Only beam component of solar radiation can be effectively concentrated – Short-wave radiation, in the wavelength range from 0.3 to 3 μm, comes directly from the sun. It includes both beam and diffuse components. Long-wave radiation, with wavelength 3 μm or longer, originates from the sources at near-ambient temperatures – atmosphere, earth surface, light collectors, other bodies.
- The solar radiation reaching the earth is highly variable and depends on the state of the atmosphere at a specific locale.
- Two atmospheric processes can significantly affect the incident irradiation: scattering and absorption,
- Scattering is caused by interaction of the radiation with molecules, water, and dust particles in the air.
How much light is scattered depends on the number of particles in the atmosphere, particle size, and the total air mass the radiation comes through. Absorption occurs upon interaction of the radiation with certain molecules, such as ozone (absorption of short-wave radiation – ultraviolet), water vapor, and carbon dioxide (absorption of long-wave radiation – infrared).
- Due to these processes, out of the whole spectrum of solar radiation, only a small portion reaches the earth surface.
- Thus most of x-rays and other short-wave radiation is absorbed by atmospheric components in the ionosphere, ultraviolet is absorbed by ozone, and not-so abundant long-wave radiation is absorbed by CO2.
As a result, the main wavelength range to be considered for solar applications is from 0.29 to 2.5 μm, Figure 2.1. Different types of radiation at the earth surface: orange – short wave; blue – long wave. Credit: Mark Fedkin – modified after Duffie and Beckman, 2013 The amount of solar radiation on the earth surface can be instrumentally measured, and precise measurements are important for providing background solar data for solar energy conversion applications.
- Pyrheliometer is used to measure direct beam radiation at normal incidence. There are different types of pyrheliometers. According to Duffie and Beckman (2013), Abbot silver disc pyrheliometer and Angstrom compensation pyrheliometer are important primary standard instruments. Eppley normal incidence pyrheliometer (NIP) is a common instrument used for practical measurements in the US, and Kipp and Zonen actinometer is widely used in Europe. Both of these instruments are calibrated against the primary standard methods. Based on their design, the above listed instruments measure the beam radiation coming from the sun and a small portion of the sky around the sun. Based on the experimental studies involving various pyrheliometer design, the contribution of the circumsolar sky to the beam is relatively negligible on a sunny day with clear skies. However, a hazy sky or a uniform thin cloud cover redistributes the radiation so that contribution of the circumsolar sky to the measurement may become more significant.
- Pyranometer is used to measure total hemispherical radiation – beam plus diffuse – on a horizontal surface. If shaded, a pyranometer measures diffuse radiation. Most of solar resource data come from pyranometers. The total irradiance (W/m 2 ) measured on a horizontal surface by a pyranometer is expressed as follows:
\ (2.1) where θ is the zenith angle (i.e., angle between the incident ray and the normal to the horizontal instrument plane. Examples of pyranometers are Eppley 180 o or Eppley black-and-white pyranometers in the US and Moll-Gorczynsky pyranometer in Europe. These instruments are usually calibrated against standard pyrheliometers. There are pyranometers with thermocouple detectors and with photovoltaic detectors. The detectors ideally should be independent on the wavelength of the solar spectrum and angle of incidence. Pyranometers are also used to measure solar radiation on inclined surfaces, which is important for estimating input to collectors. Calibration of pyranometers depends on the inclination angle, so experimental data are needed to interpret the measurements.
- Photoelectric sunshine recorder, The natural solar radiation is notoriously intermittent and varying in intensity. The most potent radiation that creates the highest potential for concentration and conversion is the bright sunshine, which has a large beam component. The duration of the bright sunshine at a locale is measured, for example, by a photoelectric sunshine recorder. The device has two selenium photovoltaic cells, one of which is shaded, and the other is exposed to the available solar radiation. When there is no beam radiation, the signal output from both cells is similar, while in bright sunshine, signal difference between the two cells is maximized. This technique can be used to monitor the bright sunshine hours. More detailed explanation of how these instruments work and what kind of data are obtained from those measurements is available in the following Duffie and Beckman (2013) book referred below. Please spend some time acquiring basic knowledge on solar resource data. For everyone who took EME 810 and is more or less familiar with this topic, this still may be a useful refresher.
