High-flux Solar Furnace Research Articles

Development of an optical method to align a high-flux solar furnace

Development of an optical method to align a high-flux solar furnace

Experimental results from a high heat flux solar furnace with a molten metal-cooled receiver SOMMER

Liquid metals are promising candidates for highly efficient thermal receivers in concentrating solar power plants due to their excellent thermal conductivity. In the SOMMER (SOlar furnace with a Molten MEtal-cooled Receiver) facility at the Karlsruhe Institute of Technology (KIT), the cooling of a 10-kW thermal receiver by a lead–bismuth eutectic flow has been successfully demonstrated at high heat flux conditions in a solar furnace without any receiver damage. The experimental results show that peak heat fluxes of up to 4MW/m2 can be achieved in the SOMMER facility and efficiently cooled with a liquid metal flow. In this study, the experimentally determined heat flux densities in the focal point of the solar furnace and the power input by the receiver are presented. In addition, the estimation of the thermal losses of the receiver are described and the results are discussed.The test results from three different shutter blinds positions as well as three different inlet temperatures (200°C, 250°C, 300°C) and different mass flows are presented. During the measurement campaign in the summer of 2019 the Direct Normal Irradiance (DNI) values ranged from 670W/m2 to 960W/m2. Under these conditions maximum wall temperatures of 670°C were measured. All in all, the results prove the excellent cooling ability of liquid metals under high heat flux conditions of up to 4MW/m2.

Synthesis of Non-Cubic Nitride Phases of Va-Group Metals (V, Nb, and Ta) from Metal Powders in Stream of NH3 Gas under Concentrated Solar Radiation

Using a high-flux solar furnace, loosely compacted powders of Va-group transition metal (V, Nb, and Ta) were reacted with stream of NH3 gas (uncracked NH3 gas) being heated by concentrated solar beam to a temperature (T) range between 600 and 1000 °C. From V, sub-nitride V2N (γ phase) and hypo-stoichiometric mono-nitride VN possessing fcc (face-centered cubic) crystal lattice structure (δ phase) were synthesized. On the other hand, in the reaction product from Nb and Ta, hexagonal mono-nitride phase with N/M atom ratio close to 1 (ε phase) was detected. The reaction duration was normalized to be 60 min. In a conventional industrial or laboratory electric furnace, the synthesis of mono-nitride phase with high degree of crystallinity that yield sharp XRD peaks for Va-group metal might take a quite long duration even at T exceeding 1000 °C. In contrast, mono–nitride phase MN of Va-group metal was synthesized for a relatively short duration of 60 min at T lower than 1000 °C being co-existed with lower nitride phases.

Sustainable production of self-activated bio-derived carbons through solar pyrolysis for their use in supercapacitors

This work aims to propose a sustainable green process to obtain bio-derived carbons (BDCs) for utilization in supercapacitors. The process consists in carrying out solar pyrolysis to produce BDCs from abundant lignocellulosic wastes, Agave Angustifolia leaves and pruned tomato plant. Concentrated solar radiation from a high flux solar furnace was utilized to reach sample temperatures between 450 and 1564 °C in a spherical reactor. Before pyrolysis, both wastes were characterized by thermogravimetric analysis to semi-quantify cellulose and hemicellulose as well as ash content. XRD was used to determine the ash composition in both wastes, and the effect of solar pyrolysis temperature on the obtained BDCs. Additional structural properties of BDCs were analyzed by SEM, Raman spectroscopy, and physisorption. Elemental analysis and EDAX were used to determine the chemical composition of wastes, and the effect of this on BDCs. Electrochemical properties of BDCs were analyzed by cyclic voltammetry in half cells, and those showing better performance were also tested in supercapacitor cells. Results show that BDCs from tomato plant waste have higher surface areas, with well-developed microporosity, without needing an additional activation process. This is attributed to self-activation during pyrolysis, produced by the high K and Ca content of the tomato plant pruning. Ragone plots indicate that the assembled supercapacitor cells employing the best BDCs from solar pyrolysis have specific energies and power values similar to a commercial carbon designed for supercapacitors. These results indicate that the proposed green procedure is suitable for obtaining BDCs with properties suitable for supercapacitors.

