Use Of Concentrated Solar Energy Research Articles

Processing of AISI M2 high speed steel reinforced with vanadium carbide by solar sintering

This investigation addresses sintering of AISI M2 high speed steel reinforced with vanadium carbide (VC) using concentrated solar energy (CSE). CSE allows significant reduction of processing times due to its enhanced activating effect and high heating rates achieved. The M2 steel powders were reinforced with 3, 6 and 10wt% VC and sintered under a N2–5%H2 atmosphere at two solar installations: a Fresnel lens (FL) and a solar furnace (SF). The microstructures of the solar-sintered samples were studied and compared with other samples sintered using a conventional tubular furnace. Results show that the use of solar installations significantly reduces optimal sintering temperatures in more than 200°C. Furthermore, the use of CSE allows a dramatic reduction of the treatment times from cycles longer than 10h in tubular furnace (TF) to cycles of 15min in solar furnace. Finally, the addition of VC increases hardness in most of the studied cases but in the samples processed in the solar installation the increase is higher achieving 987HV in the sample reinforced with 10wt% of VC without any additional thermal treatment. The microstructural analysis together with X-ray diffraction characterization allows explaining these results.

Development of high speed steel sintered using concentrated solar energy

The steel powders were sintered under N2–H2 atmosphere in a solar furnace and in a Fresnel lens, after compaction of the green parts, using much higher heating and/or cooling rates as compared to conventional tubular furnace. The effects of processing parameters and the use of concentrated solar energy on densification and mechanical properties were analyzed. Experimental results demonstrated the activating effect of concentrated solar energy on the sintering process showing that an optimum densification is achieved at 1150°C on both solar installations in just 90min for the solar furnace and in 30min in the case of Fresnel lens installation compared with an optimum temperature of 1290°C in ∼10h of total cycle in the conventional tubular furnace. Better mechanical properties were obtained using concentrated solar energy with microhardness measurements ranging between 800 and 900HV. Microstructural analyses by scanning and transmission electron microscopy reveal the presence of submicron sized vanadium nitrides and other nanometer sized particles in the samples that could be responsible for the high hardness values obtained.

Solar Gasification of Biomass: Kinetics of Pyrolysis and Steam Gasification in Molten Salt

The use of concentrated solar energy as a heat source for pyrolysis and gasification of biomass is an efficient means for production of hydrogen rich synthesis gas. Utilizing molten alkali carbonate salts as a reaction and heat transfer media promises enhanced stability to solar transients and faster reaction rates. The present study establishes and compares the reaction kinetics of pyrolysis and gasification of cellulose from 1124 K to 1235 K in an electric furnace. Data are presented in an inert environment and in a bath of a ternary eutectic blend of lithium, potassium, and sodium carbonate salts. Arrhenius rate expressions are derived from the data supported by a numerical model of heat and mass transfer. The molten salt increases the rate of pyrolysis by 74% and increases gasification rates by more than an order of magnitude while promoting a product gas composition nearer to thermodynamic equilibrium predictions. These results justify using molten carbonate salts as a combined catalyst and heat transfer media for solar gasification.

Solar Aluminum Production by Vacuum Carbothermal Reduction of Alumina—Thermodynamic and Experimental Analyses

Thermochemical equilibrium calculations indicate the possibility of significantly lowering the onset temperature of aluminum vapor formation via carbothermal reduction of Al2O3 by decreasing the total pressure, enabling its vacuum distillation while bypassing the formation of undesired by-products Al2O, Al4C3, and Al-oxycarbides. Furthermore, the use of concentrated solar energy as the source of high-temperature process heat offers considerable energy savings and reduced concomitant CO2 emissions. When the reducing agent is derived from a biomass source, the solar-driven carbothermal reduction is CO2 neutral. Exploratory experimental runs using a solar reactor were carried out at temperatures in the range 1300 K to 2000 K (1027 °C to 1727 °C) and with total pressures in the range 3.5 to 12 millibar, with reactants Al2O3 and biocharcoal directly exposed to simulated high-flux solar irradiation, yielding up to 19 pct Al by the condensation of product gases, accompanied by the formation of Al4C3 and Al4O4C within the crucible. Based on the measured CO generation, integrated over the duration of the experimental run, the reaction extent reached 55 pct at 2000 K (1727 °C).

