Thermodynamic performance of CaCl₂ absorption heat pump thermal energy storage system with triple storage tanks

Renewable energy source is playing an increasingly important role in district heating based on cogeneration units. Absorption thermal energy storage has the characteristics of high thermal energy storage density and low heat loss in long-term storage. In this paper, an absorption heat pump thermal energy storage system with CaCl₂-water solution as the working fluid is proposed for solving the problem of insufficient wind power accommodations due to coal-fired cogeneration heat-power coupling. A steady-state thermodynamic model of the system is constructed in the Aspen Plus software. Under the considered design conditions, the volume of the tank required for one hour of operation of the system, thermal energy storage density, thermal energy storage ratio, and heating ratio are 464.61 m³, 29.81 kW h/m³, 46.50%, and 53.54%, respectively. The thermal energy storage density is 1.43 times and 1.25 times, and the tank volume is 0.7 times and 0.8 times, of those of a dual tank thermal energy storage system with H₂O and CaCl₂-water solution as the working fluids respectively. The effects of the system parameters on the thermal energy storage performance are simulated to obtain the optimal performance. At the optimum system performance, the tank volume required to operate the system for one hour, thermal energy storage density, heating ratio, and thermal energy storage ratio are found to be 439.56 m³, 37.39 kW h/m³, 44.83%, and 55.17%, respectively. Accordingly, the thermal energy storage performance of this system is also found to be superior to that of the traditional thermal energy storage system, which will be beneficial for reducing the volume of storage tanks in cogeneration units and increasing the thermal storage temperature.