Experimental testing and heat transfer analysis of a novel iris-driven solar cavity receiver

Fossil fuel depletion and climate change have been the primary foundation for finding alternatives to conventional industrial processes. Concentrated solar technologies have therefore been considered as a viable route. The processes involved in these technologies use concentrated solar energy to sustain the endothermic dissociation reaction or thermochemical cycles inside a solar reactor which leads to the production of solar fuels and commodities. One of these solar processes is solar thermal cracking of natural gas, which has attracted significant research efforts for obtaining hydrogen as an energy carrier through decarbonization of fossil fuels without greenhouse gas emission. Two valuable commodities are provided by this emission-free process, hydrogen and carbon black respectively. Carbon black is an important additive for rubbers, inks, batteries and several polymers. On the other hand, hydrogen is in great demand in the chemical industry and refineries and offers the advantage of being transportable over long distances. Thus, these storable solar fuels are an important part of the transition path towards a hydrogen economy.

While today industrial grade carbon is produced by furnace process which is inherently associated with the emission of significant amounts of CO2, solar thermal decomposition of natural gas offers an appealing alternative free of combustion byproducts. Conventional production of hydrogen is mainly performed through steam reforming of natural gas, as well as partial-oxidation reforming and auto-thermal reforming. These production methods entail environmental effects due to the resulting CO2 emission and the necessity of partial combustion of the feedstock in order to supply the endothermic process heat in case of methane reforming. Therefore, solar thermochemical decomposition of methane is a promising alternative. However, technical challenges hinder the possible commercialization of these solar thermochemical processes. One of the key challenges is the natural fluctuation of solar radiation due to the position of the sun, unavailability during night time and transient availability at various weather conditions. In addition to this transient behavior, the concentrated energy entering the solar cavity receiver should be effectively used by and transferred to the flow medium. Therefore, optimal heat transfer and temperature uniformity inside the cavity are utmost important while preventing local thermal hot spots. Another important challenge, specifically involved in the solar cracking process of methane, is the agglomeration of carbon particles. These particles are either seeded as a catalyst or formed by the decomposition reaction inside the reactor. The particles in the gas-solid flow deposit on the reactor walls and quartz window of directly heated solar reactors. This further leads to clogging at the reactor exit, with all its detrimental consequences.

The objective of this doctoral study is to address and cope with the abovementioned challenges through in-depth numerical and experimental examination of the heat transfer and thermal hydraulics of the flow inside solar reactors. The first-mentioned challenge is addressed by utilization of a variable aperture mechanism which is capable of maintaining constant thermal conditions inside the cavity receiver regardless of the changing direct normal insolation. This technique surpasses other commonly used control methods such as focusing/defocusing heliostats where careful control of heliostat field is utmost important; thermal storage of the captured energy which is highly dependent on the media used; or control of flow rate which adversely affects the flow pattern and disturbs flow dynamics inside the reactor cavity. Unlike the fixed apertures in current solar reactor designs, the variable aperture concept is capable of regulating temperature and efficiency, and thus, achieving stable operating conditions.

A thorough computational study on the fluid dynamics and thermal hydraulics of a solar cavity receiver is performed. Agreement between the numerical CFD results and the experimentally obtained results leads to optimization of the receiver design with careful consideration and investigation of the many factors and aspects of the flow, i.e. heat transfer, reactor geometry, flow pattern, flow configuration, particle seeding parameters etc. Insights into these aspects yield critical design parameters and solutions to deal with the observed thermal hot spots, temperature (non)-uniformity and carbon particle deposition.