Wind energy, used for endured of years to provide mechanical energy or for transportation, uses air currents that are created by solar heated alarm and the rotation of the earth. Today wind turbines convert wind power Into electricity as well as Its traditional uses. Even hydroelectricity is derived from the sun. Hydrophone depends on the evaporation of water by the sun, and its subsequent return to the Earth as rain to provide water in dams. Photovoltaic(often abbreviated as IV) is a simple and elegant method of harnessing the sun’s energy.
IV devices (solar cells) are unique in that they directly invert the incident solar radiation into electricity, with no noise, pollution or moving parts, making them robust, reliable and long lasting. Solar cells are based on the same principles and materials behind the communications and computer revolutions, and this CDR covers the operation, use and applications of photovoltaic devices and systems. Introduction Photovoltaic is the process of converting sunlight directly into electricity using solar cells.
Today it is a rapidly growing and increasingly important renewable alternative to conventional fossil fuel electricity generation, but compared to other electricity enervating technologies, It Is a relative newcomer, with the first practical photovoltaic devices demonstrated in the asses. Research and development of photovoltaic received its first major boost from the space industry in the asses which required a power supply separate from “grid” power for satellite applications.
These space solar cells were several thousand times more expensive than they are today and the perceived need for an electricity generation method apart from grid power was still a decade away, but solar cells became an interesting scientific variation to the rapidly expanding silicon transistor development with several potentially specialized niche markets. It took the OLL crisis In the asses to focus world attention on the desirability of alternate energy sources for terrestrial use, which in turn promoted the investigation of photovoltaic as a means of generating terrestrial power.
Although the oil crisis proved short-lived and the financial incentive to develop solar cells abated, solar cells had entered the arena as a power generating technology. Their application and advantage to the “remote” power supply area was quickly recognized ND prompted the development of terrestrial photovoltaic industry. Small scale transportable applications (such as calculators and watches) were utilized and remote power applications began to benefit from photovoltaic. In the asses research Into colon solar cells paid off and solar cells began to Increase their efficiency.
In 1 985 silicon solar cells achieved the milestone of 20% efficiency. Over the next and 20%, largely promoted by the remote power supply market. The year 1997 saw a growth rate of 38% and today solar cells are recognized not only as a means for roving power and increased quality of life to those who do not have grid access, but they are also a means of significantly diminishing the impact of environmental damage caused by conventional electricity generation in advanced industrial countries.
The increasing market for, and profile of photovoltaic means that more applications than ever before are “photographically powered”. These applications range from power stations of several megawatts to the ubiquitous solar calculators. PICADOR aims to provide an overview of terrestrial photovoltaic to furnish the non- specialist with basic information. It is hoped that having used PICADOR you will understand the principles of photovoltaic devices and system operation, you will be able to identify appropriate applications, and you will be capable of undertaking photovoltaic system design.
By gradually increasing the number of people who are familiar with photovoltaic concepts and applications, we hope to increase the use of photovoltaic in appropriate applications. Properties of sun light The light that we see everyday is only a fraction of the total energy emitted by the sun incident on the earth. Sunlight is a form of “electromagnetic radiation” and the kibbles light that we see is a small subset of the electromagnetic spectrum shown at the right. The electromagnetic spectrum describes light as a wave which has a particular wavelength.
The description of light as a wave first gained acceptance in the early sass’s when experiments by Thomas Young, Francis Aragua, and Augustan Jean Freeness showed interference effects in light beams, indicating that light is made of waves. By the late sass’s light was viewed as part of the electromagnetic spectrum. However, in the late sass’s a problem with the wave-based view of light became apparent when experiments measuring the spectrum of wavelengths from heated objects could not be explained using the wave-based equations of light. This discrepancy was resolved by the works of 1 in 1900, and 2 in 1905.
