Hydrogen production from solar thermal dissociation of natural gas: development of a 10 kW solar chemical reactor prototype

This study addresses the solar thermal decomposition of natural gas for the co-production of hydrogen, as well as Carbon Black as a high-value nano-material, with the bonus of zero CO₂ emissions. The work focused on the development of a medium-scale solar reactor (10 kW) based on the concept of indirect heating. The solar reactor is composed of a cubic cavity receiver (20 cm side), which absorbs concentrated solar irradiation through a quartz window via a 9 cm-diameter aperture. The reacting gas flows inside four graphite tubular reaction zones that are settled vertically inside the cavity. Experimental results were as follows: methane conversion and hydrogen yield of up to 98% and 90%, respectively, were achieved at 1770 K, and acetylene was the most important by-product, with a mole fraction up to about 5%. The effect of the methane mole fraction in the feed gas, the residence time and the temperature on the reaction extent was analyzed. In addition to the experimental section, thermal simulations were carried out. They showed a homogeneous temperature distribution inside the cavity receiver of the reactor and permit to draw up a thermal balance.     

Reformer and membrane modules plant powered by a nuclear reactor or by a solar heated molten salts: Assessment of the design variables and production cost evaluation

The application of hydrogen selective membranes in steam reforming plants may play an important role in converting natural gas or heavy hydrocarbons into hydrogen in a very efficient way. Providing the reaction heat by sources as solar heated molten salts or a fluid heated in a nuclear reactor may further increase the overall energy efficiency of the system and pave the way for producing large amount of hydrogen with minimum environmental impact. The new architecture proposed in this paper consists of a heat exchanger where air is heated up by molten salts or other fluids as helium, a post-combustion chamber, a reforming-membrane system based on three reaction/separation steps, a retentate recirculator, a hydrogen cooler and compressor, and a final PSA.     

A thermochemical reactor design with better thermal management and improved performance for methane/carbon dioxide dry reforming

A solar thermochemical reactor with better thermal management is proposed to improve the performance for dry reforming of methane. Conical cavity is introduced in the thermochemical reactor to adjust incident solar radiation distribution. Preheating area is adopted to recover sensible heat from gas outlet. Multiphysical model is presented for analyzing the overall performance of the reactor under different inlet flow rates. Also, local ideal reaction temperature required for maximizing local hydrogen production is analyzed according to the reaction kinetics. It is shown that better synergy between real temperature distribution and ideal temperature requirement can be achieved in this new reactor. Compared with conventional reactor, the present reactor exhibits the better performance in terms of reactant conversion, energy storage efficiency and hydrogen yield. Particularly, hydrogen yield is increased by 4.31%–17.12% at inlet flow rates between 6 and 12 L min^?1.     

A pilot-scale solar reactor for the production of hydrogen and carbon black from methane splitting

A pilot-scale solar reactor was designed and operated at the 1 MW solar furnace of CNRS for H₂ and carbon black production from methane splitting. This constitutes the final objective of the SOLHYCARB EC project. The reaction of CH₄ dissociation produces H₂ and carbon nanoparticles without CO₂ emissions and with a solar upgrade of 8% of the high heating value of the products. The reactor was composed of 7 tubular reaction zones and of a graphite cavity-type solar receiver behaving as a black-body cavity. Temperature measurements around the cavity showed a homogeneous temperature distribution. The influence of temperature (1608K–1928K) and residence time (37–71 ms) on methane conversion, hydrogen yield, and carbon yield was especially stressed. For 900 g/h of CH₄ injected (50% molar, the rest being argon) at 1800K, this reactor produced 200 g/h H₂ (88% H₂ yield), 330 g/h CB (49% C yield) and 340 g/h C₂H₂ with a thermal efficiency of 15%. C₂H₂ was the most important by-product and its amount decreased by increasing the residence time. A 2D thermal model of the reactor was developed. It showed that the design of the reactor front face could be drastically improved to lower thermal losses. The optimised design could reach 77% of the ideal black-body absorption efficiency (86% at 1800K), i.e. 66%.     