Hydrogen production by hydrogen sulfide splitting using concentrated solar energy – Thermodynamics and economic evaluation

Thermodynamic and economic analyses were carried out to evaluate the use of concentrated solar energy for driving the endothermic dissociation reaction H 2S → H 2 + 0.5S 2. Three different schemes were assessed: (1) a pure solar process; (2) a hybrid process, which uses both solar and natural gas combustion as the energy sources of high-temperature process heat; and (3) the Claus process. This study indicates that the pure solar process has the potential of lowering the disposal costs of H 2S vis-à-vis the conventional Claus process while co-producing H 2 without concomitant CO 2 emissions. An economic assessment for a 40 MWth chemical plant using solar tower technology indicates savings of approximately 45% in comparison to the Claus process. Solar H 2 production is estimated at a cost in the range of 0.061–0.086 $/kW h, based on its lower heating value and without credit attributed to H 2S disposal. A sensitivity analysis revealed that the quench efficiency represents the parameter with the highest impact on the economics of the process. A hybrid natural gas/solar plant design able to operate 24 h-a-day is predicted to reduce the H 2 production cost to 0.058 $/kW h at current fuel prices, however, at the expense of increased complexity related with the hybrid reactor design and operation plus the associated CO 2 emissions of 0.42 kg/kW h.

Dynamics and control of solar thermochemical reactors

Abstract A general dynamic model for solar-driven thermochemical processes is formulated based on unsteady mass and energy conservation equations coupled to the reaction kinetics. It is applied to two pertinent high-temperature thermochemical reactors for fuel production that make use of concentrated solar energy as the source of process heat, namely: an indirectly irradiated batch-operated packed bed reactor for the carbothermic reduction of zinc oxide, and a directly irradiated continuously operated particle flow reactor for the steam-gasification of petcoke. Model parameter identification and validation is accomplished by comparing numerically simulated and experimentally measured temperatures and outlet product concentrations. A linear feedback controller was implemented using the LQG/LTR design method. Simulations of the controlled reactor system with real solar irradiation data indicates improved quality and steadiness of product composition throughout transient solar input phases and superior solar-to-chemical energy conversion efficiency.

Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy

Hydrogen, a promising and clean energy carrier, could potentially replace the use of fossil fuels in the transportation sector. Currently, no environmentally attractive, large-scale, low-cost and high-efficiency hydrogen production process is available for commercialization. Solar-driven water-splitting thermochemical cycles may constitute one of the ultimate options for CO 2-free production of hydrogen. The method is environmentally friendly since it uses only water and solar energy. First, the potentially attractive thermochemical cycles must be identified based on a set of criteria. To reach this goal, a database that contains 280 referenced cycles was established. Then, the selection and evaluation of the promising cycles was performed in the temperature range of 900–2000 °C, suitable to the use of concentrated solar energy. About 30 cycles selected for further investigations are presented in this paper. The principles and basis for a thermodynamic evaluation of the cycles are also given.

Multitube Rotary Kiln for the Industrial Solar Production of Lime

We designed and tested a scaleable solar multitube rotary kiln to effect the endothermic calcination reaction CaCO3→CaO+CO2 at above 1300K. The indirect heating 10-kW reactor prototype processes 1-5mm limestone particles, producing high purity lime of any desired reactivity and with a degree of calcination exceeding 98%. The reactor’s efficiency, defined as the enthalpy of the calcination reaction at ambient temperature (3184kJkg−1) divided by the solar energy input, reached 30%–35% for solar flux inputs of about 2000kWm−2 and for quicklime production rates up to 4kgh−1. The use of concentrated solar energy in place of fossil fuels as the source of process heat has the potential of reducing by 20% CO2 emissions in a state-of-the-art lime plant and by 40% in a conventional cement plant.

Economic evaluation of the industrial solar production of lime

Abstract The use of concentrated solar energy in place of fossil fuels for driving the endothermic calcination reaction CaCO3 → CaO + CO2 at above 1300 K has the potential of reducing CO2 emissions by 20% in a state-of-the-art lime plant and up to 40% in a conventional cement plant. An economic assessment for an industrial solar calcination plant with 25 MWth solar input indicates that the cost of solar produced lime ranges between 128 and 157 $/t, about twice the current selling price of conventional lime. The solar production of high purity lime for special sectors in the chemical and pharmaceutical industry might be competitive with conventional fossil fuel based calcination processes at current fuel prices.

Recycling of hazardous solid waste material using high-temperature solar process heat. 2. Reactor design and experimentation.

