Abstract
Thermochemical hydrogen production is of great interest due to the potential for significantly reducing the dependence on fossil fuels as energy carriers. In a solar plant, the solar receiver is the unit in which solar energy is absorbed by a fluid and/or solid particles and converted into thermal energy. When the solar energy is used to drive a reaction, the receiver is also a reactor. The wide variety of thermochemical processes, and therefore of operating conditions, along with the technical requirements of coupling the receiver with the concentrating system have led to the development of numerous reactor configurations. The scope of this work is to identify general guidelines for the design of solar reactors/receivers. To do so, an overview is initially presented of solar receiver/reactor designs proposed in the literature for different applications. The main challenges of modeling these systems are then outlined. Finally, selected examples are discussed in greater detail to highlight the methodology through which the design of solar reactors can be optimized. It is found that the parameters most commonly employed to describe the performance of such a reactor are (i) energy conversion efficiency, (ii) energy losses associated with process irreversibilities, and (iii) thermo-mechanical stresses. The general choice of reactor design depends mainly on the type of reaction. The optimization procedure can then be carried out by acting on (i) the receiver shape and dimensions, (ii) the mode of reactant feed, and (iii) the particle morphology, in the case of solid reactants.
Highlights
Hydrogen production through solar-driven processes is of great interest due to the potential for significantly reducing the dependence on fossil fuels as energy carriers
The aim of the present review is to provide a survey of criteria that can be adopted to identify the most suitable receiver/reactor configuration for a given application and methodologies proposed to improve the reactor design
It is interesting to note that the authors pointed out that the lower limit for useful radiation, i.e., 60% of Qmax, was quite arbitrary, but the results in terms of reactor design did not change significantly if the limit was placed to 70% or 80% of Qmax, whereas noticeable differences emerged if no limitation was placed on the value of the heat flux
Summary
Hydrogen production through solar-driven processes is of great interest due to the potential for significantly reducing the dependence on fossil fuels as energy carriers. These particles may be either injected along with the methane feed at the top of the reactor, as shown, or produced by the cracking reaction itself In this configuration, solar radiation enters the receiver through a quartz window and is directly absorbed by the carbon particles, which reach a higher temperature than the reactor walls, thereby allowing the use of existing ceramic materials to realize the reactor. As described in [62], water-splitting thermochemical cycles (WSTCs) may be divided into: two-step WSTCs, in which two reactions, one endothermic and the other exothermic, are required to achieve the production of H2 and O2 These processes have the advantage of producing hydrogen and oxygen in two distinct steps, thereby avoiding the requirement of a high temperature gas separation. In the case of significant temperature swings during reactor operation and depending on the material characteristics, it may become important to describe thermal stresses generated in the reactor and analyze the possibility of material failure
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