Solar Energy on Demand: A Review on High Temperature Thermochemical Heat Storage Systems and Materials

Abstract

Among renewable energies, wind and solar are inherently intermittent and therefore both require efficient energy storage systems to facilitate a round-the-clock electricity production at global scale. In this context, concentrated solar power (CSP) stands out among other sustainable technologies because it offers the interesting possibility of storing energy collected from the sun as heat, by sensible, latent or thermochemical means. Accordingly, continuous electricity generation in the power block is possible even during off-sun periods, providing CSP plants with a remarkable dispatchability. Sensible heat storage has been already incorporated to commercial CSP plants. However, due to its potentially higher energy storage density, thermochemical heat storage (TCS) systems emerge as an attractive alternative for the design of next generation power plants, which are expected to operate at higher temperatures. Through these systems, thermal energy is used to drive endothermic chemical reactions, which can subsequently release the stored energy when needed through a reversible exothermic step. This review analyzes the status 2 of this prominent energy storage technology, its major challenges and future perspectives, covering in detail the numerous strategies proposed for the improvement of materials and thermochemical reactors. Thermodynamic calculations allow selecting high energy density systems, but experimental findings indicate that sufficiently rapid kinetics and long-term stability trough continuous cycles of chemical transformation are also necessary for practical implementation. In addition, selecting easy to handle materials with reduced cost and limited toxicity is crucial for large-scale deployment of this technology. In this work, the possible utilization of materials as diverse as metal hydrides, hydroxides or carbonates for thermochemical storage is discussed. Furthermore, especial attention is paid to the development of redox metal oxides, such as Co3O4/CoO, Mn2O3/Mn3O4 and perovskites of different compositions, as an auspicious new class of TCS materials due to the advantage of working with atmospheric air as reactant, avoiding the need of gas storage tanks. Current knowledge about the structural, morphological and chemical modifications of these solids, either caused during redox transformations, or induced wittingly as a way to improve their properties, is revised in detail. In addition, the design of new reactors concepts proposed for the most efficient use of TCS in concentrated solar facilities is also critically considered. Finally, strategies for the harmonic integration of these units in functioning solar power plants, as well as the economic aspects are also briefly assessed.

Advanced Materials for High‐Temperature Thermal Transport

“…Mater. 2020, 30,1904815 Phonon-Phonon Scattering: The most common phononphonon scattering is the three-phonon processes, including the Normal processes (or N processes) and Umklapp processes (or U processes), as shown in Figure 1c. In the N process, two phonons collide and merge into a new third phonon, or one phonon splits into two new phonons, in which the energy and momentum are both conserved, which means the N process does not directly contribute to thermal resistance.…”

“…Mater. 2020, 30,1904815 to protect the turbine blades and enable higher working temperature. [16a] The state-of-art TBC material is YSZ, among which the 7-8 mol% YSZ (called 7YSZ or 8YSZ) is most widely used, with κ of 2-3 W m −1 K −1 from room temperature to over 1000 °C in a dense form.…”

“…Mater. 2020, 30,1904815 [80] Copyright 2012, Materials Research Society. (c) Reproduced with permission.…”

“…Mater. 2020, 30,1904815 in the κ solid by increasing the porosity. Figure 5b shows that the decreasing κ solid with lower density.…”

“…The state‐of‐the‐art large‐scale TES is based on molten salts, mostly solar salts (60 wt% NaNO 3 –40 wt% KNO 3 mixture) that have the stable liquid phase within the temperature window of 228 to 650 °C . Development of future TES systems with higher hot‐side temperature requires the solutions to a host of material issues, including suitable TES media (e.g., eutectic halide molten salts, liquid metals, and thermochemical reactions), as well as chemically stable and thermally insulating containment materials that interact with them …”