The Earth and the Sun
Very powerful sources of radiant energy are the stars. However, given the enormous distance to which they are and, since the radiation they give off is attenuated as they progressively spread over a larger and larger spherical surface as it spreads through space, the effects they produce on Earth are very small.
However, one of them, the Sun, because of its proximity, is capable of bringing to us such an amount of radiant energy that has established the physical conditions that reign on the planet since its formation, including those that define what we know as life .
We are interested in the quantitative aspects of the Sun's energy, without going into a deep description of the physical-chemical transformations that it undergoes once it affects the earth's surface.
However, one of them, the Sun, because of its proximity, is capable of bringing to us such an amount of radiant energy that has established the physical conditions that reign on the planet since its formation, including those that define what we know as life .
We are interested in the quantitative aspects of the Sun's energy, without going into a deep description of the physical-chemical transformations that it undergoes once it affects the earth's surface.
Numerical data
The Sun is a fairly common star, with the only feature that it is only at a distance of about 150 million kilometers from Earth. The radiation it emits takes more than 8 minutes to reach our planet, at a rate of about 300,000 km / s. Its diameter is about 1,400,000 km and its mass is equivalent to that of about 300,000 planets equal to Earth.
Like all stars, the Sun is a gigantic nuclear furnace in which the mass is continuously converted into radiant energy, the time that will pass until it is completely extinguished is calculated in more than 5,000 million years.
Of that huge amount of radiant energy, only a very small fraction reaches our planet, although it represents a very large amount compared to the energy we need to maintain our technological civilization. The problem is not the total amount of energy available, but the difficulties for its use, since it is dispersed, distributed over the entire surface of the earth and the oceans. On average, the amount of energy that reaches our outer atmosphere is equivalent to a power of 1.4 kW per m², an amount that is reduced to about 1 kW when it passes through the atmosphere and reaches the ground.
The effective temperature of the Sun's surface is about 5,600 ° C. This data is important because the characteristics of the radiation emitted by a body are a function of its surface temperature. The temperature of 5,600 ° C is higher than the temperature normally attainable in the usual industrial processes that man can produce artificially. Hence, the characteristics of solar radiation are significantly different from those of other artificial radiation sources.
Solar radiation is formed by a mixture of electromagnetic waves of different frequencies, some of them (those whose "wavelength" is between 0.4 and 0.7 µm) can be detected by the human eye, constituting what we know As visible light. Others, although not visible, also note its effects, by giving the bodies the energy that they.
The Sun is a fairly common star, with the only feature that it is only at a distance of about 150 million kilometers from Earth. The radiation it emits takes more than 8 minutes to reach our planet, at a rate of about 300,000 km / s. Its diameter is about 1,400,000 km and its mass is equivalent to that of about 300,000 planets equal to Earth.
Like all stars, the Sun is a gigantic nuclear furnace in which the mass is continuously converted into radiant energy, the time that will pass until it is completely extinguished is calculated in more than 5,000 million years.
Of that huge amount of radiant energy, only a very small fraction reaches our planet, although it represents a very large amount compared to the energy we need to maintain our technological civilization. The problem is not the total amount of energy available, but the difficulties for its use, since it is dispersed, distributed over the entire surface of the earth and the oceans. On average, the amount of energy that reaches our outer atmosphere is equivalent to a power of 1.4 kW per m², an amount that is reduced to about 1 kW when it passes through the atmosphere and reaches the ground.
The effective temperature of the Sun's surface is about 5,600 ° C. This data is important because the characteristics of the radiation emitted by a body are a function of its surface temperature. The temperature of 5,600 ° C is higher than the temperature normally attainable in the usual industrial processes that man can produce artificially. Hence, the characteristics of solar radiation are significantly different from those of other artificial radiation sources.
Solar radiation is formed by a mixture of electromagnetic waves of different frequencies, some of them (those whose "wavelength" is between 0.4 and 0.7 µm) can be detected by the human eye, constituting what we know As visible light. Others, although not visible, also note its effects, by giving the bodies the energy that they.
Position of the Earth with respect to the Sun
Even more important than the absolute amount of energy received is the inclination with which the radiation waves (that is, the sun's rays) hit a surface, since this will cause the energy to be distributed over a more or less extensive area, decreasing or increasing its intensity.
Due to the inclination of the axis of rotation of the Earth with respect to the plane of its orbit around the Sun and its spherical shape, the same point on the earth's surface receives the rays with a different inclination, depending on the time of year, and therefore , the effective energy that affects a square meter of horizontal surface varies considerably.
