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Abstract Thermochemical energy storage (TCS) based on gas-solid reactions constitutes a promising concept to exploit reaction enthalpies for thermal energy storage. This concept facilitates the development of efficient storage solutions with higher energy densities compared to widely investigated sensible and latent thermal energy storage systems. Multivalent metal oxides are capable of undergoing a reversible redox reaction at high temperatures, which is why those storage materials are considered particularly suitable for the operating temperature range of concentrated solar power plants with central receiver systems to increase the total plant efficiency and ensure dispatchability of electricity. In the scope of this work a granular manganese-iron oxide with a Fe/Mn molar ratio of 1:3 has been selected as a potentially suitable storage material, which is non-toxic, abundant and economical. For this reason a preparation route from technical grade raw materials has been chosen. The reversible redox reaction is investigated with respect to the thermodynamic and kinetic characteristics by means of simultaneous thermal analysis in dynamic and isothermal series of measurements. Those revealed that the observed presence of a strong divergence of the reactive temperature range from the actual thermodynamic equilibrium can mainly be attributed to kinetic limitations. Expressions for the effective reaction rates are deduced from experimental data for the reduction and oxidation step, describing the dependence of the reaction rate on temperature and oxygen partial pressure, respectively. The expressions are valid for the temperature ranges in proximity to the equilibrium, which are relevant for the targeted operating conditions of the storage reactor in air. The storage material provides good cycling stability in terms of reversibility and widely maintained reactivity throughout 100 redox cycles in air. Future work comprises material modifications, which are expected to further enhance the mechanical stability of the particles. Overall, the manganese-iron oxide of the chosen composition exhibits a redox reactivity practical for regenerator-type storage systems combining a high temperature TCS zone and a lower temperature non-reactive zone merely used for sensible thermal energy storage.

Abstract High pressure storage of hydrogen is the established storage technology for automotive systems. However, around 15% of the lower heating value of hydrogen is spent to compress hydrogen up to the pressure of 700 bar. Since this energy is available on board but so far wasted, an open air-conditioning system based on metal hydrides is promising to reutilize this compression work. Here we present the experimental demonstration of a first of its kind system. The setup consists of two alternately operating plate reactors, each filled with around 1.5 kg of Hydralloy C2 ( T i 0.98 Z r 0.02 V 0.41 F e 0.09 C r 0.05 M n 1.46 ), coupled to a polymer electrolyte membrane fuel cell. The demonstration at an electrical power of 5 kW shows that the fuel cell operation is not affected by the alternately H2 desorbing reactors (half-cycle duration of 150 s). The system’s average cooling power was 662 W for an ambient temperature of 30 °C and a cooling temperature of 20 °C, reaching of specific cooling power of 227 W k g M H - 1 . Related to the maximum obtainable cooling power of 18.3% of the electrical fuel cell power, the cooling efficiency corresponds to 75%. As an innovative hydrogen pressure transducer the presented system can be transferred to all applications where an unused hydrogen pressure difference is available.

The development of new vehicle concepts, amongst others, aims to address current challenges in traffic and environmental protection. The modular vehicle concept U-Shift, which is being developed by the German Aerospace Center (DLR), promises a high operating efficiency through an on-the-road modular design and the associated possibility of distributed energy storage. The U-Shift consists of a driverless drive unit and various application-specific transport capsules. This work addresses a key research question of how the U-Shift's distributed energy storage systems must be designed to accommodate the most extreme case of a 24-hour operation without requiring idle time for grid charging. For this purpose, a simulation model of the U-Shift was developed in Dymola. The battery configuration was optimized regarding to a generic use case. As a result, a minimum energy content of the batteries of the drive unit and each capsule type was computed to enable a theoretical 24-hour operation.

Three crucial aspects still to be overcome to achieve commercial competitiveness of the solar thermochemical production of hydrogen and carbon monoxide are recuperating the heat from the solid phase, achieving continuous or on-demand production beyond the hours of sunshine, and scaling to commercial plant sizes. To tackle all three aspects, we propose a moving brick receiver–reactor (MBR2) design with a solid–solid heat exchanger. The MBR2 consists of porous bricks that are reversibly mounted on a high temperature transport mechanism, a receiver–reactor where the bricks are reduced by passing through the concentrated solar radiation, a solid–solid heat exchanger under partial vacuum in which the reduced bricks transfer heat to the oxidized bricks, a first storage for the reduced bricks, an oxidation reactor, and a second storage for the oxidized bricks. The bricks may be made of any nonvolatile redox material suitable for a thermochemical two-step (TS) water splitting (WS) or carbon dioxide splitting (CDS) cycle. A first thermodynamic analysis shows that the MBR2 may be able to achieve solar-to-chemical conversion efficiencies of approximately 0.25. Additionally, we identify the desired operating conditions and show that the heat exchanger efficiency has to be higher than the fraction of recombination in order to increase the conversion efficiency.