Optimal Design of an Absorbent-Enhanced Ammonia Synthesis Process for Solar Thermochemical Energy Storage

Concentrating solar power systems are crucial for capturing solar energy. However, the intermittent nature of sunlight necessitates effective energy storage solutions. Ammonia-based thermochemical energy storage systems have emerged as a promising option, utilizing solar energy to dissociate ammonia into hydrogen and nitrogen gas. This gaseous mixture is then employed for exothermic ammonia synthesis, releasing energy for a continuous thermal power cycle. This study focuses on the optimal design of a novel ammonia synthesis process, which uses absorption for ammonia separation instead of condensation, for solar thermochemical energy recovery. A comprehensive first-principles model of the system, encompassing ammonia synthesis and absorption, heat exchange, and gas compression and storage, was developed. An optimization problem was formulated considering standard materials and design constraints, and a nested optimization/simulation approach was employed to integrate the transient absorption behavior with steady-state design. The results provide optimal dimensions and operating conditions for all process units, minimizing the total capital cost. Various operating pressures were examined, revealing minimal differences among the optimal results. The proposed absorbent-enhanced ammonia synthesis process can heat supercritical steam from 350 to 720 °C, producing approximately 40.6 MW_t with discharging and exergetic efficiencies of around 85 and 25%, respectively. Given that the storage tank is the most expensive unit, it was replaced with underground storage, resulting in a levelized cost of heat of about 1.6 ¢/kWh_t. The case study findings highlight the potential of utilizing ammonia absorption in an ammonia-based thermochemical energy storage system.