CFD simulation and development of an improved photoelectrochemical reactor for H2 production
Introduction
Nowadays most of the world energy demand is being supplied by the fossil fuels which is easy to use and access. However, apart from depleting sources of the fossil fuels, burning them to produce energy causes devastating effects on our environment. Therefore, development of technologies for more sustainable and cleaner energy sources is inevitable. Even though hydrogen is not an ideal fuel, it is one of the promising options. Hydrogen is an energy carrier that produces water and energy when used in fuel cells. Water can be split into hydrogen and oxygen using a suitable semiconductor when subjected to sunlight [1]. Therefore, splitting water into hydrogen and oxygen provides a feasible solution to store sun's energy. Fujishima and Honda observed that the interaction of light with TiO2 produces electron–hole-pairs [2]. The holes oxidize water at the TiO2 surface to form oxygen while the electrons reduce water to form hydrogen at the counter electrode [3] (see equations (1), (2), (3)).
In recent years many photoelectrochemical researches has been done to develop new materials with increased stability and energy conversion efficiency [4], [5], [6], [7], [8], [9], [10], and little attention have been paid to the development of PEC reactors for hydrogen generation [8], [11], [12]. Even though, there are different types of photocells developed for testing the photoactive materials [8], there is dearth of research to develop more practical and optimized PEC reactors that targets scaled-up hydrogen production photoreactors.
Baniasadi et al. have proposed a hybrid water splitting reactor for H2 production [13]. They improved the energy conversion efficiency by applying an external electric potential. However, they did not consider important design criteria such as: electrolyte flow profile distribution, potential drop problem across the photoelectrods, and light absorbtion. In recent times, some researchers investigated and developed new continuous-type PEC systems for hydrogen production [14], [15], [16]. Carver et al. recently have developed a relatively new PEC reactor with a 0.01 m2 photoanode [11]. The main design criteria that they used to build the PEC reactor were light collection/absorption and improvement of the electrolyte flow distribution. In this work, we improved the photoreactor design considering the main design criteria mentioned in Section simulation results and discussion. Furthermore, the practical problems for constructing larger photoelectrodes is studied and addressed in the proposed PEC reactor. The author's main objectives in this paper was, eradication of limits imposed by potential drop across the sheet resistance of TCFs in photoelectrods, and improvement of PEC reactors for possible industrial uses.
The reset of this paper is organized as follows: 3D modeling of the reactor is explained in Section mathematical modeling, simulation results and discussions are provided in Section simulation results and discussion, and extra information for construction of the designed PEC reactor are given in Section fabrication of PEC reactor. The conclusions are stated in Section conclusions.
Section snippets
Mathematical modeling
A three-dimensional mathematical model is used to simulate the PEC reactor. The governing differential equations are continuity, momentum, Butler–Volmer, and Nernst–Plank equations. These equations are solved simultaneously by COMSOL Multiphisics (4.2) software, which is based on finite element method.
Simulation results and discussion
Both 3D and 2D models are considered and the simulation results are used to design and fabricate the photoreactor.
The parameters used in the simulation are given in Table 1.
The following important criteria for design of a practical PEC reactor were considered to improve the efficiency of PEC reactors.
- 1
Uniform electrolyte flow profile in the photoreactor
- 2
Reduce the potential drop on the anode and cathode surface
- 3
Separation of products to prevent back reaction
- 4
Using concentrated sunlight to increase
Fabrication of PEC reactor
In this paper a new dual chamber PEC reactor for water splitting is developed. The presented PEC reactor consists of two photosystems where two PEC cells are linked together and each cell has half the electrochemical potential for water splitting. An aqueous solution circulates between the two chambers. Poly methyl methacrylate (Plexiglas) was used for fabrication of photoreactor. This material has 160 °C melting point and its density is 1.18 gr cm−3. Plexiglas has very good strength both
Conclusions
In this paper, an improved PEC reactor was developed. COMSOL Multiphysics software was used to simulate the photoreactor. After studying different cases, oval shaped chamber was proposed for PEC reactor because of minimum dead zones and uniform flow profile. In addition, the potential drop problem due to sheet resistance of TCFs was addressed by introducing a golden grid layer on top of TCFs. Simulation results confirms that using a thin layer of golden mesh, will increase the conductivity of
Acknowledgment
The author would like to thank Elite Advanced Research Center (EARC) for their cooperation and support.
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