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 the calibration. On March 30, 1976, twenty-nine n/p silicon and four p/n gallium arsenide solar cells were calibrated in space and successfully recovered from a NASA-Astrobee F rocket that reached a peak altitude of 230 km. Approximately 75 IV characteristic curves were generated for thirty-two of the cells to an accuracy of ±0.2 ma and ±0.2 mV. The short-circuit currents for these cells are presented for AMO and AM1.

This paper describes a development program which uses sunlight concentration techniques to effect an immediate reduction in cost-per-unit-power for photovoltaic systems in which solar cell cost dominates the total system cost. Current examples of concentrator solar cell technologies are single crystal silicon and gallium arsenide. Implementation of cost reductions by the use of sunlight concentration is not dependent on the development of low-cost, mass-production cell technologies but emphasizes high cell efficiency and low-cost concentrator systems.

Long-term survivability of photovoltaic arrays and components in terrestrial environments will require development of adequate protective systems. Highly considered are polymeric encapsulants, a method which was successfully employed in space and aerospace applications to protect critical electrical circuitry. To be employable, however, the polymer encapsulants must themselves be chemically and mechanically resistant to failure in terrestrial service. Chemical resistance includes stability to the degrading actions of ultraviolet light, oxygen, moisture and elevated temperatures in sun rich areas. Programs are underway to identify and develop chemically stable encapsulant candidates. Chemical considerations aside, mechanical failures of the encapsulants must also be avoided in array designs. This paper discusses design considerations for avoiding mechanical failures of polymeric encapsulants, with emphasis on biaxial properties, thermal fatigue, and anisotropy and nonhomogeneity of material properties. The general principles to be presented evolved from actual failures of polymeric materials in engineering applications. Also included are brief remarks on the permeability of polymer materials to atmospheric gases.

For terrestrial applications, the cost of photovoltaic power must be reduced by orders of magnitude. The usual approaches to low cost cell fabrication have many unsolved problems. Specially-constructed AlGaAs/GaAs heterojunction solar cells operated with large factors of solar concentration have reduced the projected cell cost per watt to an economically feasible level. The electronics for a two-axis run following tracking system are also sufficiently inexpensive. Therefore the ecomonic feasibility of a concentrator GaAs solar cell based terrestrial photovoltaic power system depends on the development of low cost optics capable of high values of solar concentration.

Molybdenum thin films have been produced by chemical vapor deposition with an emittance(calculatedat 500°C) which is at least as low as the emittance of the pure, bulk metal (0.069). Tungsten films deposited by CVD have an emittance of 0.045 as compared to 0.019 reported for the bulk metal. These experimental results indicate the potential usefulness of CVD Mo or W thin films as infrared reflectors in tandem layer, spec-trally selective, solar absorber systems. In addition, a spectrally selective CVD thin film which consists of a mixture of Mo and Mo03 phases has been deposited which has a solar absorptance of 0.74 and a calculated emittance at 500°C of 0.063.

Wavelength-selective surfaces for solar energy utilization can be divided into two classes: selective-black absorbers and transparent heat mirrors. Selective-black absorbers absorb solar radiation and have low infrared thermal emis-sivity; transparent heat mirrors transmit solar radiation and reflect thermal radiation. We have used rf sputtering to prepare both selective-black absorbers (Cr-black and MgO/Au on metal substrates) and transparent heat mirrors (Ti02/Ag/Ti02, Sn-doped 1n203, and Sn-doped 1n203 microgrids) with excellent wavelength-selective properties for solar energy applications such as solar heating, solar/thermal/electric conversion, and window insulation.

Various types of coatings have been considered as selective surfaces for solar photothermal energy collectors. These include the semiconductor overcoatinq of a metal, the lossy cavity, the artificial dielectric, the graded profile medium and the infrared reflector. These coatings are briefly described. Estimates are made of the cost of producing them en masse via electroplating, chemical deposition, sputtering, or vacuum evaporation.

