Applications of carbon materials in photovoltaic solar cells

Carbon-based photovoltaic cells (PVCs) have attracted a great deal of interest for both scientific fundamentals and potential applications. In this paper, applications of various carbon materials in PVCs, especially in silicon-based solar cells, organic solar cells and dye-sensitized solar cells, are reviewed. The roles carbon materials played in the PVCs are discussed. Further research on solar cells comprised solely of carbon is prospected.     

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.     

Lagrangian characterization of multi-phase turbulent flow in a solar reactor for particle deposition prediction

Solar cracking of methane is a promising technology for emission free hydrogen production. One of the major problems affecting methane cracking solar reactors' performance is the carbon particle deposition on the window, walls, and at the exit. In present study, a Lagrangian particle dispersion model has been implemented for predicting the particle deposition on the window of a seeded solar thermal reactor. A three-dimensional Computational Fluid Dynamics (CFD) analysis using Discrete Phase Model (DPM) has been done for qualitative validation of the experimental observations. In order to evaluate the turbulent quantities in the solar reactor; RNG k–? model has been applied. Species transport has been solved by taking the gas for window screening as different from that used in the main flow. In addition, this paper presents a thorough parametric study predicting the particle deposition on reactor window for various flow configurations and flow conditions, which can be summarized as; (1) when the inlet flow angle is smaller, higher tangential velocities or swirl strength is obtained, (2) higher tangential velocities help in maintaining a stronger swirl, which keeps the screening flow close to the reactor window, (3) by increasing the main flow and the screening flow rates, the particle deposition on window is reduced, (4) when a lower density fluid is used as window screening gas, the particle deposition is reduced because the Taylor instabilities are avoided. The CFD work and the findings presented in this paper would be used as a guide in designing a solar reactor or improving the configuration of existing reactor.     

Metallic and carbonaceous –based catalysts performance in the solar catalytic decomposition of methane for hydrogen and carbon production

Solar catalytic decomposition of methane (SCDM) was investigated in a solar furnace facility with different catalysts. The aim of this exploratory study was to investigate the potential of the catalytic methane decomposition approach providing the reaction heat via solar energy at different experimental conditions. All experiments conducted pointed out to the simultaneous production of a gas phase composed only by hydrogen and un-reacted methane with a solid product deposited into the catalyst particles varying upon the catalysts used: nanostructured carbons either in form of carbon nanofibers (CNF) or multi-walled carbon nanotubes (MWCNT) were obtained with the metallic catalyst whereas amorphous carbon was produced using a carbonaceous catalyst. The use of catalysts in the solar assisted methane decomposition present some advantages as compared to the high temperature non-catalytic solar methane decomposition route, mainly derived from the use of lower temperatures (600–950 °C): SCDM yields higher reaction rates, provides an enhancement in process efficiency, avoids the formation of other hydrocarbons (100% selectivity to H₂) and increases the quality of the carbonaceous product obtained, when compared to the non-catalytic route.     

Silicon solar cells with antireflection diamond-like carbon and silicon carbide films

Optical properties of diamond-like carbon and silicon carbide (SiC) films in dependence on deposition conditions were investigated. It was established that the films having refractive index from 1.6 to 2.3 may be obtained. The film optical bandgap and hardness may be changed from 1.5 to 4 eV and from 1 to 20 GPa, correspondingly. The films were deposited onto the front side of silicon solar cells (SCs). It has been shown that deposition of single- or two-layer diamond-like carbon antireflection (AR) coatings enables the SCs efficiency to be improved 1.35–1.5 times. The improvement is connected with decreasing of reflection losses and passivation of recombination active centers. SiC AR coatings improve the solar cell efficiency up to 1.3 times.     

A solar energy storage and power generation system based on supercritical carbon dioxide

This paper proposes a new type of solar energy based power generation system using supercritical carbon dioxide and heat storage. The power generation cycle uses supercritical carbon dioxide as the working fluid and integrates the supercritical carbon dioxide cycle with an efficient high-temperature heat storage. The analysis shows that the new power generation system has significantly higher solar energy conversion efficiency in comparison to the conventional water-based (steam) system. At the same time, the heat storage not only overcomes the intermittent nature of solar energy but also improves the overall system efficiency. The study further reveals that the high temperatures and high pressures are favorable for solar energy storage and power generation. Moreover the expander and the heat storage/regenerator are found to be the key components that determine the overall system performance.     

