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Advanced CSP Teaching Materials

Chapter 11

Quality Control

Author Matthias Günther1 Simon Csambor1

Reviewers Ahmed Chikouche2 Björn Schiricke3

1 Institute for Electrical Engineering, Rational Energy Conversion, University of Kassel, Wilhelmshöher

Allee 73, 34121 Kassel 2 L’Unité de Développement des Equipements Solaires (Algeria)

3 German Aerospace Center (DLR), Institute of Solar Research, Linder Höhe, 51147 Köln, Germany

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Table of Contents

Summary ................................................................................................................................3

1. The importance of quality control .......................................................................4

2. Analysis of geometrical properties .....................................................................4

2.1 Photogrammetry 5

2.2 Alternative coordinate measurement techniques 10

2.3 Deflectometry 11

3. Further measurements in CSP plants ..............................................................14

Reference list .......................................................................................................................15

Questions .............................................................................................................................16

Answers ...............................................................................................................................17

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Summary

In this section we shall explain some methods for the quality control of CSP plant components. We

will concentrate on the analysis of the geometrical collector exactness and on the techniques

photogrammetry and deflectometry.

Key questions

• Why is quality assurance of CSP components important?

• Which techniques exist to measure the geometrical collector exactness?

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1. The importance of quality control

Special control methods had to be developed in order to compare and assure the quality of components

of CSP plants. As there is not much experience in the fabrication and long term operation, it is

important to ensure the quality and to reduce energy losses, which would also affect the profitability of

the solar field project. Measurements showed that without proper quality assurance 3-10% and in

some cases even more of the field performance can be lost. A performance loss of a Spanish 50 MW

plant with thermal storage by 1% corresponds to a yearly economic loss of about 500,000 Euros.4 Also

the quality assurance and final acceptance tests of collector fields are necessary to control the

subcontractors and because of warranty claims.The corresponding measurements should reduce risks

and costs and enable a long term economic power plant operation.

Measurement procedures have been developed for the analysis of geometrical properties of the

collectors, thermal properties of the receivers and for the performance analysis of whole solar fields.

2. Analysis of geometrical properties

There are several measurement methods for the analysis of the geometrical properties of the CSP plant

collector. Geometrical inaccuracies in the collector imply high optical losses because they make

decrease the intercept factor, i.e. the ratio of the reflected radiation that hits the absorber to the total

reflected radiation.

There are two types of geometrical deviations from the design shape of a mirror that can be detected in

control processes: slope errors and form deviations. Form deviations are positioning errors of the

mirror surface and slope errors are angular aberrations of the mirror surface. A method that is

especially appropriate to detect form deviations is photogrammetry. A special method to detect slope

errors is deflectometry.

Figure 1: Geometrical collector errors and associated evaluation methods (source: Ulmer, DLR)

4 See Ulmer.

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2.1 Photogrammetry

High precision close-range photogrammetry has been proven to be an efficient tool to measure 3D

coordinates of collector support points as well as mirror surfaces themselves.5 In photogrammetry, the

coordinates of reference points on the measurement object are calculated by a software from a set of

digital pictures taken from different observation points. This happens via triangulation6 of the light

rays between the camera positions and reference points at the observed object. For a precise

measurement, every object reference point should be pictured on about six photos.

Photogrammetry is not only used for the quality assessment of prototyps or for sample surveys, but it

has also been applied in permanent quality control processes in collector production lines where the

exactness of the assembled collector space frames is measured.

Figure 2: Measuring camera in a control device for parabolic trough collectors (Source: Lüpfert, DLR)

Figure 3: With one photo, a ray is determined on which the point is situated. (Source: Prahl, DLR)

5 See Shortis et al. 2008.

6 Triangulation is the process of determining the location of a point by measuring the angles under

which it appears from different viewpoints (camera positions, in our case).

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Figure 4: With two photos the position can be defined as the intersection of two rays. In real measurements, more photos are taken.

Figure 5: Complex objects can be measured with several measuring points.

In the following, we will mention the following CSP relevant applications of photogrammetry:

Determination of space frame exactness (mirror support point coordinates, absorber

tube support points, vertex, rotation axis)

Determination of mirror surface coordinates at the complete collector

Deformation analysis under different conditions (varying gravitational loads because

of tracking, temperature variations)

The most important application of photogrammetry in parabolic trough technology is the test of the

geometrical exactness of assembled trough frames. The coordinates of the mirror support points, of the

absorber tube support points as well as of the vertex and the rotation axis can be determined.

