Feasibility analysis of a novel solid-state H2 storage reactor concept based on thermochemical heat storage: MgH2 and Mg(OH)2 as reference materials

Abstract

This paper discusses the feasibility of a novel adiabatic magnesium hydride (MgH2) reactor concept based on thermochemical heat storage. In such a concept, the heat of reaction released during the absorption of hydrogen is stored by a thermochemical material in order to be reused in a subsequent desorption stage. Magnesium hydroxide (Mg(OH)2) has been selected as the suitable material for integration into the MgH2 storage system due to its thermodynamic properties. An analytical formulation of hydrogen absorption time is used to determine the range of the geometrical characteristics of the two storage media, their properties and their operating conditions. The advantage of the proposed new concept is the possibility to reduce the mass of the heat storage media by a factor of 4 compared to phase change material, improving then the gravimetric system capacity as well as its total cost. The second advantage is an improved flexibility of the operating pressure conditions for MgH2 absorption reaction and Mg(OH)2 dehydration reaction that enables shorter hydrogen absorption times by ensuring larger temperature gradients between the two storage media.

Numerical investigation of H2 absorption in an adiabatic high-temperature metal hydride reactor based on thermochemical heat storage: MgH2 and Mg(OH)2 as reference materials

“…Although the complexity of the MgH2 tank was simplified with the use of PCM to store and recover the heat of reaction, the gravimetric capacity of the system was found to be 0.315 wt.% versus 6 wt.% for the MgH2 + 5 wt.% ENG (expanded natural graphite) pellets incorporated into the storage tank, due to the weight of the PCM [13]. In a recent publication [17], the feasibility of an adiabatic MgH2 reactor based on thermochemical heat storage has been discussed; and preliminary analytical results have shown that the combination of MgH2 with magnesium hydroxide (Mg(OH)2) can lead to a competitive hydrogen absorption time compared to the MgH2-PCM reactor, while reducing the mass of the heat storage media by a factor of 4, using an inexpensive thermochemical heat storage material with a higher energy density compared to the metallic PCM, and operating at more flexible pressure and temperature conditions.…”

“…The two described operating modes are governed by different conditions of temperature and pressure to be imposed to the hydrogen and thermochemical heat storage media as discussed in Ref. [17].…”

“…Despite the similarities between the two systems described in [17][18][19], the concept of the hydrogen storage reactor based on thermochemical heat storage described here utilizes different hydrogen and heat storage materials, and involves different reactions mechanisms. Besides, there has been no research conducted so far to investigate the H2 storage process in such a combination reactor.…”

“…Besides, there has been no research conducted so far to investigate the H2 storage process in such a combination reactor. Therefore, as a continuity of the analytical study presented in [17], the aim of this paper is to predict numerically the dynamic performance of the MgH2-Mg(OH)2 system during the uptake of hydrogen. To this end, a two-dimensional mathematical model describing the kinetics and heat transfer in the two storage media packed in a cylindrical reactor, is first developed.…”

“…To this end, a two-dimensional mathematical model describing the kinetics and heat transfer in the two storage media packed in a cylindrical reactor, is first developed. Then, the simulation results are presented to better understand the thermal interaction of both materials; and the resulting H2 absorption time is compared to the one obtained from the analytical study in order to assess the validity of the assumptions adopted in [17]. Finally, the dependence of the hydrogen storage process on the H2 absorption pressure and the thermal conductivity of the thermochemical heat storage media is presented and discussed.…”