Planck proposed that the total energy of light is made up of indistinguishable energy elements, or a quanta of energy. Einstein, while examining the photoelectric effect (the release of electrons from certain metals and semiconductors when struck by light), correctly distinguished the values of these quantum energy elements. For their work in this area Planck and Einstein won the Nobel prize for physics in 1918 and 1921, especially and based on this work, light may be viewed as consisting of “packets” or particles of energy, called photons..
Today, quantum-mechanics explains both the observations of the wave nature and the particle nature of light. In quantum mechanics, a photon, like all other quantum-mechanical particles such as electrons, protons etc, is most accurately pictured as a “wave-packet”. A wave packet is defined as a collection of waves which may interact in such a way that the wave-packet may either appear spatially localized (in a similar fashion as a square wave which results room the addition of an infinite number of sine waves), or may alternately appear simply as a wave.
In the cases where the wave-packet is spatially localized, it acts as a particle. Therefore, depending on the situation, a photon may appear as either a wave or as a particle and this concept is called “wave-particle duality”. A wave-packet, or photon is pictured as used in PICADOR below. A complete physical description of a type of quantum-mechanical particle called a photon. For photovoltaic applications, this level of detail is seldom required and therefore only a few sentences on the quantum nature of light are given here.
However, in some situations (fortunately, rarely encountered in IV systems), light may behave in a manner which seems to defy common sense, based on the simple explanations given here. The term “common sense” refers to our own observations and cannot be relied on to observe the quantum-mechanical effects because these occur under conditions outside the range of human observation. For further information on the modern interpretation of light please refer to 3. There are several key characteristics of the incident solar energy which are critical in determining how the incident sunlight interacts with a hotfooting converter or any other object.
The important characteristics of the incident solar energy are: * the spectral content of the incident light; * the radiant power density from the sun; * the angle at which the incident solar radiation strikes a photovoltaic module; and * the radiant energy from the sun throughout a year or day for a particular surface. Energy of Photon A photon is characterized by either a wavelength, denoted by or equivalently an energy, denoted by E. There is an inverse relationship between the energy of a photon (E) and the wavelength of the light (X) given by the equation: where h is
Plank’s constant and c is the speed of light. The value of these and other commonly used constants is given in the constants page. H = 6. 626 x 10-34 Joule;s c = 2. 998 x 108 revs By multiplying to get a single expression, HCI = 1. 99 x 10-25 Joules-m The above inverse relationship means that light consisting of high energy photons (such as “blue” light) has a short wavelength. Light consisting of low energy photons (such as “red” light) has a long wavelength. When dealing with “particles” such as photons or electrons, a commonly used unit of energy is the electron-volt (eve) rather than the joule 0).
An electron volt is the energy required to raise an electron through 1 volt, thus a photon with an energy of 1 eve= 1. 602 x 10-19 J. Therefore, we can rewrite the above constant for HCI in terms of eve: HCI = (1. 99 x 10-29 Joules-m) x (level/l . 602 x 10-19 joules) = 1. 24 x 10-6 eve-m Further, we need to have the units be in in pm (the units for X): HCI = (1. 24 x 10-6 eve-m) x(l . Xx 106 pm/ m) = 1. 24 eve-pm By expressing the equation for photon energy in terms of eve and pm we arrive at a commonly used expression which relates the energy and wavelength of a photon, as shown in the following equation: E(eve)=1. Pm) The exact value of 1 x 106(HCI/q) is 1. 2398 but the approximation 1. 24 is sufficient for most purposes. Photon Flux The photon flux is defined as the number of photons per second per unit area: The photon flux is important in determining the number of electrons which are generated, and hence the current produced from a solar cell. As the photon flux does not give information about the energy (or wavelength) of the photons, the energy or wavelength of the photons in the light source must also be specified. At a given wavelength, the combination of the photon wavelength or energy and the photon flux reticular wavelength.
