Showing papers on "Solar energy published in 2005"

High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends

Abstract: Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Polymer solar cells have shown potential to harness solar energy in a cost-effective way. Significant efforts are underway to improve their efficiency to the level of practical applications. Here, we report highly efficient polymer solar cells based on a bulk heterojunction of polymer poly(3-hexylthiophene) and methanofullerene. Controlling the active layer growth rate results in an increased hole mobility and balanced charge transport. Together with increased absorption in the active layer, this results in much-improved device performance, particularly in external quantum efficiency. The power-conversion efficiency of 4.4% achieved here is the highest published so far for polymer-based solar cells. The solution process involved ensures that the fabrication cost remains low and the processing is simple. The high efficiency achieved in this work brings these devices one step closer to commercialization.

Design Considerations for Solar Energy Harvesting Wireless Embedded Systems

Abstract: Sustainable operation of battery powered wireless embedded systems (such as sensor nodes) is a key challenge, and considerable research effort has been devoted to energy optimization of such systems. Environmental energy harvesting, in particular solar based, has emerged as a viable technique to supplement battery supplies. However, designing an efficient solar harvesting system to realize the potential benefits of energy harvesting requires an in-depth understanding of several factors. For example, solar energy supply is highly time varying and may not always be sufficient to power the embedded system. Harvesting components, such as solar panels, and energy storage elements, such as batteries or ultracapacitors, have different voltage-current characteristics, which must be matched to each other as well as the energy requirements of the system to maximize harvesting efficiency. Further, battery nonidealities, such as self-discharge and round trip efficiency, directly affect energy usage and storage decisions. The ability of the system to modulate its power consumption by selectively deactivating its sub-components also impacts the overall power management architecture. This paper describes key issues and tradeoffs which arise in the design of solar energy harvesting, wireless embedded systems and presents the design, implementation, and performance evaluation of Heliomote, our prototype that addresses several of these issues. Experimental results demonstrate that Heliomote, which behaves as a plug-in to the Berkeley/Crossbow motes and autonomously manages energy harvesting and storage, enables near-perpetual, harvesting aware operation of the sensor node.

Latent Heat Storage Materials and Systems: A Review

Abstract: The use of a latent heat storage system using Phase Change Materials (PCM) is an effective way of storing thermal energy (solar energy, off-peak electricity, industrial waste heat) and has the advantages of high storage density and the isothermal nature of the storage process. It has been demonstrated that, for the development of a latent heat storage system, choice of the PCM plays an important role in addition to heat transfer mechanism. The information on the latent heat storage materials and systems is enormous and published widely in the literatures. In this paper, we make an effort to gather the information from the previous works on PCMs and latent heat storage systems. This review will help to find a suitable PCM for various purposes a suitable heat exchanger with ways to enhance the heat transfer, and it will also help to provide a variety of designs to store the heat using PCMs for different applications, i.e. space heating & cooling, solar cooking, greenhouses, solar water heating and waste heat recovery systems. Measurement techniques of thermophysical properties, studies on thermal cycles for long term stability, corrosion of the PCMs and enhancement of heat transfer in PCM are discussed. New PCM innovations are also included for the awareness of new applications. This paper contains a list of about 250 PCMs and more than 250 references.

