Hydration characteristics of lithium, zinc, potassium and sodium salts

Abstract

A new inter-seasonal process for storing solar energy is based on absorption of water by a saline solution [1]. For this purpose, many salt hydrates proposed for thermal energy storage application have been studied. In a previous study [2, 3], it was proved that crystallization of the solution inside a storage tank was necessary to increase the storage density, and thus reduce the storage tank size and cost. Crystallization is possible due to a little circulation of fluid in the tank [4] and it must be reversible and controlled. Enhancement of the storage capacity can be the increase of the mass fraction of absorbent in the concentrated solution. Another criterion taken into account in this process is the appearance (size and form) of the crystals which can impact on the design of the storage tank. As a consequence, the salt needs to be in an anhydrous or at least monohydrate form at the crystallization temperature (278 K). The first salt chosen for the system was lithium bromide. It presents interesting thermal properties. However, LiBr crystallizes in dihydrated form at the temperature of the storage tank surrounding (5-10°C) with a water content of 29 wt% of water [2-5]. In view of replacement of LiBr, it was selected six other salts according to their literature data on their hydrated forms at low temperature, their solubility in water and their low price. These salts are lithium chloride, zinc bromide, potassium hydroxide, potassium formate, sodium hydroxide and sodium thiocyanate. Lithium chloride is a well-known desiccant salt principally used in absorption system for cooling and dehumidification [6]. Because of their good thermal properties, potassium formate and sodium hydroxide were previously studied respectively as a secondary refrigerant in an aqueous solution [7] and as the sorbent in a long term heat storage process. Flamensbeck [8] associated potassium hydroxide with sodium hydroxide to replace lithium bromide in absorption chiller. Eventually, sodium thiocyanate was generally combined with ammonia to create the couple absorbent/absorbate as a refrigerant in absorption cooling system [9]. In this work, hydration characteristics, enthalpies and transition temperature, of these seven salts, were determined using a thermogravimetry analysis (TG) and differential scanning analysis (DSC). Results of these experiments were compared to those of lithium bromide.