Solar Thermal Power Plants - Linear Concentrating Systems Part 1
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00:00
Computer animationLecture/Conference
Transcript: English(auto-generated)
00:12
Hello, and welcome to the second chapter of the lecture on solar thermal power plants. In this chapter, we will deal with linear concentrating
00:21
systems. First, we will look at the technology of parabolic trough collectors. Then we will discuss how to calculate the efficiency and the output of such a collector. We will then also look at the Fresnel technology as an alternative to the parabolic troughs.
00:40
And finally, I will show you some examples of entire power plants that use this technology. Let's now start with the technology of parabolic troughs. As you can see here from this photo, this technology is very, very old. It's more than 100 years old.
01:02
It goes back to a germ inventor and a germ patent from 1907. It was a patent for a device for the direct use of solar radiation for steam generation. So this is nowadays still the purpose of a parabolic trough collector, the generation of steam.
01:22
In this photo, you see the first known application of this technology. It's a plant built in 1912 or 1913 in Egypt. The power of the system was 38 kilowatts. But the system was not used to generate electricity, but to operate a steam engine, which was then used to drive
01:42
a water pump for irrigation. Interestingly, the power of this technology of concentrating solar radiation had been completely underestimated at that time. When the plant was put into operation, the radiation receivers, which are installed here
02:00
in the focal line, melted down relatively quickly. Then the material was replaced. The copper pipes were exchanged by cast iron pipes. And then the plant was able to continue operating smoothly for several years. In the meantime, of course, the plant does no longer exist.
02:21
The city of Cairo has expanded so that the suburb of the city now stands on the side where this plant was previously located. In the next slide, we see what a parabolic trough collector looks like nowadays. The picture shows the so-called Eurotrough collector,
02:41
one of the most common parabolic trough collectors. Surprisingly, you don't see much difference to the old collectors. At first look, the technology hasn't changed much in the last 100 years. I want to use this picture to briefly walk you through the essential components.
03:02
Of course, the most important component is the solar concentrator. Here we see the mirrors, or the more appropriated terminology for this is reflectors. The reflector reflects and concentrates the radiation. It is supported by a metal support structure.
03:22
The metal structure holds the reflector in its position. And it also ensures that the shape of the mirror, which is relatively thin, is kept in its ideal form. The radiation from the reflector is concentrated in the focal line. And in the focal line, the radiation receiver is located.
03:43
The receiver is composed out of a steel tube. And through this tube, a heat transfer fluid is pumped, which takes over the heat and transports it to the power block. As already mentioned in the previous chapter, these collectors must track the position of the sun.
04:01
Therefore, every collector also has a drive unit to move the collector. The drive unit also has a control unit that tracks the collector according to the position of the sun and ensures that the collector is always oriented perfectly towards the sun. I would like to go into a little bit more detailed
04:23
about the individual components, starting with the reflector, which is one of the most important components of all. The reflector, on one hand, of course, has to reflect the radiation, but on the other hand, it has to be very precisely curved so that the radiation cannot only be reflected,
04:42
but also concentrated exactly in the focus. In order to achieve this, the mirror has a few special characteristics. The first quality lies in the glass type used. We use here a particularly pure glass, a so-called white glass,
05:01
which differs from normal mirror glass. If, for example, you look at a bathroom mirror and look at the edges, you will notice that these mirrors have a slightly greenish shine. This is caused by impurities in the glass, mainly by iron, and this reduces the reflectance of the glass,
05:22
or actually, it reduces not the reflectance, but the transmissivity of the glass. And the transmissivity then affects reflectivity. This can be explained by the fact that the radiation is not reflected by the glass itself. The actual reflection occurs at the silver layer,
05:42
which is deposited on the backside of the mirror. But if the silver layer is the actual reflective layer, what is then the purpose of the glass? Glass is used here for two reasons. On the one hand, the glass is used as a carrier, because after all, silver is very expensive,
06:03
and therefore is only applied in very, very, very thin layers, and this layer itself is not stable. So we need a supporting structure for the silver layer, and since this supporting structure has to be transparent, glass is used. And on the other hand, the glass also serves
06:22
to protect the silver layer from environmental influences, at least from one side. Hence, the silver layer is the actual reflective layer, and the glass only serves as a support and a protection. The radiation then must first pass through the glass,
06:42
is then reflected by the silver layer on the backside, and then passes through the glass again. Consequently, the radiation must pass through the glass twice. Therefore, it is very important that the glass is clear and transparent, particularly white and particularly pure,
07:01
so that the transmissivity is very high, and the mirror as a whole achieves a high degree of reflection. Also, the use of thin glasses can improve the transmissivity. Therefore, for determination of the optimum glass thickness, you have to find a compromise between high transmissivity,
07:23
which would require a thin glass, and high stability, which would require a thick glass. The silver layer is, as I said, protected by the glass on the front side. On the backside, the silver is protected from environmental influences by certain layers of coatings.
