Solar Thermal Power Plants - Point Focusing Systems Part 2
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00:00
Computer animationLecture/ConferenceProgram flowchartEngineering drawing
Transcript: English(auto-generated)
00:00
Let us now have a closer look at the Heliostarts. The Heliostarts is a mirror that concentrates the solar radiation like the little mirror
00:21
I got here. But of course a real Heliostart is a little more complex than this piece of mirror. Here in the next slides we see a photo of a real Heliostart. Heliostarts usually consists out of several individual mirror elements mounted on a common structure.
00:41
The mirror elements are usually flat but can also be slightly curves. Heliostarts can vary greatly in size from about 1 to over 100 square meter. The photo here shows a rather large example. Depending on the size the cost can also vary significantly from 80 to 200 euros per square
01:02
meter. The solar mirrors are mostly out of glass with silver coating on the back similar to the one used for borobolic drafts and the reflectivity of white glass mirror is usually higher than 90%.
01:20
The next picture shows such a Heliostart from the back. Again we see a metal structure holding the mirrors and in this example here we have a torque tube that is used to transmit the forces. We have two motors here since Heliostarts have to be moved in two axes to follow the path of the sun and to concentrate the solar radiation on a fixed point.
01:46
While for borobolic drafts the shape is more or less fixed in case of Heliostarts we see many variations not only in size but also in shape. Besides the classical square mirrors they are also round, hexagonal or one of the latest
02:03
developments of the company SPP, pentagonal mirrors as can be seen here on the bottom right. This Heliostart has got its special shape through an optimization of these statics. We know that in statics the levers play an important role.
02:21
The longer the lever the larger the force. In the pentagonal Heliostart the span is shorter compared to a square mirror thus reducing the forces which saves material and therefore cost. Cost is the critical factor in Heliostart design. The Heliostart field is the most expensive subsystem of a power tower and therefore design
02:46
and cost must be carefully optimized. This is also the reason why you can see so many different shapes and sizes on the market and there are many different ways for optimization.
03:01
From the cost aspect large Heliostarts are better. You need less individual units which all have their own drive, their own control and their own foundation. There are also fewer units to assemble and to maintain during operation. All this saves cost. On the other hand size brings also disadvantages.
03:23
Installing large units is more costly and so is maintenance if it has to take place at a great height. But the most important argument is the challenge regarding accuracy and the associated required rigidity for large Heliostarts.
03:40
Heliostarts have to hit their target very accurately and they have to do so even under wind loads. In large installations the distance between the Heliostart and the tower can be several hundred meters and even the smallest deviation can cause the Heliostart to miss the target. Even a small gust of wind can cause the accuracy to suffer.
04:04
Large Heliostarts naturally offer a much larger attack surface to wind and must therefore withstand much greater forces and bending. This is the major disadvantage of large Heliostarts. Therefore the design of a Heliostart is always a compromise between cost and accuracy.
04:27
A very radical approach with respect to size we see here in the design of the micro Heliostart a development of the solar institute Julich. Here very very small mirrors are used hence the name micro Heliostart.
04:45
The cost reduction is to be achieved here by having simple individual elements and due to the higher number of pieces further cost reduction can be achieved by rapid mass production. Large micro Heliostarts would be particularly suitable for smaller plants and possibly also
05:05
for other applications for example in the architectural field. Next we want to look at the efficiency of Heliostarts. Since the subsystem Heliostart field is independent of the other subsystems the efficiency can
05:21
also be considered independently of the other components. The field efficiency is composed of different individual parameters which we will now discuss in detail. Let's start with the cosine losses which we have already learned about in case of parabolic troughs.
05:41
They can be calculated also here with the cosine of the angle of incident. The difference to line focusing systems is that a Heliostart is never aligned perpendicular towards the sun because otherwise it would reflect the radiation directly back towards the sun and not onto the tower.
06:01
Thus cosine losses occur all the time. The cosine of the angle of incident can be determined with the position relative to the tower and with the help of the solar athemus and the Denes angle. Since the position relative to the tower is different for each Heliostart the angle of
06:20
incident is also different and each Heliostart must be tracked individually through the sun. This is of course done by a computer program. The next effect to consider is neutral shading. Since the mirrors are relatively close together they can shadow each other at certain sun
06:41
angles as you can see in this graph here. Of course one could just place the Heliostarts further apart from each other to avoid this effect but then the field needs more space and also the distance of the individual Heliostarts to the tower becomes larger which again makes aiming at the target more difficult.
07:02
Therefore a compromise has to be made again. The optimum distance and positioning of a Heliostart is determined individually for each system size and location using an appropriate computer algorithm. In addition to shading mutual radiation blocking must also be considered.
