Solar Thermal Power Plants - Linear Concentrating Systems Part 4
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00:00
Computer animationLecture/ConferenceDiagramProgram flowchart
Transcript: English(auto-generated)
00:11
In the last subchapter we want to look at complete solar thermal power plants using the parabolic trough collector technology.
00:21
First let's have a quick look again at the general scheme of a solar thermal power plant. We know from the first chapter that a solar thermal power plant is divided into two areas. On the left we got the heat source with the collectors that we have just discussed.
00:40
And then the heat that is collected by the collectors is used to provide steam to a steam cycle. How such plants look like in reality we can see in this aerial photo that shows a complex of three parabolic trough power plants.
01:00
The total size of each field is more than one square kilometer which means more than 20 soccer fields. These plants here are located in southern Spain each with an output of 50 megawatts of electricity. We see the collectors arranged here in parallel rows and in the middle of
01:21
each solar field is the power block where the heat is converted into electricity. In the next slide we have a schematic representation of the solar field. We see the collectors connected in series and in parallel rows. The connection in series is required to achieve the desired temperatures.
01:43
Such a series of collectors is called a collector loop. And then we have connected several loops in parallel to achieve the desired power output. The individual loops are connected by headers in order to distribute the cold heat transfer fluid to the loops and to collect the hot fluid at the outlet.
02:06
Most commercial parabolic trough plants use a synthetic thermal oil as heat transfer fluid. The typical temperature spread in such a plant is from about 300 degrees C at the cold end to almost 400 degrees C at the outlet.
02:24
The thermal oil has the decisive advantage that it remains liquid at these temperatures even at 400 degrees C. This is the reason why this type of synthetic oil was chosen as heat transfer fluid. The disadvantage is that this thermal oil is only stable up to 400 degrees C because at higher temperatures the oil starts to decompose.
02:49
Consequently we have to monitor carefully that this temperature is not exceeded at the outlet of the solar field. This limitation on the other hand also affects the efficiency of the power block.
03:04
State of the art turbines rather work at higher temperatures of more than 500 degrees C. And of course as higher the temperature as higher the efficiency. That we know from Kano. A parabolic trough collector is in principle able to reach higher temperatures.
03:20
But we don't have the suitable fluid for that or at least not a fluid that is still liquid at moderate pressure in this temperature range. Even with oil I have to maintain a certain pressure to keep the fluid liquid. But with 11 bar this pressure is relatively moderate.
03:40
Nevertheless 11 bar is already a pressure which requires a certain investment for the design of the piping system. And as you have seen in the aerial view before a solar field is very large and therefore a lot of piping is required. Another disadvantage is that this oil is also not necessarily environmentally friendly.
04:04
Therefore you have to be careful that the oil does not get into the groundwater in the event of any damage or the event of possible leakages. But meanwhile there are new synthetic oils on the market that are less harmful to the environment and that can even go up to slightly higher temperatures of up to 430 degrees.
04:28
The hot oil which is collected here in the hot header is then used to generate the steam for the steam process. The schematic for the steam cycle is shown in the next slide.
04:41
The oil transfers the heat to the water steam in counter flow. The hot oil enters the steam generator unit here from the top and the cold water enters here from the bottom. The water first is fed into the preheater the so called economizer where the water is heated up to the saturation temperature.
05:04
Then it goes into the evaporator where saturated steam is generated. And finally it is overheated with the hot oil in the super heater. In order to increase the efficiency of the steam cycle a two stage turbine with an intermediate reheating is used.
05:24
A parallel flow of oil is used here to reheat the steam. Another measure to improve the efficiency is to use extract steam for the turbine for preheating the water. Here in this scheme two steam extractions are shown.
05:42
However the state of the art turbines for solar power plants use up to six steam extractions which further improves the efficiency. To close the water steam cycle a condenser is used to condense the steam at the outlet of the turbine so that it can be pumped back into the steam generator.
06:02
The efficiency of the overall system is calculated by the product of the solar field efficiency and the electrical efficiency of the water steam cycle. And in case the heat of the solar field is first stored in the heat storage the efficiency of the storage has to be taken into account also.
06:23
One advantage of such a solar thermal power plant is that there is the possibility of hybridization. Hybridization means that I can drive the steam system with two different sources. With the sun as a fuel as well as with fossil fuel.
