Incident Solar Flux Research Articles

Solar flux incident on an orbiting surface after reflection from a planet

Algorithms describing the solar radiation impinging on an infinitesimal surface after reflection from a gray and diffuse planet are derived. The following conditions apply: only radiation from the sunny half of the planet is taken into account; the radiation must fall on the top of the orbiting surface, and radiation must come from that part of the planet that can be seen from the orbiting body. A simple approximate formula is presented which displays excellent accuracy for all significant situations, with an error which is always less than 5% of the maximum possible reflected flux. Attention is also given to solar albedo flux on a surface directly facing the planet, the influence of solar position on albedo flux, and to solar albedo flux as a function of the surface-planet tilt angle.

100 – 1000 kW(el) medium-power distributed-collector solar system

A systems analysis of medium-power 100 – 1000 kW(el) heliothermal stations was performed to minimize kWh(el) costs. The following design choices resulted: distributed concentrating collectors; collection of heat by organic heat-transfer fluid permitting storage at acceptable cost; diurnal-nocturnal storage to permit 24-hour generation of electricity from the heat absorbed during periods of insolation; a turbogenerator utilizing heavy organic fluid well-suited to the temperature of the heat source (200 – 250 °C) and the desired power level. Component development has given rise to the construction and testing of prototype modules of the collectors and to a pilot storage and turbine installation. The test results compare favourably with the theoretical performance: the thermal flux at the ends of the collector is equal to 40 – 50% of the incident solar flux at an operating temperature of 200 °C; the mechanical energy produced by the turbogenerator is greater than 20% of the thermal energy supplied to the heat engine. Two direct spin-offs of this project appear promising: the collector and storage system together form a medium-temperature (100 – 250 °C) heat generator which should find applications in the production of industrial heat, for example for food-processing plants; the heat engine can be employed not only with solar heat but also with heat recovered in factories having excess heat at 100 – 250 °C.

Effects of anisotropic multiple scattering on solar radiation in the troposphere and stratosphere

Accurate determination of photodissociation rates in the troposphere and stratosphere in the 280-800-nm wavelength range requires calculation of the effects of anisotropic multiple scattering, pure absorption, and ground reflection on the solar flux incident at the top of the atmosphere. Past attempts to model these effects have led to approximate, sometimes time-consuming, models whose reliability is unknown. A general algorithm has been developed which includes (1) Rayleigh and Mie scattering by molecules and aerosols, (2) ground reflectivity, and (3) pure absorption by trace constituents such as ozone. Selected comparisons with existing approximate calculations are discussed, and a region of validity in wavelength and optical depth for isotropic scattering is presented.

The seasonal‐latitudinal variation of exospheric helium from He 584‐A Dayglow emissions

The transport of solar 584‐A photons through the spherical, asymmetric helium geocorona is discussed in detail. The analysis shows that with a favorable geometry and a moderately optically thick atmosphere it is possible to determine the helium abundance from dayglow observations without knowing the incident solar flux or the instrument calibration. Comparison of theoretical intensities with 584‐A airglow observations made from the STP 72‐1 satellite shows that recent satellite‐drag and mass spectrometer‐incoherent scatter empirical models predict helium densities which have seasonal‐latitudinal dependences which approximate the airglow‐determined densities in shape, but in general are high by as much as a factor of 2 in absolute density. Airglow‐determined densities in the polar regions suggest a decrease in the exospheric helium density relative to the empirical model estimates.

An analysis of the infrared continuum of comets. II - Comet Kohoutek

An analysis of the infrared observations of comet Kohoutek has been carried out on the basis of silicate grain models. The 'observed' grain temperature has been obtained as a function of the heliocentric distance, and it is compared with the expected distribution. In the visible region, one requires for the refractive index, (m = n-ik), n of about 1.6 and k less than 0.05. The total number of grains varies essentially as the inverse square of radial distance; the size of the particles in the antitail of the comet Kohoutek is greater than 5 microns. The ratio of infrared flux from grains to the incident solar flux at wavelengths of a few microns is calculated.