In winter the sun's rays fall with a small angle to the horizontal, the opposite of summer, when the angle is much greater, reaching the perpendicular in the areas near Ecuador and at the central moments of the day. For that reason, the total incident energy is much higher in summer than in winter and, if we consider the incident energy in a certain period of time - for example, one hour - it is also much higher in the central hours of the day (around noon ) that in the hours near sunrise or sunset.
Although we all know that it is the Earth that revolves around the Sun, and not vice versa, for practical purposes it is still useful, and leads to the same results, to assume that it is the Sun that revolves around our planet, describing an orbit approximately circular (actually describes a very little pronounced ellipse).
With this fictitious model, the Sun behaves like a luminaire that rises every day from the East and towards the West, describing in the sky a more or less wide arc, according to the time of the year.
In spring and summer the arc of the solar path is larger, the Sun rises more above the horizon and stays longer shining in the sky. On the contrary, in winter the points of the horizon where it leaves and is hidden are closer to each other, the path is shorter and less elevated, and the time (duration of the solar day) that elapses between sunrise and sunset It is much smaller.
Logically, the longer the duration of the solar day, the more energy can be collected throughout the day. In addition, another factor even more important than the duration of the day, is the fact that the lower the solar path, the lower the angle will affect the horizontal ground and, as stated, the intensity will be lower , having to distribute the energy in a larger area.
The weather
Another factor that determines the lesser or greater amount of energy that reaches the surface is the degree of cloudiness in the area. The clouds absorb most of the solar energy, reflecting it from the top and returning it to space. On a typical covered day the energy that manages to cross the cloud layer is only a small fraction of what would reach the surface if the sky were clear, being usually insufficient for solar energy utilization systems (with the possible Except for those based on the photovoltaic effect) they may be operational.
The clinical conditions of a given region are thus the most important factor when evaluating the practical possibilities of a solar installation. If the weather is cloudy, the chances of getting the system profitable will be slim.
Also the average air temperature and wind speed influence, although in a smaller amount than the cloudiness, especially in the flat collectors intended to heat water, because if said temperature is too low or the prevailing wind is strong, the collector will tend to Quickly lose the heat produced by solar radiation, making it difficult to transmit to the water we want to heat.
Regions with low cloudiness and not too cold constitute the ideal area where, with current technology, it is possible to take full advantage of the usual systems of thermal use of solar energy. However, it is also possible to take reasonable advantage of the low energy that can be collected in regions of high latitudes and very low temperatures.
Even more important than the absolute amount of energy received is the inclination with which the radiation waves (that is, the sun's rays) hit a surface, since this will cause the energy to be distributed over a more or less extensive area, decreasing or increasing its intensity.
Due to the inclination of the axis of rotation of the Earth with respect to the plane of its orbit around the Sun and its spherical shape, the same point on the earth's surface receives the rays with a different inclination, depending on the time of year, and therefore , the effective energy that affects a square meter of horizontal surface varies considerably.
In winter the sun's rays fall with a small angle to the horizontal, the opposite of summer, when the angle is much greater, reaching the perpendicular in the areas near Ecuador and at the central moments of the day. For that reason, the total incident energy is much higher in summer than in winter and, if we consider the incident energy in a certain period of time - for example, one hour - it is also much higher in the central hours of the day (around noon ) that in the hours near sunrise or sunset.
Although we all know that it is the Earth that revolves around the Sun, and not vice versa, for practical purposes it is still useful, and leads to the same results, to assume that it is the Sun that revolves around our planet, describing an orbit approximately circular (actually describes a very little pronounced ellipse).
With this fictitious model, the Sun behaves like a luminaire that rises every day from the East and towards the West, describing in the sky a more or less wide arc, according to the time of the year.
In spring and summer the arc of the solar path is larger, the Sun rises more above the horizon and stays longer shining in the sky. On the contrary, in winter the points of the horizon where it leaves and is hidden are closer to each other, the path is shorter and less elevated, and the time (duration of the solar day) that elapses between sunrise and sunset It is much smaller.
Logically, the longer the duration of the solar day, the more energy can be collected throughout the day. In addition, another factor even more important than the duration of the day, is the fact that the lower the solar path, the lower the angle will affect the horizontal ground and, as stated, the intensity will be lower , having to distribute the energy in a larger area.
Another factor that determines the lesser or greater amount of energy that reaches the surface is the degree of cloudiness in the area. The clouds absorb most of the solar energy, reflecting it from the top and returning it to space. On a typical covered day the energy that manages to cross the cloud layer is only a small fraction of what would reach the surface if the sky were clear, being usually insufficient for solar energy utilization systems (with the possible Except for those based on the photovoltaic effect) they may be operational.
The clinical conditions of a given region are thus the most important factor when evaluating the practical possibilities of a solar installation. If the weather is cloudy, the chances of getting the system profitable will be slim.
