Characterization and modelling of K2CO3 cycles for thermochemical energy storage applications

Thermochemical heat storage in salt hydrates is a promising concept to bridge the gap between supply and demand of solar thermal energy in the built environment. Using a suitable thermochemical material (TCM), a heat battery can be created to supply low-temperature thermal energy during colder time periods. The principle is based on a reversible hydration-dehydration reaction with water vapour. The TCM can be charged (dehydrated) at a temperature of 120°C by using solar thermal collectors. Conversely, the discharge (hydration) occurs at room temperature using a constant water vapour pressure of 12 mbar. Previous studies have indicated that potassium carbonate (K2CO3) is a good candidate to fulfil the role of TCM in built environment applications. To generate adequate power from a heat battery for hot tap water or space heating, the kinetics of the TCM need to be sufficiently fast. It is hypothesized that the kinetics of the material improve over multiple charge and discharge cycles due to crack formation and volume increase of the grains. The aim of this work is to evaluate the kinetics of 500-700 µm K2CO3 grains using thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), and to quantify the improvement in kinetics over multiple charge and discharge cycles. The kinetics serve as input for an existing nucleation and growth model, simulating the fractional conversion at grain level. In the TGA/DSC experiments, the material was charged and discharged numerous times under a constant water vapour pressure of 12 mbar. The cycling temperature varies from room temperature to a maximum temperature of 120°C. The conversion time of each cycle was monitored. Additionally, using an optical microscope, cycling experiments of K2CO3 were performed in a micro climate chamber with the same conditions as in the TGA/DSC experiments. This allows tracking of the apparent surface area of the grains and the observation of crack formation for each cycle. The existing nucleation and growth model is enhanced by incorporating grain growth and crack formation observed from the optical experiments. Thermal characterization by means of TGA/DSC has indicated that indeed the kinetics of the material improve over multiple cycles. Typical conversion rates are increased by a factor 10 comparing the first and the 12th cycle. Preliminary optical microscope experiments show an increase of the apparent grain surface area of approximately 55%. Additionally, crack formation is observed over multiple hydration and dehydration cycles leading to increased inter-particle porosity, likely adding to the improved kinetics.