Showing papers on "Solar energy published in 1972"

Solar radiation and productivity in tropical ecosystems

Abstract: In thermodynamic terms, ecosystems are machines supplied with energy from an external source, usually the sun. When the input of energy to an ecosystem is exactly equal to its total output of energy, the state of equilibrium which exists is a special case of the First Law of Thermodynamics. The Second Law is relevant too. It implies that in every spontaneous process, physical or chemical, the production of 'useful' energy, which could be harnessed in a form such as mechanical work, must be accompanied by a simultaneous 'waste' of heat. No biological system can break or evade this law. The heat produced by a respiring cell is an inescapable component of cellular metabolism, the cost which Nature has to pay for creating biological order out of physical chaos in the environment of plants and animals. Dividing the useful energy of a thermodynamic process by the total energy involved gives a figure for the efficiency of the process, and this procedure has been widely used to analyse the flow of energy in ecosystems. For example, the efficiency with which a stand of plants produces dry matter by photosynthesis can be defined as the ratio of chemical energy stored in the assimilates to radiant energy absorbed by foliage during the period of assimilation. The choice of absorbed energy as a base for calculating efficiency is convenient but arbitrary. To derive an efficiency depending on the environment of a particular site as well as oil the nature of the vegetation, dry matter production can be related to the receipt of solar energy at the top of the earth's atmosphere. This exercise was attempted by Professor William Thomson, later Lord Kelvin, in 1852. 'The author estimates the mechanical value of the solar heat which, were none of it absorbed by the atmosphere, would fall annually on each square foot of land, at 530 000 000 foot pounds; and infers that probably a good deal more, 1/1000 of the solar heat, which actually falls on growing plants, is converted into mechanical effect.' Outside the earth's atmosphere, a surface kept at right angles to the sun's rays receives energy at a mean rate of 1360 W m-2 or 1f36 kJ m-2 s-1, a figure known as the solar constant. As the energy stored by plants is about 17 kJ per gram of dry matter, the solar constant is equivalent to the production of dry matter at a rate of about 1 g m-2 every 12 s, 7 kg m-2 per day, or 2 6 t m-2 year-'. The annual yield of agricultural crops ranges from a maximum of 30-60 t ha-' in field experiments to less than I t ha-' in some forms of subsistence farming. When these rates are expressed as a fraction of the integrated solar constant, the efficiencies of agricultural systems lie between 0-2 and 0 004%, a range including Kelvin's estimate of 0-1%. Conventional estimates of efficiency in terms of the amount of solar radiation incident at the earth's surface provide ecologists and agronomists with a method for comparing plant productivity under different systems of land use and management and in different * Opening paper read at IBP/UNESCO Meeting on Productivity of Tropical Ecosystems, Makerere University, Uganda, September 1970.