Design considerations for solar energy harvesting wireless embedded systems

Abstract: Sustainable operation of battery powered wireless embedded systems (such as sensor nodes) is a key challenge, and considerable research effort has been devoted to energy optimization of such systems. Environmental energy harvesting, in particular solar based, has emerged as a viable technique to supplement battery supplies. However, designing an efficient solar harvesting system to realize the potential benefits of energy harvesting requires an in-depth understanding of several factors. For example, solar energy supply is highly time varying and may not always be sufficient to power the embedded system. Harvesting components, such as solar panels, and energy storage elements, such as batteries or ultracapacitors, have different voltage-current characteristics, which must be matched to each other as well as the energy requirements of the system to maximize harvesting efficiency. Further, battery non-idealities, such as self-discharge and round trip efficiency, directly affect energy usage and storage decisions. The ability of the system to modulate its power consumption by selectively deactivating its sub-components also impacts the overall power management architecture. This paper describes key issues and tradeoffs which arise in the design of solar energy harvesting, wireless embedded systems and presents the design, implementation, and performance evaluation of Heliomote, our prototype that addresses several of these issues. Experimental results demonstrate that Heliomote, which behaves as a plug-in to the Berkeley/Crossbow motes and autonomously manages energy harvesting and storage, enables near-perpetual, harvesting aware operation of the sensor node.

Basic Research Needs for Solar Energy Utilization: report of the Basic Energy Sciences Workshop on Solar Energy Utilization, April 18-21, 2005

Abstract: This report of the Basic Energy Sciences Workshop on Solar Energy Utilization identifies the key scientific challenges and research directions that will enable efficient and economic use of the solar resource to provide a significant fraction of global primary energy by the mid 21st century. The report reflects the collective output of the workshop attendees, which included 200 scientists representing academia, national laboratories, and industry in the United States and abroad, and the U.S. Department of Energy’s Office of Basic Energy Sciences and Office of Energy Efficiency and Renewable Energy. Solar energy conversion systems fall into three categories according to their primary energy product: solar electricity, solar fuels, and solar thermal systems. Each of the three generic approaches to exploiting the solar resource has untapped capability well beyond its present usage. Workshop participants considered the potential of all three approaches, as well as the potential of hybrid systems that integrate key components of individual technologies into novel cross-disciplinary paradigms.

Physics of solar cells : from principles to new concepts

Abstract: List of SymbolsPreface1 Problems of the Energy Economy11 Energy economy12 Estimate of the maximum reserves of fossil energy13 The greenhouse effect2 Photons21 Black-body radiation22 Kirchhoff's law of radiation for non-black bodies23 The solar spectrum24 Concentration of the solar radiation25 Maximum efficiency of solar energy conversion3 Semiconductors31 Electrons in semiconductors32 Holes33 Doping34 Quasi-Fermi distributions35 Generation of electrons and holes36 Recombination of electrons and holes37 Light emission by semiconductors4 Conversion of Thermal Radiation into Chemical Energy41 Maximum efficiency for the production of chemical energy5 Conversion of Chemical Energy into Electrical Energy51 Transport of electrons and holes52 Separation of electrons and holes53 Diffusion length of minority carriers54 Dielectric relaxation55 Ambipolar diffusion56 Dember effect57 Mathematical description6 Basic Structure of Solar Cells61 A chemical solar cell62 Basic mechanisms in solar cells63 Dye solar cell64 The pn-junction65 pn-junction with impurity recombination, two-diode model66 Hetero-junctions67 Semiconductor-metal contact68 The role of the electric field in solar cells7 Limitations on Energy Conversion in Solar Cells71 Maximum efficiency of solar cells72 Efficiency of solar cells as a function of their energy gap73 The optimal silicon solar cell74 Thin-film solar cells75 Equivalent circuit76 Temperature dependence of the open-circuit voltage77 Intensity dependence of the efficiency78 Efficiencies of the individual energy conversion processes8 Concepts for Improving the Efficiency of Solar Cells81 Tandem cells82 Concentrator cells83 Thermo-photovoltaic energy conversion84 Impact ionization85 Two-step excitation in three-level systems9 Prospects for the FutureAppendixIndex

Design of Commercial Solar Updraft Tower Systems—Utilization of Solar Induced Convective Flows for Power Generation

Abstract: A solar updraft tower power plant-sometimes also called solar chimney, or just solar tower-is a solar thermal power plant utilizing a combination of solar air collector and central updraft tube to generate a solar induced convective flow which drives pressure staged turbines to generate electricity The paper presents theory, practical experience, and economy of solar updraft towers: First a simplified theory of the solar tower is described Then results from designing, building and operating a small scale prototype in Spain are presented Eventually technical issues and basic economic data for future commercial solar tower systems like the one being planned for Australia are discussed

