ABraytCSPfuture concept is based on a Thermochemical Energy Storage (TCES) system that exploits reversible reduction/oxidation (redox) reaction schemes of earth-abundant, inexpensive, non-toxic oxides in direct contact with air.
These schemes operate between the oxidized and reduced form of a metal oxide with multiple oxidation states in a cyclic process. The heat received by the solar field is used to carry out the thermal reduction step, where the higher-valence oxide state releases oxygen and transforms to the lower-valence form (1). When it is required, exothermic oxidation (2) takes places and the reduced form is transformed to the oxidized form, releasing the stored heat.
The ambition of ABraytCSPfuture is to demonstrate for the first time ever and at a proof-of-concept level this radically new idea by the development and operation of a first-of-its-kind, compact, dual-bed thermochemical reactor/heat exchanger, comprised of non-moving, flow-through porous ceramic structures (honeycombs and/or foams) based on such oxides. The project is focus on inexpensive, non-critical, abundant and environmentally safe Ca-, Mn- and recycled Fe-based perovskite materials.
ABraytCSPfuture concept’s solar plant operation
Step 1: During on-sun operation a part of the high-temperature air stream at atmospheric pressure from the receiver (red lines) is diverted to the «Off-sun storage unit«. The other part of the air from the receiver is also charging sensibly one of the regenerator beds introduced from the top; the right one, which contains porous structures with oxide in its fully oxidized state. Provided that the hot air stream temperature is higher than the reduction onset temperature, the oxide will be reduced according to the endothermic reaction (1) in a “stratified” pattern. Due to the constant supply of hot air, eventually all the solid will acquire the inlet air stream temperature and will be transformed to its reduced state (charging completed). The oxide will remain reduced as long as its temperature remains higher than its reduction onset one.
It is foreseen that this procedure has been implemented in the left chamber in the previous cycle; thus, when it starts on the right chamber, the oxide in the left chamber is at its reduced state, at a temperature above its reduction onset and can now be exploited for the reverse, exothermic oxidation reaction (2) for heat generation (discharging). Thus, at the same time, compressed air from the compressor at low temperature is introduced in the lower end of the left chamber. Simultaneously to sensibly heating the colder air by the hot oxide mass, the oxygen of the air stream oxidizes the material according to reaction (2). The enthalpy of the exothermic reaction is used to further heat the air stream as it flows towards the top end raising its temperature beyond the level achievable by only sensible heating («thermal boosting”). In addition, the high pressure under which oxidation is performed raises temperature sufficiently to operate the power block.
Step 2: The flow to the two chambers is switched. Pressurized discharging takes place in the right chamber and atmospheric pressure-charging in the left. As a result, the gas turbine is always fed with a high-pressure, high-temperature air stream from the discharged chamber. During off-sun operation it is assumed that hot air of temperature similar to that coming out of the receiver is provided from the storage system which operates with exactly the same sensible-TCS principle as above, yet only with atmospheric-pressure air; hence the dual-bed operation principle of the thermal booster is identical.