###### 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 \({}^{\circ}\)C. Such solution is definitely competitive, provided that some difficult issues are solved--issues that are discussed throughout this paper. chemical sorption; heat storage; system efficiency; reaction enthalpy; theoretical limits 2020 10 489 10.3390/app10020489 10.  
limit will be considered. During the discharge, the system must provide the heat required for heating and/or domestic hot water production: these constraints will drive the power and temperature during the release of heat. The two main objectives of heat storage in buildings are 1) to store heat during the summer and release heat during cold days: interseasonal storage or 2) to store heat during electricity off-peak period and release heat during peak load: peak load shaving. Now, let us have a look at the literature dealing with chemical reaction system efficiency. MgCl\({}_{2}\cdot 6\) H\({}_{2}\)O was selected by [10] to be tested in a 17 L open reactor. Under realistic operating condition, the energy storage capacity of the reactor reached about 139 kWh m\({}^{-3}\). The energy efficiency was characterized by the instantaneous electrical _COP_ of the system evaluated via:  
\[COP_{el}=\frac{P_{heating}}{P_{fan}} \tag{1}\]  
where \(P_{heating}\) is the heat released and \(P_{fan}\) the fan electrical energy. A maximum electrical coefficient of performance of 12 was found, but the authors expected to reach up to 30 with by optimizing the heat recovery and decreasing the pressure drop in the storage. Of course this high _COP_ value is a direct consequence of the \"home-made\" definition. For the sake of clarity and generalizability, the present study will only deal with thermodynamic efficiency of the system. A laboratory scale closed chemical heat storage system was developed in [11], capable of holding about 974 g of material. The storage material used was SrBr\({}_{2}\cdot 6\) H\({}_{2}\)O and 13 dehydration and hydration cycles were conducted. The energy storage capacity of the reactor was about 65 kWh. The reactor thermal energy efficiency was 0.77 meaning a global heat loss of 23%. It is worth mentioning that other reactors have been experimentally tested but no data are available concerning the energy efficiency:  
* SrBr\({}_{2}\cdot 6\) H\({}_{2}\)O in an open reactor--[12],
* Na\({}_{2}\)S \(\cdot 5\) H\({}_{2}\)O in a closed reactor--[13],
* SrBr\({}_{2}\cdot 6\) H\({}_{2}\)O in an open reactor--[14; 15],
* CaCl\({}_{2}\cdot 2\) H\({}_{2}\)O in a closed reactor--[16],
* CaCl\({}_{2}\cdot 2\) H\({}_{2}\)O in an open reactor--[17]. The experimental investigation was focused on the reactor alone and the storage capacity and efficiency were based on a perfect system integration. For example, heat required for water vapor generation is never evaluated nor discussed.