Showing papers on "Solar energy published in 1996"

The Performance and Stability of Ambient Temperature Molten Salts for Solar Cell Applications

Abstract: Room temperature molten salt systems based on methyl-hexyl-imidazolium iodide (MHImI) have been used to scrutinize the performance characteristics, the stability and the mass-transfer effects in a photoelectrochemical regenerative device, as the latter is influenced and can even be limited by local concentration and mass-transport of the electroactive redox mediator species in the electrolyte phase. These salts appear to afford particular advantages over organic liquids as solvents for solar cell electrolytes. Cell performance showed outstanding stability, with an estimated sensitizer turnover in excess of 50 million. An investigation has been carried out on the physical-electrochemical properties of MHImI and its mixtures with organic solvents such as n-methyl-oxazolidinone, acetonitrile and with other lower viscosity molten salts such as methyl-butyl-imidazolium triflate. The repercussions of these properties on solar cells is described experimentally by the performance of practical application devices. Simulation models of mass transport in the nanocrystalline solar cell help illustrate operational aspects such as concentration profiles, limiting currents, anticipated mass-transfer overpotential as a function of current density, and they help to make projections as to how the properties of molten salt electrolytes can be better exploited toward this practical end.

Photovoltaic module and array performance characterization methods for all system operating conditions

Abstract: This paper provides new test methods and analytical procedures for characterizing the electrical performance of photovoltaic modules and arrays. The methods use outdoor measurements to provide performance parameters both at standard reporting conditions and for all operating conditions encountered by typical photovoltaic systems. Improvements over previously used test methods are identified, and examples of the successful application of the methodology are provided for crystalline- and amorphous-silicon modules and arrays. This work provides an improved understanding of module and array performance characteristics, and perhaps most importantly, a straight- forward yet rigorous model for predicting array performance at all operating conditions. For the first time, the influences of solar irradiance, operating temperature, solar spectrum, solar angle-of- incidence, and temperature coefficients are all addressed in a practical way that will benefit both designers and users of photovoltaics.

Absorption of solar energy in the atmosphere : Discrepancy between model and observations

Abstract: An atmospheric general circulation model, which assimilates data from daily observations of temperature, humidity, wind, and sea-level air pressure, was compared with a set of observations that combines satellite and ground-based measurements of solar flux. The comparison reveals that the model underestimates by 25 to 30 watts per square meter the amount of solar energy absorbed by Earth's atmosphere. Contrary to some recent reports, clouds have little or no overall effect on atmospheric absorption, a consistent feature of both the observations and the model. Of several variables considered, water vapor appears to be the dominant influence on atmospheric absorption.

A perspective on thermal energy storage systems for solar energy applications

Abstract: The use of thermal energy storage (TES) systems is essential for solar power systems because of fluctuations in the solar energy input. Several classes of storage may be required for a single installation, depending on the type and scale of the solar power plant itself, and the nature of its integration with conventional utility systems. For heating and hot water applications, water and phase change materials (PCMs) constitute the principle storage media. Soil, rock and other solids are used as well. Water has the advantage of approximately 80% less volume than that of water for a temperature variation of 10°C, which is the difference between temperatures of a fully charged and a fully discharged storage tank. Some PCMs are viscous and corrosive, and must be segregated within the container in order to be used as a heat transfer medium. For heat storage, two PCMs must be available, unless heat pumping is employed. A variety of solids is also used; rock particles of 20 to 50 mm in size are most prevalent. Well-designed packed rock beds have several desirable characteristics for energy storage. The heat transfer coefficient between the air and the solid is high, the cost of the storage material is low, the conductivity of the bed is low when air flow is not present and a large heat transfer area can be achieved at low cost by reducing the size of particles. TES systems have also been suggested for storing thermal energy at medium (38–304°C) and high temperatures (120–566°C). For instance, systems in an oil-rock system for hot water and heat-recovery applications are examples of medium-temperature applications, while those in molten nitrate salt systems (an excellent storage medium) for steam production for process applications are for high temperatures. Oil-rock TES, in which the energy is stored in a mixture of oil and rock in a tank, is less expensive than molten nitrate salt TES, but is limited to low-temperature applications. However, this oil-rock TES has been proven successful for solar thermal applications. The selection of the type of TES depends on various factors such as the storage period (diurnal or seasonal), economic viability, operating conditions, etc. In this article, a study of TES systems (including materials) is presented particularly with solar thermal applications in mind. The evaluation of their performances, cost and economic viability, ease of installation, cleanliness, environmental impact, safety factors, technological usability and applicability are discussed.

