Thermodynamic Efficiency of Water Vapor/Solid Chemical Sorption Heat Storage for Buildings: Theoretical Limits and Integration Considerations

Abstract

The theoretical limits of water sorbate-based chemical sorption heat storage are investigated in this study. First, a classification of thermochemical heat storage is proposed based on bonding typology. Then, thermodynamics of chemical solid/gas sorption is introduced. The analysis of the reaction enthalpy from the literature indicates that this value is only slightly varying for one mole of water. Using this observation, and with the help of thermodynamic considerations, it is possible to derive conclusions on energy efficiency of closed and open heat storage systems. Whatever the salt, the main results are (1) the energy required for evaporation of water is, at least, 65% of the available energy of reaction, and (2) the maximum theoretical energy efficiency of the system, defined as the ratio of the heat released to the building over the heat provided to the storage, is about 1.8. Considering the data from literature, it is also possible to show that perfectly working prototypes have an energy efficiency about 49%. Based on those results, it is possible to imagine that for the best available material, a perfect thermochemical heat storage system would correspond to 12 times water with a temperature difference about 50 °C. Such solution is definitely competitive, provided that some difficult issues are solved—issues that are discussed throughout this paper.

Thermochemical Energy Storage: an approach to integration pathways

“…Sorption technologies include adsorption, absorption, or both, and entails fixation or capture of gas or vapor by a sorbent. [14] Chemical reactions can be divided into solid-gas and solidliquid processes and involve a change in the molecular configuration of the compounds involved. [15] B. Water-sorption TCES Of the TCES methods, water-sorption is the most promising and feasible system due to its relatively easy operation, high reliability, and safety compared to others such as carbon dioxide or ammonia sorption.…”

A Review of Thermochemical Energy Storage Systems for Power Grid Support

“…A schematic classification of thermochemical heat storage principles is shown in Figure 2. With respect to type of reaction, thermochemical processes are divided into reversible chemical reactions and sorption processes [124]. The fixation or capture of a gas or a vapor by a sorbent is referred as sorption (adsorption and absorption) [125].…”

“…The thermochemical process consists of desorption, storage and sorption [129]. Desorption is the charging process during which heat, supplied to the storage material, is stored in the form of chemical potential by breaking the binders between the sorbent and the sorbate [124]. Storage is the phase in which the sorbent and the sorbate are separated [124].…”

“…Desorption is the charging process during which heat, supplied to the storage material, is stored in the form of chemical potential by breaking the binders between the sorbent and the sorbate [124]. Storage is the phase in which the sorbent and the sorbate are separated [124]. Sorption is the discharging process aimed at recovering heat by contacting the sorbent and the sorbate [130].…”