1
Materials for Thermochemical and Sorption Heat Storage

Kokouvi Edem N'TSOUKPOE

2iE, Ouagadougou, Burkina Faso

1.1. Introduction

It is easy to understand what a material for storage in the form of chemical potential is: we can find examples in our everyday experience. Just take a look at an application that dates back to the Stone Age: the preparation of whitewash using lime obtained from quicklime. What happens when you mix quicklime with water? We can observe that heat is released, which becomes all the more intense when a greater quantity of quicklime is mixed with water. The water may start to boil and give off lime, which has a very base pH level and is in fact corrosive. This is why we are often told to pour the lime powder into the water gradually, and not vice versa. After the reaction of quicklime with water is complete, a paste of slaked lime is obtained. What happens next if we dry this paste by bringing it up to a high temperature? The paste releases water, and we end up with the same quicklime we started with. It is therefore possible to wet it again and obtain a release of heat. In summary, when you have quicklime in a bag at room temperature, you can say that you have heat at hand, since at any time when you want to obtain heat, you can just mix it with water. It does not matter how long the lime is stored for; the main issue is to protect it from moisture and air. In this way, the drying process, during which you apply heat to allow the slaked lime to release water and thus to regenerate the potential for water absorption of the lime, can be considered as a process of recharging the lime with heat. This means that lime is a material that allows heat to be stored.

Quicklime is a powder that is essentially made up of calcium oxide (CaO), while slaked lime is made up of calcium hydroxide (Ca(OH2). The chemical reaction that occurs during the hydration operation is the following:

If the amount of water used is not too great - an amount no greater than what is necessary for it to be hydrated (1 L of water for every ~3.1 kg of pure quicklime), the slaked lime that is obtained is also in the form of a powder. This result is generally obtained by spraying the quicklime with water. The amount of heat released is ?H = -65 kJmol-1 of CaO.

Because of its "thirst for water", lime is sometimes spread onto soils that are too humid in order to reduce their water content, and thus facilitate working on them: it is a hydrophilic material.

Another hydrophilic material that can be found frequently in our daily lives is silica gel (SiO2·H2O). It is made up of small translucent pellets that can be found in small packets, which are placed in shoe boxes, electronic equipment, or in bottles containing medical tablets. By absorbing the surrounding moisture, these pellets protect the products from damages that might be caused by the presence or introduction of moisture during the transporting or storage of the product. The water attraction capacity or water sorption capacity of the silica gel is due to its large specific surface area; in other words, the area of the real surface relative to its mass. This area is equal to 600-800 m2·g-1 (Sun and Meunier 2003), meaning that every 10 g of silica gel, the amount contained in a small spoon, has a surface area equivalent to that of a soccer field (which totals roughly 7,000 m2 for international competitions)! How is this possible? Apart from the size, shape and volume, which have an obvious impact on the specific surface area, irregularities or roughness on the surface can greatly influence the actual surface area offered by a particle, and therefore its specific surface area. To demonstrate this, let us consider the beads from the same material, as shown in Figure 1.1. The sizes of the irregularities have been exaggerated for ease of viewing, since at the macroscopic scale, observers perceive them as identical spherical balls (shown with a dotted outline). Thus, the bead presented in Figure 1.1(b) has an irregular contour, which is longer than the bead shown in Figure 1.1(a), and the bead (b) contains a larger external surface area than the ball (a). This is considered to be external surface roughness. In the case of the bead (c), there are also "internal" surfaces connected to the external surface by "passages" which are known as p???? (poros) in Greek, a name which also evokes words such as "portal", in the sense of a passageway, or "to carry", referring to transportation. Because of this, we use the word pores to refer to cavities, canals, or ducts that are deeper than they are wide (Rouquerol et al. 1994), according to the definition established by the International Union of Pure and Applied Chemistry (IUPAC). Bead (d) contains a certain number of closed pores, which are totally isolated from the others in such a way that no external flow of fluids can access them, and no external gas adsorption can occur there. These closed pores do not contribute to the specific surface area, which focuses on accessible surfaces (Rouquerol et al. 1994).

Figure 1.1. Origin of the specific surface area for a grain of a fixed size of a given material

One last everyday experience - it depends a lot on where you are! - is the ignition of charcoal. In some large African cities, especially in the West African coastal countries, cooking is primarily done using charcoal as fuel. For ignition, the charcoal is lightly soaked with lamp oil, and it needs to be quickly ignited with a match - or another igniter - otherwise, the flame does not catch and the charcoal quickly seems dry. Indeed, the oil used as lighter fluid is quickly "absorbed" by the coal and disappears in a short time. It is then necessary to soak it again when the fuel is on the outer surface, or not too far down in the pores. This experiment shows that while the charcoal is not specially prepared in such a way as to have a high sorption capacity, it is sufficiently porous and absorbs oil quickly. On the other hand, if any water is accidentally poured onto the coal, the moisture remains apparent (the coal appears wet) and it must then be dried until the coal is fully dry. Coal has a high affinity for lamp oil but not for water.

This chapter is devoted to materials for thermochemical and sorption storage, and begins with the presentation of the key concepts and terminology used in the study of storage materials. It continues with a presentation and a critical analysis of the main criteria for the selection of materials before a presentation on the thermodynamic equilibria of these materials. The next section addresses the materials currently used in the processes developed up to this time, with a particular emphasis on those that are used most frequently. The chapter ends with an introduction to the problem of mass and heat transfers in solid-gas systems and an overview of the material classification techniques for thermochemical and sorption storage.

1.2. Definitions and key concepts

Chemical energy storage uses invertible chemical reactions1 to store heat. Depending on the form of the energy supplied to the reaction, we will use the terms thermochemical storage (thermal energy), electrochemical storage (electrical energy) or photochemical storage/photosynthesis (electromagnetic radiation) (Figure 1.2).

Sorption is the retention of gas (the sorbate) by a solid or a liquid (the sorbent). It can involve either absorption or adsorption. The latter, a surface phenomenon, is generically defined as the retention of gas or liquid by a solid or a liquid by surface adhesion. More typically, it is the retention of gas at the surface of a solid or a porous material. We use the terms chemical adsorption (chemisorption) when the phenomenon involves covalent bonds, and physical adsorption (physisorption) when it instead involves Van der Waals forces. Absorption refers to a phenomenon in which a substance (a liquid or gas) penetrates into another (a solid or liquid). In heat storage applications, absorption generally corresponds to the absorption of a vapor by a liquid. Chemical sorption involves more energy than physical sorption. It is characterized by a variance equal to 1. Thus, a single state variable (such as temperature or pressure) is all that is needed to describe a chemisorption equilibrium, while two are required (such as temperature, pressure, or concentration of the sorbate in the sorbent) to describe a physical sorption equilibrium. Section 1.4 will be devoted to the thermodynamic equilibria that are relevant to thermochemical heat storage materials.

Figure 1.2. Classification of chemical storage and sorption storage using illustrations: s = solid; aq = aqueous solution; g = gas. Some phenomena, such as storage through photosynthesis, are not reversible

Several adsorbents are obtained through the agglomeration of microporous crystals to obtain macroporous pellets or granules (Ruthven 1984; Sun and Meunier 1987; Thomas and Crittenden 1998). In this arrangement, the micropores are inside crystals or microparticles, while macropores exist between the crystals, which is to say that they constitute the inter-crystalline space. In the case of the physisorption, depending on whether the physisorbent has one level of porosity...