Transmission Of Diffuse Radiation Research Articles

Comparison and Optimization of Light Use Efficiency-Based Gross Primary Productivity Models in an Agroforestry Orchard

The light-use efficiency-based gross primary productivity (LUE-GPP) model is widely utilized for simulating terrestrial ecosystem carbon exchanges owing to its perceived simplicity and reliability. Variations in cloud cover and aerosol concentrations can affect ecosystem LUE, thereby influencing the performance of the LUE-GPP model, particularly in humid regions. In this study, the performance of six big-leaf LUE-GPP models and one two-leaf LUE-GPP model were evaluated in a humid agroforestry ecosystem from 2018–2020. All big-leaf LUE-GPP models yielded GPP values consistent with that derived from the eddy covariance system (GPPEC), with R2 ranging from 0.66–0.73 and RMSE ranging from 1.81–3.04 g C m−2 d−1. Differences in model performance were attributed to the differences in the quantification of temperature (Ts) and moisture constraints (Ws) and their combination forms in the models. The Ts and Ws algorithms in the eddy covariance-light-use efficiency (EF-LUE) model well characterized the environmental constraints on LUE. Simulation accuracy under the common limitation of Ts and Ws (Ts × Ws) was higher than the maximum limitation of Ts or Ws (Min (Ts, Ws)), and the combination of the Ts algorithm in the Carnegie–Ames–Stanford Approach (CASA) and the Ws algorithm in the EF-LUE model was optimized in combination forms, thereby constraining LUE for GPP estimates (GPPBLO, R2 = 0.76). Various big-leaf LUE-GPP models overestimated or underestimated GPP on sunny or cloudy days, respectively, while the two-leaf LUE-GPP model, which considered the transmission of diffuse radiation and the difference in photosynthetic capacity of canopy leaves, performed well (R2 = 0.72, p < 0.01). Nevertheless, the underestimation/overestimation for shaded/sunlit leaves remained under different weather conditions. Then, the clearness index (Kt) was introduced to calculate the dynamic LUE in the big-leaf and two-leaf LUE-GPP models in the form of exponential or power functions, resulting in consistent performance even in different weather conditions and an overall higher simulation accuracy. This study confirmed the potential applicability of different LUE-GPP models and emphasized the importance of dynamic LUE on model performance.

Model of solar diffuse radiation transmission through circular perforated louvers and experimental verification

The main problem of previous methods for diffuse radiation transmission calculation of shading louvers is that they can only deal with flat slats. Taking the radian and perforation rate of louvers into consideration, a new model of solar diffuse radiation transmitted through circular perforated louvers was established and experimentally verified in this paper. Firstly, taking two adjacent circular perforated louvers as the research object, the new calculation model was established, and based on the model, a computer program was developed with MATLAB, which can be used to study the impact of shape, size, rotation angle, and surface optical properties of circular perforated louvers on diffuse radiation transmission. Secondly, an experimental set-up was built to verify the new model, and the results showed that the model predictions compared well with the experimental data. Finally, the error of the conventional calculation methods was analyzed, and the results showed that, the “flat slat model” led to overestimation of the transmittance of diffuse radiation, and the relative error increased from 2.9% to 13.8% when the rotation angle increasing from 0 to 85°.

Model of solar diffuse radiation transmission through circular perforated louvers and program realization

The main problem of previous methods for diffuse radiation transmission calculation of shading louvers is that they can only deal with flat slats. Taking the radian and perforation rate of louvers into consideration, a new model of solar diffuse radiation transmitted through circular perforated louvers was established and experimentally verified in this paper. Firstly, taking two adjacent circular perforated louvers as the research object, the new calculation model was established, and based on the model, a computer program was developed with MATLAB, which can be used to study the impact of shape, size, rotation angle, and surface optical properties of circular perforated louvers on diffuse radiation transmission. Secondly, the correctness of the calculating program was verified, and sensitivity analysis was carried out on the partition number of circular translucent louvers, and then find that the relative error rate decreases with the partition number, results no longer change when partition number over 7; Finally, the error of the conventional calculation methods was analyzed, and the results showed that, the “flat slat model” led to overestimation of the transmittance of diffuse radiation, and the relative error increased from 2.9% to 13.1% when the rotation angle increasing from 0 to 85°.