Performance evaluation of a passive solar building in Western Himalayas

Under the Passive Solar Building Programme, more than 100 buildings have been constructed in the high altitude region of the Indian State of Himachal Pradesh. A policy decision has been taken by the State that all government/semi-government buildings are to be designed and constructed as per passive solar housing technology. The evaluation studies of some of these buildings have been carried out by our group. In the present study, the thermal performance of a passive solar bank building at Shimla, has been evaluated. This solar building incorporates a heat-collecting wall and a roof-top solar air heater with an electric heating backup, sunspaces and double-glazed windows. The monitoring of the building shows that the solar passive features in the building results in comfortable living conditions. The study shows that the high cost central electric/gas/wood-fired heating systems can be replaced by a low cost solar heating system with backup heaters. This will result not only in reducing higher installation costs of these systems but also the annual running and maintenance costs. It is shown that the solar passive features save electricity required for space heating and reduce the heat losses in the building by about 35%. The strategy to be followed for the propagation of passive solar technology on large scale in this Himalayan State or in any other cold hilly region is also presented.     

Two novel high-porosity materials as volumetric receivers for concentrated solar radiation

This article reports on two novel porous materials, which have been foreseen as volumetric receivers for concentrated solar radiation: a double-layer silicon carbide foam and a screen-printed porous silicon carbide material. Volumetric receivers are used in the solar tower technology. In this technology ambient air flows through the porous solid, which is heated by concentrated solar radiation. A heat exchanger then transfers the energy to a conventional steam turbine process. The general thermophysical and permeability properties of materials required for this application are reviewed. Experimental set-up and results of pressure loss and laboratory scale tests in concentrated solar radiation are reported. For the foam, efficiency data could be determined from the test results. Finally, a comparison is presented between the efficiency properties of the foam and those of materials used for the same application until now. This comparison shows, that the efficiency of the double-layer foam material is significantly higher. Up to now, porous materials consisting of a parallel channel geometry with thin walls showed disadvantageous permeability properties. By applying a new manufacturing process and modifying the channel geometry, the permeability properties of the printed material could be significantly changed, so that it now meets the requirements for an application as a volumetric receiver.     

Performance of a solar crop dryer under PNG climatic conditions

A small solar crop dryer, consisting of a drying unit, thermal storage and solar collector, was designed for the climatic conditions of Papua New Guinea, and was constructed and tested at the Energy Research site of the University of Papua New Guinea. Detailed experimental studies were carried out for drying of tapioca, as well as the testing of the drying unit with and without thermal storage. Based on economic analysis, it is observed that the annual cost per kilogram of dried tapioca is about K0.27 ≠.     

Methane steam reforming in a novel ceramic microchannel reactor

Microchannel heat exchangers and reactors can deliver very high performance in small packages. Such heat exchangers are typically fabricated from aluminum, copper, stainless steel, and silicon materials. Ceramic microchannel reactors offer some significant advantages over their metallic counterparts, including very-high-temperature operation, corrosion resistance in harsh chemical environments, low cost of materials and manufacturing, and compatibility with ceramic-supported catalysts. This work describes a ceramic microchannel reactor that achieves process intensification by combining heat-exchanger and catalytic-reactor functions to produce syngas. A complete computational fluid dynamics (CFD) model as well as a geometrically simplified hybrid CFD/chemical kinetics model is used in conjunction with experimentation to examine heat transfer, fluid flow, and chemical kinetics within the ceramic microchannel structure. Heat-exchanger effectiveness of up to 88% is experimentally demonstrated. Reactive heat-exchanger performance for methane-steam reforming reaches 100% methane conversion and high selectivity to syngas at a gas hourly space velocities (GHSV) of 15,000 h⁻¹. Model results agree well with experimental data and provide insight into physical processes underway during reactor operation.     

Solar hydrogen production from the thermal splitting of methane in a high temperature solar chemical reactor

This study addresses the single-step thermal decomposition (pyrolysis) of methane without catalysts. The process co-produces hydrogen-rich gas and high-grade carbon black (CB) from concentrated solar energy and methane. It is an unconventional route for potentially cost effective hydrogen production from solar energy without emitting carbon dioxide since solid carbon is sequestered.A high temperature solar chemical reactor has been designed to study the thermal splitting of methane for hydrogen generation. It features a nozzle-type graphite receiver which absorbs the solar power and transfers the heat to the flow of reactant at a temperature that allows dissociation. Theoretical and experimental investigations have been performed to study the performances of the solar reactor. The experimental set-up and effect of operating conditions are described in this paper. In addition, simulation results are presented to interpret the experimental results and to improve the solar reactor concept. The temperature, geometry of the graphite nozzle, gas flow rates, and CH₄ mole fraction have a strong effect on the final chemical conversion of methane. Numerical simulations have shown that a simple tubular receiver is not enough efficient to heat the bulk gas in the central zone, thus limiting the chemical conversion. In that case, the reaction takes place only within a thin region located near the hot graphite wall. The maximum CH₄ conversion (98%) was obtained with an improved nozzle, which allows a more efficient gas heating due to its higher heat exchange area.     