The power density is calculated by multiplying the photon flux by the energy of a single photon. Since the photon flux gives the number of photons striking a surface in a given time, multiplying by the energy of the photons comprising the photon flux gives the energy striking a surface per unit time, which is equivalent to a power density units of W/mm, the energy of the photons must be in Joules. The equation is: using SSL units for wavelength in pm for energy in eve where is the photon flux and q is the value of the electronic charge 1. 6 X 10-19 Radiant Power Density
The total power density emitted from a light source can be calculated by integrating the spectral radiance over all wavelengths or energies where: H is the total power density emitted from the light source in W m-2; F(X) is the spectral radiance in units foam-pm-1; and dc is the wavelength. However, a closed form equation for the spectral radiance for a light source often does not exist. Instead the measured spectral radiance must be multiplied by a wavelength range over which it was measured, and then calculated over all wavelengths.
The following equation can be used to calculate the total power density emitted from a light source. AX is the wavelength. Calculating the total power density from a source requires integrating over the spectrum by calculating the area of each element and then summing them together. Measured spectra are typically not smooth as they contain emission and absorption lines. The wavelength spacing is usually not uniform to allow for more data points in the rapidly changing parts of the spectrum. The spectral width is calculated from the mid-points between two the adjacent wavelengths.
Power in each segment is then: Summing all the segments gives the total power H as in the equation above. Blackbody Radiation Many commonly encountered light sources, including the sun and incandescent light bulbs, are closely modeled as “blackbody” emitters. A blackbody absorbs all radiation incident on its surface and emits radiation based on its temperature. Blackbodies derive their name from the fact that, if they do not emit radiation in the The blackbody sources which are of interest to photovoltaic, emit light in the visible region.
The spectral radiance from a blackbody is given by Plank’s radiation law, shown in the following equation: is the wavelength of light; T is the temperature of the blackbody (K); F is the spectral radiance in Whom-pm-1; and h,c and k are constants. Getting the correct result requires care with the units. The simplest is to use SSL units so that c is in m/s, h is in Joule;seconds, T is in Kelvin, k is in Joule/Kelvin, and is in meters. This will give units of spectral radiance in Whom-3.
Dividing by 106 gives the conventional units of spectral radiance in Whom-pm-1. The notation of F(X) denotes that the spectral radiance changes with wavelength. The total power density from a blackbody is determined by integrating the spectral radiance over all wavelengths which gives: where o is the Stefan-Balletomane constant and T is the temperature of the blackbody in Kelvin. An additional important parameter of a blackbody source is the wavelength where the spectral radiance is the highest, or, in other words the wavelength where most of the power is emitted.
The peak wavelength of the spectral radiance is determined by differentiating the spectral radiance and solving the derivative when it equals O. The result is known as Wine’s Law and is shown in the following equation: where Xp is the wavelength where the peak spectral radiance is emitted and T is the temperature of the blackbody (K). The above equations show that as the temperature of a blackbody increases, the spectral distribution and power of light emitted change.
For example, near room temperature, a blackbody emitter (such as a human body or light bulb which is turned of will emit low power radiation at wavelengths predominantly greater than 1 pm, well outside the visual range of human observation. If the blackbody is heated to 3000 K, it will glow red because the spectrum of emitted light shifts to higher energies and into the visible spectrum. If the temperature of the filament is further increased to 6000 K, radiation is emitted at avalanches across the visible spectrum from red to violet and the light appears white.
The graphs below compare the spectral radiance of a blackbody at these three temperatures. The room temperature case of KICK (the black dotted line) has essentially no power emitted in the visible and near infrared portions of the spectrum shown on the graph. Because of the huge variation in both emitted power and the range of wavelengths over which the power is emitted, the log graph below demonstrates more clearly the variation in the emitted blackbody spectrum as a function of temperature. Spectral intensity of light emitted from a black body on a log-log scale.
At room temperature the emission is very low and centered around 10 GM. The Sun The sun is a hot sphere of gas whose internal temperatures reach over 20 million degrees Kelvin due to nuclear fusion reactions at the sun’s core which convert hydrogen to helium. The radiation from the inner core is not visible since it is transferred through this layer by convections . The surface of the sun, called the photosphere, is at a temperature of about KICK and closely approximates a blackbody (see graph).
