Hydration of Magnesium Carbonate in a Thermal Energy Storage Process and Its Heating Application Design

Abstract

First ideas of applications design using magnesium (hydro) carbonates mixed with silica gel for day/night and seasonal thermal energy storage are presented. The application implies using solar (or another) heat source for heating up the thermal energy storage (dehydration) unit during daytime or summertime, of which energy can be discharged (hydration) during night-time or winter. The applications can be used in small houses or bigger buildings. Experimental data are presented, determining and analysing kinetics and operating temperatures for the applications. In this paper the focus is on the hydration part of the process, which is the more challenging part, considering conversion and kinetics. Various operating temperatures for both the reactor and the water (storage) tank are tested and the favourable temperatures are presented and discussed. Applications both using ground heat for water vapour generation and using water vapour from indoor air are presented. The thermal energy storage system with mixed nesquehonite (NQ) and silica gel (SG) can use both low (25-50%) and high (75%) relative humidity (RH) air for hydration. The hydration at 40% RH gives a thermal storage capacity of 0.32 MJ/kg while 75% RH gives a capacity of 0.68 MJ/kg.

Simulations on Design and System Performance of Building Heating Boosted by Thermal Energy Storage (TES) with Magnesium Hydro Carbonates/Silica Gel

“…However, based on data from a mixture with NQ and SG heated to 65 • C for dehydration, the maximum heat capacity for SG alone equals 0.52 MJ/kg [17,18]. Shown in our earlier studies, the main advantage of including SG in the materials is that it increases the hydration (according to R1) conversion by creating a better transport and contact structure for the water vapour to react with the NQ while formation or presence of liquid water is avoided [8]. The low desorption temperature (compared to e.g., zeolite) for NQ, being preferably at least 60 • C and, most likely, the low price are very important advantages in house or office building heating systems.…”

“…Studies show that a carbon capture and storage by mineralisation (CCSM) process [3] can leach mineral magnesium (in aqueous [4] and non-aqueous [5] solution) and produce NQ abundantly [6] which is then further mixed with silica Energies 2020, 13, 4520 2 of 14 gel [7]. The concept of the simulations in this paper is essentially based on is an exhaust air heat pump (EAHP) heating system with a STES reactor added, resulting in the required increase in the relative humidity (RH) in the reactor, as described in detail below in Section 2 of this paper [8].…”

“…Besides NQ, studies by others have shown that the TES capacity of some materials (e.g., NQ, SG, MgSO 4 and Al 2 (SO 4 ) 3 ) is reduced in conventional open systems using exhaust/outlet air with lower RH [8][9][10]. However, the lower water vapour content as specific humidity (SH, kg moisture/kg dry air, not the same as low RH) mostly affects the more costly zeolite.…”

“…The low desorption temperature (compared to e.g., zeolite) for NQ, being preferably at least 60 • C and, most likely, the low price are very important advantages in house or office building heating systems. Moreover, it should be mentioned that NQ is known to have flame retardant properties as a result of releasing CO 2 and H 2 O at elevated temperatures [8,19]. The lower operating temperatures for a TES process decrease heat losses and enable more types of heat sources such as modern low temperature district heat (30 or 70 • C) or solar collectors during cloudy days and morning/evening sun) [12].…”

“…Moreover, a method for improving the efficiency of an adsorption STES reactor was developed and presented. The process concept was earlier developed and published by the authors [2,8].…”