Showing papers on "Solar energy published in 2014"

Metal–organic frameworks for artificial photosynthesis and photocatalysis

Abstract: Solar energy is an alternative, sustainable energy source for mankind. Finding a convenient way to convert sunlight energy into chemical energy is a key step towards realizing large-scale solar energy utilization. Owing to their structural regularity and synthetic tunability, metal–organic frameworks (MOFs) provide an interesting platform to hierarchically organize light-harvesting antennae and catalytic centers to achieve solar energy conversion. Such photo-driven catalytic processes not only play a critical role in the solar to chemical energy conversion scheme, but also provide a novel methodology for the synthesis of fine chemicals. In this review, we summarize the fundamental principles of energy transfer and photocatalysis and provide an overview of the latest progress in energy transfer, light-harvesting, photocatalytic proton and CO2 reduction, and water oxidation using MOFs. The applications of MOFs in organic photocatalysis and degradation of model organic pollutants are also discussed.

Solar steam generation by heat localization

Abstract: Currently, steam generation using solar energy is based on heating bulk liquid to high temperatures. This approach requires either costly high optical concentrations leading to heat loss by the hot bulk liquid and heated surfaces or vacuum. New solar receiver concepts such as porous volumetric receivers or nanofluids have been proposed to decrease these losses. Here we report development of an approach and corresponding material structure for solar steam generation while maintaining low optical concentration and keeping the bulk liquid at low temperature with no vacuum. We achieve solar thermal efficiency up to 85% at only 10 kW m(-2). This high performance results from four structure characteristics: absorbing in the solar spectrum, thermally insulating, hydrophilic and interconnected pores. The structure concentrates thermal energy and fluid flow where needed for phase change and minimizes dissipated energy. This new structure provides a novel approach to harvesting solar energy for a broad range of phase-change applications.

Solar-energy conversion and light emission in an atomic monolayer p-n diode

Abstract: The limitations of the bulk semiconductors currently used in electronic devices-rigidity, heavy weight and high costs--have recently shifted the research efforts to two-dimensional atomic crystals such as graphene and atomically thin transition-metal dichalcogenides. These materials have the potential to be produced at low cost and in large areas, while maintaining high material quality. These properties, as well as their flexibility, make two-dimensional atomic crystals attractive for applications such as solar cells or display panels. The basic building blocks of optoelectronic devices are p-n junction diodes, but they have not yet been demonstrated in a two-dimensional material. Here, we report a p-n junction diode based on an electrostatically doped tungsten diselenide (WSe2) monolayer. We present applications as a photovoltaic solar cell, a photodiode and a light-emitting diode, and obtain light-power conversion and electroluminescence efficiencies of ∼ 0.5% and ∼ 0.1%, respectively. Given recent advances in the large-scale production of two-dimensional crystals, we expect them to profoundly impact future developments in solar, lighting and display technologies.

Meso-Substituted Porphyrins for Dye-Sensitized Solar Cells

Abstract: Among the several approaches for harnessing solar energy and converting it into electricity, dye-sensitized solar cells (DSSC) represent one of the most promising methods for future large-scale power production from renewable energy sources. In these cells, the sensitizer is one of the key components harvesting solar radiation and converting it into electric current. The electrochemical, photophysical, and ground and excited state properties of the sensitizer play an important role for charge transfer dynamics at the semiconductor interface. Moreover, for long-term stability and practical applications, electrolytes based on the iodine/triiodine couple also suffer from two other disadvantages: the corrosive effect toward the metal electrodes, and the partial absorption of the visible light by triiodine anions. These issues hence constitute one of the reasons that have encouraged the development of alternative iodine-free redox couples in liquid electrolytes for DSSCs.

Review of software tools for hybrid renewable energy systems

Abstract: Hybrid energy systems are being utilized for supplying electrical energy in urban, rural and remote areas to overcome the intermittence of solar and wind resources. A hybrid renewable energy system incorporates two or more electricity generation options based on renewable energy or fossil fuel unit. The techno-economic analysis of the hybrid system is essential for the efficient utilization of renewable energy resources. Due to multiple generation systems, hybrid system analysis, is quite complex and requires to be analyzed thoroughly. This requires software tools for the design, analysis, optimization, and economic viability of the systems. In this paper, 19 softwares with their main features and current status are presented. The softwares studied are HOMER, Hybrid2, RETScreen, iHOGA, INSEL, TRNSYS, iGRHYSO, HYBRIDS, RAPSIM, SOMES, SOLSTOR, HySim, HybSim, IPSYS, HySys, Dymola/Modelica, ARES, SOLSIM, and HYBRID DESIGNER. The research work related to hybrid systems carried out using these softwares at different locations worldwide is also reviewed. The main objective of the paper is to provide the current status of these softwares to provide basic insight for a researcher to identify and utilize suitable tool for research and development studies of hybrid systems. The capabilities of different softwares are also highlighted. The limitations, availability and areas of further research have also been identified.