Continuous on-sun solar thermochemical hydrogen production via an isothermal redox cycle

Solar thermochemical hydrogen production from water is a path towards a carbon-free sustainable hydrogen economy. Here, isothermal on-sun hydrogen production is demonstrated using active iron aluminate (hercynite) particles contained in dual fluidized bed reactors. The two fluidized beds were held in a single cavity solar receiver that was heated with a 10 kW high flux solar furnace. During 8 h of on-sun testing, 5.3 L of H2 were generated with an average productivity of 597 µmol H2/g using an intermittent process with optimized redox cycle times. Redox cycling was performed isothermally and continuously with equivalent oxidation and reduction times producing 547 µmol H2/g over two cycles. These results show excellent agreement with the H2 production measured in an 800X scaled-down electrically heated laboratory stagnation flow reactor and surpass on-sun H2 production of current benchmark materials. The effect of environmental variability on the incident solar radiation and hydrogen production is evaluated during on-sun testing. Finally, a discussion of the important factors for scalability of the process to commercial applications is provided, highlighting the importance of robust containment and active materials. This work links current materials research and development with commercial implementation in an on-sun process and demonstrates the viability of solar thermochemical hydrogen production that leverages continuous isothermal redox cycling.

Correction of the concentrated sunlight spot's drift of the IER-UNAM's solar furnace

This paper discusses the methods implemented for the solution of the drift and backlash problems in the heliostat of the High Radiative Flux Solar Furnace recently built at the Renewable Energy Institute (former Center for Energy Research) of the National University of Mexico (IER-UNAM), located at the geographical coordinates 18°50′24″ North latitude and 99°15′00″ West longitude. To solve the observed drift, several algorithms were analyzed for the calculation of the solar vector, and a closed loop through an electronic device (peephole) was implemented which makes corrections to the position of the heliostat. Using the peephole, drift heliostat decreased significantly; 70% in the horizontal direction and 63% in the vertical direction relative to the no use of peephole. The backlash was corrected by means of unbalancing the heliostat.

A High-temperature, High-efficiency Solar Thermoelectric Generator Prototype

Solar thermoelectric generators (STEGs) have the potential to convert solar energy at greater than 15% efficiency. This project investigates the system design, the necessary thermoelectric and optical technologies, and the economic feasibility of the STEG approach. A STEG is a solid-state heat engine that converts sunlight directly into DC electricity through the thermoelectric effect. STEGs consist of three subsystems: the solar absorber, the thermoelectric generator (TEG), and the heat management system (insulation, heat exchanger, vacuum enclosure, etc.). This project will integrate several state-of-the-art technologies to achieve high efficiency, including next- generation materials for TEGs, high-temperature solar-selective absorbers, and thermal cavities. We will test STEGs at NREL's high flux solar furnace (HFSF) and perform analysis of parasitic losses and lifetime analysis to optimize prototype operation. Equally important for this technology is the development of a cost model to determine the economic competitiveness and possible application niches for STEG technologies. We report on first-order economic analysis to identify the most promising pathways for advancing the technology.

Dynamics of a solar-thermal transport-tube reactor

Solar-thermal reactors make use of concentrated solar irradiation to drive endothermic reactions. One example of processes that can be carried out in solar-thermal reactors is the solar-thermal conversion of biomass into synthesis gas (H2, CO and CO2). The operation of these reactors is dependent on available sunlight and is affected by the presence of clouds, which act as disturbances. During a partly cloudy day, operation of solar-thermal reactors is still possible but flow rate adjustment is required in order to maintain consistent operation. Thus, a robust control system that will allow continuous high performance operation of the reactor is required in order to make the process financially viable. A model predictive control system (MPC) is proposed, which uses a model of the process to determine the required control signal. The first step in the development of the control system is the formulation of a dynamic mathematical model that describes process behavior adequately yet can be solved in real time. A simplified dynamic model for a solar-thermal transport-tube reactor has been developed based on unsteady mass and energy balances. The model was solved using MATLAB™ and validated with experimental data. Model validation was carried out at the High Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory (NREL) in Golden, CO, using different solar power level inputs. In this work, a description of the model and a comparison with the experimental results are presented. Simulations of the model were performed for a reflective and an absorbing cavity. The former allows for faster heating during experimental runs, while the latter is more representative of an industrial setting.

Optical design of a high radiative flux solar furnace for Mexico

In the present work, the optical design of a new high radiative flux solar furnace is described. Several optical configurations for the concentrator of the system have been considered. Ray tracing simulations were carried out in order to determine the concentrated radiative flux distributions in the focal zone of the system, for comparing the different proposals. The best configuration was chosen in terms of maximum peak concentration, but also in terms of economical and other practical considerations. It consists of an arrangement of 409 first surface spherical facets with hexagonal shape, mounted on a spherical frame. The individual orientation of the facets is corrected in order to compensate for aberrations. The design considers an intercepted power of 30 kW and a target peak concentration above 10,000 suns. The effect of optical errors was also considered in the simulations.