Mid‐ocean observations of atmospheric radiation

AbstractMid‐ocean (35°N 155°W) observations of the various components of radiative flux were made from the R/P FLIP during the period 2 through 13 February 1974. Cloud cover ranged from clear skies to overcast, and water vapour pressure varied between 9 and 18 mb, with sea surface temperature near 15.0°C.The net longwave radiative fluxes reported here were obtained: (1) by taking the difference between simultaneous measurements of net all‐wave and net solar fluxes; and (2) by direct measurements with a net longwave radiometer designed by G. W. Paltridge. When observations during rain and fog are excluded, the difference between night‐time 15‐minute averages by the two methods is generally less than lmWcm−2. During day‐time, indirect measurements are often larger than direct by about 5mWcm−2; from internal evidence we prefer the direct values.The albedo of the sea surface was calculated from simultaneous measurements of downward and upward solar fluxes. The observations were analysed to represent albedo as a function of solar altitude and atmospheric transmittance, following the work of R. E. Payne; our results suggest that Payne's smoothed representation is suitable for use over the open ocean. Albedo was observed to decrease with increasing wind speed for clear skies with solar altitude between 15 and 30 degrees but no variation was discernible at higher solar altitudes.Empirical formulae for calculating both shortwave and net longwave components of the radiative flux were compared with measurements. A formula due to F. E. Lumb for determining the incident solar flux given solar altitude, cloud amount and cloud type, consistently yields good agreement with the measurements, within about lmWcm−2. Daily averages of net longwave flux calculated from several empirical formulae using a linear correction for clouds are within 2mWcm−2 of the observations reported in this paper. Since daily radiation balance values were measured as only 5mWcm−2, the limitations of the best current empirical formulae are evident.

A theoretical study of laminar free convection in 1-D solar induced flows

The effectiveness of natural circulation and ventilation systems based on the “solar chimneys” principle has been tested for a number of years at numerous sites, but thermal and fluid mechanical aspects of their operation need to be fully understood before conclusive results can be supplied; a long term research work has therefore been undertaken by the authors, including both experimentation in two test sites in Italy and numerical simulation. In the present paper the heat transfer equation for one dimensional laminar flow is numerically solved with the appropriate radiative boundary conditions. Temperature fields calculated in this way are then employed to evaluate air density variations and consequently the upward driving force available to overcome head losses in the air circulation. Optical plate spacing in steady state conditions appears to be strongly dependent on head losses occurring outside the collecting section and on winter or summer use of the system; it shows a softer dependence on incident solar energy fluxes.

The radiative effect of atmospheric pollution: Latitudinally dependent radiation calculations including cloud contribution

In this paper, we have developed a radiation scheme based on the discrete-ordinate method into which a comparatively thick cloud layer can be incorporated for inhomogeneous aerosol atmospheres. Using the above radiation scheme, we performed calculations of the effect of clouds upon the inhomogeneous aerosol atmospheres, including the effects of optical depth, vertical distribution and extent of cloud layer. Calculations were also carried out for the local albedo, total absorption and diffuse transmission at each latitude belt, considering a land-sea distribution and a latitudinal variation of cloudiness, for six realistic model aerosol atmospheres. A 0.50 μm flux of the solar radiation is used for this study. The main conclusions of the radiative calculations may be summarized as follows. 1. (1) For the same optical thickness, a densely stratified cloud layer within the lower aerosol troposphere is apt to reflect the solar radiation much less effectively than other stratifications of cloud. If cloud layers are present at higher levels, absorption of solar radiation within the atmosphere would decrease considerably. 2. (2) Cloudiness and/or cloud thickness play a very important role upon the global heat balance problems and should never be ignored in studying the effects of increased aerosols upon climate, because contrary to the cloudless condition, heating of the earth-atmosphere system would tend to be induced by an increase of aerosols in the atmosphere where clouds are present. Because of the high surface albedo, an increase of aerosols reduces the reflectivity at the snow-covered high latitude belts, regardless of the effects of cloud. 3. (3) By the perturbation of adding aerosols into the troposphere or the stratosphere the diffuse transmission increases at the ground level. However, this effect is offset by the direct exponential attenuation of solar flux, and as a result, the total solar radiation reaching the ground is somewhat reduced. 4. (4) The absorption of solar radiation within the atmospheres due to aerosols would reach near 10% of incident solar flux for injection of a fairly massive amount of aerosols into the troposphere or the stratosphere due to great volcanic eruptions or man's impact.

Selective surfaces for solar-thermal conversion

Selective surfaces are used to obtain high absorption of the incident solar flux and simultaneously suppress losses due to thermal radiation from the hot converter surface. Whether or not the reduction of these losses leads to more economical energy production depends on several factors, the most important being the cost of the selective surface and the concentration ratio of the collector system. The allowable limits for the costs of a selective surface are derived for examples of a flat plate hot water heater, a distributed collector, and a central receiver system. The several basic methods for obtaining selectivity are discussed. Material science and physical chemistry enter strongly into their further development and reduction in costs. Even so, the best estimates for fabrication costs using presently available mass production techniques indicate that selective surfaces can be economically competitive for each of the discussed applications.