Organic Photovoltaics : Mechanisms, Materials, and Devices

Abstract: Foreword 1 Alan J. Heeger, Nobel Laureate, University of California at Santa Barbara Foreword 2 Aloysius F. Hepp and Sheila G. Bailey, Photovoltaic and Space Environments Branch, NASA Glenn Research Center Preface Acknowledgements Editors Contributors General Overviews The Story of Solar Cells J. Perlin Inorganic Photovoltaic Materials and Devices: Past, Present, and Future A.F. Hepp, S.G. Bailey, and R.P. Raffaelle Natural Organic Photosynthetic Solar Energy Transduction R.E. Blankenship Solid-State Organic Photovoltaics: A Review of Molecular and Polymeric Devices P.A. Lane and Z.H. Kafafi Mechanisms and Modeling Simulations of Optical Processes in Organic Photovoltaic Devices N-K. Persson and O. Inganas Coulomb Forces in Excitonic Solar Cells B.A. Gregg Electronic Structure of Organic Photovoltaic Materials: Modeling of Exciton-Dissociation and Charge-Recombination Processes J. Cornil, V. Lemaur, M.C. Steel, H. Dupin, A. Burquel, D. Beljonne, and J-L. Bredas Optimization of Organic Solar Cells in Both Space and Energy-Time Domains S-S. Sun and C.E. Bonner Materials and Devices Bulk Heterojunction Solar Cells H. Hoppe and N.S. Sariciftci Organic Solar Cells Incorporating a p-i-n Junction and a p-n Homojunction M. Hiramoto Liquid-Crystal Approaches to Organic Photovoltaics B. Kippelen, S. Yoo, J.A. Haddock, B. Domercq, S. Barlow, B. Minch, W. Xia, S.R. Marder, and N.R. Armstrong Photovoltaic Cells Based on Nanoporous Titania Films Filled with Conjugated Polymers K.M. Coakley and M.D. McGehee Solar Cells Based on Cyanine and Polymethine Dyes H. Tian and F. Meng Semiconductor Quantum Dot Based Nanocomposite Solar Cells M.H. Wu, A. Ueda, and R. Mu Solar Cells Based on Composites of Donor Conjugated Polymers and Carbon Nanotubes E. Kymakis and G.A.J. Amaratunga Photovoltaic Devices Based on Polythiophene/C60 L.S. Roman Alternating Fluorene Copolymer-Fullerene Blend Solar Cells O. Inganas, F. Zhang, X. Wang, A. Gadisa, N-K. Persson, M. Svensson, E. Perzon, W. Mammo, and M.R. Andersson Solar Cells Based on Diblock Copolymers: A PPV Donor Block and a Fullerene Derivatized Acceptor Block R.A. Segalman, C. Brochon, and G. Hadziioannou Interface Electronic Structure and Organic Photovoltaic Devices Y. Gao The Influence of the Electrode Choice on the Performance of Organic Solar Cells A.B. Djurisic' and C.Y. Kwong Conducting and Transparent Polymer Electrodes F. Zhang and O. Inganas Progress in Optically Transparent Conducting Polymers V. Seshadri and G.A. Sotzing Optoelectronic Properties of Conjugated Polymer/Fullerene Binary Pairs with Variety of LUMO Level Differences S. Sensfuss and M. Al-Ibrahim Polymer-Fullerene Concentration Gradient Photovoltaic Devices by Thermally Controlled Interdiffusion M. Drees, R.M. Davis, and R. Heflin Vertically Aligned Carbon Nanotubes for Organic Photovoltaic Devices M.H-C. Jin and L. Dai Index

Hybrid generation with alternative fuel sources

Abstract: A generating facility is provided for generating electricity from both solar and non-solar energy sources. The solar generating portion of the facility includes capability to directly generate electricity from solar insolation, or to store the solar energy in a tangible medium, including stored heat, or solar generating fuel. The generating facility is configured to generate electricity simultaneously from both solar and non-solar sources, as well a solely from immediate solar insolation and from solar energy stored in a tangible medium. Additionally, the solar generating capacity may be segregated; such that separate spectra of solar insolation are used to capture heat for steam turbine based electrical generation, capture light energy for photovoltaic based electrical generation, and to grow biomass to generate a solar fuel.

