Without question, energy is the key issue for this decade. Historians will remember this generation for its energy policy and how that policy influenced the course of human events. Our newspapers, magazines and broadcast media are filled with the interplay between politicians and nations as policy is formed on a day-to-day basis.
This paper is an update of part of an article in the SPIE Journal for February-March 1966, entitled "Symbols, Units, and Nomenclature for Atmospheric Transmission," by Irving J. Spiro, R. Clark Jones, and David Q. Wark. In the interim 1966-1975, Fred Nicodemus, lately of the Naval Weapons Laboratory at China Lake, California and now with the National Bureau of Standards (NBS) at Gaithersburg, Maryland, has done an outstanding job in solidifying the concepts proposed (as a committee action) in 1966.
The NOAA Solar Radiation Program is discussed in terms of the past data and future plans. Attempts are being made to renovate some of the old data by applying known instrument calibration changes and an analysis technique which uses cloudless, true solar noon, solar transmission values. These corrected data will be used to develop models to predict irradiance from cloudiness data. The new network and its support are described in detail.
A method is presented for determining solar insolation at the earth's surface using satellite broadband visible radiance and cloud imagery data, along with conventional in situ measurements. Conventional measurements are used to both tune satellite measurements and to develop empirical relationships between satellite observations and surface solar insolation. Cloudiness is the primary modulator of sunshine. The satellite measurements as applied in this method consider cloudiness both explicitly and implicitly in determining surface solar insolation at space scales smaller than the conventional pyranometer network.
Hourly solar radiation (insolation) data bases have been prepared for use in the computer simulation of solar energy systems. These data collections have been based on actual insolation observations or closely related weather observations. The inadequacies of the presently available insolation data have required that portions of the data be estimated by the use of the statistical procedures discussed in detail in the paper.
The Active Cavity Radiometer (ACR), an accurate absolute pyrheliometer, has been developed at the Jet Propulsion Laboratory for measurements of total solar irradience. They have been used to discover a -2.2% error in the International Pyrheliometric Scale and to make measurements in balloon flight experiments yielding a solar constant value of 136.6 (±0.7) mw/cm2. New ACR's are being developed to monitor the total output of solar optical radiation in balloon, satellite and space shuttle experiments with longterm absolute uncertainty of ±0.1% or less. In a separate program, instrumentation for the measurement of the scattering and extinction of solar radiation by the atmosphere is being constructed to provide data for modeling atmospheric aerosol content. The aerosol models will facilitate computation of radiative transfer effects yielding quantitative net fluxes useful in evaluating the climatological impact of aerosols.
An instrument system has been developed to measure the flux of energy from the sun and the circumsolar region (the small-angle region around the sun) as a function of angle, wavelength, and atmospheric conditions. The measurements are necessary to accurately predict the performance of solar thermal conversion systems using focusing collectors, and to determine whether pyrheliometer data are adequate for estimating this performance. The instrument system consists of a "scanning telescope", several conventional solar instruments, and a digital electronics control and recording system. The telescope makes a scan of ±3°, passing through the center of the sun. The angular resolution is approximately 1.5 minutes of arc from the center of the sun out to two solar radii, and 5 minutes from there out to 3°. The background caused by direct solar light scattering within the telescope itself has been reduced to less than 1% of the intensity of the circumsolar radiation. Spectral dependence of the radiation is determined using filters. A measurement program using this telescope will be carried out at various locations in the West and Southwest. Additional instruments are being constructed for deployment for the accumulation of data at potential sites for future solar power plants.
A number of areas of current research interest require measurements of solar irradiance made with greater absolute accuracy than is currently being achieved at various monitoring sites. By using currently available instrumentation and careful measuring techniques it is shown that it is possible to measure the solar irradiance with considerable precision on a routine basis. The major problem that re-mains is making the measurements absolute; and an investigation of the standards used in pyrheliometry is required before a significant increase in absolute accuracy can be obtained.
This presentation contains a description of solar radiation instrumentation. The characteristics of pyrheliometers and pyranometers are discussed. Some examples of terrestrial and extra-terrestrial solar measurements are presented.
The use of sounding rockets for calibrating solar cells offers two principal advantages: (1) there is no effect due to the terrestrial atmosphere and (2) the cells are recoverable immediately after calibration. On 18 September 1974, four n/p photovoltaic cells were calibrated in space and successfully recovered from a NASA Aerobee rocket that reached a peak altitude of 251 km. Two of the cells were optically filtered with 0.6 to 0.9-p band-pass filters. The short-circuit current agreed to within 4% of the laboratory calibration; variation of cell output due to atmospheric attenuation in the altitude range of 80 to 251 km was 0.4% for the unfiltered cells.