Contents
- 1 What instrument measures solar radiation?
- 2 What is the intensity of solar radiation?
- 3 How do you calculate solar radiation from temperature?
- 4 Which of the following devices is used for measuring beam radiation?
- 5 What is the formula of solar energy?
- 6 What is the relationship between radiation and temperature?
- 7 Why is mean intensity divided by 4?
- 8 How is irradiance calculated?
How do you calculate solar radiation intensity?
Solar Radiation outside the Atmosphere – The spectrum of the radiation emitted by the sun is close to that of a black body at a temperature of 5,900K. About 8% of the energy is in the ultra-violet region, 44% is in the visible region, and 48% is in the infra-red region.
- The solar constant I 0 is the beam solar radiation outside the Earth’s atmosphere when the sun is at its mean distance from the Earth.
- Its value is I 0 = 1.37 ± 0.02kW/m 2,
- Variations in the distance of the sun from the Earth due to the ellipticity of the Earth’s orbit cause the actual intensity of solar radiation outside the atmosphere to depart from I 0 by a few percent.
Allowance for these variations can be made by means of the factor F = 1 – 0.0335 sin 360(n d – 94)/365, where n d is the day of the year (on 1 January n d = 1; on 31 December n d = 365); the argument of the sine function is in degrees. All the values of solar radiation intensity given below, which are for the sun at its mean distance from the Earth, must be multiplied by F to obtain the actual values on day n d,
What instrument measures solar radiation?
A pyrheliometer is an instrument for measurement of the direct solar radiation flux at normal incidence.
What is the intensity of solar radiation?
Solar Radiation Solar radiation is measured atop the meteorological mast at the shore laboratory using an Eppley Model PSP (Precision Spectral Pyranometer). Approximately 99% of solar, or short-wave, radiation at the earth’s surface is contained in the region from 0.3 to 3.0 µm, which corresponds to wavelength between the ultraviolet and near infrared.
- Above the earth’s atmosphere, solar radiation has an intensity of approximately 1380 watts per square meter (W/m2).
- This value is known as the Solar Constant.
- At our latitude, the value at the surface is approximately 1000 W/m2 on a clear day at solar noon in the summer months.
- The difference between this value and the Solar Constant is due to transmission loss to the atmosphere.
The clear sky value is considerable less in the winter. Clouds can dramatically reduce this value by reflecting the solar radiation back out to space. The PSP senses this radiation with a thermopile that produces a millivolt signal that is directly proportional to the downwelling solar irradiance.
How do you calculate solar radiation from temperature?
The related computer equation (radiation vs temperature) is R=3E-26e0.2117T (3), where R is the instant solar radiation and T is the moment temperature.
How do you calculate solar radiation from sunshine hours?
The correlation is given as S = – 22.424 + 0.272RH + 1.388T – 9.791RF – 0.623W Where RH is the Relative Humidity, T is the difference of the Maximum and Minimum Temperature, RF is the average rainfall and W is the Wind Speed.
What is solar irradiance and How Is It measured?
Units – The SI unit of irradiance is watts per square metre (W/m 2 = Wm −2 ). The unit of insolation often used in the solar power industry is kilowatt hours per square metre (kWh/m 2 ). The Langley is an alternative unit of insolation. One Langley is one thermochemical calorie per square centimetre or 41,840 J/m 2,
Which of the following devices is used for measuring beam radiation?
The instrument used for measuring the intensity of direct solar radiation, that is, beam radiation is called a pyrheliometer.
Why is mean intensity divided by 4?