Elucidating the charge carrier separation and working mechanism of CH 3 NH 3 PbI 3− x Cl x perovskite solar cells

Abstract: Developments in organic-inorganic lead halide-based perovskite solar cells have been meteoric over the last 2 years, with small-area efficiencies surpassing 15%. We address the fundamental issue of how these cells work by applying a scanning electron microscopy-based technique to cell cross-sections. By mapping the variation in efficiency of charge separation and collection in the cross-sections, we show the presence of two prime high efficiency locations, one at/near the absorber/hole-blocking-layer, and the second at/near the absorber/electron-blocking-layer interfaces, with the former more pronounced. This 'twin-peaks' profile is characteristic of a p-i-n solar cell, with a layer of low-doped, high electronic quality semiconductor, between a p- and an n-layer. If the electron blocker is replaced by a gold contact, only a heterojunction at the absorber/hole-blocking interface remains.

Solar radiation prediction using Artificial Neural Network techniques: A review

Abstract: Solar radiation data plays an important role in solar energy research. These data are not available for location of interest due to absence of a meteorological station. Therefore, the solar radiation has to be predicted accurately for these locations using various solar radiation estimation models. The main objective of this study is to review Artificial Neural Network (ANN) based techniques in order to identify suitable methods available in the literature for solar radiation prediction and to identify research gaps. The study shows that Artificial Neural Network techniques predict solar radiation more accurately in comparison to conventional methods. The prediction accuracy of ANN models is found to be dependent on input parameter combinations, training algorithm and architecture configurations. Further research areas in ANN technique based methodologies are also identified in the present study.

Enhanced photoelectrochemical water-splitting performance of semiconductors by surface passivation layers

Abstract: An important approach for solving the world's sustainable energy challenges is the conversion of solar energy to chemical fuels. Semiconductors can be used to convert/store solar energy to chemical bonds in an energy-dense fuel. Photoelectrochemical (PEC) water-splitting cells, with semiconductor electrodes, use sunlight and water to generate hydrogen. Herein, recent studies on improving the efficiency of semiconductor-based solar water-splitting devices by the introduction of surface passivation layers are reviewed. We show that passivation layers have been used as an effective strategy to improve the charge-separation and transfer processes across semiconductor–liquid interfaces, and thereby increase overall solar energy conversion efficiencies. We also summarize the demonstrated passivation effects brought by these thin layers, which include reducing charge recombination at surface states, increasing the reaction kinetics, and protecting the semiconductor from chemical corrosion. These benefits of passivation layers play a crucial role in achieving highly efficient water-splitting devices in the near future.

Metallic nanostructures for light trapping in energy-harvesting devices

Abstract: Solar energy is abundant and environmentally friendly. Light trapping in solar-energy-harvesting devices or structures is of critical importance. This article reviews light trapping with metallic nanostructures for thin film solar cells and selective solar absorbers. The metallic nanostructures can either be used in reducing material thickness and device cost or in improving light absorbance and thereby improving conversion efficiency. The metallic nanostructures can contribute to light trapping by scattering and increasing the path length of light, by generating strong electromagnetic field in the active layer, or by multiple reflections/absorptions. We have also discussed the adverse effect of metallic nanostructures and how to solve these problems and take full advantage of the light-trapping effect. In recent years, researchers have demonstrated a number of new schemes for enhancing the absorption of light in solar cells. Chuan Fei Guo and colleagues from the University of Houston in the USA and National Center for Nanoscience and Technology of China in Beijing have now reviewed the use of metallic nanostructures for trapping light in photovoltaic devices. In particular, the presence of metallic nanoparticles in a solar cell or a solar absorber can aid light absorption by inducing strong, local field-enhancement effects or coupling to resonant plasmon modes. Such particles can also promote scattering and thus increase path lengths for light within the device. Solar cells that utilize this approach are either more efficient or substantially thinner than those that do not, thus reducing material costs and creating the opportunity for ultrathin, flexible devices.