The design and operation of a multipurpose instrument for the measurement of solar radiation at ground level is presented. The instrument measures insolation in seven spectral bands from 427.8 nm to 940.0 nm. The field of view of the photometer is 3° and the basic mode of operation is that of scanning the sky in a series of almucantars. In addition, the instrument measures the direct beam of the sun at fifteen minute intervals. The basic data, therefore, consists of both direct and diffuse components of insolation which can be synthesized to yield the sum total falling on a surface of arbitrary orientation. The detection mode will allow a basic stability in the measurement of ±2% and the instrument module is portable and suitable for remote siting. Control of the instrument is accomplished with a dedicated minicomputer, and data logging on digital magnetic cassette tape allows unattended operation for periods of 1-2 weeks.
The numerous potential applications of solar energy have been recognized widely for the past few decades. However, the sophisticated optical problems of an efficient collector, its integration with the overall system and the dominant economic constraints are not widely appreciated. This paper will briefly review the state-of-the-art of the non-tracking solar collectors with various performance criteria in mind. Also described in this paper are developments of some new components capable of producing high efficiency, lightweight and low cost non-tracking solar collectors. Windows with 'optical ribs' have been developed to yield high transmission, lightweight, ruggedness and efficient radiative and convective trapping. On the other hand, a new component termed 'optical valve' has been developed which is capable of transmitting the incident solar radiation and reflecting back to the absorber much of the emitted infrared radiation. Furthermore, an embossed absorber is shown to achieve almost complete absorption of solar radiation at large angles of incidence.
We describe a second generation central receiver solar thermal power conversion system that may offer more cost effective plant operation than plants presently under study for early POCE development. It is, however, technologically more risky. Our concept uses formed focusing heliostats more extensively, secondary reflection to a boiler at ground level, a flux concentrator at the boiler entrance, transpiration cooling of the boiler heat exchanger, and high energy density storage devices to smooth plant output. We report tests of some of the critical subcomponents in an actual intense solar environment. Finally, we summarize the potential system advantages for each element of the second generation plant.
Design and construction considerations are given for a 1.8 x 3.7 m cylindrical parabolic focusing collector having a concentration ratio = 28. Built of commonly available materials, this tracking collector is intended to feed, by the exchange medium of oil, an Abbot-type solar oven. At 80°C the collection efficiency is .33, declining to .05 at 260°C (500°F). The dependence of black coatings on receiver pipe efficiency is studied. The reflectivity of vacuum deposited aluminum, buffed aluminum plate and shiny aluminum foil are compared. The greatest improvement in performance has come about by placing the collector on a shade free roof. Abbot's mirror was shaded past 1 o'clock.
This is a general development of an equation to describe the optimum cavity-type stationary solar energy collector in two-dimensional space, and then the expansion into three-dimensional space. The intent is to establish a starting point for those who wish to do research in the area of concentrating collected solar energy. The final design of the collector is dimensionally optimized to collect and concentrate as much energy from the sun that is physically and economically feasible.
This paper describes the design and measured performance characteristics of a focusing solar collector that requires neither single nor dual axis tracking of the sun, but achieves solar concentration gains exceeding 2:1 for both direct and diffuse solar energy. Both sides of a conventional flat plate collector absorbing plate are used to collect solar energy using a compound cylindrical reflector and no thermal insulation is required in the collector, thus effecting significant material and fabrication cost economies. The focusing capability allows improved high temperature performance as compared to conventional flat plate collectors without requiring a selective absorber coating on the collector plate. Ray traces defining the collector's acceptance field of view for both direct and diffuse solar energy are presented and the theoretical and experimentally ob tained optical efficiency are compared. Measured results obtained from an instrumented prototype collector operating since February 1975 are presented, showing that a single collector collects approximately 50,000 Btus per average sunny day throughout the year. Applications of the collector as a transparent window or curtain-wall replacement in buildings are discussed, and large production quantity cost estimates are presented.
Itek's objective in the field of high temperature solar energy has been to develop an efficient and inexpensive method of collecting the sun's energy - ultimately for use in a solar-thermal-electric power plant. A unique and simple concept has emerged in the development process. On the basis of that concept, Itek received a grant from the National Science Foundation to conduct a Solar Power Collector Breadboard Test Program. *This paper describes Itek's collector approach and discusses the test program. A summary is given including the program objectives, hardware problems encountered, corrective actions taken, performance measurements and evaluation, and results of a preliminary operational system cost analysis.
