Solar Radiation Part 1
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00:00
Tire balanceStationeryScoutingInsulator (electricity)Engine-generatorOutsourcingHot workingRoll formingComputer animationLecture/Conference
Transcript: English(auto-generated)
00:12
Hello everybody in front of your screens and welcome to this first video of the lecture on solar radiation. This lecture belongs to the course on solar technologies and solar thermal power generation.
00:26
If you want to design solar systems, if you want to improve them, if you want to develop them, it is not just enough to have a good knowledge of the technology itself, it is also very important to have a good understanding of the energy source that you are going to use, about its
00:43
characteristics and by what it is affected. And this is what we are going to learn in this chapter. But before we start, a quick remark on the terminology. Here our heading says solar radiation, but instead of radiation you see also other expressions in the
01:04
literature like irradiation, irradiance or insulation. All this basically means the same. I'm using here mainly the term radiation, but in some of the graphs or pictures you might also find one of the other expressions.
01:24
Our lecture is structured into four main sections. First we will have a look at what is going on on the sun itself. Then we will discuss what is happening with the solar radiation while it travels through space from sun to earth. Next we will talk about what the atmosphere does to the
01:44
radiation. And finally we will discuss in detail what is important to consider on the surface of the earth itself. But let's now start with the first chapter and let's have a look at the sun itself.
02:01
The sun is basically a large fusion erector. The sun consists mainly out of hydrogen, the share is about 80%. And the energy from the sun is generated by a fusion process where this hydrogen atoms fuse together and build helium atoms. The mass of the product of this fusion process,
02:26
which is helium, is lower than the mass of the reactant, which was hydrogen. To put this in numbers, every second 650 million tons of hydrogen are converted into 646 million tons of helium.
02:45
So there is a gap of 4 million tons. So the sun loses every second 4 million tons of its mass. This sounds quite a lot, but the sun lost only 0.03% of its mass over the last 5 billion years.
03:05
So what is now happening with this lost mass? This mass is converted into energy according to this very famous law, which everybody of you knows, the famous law of Einstein, which says that the energy equals mass multiplied by the speed of light squared. So this is in very, very
03:28
brief the process of how energy is generated by the sun. This process is estimated to go on for another 7 billion years until all of the hydrogen is used up and the sun will vanish. 7 billion
03:44
years, this is quite a long time. Therefore, we don't have to be very concerned about the energy reserves of the sun. So we now know the process. We can then do the numbers. Since we know how much
04:00
mass the sun is losing every second, and we also know the value for the speed of light, we can calculate the radiated power of the sun, which results to be 3.8 times 10 to the power of 20 megawatts. This is a really huge power station, much bigger than any power station that mankind
04:23
have built on earth. So far, the largest power station is the Three Gorge Dam in China, which is hydropower plant and which has a capacity of 22 gigawatts. So much, much smaller than the power station sun. Knowing the radiated power, we can also calculate the intensity of the solar radiation
04:47
by dividing the radiated power by the surface of the sun. The surface of the sun is known. And if we insert the number, we get for the intensity of the solar radiation IS a value of 63 times 10 to
05:02
the power of 6 watts per square meter. This is the intensity of the solar radiation that the sun emits into space in form of electromagnetic radiation every second continuously. So there is no difference between day and night, or between summer or winter, or between working day or weekend and
05:23
holiday. This process is going on 24-7. The sun never stops working. The sun never makes a break. Having done this math, we can now do another calculation by taking another well-known law,
05:41
the Stefan-Boltzmann law, which is used to calculate the effective surface temperature of the sun. The Stefan-Boltzmann law says that the radiation intensity IS equals to the product of the Stefan-Boltzmann constant and the temperature to the power of 4.
06:03
If we solve this equation for t, we get for the effective surface temperature of the sun a value of 5,777 Kelvin. This value of 5,777 Kelvin is the number calculated by this very simple approach
06:21
of the Stefan-Boltzmann law. This is definitely a simplification. Other approaches, which are more sophisticated, come to slightly different numbers. So in the literature, you can find values between 5,600 to 5,800 Kelvin. And our value is perfectly within this range.
06:44
So what have we learned in this first chapter? The sun's energy originates from a nuclear fusion process of hydrogen atoms to helium atoms. The fusion process will continue for another 7 billion years. The intensity of the solar radiation IS is 63 times 10 to the power of 6
07:06
watts per square meter. And the effective surface temperature of the sun TS equals 5,777 K. So this was the introduction on the solar fusion process.
07:21
In the next chapter, we will learn what is happening to this radiation while traveling through space.
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