07:43
Because what we don't want to see here is that the mirror goes blind, as it happens sometimes with old bathroom mirrors. Then the silver layer is degraded, and the reflection properties are lost. After applying the protection layers,
08:00
a fastening element has to be attached with which the mirror can be connected to the metal support structure. Here on the photo, you can see the commonly used ceramic mounting pad. Solar mirrors can achieve reflectance of 93 or even 96%, at least if they are clean.
08:22
In comparison, the reflectance of simple bathroom mirrors is only about 80%, and considerably more radiation is lost. So we use here really high-performance glasses and materials, and this is why experts do not like to call this just a mirror,
08:41
but prefer to talk about reflectors because this sounds more technical and not just like a simple bathroom mirror. The other important component of a parabolic trough collector is the solar receiver. The picture on the right shows such a receiver. It looks very similar to a vacuum tube collector
09:03
we know from low-temperature applications. It consists of a black steel tube. This tube is the axial radiation absorber. As you know, from low-temperature applications, the absorber has a selective coating to improve the efficiency.
09:21
And this is where the heat transfer fluid flows through. It enters on one side and leaves the tube on the opposite side. The absorber tube itself is surrounded by a glass tube, and the space in between is evacuated to reduce the convection losses. This is a typical vacuum insulation.
09:43
The glass tube itself is highly transparent in order to let through as much radiation as possible, and it also has an anti-reflective coating like my glasses. Since the glass tube and the absorber tube heat up differently and have very different
10:02
expansion coefficients, they expand differently when they are hot. Therefore, a bellow is attached at the end to compensate for the different expansion lengths. This bellow must be connected to the glass tube by a special welding process. This connection is always the weak point
10:21
of such a pipe where it can easily break. If the connection fails, this then leads to a loss of vacuum and consequently to an increase of heat loss. Another feature to keep the vacuum is the hydrogen getter. This is required if thermal oil
10:40
is used as heat transfer fluid. Most commercial parabolic draft plants use such type of thermal oils, and these thermal oils unfortunately have an upper temperature limit. If they are overheated, they start to decompose. One of the decomposition products is hydrogen, and since hydrogen has very, very small molecules,
11:03
these molecules can then penetrate through the steel pipe into the vacuum, and consequently, the vacuum will be destroyed. Therefore, getters are used which can absorb the hydrogen if it penetrates through the steel pipe.
11:22
Also shown on the slide here are some typical optical property values. The absorptivity of such a pipe is around 95%, the emissivity at 400 degrees is 14%. This low value can be achieved by a selective coating, which we will discuss in more detail in the next slide.
11:44
The glass pipe has a degree of transmission of at least 96%. A typical length of this pipe is around four meters, and the operating pressure is designed to be at maximum of 40 bar. Let's now have a detailed look
12:00
at the optical properties of a selective coating. First, I will draw a graph with the solar spectrum. On the X-axis, we have the wavelengths in micrometer,
12:23
this is the wavelengths in micrometer, and on the Y-axis, we got the spectral irradiance level,
12:41
the spectral error, and then I will draw the solar spectrum
13:00
with a green curve, this is this curve, and that is you already know, the solar spectrum covers the short wavelength range from about 0.3 to 2.5 micrometer.
13:31
I will also here add the curve of the heat radiation of a black body here in red,
13:43
and the heat radiation covers the infrared wavelength range. So very, very important to emphasize the solar radiation, the green curve occur at different wavelengths than the heat losses, the red curve.