A novel high-temperature solar chemical reactor is proposed for the thermal recycling of hazardous solid waste material using concentrated solar power. It features two cavities in series, with the inner one functioning as the solar absorber and the outer one functioning as the reaction chamber. The solar reactor can handle thermochemical processes at temperatures above 1,300 K involving multiphases and controlled atmospheres. It further allows for batch or continuous mode of operation and for easy adjustment of the residence time of the reactants to match the kinetics of the reaction. A 10-kW solar reactor prototype was designed and tested for the carbothermic reduction of electric arc furnace dusts (EAFD). The reactor was subjected to mean solar flux intensities of 2,000 kW m(-2) and operated in both batch and continuous mode within the temperature range of 1,120-1,400 K. Extraction of over 90% of the toxic compounds originally contained in the EAFD was achieved while the condensable products of the off-gas contained mainly Zn, Pb, and Cl. The use of concentrated solar energy as the source of process heat offers the possibility of converting hazardous solid waste material into valuable commodities for processes in closed and sustainable material cycles.

Recycling of Hazardous Solid Waste Material Using High-Temperature Solar Process Heat. 1. Thermodynamic Analysis

The thermochemical conversion and recycling of hazardous solid waste materials is investigated using high-temperature solar process heat. Two important sources of wastes contaminated with heavy metal oxides are considered: (1) electric are furnace dust (EAFD) and (2) automobile shredder residue (ASR). The chemical equilibrium composition of these complex materials and the energy required to process them, using carbon, methane, or pyre-coke as reducing agents, are computed for temperatures in the range 300-2000 K. Metals can be extracted from their oxides in reducing atmospheres at above 1300 K for both EAFD and ASR: Zn is obtained in the gas phase, while Fe, Pb, and Cu are obtained in the condensed phase. The thermal energy requirements for converting EAFD at 1500 K are 3008 kJ/ kg and 4143 kJ/kg using C(gr) and CH4 as reducing agents, respectively. For converting ASR at 1500 K, 2455 kJ/kg are required. The solar exergy conversion efficiency, i.e., the efficiency of converting solar energy into the chemical energy of the reaction products (given by the Gibbs free energy change of product oxidation), can be as high as 69% for the EAFD conversion and 87% for the ASR conversion. Major sources of irreversibilities are those associated with the reradiation losses of the solar reactor and the heat rejected during the quenching. The use of concentrated solar energy as the source of process heat avoids emissions of greenhouse gases and other pollutants derived from the combustion of fossil fuels and further offers the possibility of converting waste materials into valuable commodities for processes in closed and sustainable materials cycles.

Economic evaluation of the solar thermalco-production of zinc and synthesis gas

The use of concentrated solar energy for co-producing Zn and synthesis gas from ZnO and natural gas upgrades the calorific value of the initial reactants by 39% and, when compared to the traditional carbothermic reduction of ZnO, has the potential of reducing CO 2 emissions by up to 78%. An economic assessment for a 50,000 mt/yr chemical plant, 30 to 51 MW solar input, indicates that the cost of solar thermal production of zinc ranges between 89–133 $/mt (excluding the cost of ZnO feed and credit for pollution abatement) and, thus, might be competitive with conventional fossil fuel based processes at current fuel prices.

Oil-shale gasification by concentrated sunlight: An open-loop solar chemical heat pipe

Gasification of oil shales followed by catalytic reforming can yield synthesis gas, which is easily transportable and may also be used for producing liquid fuels. We report here on experiments performed at the Weizmann Institute's Solar Central Receiver involving the use of concentrated solar energy as the heat source for the gasification. A number of dual-axis tracking heliostat mirrors provided primary concentration, and compound parabolic concentrators (CPCs) were used for secondary concentration. In addition to being the first experimental study on this particular type of chemical storage of concentrated sunlight, new aspects of this work include demonstrations of the following: 1. (i)significantly higher synthesis gas yields than previously reported; 2. (ii) marked improvements in conversion efficiency in transparent reactors, as opposed to opaque reactors, both as fluidized beds, primarily due to better heat transfer; 3. (iii) a substantial increase in deliverable energy compared to the conventional retorting of oil shales, and considerable reduction in the resulting spent shale.

Structural observations during amorphization of chromium-iron nitride powders by mechanical alloying : T. Fukunaga et al. (Nagoya University, Nagoya, Japan.) J. Japan Soc. Powder and Powder Metallurgy, Vol 38, No 7, 1991, 940–943. (In Japanese.)

