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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SESPer (Solar Energy Storage PERovskites)

Teaser

Summary

Solar energy, being virtually free and endless, is an attractive renewable source of energy that can be converted into electricity in Concentrating Solar Power (CSP) plants. However, such technology is limited by the intermittency and the diurnal nature of the solar light. So, future CSP plants need to be coupled with an inexpensive and efficient mean of storing the energy produced. In this regards, Thermal Storage (TS) systems can directly store solar heat produced by the CSP plant and reuse it on demand during off-sun operation mode. Such technology offers the notable advantage of being less expensive than battery storage.
Among the existing TS systems, ThermoChemical Storage (TCS) is one of the most promising technology. It is based on the exploitation of the reaction heat interchanged in a reversible chemical reaction:

A + Î”Hr â†” B + C (1)

The thermochemical heat stored is linked to the reaction enthalpy. During the charging step, thermal energy is used to dissociate a chemical A, into products B and C. During the releasing step, the products of the endothermic reaction (B and C) react to form the initial reactant A. When required, this reaction releases heat, which is stored in the chemical bonds in addition to sensible heat.
The TCS system has higher energy density and longer-term storage duration respect to other thermal energy storage technologies. Different kind of substances can be used in a TCS system, but in the last years a growing interest focussed on metal oxide reduction/oxidation (redox) pairs due to their reversibility, higher temperature of operation and the use of air as reagent. However, TCS materials development is still in a laboratory stage and production is far from commercial scale. Hence, a material that satisfies the following requirements of the TCS has still to be identified:
1. high energy density
2. complete reversibility for many charging/discharging cycles
3. fast kinetics to facilitate the energy charging and discharging
4. easily separated and stably stored reaction products
5. non-toxic, non-corrosive, and safe reactants and products
6. large-scale availabilities and inexpensive
The temperature storage range in which a TCS system coupled with a CSP plant will operate depends on the technology used. A maximum temperature of about 550 Â°C is reached for parabolic trough, while for tower technology temperatures >700 Â°C are achieved. However, operating temperature exceeding 1000 C are required for increasing the efficiency of future plants and for coupling TS with Air Bryton turbines. These temperature ranges are very broad to be covered by a single system and, further research efforts are needed to develop materials able to operate in a wider temperature range.
Recently, perovskite oxides have drawn interest as potential candidates for TCS systems. Perovskites are solids with the crystal structure of CaTiO3 and general formula ABO3, where A and B are the two cations of the structure. They exhibit a continuous oxygen release/uptake within a very wide temperature range, through the creation/destruction of oxygen vacancies in the crystal lattice. The working principle of a TCS system based on perovskite consists in the following reaction:

ABO3 (s) â†” ABO3-Î´ (s) + Î´/2 O2 (g) (2)

The reduction, being endothermic, is the heat storage step, while oxidation releases heat when it is required. The amount of reversibly exchangeable oxygen, Î´, is a function of temperature and oxygen partial pressure. One of the most interesting characteristic of such materials is that the cations can be easily replaced by similar elements, without undergoing any phase change. This means that the material can present a wide array of possible behaviors, with the extent of reduction (Î´) varying broadly. Many of the perovskite types until now studied contain rare earth elements, which makes them costly and unavailable for large-scale amounts. The OVERALL OBJECTIVE of this project is to study p

Work performed

Perovskites exhibit a variety of useful properties defined by their composition. According to computational studies, the number of possible compositions are over 5,000. Then, the first task of this project was to identify, through a broad literature survey, promising compositions for thermochemical heat storage application that could satisfy some fundamental requirements, such as large oxygen release, high reduction/oxidation enthalpy, non-toxicity, abundance and low cost. In this way, the expertise of the research group of Northwestern University about synthesis and characterization of perovskites was beneficial to the Researcher. During the last 12 months of this project several perovskites were synthesized and, after a preliminary screening, the better performing compositions have been identified and fully characterized for thermochemical heat storage application. The methodology for the extraction of the thermodynamic data, that is the enthalpy and entropy, was optimized in order to obtain a complete characterization of the sample. The strengths and weaknesses of the investigated compositions were identified in order to overcome the limitation of the materials (such as operating temperature range of stability) through the doping mechanism. The doped perovskites were successfully synthesized and analyzed for the extraction of the thermodynamic data. The experimental results obtained showed that the limitations of the un-doped compositions were successfully overcome through the optimal doping level for both A and B sites. The results achieved so far will be used for meeting the future milestones: (i) scale up of the most promising candidate materials for thermochemical heat storage, (ii) the preparation of monoliths and (ii) testing in a solar receiver simulator for studying the thermochemical heat storage capacity in larger scale. All the milestones reported of the project proposal were achieved. The Supervisors from Northwestern University and Institute of Catalysis and Petrochemistry of CSIC provided guidance to the research tasks during this period.

Final results

Only few studies on perovskites as thermochemical heat storage materials are available in literature, some of which containing expensive and heavy rare earth and/or metal cations that increase costs and decrease the mass-specific energy storage densities. For this reason, here the attention has been focused on more cost-effective and lighter perovskites compositions.
The development of this project is successfully leading to the identification of promising candidate storage materials with high stability and energy density for the development of a multilevel-cascaded TCS system, which aims at bringing the development of this technology to a level closer to the commercial scale for applications in CSP facilities-
This project is totally in line with the current ambitious energy and environmental goals of Horizon 2020 and it can have an impact on future developments of more efficient TCS plants. Due to the practical interest this research has been funded by EU and United States Government through research projects like the â€œSunShotâ€ and â€œStolarfoamâ€.