07:22
It may happen that also one Heliostart is completely illuminated by the sun the reflected beam is then blocked on its way to the tower by a Heliostart in front of it as it is shown in this illustration. Of course similar optimization issues apply here as with shading.
07:43
Therefore both factors are usually combined into shading and blocking losses. This effect is also visualized very clearly here on this photo of a micro Heliostart. On the left side you can see areas where the back of the front mirror is reflected.
08:05
This area is in the shadow of the front mirror and cannot be seen by the sun. On the right side the photo is taken from behind. Here we see partially brightly illuminated stripes at the edges.
08:21
Here the solar radiation is reflected from the neighboring mirror onto the back side of the front mirror. So here the radiation is blocked by the front mirror. The next effect that has to be considered is the spillage loss. I have already mentioned several times the accuracy requirements for Heliostarts.
08:45
How good they really are at the end or how much reflected radiation reaches the receiver and does not pass the target is described by the spillage loss. The reason for these losses can be either inaccuracies in manufacturing of the Heliostarts,
09:02
inaccuracy in assembly or poor alignment or tracking errors. For the alignment and calibration of Heliostarts there are different optical measurement methods available for example camera or laser based methods. Due to the rapid development of computer technology in recent years new in-situ calibration methods
09:24
are being developed. Next we have to consider extinction losses. The reflected beam can be further attenuated by particles in the air on its way to the tower. This is particularly relevant in dusty areas such as sandy deserts and for very large plants
09:45
where the path from the Heliostart to the tower is extremely long. This somewhat limits the optimal size of solar tower systems. This is one of the reasons why current commercial plant sizes tend to be in the 100 MW range
10:02
while parabolic trough power plants have already exceeded the 200 MW size. Then of course reflectivity of the mirror plays a role which can be further affected by soiling as we have already seen for parabolic troughs. As previously mentioned due to the long distances involved wind plays a particular role for
10:25
the accuracy and efficiency of a Heliostat. Wind can affect the targeting accuracy of the Heliostats but also the target itself. Towel towers are not 100% rigid and can easily swing with a tip at windy conditions.
10:43
This can result in further increase of spillage. With that we have gone over the key factors in calculating the efficiency of a Heliostat. Here the annual factors are listed and also typical values are given.
11:03
To calculate the overall efficiency of a Heliostat the product of the individual efficiencies has to be taken. In the following figure one can see the result of a calculation of the individual efficiencies for a field of Heliostats.
11:22
Simply seen here is the distribution of the efficiency across the complete solar field. Near the tower and on the north side the efficiency tends to be higher than at the edges and on the south side. Of course the distribution in north-south direction is only valid for plants in the
11:42
northern hemisphere. In the southern hemisphere of course it's exactly the opposite. It should be mentioned that the result shown here is of course only valid for a certain point in time and a certain position of the sun and radiation. For the exact determination of the efficiency therefore a whole year must be considered
12:04
and calculations have to be done for each hour or each minute of a year depending on the accuracy you want to achieve. Of course the result also depends on the location of the plant. If the efficiency of the entire field is to be determined it should be noted that the
12:25
efficiency for each individual Heliostat is different since each Heliostat has a different position in the field and therefore different losses. The efficiency can vary greatly depending on the position and can be as high as 85%
12:41
near the tower and as low as 45% at the edge. To determine the efficiency the average value of all single Heliostats has to be calculated. The exact calculation of the individual and field efficiencies is usually performed with the help of computer programs.
13:05
Once the efficiency is known the power and power density at the receiver can be calculated with the equation shown here below. If a location is close to the equator a field that surrounds the tower is the best choice.
13:23
However if the plant is very far north for example in Germany then a north field setup makes more sense. The determination of the optimum field size and the optimum field layout is therefore always done with the help of annual optimizations.
13:41
By this it can be considered that the performance varies over the day and the year and also from location to location. It should be noted that the power cycle is usually not dimensioned in such a way that it can always completely absorb the power of the Heliostat field. This would mean that it would only be fully utilized at a maximum load which only occurs
14:05
in summer during certain hours. In the rest of the operation period the power block would operate then in part load. Such an operation is not preferred by a power block because the efficiency suffers at part load. Therefore the power block and the receiver are usually designed to be somewhat smaller
14:26
than the maximum power of the solar field or to put it the other way around the maximum power of the field is larger than the thermal power of the power block. The over-sizing of the solar field compared to the power block is described by the so-called
14:43
solar multiplier. The solar multiplier is the ratio of the equivalent thermal capacity of the solar field related to the thermal capacity of the power block under design conditions. Usually it is always greater than one and of course especially when the thermal storage
15:03
is integrated and has to be charged. With this overview of the calculation of the Heliostat efficiency that was shown already before I would like to conclude the chapter on Heliostats. In the next chapter we will deal with what happens with the reflected radiation when
15:21
it reaches the top of the tower.