A new black nickel formula was developed which had a solar absorptance of 0.92 and an infrared emittance (at 100° C) of <0.10 after 14 days at 38° C (100° F) and 95 percent relative humidity. This compares to coatings prepared with a previous NASA formula which had their solar absorptance degraded from 0.90 to 0.72 after the same period of humidity exposure. The electroplating bath and conditions were changed to obtain the more stable coating configuration. The effect of bath composition, temperature, pH, and plating current density and time on the coating composition, optical properties and durability were investigated.

The testing and rating of solar collectors are the subject of these brief remarks. The papers to be presented today are concerned ultimately with the end product and its efficiency in converting incoming radiation into a useful form, primarily heated fluids at useful temperature levels.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), working through a Task Force appointed from Technical Committee 6.7, Solar Energy Applications, has completed and is now ready to distribute Proposed Standard 93-P, "Method of Testing for Rating Solar Collectors Based on Thermal Performance". This document is based on an earlier draft prepared in 1974 by J. E. Hill and Tamami Kusuda of the National Bureau of Standards for the National Science Foundation which then was the Federal agency responsible for solar energy research. The Hill-Kusuda proposal was carefully reviewed by competent groups drawn from ASME, ASHRAE and the International Solar Energy Society (ISES) and a number of changes were made by the authors before their proposed test procedure was published in December, 1974 as NBSIR 74-635. This proposal has already been used widely and specifications are being written requiring that it be followed in testing collectors that are being offered for sale to government agencies. However, NBSIR 74-635 is not an officially adopted standard and it will undoubtedly be superceded by ASHRAE Proposed Standard 93-P which is the subject of the following paper. 93-P, prepared by a group of specialists drawn from universities, government agencies, private testing laboratories, manufacturers and users, provides for outdoor testing under the sun and indoor testing using a solar simulator. It also covers methods of determining collector time constants and incident angle modifiers. It is equally appropriate for use with concentrating and with flat plate collectors. It is anticipated that the new Standard will be issued early next year, following the usual review procedure, bearing the identifying number 93-77 to designate that it is the 93rd ASHRAE Standard and that its year of adoption is 1977.

Interest and activity in solar energy as an alternative source of energy are rapidly increasing the world over, and this is likely to continue for years to come. No solar energy system, if it is to be efficient and economic, can be designed or constructed prior to detailed engineering study and a complete systems analysis. All such analytic work requires a detailed and accurate knowledge of insolation and other parameters. For efficient computer analysis these parameters should be expressed in the form of a probabilistic insolation model based on precision measured data over extended periods of time. Central solar station siting surveys also require accurate measured data, as do efficiency verifications and demand-load analyses. The present lack of accurate historic insolation is due mainly to the lack of precision insolation instrumentation for field work. This paper describes a precision insolation instrument developed for field survey work and for use as a calibration transfer standard. Two of these instruments have operated in the field for two years and a calibration stability analysis over this period of time is discussed.

The efficiencies of thermal collectors are traditionally expressed with respect to the total (direct beam plus diffuse) insolation at the collector plane. Evaluation of the performance of concentrating collectors, however, requires, in addition, a knowledge of the insolation which is within the angular field of view of the collector. The use of pyranometers to measure the total insolation as well as the insolation within specified angular fields of view, together with a normal incidence pyrhe iometer to measure the direct beam component, is described. The instrument calibration procedure is discussed, and several data are presented.

The current development of large focusing heliostats to concentrate solar energy on a boiler or central receiver requires a method for measuring the overall efficiency/performance of the heliostat. A flat plate calorimeter, using the temperature rise in water at a measured flow rate, has been used to test a 22.3 - square meter heliostat focused at a distance of 31 meters. These measurements represent the first test of a large area concentrating heliostats in this country. This type of testing also established the requirement for verifying the accuracy of the collected calorimeter data. This paper presents one of the methods used to accomplish this verification. The subject method to be discussed is the use of an AGA Thermovision Model 665 for determination of temperature gradients and flux patterns on the water filled plates of the calorimeter both with and without solar beam impingement. The initial thermovision testing has established that the infrared television can be a potentially valuable tool in the assessment of flux patterns, measurement of losses within the central receivers, and eventually for determination of energy concentrations within a reflected solar beam.

