Integrated Mg-Cl hydrogen production process and CaO/CaCO3-CaCl2 thermochemical energy storage phase change system using solar tower system

Abstract

Hydrogen has been considered as a clean and sustainable fuel to meet heat requirements. The Mg-Cl hydrogen production cycle has been introduced as a promising thermochemical low-temperature cycle. In this paper, a solar system is integrated with a Mg-Cl cycle and a phase change CaO/CaCO3-CaCl2 thermochemical energy storage system (TCES). The heat generated by the solar tower is stored by the TCES in the charge mode. In the cases of low solar radiation, the TCES cycle stands in discharge mode and, if necessary, an auxiliary heat exchanger is utilized to provide the heat for the Mg-Cl cycle. The solar system is modeled by EES software and the other two cycles are simulated by Aspen Plus. Sensitivity and exergy analyses were performed for the system and reported along with the annual results of the whole system. The amount of 11.01 MW heat is stored in the charge mode and 9.536 MW is released in the discharge mode and 35.6 mol/s of hydrogen is acquired by the Mg-Cl cycle. The total exergy efficiencies of the TCES system and Mg-Cl cycle are 62.2% and 84.3%. The Mg-Cl cycle efficiency is enhanced due to the heat recovery of the internal streams and the TCES cycle efficiency is elevated due to the phase change process. The most exergy destroyer components of the TCES and Mg-Cl units belong to Carbonation and Electrolysis1 reactors with the value of 663.7 kW and 3747.4 kW, respectively. According to annual analysis, the most exergy destructions are related to the Electrolysis1 reactor, followed by solar system, which comprise 29.73% and 23.4% of total annual exergy destruction, respectively. Moreover, the annual system efficiency, exergy efficiency and solar fraction are 63.74%, 54.76, and 83.59%, respectively.