06:41
Thus such a hybrid plant can guarantee a secure output as shown in the next graph. In this diagram a 24 hour operation of a hybrid plant is shown exemplary. During the day when the sun is shining the plant can operate with the solar resource only.
07:02
And depending on the requirements electricity production can continue after sunset in the morning and in the evening hours and if necessary during the whole night as well using fossil fuel. Usually production requirements during the night are lower. That is why a reduction of output is shown in this example.
07:23
But of course the production can be adapted to the required demand profile. In such a configuration as shown here the solar field would serve as a so called fuel saver. Since fuel can be saved during daytime operation.
07:43
Much more interesting than the possibility of hybridization however is the possibility to integrate a storage system into the plant. A solar power plant first generates heat at a high temperature level that is then converted in the next step into electricity in the steam cycle.
08:01
Therefore we have the possibility to integrate a storage system already on the thermal side of the system before the conversion step into electricity takes place. So we do not have to store the electricity itself as is the case with photovoltaic systems or wind turbines.
08:21
Thus we don't need expensive batteries that also require a lot of resources but we can store the solar heat directly before it is converted. And the thermal storage is much cheaper than a battery. One of the most commonly used thermal storage concepts in solar power plants is the two tank molten salt storage concept.
08:46
In this storage concept the heat is stored in liquid salt. Now the question arises why do I not take the oil directly and store the hot oil in a storage tank? There are two reasons why this is not done.
09:01
On the one hand it is due to the operating pressure of the oil system. As I have explained earlier the whole oil system must be operated under a minimum pressure of 11 bar to keep it always liquid. If we want to store the hot oil in a storage tank also this has to be designed to withstand 11 bar.
09:22
Consequently relatively large wall thickness would be required and also the maximum feasible tank size would be restricted. All this is associated with cost. On the other hand large quantities of oil would be required for this and the cost of the thermal oil is very high.
09:41
Consequently the use of oil as a storage medium is not economic and also technical challenging due to the pressure. Salt is much cheaper and liquid salt is still liquid at ambient pressure even at high temperatures and therefore the system does not have to be designed with overpressure.
10:03
This makes a molten salt storage system much more attractive and economical. So how does the two tank system work now? During the day when the oil is heated up in the solar field, cold molten salt will be taken from
10:21
the cold tank and pumped through a heat exchanger where the heat is transferred from the oil to the salt. The salt heats up and is then stored in the hot tank. For discharging during the night when there is no more solar radiation available the process in the salt system can be reversed.
10:43
Then the hot salt is taken from the hot storage tank and the heat will be returned to the thermal oil and the turbine process continues to operate with hot thermal oil heated up by the storage instead of the solar field. Hence electricity production can continue after sunset or during cloudy periods.
11:07
These storage tanks are usually dimensioned that the turbine can be operated for several hours. Typical capacities correspond to 6 to 10 hours of full load operation. The first commercial power plant based on this concept was commissioned in 2008 in Spain.
11:26
Here you can see a photo of this plant. The first plant was the one here on the right. The photo shows two other plants that were built with exactly the same concept one after the other. Each of these systems has an electrical capacity of 50 MW.
11:45
All three plants use the same two tank storage technology with molten salt. The storage tanks which are located in the center of each plant have a capacity to keep the turbine running at full load for 7.5 hours.
12:02
The plants are located in southern Spain in the province of Coronada where we have one of the highest radiation levels in Spain. Several years of radiation measurements have been performed throughout Spain to identify the best locations. The plants can generate around 160 million kilowatt hours of electricity per year.
12:24
Of course depending on the actual weather in that specific year. In better years it can be a little bit more, in bad years it will be less. The investment cost of these systems at that time were around 260 million euros per plant.
12:41
Since then many other plants have been built according to this principle and cost went down significantly. Nowadays hardly any system is built without storage because that is one of the great advantages of this technology. I would now like to briefly summarize the historical development of the technology of parabolic trough power plants.
13:07
The very first commercial parabolic trough power plant was installed in 1984 and was operated successfully for 30 years. The very first plant was not yet able to run on solar only but fossil fuel was used to reach the required temperatures for operating the steam cycle.
