Reversible redox reactions for high temperature thermochemical energy storage

Abstract

Thermochemical energy storage (TCS) constitutes a promising concept to exploit reaction enthalpies for the storage of thermal energy, which offers great potential for the development of more efficient storage solutions with higher energy densities compared to sensible and latent thermal energy storage. Reversible redox reactions of multivalent metal oxides with oxygen allow the use of ambient air simultaneously as heat transfer fluid (HTF) and source of the reactant O2. Based on a literature review regarding decisive TCS properties such as thermodynamic equilibrium, energy storage density, reactivity as well as material availability and a general health & safety rating, manganese-iron oxide with a Fe/Mn molar ratio of 1:3, prepared in granular form using low-cost, technical grade raw materials, has been selected as a suitable reference material for detailed examinations in this thesis. Redox cycles of (Mn0.75Fe0.25)2O3/(Mn0.75Fe0.25)3O4 performed by means of thermal analyses (TG-DSC) in the temperature range between 750 °C and 1060 °C in air showed, that the material exhibits a narrowed thermal hysteresis, a higher reaction enthalpy, enhanced reaction rates, as well as a better long-term cycling stability than the corresponding redox transition Mn2O3/Mn3O4 of pure manganese oxide, which served as underlying base material for comparison. Thereupon the thermodynamic characteristics including the thermal hysteresis behavior of the redox reaction as well as thermophysical material properties of the Mn-Fe oxide were characterized in detail. Effective reaction rate equations for the reduction and oxidation step have been derived, describing the dependence of the reaction rate not only on temperature but for the first time also on the prevalent oxygen partial pressure. A lab-scale test rig with a packed bed storage reactor for about 470 g of Mn-Fe oxide filling (128 kJ chemical storage capacity, energy density of 102 kWh/m3) has been developed and operated with ambient air as HTF and reaction gas carrier using direct contact heat transfer between the two phases. The redox reaction based storage concept could be demonstrated over consecutive storage cycles between 400 °C and 1040 °C. The proceeding redox reactions disclosed the development of distinct temperature profiles with a plateau formation, characteristic for exploiting the heat effect of reversible reactions for TCS. Parametric studies regarding the influence of essential operating parameters revealed the main limitations affecting the reaction progress. At higher HTF flow rates the observed charging and discharging rate (reaction rate of packed bed) was limited by the present rate of heat input and rate of heat dissipation by the HTF. At low HTF flow rates the discharging performance was dominated by a reduced packed bed reaction rate due to a shortened availability of O2 as well as by diminished heat transfer and heat transport capabilities of the HTF. Owing to the high operating temperatures of a redox reaction based TCS reactor, a high proportion of thermal energy is stored as sensible energy, particularly in the case of the comparatively low chemical storage density of manganese-based oxides. This circumstance equally necessitates the utilization of the sensible thermal energy in the overall storage concept, in order to increase the total storage capacity and exploit the energy storage system efficiently.