Design, analysis and optimization of novel photo electrochemical hydrogen production systems

Abstract

Hydrogen is a green-energy carrier with a high heat of combustion. If obtained from renewable sources, it could be the ultimate and environmentally benign solution for the future energy requirements. This thesis presents the candidate’s research on the photochemical hydrogen production cycle, motivations and objectives, literature survey, thermodynamic modeling, exergoeconomic modeling, electrochemical modeling, statistical modeling and preliminary results of the photochemical research. The process selected for the present study is known as the “chloralkali process”. The main objective of this research is, in this regard, to develop a hybrid system that produces hydrogen by photo chemically splitting water and neutralizing the by-products (i.e. hydroxyl ions) into a useful industrial process. To be more specific, the objective of this present study is to develop a system that combines photochemical hydrogen production with electrochemical chlorine and sodium hydroxide production in a photo electrochemical chloralkali process. Initially, a series of experiments are performed by using an electric power supply. These initial experiments are followed by photo electrochemical experiments, experiments with salt water, experiments without hole scavenger material and solarium experiment. The results of electrochemical experiments show that the concentration of brine in the anolyte compartment and the concentration of electrolyte in the catholyte compartment do not affect the rate of chlorine and hydrogen production. The applied voltage, reactor temperature, and current density have a significant effect on the rate of hydrogen production. The optimal brine concentration is 225 g/L and the optimal electrolyte concentration is 25g/450ml. The increasing temperature reduces the solubility, thus increasing the rate of hydrogen production. Also, increasing the electrode surface area in contact with the working fluid increases the rate of hydrogen production. During the photo electrochemical experiments, three different process parameters are studied, namely light intensity, catalyst concentration, and applied voltage. Using statistical models and experimental data, correlations for the production rate of products are developed. Energy and exergy efficiencies are calculated to assess the performance. An optimization study for photo electrochemical experiments is performed in order to find the optimal catalyst concentration. Using the optimal catalyst concentration from the photo electrochemical experiment results, experiments with salt water in catholyte are performed. The results show that salt concentration does not have any significant effect on the rate of hydrogen production. During the photo electrochemical experiments, it is observed that applied voltage has a significant effect on the rate of photochemical hydrogen production. This fact is further explored by performing experiments without hole scavenger material. The results show the continuous production of hydrogen. Ultimately, this means that a solid electrode can replace the hole scavenger material. Energy, exergy, and exergoeconomic models for a heliostat based hybrid system are developed. A radiation model is coupled with a thermodynamic model in order to predict the rate of hydrogen, chlorine, and sodium hydroxide production for a given light intensity at a particular time. The parameters of the radiation model are set to simulate two varied weather conditions-namely a clear sky and a turbid sky environmental setting. Toronto is assumed to be a place where photo electrochemical chloralkali plant is located. The result shows that the maximum intensity occurs at noon time with the surface angle of 22° in an environment with a clear sky. The cost of hydrogen is calculated from the exergoeconomic model. The average annual cost for the hydrogen based on this model is calculated to be 0.7$/kg for a clear sky environment and 1.3$/kg for a turbid sky environment. To find the minimum required potential, the electrochemical model is developed. The parametric study of different processing parameters shows that the brine concentration and electrolyte concentration do not have significant effect on required cell potential. Current density, temperature, and distance between electrodes, however, have a significant effect on cell potential. The results of the electrochemical model are consistent with the electrochemical experimental results.