Experimental characterization of the LiCl/vermiculite composite for sorption heat storage applications

Highlights

• Experimental characterization of sorption composite for thermal energy storage applications.

• Evaluation of sorption enthalpy, cycling stability and sorption kinetic.

• Results validation by means of a lab-scale prototype operating under seasonal storage conditions.

Abstract

The present paper reports about the experimental characterization of a recently developed composite sorbent of water, LiCl/vermiculite, for thermal energy storage applications. The sorption ability as well as the thermal storage capacity (TSC) of the material itself were tested in a dedicated TG/DSC apparatus, under two relevant boundary conditions, namely, seasonal (SS) and daily (DS) storage applications. The dynamic behavior of the composite sorbent was tested by means of a G-LTJ apparatus in flat-plate adsorber configuration, under both SS and DS working conditions. Finally, preliminary tests on a lab-scale TES configuration were performed and reported. The main outcomes confirm that the composite is promising for TES applications, reaching the TSC up to 2.15 kJ/g under SS conditions. The stability of the composite was proven for 14 consecutive sorption/desorption cycles under conditions similar to those at real SS and DS cycles. The kinetic adsorption tests confirmed a slowdown of the sorption dynamics when passing from 1.7–2.0 mm to 2.36–2.80 mm of the grain size. Furthermore, the adsorption kinetic under SS mode is faster than of DS one. The preliminary testing in the lab-scale TES at SS cycle allowed getting TSC up to 1.25 kJ/g with a specific power up to 2.1 kW/kg.

Introduction

Nowadays heating and cooling demands are accounted for about 50% of the primary energy consumption. Only 18% of this energy is produced from renewable sources (solar, geothermal etc.) and 75% – from fossil fuels (European Commission, 2016). The increasing greenhouse gas emission related to the extensive exploitation of the fossil fuels, is recognized as the first cause of the temperature rise on the Earth and the global environment pollution. Furthermore, industrial activity is a source of immense amount of waste heat, which rejected to the environment by means of hot exhaust gases, cooling media, etc. (Miró et al., 2016). In order to limit these negative factors, the utilization of renewable sources, like solar thermal (Al-Alili et al., 2014) (Döll et al., 2014), as well as the increasing in energy efficiency of processes through waste heat recovery, are strongly promoted during recent years (Scapino et al., 2017). However, the broader utilization of the renewable and waste heat sources is hindered by temporary or geographical mismatch between the production of and demand for the energy they generate. To overcome this barrier, the development of advanced thermal energy storage (TES) systems is mandatory. The TES can be divided into several groups in accordance with the storage mode, namely as sensible (Navarro et al., 2012), latent (Kant et al., 2017) (Veerakumar and Sreekumar, 2016) and chemical reaction (Aydin et al., 2015) (Li et al., 2014). In thermochemical TES the heat is stored as the heat of the broken chemical bonds in reversible chemical reaction that provides some important benefits, namely large thermal storage capacity (TSC) and minor heat losses during the storage period (Cot-Gores et al., 2012). The adsorption TES, belonging to the wider class of thermochemical TES, could represent a promising solution, especially for low temperature energy sources (Frazzica and Freni, 2017). However, contrary to the sensible and latent TES, being in an advanced stage of development, and already applied at large scale in solar thermal plant applications (Kousksou et al., 2014), (Zhang et al., 2016a), the sorption TES is still being under research level with several lab-scale prototypes (Palomba et al., 2017). The researches related to sorption TES are focused substantially on the development of new low cost sorption materials, with enhanced TSC and high cycling stability (Cabeza et al., 2017). The composite sorbents, so-called CSPMs (i.e. Composites “Salt in Porous Matrix”), which contain an inorganic salt inside pores of a matrix, firstly suggested for TES in Levitskij et al. (1996), are considered promising TES materials due to large TSC and tunable sorption behavior (Aydin et al., 2015), (Gordeeva and Aristov, 2012). The inorganic salt, being the main sorbing component, reacts with water vapor and forms crystalline hydrates or/and aqueous salt solution inside pores. The porous matrix is the structural support for the salt; it promotes faster mass- and heat-transfer in the sorbent bed, and accommodates the salt swelling during the hydration. To date, a large number of CSPMs based on various salts, namely CaCl₂ (Jänchen et al., 2004), (Levitskij et al., 1996), (Casey et al., 2014), (Permyakova et al., 2017), Al₂(SO₄)₃ (Jabbari-Hichri et al., 2016), MgSO₄ (Hongois et al., 2011), MgCl₂ (Whiting et al., 2014), SrBr₂ (Zhang et al., 2016c), (Courbon et al., 2017), LiBr (Gordeeva et al., 1998) and CaNO₃ (Simonova et al., 2009), (Henninger et al., 2017), have been developed as water sorbents for TES. An innovative composite sorbent LiCl/vermiculite has been recently suggested for seasonal and daily TES cycles (Zhang et al., 2016b), (Grekova et al., 2017). LiCl successively reacts with water according to the reactions

$$\text{LiCl}+{\mathrm{H}}_2\mathrm{O}=\text{LiCl}·{\mathrm{H}}_2\mathrm{O}$$

$$\text{LiCl}·{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}=\text{LiCl}·2{\mathrm{H}}_2\mathrm{O}$$

When temperature decreases, the hydrate LiCl⋅2H₂O deliquesces forming a LiCl aqueous solution inside the vermiculite pores, which further absorbs water vapor (Grekova et al., 2017). The expanded vermiculite is a cheap natural mineral with extremely large pore volume of 2.4–2.8 cm³/g. This allows a large amount of the salt to be encapsulated and promotes large sorption ability of the composite. The study of water adsorption equilibrium on the composite LiCl (59)/vermiculite with 59 wt% of the salt demonstrated that it adsorbs large amount of water up to 0.85 g/g and can be regenerated at low charging temperature 75–85 °C. That provides promising avenues for its application in TES systems. This paper addresses the characterization of TES ability of the LiCl/vermiculite composite, both at material and prototype levels, as well as the water sorption dynamics, performed under conditions of the TES cycle.