Double-walled reformer tubes using high-temperature thermal storage of molten-salt/MgO composite for solar cavity-type reformer

Composite materials with alkali carbonate and magnesia have been examined for high-temperature thermal storage in solar tubular reformers. The concept of a double-walled reactor tube involves packing a molten-salt/ceramic composite material into the annular region between internal catalyst tube and exterior solar-absorber wall. In this paper, the shape and interior structure of the reactor tube are newly designed for use in solar cavity-type reformers using straight reactor tubes. Na₂CO₃, K₂CO₃, and Li₂CO₃ composite materials with magnesia were tested as thermal storage media for CO₂ reforming of methane during cooling mode of the reactor tube at a laboratory scale. The efficiency of Na₂CO₃/MgO composite with various MgO contents was also estimated. Composite materials of Na₂CO₃ 80–90 wt% and MgO 20–10 wt% were successfully delayed the cooling of the catalyst bed and sustained methane conversion at >90%. A solar cavity-type reformer consisting of multiple straight reactor tubes is expected to enable stable operation of the solar reforming process under fluctuating solar insolation during cloud passage.     

Step-by-step methodology of developing a solar reactor for emission-free generation of hydrogen

This study presents a methodology to develop a solar reactor based on the thermodynamics and kinetics of methane decomposition to produce hydrogen with no emissions. The kinetic parameters were obtained in the literature for two cases; methane laden with carbon particles and methane without carbon particles. Results show that there is significant difference in experimentally obtained and theoretically predicted methane conversion. The paper also presents a parametric study on the effects of temperature, pressure and the influence of inert gas composition, which is fed along with methane, on the thermodynamics of methane decomposition. Results show that there is significant effect of the inert gas presence in the feeding gas mixture on the equilibrium of methane conversion and product gas composition. Results also show that higher conversions are obtained when the carbon particles laden with methane. The step-by-step reactor design methodology for homogenous methane decomposition and the parametric study results presented in this paper can provide a very useful tool in guiding a solar reactor design and optimization of process operating conditions.     

Energy management in solar thermal power plants with double thermal storage system and subdivided solar field

In the paper, two systems for solar thermal power plants (STPPs) are devised for improving the overall performance of the plant. Each one attempts to reduce losses coming from two respective sources. The systems are simulated and compared to a reference STPP.     

The passive solar heated school in wallasey. III model studies of the thermal response of a passive school building

Using information on the dimensions and materials of a room in the Wallasey School, estimates are presented of its response to heat inputs due to the lighting system and the sun. The daily variation due to these influences is calculated using transient and harmonic approaches.     

Evaluation of porous silicon carbide monolithic honeycombs as volumetric receivers/collectors of concentrated solar radiation

Porous monolithic multi-channeled silicon carbide (SiC) honeycombs employed as open volumetric receivers of concentrated solar radiation, were evaluated with respect to their porous structure and thermomechanical properties before and after long-time operation. Proper “tuning” of porosity, pore size distribution and microstructure can provide SiC honeycombs with improved mechanical properties (higher bending and compressive strength) in the “as-manufactured” state. Exposure under solar irradiation was found to affect both their pore structure and their mechanical characteristics. During the first stages of exposure, a re-structuring of the porous structure takes place shifting the mean pore size to higher values and slightly decreasing the total porosity; this re-structuring ceases after some “characteristic” exposure time. After solar exposure the honeycombs become harder and exhibit significantly higher compressive strength. Extension of anticipated lifetime can be achieved by materials with enhanced mechanical properties like silicon-infiltrated (siliconized) SiC.     

Co-production of hydrogen and carbon black from solar thermal methane splitting in a tubular reactor prototype

This study addresses the solar thermal decomposition of natural gas for the co-production of hydrogen and carbon black (CB) as a high-value nano-material with the bonus of zero CO₂ emission. The work focused on the development of a medium-scale solar reactor (10 kW) based on the indirect heating concept. The solar reactor is composed of a cubic cavity receiver (20 cm-side), which absorbs concentrated solar irradiation through a quartz window by a 9 cm-diameter aperture. The reacting gas flows inside four graphite tubular reaction zones that are settled vertically inside the cavity. Experimental results in the temperature range 1740-2070 K are presented: acetylene (C₂H₂) was the most important by-product with a mole fraction of up to about 7%, depending on the gas residence time. C₂H₂ content in the off-gas affects drastically the carbon yield of the process. The effects of temperature and residence time are analyzed. A preliminary process study concerning a 55 MW solar chemical plant is proposed on the basis of a process flow sheet. Results show that 1.7 t/h of hydrogen and 5 t/h of CB could be produced with an hydrogen cost competitive to conventional steam methane reforming.     