Experimental Evaluation of a Cylindrical Parabolic Solar Collector

Results are presented for a series of solar collector experiments in which the incident solar flux was concentrated by a single-axis tracking parabolic trough mirror. The concentrated solar flux was directed onto an absorber tube whose axis coincided with the focal axis of the concentrator. The performance of the collector was evaluated using three different absorbers; a black painted tube designed to operate near ambient temperature, a heat pipe which had a selective solar absorber coating applied to its surface, and a heat pipe which had its surface coated with a nonselective black paint. The peak efficiency for the collector in the absence of heat losses is approximately 62 percent when the incoming solar energy is normal to the collector aperture. The heat losses which occurred at elevated temperatures (300°C) decreased the peak efficiencies to 50 and 30 percent, respectively, for the selectively coated and black painted tubes. The experimental results establish the technical feasibility of using parabolic trough collectors for applications requiring thermal energy at temperatures up to 300°C.

Solar radiation incident upon slopes of different orientations

By means of a pyranometer oriented at various tilt angles and azimuth angles with respect to the Sun, measurements of the direct and diffuse solar flux incident on slopes of various orientations have been made for clear sky conditions. The resulting data have been used to evaluate the validity of the solar flux model of Robinson[1] and to introduce certain modifications of that model to achieve better agreement of computations with measurements. It is concluded that the modified model yields significantly better results than the original for slopes oriented in the azimuth of the Sun, but results are comparable for other azimuthal orientations of the slope. The accuracy attainable by this modeling approach is considered sufficient for many practical applications.

On the Absorption, Reflection and Transmission of Solar Radiation in Cloudy Atmospheres

Abstract Band-by-band calculations have been carried out to evaluate the reflection, absorption and transmission of solar radiation by cloud layer and model cloudy atmospheres in the entire solar spectrum. The radiation transfer program is based on the discrete-ordinate method with applications to inhomogencous atmospheres. The gaseous absorption in scattering atmospheres is taken into account by means of exponential fits to the total band absorption based on laboratory measurements. Thick clouds such as nimbostratus. and cumulonimbus reflect 80–90% and absorb 10–20% of the solar radiation incident upon them. The reflection and absorption of a fair weather cumulus with a thickness of 0.45 km are about 68–85% and 4–9%7p, respectively. A thin stratus, whose thickness is 0.1 km, reflects about 45–72% and absorbs about 1–6% of the solar flux incident on the cloud top. The reflection of a 0.6 km thick altostratus is about 57–77%, with a larger absorption of 8–15%. A number of aircraft observations reveal that ...

Gas Phase Chemistry in Comets

Present theories for the formation of molecular species observed in comets predict the sublimation of parent molecules such as H2O, CH4, CO2, and NH3, from the surface of the nucleus and their subsequent photodissociation and ionization to form the observed species (Delsemme 1973). It can be shown (Oppenheimer 19 75) that gas phase chemical reactions occur between these fragments which have characteristic timescales which are short compared to the timescale for significant variation in the solar flux incident on the comet. Hence, a steady-state approximation may be used for determining the densities of many species. It can also be shown (Oppenheimer 1975) that the rate of formation of many species is faster by gas phase reactions than by photoprocess. For instance, the formation of OH+ from H2O+ by the reactionH2O+ + O + → OH+ + OH

Thick semiconductor films for photothermal solar energy conversion

Efficient and economical photothermal conversion of solar energy requires the use of spectrally selective surfaces for collection and retention of incident solar flux. This spectral selectivity can be obtained from an absorber–reflector tandem by overcoating an opaque metal film having high infrared reflectance with a thick film (1–5μm) of semiconductor having an appropriate band gap. The practical implementation of this design using films of silver and silicon on a variety of substrate materials is described. Chemical vapor deposition (CVD) is used for depositing the absorber and the antireflection layers. The spectral reflectance of coated samples has been measured at temperatures up to 500 °C and gives α/ε values in the range of 12–15, with a solar absorptance of 75%. The solar absorptance increases with temperature, while the thermal emittance typically increases by 2%–4% at 500 °C. Both the film adhesion and the spectral reflectance have remained stable after over 2000 cycles to 450 °C.

Evidence for an internal heat source in Neptune

Analysis of Morrison and Cruikshank's (1974) infrared flux measurements of Uranus and Neptune shows that Uranus is probably in equilibrium with the incident solar flux, while Neptune probably radiates 2.4 times as much energy as it receives from the sun, implying an internal heat source of 1.4 times the solar input. On the basis of very broadband data (1.5 to 300 micron) Jupiter has been shown to radiate more energy than it receives from the sun. Of the Jovian planets, only Uranus, the least massive, appears to lack an internal heat source.

The Apollo X-Ray Fluorescence Spectrometer

Located in the Science Instrument Module of Apollo 15 and 16, the X-ray Fluorescence Spectrometer measured the X-ray fluorescence induced in the lunar surface by the incident solar flux. The fluorescing radiation energy spectra is characteristic of the elemental composition of the lunar surface allowing qualitative and quantitative assessments of the lunar surface. This paper provides a description of the detectors, circuit functions, on-board data processing techniques, in-flight calibration, hardware and flight results of the spectrometer.

