Solar Thermal Power Plants - Point Focusing Systems Part 3
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00:00
Computer animationLecture/Conference
Transcript: English(auto-generated)
00:11
The next component we want to deal with in detail is the solar receiver, which is mounted on the top of the tower. In this photo, the receiver is clearly visible here
00:23
at the top of the tower. It is brightly illuminated by the solar radiation reflected by the heliostats. Why do we dedicate a separate subchapter to the receivers? This is based on the fact that there are several different concepts for solar power systems
00:41
on the market and under development. With the parabolic trough, it was all quite straightforward and easy. There is the evacuated tube as a receiver. That's it. The receiver can vary only in size and in the optical properties, but nothing else. There are also not much variation
01:01
in the heat transfer fluids that are currently used. Most commercial power plants use thermal oil. We remember thermal oil has a limit at about 400 degrees C for the operating temperature. The main motivation to build power towers is to reach higher temperatures.
01:20
Oil then can no longer be used. There are several approaches in choosing the appropriate fluid and the appropriate receiver technologies for this kind of application. Here is a list of various concepts, which we will discuss in detail below.
01:40
The most common receiver is the external tube receiver. It consists of a series of vertically parallel tubes arranged around the tower. The flow through the tubes is usually from bottom to top. Liquid fluids are usually selected here as the heat transfer medium. Liquid salt is widely used, but also water,
02:03
which is then evaporated in the receiver. The use of sodium as a heat transfer fluid has also been explored in the past. However, sodium is difficult to handle because it is very flammable. Here we see a few designs of realized external tube receivers.
02:22
There are basically two approaches. First here, the cylindrical arrangement and then the square arrangement. The square arrangement is somewhat easier to manufacture, but the radiation distribution on the individual panels is somewhat less uniform.
02:41
Currently, research is also being done on star-shaped receivers, which can then be irradiated from both sides. One of the disadvantages of an external tube receiver is that the tube are freely exposed to the environment and thus the heat losses are relatively high.
03:00
Vacuum insulation, as with parabolic troughs, is technically not feasible due to the higher temperatures and larger dimensions. When establishing the heat balance and heat losses, all relevant mechanisms must be taken into account. G-REC describes the irradiance that reaches the receiver.
03:23
Of this, only a portion is absorbed while a small share is reflected directly at the surface. As a result of the absorbed radiation, the receiver heats up and radiation and convection losses occur. The efficiency of the receiver is then calculated
03:41
from the ratio between the useful heat from the receiver and the concentrated radiation onto the receiver. The useful heat, in turn, is the difference between incident radiation and the heat losses I mentioned earlier. The reflected radiation losses are calculated
04:01
using the reflectivity rho of the receiver tube. The radiation losses are calculated according to the Stefan-Boltzmann law. In comparison to parabolic troughs, the emission coefficient is relatively high for a power tower since there is no selective coating available for such high temperatures.
04:22
Convection losses are calculated using the heat transfer coefficient, which is significantly influenced by wind. Due to the exposure, the heat losses are relatively high, as already mentioned. In return, however, the design is quite simple.
04:42
One option to reduce the losses to a certain extent is the approach of the cavity receiver. Here, the tube bundle is not mounted on the outside of the tower, but inside the cavity with an opening through which the radiation enters. On the one hand, this can reduce convection losses somewhat
05:01
since the tubes are not directly exposed to the environment, but most importantly, radiation losses can be reduced. Of course, the receiver, the cavity, also radiates heat, but most of this heat is reflected back into the cavity and is therefore not completely lost. The disadvantage here, however,
05:21
is that the opening angle of a cavity receiver is very limited and thus only relatively small and the smaller solar fields can be used. Again, I would like to show you a few photos of executed cavity receivers. There are single receivers on the one hand
05:41
and on the other hand also versions with several receivers, which are arranged around the top of the tower. In principle, the same heat transfer fluids can be used in cavity receivers with tube bundles as in external tube receivers. As already mentioned, in most cases, liquid salt is being used
06:01
that is pumped through the tubes. An approach that is still being investigated and that also uses salt is this so-called direct absorption receiver. Here, the salt does not flow through a pipe, but flows down a wall in a very thin film
06:21
in an open flow and is then directly irradiated and heated by the solar radiation, hence the name direct absorption receiver. More interesting than salt for direct absorption receivers are particles as heat carriers, for example, a kind of sand.
