An overall assessment of hydrogen production by solar water thermolysis

Major schemes in process designs for water thermolysis are described, thermodynamically modelled and rated in view of certain process evaluation parameters. The process in which the product gases are quenched with water and separated at low temperature by solid diffusion methods is found to have the highest overall efficiency. This process is further studied to find the most suitable operating conditions to reach a satisfactory level of hydrogen yield avoiding material problems generally encountered at high temperature levels. Solar self sufficiency is achieved through heat recovery within the process. Hydrogen production rate is increased by 100% upon converting the recovered heat into electricity in a steam turbine and performing water electrolysis. A feasibility analysis is carried out for an industrial size plant module for thermolysis only at a yearly capacity of 172 GJ, and for hybrid operation coupled with unipolar electrolysis at a total capacity of 344 GJ. Experimental results are reported for the study of a reactor module using the solar furnace at the École Polytechnique. The first and second law efficiencies and the cost of hydrogen are determined as 4.11%, 3.39% and $68 GJ⁻¹ for thermolytic operation and as 5.39%, 4.44% and $46 GJ⁻¹ for hybrid operation.