Position of the Sun

Summary

The position of the Sun in the sky is a function of both the time and the geographic location of observation on Earth's surface. As Earth orbits the Sun over the course of a year, the Sun appears to move with respect to the fixed stars on the celestial sphere, along a circular path called the ecliptic. Earth's rotation about its axis causes diurnal motion, so that the Sun appears to move across the sky in a Sun path that depends on the observer's geographic latitude. The time when the Sun transits the observer's meridian depends on the geographic longitude. To find the Sun's position for a given location at a given time, one may therefore proceed in three steps as follows:

calculate the Sun's position in the ecliptic coordinate system,

convert to the equatorial coordinate system, and

convert to the horizontal coordinate system, for the observer's local time and location. This is the coordinate system normally used to calculate the position of the Sun in terms of solar zenith

Official source

https://en.wikipedia.org/wiki/Position_of_the_Sun

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A new method to determine multi-angular reflectance factor from lightweight multispectral cameras with sky sensor in a target-less workflow applicable to UAV

Manuel Simon Paul Cubero-Castan, Dai Shi, Christoph Strecha

A new physically based method to estimate hemispheric-directional reflectance factor (HDRF) from lightweight multispectral cameras that have a downwelling irradiance sensor is presented. It combines radiometry wilh photogrammetric computer vision to derive geometrically and radiometrically accurate data purely from the images, without requiring reflectance targets or any other additional information apart from the imagery. The sky sensor orientation is initially computed using photogrammetric computer vision and revised with a non linear regression comprising radiometric and photogrammetry-derived information. It works for both clear sky and overcast conditions. A ground-based test acquisition of a Spectralon target observed from different viewing directions and with different sun positions using a typical multispectral sensor configuration for clear sky and overcast showed that both the overall value and the directionality of the reflectance factor as reported in the literature were well retrieved. An RMSE of 3% for clear sky and up to 5 for overcast sky was observed.

ELSEVIER SCIENCE INC2019

Transmittance thresholds of electrochromic glazing to achieve annual low-glare work environments

Marilyne Andersen, Sneha Jain, Jan Wienold

A recent study on the glare protection performance of electrochromic (EC) glazing showed that visible transmittance levels lower than 0.6% are necessary to achieve comfortable situations for sun positions, that were close to the central field of view. However, the question that arises is how often such situations occur throughout the year and how the glare protection performance of EC systems is for typical office situations for different climates and orientations. This study aims to quantify the annual performance for such configurations by applying improved simulation methods to conduct annual glare simulations and comparing them to the EN17037 classifications. The enhancement of the simulation method compared to existing methods was necessary to correctly consider the blurring effect in the lens of the eye – neglecting this would lead to an overestimation of glare. We found that for mid and north European climates the extreme situations do not occur such often, so that the EC- glazing systems being able to switch to transmittance levels of 1% can mitigate glare throughout the year reasonably well for typical office situations and reaches typically the highest glare protection category according to EN17037 for a viewing direction, that is parallel to the facade. For more sunny climates such as Rome, slightly lower transmittance levels (around 0.5%) would be necessary to achieve a similar glare protection level. The study also revealed that tables E.7 and E.8 of EN17037 with pre-calculated 95-percentile Daylight Glare Probability (DGP) values should be re-calculated.

2022

This work discusses solar power towers and especially the simulation and optimization of heliostat fields. The technological stateof the art is given through typical characteristic values and dimensions of some existing central tower power plants such as th so-called Solar Two in California and the PS10 in Spain. Leaving cycle losses aside, the field losses involved are described and roughly estimated thanks to commonly measured and computed values, typically coming to an 80% overall field efficiency. On the other side, the coats that must be taken into consideration when designing a heliostat field are listed and approached through correlations, giving for instance a total investment cost of $15 million for relatively large fields ( approximately 700 [m]). The raytracing software Radiance is then chosen for modeling an entire heliostat field with a central receiver, and thus for running simulations over time for different sun positions. Some given field layouts such as those obtained in published cases can be visualized and heat flux at the receiver‘s surface is computed thanks to Radiance tools before being displayed through a MATLAB post-processing. For validating Radiance calculations, the results are first compared with published cases: the receiver‘s total thermal energy produced over a year is generally between 5 and 10 [%] lower than these published results. Secondly, the Radiance heat flux profile has to be compared with some published experimental results: based on measurements led at the PSA Solar de Almeria). the heat flux profile (gradient) is close to measurements, but computed and measured total thermal powers do not match when computed and measured peak values do match. Therefore the Radiance heat flux still requires further validation probably involving some model adjustments. Nevertheless, this does not prevent from implementing some very large fields (more than 3000 heliostats) optimized with the MOO multi-objective optimizer after having modified routines for generating surrounding fields. and thus resulting in a first overview of what the heat flux profile on a cylindrical receiver may look like for a total thermal power of 300 [M Wth].

2008