First-principles identification of the origin for higher activity of surface doped carbon nanohorn: Impact on hydrogen storage

Presence of curvature is considered as a tuning parameter to activate the hydrogen storage capability of carbon nanostructures. Here, we explicate the role of ‘intra-curvature’ in a set of single-walled carbon nanohorns (SWCNHs), to adsorb light metal ad-atoms (M) e.g. Li, Na, Ca and subsequently explore the metal-doped systems for hydrogen storage application using density functional theory. The binding strength of ad-atoms on SWCNHs of different curvature is correlated with the π electron occupancy of the corresponding carbon ring. Higher π electron occupancy causes significantly high binding energy of the metal ad-atoms (M), thereby indicating high stability of those M−C bonds for intra-curvature values more than 11⁰, even at a higher temperature. After full hydrogenation, Li-doped SWCNHs are found to contain a maximum of 7.5 wt % of hydrogen. Overall, our results indicate that Li-doped SWCNHs with intra-curvature values higher than 11⁰, is a potential candidate for hydrogen storage.     

All-ceramic solar collector and all-ceramic solar roof

A type of all-ceramic solar collector is introduced. These all-ceramic solar collectors are made from ceramics. The material of absorber coating is V–Ti black ceramic. The solar absorptance of absorber coatings with a reticular formation is in the range of 0.93–0.97, without the attenuation of solar absorptance. The fluid passages are integrated with the absorber plate, which naturally formed in the process of shaping. The integration between fluid passage and absorber plate is good to transfer heat from the absorber plate to the fluid. The thermal efficiency of all-ceramic solar system is more than 50%. The all-ceramic solar system can integrate well with building roof. All-ceramic collector and system are characterized by low cost and long lifetime. Such characteristics reduce the cost of solar energy utilization.     

Particle–gas reacting flow under concentrated solar irradiation

A transient heat transfer model is developed for a reacting flow of CH₄ laden with carbon particles directly exposed to concentrated solar radiation and undergoing thermal decomposition into carbon and hydrogen. The unsteady mass and energy conservation equations, coupling convective heat and mass transfer, radiative heat transfer, and chemical kinetics for a two-phase solid–gas flow, are formulated and solved numerically for both phases by Monte Carlo and finite volume methods using the explicit Euler time integration scheme. Parametric study is performed with respect to the initial particle diameter, volume fraction, gas composition, and velocity. Validation is accomplished by comparing temperatures and reaction extent with those measured experimentally using a particle-flow solar reactor prototype subjected to concentrated solar radiation. Smaller particles and/or high volume fractions increase the optical thickness of the medium, its radiative absorption and extinction coefficients, and lead to higher steady-state temperatures, reaction rates, and consequently, higher extent of chemical conversion.     

Tandem structure of aligned carbon nanotubes on Au and its solar thermal absorption

Parallel, close-arranged carbon nanotubes are promising candidates for sunlight trapping. A tandem structure was made by synthesizing aligned carbon nanotube arrays on an Au film. With the nanotube length greater than 10 μm, the tandem structure shows an ultra-high absorption in both the visible and infrared regions. With shorter length (less than 5 μm), infrared reflectance is enhanced due to the presence of the underlying Au film; however, the visible absorptance also decreases at the same time. Carbon nanotubes themselves have no selectivity of solar absorption. They could be used in solar thermal plants where sunlight is collected with high concentration ratios.     

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.     

