Screening and experimental evaluation of materials for sorption storage applications for sector coupling

The use of sorption storage for mid to long-term applications is gaining interest, not only for the residential areas but also in combination with district heating and cooling (DHC). Indeed, one of the most interesting fields of application is thermal-electric sector coupling. In this case, it is possible to use sorption storages as decentralized storages at district/substation levels or even at building levels, charging them directly from the main district heating/cooling ring or in combination with heat pumps. This is especially useful in the new generation of DHC, that approaches the concept of a "temperature neutral" network, in which the temperature is kept as close to ambient temperature as possible. At the same time, the new generation of DHC is intended to accommodate a larger share of renewables inside the grids (thermal and electric), which requires suitable storage systems for peak shaving and load shifting. Sorption storage allows answering to such challenges, thanks to its high flexibility (it can be operated for heating and cooling purposes), virtually lossless operation and the possibility of being installed also in residential areas or in buildings (differently from underground storage or aquifer storage). The main challenges that sorption technology has to face for the specific application are: the possibility of using non-toxic materials and refrigerants, that would hinder installation in buildings; the need for high energy density and a charging temperature lower than the state-of-art application of sorption system, which usually intended for charging with solar heat or industrial heat at temperatures higher than 80°C. Starting from this context, a preliminary screening of materials available was carried out and the most performant were selected and acquired: three ionic liquids, namely [EMIM] [Ac], [EMIM] [DEP] and [DMIM] [DMP], one salt hydrate- FeCl2·4H2O and one hygroscopic inorganic salts- K2CO3. Sorption experimental tests were carried out on the materials using the Dynamic gravimetric vapor sorption analyzer (DVS) equipment from Surface Measurement Systems, available at CNR ITAE. It allows a gravimetric measurement of the sorption capacity of solids and liquids. Results from the tests highlighted that the ionic liquids show a higher sorption capacity at lower relative pressures (up to 0.9 kg/kg for a 40°C isotherm for the [DMIM] [DMP] and up to 0.6 kg/kg considering the application in heat pump-assisted operation). The salts showed up to 1.5 kg/kg (K2CO3) and 1.9 kg/kg (FeCl2) of sorption capacity, but the vast majority of adsorption is shifted towards higher pressures (2 to 3 kPa for the K2CO3 isotherm and 2.5 to 3 kPa for the FeCl2 isotherm at 30oC), thus indicating that a higher charging temperature might be needed. Even though the sorption behavior of the ionic liquids is very interesting, their cost does not allow for application in a large scale, as is the case of the mid to long-term storage. On the other side, the salt showed consistent swelling issues during the sorption tests. Nevertheless, to solve such issues, composite materials will be prepared using silica gel as the matrix and salts as fillers. The dry impregnation method will be adopted to prepare the composites. Composites with different salt contents varying in the range of 20-40% will be developed and characterized to select the optimized salt content.