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Thermochemical energy storage for high-temperature concentrating solar applications

Stéphane Abanades

Processes, Materials and Solar Energy Laboratory, PROMES-CNRS, 7 Rue du Four Solaire, 66120 Font-Romeu, France


Being an intermittent and variable renewable energy, solar energy storage in the form of heat is a key issue. Thermochemical energy storage (TCES) of solar energy at high temperatures can be performed by the means of reversible solid-gas reactions: AB(s) + ΔH ⇄ A(s) + B(g). This type of thermal energy storage can be associated to concentrating solar thermal power plants (concentrated solar power, CSP) for continuous electricity generation, or more generally to any type of industrial processes requiring high-temperature process heat (e.g., chemical industry, cement production, metallurgy, thermochemical fuels production). The main interest of such a storage is the possibility to operate the solar process continuously by overcoming the inherent limitations of solar energy (intermittency and solar resource variability). The thermal effect of endothermal/exothermal reactions can be exploited provided that the reaction is reversible. The storage step (heat charge) corresponds to the solid material decomposition (endothermal step) that is carried out with the supply of solar energy. The energy release step (heat discharge) corresponds to the reverse reaction to recover the energy stored as chemical bonds in the solid. This energy can be transferred to a heat transfer fluid (HTF) that can be then used to operate a thermodynamic cycle for electricity generation or directly fed to an industrial process for external energy supply (Fig. 1). For instance, the utilization of redox pairs of oxides or mixed oxides (MO2x+1 + ΔH ⇄ MO + xO2) allows to perform cycles under air in open loop. The main challenges in this field are related to the selection of the most suitable thermochemical systems depending on the downstream process requirements.

Principle of the energy storage concept for supplying high-temperature process heat
Application of thermochemical energy storage to concentrating solar power plants
(a)(b)

Figure 1. (a) Principle of the energy storage concept for supplying high-temperature process heat, (b) Application of thermochemical energy storage to concentrating solar power plants

TCES systems exhibit different energy storage properties including temperatures of the heat charge/discharge, heat storage capacity (energy storage density), reaction reversibility and kinetics for both steps. In addition, suitable solar reactor concepts for the solid-gas reactions need to be designed as well as the heat exchangers to transfer the solar energy to the reactive storage materials (solar receiver) and then to the heat transfer fluid. Possible solar reactor concepts can be based either on the continuous-flow particles reactor or monolithic reactor concepts. In such reactors, porous monolithic absorbers or particles play the role of the solar interface. The main benefits of the former are the high surface area, effective volumetric absorption and heat transfer, and straightforward integration of concentrated sunlight, while the advantages of the latter are the possible transport and storage of particles, performing as both heat transfer and storage medium at the same time, requirement of particulate material in various industrial processes, and possible tuning of mass flow rate as control parameter. Integration of the TCES system and coupling of the solar interface to the industrial process are key aspects for large-scale deployment and process intensification.

State-of-the-art, screening and selection of thermochemical reactions and candidate materials with high potential for energy storage at high temperatures (400–1200°C) have been conducted (André et al., 2016). Most of TCES systems have been assessed and tested only at laboratory-scale so far. The energy storage density of such thermochemical pathway is usually 5 to 10-fold higher in comparison with sensible and latent energy storage systems. TCES systems based on reversible solid-gas reactions thus appear to be the most promising candidates for long-term stable storage of solar energy. Accordingly, the reaction products can be stored at room temperature without energy losses during their storage and therefore, the storage duration and transportation distance are theoretically unlimited. Regarding the key characteristics targeted when developing such systems, the process must be reversible with a constant conversion without performance losses during cycling to avoid a decrease of the material storage capacity. Another challenge is the optimization of the temperature gap between the charge/discharge steps that should be lowered to improve process efficiency and facilitate reactions control.

