Recent developments in thin film polycrystalline cells based on CdS/Cu2S have established this cell as a major contender for large scale terrestrial application. Conversion efficiencies for CdS/Cu2S now exceed 8%, with firm reason to believe that 10% is achievable. A modified cell using (CdZn)S is expected to be capable of about 15% conversion efficiency.
Photovoltaic effects have been investigated in II-VI heterojunctions prepared by close-space vapor transport, vacuum evaporation, spray pyrolysis and sputtering. Solar efficiencies of about 8% have been measured for the following systems: (a) n-CdS film deposited on single crystal p-CdTe by vacuum evaporation, (b) n-ZnCdS film deposited on single crystal p-CdTe by spray pyrolysis, and (c) n-Indium-Tin Oxide film deposited on single crystal p-CdTe by sputtering in an inert atmosphere. Open-circuit voltages greater than 0.8 V have been measured in heterojunctions of type (b) and (c), as well as in CdTe p-n homojunctions produced by ion implantation.
We report single crystal InP-CdS solar cells having AM2 efficiencies of 15 percent and polycrystalline thin-film cells having AM2 efficiencies of 5.7 percent. Basic studies of the interface reveal that the thin-film efficiency is presently limited at least in part by the quality of the InP within the grains, and not exclusively by interface phenomena intrinsic to a polycrystalline cell.
This article outlines the impetus and form for one of the research tasks to be undertaken at the Solar Energy Research Institute. Two options, the high temperature option and the intermediate temperature option, are usually specified for solar energy development. Evidence is presented which supports the need for immediate research into the intermediate temperature, supplementary source role for solar thermal power in the United States. Particularly promising is a technological, approach which will match industrial process heating demands to currently available solar concentrating collector systems. The impact appears to be strong, with 19% of U.S. energy input falling at temperature demand levels less than 1770 C (3500 F). A detailed industrial process heat data base is to be assembled and matched to the system performance of major concentrator types. Solar concentrator characteristics are generally outlined and areas for immediate improvement suggested. An example of the matching of a concentrator system to a specific task is included.
Space heat, hot water, air conditioning and process heat utilize more than 40% of U. S. energy consumption. These uses provide the greatest opportunity for solar energy to contribute significantly to the U. S. and world energy needs. In order to compete economically with other energy sources for these uses and also achieve the higher operating temperatures required for air conditioning and process heat (up to - 200°C), a solar collector must have low energy loss and low cost. Low loss requires vacuum to eliminate conduction and convection losses, and concentration onto a selective absorber to minimize radiation loss. Optimum concentration, or minimum ratio of absorbing surface area to collecting area, requires Winston concentration. The apparent motion of the sun indicates the use of cylindrical concentration with axis east-west. A series of arguments leading to optimum performance at near minimum cost indicate the collector should be of all glass tubular construction. A mirror coating on side and bottom inside surfaces provides concentration onto an internal glass tube coated with selective absorber. The window is only slightly convex to withstand atmospheric pressure while keeping reflection losses low. Heat transfer calculations, which are used to minimize losses help fix the collector length and width. Radiation collection calculations determine the acceptance angle and other parameters.
Reflecting solar concentrators must possess good specular reflectance and optical quality. Quantitative techniques to evaluate beam spreading have been developed to allow analysis of concentrator performance. These techniques, known as bidirectional reflectometry and laser ray trace inspection, are summarized and examples of their usefulness for evaluation of solar concentrators are given. Data on contour errors found in model parabolas of various constructions and data on the specular reflectance of common mirror materials are included. A data format is outlined which allows the experimental results to be used for collector design, quality control and/or concentrator specification.
A unique system has been developed to focus and align the nearly 8000 mirrors for our nation's first Central Receiver Solar Thermal Test Facility, located at Albuquerque, New Mexico. The computer controlled system utilizes a laser light source which is scanned over a 61 inch diameter collimating mirror to simulate a collimated solar beam. The collimator and laser source are mounted on a microprocessor controlled precision pointing mount with 0.1 milliradian pointing accuracy. Control of the system is achieved through a vehicle mounted terminal plugged directly into the control electronics of the heliostat undergoing alignment. The simulated solar beam is reflected from the heliostat mirror and imaged onto a retro-reflective target screen. The screen is observed by an integrating video system over the period of a complete laser scan. Alignment is achieved through computer simulation of the sun/heliostat/receiver geometry for a selectable time and day of the year. This paper gives an overview of the Solar Thermal Test Facility and a detailed description of the Focus and Alignment System.
This paper describes the class of nonimaging concentrators that achieve the maximum concentration of light permitted by physical principles. Examples of the concentrators for different absorber shapes are shown, and the modifications necessary to make a practi-cal collector are discussed. The performance of an experimental collector is also shown.
The concentrating properties of specularly reflecting pyramids, hexagons and circular cones are examined. The concentration factor is determined as a function of the coefficient of reflection and the shape and orientation of the incident sunlight. Reflector designs allowing multiple reflections for both normal and oblique incidence are considered.