Also the average air temperature and wind speed influence, although in a smaller amount than the cloudiness, especially in the flat collectors intended to heat water, because if said temperature is too low or the prevailing wind is strong, the collector will tend to Quickly lose the heat produced by solar radiation, making it difficult to transmit to the water we want to heat.
Regions with low cloudiness and not too cold constitute the ideal area where, with current technology, it is possible to take full advantage of the usual systems of thermal use of solar energy. However, it is also possible to take reasonable advantage of the low energy that can be collected in regions of high latitudes and very low temperatures.
The photons
The Quantum Theory applied to electromagnetic radiation and in particular to electromagnetic solar radiation, explains that said radiation is configured in a peculiar way, being able to be treated in a very simplified first view, as a set formed by a very high number of energy "clusters" discrete called photons, which constitute the natural energy transport units.
Thus, the rays of light would be a kind of "rain" of photons, each of them carrying a tiny amount of energy, but given the huge number of photons that cross a certain section or area in every second, the net result is a considerable energy transport.
The photons only differ from each other by the value of their wavelength (or their frequency, defined as the ratio between their speed - the speed of light -, and said wavelength).
The greater or lesser intensity of the photon flow, that is, the number of photons that the unit of area passes perpendicular to the direction of its movement in the unit of time, will define the intensity of the solar radiation.
If all photons had the same wavelength, the total energy could easily be calculated by simply multiplying the unit energy of each photon (which, according to the Quantum Theory, is simply the product of a constant quantity, called Planck's Constant , by the frequency of the photon) by the number of them. The reality is more complex, since the light emitted by the Sun is formed by a very unequal mixture of photons of different wavelengths.
In the same ray of the Sun there are photons whose wavelength - to name a few figures - of half a micron, a micron, 1.2 microns, 1.5 microns, etc.
Fortunately, the relative proportion of photons according to their wavelength is always approximately the same, at least before entering the Earth's atmosphere, in which a series of phenomena can alter this proportion, although it still retains a more or less defined profile.
The relative distribution of the frequencies (or wavelengths) of the set of photons that make up the solar radiation is what is known as the solar spectrum.
Only some of the photons - those whose wavelength is between 0.3 and 3 thousandths of a millimeter - are likely to be detected by the human eye, forming what we call "visible light."
Each individual photon has a very small amount of energy, but when considering the sum of the energies of all the photons that, for example, in a minute affect a certain surface (for example that of the roof of a house), you get a amount of energy of some consideration, given the very high number of photons that, as has been said, are in a beam of solar radiation.
Solar energy reaches the outermost layers of the atmosphere approximately constantly, since at that height there are no clouds or obstacles that can mitigate it. With slight variations according to the time of the year, because the distance between the Earth and the Sun is not always the same, the value of what is known as the Solar Constant is about 1.35 kW of power for each square meter of surface perpendicular to the rays. This means that in a second about 1350 joules / m² of energy reach the stratosphere.
To the ground, the radiant power that arrives, even on favorable days (sky completely free of clouds and with clean air), does not usually exceed 1,000 W / m², always measured on a surface perpendicular to the path of the rays. This means that the attenuating effect of the atmosphere, which absorbs and deflects many of the incident photons, is considerable.
The amount of energy that solar radiation provides is not really too impressive in relative terms, compared to that generated with other commonly used devices. For example, a small plate or electric heater for heating of 1 kW of power produces the same energy, in the same period of time, as the maximum solar radiation that could be obtained on a day when the Sun shines high, on a surface of 1 m² more or less perpendicular to these rays. Taking into account that the sun's rays have a certain angle, which varies throughout the day from when it leaves until it sets, it can be calculated that the total energy incident on a full summer day does not usually exceed, at mid latitudes , the 8 kilowatt hours (in winter it is much lower). The equivalent electrical energy would represent a value of a couple of dollars, at current prices.
However, and despite its moderate value, solar energy can be very useful if properly utilized, as will be seen later.
Additionally, if the rays have to pass through the atmospheric air layer, the less perpendicular they make it the longer the path will be and the greater the mass of air they will have to overcome to reach the ground, their intensity being attenuated by the absorption effect.
All these factors are primarily responsible for, for example, the solar energy collected over a day in late autumn or early winter is much less than in a day in late spring or early summer, even when in both cases there was no cloudiness.
If, finally, to this is added that the cloudiness is greater in winter, it is easy to understand the small amount of useful energy that, on average, we can expect to use in the most unfavorable months of the year (in the Northern Hemisphere, November, December and January).
Direct and diffuse radiation
A good part of the photons that finally reach the ground have suffered deviations from their original trajectory (a straight line from the Sun) when interacting with the atoms present in the air.