CFD analysis on the influence of helical carving in a vortex flow solar reactor

Solar thermal cracking of natural gas is a promising technology, which has attracted researchers in recent years for its potential to lead to the development of CO₂ free hydrogen production process. However, experimental access to the reaction chamber of solar cracking reactors is a challenge due to the high temperature process as the instruments capable of measuring fluid flow cannot survive the medium inside the reactor. However, computational fluid dynamics (CFD) can provide an insight into the flow, where experimental access is limited or not possible. This paper presents a CFD analysis for directly irradiated solar thermochemical reactor to characterize the influence of flow behavior on the heat transfer and solar cracking process. The heat transfer by radiation from carbon particles is considered by providing global absorption and scattering coefficients in the computational domain obtained from Mie code. The flow field is based on RNG k–? model derived using renormalization group theory. This technique accounts for the effect of swirl on turbulence thereby enhancing accuracy for the swirl flows. Validation of the numerical results is carried out by making a comparison with the experimental results. Highlighting the effects of carving on the solar reactor walls, this study presents numerical analyses of solar reactor geometry for two cases; namely, when there is no vortex forming carving in the cavity, and when there is vortex forming helical carving. The results show that carving has significant influence on the flow behavior, however, it has very little effect on the outlet temperature. The numerical results also show that the radiative heat transfer mechanism is the dominant means of heat transfer compared to the effects of conduction and convection.     

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.     

Hydrogen production from thermo-catalytic decomposition of methane using carbon black catalysts in an indirectly-irradiated tubular packed-bed solar reactor

The solar thermo-catalytic decomposition of methane using carbon black catalysts for CO₂-free hydrogen production is studied in a packed-bed reactor. The indirectly-irradiated reactor is based on a cavity receiver and a tube-type absorber in which a given load of particle catalyst is injected during on-sun operation, while enabling multiple refilling for catalyst replacement. Concentrated solar power is used as an external radiative source for supplying the high temperature process heat and for driving the endothermic reaction. The indirect irradiation via the intermediate opaque tubular absorber results in a more uniform heating of the whole reacting bed volume and thus an easier reaction temperature control and determination. Carbon particles are used for enhancing the rate of the heterogeneous decomposition reaction and the coupling of the reactor with a particle injection system is implemented to operate in semi-continuous mode with possibility of catalyst load renewal after deactivation.     

The analysis of renewable energy policies for the Taiwan Penghu island administrative region

Taiwan dependents on thermal power for 70% of its total energy supply. The high consumption of fossil fuel increases the carbon dioxide (CO₂) emissions and consequently causes global warming and climate change. Thus, Taiwan has proposed new regulations and measures such as “The Framework for Sustainable Energy Policy - An Energy Saving and Carbon Reduction Action Plan“and” The Master Plan of Energy Conservation and Carbon Mitigation” for domestic carbon reduction. These regulations indicate that the urgency to promote renewable energy to the public to achieve sizable reduction of CO₂ emissions. The objective of this paper is to develop a cost-benefit evaluation methodology based on system dynamics (SD) modelling for any given administrative region to evaluate renewable energy policies. This research develops specific SD models with causal feedback loops to assess the effectiveness of policies and the corresponding benefits for solar energy carbon reduction. The solar energy applications on Taiwan's largest island, Penghu, are used to demonstrate the proposed methodology. The SD approaches and the evaluation of the results serve as a reference to promote solar energy in the other regions with reduced costs and reliability.     

Effect of seeding on hydrogen and carbon particle production in a 10 MW solar thermal reactor for methane decomposition

Solar methane decomposition reactors are a novel technology for the production of carbon neutral hydrogen; however, the impact of this technology depends greatly on the ability to co-produce carbon black particles of commercial grade in order to offset the cost of hydrogen production and, therefore, the control of the reactor is very important. To this end, the seeding of indirect heating concept reactors using the product particles themselves could be used to control heat transfer inside the reactor. In this work, a previously developed one-dimensional reactor – particle population model was used to simulate the effect of seeding on the hydrogen and carbon particle production rates in the absorber tubes of a 10 MW indirect heating concept solar reactor. It was found that seed particle feed rates less than 10% of the methane-contained carbon feed rate allowed the hydrogen and fresh particle production rates to be doubled while keeping the rate of carbon growth on the tube walls constant. It was also found that similar seed fee rates could be used to maintain the hydrogen and particle production rates constant, given variations in the absorber tube wall temperature within a 100 °C range, for example due to cloud passage. Furthermore, it was found that the size characteristics of the freshly produced particles were not affected at these seed feed rates. Thus, seeding could be an effective means for increasing and controlling the hydrogen and carbon particle production rates in industrial scale indirect heating concept solar methane decomposition reactors, while also reducing carbon growth on the walls of the absorber tubes.     

