Neutrons for Thermochemical Heat Storage System Modelling

Description

According to the European Commission, Thermal Energy Storage is expected to play an important role in increasing the overall efficiency of energy systems. This is particularly true for systems producing waste heat at temperatures below 100˚C, so-called low-grade waste heat, which, according to the recent studies, accounts for more than 45% of the global primary energy consumption [1].
Large quantities of low-grade heat can be stored reversibly in a composite material made of expanded natural graphite (ENG) impregnated with SrCl2, upon exo-/endothermal ab-/desorption of ammonia [2]. In addition to the high energy density of the material, it has enhanced heat and mass transfer properties thanks to the high thermal conductivity of ENG and its porous structure. For the aforementioned reasons, SrCl2-ENG was selected to be used in a thermochemical heat storage (TCS) system investigated in this work.
To achieve higher energy efficiency in the TCS system, its heat reactor design may be optimized using numerical modeling. To build a reliable numerical model of the reactor, a comprehensive knowledge of the processes occurring during the reactor operation and accurate characterization of the active material, at micro- and nanoscales, are required. To acquire such knowledge, we have used neutron imaging (NI) and Small Angle Neutron Scattering (SANS) techniques. The in-situ NI experiment was carried out at the instrument D50 at the neutron source ILL (Grenoble, France), and the in-situ SANS experiments were performed at the instruments vSANS and Larmor at the neutron facilities NIST (USA) and ISIS (UK), respectively.
With the results from the SANS experiments, the composite material nanostructure and its evolution over ammonia cycling was studied. The knowledge of the material structural behavior under ammonia cycling was coupled to its heat and mass transfer properties, i.e. thermal conductivity and permeability, which resulted in acquiring the dependences between the above-mentioned properties and the degree of absorption/desorption. Using NI technique, we studied the progression of ammonia absorption/desorption into/from the salt under different pressure-temperature conditions at each point of the reaction medium. As a result, the plots presenting the dependence of the reaction advancement on time were derived and subsequently compared with the results from the heat reactor model (COMSOL Multiphysics), which was updated with thermal conductivity and permeability extracted from the results of the SANS experiments.
The kinetics curves from the heat reactor model appear to be in a good agreement with the results from the NI experiments, which confirms that SANS and NI are techniques of choice for an accurate characterization of thermochemical materials at nano- and micro-levels, respectively.

[1] C. Forman, I. K. Muritala, R. Pardemann, and B. Meyer, “Estimating the global waste heat potential,” Renew. Sustain. Energy Rev., vol. 57, pp. 1568–1579, 2016.
[2] Y. Yuan, H. Bao, Z. Ma, Y. Lu, A. P. Roskilly, “Investigation of equilibrium and dynamic performance of SrCl2-expanded graphite composite in chemisorption refrigeration system,” Applied Thermal Engineering, vol. 147, pp. 52–60, 2019.

Period	22 Sept 2020 → 25 Sept 2020
Event title	Materials Science and Engineering Conference 2020
Event type	Conference
Degree of Recognition	International