Passive solar array orientation devices for terrestrial application

The output power of a solar array is directly related to orientation of the solar cells with respect to the incident solar flux. In this paper, a passive solar array orientation device, called a thermal heliotrope, is described, and several terrestrial applications are illustrated. The thermal heliotrope consists of a bimetallic helical coil that serves as the motor element, producing torque and angular displacement. A control mechanism in the form of one or more shades completes the basic device. In comparison with electromechanical tracking systems, the thermal heliotrope is electrically passive, has relatively few parts, and is low cost. After describing the principle of operation and several models built for space applications, the design considerations for several terrestrial thermal heliotrope units are presented. It is suggested that the use of the thermal heliotrope for solar array orientation could significantly reduce array cost, thereby increasing the competitive economic posture of solar arrays for terrestrial applications. The thermal heliotrope modified for terrestrial use is readily adaptable to orient solar energy concentrators, such as furnace, and stills.

A nongrey calculation of the runaway greenhouse: Implications for Venus' past and present

Abstract We obtain the dependence of the surface temperature of a planet covered with bodies of liquid water upon the amount of incident solar flux. To determine this relationship we carry out nongrey calculations of both the thermal flux emitted to space and the planetary albedo. Water vapor is the only source of infrared opacity in our models. For most of the model atmospheres with a 50% cloud cover, the solar flux incident upon Venus at the present time is sufficiently high to cause complete evaporation of any hypothetical oceans, thus assuring that any carbon dioxide outgassed into the atmosphere remains there. However, our models with a 100% cloud cover do not achieve runaway conditions. Our calculations indicate that four and a half billion years ago, when the solar luminosity was 30% lower than its present value, Venus may have had a very moderate surface temperature and bodies of liquid water on its surface.

The extreme ultraviolet dayglow of Jupiter

Processes involving both resonant scattering and fluorescence of the incident solar flux in the wavelength region 300 Å–1300 Å have been considered for the upper atmosphere of Jupiter, and calculations have been performed for the intensities of the spectral features that would be observed from outside the atmosphere. The principal features were found to be the resonance lines of atomic hydrogen and helium at 1216 Å and 584 Å respectively. It is noted that a significant amount of radiation can be carried in the wings of the line when there are large amounts of scattering atoms present. The radiative transfer problem for this case is formulated and solved for the hydrogen and helium radiations. The emerging intensity of the resonantly scattered helium line depends upon the relative total amounts of H 2 and He in the atmosphere, the dependence arising through photoabsorption of the helium line by molecular hydrogen. The emerging intensity also depends upon the temperature and diffusion processes. The hydrogen Lyman alpha line will appear due to resonance scattering with a minor contribution due to the decay of excited hydrogen atoms formed through photodissociative excitation. The intensity of the HI Lyman alpha glow is a function of the amount of atomic hydrogen present, which in turn depends upon diffusion processes in the atmosphere and is less dependent upon the relative amounts of hydrogen and helium. Simultaneous measurements of these lines may be used to determine the relative total amounts of molecular hydrogen and helium in the atmosphere. Minor features in the Jovian ultraviolet spectrum will be the following. The HeII λ303 Å line is formed primarily through photoionization excitation and is found to be less than 5 R in intensity. The H 2 Lyman bands, produced through resonance fluorescence, will be of approximately 25 R in intensity. The Rayleigh and Raman scattered HI Lyman-α lines will be virtually unobservable, being of order 0.05 R or less.

Decrease inD-region O2(1Δg) Photoionization rates resulting from CO2absorption

Photoionization of the metastable O2 (1Δg) molecule has been proposed as an important source of ionization in the D region. The wavelength region of interest is from 1027 to 1118 A, where ground state O2 is the primary absorber of the incident solar flux but is not ionized itself. This paper presents improved ion production rates based on new laboratory data, including new O2 (1Δg) photoionization cross sections and more detailed curves in several ground state O2 windows. Recent solar flux measurements reduce the continum intensity by about a factor of 5. This reduction is partly compensated by including in the calculations the Si III multiplet, which is at the deepest O2 window (1108.2 A, minimum cross section =4.4×10−21 cm2) and by the new ionization cross sections, which are generally larger than the previously assumed values. An important factor not previously considered is absorption by carbon dioxide, which has a much larger absorption cross section than ground state O2 throughout this region. The ground level mixing ratio has been assumed. The ion production rates are somewhat less than Hunten and McElroy's [1968] curves if CO2 is not included. With CO2, a production rate of 1 ion cm−3 sec−1 is reached at approximately 80, 84, and 86 km for zenith angles of 0°, 45°, and 60°, respectively. Hunten and McElroy find this rate to be 10 to 15 km lower. Concentrations of O2+ are calculated for a zenith angle of 45° for a variety of conditions, including a dry atmosphere and an atmosphere with 5 ppm H2O.