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Fundamentals of Solar Radiation
Published in D. Yogi Goswami, Principles of Solar Engineering, 2023
For the purposes of this book, the Ptolemaic view of the sun’s motion provides a simplification to the analysis that follows. It is convenient to assume the earth to be fixed and to describe the sun’s apparent motion in a coordinate system fixed to the earth with its origin at the site of interest. Figure 2.7 shows an apparent path of the sun to an observer. The position of the sun can be described at any time by two angles, the altitude and azimuth angles, as shown in Figure 2.7. The solar altitude angle, α, is the angle between a line collinear with the sun’s rays and the horizontal plane. The solar azimuth angle, as, is the angle between a due south line and the projection of the site to sun line on the horizontal plane. The sign convention used for the azimuth angle is positive west of south and negative east of south. The solar zenith angle, z, is the angle between the site to sun line and the vertical at the site: z=90°−α.
Environmental and Social Impact
Published in Ajai, Rimjhim Bhatnagar, Desertification and Land Degradation, 2022
Emissivity is defined as the ratio of the actual radiation emission from a body to the maximum possible radiation emission (black-body emission). For air, it depends upon the vapour pressure and air temperature (Moran et al. 1989). The second term at the right-hand side (RHS) of the equation represents the downward long-wave radiation emitted by the atmosphere (the gases, particulates and clouds) and incident at the surface. The third term at RHS represents the solar radiation reflected by the surface (outgoing short-wave radiation). The fourth term at RHS is the outgoing long-wave radiation emitted by the land surface. The amount of net radiation available at a particular place on the earth surface depends on the magnitude/intensity of solar insolation, the air temperature and vapour pressure, surface characteristic properties (albedo, emissivity, land surface temperature, vegetation cover/land cover). Thus, the net radiation has diurnal, seasonal and annual variations and also varies with solar zenith angle. During daytime and under normal (sunny) conditions, net radiation is positive (directed towards the surface) as incoming radiation is more than the outgoing radiation, hence land surface gains energy. During night-time, the outgoing long-wave infrared flux (emitted by land surface) dominates as there is no incident solar radiation. This results in negative net radiation, and there is a net loss of energy from the land surface to the atmosphere.
Definitions and Terminology
Published in Frank Vignola, Joseph Michalsky, Thomas Stoffel, Solar and Infrared Radiation Measurements, 2019
Frank Vignola, Joseph Michalsky, Thomas Stoffel
A coordinate system that can be used for defining positions with respect to an observer fixed on the surface of the Earth is shown in Figure 2.7. The solar zenith angle is defined as the angle between the zenith and the Sun. The cosine of the solar zenith angle with respect to the observer on the surface of the Earth can be obtained by calculating the vector product of the normal to the surface and the unit vector coming directly from the Sun. Consider a reference coordinate system with the x^ and y^ axes defined in the equatorial plane with the east–west axis for the x^ axis and the intersection with the local meridian plane for the y^ axis. The celestial polar axis is the x^ axis. The normal to the surface at a location of latitude (lat) degrees is then elv is the elevation angle, measured up from horizonsza is the zenith angle, measured from verticalaz is the azimuth angle, measured clockwise from north
Mapping high-resolution surface shortwave radiation over East Asia with the new generation geostationary meteorological satellite Himawari-8
Published in International Journal of Digital Earth, 2023
Jun Li, Wenjun Tang, Jingwen Qi, Zhenyu Yan
Based on the estimated SSR data in 2017, we calculated the seasonal and annual mean SSR to observe the SSR distribution over East Asia. As shown in Figure 7, the SSR in the Western Pacific between the equator and 30°N exceeds 260 W m−2 in summer, whereas the SSR in the region between 40°N and 60°N is below 40 W m−2 in winter. This discrepancy proves that the distribution of SSR is significantly influenced by season and latitude, mainly caused by variations in the solar zenith angle. As the season changes and latitude increases, the solar zenith angle becomes lower, resulting in a corresponding decrease in radiation values. In addition, it can be seen that areas located in the subtropical high-pressure belt are rich in SSR throughout the year, especially the Western Pacific and Australia. In the western Pacific, the central values of the two high SSR centers were greater than 280 W m−2. The Australian continent had the highest SSR value in excess of 300 W m−2.
Tilt sensitivity for a scalable one-hectare photovoltaic power plant composed of parallel racks in Muscat
Published in Cogent Engineering, 2022
Because the latitude is fixed here (φ = 23.6°), the solar zenith angle at solar noon (SZAn) becomes a function of the solar declination (δ) only. Figure 9 demonstrates the variation of the solar-noon solar zenith angle over the year, for both the sinusoidal (realistic) declination function and the linear (simplified) declination function. For sun rays to fall as perpendicularly as possible on the PV panels, the tilt of the PV panel should be equal to the solar zenith angle (SZA) at all times during the day, not just during the solar noon (Hailu & Fung, 2019). Regardless of the exact shape of the declination variation during the year, the solar zenith angle at solar noon (SZAn) at the latitude of interest (23.6°) varies from a minimum of 0.1° (at the summer solstice, occurring annually on June 20 or 21 (Britannica, 2021)) to a maximum of 47.1° (at the winter solstice, occurring annually on December 21 or 22). Outside the solar noon time, the solar zenith angle is larger than its solar-noon value (SZA ≥ SZAn), becoming close to 90° near the sunrise and the sunset times, when the sun is near the horizon, being in a very low position in the sky. However, given that the solar irradiation is highest around the solar noon for a clear sky (Yang & Koike, 2005), the solar-noon value of the solar zenith angle should be given priority when considering the tilt angle of PV panels.
Time Series-Based Photovoltaic Power Forecasting to Optimize Grid Stability
Published in Electric Power Components and Systems, 2021
Parthasarathy Seshadri, Bagavat Perumaal T.S., Ashok Kumar B., Keerthana H., Kavinmathi G., Senthilrani S.
Solar zenith angle is the angle between the sun’s incident radiation with the vertical axis from the Earth’s surface. It determines the effective component of incident irradiation falling on the surface [32, 33]. It characterizes the final irradiation level on the surface of PV modules as well as plays an important role in characterizing the output power from the PV module [34]. The datasets fetched from PyOWM library and online database (NSRDB) are presented in Table 2 [31].