Directing and concentrating solar energy collectors

Abstract: A directing and concentrating solar energy collector with a concentrating reflector and one or more cooperating directing reflectors concentrates solar radiation to a substantially narrow and straight line of concentration The directing reflectors, being perpendicular to the concentrating reflector, confine the zones of concentration at the line of concentration A stationary optimizing energy converter having multiple energy conversion sections receives the concentrated radiation, converts part of it to electricity, with another part to thermal energy A building includes multiple collectors in an energy collection system, with one variation of the collectors used under a transparent roof and another inside a transparent wall The concentrating reflector is provided either with single axis tracking, or with no tracking at all, enabling different installations and having different advantages For the tracking versions, a radiation scanner digitally measures and computes the most optimal angular tracking position for accurate adjustment of the concentrating reflectors, yielding the highest available energy level Software in a micro processor controls the collection system and the conversion sections, independently from each other, optimizing the energy collection process The software also reads radiation energy levels from the radiation sensors of the scanner, at regular angular intervals of the rotation of the rod, and computes the most optimal angular tracking adjustment for positioning of the concentrating reflectors of all collectors

Quantum efficiency analysis of thin-layer silicon solar cells with back surface fields and optical confinement

Abstract: Thin-layer silicon solar cells utilize surface textures to increase light absorption and back surface fields to prevent recombination at the silicon-substrate interface. We present an analytical model for the internal quantum efficiency that accounts for light trapping and also considers carrier generation and recombination in back surface fields or substrates. We introduce a graphical representation of experimental data, the so-called Parameter-Confidence-Plot, which allows one to draw maximum information on diffusion lengths and surface recombination velocities from quantum efficiency measurements. The analysis is exemplified for state of the art thin-layer silicon solar cells with and without back surface fields.

Optimization and control of autonomous renewable energy systems

Abstract: Recently, there has been a growing interest in harnessing renewable energy resources particularly for electricity generation. One of the main concerns in the design of an electric power system that utilizes renewable energy sources, is the accurate selection of system components that can economically satisfy the load demand. This depends on the load that ought to be met, the capacity of renewable resources, the available space for wind machines and solar panels, and the capital and running costs of system components. Once size optimization is achieved, the autonomous system must be controlled in order to correctly match load requirements with instantaneous variation of input energy. In this paper, a new formulation for optimizing the design of an autonomous wind-solar-diesel-battery energy system is developed. This formulation employs linear programming techniques to minimize the average production cost of electricity while meeting the load requirements in a realiable manner. The computer program developed reads the necessary input data, formulates the optimization problem by computing the coefficients of the objective function and the constraints and provides the optimum wind, solar, diesel, and battery ratings. In order to study the effect of parameters predefined by the designer on the optimum design, several sensitivity analysis studies are performed, and the effects of the expected energy not served, the load level, the maximum available wind area, the maximum available solar area, and the diesel engines' lifetime are investigated. A controller that monitors the operation of the autonomous system is designed. The operation of this controller is based on three major policies; in the first, batteries operate before diesel engines and hence the storage system acts as a fuel saver, while in the second diesel engines are operated first so that the unmet energy is lower but the fuel cost is high. According to the third policy, the supply is made through diesel engines only. This is done for the purpose of making a performance comparison between the isolated diesel system and the hybrid renewable energy system. The proposed optimization and control techniques are tested on Lebanese data. Although three different control policies have been adopted in this work, the software is able to accommodate other policies.

Integrated power source

Abstract: A self-contained, small, lightweight, portable, renewable, modular integrated power source (10). The power source consists of solar cells (18, 20) that are laminated onto a solid state polymer battery (12) which in turn is laminated onto a substrate containing circuits (26) which manage the polymer battery charging. Charging of the battery can occur via solar energy or, alternatively, via RF coupling using external RF charging equipment (3O) or a hand held generator. For added support, the integrated power source is then bonded to an applications housing or structure. This integrated power source can independently power the electronic application. It can also serve as casing or housing by taking the shape of the application enclosure.