With strong governmental support and broad based public approval, the United States solar energy program has expanded rapidly in the last few years. In particular, we have traced the history and development of the solar tower from an idea in 1969 through first federal funding in 1973, to a program to initiate pilot plant construction in 1976. About 3,000 heliostats will reflect sunlight onto a central receiver, in which 500°C steam will be generated to drive a turbo-generator. With this 10 MWe plant scheduled to feed electricity into a utility grid in 1980, a 50-100 MWe demonstration plant is proposed to be on line in 1985.

We describe an approach to the analysis of the optical system of the solar tower concept, which is designed to provide maximum design information. By inputing all quantities in terms of angles or dimensionless ratios the results remain relatively scale independent. First the power redirected from the heliostat field to the central receiver is computed, then solar images are projected from each heliostat to the central receiver and interception factors are obtained. This information allows us to size the central receiver, to enhance performance by redistributing heliostats in the field, and finally to define the rim angle of the field. The procedure for this step by step definition of the solar tower optical system results in optimal heliostat fields, receiver flux distributions and diurnal power curves.

A solar concentrator system is described which is based on the use of a long array of east-west mounted focusing reflectors. Each of the reflective segments projects a line image of the sun into the entrance aperture of a stationary receiver through which a working fluid flows. Typically, ten segments in parallel are initially aligned to focus on the same receiver aperture; the mean solar flux across the aperture can be on the order of forty times that falling on the mirrors. The mirrors are steered by linked rotation about their individual centers to track the sun in elevation. Simple logic is also incorporated to rotate the mirrors into a face-down position on lack of sun or lack of demand. This provides for protection during storms and at night, and for defocusing in case the system overheats. The system is simple, well suited to installation and maintenance operations, and offers the potential for high efficiency in the intermediate temperature range at a low life-cycle cost.

A fixed segment of a concave spherical mirror can be used to concentrate beam radiation onto a tracking absorber which pivots about the center of curvature of the mirror. A possible economic advantage of this solar collector over concentrating collectors with tracking mirrors is reduced mirror cost. The objective of the investigation is to determine the potential of this system for electrical power production. Special emphasis is given to identifying the penalty associated with the fixed reflector. The results showed appreciable cosine losses even at the best times of the year. The overall system efficiency was found to be strongly dependent on the rim angle, the optical efficiency, the absorber temperature, and the degree of selectivity of the absorber surface.

A new concept for a fixed-mirror solar concentrator is described. The fixed, stepped surface of the proposed cylindrical mirror is designed to produce a sharply focused line image regardless of the incident sun direction. This is in contrast with the severe off-axis aberration (hence, poor concentration factor) of a parabolic mirror for all sun directions other than on-axis. It is shown that 1) the heat absorber follows a circular path to remain in the focus as the sun moves, 2) the annual light losses from self-shadowing of the "stepped structure of the mirror surface can amount to less than %in for appropriate mirror orientation, 3) by facing the mirror in a southerly direction in the northern hemisphere) , the winter deficit in sun input can be significantly compensated, and 4) practical concentration factors of around 50 can be achieved with a single stage of concentration, 80 with a second stage of concentration.

This paper describes various configurations of solar collectors and components employing the phenomenon of total reflection. A double glazed window using "Optical Ribs" has been developed to yield high light transmission and ruggedness. Conical wedges are developed for use as energy concentrators as well as "Optical Valves" Dielectric compound concentrators with flat, parabolic, and elliptical reflecting surfaces will be described with various applications.