four reference days;
the dynamic update of the height of the thermocline allows this new model to fit
very well any weather condition.
The model developed has fixed dimensions and parameters, which limits its generality.
As such, a fourth model of tank is developed, based on dimensionless numbers.
This last model is validated in various conditions, and is therefore suitable to any
situation, with no constraint regarding weather conditions, geometry of the tank or
working fluid. The simulation time required by this model is between 75 and 180
times less than that of the first complex model, and the robustness of the model is
flawless, which makes it a very powerful tool. Finally, a new control strategy for the
solar power plant is assessed : it allows validation of the new model of tank in yet another
set of working conditions, as well as investigation of advantages and drawbacks
of one strategy over an other. An unexpected observation is that the thermocline
height at the end of the day does not depend on the strategy used, even though the
evolution is different in both cases. Some numerical issues that have been tackled to
bring the model to a perfect robustness are also discussed.