Hydrogen Production by Thermochemical Water-Splitting

Thermochemical processes utilize thermal energy, either directly or through different chemical reactions, to carry out water splitting for hydrogen generation. The net reaction is 

A portion of the supplied thermal energy is expelled with the products. The overall water splitting process, see Fig. 2.6, in general requires both heat and work input. In Fig. 2.6 qr represents the heat energy supplied at high temperature Tr, qo represents the heat rejected at lower temperature To, and Wi is the useful work input, if any, for the process. The enclosed region contains only water, or water and materials involved in different 

As the water splitting process is cyclic (reversible behavior shown by dotted lines in Fig. 2.6) it has limitations imposed by second law of thermodynamics [60]. Hence the operating temperatures Tr and To are crucial in determining the thermal efficiency of the process. In a water splitting process using both heat and work inputs, the thermal efficiency in general is defined as [60,61] 

Where gr represents the heat input from different sources, Wi the useful work input and denoting the heat to useful work conversion efficiency associated with it. Depending upon the process, work input Wi may be required for different process steps like driving the reaction, reaction-product separation, and mass transfer. AHo is the 

Equation (2.3.2) shows that a reduction in work input Wi is necessary if, realistically, high operating efficiencies are to be obtained. If the work input is zero, i.e. if only thermal energy is supplied, using the first and second laws of thermodynamics it can be shown that [2,4,60,61] 

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In tire hydrogen production by nuclear heat, there is the limitation by thermodynamic law (the Carnot efficiency at the highest), because either the electrolysis or the thermochemical water splitting process has to go through the "heat engine path. 

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