14:02
And what we want to achieve is that the solar radiation absorption is as high as possible and the emission which is responsible for the heat losses is as low as possible. Unfortunately, at certain wavelengths, the absorption coefficient equals the emission coefficient,
14:25
alpha equals epsilon. Therefore, what I finally would like to achieve is a high absorption coefficient only in the short wavelength range here
14:48
and a low absorption, meaning a low emissivity in the infrared wavelength range here. And this is what a selective coding does. Selective coding has changing optical properties
15:02
depending on the wavelengths. An ideal selective coding would have a high absorptivity in the visible wavelength range, preferably one. And then here is sudden drop to a low absorptivity,
15:24
which means a low emissivity in the infrared wavelength range. So this is the curve which I would like to get. Usually such diagrams don't show the absorptivity,
15:43
but instead the reflectivity because reflectivity is easier to measure. And as we know, reflectivity, absorptivity and transmissivity add up to one. So alpha plus rho plus tau equals one.
16:13
In our case, the transmissivity is zero. It can be neglected. So alpha and rho is left.
16:23
So the reflectivity rho is just one minus the absorptivity. So I can draw here in yellow, the ideal curve for the reflectivity.
16:45
The reflectivity would then first be zero in the short wavelength range and then go up to one at higher values.
17:03
This is of course the ideal world and the ideal world is never reality. And therefore I also will show here the more realistic curve, a real curve. And this I will draw in blue
17:21
and it would look like more like this. So in the short wavelength range, we have a low reflectivity, which means a high absorptivity, good codings reach values of about 95%,
17:40
which means the reflectivity here would be 5% and consequently absorptivity 95%. And then for a wavelength larger than about one micrometer, the value increases
18:03
so that in the infrared range, the reflectivity is high, which means the emissivity is low. A typical value for a commercial tube would be about 30%. So the reflectivity would then be 87%,
18:22
which means the emissivity is then here 30%. So this is how a real curve would look like. Selective codings are not only used for concentrating technologies, but also for low temperature technologies
18:41
like flat blade collectors. The next component we want to look at is the metal support structure. The support structure has to keep the reflectors in shape and has to provide the required torsion stiffness. This can be realized either by a space frame structure,
19:02
as you can see in the example below, or as in the example on the top right by a torque tube. The shape of the collector is responsible for the accuracy and therefore for the optical performance of the collector. And the shape has to be maintained
19:20
under all different load conditions. So the structure has to be stiff enough and because of the rotation of the collector, it also has to be rigid against torsion. In addition, it has to maintain the perfect shape even under high wind loads. If the collector would bend under loads,
19:42
then the receiver tube would no longer be in the focal line and we would lose part of the solar radiation. In the examples shown here, the stiffness is provided either by the space frame or by the torque tube. The supporting structure also includes the connector
20:00
for the reflectors and the receiver tubes. As you can see here in these photos, metal arms are connected to the space frame or the torque tube that holds the reflectors and the receivers. The structure itself is carried by pylons that can be seen here and which in turn require appropriate foundations
20:22
that transfer the loads into the ground. Of course, the whole structure should be as light as possible in order to save material and thus cost, but without losing the stiffness and accuracy because this then would penalize the performance.
20:40
Therefore, optimization between cost and performance has to be done. As can be seen in this example, these collectors are relatively large. The typical span here is about six meters and the length is at least 50 meters. The largest collector available on the market to date
21:03
is the so-called Altimitraff developed by the company Flavec and that is now marketed by the German company SBP. In this slide here, you'll see a photo of the Altimitraff. It has a span of seven and a half meters and the total length of the collector
21:22
that is driven by just one motor is 200 meters. The motor has to move the collector to track the sun. The tracking is also illustrated here in this picture. If the collector axis is aligned in north-south direction,
21:41
the collector has to move from east in the morning to west in the evening. The tracking of the collectors is realized individually by each collector by the drive unit which is installed in the middle of each collector. Usually, this is a hydraulic drive
22:00
and the drive is controlled by a so-called local controller. Thus, each collector can be controlled individually. If necessary, this local controller is then connected to a higher level field control which then monitors the complete field of collectors. We have now discussed all main components
22:22
of a parabolic drive collector in detail. And in the next chapter, we will then deal with performance and the calculation of the efficiency of such a collector.