Rapid High Temperature Solar Thermal Biomass Gasification in a Prototype Cavity Reactor

High temperature biomass gasification has been performed in a prototype concentrated solar reactor. Gasification of biomass at high temperatures has many advantages compared with historical methods of producing fuels. Enhancements in overall conversion, product composition ratios, and tar reduction are achievable at temperatures greater than 1000°C. Furthermore, the utilization of concentrated solar energy to drive these reactions eliminates the need to consume a portion of the product stream for heating and some of the solar energy is stored as chemical energy in the product stream. Experiments to determine the effects of temperature, gas flow rate, and feed type were conducted at the high flux solar furnace at the National Renewable Energy Laboratory, Golden, CO. These experiments were conducted in a reflective cavity multitube prototype reactor. Biomass type was found to be the only significant factor within a 95% confidence interval. Biomass conversion as high as 68% was achieved on sun. Construction and design considerations of the prototype reactor are discussed as well as initial performance results.

Heat and mass transfer analysis of a suspension of reacting particles subjected to concentrated solar radiation – Application to the steam-gasification of carbonaceous materials

A chemical reactor for the steam-gasification of carbonaceous materials (e.g. coal, coke, biomass) using high-temperature solar process heat is modeled by means of a two-phase formulation that couples radiative, convective, and conductive heat transfer to the chemical kinetics for polydisperse suspensions of reacting particles. The governing mass and energy conservation equations are solved by applying advanced Monte–Carlo and finite-volume techniques with smoothing and underrelaxation. Validation is accomplished by comparing the numerically calculated temperatures, product compositions, and chemical conversions with the experimentally measured values obtained from testing a 5 kW solar reactor prototype in a high-flux solar furnace. A unique feature of the reactor concept is that the gas-particle flow is directly exposed to concentrated solar radiation, providing efficient radiative heat transfer to the reaction site for driving the high-temperature highly endothermic process.

Solar-Driven Coal Gasification in a Thermally Irradiated Packed-Bed Reactor

Coal gasification for high-quality synthesis gas production is considered using concentrated solar energy as the source of high-temperature process heat. The solar reactor consists of two cavities separated by a radiant emitter plate, with the upper one serving as the solar absorber and the lower one containing the reacting packed bed that shrinks as the reaction progresses. A 5 kW prototype reactor with an 8 cm depth, 14.3 cm diameter cylindrical bed was fabricated and tested in a high-flux solar furnace, subjected to solar flux concentrations up to 2600 suns and packed-bed temperatures up to 1440 K. The reactor is modeled by formulating the 1D unsteady energy conservation equation that couples conductive−radiative heat transfer with the reaction kinetics and solving it by the finite volume technique for a transient shrinking domain. The overall reaction rate was determined experimentally by thermogravimetry, while the effective thermal conductivity was determined experimentally in a radial heat flow ove...

Hydrogen production by steam-gasification of carbonaceous materials using concentrated solar energy—IV. Reactor experimentation with vacuum residue

The combined pyrolysis and steam-gasification of petroleum vacuum residue was performed using a 5-kW aerosol-flow solar chemical reactor tested in a high-flux solar furnace in the temperature range 1420–1567 K. The feedstock was continuously injected as a liquid spray at 423 K into the reactor's cavity along with a coaxial steam flow at a H 2 O / C molar ratio in the 1.1–7.2 range, and directly exposed to concentrated solar radiation at concentration ratios exceeding 1800 suns. The degree of chemical conversion after a single pass of 1-s residence time attained 50% at 1472 K, producing high quality syngas of composition 68% H 2 , 15% CO, 14% CO 2 , and 2% CH 4 . The process performance indicators, namely, carbon conversion and energy efficiency were generally inferior to those obtained for the solar steam-gasification of petcoke under comparable operating conditions.

A two-phase reactor model for the steam-gasification of carbonaceous materials under concentrated thermal radiation

A chemical reactor for the steam-gasification of carbonaceous materials (e.g., coal or coke) is modeled by means of a two-phase formulation that couples radiative, convective, and conductive heat transfer to the chemical kinetics. A unique feature of the reactor is that the gas-particle flow is directly exposed to concentrated solar radiation, providing efficient radiative transfer to the reaction site for the high-temperature endothermic process. The governing mass, momentum, and energy conservation equations are solved by applying Monte Carlo, two-flux, and finite-volume techniques. Validation is accomplished by comparing the numerically calculated temperatures, product composition, and chemical conversions with the experimentally measured values obtained from testing a 5 kW prototype reactor in a high-flux solar furnace.

Hydrogen production by steam-gasification of petroleum coke using concentrated solar power—III. Reactor experimentation with slurry feeding

We report on the experimental evaluation of a 5 kW solar chemical reactor for the steam-gasification of petcoke, carried out in a high-flux solar furnace. A petcoke–water slurry was continuously injected into a solar cavity-receiver to create a vortex flow directly exposed to concentrated solar radiation. For a nominal reactor temperature of 1500 K , a water–petcoke molar ratio of 4.8, and a residence time of 2.4 s , the maximum degree of petcoke conversion was 87%. Typical syngas composition produced was 62% H 2 , 25% CO, 12% CO 2 , and 1% CH 4 . The solar energy conversion efficiency—defined as the portion of solar energy absorbed as chemical energy and sensible heat—attained 17%. The effect of varying the particle size (range 8.5– 200 μ m ) and slurry stoichiometry (range 2.1–6.3) on the degree of chemical conversion and energy conversion efficiency was examined.