Aerosol and solar radiation in Britain

Abstract: SUMMARY The irradiance of the solar beam was measured on cloudless days at Sutton Bonington in the English Midlands and at sites in north-west Scotland. Total and diffuse fluxes were also measured on some days. An attenuation coefficient for aerosol T. was defined by S(T~) = S(o) exp ( - Ta m) relating the measured flux at normal incidence S(T.) to the Rux calculated for adust-free atmosphere when the air mass number is m. Changes of T. from day to day were related to changes of air mass origin; local sources of aerosol were relatively unimportant. In maritime air, ra ranged from 0.05 to 0.15, and in continental air, from 0.1 to 0.5. In a tropical maritime air mass, T~ decreased from 0.13 at sea level to 0.07 at 1,340 m. The fraction of (ultra-violet f visible) to total radiation was (0.54 - 0.28 7.) and the ratio of diffuse to total radiation (m < 2) was (0.1 + 0.7 7.). The ratio of total scattering to absorption by aerosol decreased from 4 at m = 1.1 to 0.5 at m = 2. Mean monthly values of T. at four Meteorological Office stations were calculated from records of solar radiation and hours of sunshine and corresponding values of total and diffuse flux were tabulated for ‘ isolated ’, ‘ rural ’ and ‘ urban ’ sites. The presence of solid particles in the Earth’s atmosphere has important consequences for the transmission of solar radiation and for the nature of the radiation rkgime at the ground. The absorption of solar energy by a layer of aerosol increases the radiative heating of the atmosphere and decreases the amount of energy available at the surface. Scattering by aerosol increases the amount of radiation which is reflected by the atmosphere into space and increases the downward flux of diffuse radiation at the Earth’s surface. Attenuation also produces changes in the spectral composition of solar radiation which are significant biologically. To estimate the amount of radiant flux which is absorbed and scattered by aerosol as distinct from other atmospheric constituents, measurements of direct and diffuse radiation at, the ground may be compared with the fluxes predicted below a model atmosphere containing appropriate amounts of ozone, water vapour, and carbon dioxide (G. D. Robinson 1962, 1966). The height distribution of aerosol can be inferred by measuring solar radiation from aircraft (Roach 1961), and the presence of particles as high as 50 km has been demonstrated by measuring the scattering of light from searchlight beams (Elterman, Wexler and Chang 1969). Concern about possible changes in global climate has stimulated new interest in the radiative effects of aerosol. Recent calculations by Rasool andSchneider (1971) imply that any future increases of aerosol content will decrease the mean surface temperature of the Earth and that the heat balance of the atmosphere may become increasingly sensitive to changes of aerosol content. To be able to detect the radiative effects of changing aerosol content, it is essential to establish baselines for the income of solar radiation in different parts of the world and to show how this income is related to aerosol load. This kind of exercise has been attempted at relatively few sites. Valko (1963) analysed turbidity measurements at LocarnoMonti in Switzerland and demonstrated marked daily and annual changes in the strength of the direct solar beam which he ascribed to differences in the composition of aerosol in different air masses. Flowers, McCormick and Kurfis (1969) reported measurements of turbidity from a network of stations in the USA equipped with sun photometers. These instruments record the irradiance of the direct beam at a wavelength of 0.55 pm but they

A Proposed New Concept for a Solar-Energy Converter

Abstract: The potential advantages of terrestrially utilizing nonpolluting solar energy are well-known. One of the more promising approaches to achieving the desired high solar-electrical conversion efficiency is the direct process, avoiding Carnot efficiency limitations of heat engines. The chief present-art example is the large area silicon solar cell based on utilization of quantum properties of light. In the United States it is now an established technology created from the space program. This paper presents the results of some preliminary new research exploring the possibilities of creating high efficiency solar-electricity converters utilizing wave-like properties of radiation interacting with absorber-converter elements. The concept is revealed for what is believed to be a new, unique, and potentially useful pyramidical solar radiation absorber-converter structure. It is based on the possibility of extending concepts of power absorbing antennas and converters to the visible light range. The resulting proposed converter structure would have a rough surface texture and yield a d-c output. It may have, if subsequently researched, significant efficiency, cost, and fabricating advantages, particularly for large-scale terrestrial utilization of solar energy. The concept is called an Electromagnetic Wave Energy Converter (EWEC). Concept validity evidences of a preliminary kind at both microwave and near light wavelengths are presented.

High‐efficiency Ga1−xAlxAs–GaAs solar cells

Abstract: Heterojunction solar cells consisting of pGa1−xAlxAs–pGaAs–n GaAs are grown by liquid‐phase epitaxy and exhibit power conversion efficiencies of over 16% (corrected for contact area) measured in sunlight for air mass 1 at sea level, while efficiencies of 19–20% are obtained for an air mass value of 2 or more. The improved efficiencies compared to conventional homojunction (Si and GaAs) cells are attributed to the reduction of series resistance and the reduction of surface recombination losses resulting from the presence of the heavily doped Ga1−xAlxAs layer. Open‐circuit voltages of 0.98–1.0 V and short‐circuit currents of 18–21 mA/cm2 (corrected for contact area) are observed for a solar input intensity of 98.3 mW/cm2.

Free Oscillations of the Sun and Their Possible Stimulation by Solar Flares

Abstract: The main characteristics of free oscillations of the sun are described, and the sources and sinks of energy are estimated. Adiabatic oscillations and damping are considered. It is shown how a large solar flare can exert a significant mechanical impulse on the sun by causing a wave of compression to move subsonically into the interior. The solar interior does not easily dissipate low-amplitude acoustical energy, and therefore a relatively large fraction of the energy should remain available to go into the normal modes. An estimate is made of how this energy might be distributed among the more interesting modes.