06:40
The particles are poured from the top to the bottom by gravity and are directly irradiated and heated. If the particles are dark colored, the absorption is particularly good. The main advantage of using particles is that they have virtually no temperature limit and thus higher temperatures can be achieved
07:01
than with salt. Also, the particles can be easily stored directly. However, approaches using direct absorption receivers are still under development. Here are some pictures how such a particle receiver looks like and how the particle film is formed.
07:21
We see here a flat particle stream which can also be done in different planes or in zigzag shape so that the surface area exposed to the radiation is enlarged. Another option for a receiver design is the volumetric receiver. It is called volumetric because the radiation
07:42
is not only absorbed on the surface but inside a honeycomb structure. Here we see some pictures of such a receiver and it is implemented at the solar tower in Jülich. It consists of single porous ceramic elements as shown in the upper left of the picture.
08:01
And I got such an element also here. We can clearly see the porous structure of this element. So what is the idea behind this structure? I will make a little sketch to explain this a little better. Usually for a standard absorber, the radiation is absorbed by the surface of a receiver.
08:29
So here I have the surface of the receiver and then I have the incoming solar radiation and a part of this radiation would be directly reflected.
08:51
So and to minimize the reflection, here come the channels of the volumetric receiver into the game. I will wipe out here the standard receiver quickly
09:04
and I will draw here the volumetric element. A volumetric element with little channels here.
09:39
These channels are not to scale.
09:42
Usually these channels are much thinner. So and now we have the incoming radiation that is now not absorbed only at the surface but falls here into the channel
10:02
and is then absorbed on the inside wall of this channel. And the part of course is reflected but it's not re-reflected back into the environment but further inside the structure and can then be absorbed by the opposite wall and the small part is again reflected
10:23
to the opposite wall and so on. So the channel works as a radiation trap so to speak and can almost completely absorb the radiation. The structure does acts like a kind of black body.
10:42
In addition, the inside of the honeycomb heats up more than the outside and as a result, the radiation losses are not quite as high. But once the ceramic structure has heated up, how is the heat then transported away?
11:00
This is done by means of air. Air is drawn in from the outside here and is sucked through the porous structure.
11:24
The cold air is hereby heated up. So we got cold air at the inlet, cold air in and hot air at the outlet.
11:46
And hot air here at the outlet. Air is a very cheap and environmentally friendly heat transfer medium and also has no temperature limitation. Another advantage of this concept
12:00
is that convection losses can also be minimized. Normally, a convection flow would develop here on the outside which cools the surface and takes away the heat. Here, the air is sucked in from the outside
12:20
so no air flow can develop on the outside and thus convection can be minimized. Hence, the advantages of the volumetric receiver are the following. High absorptivity and therefore lower reflectivity. The highest temperature occurs in the inside
12:41
and not on the outside which reduces radiation losses. And there are hardly any convection losses since the air flow is sucked in and no convection flow can develop. Now let's take a look at how this changes and simplifies the heat balance at the receiver.
13:03
Since the structure acts as a blackbody, the absorption coefficient is equal to one and thus the reflection losses are zero. However, since the receiver has no selective coating, epsilon is then also equal to one which must be taken into account in the radiation losses.
13:24
And finally, we can neglect the convection losses. All of this improves the efficiency of such a volumetric receiver. Finally, I would like to briefly discuss the closed volumetric receiver.
13:40
This differs from the open volumetric receiver in so far as it is closed by a glass window. This reduces the efficiency, but it offers the possibility of pressurizing the air flow inside so that it can be sent directly to a gas turbine. All gas mixtures can be used
14:00
for certain chemical reactions. By running them in a closed loop, also toxic mixtures can be used. However, the technology of the closed volumetric receiver is an early stage of development. The figures shown here are a development of the Solar Institute Jülich.
14:22
With that having said, we have now discussed all different forms and variation of receiver technologies. We have learned about the external tube receiver, the cavity receiver, the dilute absorption receiver, and the open and closed volumetric receiver. Currently, the external tube receiver with liquid salt
14:42
and heat transfer fluid is the most widely used technology. However, the development of the particle receiver, which is a direct absorption receiver, is also very promising, since higher temperatures can be achieved by using particles, and the hot particles can also be easily stored directly.
15:03
And storage, as we know, plays a significant role in the dissemination of solar technologies.