energy conversion and management

Energy Conversion and Management 86 (2014) 11341146

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Energy Conversion and Management

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Optical and thermal evaluations of a medium temperature parabolictrough solar collector used in a cooling installation

http://dx.doi.org/10.1016/j.enconman.2014.06.0950196-8904/ 2014 Elsevier Ltd. All rights reserved.

Corresponding author. Tel.: +216 97570277; fax: +216 430934.E-mail addresses: [email protected], [email protected].

tn (M. Balghouthi).

Moncef Balghouthi , Abdessalem Bel Hadj Ali, Seif Eddine Trabelsi, Amenallah GuizaniCentre des Recherches et des Technologies de lEnergie CRTEn Bordj Cdria, Tunisia

a r t i c l e i n f o

Article history:Received 23 December 2013Accepted 29 June 2014

Keywords:Parabolic trough solar collectorPhotogrammetryDeformationSlope errorsIntercept factorOptical performanceThermal efficiency

a b s t r a c t

Concentrated solar power technology constitute an interesting option to meet a part of future energydemand, especially when considering the high levels of solar radiation and clearness index that are avail-able particularly in Tunisia. In this work, we study a medium temperature parabolic trough solar collectorused to drive a cooling installation located at the Center of Researches and Energy Technologies (CRTEn,Bordj-Cedria, Tunisia). Optical evaluations of the collectors using photogrammetric techniques were per-formed. The analysis and readjustments of the optical results were conducted using a Matlab code. There-fore, slope errors ranged from 3 to +27 milliradian and the height deviations from the ideal shapes ofthe parabolic trough collector were 2.5 mm in average with a maximum of 7.5 mm. The intercept factorwas determined using both the method of the total optical errors and the camera target method leadingrespectively to 0.62 and 0.7. Thus, the values of the overall optical efficiency were 0.48 and 0.514.Conversely, a thermal performance testing of the parabolic trough collector was conducted leading tothe thermal efficiency and the heat losses evaluations. The instantaneous thermal efficiency reached amaximum of 0.43 but it did not exceed the value of 0.30 when the reflector becomes dirty by dust depo-sition. This study was also an opportunity for suggesting some recommendations for the enhancement ofthe PTC performances.

2014 Elsevier Ltd. All rights reserved.

1. Introduction

In the solar thermal applications needing relatively high tem-peratures, the energy is optically concentrated before being con-verted into heat. The sunlight is concentrated in the focal plane,with the aim of maximizing the energy flux on the absorber sur-face. At present the Parabolic Trough Collector (PTC) can be consid-ered as the most advanced solar thermal technology. It representsthe most mature solar technology to generate heat at temperaturesup to 400 C for solar thermal electricity generation [1]. The otherkind of PTC is destined to provide heat to processes that need tem-peratures between 100 and 250 C. These applications are mainlyindustrial process heat, such as cleaning, drying, evaporation, dis-tillation, pasteurization, sterilization, cooking, among others, aswell as heat driven refrigeration and cooling. Typical aperturewidths are between 1 and 3 m, total lengths vary between 2 and10 m by row and geometrical concentrating ratios are between15 and 20 [2]. The PTCs of this group are called mediumtemperature collectors.

As far as the importance of the medium temperature parabolictrough solar collector applications, special concerns were attrib-uted by some organizations and researchers to this kind of solarcollectors. In fact, the International Energy Agencys (IEA) devel-oped the Task 33/IV program to improve and optimize mediumtemperature solar-thermal collectors for solar industrial processesheat. They reported that most solar applications for industrial pro-cesses have been used on a relatively small scale and are mostlyexperimental in nature [3]. In addition, Cabrera et al. [4] performeda literature survey on worldwide applications of the medium tem-perature PTCs to drive air conditioning and refrigeration facilities.They reported that, despite the relatively important solar fractiongiven by the PTCs compared to other solar collector technologies,the yearly rate of grow of this type of installations is still low.Recently, Minder [5] presented a medium temperature CSP fieldfor indirect steam generation used for milk process industry inSwitzerland. The field area is 115 m2 and the system uses thermaloil as heat transfer fluid and works up to 190 C. Besides, Sagadeet al. [6] described the experimental results of the prototypeparabolic trough destined for process heat applications made offiberglass-reinforced plastic with its aperture area coated by alu-minum foil. They tested the steel receiver coated with black proxymaterial. They achieved an instantaneous efficiency of 51% and 39%
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Nomenclature

C the geometric concentrating factorDNI the incident normal solar radiation (W/m2)F focal plan position of the parabolaf focal length of the parabolap Parameter of the parabolaSx the mirror surface local slopeTm average temperature of the receiver fluid (C)Ta ambient temperature (C)Xa, Ya, Za

absolute coordinatesX, Y, Z local coordinatesDSx the local slope errorDZ the height deviations regarding the ideal collectorDa the slope deviationsa the surface curvatureroptical standard deviation of the total optical errors (mrad)rslope standard deviation of slope errors (mrad)rspecular standard deviation of specular errors (mrad)

rdisplacement standard deviation of receiver displacement errors(mrad)

rtracking standard deviation of tracking errors (mrad)rsun Gaussian distribution for the errors caused by the sun

shapertot the standard deviation of the total errorscn the intercept factor at normal incidence angleW rim anglesgO the optical efficiencyqm average specular reflectance of the mirrors transmittance of the glass envelopeac absorptance of the absorber surface coating