A comparative technical evaluation of the four primary solar collecting configurations, namely, the central receiver, parabolic trough, paraboloidal dish, and "Winston" collector, is made. Subsystem and overall efficiencies are presented, and technical descriptions and performance characteristics are discussed. Comparisons are made on a solar thermal conversion basis. Applicability of the "Winston" collector for photovoltaic conversion is summarized. The most attractive candidate for solar thermal applications from a technical viewpoint is the central receiver configuration with the overall efficiency of 19.2%. The "Winston" collector is more applicable for photovoltaic conversion than for solar-thermal conversion.
We describe principles of design and experimental tests of the compound parabolic collector (CPC) concept for concentrating solar energy onto a receiver. Because of its wide angle of acceptance, the CPC can concentrate solar energy up to a factor ten without diurnal tracking of the sun, making it well suited for use in applications as a source of relatively high temperature (150 - 400°F) heat or an optical concentrator to reduce surface area requirements of photovoltaic cells.
A selective surface that absorbs the incident solar energy but suppresses its thermal reradiation can in some cases substantially improve the efficiency of a solar energy converter. In high-temperature applications, selective surfaces are shown to be most beneficial to systems using low or intermediate concentration ratios, although substantial economic savings can arise when used with large power systems having high concentration ratios. The optical performance and high temperature stability are discussed for coatings that rely on each of the five different methods for producing spectral selectivity. Each method has disadvantages as well as advantages, with most of the successful coatings using two or more of the methods. A particular coating that utilizes a silicon absorber over a silver reflector layer is described. The silicon is added by chemical vapor deposition (CVD) at elevated temperatures (>600°C). This is a unique application for CVD and has the potential for large-scale production in a flow-through system. At 500°C the solar absorptance and total normal emittance for these coatings are 0.76 and 0.07, respectively. They have withstood lifetime testing for 1000 hours of cycling (2/h) between 150°C and 450°C, plus 100 h continuously at 600°C with little degradation. Future improvements should allow the entire stack to be fabricated by CVD and at the same time achieve increases in both optical performance and high-temperature durability.
The shortfall between our nation's supply and demand for energy has created problems of critical proportions. One solution is to collect, retain, and utilize the energy of the sun. Thin films can be used to modify the amount of solar energy that is reflected, absorbed, or transmitted at various collector surfaces. Three types of films that have direct relation to solar energy are antireflection coatings (p < 2% per surface), selective absorber coatings (a = 0.96/s = 0.06), and transparent-low emittance coatings (T = 0.85/s = 0.14). Studies have shown that through the use of coatings the performance of flat plate collectors can be improved as much as 12.5% and the performance of linear focusing collectors can be improved as much as 22.5%. The resultant changes in the economic viability of the collector systems indicate that the coatings can make a positive contribution to the solar energy collection systems. The implementation of these systems is close at hand and now is the time to press for development.
For maximum effectiveness of solar-thermal conversion, a solar absorber must be a low emitter of thermal radiation. Thin film absorber stacks which possessed such spectrally selective characteristics were developed by the Bureau of Mines. The stacks consisted of zirconium suboxide or subnitride absorber films on a reflective film of silver or copper. The absorber films were prepared by reactively sputtering zirconium in argon with small amounts of oxygen, nitrogen, or both, or by oxidation of zirconium films in air. The solar absorptance and the thermal emittance were calculated from reflectance data. The solar absorptance, a(s), ranged from 0.60 to 0.93 and the emittance, e(T), at 600 K, ranged from 0.024 to 0.28. Ratios of a(s)/e(T) extended from about 2 to 28. The stacks appear relatively stable under vacuum at temperatures up to about 600 K.
The PbS-Al solar panel absorbs 0.90 of the incident solar energy at sea level while having a 258°C blackbody emissivity of 0.038. This value can be achieved in a reproducible manner on aluminum-coated aluminum substrates with a single layer of PbS evaporated in standard vacuum to a thickness of 1100 Α.
The metal-semiconductor solar cell is a possible candidate for converting solar to electrical energy for terrestrial application. In this paper, a method for obtaining optical parameters of practical anti-reflection coatings for the metal-semiconductor solar cell, is given. This method utilizes the measured index of refraction obtained from ellipsometry since the surface to be AR coated has a multi-layer structure. Both the experimental results and theoretical calculation of optical parameters for Ta205 anti-reflection coatings on Au-GaAs and Au-GaAs.78P.22 solar cells are presented for comparison.
This paper presents the effects of optical properties on the performance of flat plate solar collectors. Efficiency is presented for flat plate designs utilizing a single glass cover, two glass covers, two transparent plastic covers, and honeycomb between the cover and absorber plate. The performance of evacuated systems with a single glass cover are compared to the other designs. The effects due to variation in solar transmittance and infrared reflectance of the cover plate and in solar absorptance and emittance of the absorber plate are discussed.