Figure 6 shows a Eurotrough frame with retro-reflective targets placed on the mirror support points.

These targets reflect the camera flash. Several photos of the targets are taken from different positions

along the construction. The photos are analysed in a special software and their coordinates are

calculated. This allows to detect deviations from the design values.

Measuring the space frame before the mirror facets are mounted has the advantage that possible

deviations can be corrected more easily.

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Figure 6: Parabolic through frame (Euro Trough) with measurement targets on the mirror support points (Source: Shortis, RMIT University)

In the following graphic, the result of a measurement of the mirror support point exactness is shown.

Position deviations (height deviations) are represented with different coulors. The dark points are the

measurement points. The form deviation of the points between the measurement points are colloured

according to an interpolation between the measured points. Noticeable is the bending of the structure

due to the fact that it is supported at both ends: The measurement points at the ends are slightly higher

than they should be and the points in the centre are slightly lower.

Figure 7: Representation of the mirror support point height deviations in [mm] (Source: Shortis, RMIT University)

The next figure shows the absorber tube support points the position of which can be determined with

retro-reflective targets.

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Figure 8: Reflective targets on the absorber tube for absorber tube position measurement (source: Prahl, DLR)

Another application of photogrammetry is the measurement of the mirror surface of a complete

parabolic trough. Also for that, retro-reflective targets have to be placed on the mirror surface. The

surface is measured only at the points where such targets are placed. It is, so to say, only an exemplary

form control of the mirror surface.

Figure 9: Photogrammetric measurement of a parabolic trough collector with measurement targets on the mirror facets (Source: Lüpfert, DLR)

The next figure shows a representation of a possible result of such a measurement. The dark points are

the measurement points. The areas between the dark points are collored according to an interpolation

that is done for the points between the measured points.

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Figure 10: Height deviations (not to scale) of a EuroTrough mirror surface (Source: Shortis, RMIT University)

A further interesting application of photogrammetry is the determination of the shape effects of

thermal and gravitational (positional) changes. Glass mirrors bend under gravitational forces. That

means that the collector shape changes slightly with orientation. So, sun tracking may affect the flux

distribution in the focal line (or focal spot, according to the concentration geometry).

Finally, photogrammetry can also be applied to measure angles, for instance to measure the inclination

of the mirror support points. An exact inclination of the mirror support points is important to achieve

accurate mirror slopes. For the measurement of the inclination of the mirror support points, bars with

two retro-reflective targets at their ends have to be placed on them. The positions of the end points is

measured with photogrammetry and the inclination of the bars and, consequently, of the support points

is calculated.

However, for the measurement of mirror slopes, the more appropriate technique, compared to

photogrammetry, is deflectometry.

Figure 11: Cantilever arm of a parabolic trough module with glass bracket retaining points (GBRT) and surface orientation angles (Source: Prahl, DLR)

Measurement precisions of ±0.2 mm for coordinate measurement and ±1.0 mrad for angle

measurement were reached by DLR.

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2.2 Alternative coordinate measurement techniques

There are more techniques for non-contact coordinate measuring that can be (and have been) used for

the test of the geometrical collector exactness. While photogrammetry determines the coordinates of a

point via the evaluation of photo series, there are other techniques that work with laser rays and that

apply time-of-flight measuring or interferometry.7 We will mention them here in a rather short form:

a) Laser scanning measures the time a laser beam needs to travel from the measurement instrument to

the measured object and back. Contrary to photogrammetry, no reflecting targets have to be placed.

Precisions of about are reached.

b) Total station measurements operate with an electronic theodolite that has an integrated electronic

distance meter. They achieve a similar measurement precision like laser scanning. A disadvantage is

that, for the reflection of the laser beam, it is necessary to place a prism on all points that shall be

measured. The manual orientation of the prism implicates an additional possible error.

c) Laser tracking operates with interferometry. It is more precise than the techniques mentioned

before, but it is also more expensive. This method also requires the manual usage of a prism and is

therefore also susceptible to additional errors.

d) Laser radar measurements combine time-of-flight measurement and interferometry. The laser

beam is focused on the object to be measured. Without additional prisms, the distance measurement

can be carried out in the micrometer range.

The different techniques are resumed in the following table:

Table 1: Non-contact coordinate measurement techniques

7 “Time of flight” refers to the time the ray needs to propagate from the source to the object and finally

to the receiver of the measurement device.

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2.3 Deflectometry

The techniques mentioned in the previous section measure the coordinates of an object. However, the

optical performance of a CSP collector depends also strongly on the slope exactness of the mirror

surfaces. It is possible that a mirror support point is in the right position, but the slope of the mirror

surface does not correspond to the design values and a part or all of the reflected radiation misses the

absorber tube.