Hetero-nanostructured suspended photocatalysts for solar-to-fuel conversion

Abstract: Converting solar energy into valuable hydrogen and hydrocarbon fuels through photocatalytic water splitting and CO2 photo-reduction is highly promising in addressing the growing demand for renewable and clean energy resources. Developing efficient photocatalysts for solar-driven H2 production and CO2 reduction is the most essential part in achieving this goal. For the purpose of attaining high photocatalytic efficiency, hetero-nanostructures formed by multiple material components have been demonstrated as an effective strategy. Within this heterostructure, its interface is a critical consideration, whereby it determines the principle of charge transfer across the heterojunctions and consequent surface reactions. This article reviews the recent developments of hetero-nanostructures for photocatalytic H2 production and CO2 reduction based on material compositions that form heterojunctions.

Black silicon: fabrication methods, properties and solar energy applications

Abstract: Black silicon (BSi) represents a very active research area in renewable energy materials. The rise of BSi as a focus of study for its fundamental properties and potentially lucrative practical applications is shown by several recent results ranging from solar cells and light-emitting devices to antibacterial coatings and gas-sensors. In this paper, the common BSi fabrication techniques are first reviewed, including electrochemical HF etching, stain etching, metal-assisted chemical etching, reactive ion etching, laser irradiation and the molten salt Fray-Farthing-Chen-Cambridge (FFC-Cambridge) process. The utilization of BSi as an anti-reflection coating in solar cells is then critically examined and appraised, based upon strategies towards higher efficiency renewable solar energy modules. Methods of incorporating BSi in advanced solar cell architectures and the production of ultra-thin and flexible BSi wafers are also surveyed. Particular attention is given to routes leading to passivated BSi surfaces, which are essential for improving the electrical properties of any devices incorporating BSi, with a special focus on atomic layer deposition of Al2O3. Finally, three potential research directions worth exploring for practical solar cell applications are highlighted, namely, encapsulation effects, the development of micro-nano dual-scale BSi, and the incorporation of BSi into thin solar cells. It is intended that this paper will serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in research currently being undertaken for renewable energy applications.

Tantalum-based semiconductors for solar water splitting

Abstract: Solar energy utilization is one of the most promising solutions for the energy crises Among all the possible means to make use of solar energy, solar water splitting is remarkable since it can accomplish the conversion of solar energy into chemical energy The produced hydrogen is clean and sustainable which could be used in various areas For the past decades, numerous efforts have been put into this research area with many important achievements Improving the overall efficiency and stability of semiconductor photocatalysts are the research focuses for the solar water splitting Tantalum-based semiconductors, including tantalum oxide, tantalate and tantalum (oxy)nitride, are among the most important photocatalysts Tantalum oxide has the band gap energy that is suitable for the overall solar water splitting The more negative conduction band minimum of tantalum oxide provides photogenerated electrons with higher potential for the hydrogen generation reaction Tantalates, with tunable compositions, show high activities owning to their layered perovskite structure (Oxy)nitrides, especially TaON and Ta3N5, have small band gaps to respond to visible-light, whereas they can still realize overall solar water splitting with the proper positions of conduction band minimum and valence band maximum This review describes recent progress regarding the improvement of photocatalytic activities of tantalum-based semiconductors Basic concepts and principles of solar water splitting will be discussed in the introduction section, followed by the three main categories regarding to the different types of tantalum-based semiconductors In each category, synthetic methodologies, influencing factors on the photocatalytic activities, strategies to enhance the efficiencies of photocatalysts and morphology control of tantalum-based materials will be discussed in detail Future directions to further explore the research area of tantalum-based semiconductors for solar water splitting are also discussed

Compact Layer Free Perovskite Solar Cells with 13.5% Efficiency

Abstract: The recent breakthrough of organometal halide perovskites as the light harvesting layer in photovoltaic devices has led to power conversion efficiencies of over 16%. To date, most perovskite solar cells have adopted a structure in which the perovskite light absorber is placed between carrier-selective electron- and hole-transport layers (ETLs and HTLs). Here we report a new type of compact layer free bilayer perovskite solar cell and conclusively demonstrate that the ETL is not a prerequisite for obtaining excellent device efficiencies. We obtained power conversion efficiencies of up to 11.6% and 13.5% when using poly(3-hexylthiophene) and 2,2′,7,7′-tetrakis(N,N-di(4-methoxyphenyl)amino)-9,9′-spirobifluorene, respectively, as the hole-transport material. This performance is very comparable to that obtained with the use of a ZnO ETL. Impedance spectroscopy suggests that while eliminating the ZnO leads to an increase in contact resistance, this is offset by a substantial decrease in surface recombination.