13:29
The first solar only system was then put into operation one year later at the same location. That plant had a capacity of 30 megawatts, it achieved an overall efficiency of 29.4% in solar only operation.
13:46
This system was also in operation for about 30 years and has now been shut down a couple of years ago. Then we saw the next development step a few years later with systems that were built in 1989 in California, also 30 megawatt plants.
14:04
However the efficiency of these plants could be significantly improved. The primary reason for this improvement was the improved steam cycle process. Turbines with preheating and reheating were used here for the first time in
14:23
solar application and thus the entire cycle efficiency could be increased to 37.5%. These systems are still in operation and have thus reached an operating time of more than 30 years now.
14:42
In 2008 the next big development step was achieved in the plants built in Spain. They achieved a slightly improved efficiency of 38.5%. But what is more important that for the first time a thermal storage system was integrated so that evening and night operation can be covered without using fossil fuel.
15:06
The efficiency then could be further increased a little through improved preheating from 38.5% to around 40%. The next development step was observed then in 2013 when the first plant in the 100 megawatt range was put into operation.
15:27
This plant was built in Abu Dhabi. And then in 2018 the first 200 megawatt plant was put into operation, a plant in Morocco. And of course in this plant also a thermal storage was integrated.
15:41
In the next slide we see some pictures of this plant. The plant belongs to the currently largest solar complex in the world. It was built in Morocco and is called Noor. It consists of four solar power plants, two systems according to the concept discussed in this chapter.
16:03
The third system is a solar tower plant that you will get to know in the next chapter. And the fourth system is a PV plant. Next I would like to show you the current important developments in this field.
16:22
Most development activities are currently focused on raising the upper temperature because the upper process temperature has a decisive influence on the efficiency of the cycle. Via the Kano efficiency there is a direct relation between the upper process temperature and the efficiency of the water steam cycle.
16:43
Unfortunately the upper temperature is limited by the thermal oil. The oil that is currently used in most plants has a maximum limit of about 400 degrees C. Above the temperature we cannot operate this oil because the oil then begins to decompose.
17:03
There are some developments ongoing to push this limit a bit higher. But here we are just talking about 10 to 30 degrees of increase, which only has a slight effect on the efficiency. If you want to take bigger steps then you have to change the medium and you have to go for a completely different type of heat transfer fluid.
17:29
One option would be to generate the steam directly in the solar field. For steam there is no temperature limit and we would also save some equipment for the thermal oil cycle.
17:42
But steam has the disadvantage that we have a problem in terms of storage. Steam is difficult to store and there is no large scale storage available for superheated steam. Of course there is a so called roof storage, but that is only for saturated steam.
18:00
However steam turbines only run at very low efficiencies with saturated steam. Therefore this is not a good option for large scale commercial applications. So the approach of direct steam generation in parabolic trough collectors is currently not pursued further. But there is another obvious choice.
18:22
You may use the liquid salt that is already used as a storage medium. And why not using it also as a heat transfer medium in the solar field. This is an approach that is currently under development. Such a system would look like the one shown in this scheme.
18:43
At first glance we directly see a significant difference from the standard scheme. There is one cycle less. In the state of the art concept we have three cycles. One for the thermal oil, one for the salt storage and one for the water steam cycle.
19:00
Here in this case we can save one cycle, the oil cycle. We replace the oil by salt which is then pumped directly through the solar field and we can integrate the storage tanks directly into this cycle. This has the additional advantage that loading and unloading can be decoupled.
19:23
The charging and the discharging can be done independent from each other with separate pumps. For charging the salt is taken from the cold tank, heated in the solar field and then stored directly in the hot tank. And by using another pump we can take the hot salt out of the hot
19:42
storage tank and feed it to the steam cycle independent from the solar field operation. Very obvious this approach has significant advantages. These are listed in the next slide. One advantage of such a concept is that we save costs by eliminating one cycle with all the related equipment.
20:05
The salt is also significantly more environmentally friendly than oil. That means we no longer have the problem that in the case of leakage groundwater could be potentially contaminated. But the main advantage and the driver for this development is that we can raise the upper temperature level.
20:25
Currently researchers are working on upper temperatures of over 500°C which would improve the water steam cycle efficiency significantly. Another advantage is that salt is also much cheaper than oil.