Performance of a direct steam generation solar thermal power plant for electricity production as a function of the solar multiple

This paper describes the influence of the solar multiple on the annual performance of parabolic trough solar thermal power plants with direct steam generation (DSG). The reference system selected is a 50 MW_e DSG power plant, with thermal storage and auxiliary natural gas-fired boiler. It is considered that both systems are necessary for an optimum coupling to the electricity grid. Although thermal storage is an opening issue for DSG technology, it gives an additional degree of freedom for plant performance optimization. Fossil hybridization is also a key element if a reliable electricity production must be guaranteed for a defined time span. Once the yearly parameters of the solar power plant are calculated, the economic analysis is performed, assessing the effect of the solar multiple in the levelized cost of electricity, as well as in the annual natural gas consumption.     

Thermal modelling of a few novel solar collector-cum-storage units for passive heating

Analytical models have been put forward to predict the thermal performance of passive heating systems, which have previously been suggested. The systems consist of a water vessel for heat storage and a structure positioned on its outside wall, which act as a solar collector and a thermal insulation for the storage, respectively. Four different variations of structures have been considered and numerical calculations performed corresponding to the physical parameters of an earlier reported experimental study. The analysis is able to predict the experimental results fairly satisfactorily.     

The passive solar heated school in wallasey. IV. An observational study of the thermal response of a passive school building

An observational study on the Wallasey School has demonstrated its ability to maintain in most conditions of climate an equitable indoor climate both in regard to daily mean temperatures and daily variations, through use of solar gain and heat from the lights, and the appropriate control of ventilation. During occupied periods, air temperatures are usually between 17°C in winter and 23°C in sunny summer periods. The room provides a mainly ‘cold wall’ environment. The observational data and a series of model estimates have been compared. The general level of temperature within the building is known to depend strongly on ventilation rate, but since ventilation rate was not measured, steady-state comparisons as such are not possible. The observed and estimated temperature profiles for air and various surfaces including that of the furnishings during a very sunny period are in broad agreement. Analyses of the transient response of the structure in winter conditions has demonstrated a long response time (several days) describing the response of the enclosure, and a shorter response time of about half a day which describes the rate of settlement of internal temperature differences which may be initially present. Evidence is presented indicating low values for the convective heat transfer coefficient. An autocorrelational technique demonstrates that the thermal ‘memory’ of the classroom is much longer in winter than in summer. The response of the room during occupied and unoccupied periods is broadly similar, but conditions are rather more variable during occupation.     

Numerical investigation of flow and heat transfer in a volumetric solar receiver

Volumetric solar receivers are used in solar power plants to convert concentrated solar radiation into high temperature heat to operate a thermal engine. In general, porous high temperature materials are used for this purpose. Since the pore geometry is important for the efficiency performance of the receiver, current R&D activities focus on the optimization of this quantity. In this study, the influence of slight geometry changes of this component on its temperature distribution and efficiency has been investigated with the objective of an overall improvement. A numerical analysis of the mass and heat transfer through the receiver has been performed. The investigated receiver was an extruded honeycomb structure made out of Silicon Carbide. Additionally, experimental tests have been performed. In these tests, selected receiver samples have been exposed to concentrated radiation. From these tests solar-to-thermal efficiency data have been derived, which could be compared with the calculated data. Two numerical models have been developed. One makes use of the real geometry of the channel (single channel model), the other one considers the receiver to be “porous continuum”, which is described by homogenized properties such as permeability and effective heat conductivity. The experimental parameters such as the average solar heat flux and the mass flow were taken into account in the models as boundary conditions. Various quantities such as the average air outlet temperatures, the temperature distributions and the solar-to-thermal efficiency were used for the comparison. The correspondence between the experimental and numerical results of both numerical models confirms the capability of the approaches for further studies.     

Performance analysis of a solar vapour thermal compression chiller

The influence of the design conditions on the coefficient of performance (COP) of a solar vapour thermal compression chiller was investigated from an operational point of view. Based on a well known model, a number of COP evaluation runs were conducted and the results are presented in graph form as a function of the main design parameters. Through these graphs the influence of the main operating parameters on the performance is quantitatively shown. Finally, the calculated COP is compared to that of a solar absorption system operating under similar conditions.