TCES systems involving metal oxides (e.g., Co3O4, Mn2O3, BaO2, CuO, Fe2O3) are particularly attractive for CSP applications because air can be directly used as the heat transfer fluid in open loop. The tuning of the redox properties (transition temperatures, oxygen storage capacity, reaction reversibility…) is made possible via synthesis of mixed oxides (André et al., 2017, 2018a,b). Accordingly, the reversibility and cycling stability of Mn2O3/Mn3O4 system can be noticeably improved via Fe addition with a content higher than 15 mol% (André et al., 2017), as well as Co or Cu beyond 30 mol% (André et al., 2017, 2018a,b). The Co3O4/CoO system also offers noteworthy performance as a pure material. Cu addition to Co3O4 allows to maintain a good cycling capability and high enthalpies at the expense of increasing sintering, while Fe addition to Co3O4 impacts its performances such as reaction enthalpy (André et al., 2017). The energy storage density of oxides is correlated to the oxygen storage capacity (André et al., 2018b).

Other attractive candidates for TCES have been studied such as perovskites (ABO3 structure) featuring a continuous redox activity over a variety of temperature and oxygen partial pressure, thanks to vacancies that facilitate oxide ion transport (Zhang et al., 2016; André and Abanades, 2018). As opposed to the oxides with discrete transitions in the metal oxidation state during redox reactions, perovskites offer continuous oxygen mobility in non-stoichiometric reactions and thus can adapt to various operating temperatures, which is convenient for high-temperature applications (> 1000°C).

Alternatively, TCES systems based on carbonates show a practical interest (André and Abanades, 2017, 2018) and have been largely developed for post-combustion CO2 capture (e.g., Ca-looping cycles). The CaCO3/CaO system involving successive calcination/carbonation reactions is advantageous as the natural mineral resource is abundant, low cost and offers a high energy storage potential. The loss-in-capacity due to sintering and pore blockage during cycling can be overcome by using composite materials or stabilizing agents, to maintain the system capacity during charge/discharge steps. Among alkaline earth carbonates, CaCO3 and SrCO3 are the most suitable candidates for energy storage, with a high gravimetric energy storage density. In contrast, BaCO3/BaO is more challenging due to a melting issue that impacts reaction reversibility. The addition of an inert material such as MgO can be used to improve the stability of carbonates during cycles (André and Abanades, 2017). Finally, hydroxides (M(OH)2/MO) (André and Abanades, 2018) and sulfates (MSO4/MO) also represent appropriate TCES systems thanks to their high energy storage density, at the expense of additional issues related to corrosive gas products.

The integration of TCES systems to industrial processes is thus currently a great opportunity to supply solar heat on demand and warrant round-the-clock operation. Continuous-flow reactor concepts for solar energy storage are being proposed and demonstrated (Abanades and André, 2018). The variety of candidate materials for thermochemical energy storage with relevant adaptable properties makes possible their combination with various industrial processes. In comparison with sensible and latent heat storage technologies, TCES is a more flexible approach offering a wide range of operating temperatures adaptable to various high-temperature processes, a high storage capacity without thermal losses during long periods including seasonal energy storage, and thus a strong potential for development of concentrating solar applications.

REFERENCES

Abanades, S. and André, L. (2018) Design and demonstration of a high temperature solar-heated rotary tube reactor for continuous particles calcination, Applied Energy, 212, 1310–1320. DOI: 10.1016/j.apenergy.2018.01.019

André,L., Abanades, S., and Flamant, G. (2016) Screening of thermochemical systems based on solid-gas reversible reactions for high temperature solar thermal energy storage, Renewable & Sustainable Energy Reviews, 64, 703–715. DOI: 10.1016/j.rser.2016.06.043

André, L., Abanades, S., and Cassayre, L. (2017) High-temperature thermochemical energy storage based on redox reactions using Co-Fe and Mn-Fe mixed metal oxides, Journal of Solid State Chemistry, 253, 6–14. DOI: 10.1016/j.jssc.2017.05.015