Using HOMER Software, NREL's Micropower Optimization Model, to Explore the Role of Gen-sets in Small Solar Power Systems; Case Study: Sri Lanka

Abstract: This paper discusses using HOMER Software, NREL's Micropower Optimization Model, to explore the role of gen-sets in small solar power systems in Sri Lanka

Organic and Nano-Structured Composite Photovoltaics: An Overview

Abstract: Organic photovoltaic devices are poised to fill the low-cost, low power niche in the solar cell market Recently measured efficiencies of solid-state organic cells are nudging 5% while Gratzel’s more established dye-sensitized solar cell technology is more than double this A fundamental understanding of the excitonic nature of organic materials is an essential backbone for device engineering Bound electron-hole pairs,“excitons,” are formed in organic semiconductors on photo-absorption In the organic solar cell, the exciton must diffuse to the donor-accepter interface for simultaneous charge generation and separation This interface is critical as the concentration of charge carriers is high and recombination here is higher than in the bulk Nanostructured engineering of the interface has been utilized to maximize organic materials properties, namely to compensate the poor exciton diffusion lengths and lower mobilities Excitonic solar cells have different limitations on their open-circuit photo-voltages due to these high interfacial charge carrier concentrations, and their behavior cannot be interpreted as if they were conventional solar cells This article briefly reviews some of the differences between excitonic organic solar cells and conventional inorganic solar cells and highlights some of the technical strategies used in this rapidly progressing field, whose ultimate aim is for organic solar cells to be a commercial reality

Engineering metal-impurity nanodefects for low-cost solar cells

Abstract: As the demand for high-quality solar-cell feedstock exceeds supply and drives prices upwards, cheaper but dirtier alternative feedstock materials are being developed. Successful use of these alternative feedstocks requires that one rigorously control the deleterious effects of the more abundant metallic impurities. In this study, we demonstrate how metal nanodefect engineering can be used to reduce the electrical activity of metallic impurities, resulting in dramatic enhancements of performance even in heavily contaminated solar-cell material. Highly sensitive synchrotron-based measurements directly confirm that the spatial and size distributions of metal nanodefects regulate the minority-carrier diffusion length, a key parameter for determining the actual performance of solar-cell devices. By engineering the distributions of metal-impurity nanodefects in a controlled fashion, the minority-carrier diffusion length can be increased by up to a factor of four, indicating that the use of lower-quality feedstocks with proper controls may be a viable alternative to producing cost-effective solar cells.

Testing of Thermocline Filler Materials and Molten-Salt Heat Transfer Fluids for Thermal Energy Storage Systems in Parabolic Trough Power Plants

Abstract: Parabolic trough power systems that utilize concentrated solar energy to generate electricity are a proven technology. Industry and laboratory research efforts are now focusing on integration of thermal energy storage as a viable means to enhance dispatchability of concentrated solar energy. One option to significantly reduce costs is to use thermocline storage systems, low-cost filler materials as the primary thermal storage medium, and molten nitrate salts as the direct heat transfer fluid. Prior thermocline evaluations and thermal cycling tests at the Sandia National Laboratories' National Solar Thermal Test Facility identified quartzite rock and silica sand as potential filler materials. An expanded series of isothermal and thermal cycling experiments were planned and implemented to extend those studies in order to demonstrate the durability of these filler materials in molten nitrate salts over a range of operating temperatures for extended time frames. Upon test completion, careful analyses of filler material samples, as well as the molten salt, were conducted to assess long-term durability and degradation mechanisms in these test conditions. Analysis results demonstrate that the quartzite rock and silica sand appear able to withstand the molten salt environment quite well. No significant deterioration that would impact the performance or operability of a thermocline thermal energy storage system was evident. Therefore, additional studies of the thermocline concept can continue armed with confidence that appropriate filler materials have been identified for the intended application.