Hydrogen production by steam-gasification of petroleum coke using concentrated solar power—II Reactor design, testing, and modeling

The solar chemical reactor technology for the steam-gasification of petcoke is presented. The reactor features a continuous vortex flow of steam laden with petcoke particles confined to a cavity receiver and directly exposed to concentrated solar radiation. A 5 kW prototype reactor tested in a high-flux solar furnace in the range 1300–1800 K yielded up to 87% petcoke conversion in a single pass of 1 s residence time. The solar-to-chemical energy conversion efficiency attained 9% without accounting for the product's sensible heat, and 20% when the sensible heat is recovered for steam generation and pre-heating. A steady-state process model that couples radiative heat transfer with the reaction kinetics is validated with experimental data and used for optimization and scale-up of the reactor design.

Solar Carbothermal Reduction of ZnO: Shrinking Packed-Bed Reactor Modeling and Experimental Validation

The thermodynamics and kinetics of the carbothermic reduction of ZnO are examined over the temperature range 400−1600 K. Above 1340 K, the equilibrium composition of the stoichiometric chemical system consists of an equimolar gas mixture of Zn (vapor) and CO. Assuming a first-order rate constant for the surface reaction kinetics between ZnO(s) and CO and further applying a shrinking spherical particle model with an unreacted core, the apparent activation energy obtained by linear regression of the thermogravimetric data is EA = 201.5 kJ/mol. A numerical model is formulated for a solar chemical reactor that uses concentrated solar radiation as the energy source of process heat. The model involves solving, by the finite-volume technique, a 1D unsteady-state energy equation that couples heat transfer to the chemical kinetics for a shrinking packed bed exposed to thermal radiation. Validation is accomplished by comparison with experimentally measured temperature profiles and Zn production rates as a function of time, obtained for a 5-kW solar reactor tested in a high-flux solar furnace. Application of the model for a scaled-up reactor predicts a large temperature gradient at the top layer, which is typical of ablation processes where heat transfer through the bed becomes the rate-controlling mechanism. Once the temperature of the top layer exceeds 1200 K, the bed shrinks at an approximately constant speed of 2.8 × 10-5 m/s as the reaction proceeds under a constant radiative flux input.

Solar hydrogen production by thermal decomposition of natural gas using a vortex-flow reactor

The endothermic decomposition of natural gas is experimentally investigated using concentrated solar radiation as the source of high-temperature process heat. The solar chemical reactor features a vortex flow of CH 4 confined to a cavity-receiver and laden with carbon particles that serve simultaneously as radiant absorbers and nucleation sites for the heterogeneous decomposition reaction. A 5 kW reactor prototype was fabricated and tested in a high-flux solar furnace with power flux intensities exceeding 3500 kW/ m 2 . The chemical reactor engineering, experimental setup, and test results are described. Maximum chemical conversion of CH 4 to H 2 and C(gr) was 67% at 1600 K and 1 bar . Carbon formed was of nano-filamentary nature. The proposed solar hybrid chemical process conserves natural gas, reduces CO 2 emissions, and provides a transition path to solar hydrogen.

Solar-Thermal Processing of Methane to Produce Hydrogen and Syngas

A solar-thermal aerosol flow reactor has been constructed, installed, and tested with the High-Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory (NREL). Experiments were successfully carried out for the dissociation of methane to produce hydrogen and carbon black and for the dry reforming of methane with carbon dioxide to form syngas (hydrogen and carbon monoxide). Approximately 90% dissociation of methane was achieved in a 25-mm diameter quartz reaction tube illuminated with a solar flux of 2400 kW/m2 (or suns). The carbon black produced was amorphous and had a particle size of 20 to 40 nm. Approximately 70% conversion was achieved for dry reforming using a solar flux of 2000 kW/m2. The experimental results for both processes are very encouraging and support further work to address the technical issues and to develop the processes.

Solar chemical engineering and solar materials research into the 21st century

“Solar Chemistry and Solar Materials Research” is one important task of the “Solar Energy Association North Rhine–Westphalia, Germany”. Numerous individual projects have been carried out which address the construction and operation of a high-flux solar furnace, solar chemical engineering, and solar materials research. Almost 10 years of research and development have led to significant progress. This paper reviews the scope of work in solar chemistry and summarizes the results. The authors present perspectives for commercialization and address open questions and needs for further research and development.