مستخدم:هيثم الصعب/الإشعاع الشمسي

الإشعاع الشمسي هو مقدار الأشعة الشمسية الساقطة علة مساحةٍ معينة ،القادرة على توليد قدرة الكهربائية . الإشعاع قد يقُياس في الفضاء أو على سطح الأرض بعد امتصاص الغلاف الجوي و نثر. ويقاس عمودي على أشعة الشمس الواردة. مجموع الإشعاع الشمسي (TSI) هو مقياس الطاقة الشمسية على جميع الأطوال الموجية لكل وحدة منطقة الحادث في الغلاف الجوي العلوي للأرض. فإن الشمسية ثابتة تقليدية لقياس يعني TSI على مسافة واحدة الوحدة الفلكية (AU). الإشعاع هو وظيفة من المسافة من الشمس ، الدورة الشمسية ، عبر دورة التغييرات. الإشعاع على الأرض هو أيضا قياس عمودي على أشعة الشمس الواردة. تشميس هو القوة التي تلقى على الأرض في وحدة المساحة على سطح أفقي. فإنه يعتمد على ارتفاع الشمس فوق الأفق.

المتوسط السنوي للإشعاع الشمسي في الجزء العلوي من الغلاف الجوي للأرض (TOA) على سطح الكوكب

تشميس

The solar irradiance integrated over time is called solar irradiation, solar exposure or insolation. However, insolation is often used interchangeably with irradiance in practice.

Units

The SI unit of irradiance is watt per square meter (W/m²).

An alternate unit of measure is the Langley (1 thermochemical calorie per square centimeter or 41,840 J/m²) per unit time.

The solar energy industry uses watt-hour per square metre (Wh/m²) divided by the recording time. 1 kW/m² = 24 kWh/(m² day).

Irradiance can also be expressed in Suns, where one Sun equals 1000 W/m² at the point of arrival.

Absorption and reflection

Solar irradiance spectrum above atmosphere and at surface

Part of the radiation reaching an object is absorbed and the remainder reflected. Usually the absorbed radiation is converted to thermal energy, increasing the object's temperature. Manmade or natural systems, however, can convert part of the absorbed radiation into another form such as electricity or chemical bonds, as in the case of photovoltaic cells or plants. The proportion of reflected radiation is the object's reflectivity or albedo.

Projection effect

One sunbeam one mile wide shines on the ground at a 90° angle, and another at a 30° angle. The oblique sunbeam distributes its light energy over twice as much area.

Categories

Solar potential – global horizontal irradiation

Earth

A Pyranometer, a component of a temporary remote meteorological station, measures insolation on Skagit Bay, Washington.

Top of the atmosphere

Spherical triangle for application of the spherical law of cosines for the calculation the solar zenith angle Θ for observer at latitude φ and longitude λ from knowledge of the hour angle h and solar declination δ. (δ is latitude of subsolar point, and h is relative longitude of subsolar point).

The distribution of solar radiation at the top of the atmosphere is determined by Earth's sphericity and orbital parameters. This applies to any unidirectional beam incident to a rotating sphere. Insolation is essential for numerical weather prediction and understanding seasons and climate change. Application to ice ages is known as Milankovitch cycles.

Distribution is based on a fundamental identity from spherical trigonometry, the spherical law of cosines:

$\cos(c)=\cos(a)\cos(b)+\sin(a)\sin(b)\cos(C)\,$

where a, b and c are arc lengths, in radians, of the sides of a spherical triangle. C is the angle in the vertex opposite the side which has arc length c. Applied to the calculation of solar zenith angle Θ, the following applies to the spherical law of cosines:

$C=h\,$

$c=\Theta \,$

$a={\tfrac {1}{2}}\pi -\phi \,$

$b={\tfrac {1}{2}}\pi -\delta \,$

$\cos(\Theta )=\sin(\phi )\sin(\delta )+\cos(\phi )\cos(\delta )\cos(h)\,$

The separation of Earth from the sun can be denoted R_E and the mean distance can be denoted R₀, approximately 1 AU. The solar constant is denoted S₀. The solar flux density (insolation) onto a plane tangent to the sphere of the Earth, but above the bulk of the atmosphere (elevation 100 km or greater) is:

$Q=S_{o}{\frac {R_{o}^{2}}{R_{E}^{2}}}\cos(\Theta ){\text{ when }}\cos(\Theta )>0$

and

$Q=0{\text{ when }}\cos(\Theta )\leq 0\,$

The average of Q over a day is the average of Q over one rotation, or the hour angle progressing from h = π to h = −π:

${\overline {Q}}^{\text{day}}=-{\frac {1}{2\pi }}{\int _{\pi }^{-\pi }Q\,dh}$

Let h₀ be the hour angle when Q becomes positive. This could occur at sunrise when $\Theta ={\tfrac {1}{2}}\pi$, or for h₀ as a solution of

$\sin(\phi )\sin(\delta )+\cos(\phi )\cos(\delta )\cos(h_{o})=0\,$

or

$\cos(h_{o})=-\tan(\phi )\tan(\delta )$

If tan(φ)tan(δ) > 1, then the sun does not set and the sun is already risen at h = π, so h_o = π. If tan(φ)tan(δ) < −1, the sun does not rise and ${\overline {Q}}^{\mathrm {day} }=0$.

${\frac {R_{o}^{2}}{R_{E}^{2}}}$ is nearly constant over the course of a day, and can be taken outside the integral

$\int _{\pi }^{-\pi }Q\,dh=\int _{h_{o}}^{-h_{o}}Q\,dh=S_{o}{\frac {R_{o}^{2}}{R_{E}^{2}}}\int _{h_{o}}^{-h_{o}}\cos(\Theta )\,dh$

$\int _{\pi }^{-\pi }Q\,dh=S_{o}{\frac {R_{o}^{2}}{R_{E}^{2}}}\left[h\sin(\phi )\sin(\delta )+\cos(\phi )\cos(\delta )\sin(h)\right]_{h=h_{o}}^{h=-h_{o}}$

$\int _{\pi }^{-\pi }Q\,dh=-2S_{o}{\frac {R_{o}^{2}}{R_{E}^{2}}}\left[h_{o}\sin(\phi )\sin(\delta )+\cos(\phi )\cos(\delta )\sin(h_{o})\right]$

${\overline {Q}}^{\text{day}}={\frac {S_{o}}{\pi }}{\frac {R_{o}^{2}}{R_{E}^{2}}}\left[h_{o}\sin(\phi )\sin(\delta )+\cos(\phi )\cos(\delta )\sin(h_{o})\right]$

Let θ be the conventional polar angle describing a planetary orbit. Let θ = 0 at the vernal equinox. The declination δ as a function of orbital position is

$\sin \delta =\sin \varepsilon ~\sin(\theta -\varpi )\,$

where ε is the obliquity. The conventional longitude of perihelion ϖ is defined relative to the vernal equinox, so for the elliptical orbit:

$R_{E}={\frac {R_{o}}{1+e\cos(\theta -\varpi )}}$

or

${\frac {R_{o}}{R_{E}}}={1+e\cos(\theta -\varpi )}$

With knowledge of ϖ, ε and e from astrodynamical calculations and S_o from a consensus of observations or theory, ${\overline {Q}}^{\mathrm {day} }$can be calculated for any latitude φ and θ. Because of the elliptical orbit, and as a consequence of Kepler's second law, θ does not progress uniformly with time. Nevertheless, θ = 0° is exactly the time of the vernal equinox, θ = 90° is exactly the time of the summer solstice, θ = 180° is exactly the time of the autumnal equinox and θ = 270° is exactly the time of the winter solstice.

Variation

Total irradiance

Total solar irradiance (TSI) changes slowly on decadal and longer timescales. The variation during solar cycle 21 was about 0.1% (peak-to-peak). In contrast to older reconstructions, most recent TSI reconstructions point to an increase of only about 0.05% to 0.1% between the Maunder Minimum and the present.

Ultraviolet irradiance

Ultraviolet irradiance (EUV) varies by approximately 1.5 percent from solar maxima to minima, for 200 to 300 nm wavelengths. However, a proxy study estimated that UV has increased by 3.0% since the Maunder Minimum.

Milankovitch cycles

Some variations in insolation are not due to solar changes but rather due to the Earth moving between its perigee and apogee, or changes in the latitudinal distribution of radiation. These orbital changes or Milankovitch cycles have caused radiance variations of as much as 25% (locally; global average changes are much smaller) over long periods. The most recent significant event was an axial tilt of 24° during boreal summer near the Holocene climatic optimum.