A review of cermet-based spectrally selective solar absorbers

Abstract: Spectrally selective solar absorbers harvest solar energy in the form of heat. Solar absorbers using cermet-based coatings demonstrate a high absorptance of the solar spectrum and a low emittance in the infrared (IR) regime. Extensive work has been done to optimize cermet-based solar absorbers to achieve high performance by exploring different cermet (ceramic–metal composite) materials and film configurations through different preparation techniques such as electrodeposition, sputtering, pulsed laser deposition, and solution-based methods. In this article, we review the progress of cermet-based spectrally selective absorbers with high solar absorptance and low thermal emittance, such as Cr2O3, Al2O3, AlN, SiO2, and ZrO2 based cermets as absorption layers. We also present an outlook for cermet-based spectrally selective absorbers with high thermal stability and high conversion efficiency from sunlight to heat.

Radiative cooling of solar cells

Abstract: Standard solar cells heat up under sunlight. The resulting increased temperature of the solar cell has adverse consequences on both its efficiency and its reliability. We introduce a general approach to radiatively lower the operating temperature of a solar cell through sky access, while maintaining its solar absorption. We first present an ideal scheme for the radiative cooling of solar cells. For an example case of a bare crystalline silicon solar cell, we show that the ideal scheme can passively lower its operating temperature by 18.3 K. We then demonstrate a microphotonic design based on real material properties that approaches the performance of the ideal scheme. We also show that the radiative cooling effect is substantial, even in the presence of significant convection and conduction and parasitic solar absorption in the cooling layer, provided that we design the cooling layer to be sufficiently thin.

Integrated Polymer Solar Cell and Electrochemical Supercapacitor in a Flexible and Stable Fiber Format

Abstract: An all-solid-state, coaxial and self-powered "energy fiber" is demonstrated that simultaneously converts solar energy to electric energy and further stores it. The "energy fiber" is flexible and can be scaled up for the practical application by the well-developed textile technology, and may open a new avenue to future photoelectronics and electronics.

Enabling silicon for solar-fuel production

Abstract: Ke Sun,† Shaohua Shen,*,‡,§ Yongqi Liang, Paul E. Burrows, Samuel S. Mao,* and Deli Wang*,†,⊥,# †Department of Electrical and Computer Engineering, Material Science Program, and QualComm Institute, University of California at San Diego, La Jolla, California 92093, United States ‡International Research Center for Renewable Energy, State Key Lab of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China Department of Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720, United States Department of Chemistry, Chemical Biological Center, Umea ̊ University, Linnaeus vag̈, 6 901 87 Umea,̊ Sweden Samuel Mao Institute of New Energy, Science Hall, 1003 Shangbu Road, Shenzhen, 518031, China

Solar thermal reforming of methane feedstocks for hydrogen and syngas production—A review

Abstract: It is currently accepted that at least for a transition period, solar-aided reforming of methane-containing gaseous feedstocks with natural gas (NG) being the first choice, can offer a viable route for fossil fuel decarbonization and create a transition path towards a “solar hydrogen- solar fuels” economy. Both industrially established traditional reforming concepts, steam and dry/carbon dioxide reforming, being highly endothermic can be rendered solar-aided and thus offer in principle a real possibility to lower the cost for introducing renewable hydrogen production technologies to the market by a combination of fossil fuels and solar energy. They also share similar technical issues considering linking of their key thermochemistry and thermodynamics to efficient exploitation of solar energy. In this perspective, the current article presents the development and current status of solar-aided reforming of gaseous methane-containing feedstocks, focussing in particular on the reactor technologies and concepts employed so far to couple the heat requirements of the methane reforming process to the underlying principles, intricacies and peculiarities of concentrated solar power (CSP) exploitation. A thorough literature review is presented, addressing practically the whole scale of solar reactors employed so far: from small-scale reactor prototypes often tested under simulated solar irradiation up to scaled-up reformer reactors tested on solar platform sites at the level of few hundreds of kilowatts. Having presented the current state-of-the-art of the technology, topics for future work are suggested and issues to help further commercialization are addressed.

Zero-reabsorption doped-nanocrystal luminescent solar concentrators.

