Exergy analysis for the Na-O-H (sodium-oxygen-hydrogen) thermochemical water splitting cycle

Abstract

Thermochemical water splitting cycles are considered an attractive option to produce H2, a substance with many important applications for society such as ammonia production because they do not require fossil fuels. In contrast, they obtain H2 decomposing water by means of cyclic chemical reactions. There are a wide variety of thermochemical ones under research, including the relatively new Na–O–H (sodium–oxygen–hydrogen) cycle proposed in 2012 by a research group. Despite introducing a new potential thermochemical cycle, those previous researchers do not investigate the thermal performance of it. Such task can be done through an exergy analysis. So, the aim of the paper is to evaluate the exergy performance of the Na–O–H cycle and its chemical reactions. This goal is accomplished by implementing in EES (Engineering Equation Solver) software exergy efficiency (ε1) and exergy destroyed (ED_1) balances for the cycle and their reactions considering specific conditions of pressure (p) and temperature (T). This approach allows understanding how these two variables influence the system performance, which could compromise its actual implementation and economic feasibility if it has low thermal efficiency. According to the results: reaction 1 (hydrogen production step) has mean ε1=96% and ED_1 = 8.5 kJ under 100–300 °C at 0–1 bar; reaction 2 (metal separation step) has ED_2 = 280 kJ and ε2=56% at 450 °C considering vacuum condition; hydrolysis step (reaction 3) has average ε3=87% and ED_3 = 18 kJ at 25–200 °C under 0–1 bar. Then, the Na–O–H cycle has overall ε=82% and ED = 306 kJ to produce 1 mol of H2. These values of ε and ED are theoretical and maximized ones. In practical situations, such amounts probably will reduce in function of unavoidable irreversibility present in actual systems such as pressure drop and heat loss that were neglected in the work to facilitate its development. Finally, it concludes the Na–O–H cycle has relatively high exergy efficiency, making it a potential hydrogen production method. However, its practical implementation in the future must overcome some of its drawbacks, by means of more research, like the low thermal efficiency of reaction 2 when compared to reactions 1 and 3 beyond the low pressure needed to perform such step.