Solar and visible optical properties of glazing systems with venetian blinds: Numerical, experimental and blind control study

The increasing use of glazed areas in the building envelope can lead to high solar gains and glare problems that can strongly impact the entire building energy consumption, peak loads and indoor comfort. An important and fundamental strategy in sustainable building design for controlling solar heat gains and daylighting through fenestration is the use of shading devices. Therefore, it is recommended to use detailed models that can accurately estimate the optical properties of the different types of shading devices (such as roller blinds and venetian blinds) and include their effects in the glazing system analysis.This paper describes a net radiation method for determining both solar and visible optical properties of glazing with shading devices, particularly venetian blinds. Some numerical results were compared with in situ experimental measurements carried out in an outdoor test cell. The experimental work included the measurement of illuminance and irradiance fluxes and the determination of the visible and solar transmission properties of the fenestration system. The agreement between numerical predictions and experimental results was better for overcast than clear sky conditions. Moreover, a venetian blind control strategy that blocks direct solar radiation, whilst enabling the transmission of diffuse radiation to indoors, is implemented.

Variable g value of transparent façade collectors

Transparent solar thermal collectors (TSTC) represent a new development. An adequate model is needed to predict their performance. This paper presents a collector model with an advanced calculation of the transmission of diffuse radiation and a connection to the building which allows analysis of the collector gains and of the g value, also called “solar factor”, “solar heat gain coefficient (SHGC)” or “total solar energy transmittance”. The model is implemented as a TRNSYS Type and a coupled simulation between a collector and a room is presented for different façade constructions. Façade areas with glazing and venetian blinds are simulated with a second new TRNSYS Type which introduces high modelling accuracy for façades with solar control systems. An HVAC system is presented together with a first estimate of possible reductions of primary energy. It indicates primary energy savings of about 30% by replacing opaque walls with transparent collectors. The g values prove to depend not only on the irradiation, but also on the operation of the solar collectors and vary e.g. between 0.04 and 0.21. Detailed modelling of active façades like TSTC is therefore essential for accurate predictions of the collector gain, the heating and cooling loads and the thermal comfort.

Evaluation of global, photosynthetically active radiation and diffuse radiation transmission of agricultural screens

Transmittance of a material depends on the type of radiation impinging on the material (direct or diffuse), the angle of incidence of the sun’s rays (in direct radiation conditions) and the structure and characteristics of the material itself. The aim of this study was to evaluate the performance of nine agricultural screens of different densities, colours, thread diameters and porosities. A simple metal frame was used to quantify global, diffuse and photosynthetically active radiation (PAR) transmission to determine their transmittance values (as a function of the incidence angle of solar rays in direct radiation conditions) for global radiation and PAR, as well as the diffuse radiation transmitted characteristics of the screens (ratio of diffuse radiation to global radiation transmitted by screens). Non-coloured (translucent) screens contributed to a higher and more efficient proportion of diffuse radiation (Di/Gi ≈ 90%). The green screen behaved similarly to the non-coloured 20×10 type as far as global radiation was concerned, although the PAR transmittance values were lower (up to 12.4% at 15º). Variation in transmission according to diameter and density of the threads was greatest in the black screens. The non-coloured-thread screens may be the best option for maximising the transmission of diffuse radiation and the 6×6 green screen for vegetables cultivation during the summer in inland areas with latitudes close to 36ºN.

Transparent domed skylights: Optical model for predicting transmittance, absorptance and reflectance

Daylighting and thermal loads are very important design issues for skylight design, especially in large spaces such as atria. However, the trade-off between daylighting and thermal performance of skylights has been difficult to solve, due to a lack of daylighting and thermal design tools. A mathematical model was developed to predict the visible/solar transmittance, absorptance and reflectance of multi-glazed domed skylights for both direct and diffuse radiation- The model is based on tracking the beam and diffuse radiation transmission through the dome surface. Since all building energy simulation and fenestration rating tools are limited to planar skylights, the model was translated into a simple method in which domed skylights were substituted by optically equivalent planar skylights. The results showed that domed skylights yield slightly lower visible/solar transmittance at low sun zenith angles, and substantially higher visible/solar transmittance at high sun zenith angles, or near the horizon, than ...

Asymptotic solutions of the equations of radiative transfer for an optically thick layer with reflecting boundaries

An asymptotic analysis is made of the system of equations obtained in a previous paper. This system is equivalent to the radiative transfer equation and is suitable for effective solution in the case of an optically thick layer with arbitrary linear boundary conditions of reflection and transmission. Asymptotic expressions are obtained that make it possible in each specific case to find the explicit form of the laws of reflection and transmission of diffuse radiation. The spatial distribution of the radiation intensity within and on the boudaries of an optically thick layer is also investigated.

Model for Calculating Photosynthetic Photon Flux Densities in Forest Openings on Slopes

A model has been developed to calculate the spatial distribution of the photosynthetic photon flux density (PPFD) in elliptical forest openings of given slopes and orientations. The PPFD is separated into direct and diffuse components. The direct component is calculated according to the opening and radiation geometries, and pathlength of the solar beam through the forest canopy. The diffuse component is obtained from the sky, tree, and landscape view factors. In this model, the distribution of foliage area with height and the effect of foliage clumping on both direct and diffuse radiation transmission are considered. The model has been verified using measurements for six quantum sensors (LI-COR Inc.) located at different positions in a small clear-cut (0.37 ha) in a 90-year-old western hemlock-Douglas fir forest.