Energy Saving Potentials and Air Quality Benefits of Urban Heat IslandMitigation

Abstract: Energy Saving Potentials and Air Quality Benefits of Urban Heat Island Mitigation 1 Hashem Akbari Heat Island Group Lawrence Berkeley National Laboratory H_Akbari@lbl.gov http://HeatIsland.LBL.gov/ ABSTRACT Urban areas tend to have higher air temperatures than their rural surroundings as a result of gradual surface modifications that include replacing the natural vegetation with buildings and roads. The term “Urban Heat Island” describes this phenomenon. The surfaces of buildings and pavements absorb solar radiation and become extremely hot, which in turn warm the surrounding air. Cities that have been “paved over” do not receive the benefit of the natural cooling effect of vegetation. As the air temperature rises, so does the demand for air-conditioning (a/c). This leads to higher emissions from power plants, as well as increased smog formation as a result of warmer temperatures. In the United States, we have found that this increase in air temperature is responsible for 5– 10% of urban peak electric demand for a/c use, and as much as 20% of population- weighted smog concentrations in urban areas. Simple ways to cool the cities are the use of reflective surfaces (rooftops and pavements) and planting of urban vegetation. On a large scale, the evapotranspiration from vegetation and increased reflection of incoming solar radiation by reflective surfaces will cool a community a few degrees in the summer. As an example, computer simulations for Los Angeles, CA show that resurfacing about two-third of the pavements and rooftops with reflective surfaces and planting three trees per house can cool down LA by an average of 2–3K. This reduction in air temperature will reduce urban smog exposure in the LA basin by roughly the same amount as removing the basin entire on- road vehicle exhaust. Heat island mitigation is an effective air pollution control strategy, more than paying for itself in cooling energy cost savings. We estimate that the cooling energy savings in U.S. from cool surfaces and shade trees, when fully implemented, is about $5 billion per year (about $100 per air-conditioned house). 1. Introduction Across the world, urban temperatures have increased faster than temperatures in rural areas. For example, from 1930 to 1990, downtown Los Angeles recorded a growth This paper is an abridged and updated version of an earlier paper published in Solar Energy (Akbari et al 2001).

Optical Durability of Candidate Solar Reflectors

Abstract: Concentrating solar power (CSP) technologies use large mirrors to collect sunlight to convert thermal energy to electricity. The viability of CSP systems requires the development of advanced reflector materials that are low in cost and maintain high specular reflectance for extended lifetimes under severe outdoor environments. The long-standing goals for a solar reflector are specular reflectance above 90% into a 4 mrad half-cone angle for at least 10 years outdoors with a cost of less than $13.8/m 2 (the 1992 $10.8/m 2 goal corrected for inflation to 2002 dollars) when manufactured in large volumes. Durability testing of a variety of candidate solar reflector materials at outdoor test sites and in laboratory accelerated weathering chambers is the main activity within the Advanced Materials task of the CSP Program at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. Test results to date for several candidate solar reflector materials will be presented. These include the optical durability of thin glass, thick glass, aluminized reflectors, front-surface mirrors, and silvered polymer mirrors. The development, performance, and durability of these materials will be discussed. Based on accelerated exposure testing the glass, silvered polymer, and front-surface mirrors may meet the 10 year lifetime goals, but at this time because of significant process changes none of the commercially available solar reflectors and advanced solar reflectors have demonstrated the 10 year or more aggressive 20 year lifetime goal.