Cr-alloyed PM steels attract great interest in industry, especially in the automotive industry, because, due to the mechanical requirements for the parts, the parts are often subjected to hardening thermal treatments after sintering.In the present work, a sinter-hardening process was carried out in a single stage for pieces based on PM Astaloy CrA steel using concentrated solar energy. The influence of the sintering cycle and atmosphere used, as well as the content of the alloy, were studied.Among the main results obtained, it has been demonstrated that alloys based on Astaloy CrA have been successfully sintered using solar energy. Almost complete densities have been achieved, but these densities are not achieved by conventional techniques, and much shorter sintering times and temperatures (∼ 15 min of holding time at 1070 ± 10 °C) are required compared to those used in the usual processing techniques. In addition, the use of concentrated solar energy in metal treatment leads to high heating and cooling speeds that have encouraged the sinter-hardening process through the formation of bainitic and martensitic phases, which are characterized by being harder and more resistant than ferritic and pearlitic phases. Homogeneous microstructures have been obtained without the need to apply subsequent heat treatments such as tempering that it uses in the processed by conventional techniques.This research work is also intended to promote the use of concentrated solar energy in metallurgical processes that require high temperatures because it is a cheap, nonpolluting and renewable technology. Additionally, high heating and cooling speeds that often result in an improvement of the final properties of the pieces can be obtained without the need to use any external cooling medium.

Gasification of oil shales by solar energy

Gasification of oil shales, followed by catalytic reforming, can yield synthesis gas, which is easily transportable and can also be used for producing liquid fuels. We report here on the use of concentrated solar energy as the heat source for the gasification. The experiments were carried out at the Solar Central Receiver of the Weizmann Institute of Science. A number of heliostats provided primary concentration, and secondary concentration was achieved with 2D and 3D CPC's (ideal non-imaging concentrators). Reactor tubes from Inconel and from quartz were used and the shales were fluidized with argon gas. High temperatures were reached very fast and the decomposition of the shales was nearly complete.

Use of concentrated solar energy in the application of slurry coatings

Use of concentrated solar energy in the application of slurry coatings

Design and operation of a solar fired biomass flash pyrolysis reactor

The results of continuing research on the radiant flash pyrolysis of biomass as a source of fluid fuels, industrial feedstocks and chemicals are described in this paper. Windowed free fall reactors have been developed, which act as cavity receivers for the focused radiant energy and provide a means for direct use of the radiation to rapidly pyrolyze the entering biomass. Detailed results of experiments using the 400 kWth DOE Advanced Components Test Facility at Georgia Tech. are presented. These results suggest the use of concentrated solar energy as a selective means for the production of either a hydrocarbon rich synthesis gas or sugar related sirups from biomass by flash pyrolysis.

Production of hydrogen by direct thermal decomposition of water: Preliminary investigations

The present paper describes preliminary results obtained in a theoretical and experimental study of direct thermal decomposition of water for the production of hydrogen. Steam dissociates when contacting a zirconia surface heated up to high temperature with the help of an image furnace or in other high temperature furnaces simulating the future use of concentrated solar energy. Two different experimental devices were tested. In the first, the gaseous mixture resulting from steam decomposition and evolving from the high temperature zone is quenched by rapid turbulent cold jets. The amount of evolved hydrogen increases with the stirring flow rate and with quenching efficiency. The molar fraction of hydrogen decreases as the input water flow rate increases, but by a compensation effect the net production of hydrogen increases to reach almost 11. STP per hour. In the second device, in order to avoid any recombination of the dissociation products in the medium temperature part of the apparatus, one of the gaseous components issued from the steam decomposition, i.e. oxygen, was separated by means of an oxygen semi-permeable membrane. The flow rate of evolved hydrogen appears to depend primarily on the electrical characteristics of the membrane and mainly on the electronic conductivity of stabilized zirconia. Experimental data concerning the hydrogen flow rate collected at the reactor gas outlet are in good agreement with the theoretical values calculated by a reactor simulation.

Transport of solar energy with optical fibres

In order to facilitate the use of concentrated solar energy its transport in optical fibres is studied. Transmission properties of fibres as well as geometrical conditions of the association between fibres and concentrator are investigated. It is shown that modules where one fibre is associated with a small parabolic mirror may supply 2 W with an efficiency greater than 70 per cent, whilst the concentration on the exit end of a 10-m long fibre may exceed 3000. Such a device has been achieved and the experimental results are in good agreement with the preliminary study. Some further developments and intersting applications are indicated.