Relationships between transmission of solar radiation and coniferous forest stand characteristics

Transmission of global, direct, diffuse, and diffuse photosynthetically active solar radiation through coniferous forest canopies was studied on 12 plots during sunny days in summer. The proportions of radiation transmitted were a function of forest stand characteristics and differed among radiation components. The relationship between the proportion of diffuse radiation transmitted and stand characteristics differed among stand structures, but transmission of direct radiation did not differ. When direct beam radiation was scattered by open crowns of short trees and crowns extended within the height of radiation sensors, diffuse radiation within the canopy was higher than outside the stand. The extent to which different forest stand characteristics predict transmission and approximate a model analogous to Beer's law was explored. Sum of tree diameters and Reineke's stand density index were the better predictors of tranmission of global ( i 2=0.80−0.95) and direct radiation ( i 2=0.70−0.98). Overstory cover predicted transmission of diffuse radiation best ( i 2=0.74) when bases of tree crowns were above sensors. The same forest stand characteristics obtained by different sampling methods gave different predictabilities of the components transmitted. Differences were found between hemispherical photograph estimates of transmission of diffuse radiation and measured transmission of diffuse solar and diffuse photosynthetically active radiation. Photographic estimates of transmission of direct radiation differed from measured transmission of direct radiation.

Moss leaf water content and solar radiation at the moss surface in a mature black spruce forest in central Alaska

In a mature black spruce (Piceamariana (Mill.) B.S.P.) forest near the University of Alaska, Fairbanks, AK, samples of four common moss species, Polytrichumcommune Hedw., Hylocomiumsplendens (Hedw.) B.S.G., Pleuroziumschreberi (Brid.) Mitt., and Sphagnumsubsecundum Nees. were collected at intervals during the 1976 season to determine the diurnal variation in leaf water content and daily water loss as functions of temperature, moisture, radiation, and wind speed. The field measurements were followed by laboratory experiments on intact cores in an open system and on excised shoots in closed cuvettes. In these experiments, water loss rates varied by species and were affected by vapor pressure deficits and wind speed; where vapor pressure deficit is more constant than these other factors, variations in light intensities had less effect on water loss rates. Polytrichumcommune, which translocates water from the soil, avoided moisture stress to a greater extent than the other moss species which were more dependent on water absorption through the leaves. Hylocomiumsplendens was below the water content for compensation for almost 50% of the July measurement period. Observed patterns of the rates of water loss and of the moisture required to reach field capacity are correlated with the moisture status of the mosses in the field. Field measurements of photosynthetically active radiation (PAR) at the moss surface and above the forest canopy showed that the transmission of diffuse radiation through the forest canopy increased with increasing cloudiness. PAR at the moss surface was above the compensation level for photosynthesis with sunny and cloudy conditions. Sunflecks (short periods of direct sunlight), which had an intensity of about 76% of the radiation incident on the forest canopy, occurred on up to 35% of the ground surface and provided a major source of the radiation received. Species varied in their pattern of attenuation of light through the moss canopy. A mixed stand of P. commune and P. schreberi and stands of H. splendens transmit light deeply. Sphagnumsubsecundum, on the other hand, shows rapid attenuation of light high in the canopy. The observed pattern of light attenuation helps explain the vertical position of the green–brown interface and the death of green material in the moss canopy.

Transmission of diffuse radiation through CPC and flat plate collector glazings

Transmission of diffuse radiation through CPC and flat plate collector glazings

Continuous K/S minimizing distributions in Kubelka-Munk systems

The phenomenological model of Kubelka and Munk (KM) for describing the reflection and transmission of diffuse radiation in turbid, plane parallel media is utilized in investigating the nature of inhomogeneous distributions (i.e., the ratio of the absorption and scattering coefficients, K/S, is not necessarily constant) which satisfy certain constraints in finitely thick slabs. The distributions are constrained to be continuous and to minimize the integral of K2/S2 across the slab, while resulting in a specified reflectance when the slab rests upon a backing of specified reflectance. The form of the inhomogeneous distributions is obtained as the solution to the corresponding variational problem, and the associated Lagrange multiplier is found to be algebraically related to the transmittance. The sufficiency of the approach is justified a posteriori by direct comparison with the closed-form solutions of KM for homogeneous distributions. The qualitative nature of such optimal inhomogeneous distributions is discussed with regard to the effects of the boundary conditions and the scattering thickness and is found to be approximately exponential.