Fiber optical solar collector

Abstract: The invention is an improved solar collector comprising a fiber optical solar receiver which passively concentrates incident solar energy for delivery as an intensified flux to an absorptive target. In one embodiment of the invention, the present solar collector comprises a boule of fibers shaped into an arcuate collecting surface at one end, the fibers tapering to a flat output plane at the opposing end of the boule. Solar radiation entering the collector at the collecting surface is concentrated within the tapered portion of the boule and delivered as an intensified flux to an absorptive target disposed in operative relation to the output plane, the absorptive target being either a cookpot, a thermal storage mass, or the hot junction of a thermoelectric generator.

Extended study of a simplified mathematical model for predicting the nocturnal output of a solar still

Abstract: Abstract The production capacity of a solar still which converts saline water to fresh water can be increased by introducing hot feed water into the unit at night. A waste heat source, such as cooling water from a power plant, can be used to preheat the feed. The nocturnal production, i.e. the distilled water produced at night, seems to be influenced by several parameters. However, a simplified mathematical model suggests that the distillate depends only on the initial brine temperature, the drop in brine temperature and the brine depth. This was experimentally verified for different brine depths and for initial brine temperatures up to 150°F.

Physics looks at solar energy

Abstract: The idea of using the sun as a source of energy has had a long history, but so far it has been a history of bright hope and dismal failure. In the middle 1950's newspaper headlines were full of glowing predictions of what solar energy could do for mankind; the first International Conference on Applied Solar Energy had been held, and solar energy seemed ready to take its place, along with peaceful uses of atomic energy and with interplanetary exploration, on Vannevar Bush's “endless frontier of science.” And now in the 1970's nuclear power reactors and spaceflight are realities, yet solar energy, as recently as a year ago, was dismissed by a National Academy of Science–National Research Council committee as of no importance in our future—despite the admitted “energy crisis” looming ahead. Whatever happened to the grand predictions? Our search into the history of solar energy started with this question, because we were curious to know if 1970 technology might yield a different result.

Schottky barrier diodes for solar energy conversion

Abstract: Several Schottky barrier solar cells were fabricated by evaporation and sputtering of Al ohmic contacts and Cr or AuCr alloy barrier metals on 0.5-10.0 2 Ω ċ cm p-type silicon. Potential efficiencies of 4.8 to 12 percent were observed which would be realized with improved fill factors. Computer studies of the optical problem indicate an output power increase by a factor of four through the use of reduced barrier metal thickness (from 275 to 100 A) and alloy barrier metals to more effectively transmit solar energy to the Schottky junction.

Solar heat source and receiver system

Abstract: A system for collecting solar heat to meet high power heat requirements as in a Vuilleumier cycle cryogenic refrigerator, in space application. The system includes a short focal-length-to-diameter ratio collector, a heat receiver having a convex surface attached to a heat pipe further containing a heat storage element which is connected to the use, i.e. a cryogenic refrigerator. The heat receiver utilizes a novel coating allowing the system to realize an effeciency in excess of fifty percent.

Solar cell array

Abstract: A solar cell planar array fabricated by a method from the solar cells to the mounting panel. An electrically insulative, thermally conductive wafer is soft-soldered to the surface of the metallic mounting panel. The top surface of the wafer bears spaced electrically insulative, thermally conductive prominences thereon. The solar cells are attached on top of the prominences.

Economic Implications of the ‘Green Revolution’ and the Strategy of Agricultural Development in West Pakistan

Abstract: The short-stemmed varieties of wheat and rice imported from abroad and the increased use of fertilizers have dramatically enlarged the potential for rapid increases in the agricultural output of West Pakistan. This recent breakthrough in food-grains production is sometimes referred to as the ‘green revolution’. Because of the generally favourable conditions in West Pakistan in regard to irrigation water and solar energy, and of the unusually favourable weather in 1967/8 in particular, the ‘green revolution’ is spreading very rapidly. It is high time to focus our attention on some of the economic implications of the new developments in agriculture.