André, L., Abanades, S., and Cassayre, L. (2018a) Experimental investigation of Co-Cu, Mn-Co and Mn-Cu redox materials applied to solar thermochemical energy storage, ACS Applied Energy Materials, 1(7), 3385–3395. DOI: 10.1021/acsaem.8b00554

André, L., Abanades, S., and Cassayre, L. (2018b) Mixed metal oxide systems applied to thermochemical storage of solar energy: benefits of secondary metal addition in Co and Mn oxides and contribution of thermodynamics, Applied Sciences, 8(12), 2618. DOI: 10.3390/app8122618.

André, L. and Abanades, S. (2017) Evaluation and performances comparison of calcium, strontium and barium carbonates during calcination/carbonation reactions for solar thermochemical energy storage, Journal of Energy Storage, 13, 193–205. DOI: 10.1016/j.est.2017.07.014

André, L. and Abanades, S. (2018) Investigation of metal oxides, mixed oxides, perovskites and alkaline earth carbonates/hydroxides as suitable candidate materials for high-temperature thermochemical energy storage using reversible solid-gas reactions, Materials Today Energy, 10, 48–61. DOI: 10.1016/j.mtener.2018.08.007

Zhang, Z., André, L., and Abanades, S. (2016) Experimental assessment of oxygen exchange capacity and thermochemical redox cycle behavior of Ba and Sr series perovskites for solar energy storage, Solar Energy, 134, 494–502. DOI: 10.1016/j.solener.2016.05.031

Referências

  1. Abanades, S. and André, L. (2018) Design and demonstration of a high temperature solar-heated rotary tube reactor for continuous particles calcination, Applied Energy, 212, 1310–1320. DOI: 10.1016/j.apenergy.2018.01.019
  2. André,L., Abanades, S., and Flamant, G. (2016) Screening of thermochemical systems based on solid-gas reversible reactions for high temperature solar thermal energy storage, Renewable & Sustainable Energy Reviews, 64, 703–715. DOI: 10.1016/j.rser.2016.06.043
  3. André, L., Abanades, S., and Cassayre, L. (2017) High-temperature thermochemical energy storage based on redox reactions using Co-Fe and Mn-Fe mixed metal oxides, Journal of Solid State Chemistry, 253, 6–14. DOI: 10.1016/j.jssc.2017.05.015
  4. André, L., Abanades, S., and Cassayre, L. (2018a) Experimental investigation of Co-Cu, Mn-Co and Mn-Cu redox materials applied to solar thermochemical energy storage, ACS Applied Energy Materials, 1(7), 3385–3395. DOI: 10.1021/acsaem.8b00554
  5. André, L., Abanades, S., and Cassayre, L. (2018b) Mixed metal oxide systems applied to thermochemical storage of solar energy: benefits of secondary metal addition in Co and Mn oxides and contribution of thermodynamics, Applied Sciences, 8(12), 2618. DOI: 10.3390/app8122618.
  6. André, L. and Abanades, S. (2017) Evaluation and performances comparison of calcium, strontium and barium carbonates during calcination/carbonation reactions for solar thermochemical energy storage, Journal of Energy Storage, 13, 193–205. DOI: 10.1016/j.est.2017.07.014
  7. André, L. and Abanades, S. (2018) Investigation of metal oxides, mixed oxides, perovskites and alkaline earth carbonates/hydroxides as suitable candidate materials for high-temperature thermochemical energy storage using reversible solid-gas reactions, Materials Today Energy, 10, 48–61. DOI: 10.1016/j.mtener.2018.08.007
  8. Zhang, Z., André, L., and Abanades, S. (2016) Experimental assessment of oxygen exchange capacity and thermochemical redox cycle behavior of Ba and Sr series perovskites for solar energy storage, Solar Energy, 134, 494–502. DOI: 10.1016/j.solener.2016.05.031
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