We divide by 4 since the solar energy is spread over the surface of the planetary sphere. The Earth intercepts a circular area of incoming sunlight, and this area is spread over a sphere with the same radius as the circle (area of circle / area of sphere of same radius = 0.25).
How is irradiance calculated?
Spectral Irradiance The spectral irradiance as a function of photon wavelength (or energy), denoted by F, is the most common way of characterising a light source. It gives the power density at a particular wavelength. The units of spectral irradiance are in Wm -2 µm -1,
- The Wm -2 term is the power density at the wavelength λ(µm).
- Therefore, the m -2 refers to the surface area of the light emitter and the µm -1 refers to the wavelength of interest.
- In the analysis of solar cells, the photon flux is often needed as well as the spectral irradiance.
- The spectral irradiance can be determined from the photon flux by converting the photon flux at a given wavelength to W/m 2 as shown in the section on,
The result is then divided by the given wavelength, as shown in the equation below.
What is the formula of solar energy?
Globally a formula E = A x r x H x PR is followed to estimate the electricity generated in output of a photovoltaic system. Example : the solar panel yield of a PV module of 250 Wp with an area of 1.6 m² is 15.6%.
What is the relationship between radiation and temperature?
Correlation Between Temperature and Radiation – Kelvin Temperature Scale In the 19th century, Lord Kelvin created the Kelvin temperature scale to measure very low temperatures. Because zero Kelvin is considered to be the lowest temperature possible, it is described as absolute zero. There are no negative numbers in the Kelvin scale. Image Credit: Wikipedia Image Credit: Microsoft Clip Art When an object is hot enough, you can see the radiation it emits as visible light. For example, when a stovetop burner reaches 1,000 Kelvin (K) — 726° Celsius (C) or 1,340° Fahrenheit (F) — it will glow red. All objects actually emit radiation if their temperature is greater than absolute zero.
- Absolute zero is equal to zero Kelvin, which is equal to -273°C or -460°F.
- Both the sun and Earth’s surface behave as blackbodies.
- An object that absorbs and emits all possible radiation at 100 percent efficiency is called a blackbody.
- For this reason, the following two laws (Stefan-Boltzmann and Wein’s laws) can be used to explain the correlation between temperature and radiation for the sun and Earth.
The Stefan-Boltzmann law, a fundamental law of physics, explains the relationship between an object’s temperature and the amount of radiation that it emits. This law (expressed mathematically as E = σT4) states that all objects with temperatures above absolute zero (0K or -273°C or -459°F) emit radiation at a rate proportional to the fourth power of their absolute temperature.
- E = σT4 Stefan-Boltzmann Law E represents the maximum rate of radiation (often referred to as energy flux) emitted by each square meter of the object’s surface.
- The Greek letter ” σ ” (sigma) represents the Stefan-Boltzmann constant (5.67 x 10 -8 W/m 2 K 4 ); and T is the object’s surface temperature in Kelvin.
The W refers to watt, which is the unit used to express power (expressed in joules per second). Image Credit: NASA ( Sun & Earth ) Using the Stefan-Boltzmann law, let’s compare the sun’s average surface temperature of approximately 6,000K (5,727°C or 10,340°F) with Earth’s average surface temperature of just 288K (15°C or 59°F). Consistent with the Stefan-Boltzmann law, the sun emits more radiation than Earth.
Wien’s law, another law of physics, (expressed mathematically as λ max = constant/T) explains the relationship between the object’s temperature and the wavelength it emits. λ max = constant/T Wien’s Law The wavelength at which maximum radiation is emitted is expressed by the Greek letter ” λ ” (lambda).
T is the object’s temperature in Kelvin, and the constant is 2,897 μ m (micrometers). The higher the object’s temperature, the faster the molecules will vibrate and the shorter the wavelength will be. Image Credit: NASA ( Sun & Earth ) Consequently, Wein’s law explains why the hot sun emits radiation at relatively shorter wavelengths, with the maximum emission in the visible region of the spectrum, whereas the relatively cool Earth emits almost all of its energy at longer wavelengths in the infrared region of the spectrum.