20:44
But why this is not already yet applied when it has so many advantages? Well there are also some serious challenges related to this approach. And the biggest challenge is that while the salt can be operated at high temperatures there is a low temperature limit for it.
21:04
Unfortunately salt has a relatively high freezing point. That means the salt solidifies at temperatures a little over 200°C. So the freezing point is a hard limit for operating the plant.
21:21
Freezing of the salt has to be avoided under all circumstances also during night, during bad weather periods and during winter. If it comes to freezing in a pipe then this pipe would be blocked and the salt cannot circulate anymore. Once blocked there is almost no chance to unfreeze the salt again.
21:42
So freezing would lead to very serious damages. Two measures can be taken to avoid this. An auxiliary heating system can be installed that keeps the salt warm during the night and bad weather periods. This of course costs money and additional energy.
22:02
And secondly an emergency draining system has to be implemented for longer downtime periods which adds again costs. Such concepts have to be 100% reliable and 100% safety is always very expensive.
22:21
The development of reliable and cost effective concepts to avoid freezing is therefore the main focus in the development of molten salt parabolic draft systems. Another problem which is not dramatic but which needs to be considered when selecting the material or the purity level of the salt is the issue of corrosion.
22:44
Depending on the type of purity of the salt then at higher temperatures the pipe material may corrode. Therefore the pipe material and the salt mixture have to be selected carefully. But this is a problem that can be handled and therefore it is worth continuing research
23:03
and development of this concept since the temperature increase promised a big step of efficiency gains. Finally I would like to briefly summarize the advantages of parabolic drafts in comparison to photovoltaics. A very big and essential advantage is that we can integrate a storage system very cheaply with parabolic draft power plants.
23:28
We can as explained in this lecture already use the thermal energy for storing and not just the electrical which is more cost effective than using batteries. Also you don't have so many challenging materials compared to batteries where we see discussions regarding materials like lithium or cobalt.
23:51
Another advantage would be the option of hybridization for example in combination with fossil fuel or renewable fuels. And solar fields can even be integrated into existing fossil fuel power plants
24:05
thereby fossil plants can be converted step by step into renewable power plants. The disadvantage of solar thermal power plants is that if I don't need storage then such systems are more expensive than just using photovoltaics.
24:25
Solar thermal power plants only can make use of the advantage when storage is required. But with an increasing share of renewables in the electricity net the topic of storage will play an increasingly larger and decisive role.
24:42
Another disadvantage is that the parabolic draft technology like all concentrating technologies only works with direct radiation which I already explained at the beginning of this lecture. That is why such systems can only be used in regions with high solar radiation and Germany unfortunately does not belong to these regions.
25:06
I would finally like to point out that these technologies can not only be used for electricity generation also this is currently the largest area of application. Ultimately you can also use them for other processes specifically for thermal processes.
25:24
For example for thermal desalination, for metal processing and in the mining or food industry. And such systems can also be used for solar cooling such as in absorption refrigeration machines. Or they can be used for chemical processes. But these processes are not yet commercially used and a lot of research is still required here.
25:48
But this research is gaining increasing interest in the last years. That brings me to the end of this chapter. In this chapter we got to know the parabolic draft technology in great detail and I would like to summarize the main important insights.
26:08
We learned that parabolic draft is a reliable, successfully implemented and widespread technology. We have seen systems successfully in operation for more than 30 years.
26:23
We have learned that the essential components of such systems are the reflectors, the receivers that absorb the radiation, the supporting structure and the control units that are necessary for tracking the sun. We have learned that the state of the art power plant currently use thermal oil as heat transfer medium.
26:45
We have also learned that most systems have now integrated a thermal storage that is based on liquid salt as storage material. The typical size of such plants is from 50 to currently up to 200 MW of electricity.
27:02
So these are relatively large systems. We also discussed briefly that there is an alternative to parabolic draft that is the Fresnel technology. This technology is currently not that widespread because the efficiency is not quite as good as with parabolic drafts.
27:21
The necessary cost reduction to bring such systems to the market has not yet been achieved. We are now at the end of the chapter on linear concentrating systems and in the next chapter we will deal with point focusing technologies.