For simplicity, the 6000 K spectrum is commonly used in detailed balance calculations but temperatures of 5762 В± 50 K 2 and 5730 В± 90 K have also been proposed as a more accurate fit to the sun’s spectrum. The total power emitted by the sun is calculated by multiplying the emitted power density by the surface area of the sun which gives 9. 5 x 1025 W. The total power emitted from the sun is composed not of a single wavelength, but is composed of many wavelengths and therefore appears white or yellow to the human eye. These different wavelengths can be seen by passing light through a prism, or water droplets in the case of a rainbow.
Different wavelengths show up as different colors, UT not all the wavelengths can be seen since some are “invisible” to the human eye. Solar Radiation in Space Only a fraction of the total power emitted by the sun impinges on an object in space which is some distance from the sun. The solar radiance (HO in W/mm) is the power density incident on an object due to illumination from the sun. At the sun’s surface, the power density is that of a blackbody at about KICK and the total power from the sun is this value multiplied by the sun’s surface area.
However, at some distance from the sun, the total power from the sun is now spread out over a much larger reface area and therefore the solar radiance on an object in space decreases as the object moves further away from the sun. The solar radiance on an object some distance D from the sun is found by dividing the total power emitted from the sun by the surface area over which the sunlight falls. The total solar radiation emitted by the sun is given by tot multiplied by the surface area of the sun (nurse) where RSN is the radius of the sun.
The surface area over which the power from the sun falls will be TIDE. Where D is the distance of the object from the sun. Therefore, the alarm radiation intensity, HO in (W/mm), incident on an object is: where:Hush is the power density at the sun’s surface (in W/mm) as determined by Stefan-Abolition’s blackbody equation; RSN is the radius of the sun in meters as shown in the figure below; and D is the distance from the sun in meters as shown in the figure below. At a distance, D, from the sun the same amount of power is spread over a much wider area so the solar radiation power intensity is reduced.
Solar Radiation Outside the Earth’s Atmosphere The solar radiation outside the earth’s atmosphere is calculated using the radiant power density (Hush) at the sun’s surface (5. 61 x 107 W/mm), the radius of the sun (RSN), and the distance between the earth and the sun. The calculated solar radiance at the Earth’s atmosphere is about 1. 36 k/mm. The geometrical constants used in the calculation of the solar radiance incident on the Earth are shown in the figure below. Geometrical constants for finding the Earth’s solar radiance.
The diameter of the Earth is not needed but is included for the sake of completeness. The actual power elliptical orbit around the sun, and because the sun’s emitted power is not constant. The power variation due to the elliptical orbit is about 3. %, with the largest solar radiance in January and the smallest solar radiance in July. An equation 1 which describes the variation through out the year Just outside the earth’s atmosphere is: H is the radiant power density outside the Earth’s atmosphere (in W/mm); Hesitant is the value of the solar constant, 1. 53 k/mm; and n is the day of the year. These variations are typically small and for photovoltaic applications the solar radiance can be considered constant. The value of the solar constant and its spectrum have been defined as a standard value called air mass zero (AMMO) and takes a value of 1. 53 k/mm The spectral radiance is given below: Standard Solar Spectra The solar spectrum changes throughout the day and with location. Standard reference spectra are defined to allow the performance comparison of photovoltaic devices from different manufacturers and research laboratories.
The standard spectra were refined in the early sass’s to increase the resolution and to co-ordinate the standards internationally. The previous solar spectrum, ASTHMA 59, was withdrawn from use in 2005. In most cases, the difference between the spectrum has little effect on device performance and the newer spectra are easier to use. Further details on solar spectra are available at: https://www. Unreel. Gob/solar_radiation/ ASTM E-490 The standard spectrum for space applications is referred to as AMMO. It has an integrated power of 1366. W/mm ASTM 6-173-03 (International standard ISO 9845-1, 1992) Two standards are defined for terrestrial use. The AIM . 5 Global spectrum is designed for flat plate modules and has an integrated power of 1000 W/mm (100 mow/ CM). The AIM . 5 Direct (+circumpolar) spectrum is defined for solar concentrator work. It includes the the direct beam from the sun plus the circumpolar component in a disk 2. 5 degrees around the sun. The direct plus circumpolar spectrum has an integrated power density of 900 W/mm.
