Abstract
In this paper, thermal energy storage is utilized to store heat from solar energy and also use waste heat to counterbalance the demand and supply of energy. A laboratory multistage closed sorption thermal energy storage (STES) unit is designed and tested to study the performance of a cascade sorption system. The experimental unit consists of three sorbent reactors, an ammonia cycle, and an oil cycle. Composite sorbents namely, MnCl₂, CaCl₂, and Nh₄Cl are used with ammonia as sorbate. Expanded natural graphite (ENG) is mixed with sorbents to increase the thermal conductivity and improve the mass transfer performance of sorbents. Further, a computational fluid dynamic model using COMSOL Multiphysics is built to study the performance of triple, double, and single closed STES systems. For the triple experimental STES system, a charging temperature up to 155 °C is used. At Nh3 pressure of 7.5 bar, the discharging temperatures are found to be 132.9 °C, 99.1 °C, and 60.3 °C for MnCl₂-ENG , CaCl₂-ENG , and Nh₄Cl-ENG composite sorbents, respectively, while a total energy storage density (ESD) of 3474.28 kJ/kgsalt has been achieved. For a double experimental STES system, a charging temperature up to 105 °C is used. Discharging temperatures are found to be 95.3 °C and 60.1 °C for CaCl₂-ENG , and Nh₄Cl-ENG composite sorbents, respectively, and a total ESD of 2471.94 kJ/kgsalt is achieved at Nh3 pressure of 7 bar. A parametric study is carried out to study the effect of different parameters on the performance of STES system at different arrangements. Increasing the inlet oil temperature during discharge results in a reduction of total ESD. For the specified design condition, the optimum oil mass flow rate is found to be 0.04 kg/s to achieve the highest ESD.
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