Solar force modeling of block IIR Global Positioning System satellites

Abstract: Scientific applications of the Global Positioning System require that the space vehicles be located with an accuracy of a few centimeters. The most important uncertainties in position estimation are the result of direct and indirect solar forces. Perhaps as early as late 1996, Block IIR space vehicles will begin to replace the existing Blocks II and IIA. We give formulas for the solar force to be expected on Block IIR and evaluate their probable accuracy based on our previous experience. These a priori formulas include indirect solar forces, including the reradiation of sunlight in the form of heat from the space vehicle's body and solar panels, but do not include radiation-induced outgassing, especially from the multilayered insulation that wraps the space vehicle body. We discuss the best way to combine these a priori models with real-time tracking data to optimize ephemeris accuracy. MONG the many unique features of the Global Positioning System (GPS), one fact is of primary importance for its use in geodesy, studies of crustal dynamics, and support of such space missions as TOPEX/Poseidon: it is the first system of navigational satellites in which errors in knowledge of the Earth's gravitational field have negligible effect, and orbit accuracy is limited almost entirely by errors in modeling the force of solar radiation on the space vehicles (SVs). These SVs are high altitude (semimajor axes = 26,560 km = 4.16 Earth radii) and relatively insensitive to the higher-order gravitational harmonics, which are well known in any case via satellites such as LAGEOS. But GPS SVs have a large cross- sectional area (about 13.6 m2 for Block II/IIA) and are accelerated about 1 x 10~7 m/s by direct solar radiation. Table 1 shows our calculation (using the Aerospace TRACE program) of perturbations on a typical GPS orbit over 12 h. After the Earth oblateness J2, the sun, and the moon, solar radiation is the most important. An a priori model calculates the solar force to be expected on the vehicle as a function of its orientation and of its cross-sectional area and optical properties. Direct solar pressure can be pictured as the net momentum imparted to the S V by photons striking and recoil- ing from its opaque surfaces. Indirect solar pressure is caused, for example, by heat absorbed and reradiated from body surfaces and by outgassing, whereby solar energy and momentum is returned to space by volatile materials on and in the SV body. Indirect forces are of two kinds: predictable, such as the effects of Earthshine and SV heat flow; and anomalous, including outgassing and the so-called 7-bias force. Because none of the indirect effects, predictable or anomalous, were included in the early computer software devel- oped for GPS, many scientific GPS analysts have bypassed the modeling problem entirely and have determined SV accelerations directly as stochastic parameters to be estimated by tracking data.1'2 To attain highest accuracy, some such real-time estimation is nec- essary. Nevertheless, good a priori models are indispensable for three reasons: 1) many applications require not just precise fitting of orbits to tracking data already taken, but orbit prediction; 2) by us- ing a standard force model, workers more readily can intercompare ephemerides, which are calculated by many different agencies from data taken all over the world using different techniques; and 3) to filter real data accurately and reliably, one should know the statistics of the parameters measured—how fast they usually change, and how fast they can change—and this requires knowledge of the physics of the problem, and therefore, good a priori modeling. Our intention

Scaling-up of CIS technology for thin-film solar modules

Abstract: The compound semiconductor Cu(Ga,In)Se2 (CIGS) and related compounds have demonstrated their high potential for high-efficiency thin-film solar cells up to levels approaching 18%. The Center for Solar Energy and Hydrogen Research (ZSW) and the University of Stuttgart (IPE) are working together on CIS upscaling. On the module basis, ZSW is collaborating with Phototronics Solartechnik GmbH/Putzbrunn as a manufacturer for a-Si modu les supporting module technology. With the aim of developing high-volume fabrication technologies, all the laboratory deposition techniques suitable for highest device performance are applied now also on the module level to avoid physical and chemical effects that could limit device performance to a low level. All film deposition techniques are developed for high-vacuum in-line fabrication on a large area, except for the buffer layer of CdS, and monolithic integration is realized by patterning steps. A key issue for the development of modules is upscaling of the CIGS absorber deposition. Films of CIGS are prepared by simultaneous thermal evaporation of the elements. Modules are prepared on substrate areas of 7 × 7, 10 × 10 ??? 30 × 30 cm2. Actual results of modules of these sizes are shown.

A distributed SPICE-model of a solar cell

Abstract: In this paper, a distributed SPICE-model for a solar cell is worked out. Special attention is paid to the problems of nonhomogeneous current distribution and the effective series resistance. Elaborate experimental techniques have been used to determine the model parameters. They are based on the I-V characteristics, the small signal impedance, and the open-circuit voltage decay measurements. The SPICE simulation results are compared with the measured static and dynamic characteristics of a solar cell. We found satisfactory agreement concerning the static characteristics, whereas there is discrepancy in the dynamic characteristics. This is because the SPICE-model of a p-n junction diode contains only one time constant. It is found that two time constants are necessary for accurate description of the dynamic performance. The techniques and methodologies developed here are applicable to other junction diodes operating at high frequencies or under transient conditions.

Solar energy panel

Abstract: A combined solar energy panel and roof element, which provides an additional layer of weather proofing to a building, as well as providing an electric power source. The solar energy panel is comprised of a translucent polymeric pane, with electrically interconnected photovoltaic cells laminated onto the underside of the polymeric pane. A polymeric frame is attached to the underside of the pane contiguous to the edges of the pane. Anchor blocks are adhered to the roof and the panel is attached to the roof at the anchor blocks, where several anchor blocks support the frame attached to the panel and each anchor block may support the frames of more than one panel. A plurality of solar energy panels may be arranged in an array, whereby waterproofing means seals any spaces between the panels.

Solar cells with efficiencies above 21% processed from Czochralski grown silicon

Abstract: Czochralski-Si (Cz-Si) of several manufacturers and with resistivities ranging from 1 to 13 /spl Omega/cm were processed into solar cells with efficiencies higher than 20% (AM1.5) using the LBSF/PERL processing sequence. The highest efficiency was 21.7%. The investigation of high efficiency Cz-Si solar cells was augmented by computer simulation and a study of the carrier lifetime before and after processing. A small degradation of solar cell performance in the lower resistivity material is discussed. Furthermore, a much simpler processing sequence is presented revealing efficiencies well above 19% on Cz-silicon and 21% on float zone-silicon.

Modelling of tandem cell temperature coefficients

Abstract: This paper discusses the temperature dependence of the basic solar-cell operating parameters for a GaInP/GaAs series-connected two-terminal tandem cell. The effects of series resistance and of different incident solar spectra are also discussed.