The SMARTS (Simple Model of the Atmospheric Irradiative Transfer of Sunshine) program is used to generate the standard spectra and can also be used to generate other spectra as required. Solar Radiation at the Earth’s Surface While the solar radiation incident on the Earth’s atmosphere is relatively constant, the radiation at the Earth’s surface varies widely due to: * atmospheric effects, including absorption and scattering; * local variations in the atmosphere, such as water vapor, clouds, and pollution; * latitude of the location; and the season of the year and the time of day.
The above effects have several impacts on the solar radiation received at the Earth’s surface. These changes include variations in the overall power received, the spectral content of the light and the angle from which light is incident on a surface. In addition, a key change is that the variability of the solar radiation at a particular clouds and seasonal variations, as well as other effects such as the length of the day at a particular latitude.
Desert regions tend to have lower variations due to local atmospheric phenomena such as clouds. Equatorial regions have low variability teen seasons. (The amount of energy reaching the surface of the Earth every hour is greater than the amount of energy used by the Earth’s population over an entire year. ) Atmospheric Effects Atmospheric effects have several impacts on the solar radiation at the Earth’s surface.
The major effects for photovoltaic applications are: * a reduction in the power of the solar radiation due to absorption, scattering and reflection in the atmosphere; * a change in the spectral content of the solar radiation due to greater absorption or scattering of some wavelengths; * the introduction of a diffuse or indirect component into the solar radiation; and * local variations in the atmosphere (such as water vapor, clouds and pollution) which have additional effects on the incident power, spectrum and directionality. These effects are summarized in the figure below.
Absorption in the Atmosphere As solar radiation passes through the atmosphere, gases, dust and aerosols absorb the incident photons. Specific gases, notably ozone (03), carbon dioxide (CO), and water vapor (H2O), have very high absorption of photons that have energies close to the bond energies of these atmospheric gases. This absorption yields deep troughs in the spectral radiation curve. For example, much of the far infrared light above 2 pm is absorbed by water vapor and carbon dioxide. Similarly, most of the ultraviolet light below 0. Pm is absorbed by ozone (but not enough to completely prevent sunburn! ). While the absorption by specific gases in the atmosphere change the spectral content of the terrestrial solar radiation, they have a relatively minor impact on the overall power. Instead, the major factor reducing the power from solar radiation is the absorption and scattering of light due to air molecules and dust. This absorption process does not produce the deep troughs in the spectral radiance, but rather causes a power reduction dependent on the path length through the atmosphere.
When the sun is overhead, the absorption due to these atmospheric elements causes a relatively uniform reduction across the visible spectrum, so the incident light appears white. However, for longer path lengths, higher energy (lower wavelength) light is more effectively absorbed and scattered. Hence in the morning and evening the sun appears much redder and has a lower intensity than in the middle of the day. A comparison of solar radiation outside the Earth’s atmosphere with the amount of solar radiation reaching the Earth itself.
The human eye has evolved to the point where sensitivity is greatest at the most intense wavelengths 2. Direct and Diffuse Radiation Due to Scattering of Incident Light Light is absorbed as it passes through the atmosphere and at the same time it is subject to scattering. One of the which is caused by molecules in the atmosphere. Raleigh scattering is particularly effective for short wavelength light (that is blue light) since it has a X-4 dependence.
In addition to Raleigh scattering, aerosols and dust particles contribute to the scattering of incident light known as Mime scattering. I Scattered light is undirected, and so it appears to be coming from any region of the sky. This light is called “diffuse” light. Since diffuse light is primarily “blue” light, the light that comes from regions of the sky other than where the sun is, appears blue. In the absence of scattering in the atmosphere, the sky would appear black, and the sun would appear as a disk light source.