As we have seen, photogrammetry is able to measure angles, but, as we have seen too, it requires a

special preparation of the measuring points, so that only punctual measurements are possible. A high

resolution slope measurement of a whole surface, like a curved mirror facet, is not possible. This leads

us to the so-called deflectometry method, which allows a high resolution slope measurement of

surfaces.

The working principle of deflectometry is the following: We all know the experience that a deformed

mirror projects distorted pictures of objects seen through them. Sometimes this is used for public

entertainment in leisure parks where our own bodies serve as the imaged objects. Obviously, the kind

of distortion of the pictures depends on the deformation of the mirrors. That means that the knowledge

of the kind of distortion gives information about the mirror shape. What deflectometry does is exactly

this: it analyzes the deformation of mirrors on the basis of deformed images. In the following, we will

distinguish between what we call the pattern reflection method and the absorber tube reflection

method.

Pattern reflection method

In the pattern reflection method, a regular line pattern is projected onto a screen. This pattern is

registered by a camera via the mirror that is to be examined. The form of the pattern registered by the

camera allows to calculate the form of the mirror and, consequently, to know possible aberrations of

the mirror in relation to the desired mirror geometry. If parallel lines in one direction are used, the

slope is measured in relation to one dimension (figure 13). If lines in two directions are used, the slope

can be measured in relation to two dimensions (figure 12).

Additionally, the intercept factor can be determined in a further step on the basis of the known mirror

quality. Figure 12 shows the general assembly of a two-dimensional deflectometry measurement.

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Figure 12: Deflectometry measurement assembly (Source: Ulmer, DLR)

Figure 13 gives an impression on the distorting effect of slope variations on the reflected pattern

image. Figure 14 shows a representation of the calculated slope errors in one dimension.

Figure 13: Pictured pattern on the mirror Figure 14: Slope deviation in x-direction in mrad (Source: Ulmer, DLR)

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Absorber reflection method

A special case is the absorber reflection method for parabolic troughs. The known “pattern” that is

used in this method is the absorber tube, that is located in the focal line. From a distant point of view

on the optical axis of the parabolic trough, the absorber tube should fill the whole image that is to be

seen in the parabolic trough. More exactly, in a collector-receiver geometry with an intercept factor of

1 nothing else as the absorber tube is to be seen from a distante point of view on the optical axis. If the

trough is moved slightly up and down, the edges of the absorber tube will be visible. Series of photos

taken from points of view closed to the optical axis allow to analyze the slope errors of the paraboloic

trough. Irregular images of the absorber tube are caused by slope errors. These errors can be calculated

on the basis of the photos.

Figure 15: Absorber tube reflected in the parabolic mirror and seen from a point slightly below the optical axis (Source: Lüpfert, DLR)

Before starting the measurement, the collector is tilted up until the absorber tube image disappears

completely. Then a series of pictures is taken while tilting the collector down until the absorber tube

image disappears at the other side. The angular range of the collector for a measurement is of about

five degrees. The images are processed and the slope of the concentrator in the respective dimension is

calculated.

Figure 16: Reflected images of the absorber tube with the reflector facing directly the camera (center) and tilted slightly up (left) and down (right). The red marks indicate the facet to evaluate. (Source: Ulmer, DLR)

Collectors within a solar field cannot be observed from remote points on the ground, because the other

collector rows hide them. Therefore, a hoisting platform can be used to take photos from above.

However, this method is very time-consuming and can be applied only for exemplary tests in a solar

field. DLR at the Plataforma Solar de Almería is developing the application of a airborne camera

system for taking pictures of the whole solar field from the air.

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3. Further measurements in CSP plants

In the following we will mention some other measurements that are important in CSP plants. This is,

of course, only an exemplary list. There are much more measurements that can be and have been

applied to ensure the quality of CSP components.

a) For the characterization of the thermal performance of a solar thermal power plant, the

thermal efficiency of the solar field can be measured. Therefore, the incoming radiation has

to be measured as well as the heat flow in the transfer fluid. Additional environmental

conditions like air temperature and wind speed have to be taken into account because they

may influence the thermal efficiency. A meteostation can register the solar irradiance and the

mentioned environmental conditions, while the heat flow is determined on the basis of the

mass flow in the absorber tubes, the specific heat capacity of the heat transfer fluid and the

inlet and outlet temperatures.8

b) In order to evaluate the optical efficiency of the solar field of a solar tower power plant and

the thermal efficiency of the receiver, a measurement of the irradiance distribution at the

receiver may be important. A diffusely reflecting board can be located in front of the receiver,

whose illumination is registered by a camera. The grey scale value distribution on the photos

is proportional to the irradiance. The correlation between the grey scale values and the flux

density can be calibrated with a radiometer.9

c) The specularity and reflectivity of the mirrors can be measured with a spectrometer and a

reflectometer.

