Thermochemical energy storage with CaO-Ca(OH)2

Significance Statement

Thermochemical energy storage by means of the reversible gas solid reaction of calcium hydroxide to calcium oxide and water vapor has a number of advantages. Calcium hydroxide is an abundantly available, cheap, industrial mass product. The enthalpy of the reaction is high leading to significant possible energy storage densities. Moreover the charge and discharge temperature of the reaction can be adapted in a wide range. Therefore, this system can be applied for numerous high temperature processes including storage and reutilization of industrial waste heat or as an alternative storage solution for future concentrated solar power plants.

Many researchers focus on the thermal analysis of small sample masses in order to characterize the chemical reaction. However, experimental data on the operation of the reaction system in larger reactors is scarcely reported.

Matthias Schmidt, Andrea Gutierrez, and Marc Linder at German Aerospace Center were motivated to analyze the reaction system in a lab-scale reactor under operating conditions related to the real process application. Therefore, they developed a reaction bed with minimized heat and mass transfer limitations but enough mass of reactive material to analyze thermal capabilities. With the reactor, they performed a number of charging and discharging experiments at low vapor pressures and investigated the effect of various heating and cooling loads induced by a heat transfer fluid. Their work is published in Applied Energy.

The authors experimentally demonstrated, for the first time, the charging and discharging at at vapor pressures between 1.4kPa and 20kPa. It was shown that the operation of the storage system at these low vapor pressures is possible. These findings do not only improve process integration possibilities, but also increases the overall storageefficiency of the thermochemical system.

The experiments additionally revealed that the operating range of the calcium hydroxide system is partially limited because of the slow effective reaction rate of the storage material at low vapor pressures. For instance for the thermal charging at 10kPa a minimum temperature of 445 °C was determined in order to allow the operation at technically relevant thermal powers. The temperature is 45 K higher than preliminary assumptions based on theoretical values. For the discharge procedure at 8.7kPa, a maximum temperature of 383 °C was reached, 15K below theoretical values. With the experimentally determined charge and discharge temperatures the integration of the thermochemical system into real process applications can now be assed more accurate from a thermodynamic point of view. Even though the performance limitations have to be taken into account the demonstrated operation modes offer a promising potential for a wide range of applications.

Thermochemical energy storage  -Advances in Engineering

About The Author

Dipl.-Ing. Matthias Schmidt studied energy and process engineering at the University of Stuttgart, Germany and the Nanyang Technological University, Singapore. Since 2011 he is scientist at the German Aersopace Center (DLR), Germany in the research area of thermochemical systems. As a principal investigator his research activities focus on the calcium hydroxide reaction system and the development of innovative energy storage technologies.

 Reference

Matthias Schmidt, Andrea Gutierrez, Marc Linder. Thermochemical energy storage with CaO-Ca(OH)2 – Experimental investigation of the thermal capability at low vapor pressures in a lab scale reactor. Applied Energy, volume 188 (2017), pages 672–681.

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