The overall effect of these dispersions that the rays suffer is to simulate that the radiations, in addition to coming directly from the solar disk, do so more or less homogeneously from all points of the celestial vault.
The radiation that comes directly from the Sun and that reaches us without suffering deviations is called direct, and all the rest, diffuse, since the latter diffuses throughout the celestial hemisphere, pretending that it is this one that radiates it.
If there was no air, obviously all the radiation would be direct and if we looked at the sky, towards a place different from that occupied by the Sun, our eye would not receive any radiation (the sky would be black). Nor could we, for example, read a book with natural light, unless the book itself was directly exposed to sunlight.
Clouds spread solar radiation more strongly than dry air, so on a cloudy day all the radiation we can get will be diffuse radiation. On a typical clear day, direct radiation is several times higher than diffuse.
Below you can see the maps of the average solar energy incident in several areas of the world. The lines join the points at which said energy is equal. The numerical values express kW • h of energy per day per square meter of horizontal surface. To see each map in detail, click on it.
On the other hand, two radiation maps of the Iberian Peninsula can be seen below. These maps provide us with the solar energy that is received, on average, on a day of the month of July (map on the left) and December (map on the right), expressed in calories on a centimeter of horizontal soil. The great difference between the values corresponding to one month and the other can be observed.
Incident energy and usable energy
Both direct and diffuse radiation are useful for producing energy.
However, not all the radiant energy that reaches us is susceptible to being used since, as with many devices that need to start operating a stimulus greater than a certain value, solar collection devices work only from a minimum radiation value Any energy that falls under a certain minimum value will be useless for the purpose of practical use, since the sensors in charge of starting up the solar system will not detect a sufficient value to make the system work with the minimum efficiency required.
For example, during the first moments of the morning or the last of the afternoon, the incident energy is very low, not reaching the minimum threshold value to be able to be used by a thermal fluid, through a solar absorber. The same happens in times of high cloudiness: some energy always reaches the ground (so, although the day is very cloudy we can see in the streets without resorting to artificial lighting), but with insufficient intensity for, with current technology, Being able to provide useful energy. For example, although theoretically a radiation intensity of 100 W / m² affecting 6 hours would provide the same amount of energy as an intensity of 600 W / m² for one hour, in fact in the first case the net energy usable by a thermal collector It would be void.
A good part of the photons that finally reach the ground have suffered deviations from their original trajectory (a straight line from the Sun) when interacting with the atoms present in the air.
The overall effect of these dispersions that the rays suffer is to simulate that the radiations, in addition to coming directly from the solar disk, do so more or less homogeneously from all points of the celestial vault.
The radiation that comes directly from the Sun and that reaches us without suffering deviations is called direct, and all the rest, diffuse, since the latter diffuses throughout the celestial hemisphere, pretending that it is this one that radiates it.
If there was no air, obviously all the radiation would be direct and if we looked at the sky, towards a place different from that occupied by the Sun, our eye would not receive any radiation (the sky would be black). Nor could we, for example, read a book with natural light, unless the book itself was directly exposed to sunlight.
Clouds spread solar radiation more strongly than dry air, so on a cloudy day all the radiation we can get will be diffuse radiation. On a typical clear day, direct radiation is several times higher than diffuse.
Below you can see the maps of the average solar energy incident in several areas of the world. The lines join the points at which said energy is equal. The numerical values express kW • h of energy per day per square meter of horizontal surface. To see each map in detail, click on it.
On the other hand, two radiation maps of the Iberian Peninsula can be seen below. These maps provide us with the solar energy that is received, on average, on a day of the month of July (map on the left) and December (map on the right), expressed in calories on a centimeter of horizontal soil. The great difference between the values corresponding to one month and the other can be observed.
Incident energy and usable energy
Both direct and diffuse radiation are useful for producing energy.
However, not all the radiant energy that reaches us is susceptible to being used since, as with many devices that need to start operating a stimulus greater than a certain value, solar collection devices work only from a minimum radiation value Any energy that falls under a certain minimum value will be useless for the purpose of practical use, since the sensors in charge of starting up the solar system will not detect a sufficient value to make the system work with the minimum efficiency required.
For example, during the first moments of the morning or the last of the afternoon, the incident energy is very low, not reaching the minimum threshold value to be able to be used by a thermal fluid, through a solar absorber. The same happens in times of high cloudiness: some energy always reaches the ground (so, although the day is very cloudy we can see in the streets without resorting to artificial lighting), but with insufficient intensity for, with current technology, Being able to provide useful energy. For example, although theoretically a radiation intensity of 100 W / m² affecting 6 hours would provide the same amount of energy as an intensity of 600 W / m² for one hour, in fact in the first case the net energy usable by a thermal collector It would be void.
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