d) The intercept factor in parabolic troughs can be measured with the so-called camera target

method. In this method, light measurement elements are attached to the absorber tube, which

direct corresponding signals to a camera sensor.

e) For the evaluation of the sun tracking precision, the inclination angle precision during the

daily collector run is measured. This can be done with an inclinometer.10

f) The transmission coefficient of the receiver glass tube and the absorption/emission

coefficient of the absorber tube can be measured in order to assess the receiver quality.

g) The thermal insulation of the receiver can be measured in the following way: The heat

transfer fluid is heated electrically until a thermal steady state is reached. The thermal loss

power is identical to the electric heating power.11

8 See Janotte et al.

9 See Ulmer, 2004

10 An inclinometer is an instrument for measuring angles of slope of an object with respect to gravity.

11 See: Lüpfert et al

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Reference list

Shortis, M.R., Johnston, G.H.G., Pottler, K., Lüpfert, E.: “Photogrammetric analysis of solar

collectors”. The International Archives of the Photogrammetry, Remote

Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing

2008

Janotte, N., Lüpfert, E., Pitz-Paal, R., Pottler, K., März, T.: “Messung des thermischen Wirkungsgrads

im Solarfeld”. DLR

Ulmer, S. (2003): Messung der Strahlungsflussdichte-Verteilung von punktkonzentrierenden

solarthermischen Kraftwerken. PhD Thesis, DLR, Stuttgart 2003

Lüpfert, E., Schiricke, B.: “QUARZ – Übersicht der entwickelten Prüfmethoden im DLR

Qualifizierungszentrum”. DLR

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Questions

1. Already small slope errors diminish the intercept factor quite strongly. Nevertheless, only a

photogrammetric measurement of the mirror support point position is integrated in the production

line of the Spanish Andasol power plant collectors. Deflectometry, i.e. the method that is most

appropriate to determine the slope correctness and which could be applied additionally to detect

slope errors, is not used as a routine control method. Can you explain this strategy?

2. A parabolic trough can be described geometrically in a coordinate system as indicated in the

figure below. Slope deviation from the ideal parabolic trough shape may exist in relation to the x-

axis as well as in relation to the y-axis. Which slope errors are important in relation to the

intercept factor?

3.

a) The absorber reflection method was introduced as a method to detect slope errors in parabolic

troughs. Is it also possible to apply an analogue method for parabolic dishes?

b) The pattern reflection method was also introduced for line-focusing systems. Can it be used

for point-focusing systems and what has to be taken into consideration concerning the kind of

pattern?

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Answers

1. Although the correct position of the mirror support points is not sufficient for the slope exactness

of the collector12

, it is a quite a strong evidence for the slope exactness. It is not probable that the

mirror support points are in the correct position and that there are strong slope errors despite of

this. So, the application of photogrammetry and the confirmation of the exactness of the mirror

support point positions is at the same time a corroboration of the slope exactness.

Additionally, photogrammetry is applied to the collector space frame, not to the completely

assembled collector, which has the advantage that errors can be corrected more easily.

Deflectometry would have to be applied to the completely assembled collector (it could be

applied also to single mirror facets, but after the mounting they can suffer changes in their shape).

2. Slope errors in the direction of the x-axis have a stronger effect than slope errors in the direction

of the y-axis. Actually, errors in the y-axis are in most cases irrelevant for the intercept factor.

Only near the end points of the collector they may have an influence.

3.

a) The absorber method could be also applied in an analogue way. In this case, the image of the

absorber in the focal spot (contrary to the absorber in the focal line of a parabolic trough)

would be used. An important difference is that the slope exactness will be registered in two

dimensions.

b) The pattern method could also be applied in an analogue way. While the pattern for a

parabolic trough may consist of parallel lines in one direction (measurement of slope

exactness in one direction), the pattern for the examination of a point-focusing mirror must

be two-dimensional in the sense, for instance, of a two-dimensional orthogonal line pattern

(square pattern) for the measurement of slope exactness in two directions.

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It is possible, for instance, that the mirror support points are at the right position but that their angles are not correct. Additionally, there can be mirror deformations between the mirror support points.