Abstract: Optical concentration can lower the cost of solar energy conversion by reducing photovoltaic cell area and increasing photovoltaic efficiency. Luminescent solar concentrators offer an attractive approach to combined spectral and spatial concentration of both specular and diffuse light without tracking, but they have been plagued by luminophore self-absorption losses when employed on practical size scales. Here, we introduce doped semiconductor nanocrystals as a new class of phosphors for use in luminescent solar concentrators. In proof-of-concept experiments, visibly transparent, ultraviolet-selective luminescent solar concentrators have been prepared using colloidal Mn2+-doped ZnSe nanocrystals that show no luminescence reabsorption. Optical quantum efficiencies of 37% are measured, yielding a maximum projected energy concentration of ∼6× and flux gain for a-Si photovoltaics of 15.6 in the large-area limit, for the first time bounded not by luminophore self-absorption but by the transparency of the waveg...

Triplet–triplet annihilation photon-upconversion: towards solar energy applications

Abstract: Solar power production and solar energy storage are important research areas for development of technologies that can facilitate a transition to a future society independent of fossil fuel based energy sources. Devices for direct conversion of solar photons suffer from poor efficiencies due to spectrum losses, which are caused by energy mismatch between the optical absorption of the devices and the broadband irradiation provided by the sun. In this context, photon-upconversion technologies are becoming increasingly interesting since they might offer an efficient way of converting low energy solar energy photons into higher energy photons, ideal for solar power production and solar energy storage. This perspective discusses recent progress in triplet-triplet annihilation (TTA) photon-upconversion systems and devices for solar energy applications. Furthermore, challenges with evaluation of the efficiency of TTA-photon-upconversion systems are discussed and a general approach for evaluation and comparison of existing systems is suggested.

Wind and Solar Energy Curtailment: Experience and Practices in the United States

Abstract: NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Acknowledgments In 2010, the U.S. Department of Energy (DOE) awarded a grant under the American Recovery and Reinvestment Act of 2009 to the Western Governors' Association to enhance member states' capacity to participate in interconnection-wide transmission planning. Such planning in the West is done by the Western Electricity Coordinating Council (WECC), who also received a companion DOE grant to help with its own planning efforts. Associated with the Western Governors' Association is the Western Interstate Energy Board, which received Western Governors' Association grant funding for its State Provincial Steering Committee (SPSC)/Committee on Regional Electric Power Coordination (CREPC) to conduct studies and work relevant to the interests of its state electricity official members. At its October 2013 meeting, the SPSC/CREPC asked DOE's Office of Electricity Delivery and Energy Reliability to help document and present curtailment practices for bulk power wind and solar generation. NREL was asked by DOE to provide the assistance. The authors wish to thank the U.S. DOE Office of Electricity Delivery and Energy Reliability for funding the research that went into this report and in particular the support of Larry Mansueti. In addition, we thank Rebecca Johnson, who served as a liaison from the Western Interstate Energy Board. iv This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. Executive Summary Curtailment is a reduction in the output of a generator from what it could otherwise produce given available resources, typically on an involuntary basis. Curtailment of generation has been a normal occurrence since the beginning of the electric power industry. However, owners of wind and solar generation, which have …

Solar Light Driven Pure Water Splitting on Quantum Sized BiVO4 without any Cocatalyst

Abstract: Photocatalytic water splitting is the most promising process to convert solar energy into high purity chemical fuel (hydrogen), which has received significant attention in recent years. Only several photocatalysts have been reported in the literature for pure water splitting under visible light. Herein we report for the first time quantum sized BiVO4 can decompose pure water into H2 and O2 simultaneously under simulated solar light irradiation without any cocatalysts or sacrificial reagents. By electrochemical measurement, we demonstrate that the significantly different photocatalytic activity of the quantum sized BiVO4 arises from the negative shift of conduction band edge by a quantum confinement effect and a decreased overpotential for water reduction. Although the generated H2 and O2 are nonstoichiometric in the present study, these findings establish the great potential of using quantum sized BiVO4 photocatalyst and solar energy for overall water splitting.

Solution Chemistry Engineering toward High-Efficiency Perovskite Solar Cells.

Abstract: Organic and inorganic hybrid perovskites (e.g., CH3NH3PbI3) have emerged as a revolutionary class of light-absorbing semiconductors that has demonstrated a rapid increase in efficiency within a few years of active research. Controlling perovskite morphology and composition has been found critical to developing high-performance perovskite solar cells. The recent development of solution chemistry engineering has led to fabrication of greater than 15–17%-efficiency solar cells by multiple groups, with the highest certified 17.9% efficiency that has significantly surpassed the best-reported perovskite solar cell by vapor-phase growth. In this Perspective, we review recent progress on solution chemistry engineering processes and various control parameters that are critical to the success of solution growth of high-quality perovskite films. We discuss the importance of understanding the impact of solution-processing parameters and perovskite film architectures on the fundamental charge carrier dynamics in perov...