Obtaining a time series for a ${\overline {Q}}^{\mathrm {day} }$ for a particular time of year, and particular latitude, is a useful application in the theory of Milankovitch cycles. For example, at the summer solstice, the declination δ is equal to the obliquity ε. The distance from the sun is

${\frac {R_{o}}{R_{E}}}=1+e\cos(\theta -\varpi )=1+e\cos({\tfrac {\pi }{2}}-\varpi )=1+e\sin(\varpi )$

For this summer solstice calculation, the role of the elliptical orbit is entirely contained within the important product $e\sin(\varpi )$, the precession index, whose variation dominates the variations in insolation at 65° N when eccentricity is large. For the next 100,000 years, with variations in eccentricity being relatively small, variations in obliquity dominate.

Measurement

The space-based TSI record comprises measurements from more than ten radiometers spanning three solar cycles.

Technique

All modern TSI satellite instruments employ active cavity electrical substitution radiometry. This technique applies measured electrical heating to maintain an absorptive blackened cavity in thermal equilibrium while incident sunlight passes through a precision aperture of calibrated area. The aperture is modulated via a shutter. Accuracy uncertainties of <0.01% are required to detect long term solar irradiance variations, because expected changes are in the range 0.05 to 0.15 W m−2 per century.

Intertemporal calibration

In orbit, radiometric calibrations drift for reasons including solar degradation of the cavity, electronic degradation of the heater, surface degradation of the precision aperture and varying surface emissions and temperatures that alter thermal backgrounds. These calibrations require compensation to preserve consistent measurements.

For various reasons, the sources do not always agree. The Solar Radiation and Climate Experiment/Total Irradiance Measurement (SORCE/TIM) TSI values are lower than prior measurements by the Earth Radiometer Budget Experiment (ERBE) on the Earth Radiation Budget Satellite (ERBS), VIRGO on the Solar Heliospheric Observatory (SoHO) and the ACRIM instruments on the Solar Maximum Mission (SMM), Upper Atmosphere Research Satellite (UARS) and ACRIMSat. Pre-launch ground calibrations relied on component rather than system level measurements, since irradiance standards lacked absolute accuracies.

Measurement stability involves exposing different radiometer cavities to different accumulations of solar radiation to quantify exposure-dependent degradation effects. These effects are then compensated for in final data. Observation overlaps permits corrections for both absolute offsets and validation of instrumental drifts.

Uncertainties of individual observations exceed irradiance variability (∼0.1%). Thus, instrument stability and measurement continuity are relied upon to compute real variations.

Long-term radiometer drifts can be mistaken for irradiance variations that can be misinterpreted as affecting climate. Examples include the issue of the irradiance increase between cycle minima in 1986 and 1996, evident only in the ACRIM composite (and not the model) and the low irradiance levels in the PMOD composite during the 2008 minimum.

Despite the fact that ACRIM I, ACRIM II, ACRIM III, VIRGO and TIM all track degradation with redundant cavities, notable and unexplained differences remain in irradiance and the modeled influences of sunspots and faculae.

Difference Relative to TRF

Instrument	Irradiance: View-Limiting Aperture Overfilled	Irradiance: Precision Aperture Overfilled	Difference Attributable To Scatter Error	Measured Optical Power Error	Residual Irradiance Agreement	Uncertainty
SORCE/TIM ground	NA	−0.037%	NA	−0.037%	0.000%	0.032%
Glory/TIM flight	NA	−0.012%	NA	−0.029%	0.017%	0.020%
PREMOS-1 ground	−0.005%	−0.104%	0.098%	−0.049%	−0.104%	∼0.038%
PREMOS-3 flight	0.642%	0.605%	0.037%	0.631%	−0.026%	∼0.027%
VIRGO-2 ground	0.897%	0.743%	0.154%	0.730%	0.013%	∼0.025%

Applications

Buildings

In construction, insolation is an important consideration when designing a building for a particular site.

Insolation variation by month; 1984–1993 averages for January (top) and April (bottom)

Conversion factor (multiply top row by factor to obtain side column)
W/m2	kW·h/(m2·day)	sun hours/day	kWh/(m2·y)	kWh/(kWp·y)
W/m2	1	41.66666	41.66666	0.1140796	0.1521061
kW·h/(m2·day)	0.024	1	1	0.0027379	0.0036505
sun hours/day	0.024	1	1	0.0027379	0.0036505
kWh/(m2·y)	8.765813	365.2422	365.2422	1	1.333333
kWh/(kWp·y)	6.574360	273.9316	273.9316	0.75	1

References

[[تصنيف:نظام شمسي]] [[تصنيف:ظواهر شمسية]]