Design recommendations for solar organic Rankine cycle (ORC)-powered reverse osmosis (RO) desalination

This paper deals with the design recommendations for solar reverse osmosis (RO) desalination based on solar organic Rankine cycles (SORC). This technology can be the most energy-efficient technology for seawater and brackish water desalination within the small to medium power output range (up to 500 kW) of the power cycle if the system is properly designed. However, theoretical studies, design proposals and experimental works are very scarce and only very few solar reverse osmosis systems driven by ORC has been either implemented or analysed in the past. In this paper, those systems are outlined and general design recommendations from previous detailed analysis already publish are given for future RO desalination system to be designed based on SORC. Useful information is given about the selection of the working fluid and boundary conditions of the ORC, operation temperature and configuration of the solar field, suited solar collector and thermal energy storage technology, etc. Recommendations are exemplified with well selected numerical cases based on recommended working fluids and solar cycle configuration with proper values of design point parameters. Recommendations given in this paper could be helpful in future initiatives regarding the research and development of this promising solar desalination technology.     

Reduction of carbon dioxide emissions by solar water heating systems and passive technologies in social housing

Growing global concern regarding climate change motivates technological studies to minimize environmental impacts. In this context, solar water heating (SWH) systems are notably prominent in Brazil, primarily because of the abundance of solar energy in the country. However, SWH designs have not always been perfectly developed. In most projects, the installation option of the solar system only considers the electric power economy aspects and not the particular characteristics of each climatic zone. Thus, the primary objective of this paper is to assess the potential of carbon dioxide reduction with the use of SWH in comparison with electric showers in social housing in several Brazilian climatic zones. The Brazilian government authorities have created public policies to encourage the use of these technologies primarily among the low-income population. The results of this paper indicate that hot climactic regions demonstrate a low reduction of CO₂ emissions with SWH installations. Thus, solar radiation is not useful for water heating in those regions, but it does lead to a large fraction of household cooling loads, implying a demand for electrical energy for air conditioning or requiring the adoption of passive techniques to maintain indoor temperatures below threshold values.     

Thermal performance analysis for a heat receiver using multiple phase change materials

To solve the problems associated with employing the single melt point phase change material in a heat receiver for the NASA 2 kW solar dynamic power system, this paper presents a practically easy to carry-out PCM receiver model composed of three different phase change temperature materials together with the corresponding physical model. A numerical solution is also given by which the maximal temperature for heat transfer, working fluid exit temperature, and liquid PCM fraction of the total heat transfer tube in whole are calculated. Furthermore, the results are compared with those obtained from the single PCM heat receiver. The results show that it is possible to improve the receiver performance and to reduce both the fluctuation of working fluid temperature and the weight of the heat receiver. All results of the calculation can be used to guide the heat receiver design.     

One-dimensional model of solar thermal reactors for the co-production of hydrogen and carbon black from methane decomposition

The solar thermal decomposition of methane is a promising route for the large scale production of hydrogen and carbon black with zero CO₂ emissions, however careful control of the reactor is required to ensure product particles of specific sizes. A one-dimensional model employing a sectional method is developed to simulate the evolution of polydisperse fresh and seed particle populations in an indirectly heated solar reactor. The model accounts for the homogeneous nucleation of fresh particles, the heterogeneous growth of the fresh and seed particles, particle coagulation, and the growth of carbon on the walls of the reactor from heterogeneous reaction and particle deposition. The heat transport mechanisms modelled include wall-gas convection, wall-particle radiation exchange, particle-gas convection and heat release from chemical reaction. The model is validated in terms of methane conversion against a 10 kW experimental solar reactor and used to extract kinetic parameters for the homogeneous and heterogeneous reaction paths. The model shows promise as a quick and simple tool for the design and control of industrial scale solar reactors.