On a clear day, about 10% of the total incident solar radiation is diffuse. Effect of clouds and other local variations in the atmosphere The final effect of the atmosphere on incident solar radiation is due to local variations in the atmosphere. Depending on the type of cloud cover, the incident power is severely reduced. An example of heavy cloud cover is shown below. Relative output current from a photovoltaic array on a sunny and a cloudy winter’s day in Melbourne with an array tilt angle of 600 3.
Air Mass The Air Mass is the path length which light takes through the atmosphere normalized to the shortest possible path length (that is, when the sun is directly overhead). The Air Mass quantifies the reduction in the power of light as it passes through the atmosphere and is absorbed by air and dust. The Air Mass is defined as: The air mass represents the proportion of atmosphere that the light must pass through before striking the Earth relative to its overhead path length, and is equal to HIS where B is the angle from the vertical (zenith angle).
An easy method to determine the air mass is from the shadow of a vertical pole. When the sun is directly overhead, the Air Mass is Air mass is the length of the hypotenuse divided by the object height h, and from Pythagoras theorem we get The above calculation for air mass assumes that the atmosphere is a flat horizontal layer, but because of the curvature of the atmosphere, the air mass is not quite equal to the atmospheric path length when the sun is close to the horizon. At sunrise, the angle of the sun from the vertical position is 900 and the air mass is infinite, whereas the path length clearly is not.
An equation which incorporates the curvature of the earth is Standardized Solar Spectrum and Solar Irradiation The efficiency of a solar cell is sensitive to variations in both the power and the spectrum of the incident light. To facilitate an accurate comparison between solar cells measured at different times and locations, a standard spectrum and power density has been defined for both radiation outside the Earth’s atmosphere and at the Earth’s surface. The standard spectrum at the Earth’s surface is called AIM . 6, (the G stands for global and includes both direct and diffuse radiation) or AIM . AD (which includes direct radiation only). The intensity of AIM . AD radiation can be 10% to scattering). The global spectrum is 10% higher than the direct spectrum. These calculations give approximately 970 W/mm for AIM . 56. However, the standard AIM . 6 spectrum has been normalized to give 1 k/mm due to the convenience of the round number and the fact that there are inherently variations in incident solar radiation. The standard spectrum is listed in the Appendix page.
The standard spectrum outside the Earth’s atmosphere is called AMMO, because at no stage does the light pass through the atmosphere. This spectrum is typically used to predict the expected performance of cells in space. Intensity Calculations Based on the Air Mass The intensity of the direct component of sunlight throughout each day can be determined as a function of air mass from the experimentally determined equation here ID is the intensity on a plane perpendicular to the sun’s rays in units of k/ mm and AM is the air mass. The value of 1. 53 k/miss the solar constant and the number 0. 7 arises from the fact that about 70% of the radiation incident on the atmosphere is transmitted to the Earth. The extra power term of 0. 678 is an empirical fit to the observed data and takes into account the non-uniformities in the atmospheric layers. Sunlight intensity increases with the height above sea level. The spectral content of sunlight also changes making the sky ‘bluer’ on high mountains. Much of the southwest of the United States is two kilometers above sea level, adding significantly to solar isolation.
A simple empirical fit to observed data and accurate to a few kilometers above sea level is given by: where a = 0. 14 and h is the location height above sea level in kilometers. Even on a clear day, the diffuse radiation is still about 10% of the direct component. Thus on a clear day the global radiance on a module perpendicular to the sun’s rays is: Motion of the Sun The apparent motion of the sun, caused by the rotation of the Earth about its axis, hanged the angle at which the direct component of light will strike the Earth.
From a fixed location on Earth, the sun appears to move throughout the sky. The position of the sun depends on the location of a point on Earth, the time of day and the time of year. This apparent motion of the sun is shown in the figure below. Path of the sun in the southern hemisphere. This apparent motion of the sun has a major impact on the amount of power received by a solar collector. When the sun’s rays are perpendicular to the absorbing surface, the power density on the surface is equal to the incident power density.