Evaluating the light from the Sun

Abstract: Recent measurements from high altitude observing platforms have produced evidence that the values regarding the energy from the sun and its spectral distribution need significant revisions. In all ground-based measurements, the dust, haze, and smoke of the atmosphere, the permanent gases and, above all, the highly variable and absorbent water vapor are a source of uncertainty. With the advent of the space age the need for a more reliable set of values of the solar constant and the solar spectrum began to be realized. Measurements for the derivation of the solar constant were conducted with aircraft, balloons, and spacecraft. Values for solar spectral irradiance were also obtained.

An assessment of solar energy as a national energy resource

Abstract: The applications are discussed of solar energy for thermal energy for buildings; chemical and biological conversion of organic materials to liquid, solid, and gaseous fuels; and the generation of electricity. It is concluded that if solar development programs are successful, building heating for public use is possible within 5 years, building cooling in 6 to 10 years, synthetic fuels from organic materials in 5 to 8 years, and electricity production in 10 to 15 years.

Solar constant and the energy distribution in the solar spectrum

Abstract: Available data on the solar energy flux and its spectral components are reviewed and critically analyzed. On the basis of extensive measurement data obtained by various methods, it is shown that the most reliable value of the solar constant is 195 cal/sq cm-sec (or 136 mWt/sq cm). Weighted-mean data on the energy distribution in the solar spectrum from 1400 A to 0.3 mm are presented. Among the topics studied are: the attenuation of radiation in the Earth's atmosphere, instrument probiems in absolute spectrophotometry, the influence of limb darkening of the soiar disk on the energy distribution in the solar spectrum, and the energy distribution in the spectrum of the solar photosphere. (IAA)

Heat or cold and dry storage

Abstract: Storage apparatus is used for storage of large quantities of heat, or for large quantities of "cold," and "dryness," or for simultaneous storage of heat and cold, and dryness. The heat, or cold, or both, may be produced during off-peak power periods, or by solar energy, or both. One example of utility of the invention is for use during the winter to store solar produced heat, or during the summer for cold storage produced by solar energy or by nighttime off-peak electric power. And, for springtime or autumn weather, the apparatus is usable to simultaneously store heat and cold. Then, for a chilly night or a cold spell, stored heat is available, or for a hot evening or a hot spell, stored coldness is available. Thus, the system for climate control of the home or other space may be switched from heating to air conditioning instantly. Zone heating and air conditioning are provided. Also low velocity, gentle currents of warm air are provided for winter heating, and high-speed cooling breezes of air for summer cooling. Further, if no fuel could be obtained for auxiliary heat, or other unfavorable conditions were to limit the heat supply, one portion or Zone may be heated and another left unheated to conserve available heat.

Climatic Effects of Aerosol Layers in Relation to Solar Radiation

Abstract: The effect of the introduction of one or two layers of particulate matter on the heating by direct solar radiation of the earth surface-atmosphere system is calculated for a cloudless sky. It is found for a fairly wide range of absorption and backscatter coefficients of the particles with respect to.direct solar radiation that when we have but one particle layer, either near the surface or in the lower stratosphere, the combined system is cooled; the atmosphere is heated (for a finite absorption) but this heating is offset by the greater cooling of the surface. Thus, our conclusions are different in some respects from those advanced by Charlson and Pilat and, to a lesser extent, from those inferred by McCormick and Ludwig. If we have two layers of particles, one near the surface and the other in the lower stratosphere, and if we assume that both layers have the same optical characteristics with regard to solar radiation, then the earth surface-atmosphere system may actually gain heat provided tha...

Method of controlling solar heat and light in green houses

Abstract: A method of controlling the solar heat effect in glass and plastic covered greenhouses, and other windowed structures, consists of coating the greenhouses with an adhesive film consisting essentially of synthetic silicon dioxide having an average particle size of less than 10 microns and a film forming plastic resin. The film is translucent when dry but becomes transparent when wet. The film also extends the useful life of plastic coverings by reducing ultraviolet radiation.