Design and installation of a thermochemical heat storage prototype in an existing solar facility

Stefania Tescari, S. Breuer, G. Lantin, J.P. Säck, H.P. Streber, M. Roeb, C. Sattler

German Aerospace Center (DLR) – Institute of Solar Research


A present challenge for solar power plant is the efficient storage of the energy. Several commercial storage systems are available storing the heat in its sensible form, using fluid or solid storage media. As an example of the latest, ceramic bricks are employed to store the energy in air-operated solar tower power plant, like the Solar Tower in Jülich (STJ). In this system, the hot air, coming from the solar receiver, flows through the storage assembly, transferring the sensible heat to the solid material. A honeycomb-like structure provides a large heat exchange surface area between air and the solid storage medium. During “discharging” (off-sun) operation, the air flow is reversed: “cold” air is introduced in the storage medium to be heated by that before sent to the power block. The energy density of this system could be strongly increased by storing, in addition to the sensible heat, the chemical heat, absorbed or released by a reversible chemical reaction. This could offer higher energy density and possibility of storing the heat at higher temperature for longer time periods. This concept was already shown in literature, mainly using packed bed or moving particle reactor. In the present work, the material is in a structured form, similar to the one used in the STJ. To explore this concept, monolithic bodies made of a reactive material were prepared. As shown in literature, cobalt oxide (Co3O4) is a promising material for this application due to several reasons: the suitable temperature range (equilibrium temperature in air is about 900 °C under atmospheric pressure), the energy density for complete conversion is high (844 kJ/kg) and the performances do not decreases during cycling. The proof-of-concept feasibility of small monolithic bodies made of
cobalt oxide was verified in a previous study. Following the promising results, obtained by the small scale tests, the concept is scaled up to the prototype scale. The geometry and size of the system are analyzed through suitable simulation models, through successive steps. At first, the reactor requirements are defined from analytical calculations. Afterwards, a simplified model delineates the optimal reactor geometry, considering the
impact of pressure drop and thermal losses through the reactor walls. A more precise CFD model is used for simulating the gas velocity profile inside the reactor, aiming at optimization of design aspects to obtain an almost uniform gas velocity inside the reactor. Resulting from the models and from constraints related to the installation in the STJ, the design of the prototype was developed. The storage module is separated into two, identical reactor chambers, each one containing 250kg of reactive material. The reactive honeycombs are placed in the reactors centers. The gas, entering from the top of the reactor flows through a conical inlet, which guarantees a more uniform air distribution before it comes in contact with the reactive material. The complete system is insulated and placed inside an outer housing. The complete system will be implemented for the first time in an existing solar facility (STJ).