Why is mean intensity divided by 4?
We divide by 4 since the solar energy is spread over the surface of the planetary sphere. The Earth intercepts a circular area of incoming sunlight, and this area is spread over a sphere with the same radius as the circle (area of circle / area of sphere of same radius = 0.25).
How is irradiance calculated?
Spectral Irradiance The spectral irradiance as a function of photon wavelength (or energy), denoted by F, is the most common way of characterising a light source. It gives the power density at a particular wavelength. The units of spectral irradiance are in Wm -2 µm -1,
- The Wm -2 term is the power density at the wavelength λ(µm).
- Therefore, the m -2 refers to the surface area of the light emitter and the µm -1 refers to the wavelength of interest.
- In the analysis of solar cells, the photon flux is often needed as well as the spectral irradiance.
- The spectral irradiance can be determined from the photon flux by converting the photon flux at a given wavelength to W/m 2 as shown in the section on,
The result is then divided by the given wavelength, as shown in the equation below.
How do you calculate incident radiation?
5.5 Physical properties of specular reflection – Reflection describes the process whereby incident radiation bounces off the surface of a substance in a single predictable direction, which is also known as specular reflection. The angle of reflection is always equal and opposite to the angle of incidence, θ i = θ r ( Fig.5.6 ). Fig.5.6, Photon reflection. Reflection is caused by surfaces that are smooth to the wavelengths of incident photons. These smooth, mirror-like surfaces are called specular reflectors. Consequently, specular reflection causes no change to either the photon velocity or wavelength.
- Consistent with Hecht, the theoretical amplitude reflectance of a dielectric interface can be derived from electromagnetic theory.
- In this regard, E → is polarized perpendicular to the plane of incidence: (5.23) r ⊥ = n 1 cos θ i − n 2 cos θ r n 1 cos θ i + n 2 cos θ r ; For E →, a polarized parallel to the plane of incidence is then: (5.24) r ∥ = n 2 cos θ i − n 1 cos θ r n 2 cos θ i + n 1 cos θ r ; where n 1, θ i, n 2, and θ r are the refractive indices and angles of incidences and refraction, respectively.
Here, r is the ratio of the amplitude of the reflected electric field to the incident field. Consequently, the intensity of the reflected photons is the square of this value, In this regard, the incidence of the photons undergoes several mechanisms. The reflection is achieved when the angle of incidence, θ i, is equal to the angle of reflection, θ r ( Fig.5.6 ).
- Conversely, coplanarity occurs as the incident photon, the reflected photon, and the normal to the mirror n exist in the same plane.
- In this view, the frequency of the incident photon, ω i, is equal to the frequency of the reflected light, ω r, which is known as in/out frequency invariance.
- Furthermore, the photon travels on the path of least traversal time, which is presented as Fermat’s principle.
In other words, the photon travels over the shortest path. Finally, the scattering of photons by a mirror is owing to Rayleigh’s scattering processes with a cross-section dσ ∝ ω 4, This mechanism is known as the quartic frequency cross-section, Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780128181119000057
What is the intensity of the solar radiation at Jupiter’s distance?
Solar Radiation – The table below gives standardised values for the radiation at each of the planets but by entering the distance you can obtain an approximation. The distance to the sun varies for each planet since the orbits are elliptical not linear.
| Planet | Distance (x 10 9 m) | Mean Solar Irradiance (W/m 2 ) |
|---|---|---|
| Mercury | 57 | 9116.4 |
| Venus | 108 | 2611.0 |
| Earth | 150 | 1366.1 |
| Mars | 227 | 588.6 |
| Jupiter | 778 | 50.5 |
| Saturn | 1426 | 15.04 |
| Uranus | 2868 | 3.72 |
| Neptune | 4497 | 1.51 |
| Pluto | 5806 | 0.878 |
Further details on the planets are at: https://nssdc.gsfc.nasa.gov/planetary/factsheet/