Subscripts and abbreviationsd design datam measured dataPTSC parabolic trough solar collectorHTF heat transfer fluid

M. Balghouthi et al. / Energy Conversion and Management 86 (2014) 11341146 1135

with and without glass cover, respectively. Others authors [79]studied the thermal performances of medium temperature para-bolic trough particularly the thermal characterization of the recei-ver such as overall heat loss, end loss and thermal emittance of thecoating. In order to improve the performances of these kinds of PTCand their usage for industrial process heat, much more investiga-tions in the design, simulation, experimental and evaluatingtechnique ways are still required [10].

As the optical quality in particular the geometric precision ofthe solar concentrators has a significant impact on the efficiencyand thus on the performance of the PTCs plants, many studies wereperformed on the surface measurement methods of solar concen-trators. Thomas et al. [11] and Xiaoa et al. [12] presented a reviewof available methods for surface shape measurement of solar con-centrator. They gave a detailed description of the very used tech-niques: the photogrammetry, the deflectometry and the VideoScanning Hartmann Optical Test (VSHOT). The most studied mea-surement technique was the photogrammetry which is a methodbased on photographic processes and widely used for the 3-dimen-sional measurement of objects. The use of photogrammetry for theparabolic trough collector shapes evaluations was performed byGarca-Corts et al. [13], Shortis et al. [14], Fernndez-Recheet al. [15] and Pottler et al. [16]. Digital close range photogramme-try has proven to be a precise and efficient measurement techniquefor the assessment of shape accuracies of solar concentrators andtheir components. The combination of high quality mega-pixel dig-ital still cameras, appropriate software, and calibrated referencescales in general is sufficient to provide coordinate measurementswith precisions of 1:50,000 or better. The extreme flexibility ofphotogrammetry to provide high accuracy 3D coordinate measure-ments over almost any scale makes it particularly appropriate forthe measurement of solar concentrator systems. In the last years,close range photogrammetry has become a helpful tool to performthis optical evaluation, mainly due to the commercial availabilityof high resolution digital cameras and photogrammetry softwarepackages [17].

In this study, we present a medium temperature parabolictrough solar collector used to drive a cooling installation locatedat the Center of Researches and Energy Technologies (CRTEn,Bordj-Cedria, Tunisia). In a previous study [18], dealing with thedescription of the cooling installation, the results of the runningand the global performances COP were presented. Nevertheless,

in this work, we focused on the solar loop of the installation andparticularly the used parabolic trough solar concentrator. Opticalevaluations of the collectors using photogrammetric techniqueswere performed. To establish that a parabolic trough concentratorhas a good optical quality, it was documented that tolerances mustbe lower than 35 mm, and close-range photogrammetry is anaccurate enough technique to measure these surfaces as accuracieslower than 1 mm can be easily achieved [16]. In addition, thereflector of the considered parabolic concentrator is polished alu-minum without glass cover which allowed the positioning of thetargets exactly on the desired surface contrarily to the silveredglass covered mirror when the targets are placed about 4 mmabove the reflector due to the glass thickness [14]. Therefore, theuncertainties of the results are reduced. Theses raisons puttogether with the low-cost of the technique allowed us to adoptthe photogrammetry.

In the following paragraphs of the text, the procedures ofimages capturing and 3D processing were presented. These proce-dures use combination of high quality megapixel digital still cam-eras, appropriate software, suitable targeting and calibratedreference scales to provide coordinate measurements with highprecisions. The analysis and readjustments of the optical resultswere conducted using a Matlab code leading to the slope errorsand the height deviations from the ideal shapes of the parabolictrough collector. The intercept factor was determined using themethod of the total optical errors and the camera target method.The overall optical efficiency was then performed. Moreover, athermal performance testing of the parabolic trough collectorwas presented leading to the thermal efficiency and heat lossesevaluations. In addition some recommendations for the enhance-ment of the PTC performances were suggested.

2. General description of the solar cooling installation

The solar cooling installation is used to supply chilled water to aresearch laboratory building located in the Research and Technol-ogy Center of Energy in Borj Cedria, Tunisia. It consists of 39 m2 lin-ear parabolic trough solar collectors (PTSC) coupled to a 16 kWdouble effect absorption chiller, a cooling tower, a backup heater,two tanks for storage and drain-back storage and a set of fan-coilsinstalled in the building to be conditioned. A general scheme of theinstallation is presented in Fig. 1.

Fig. 1. Scheme of the solar cooling installation.

1136 M. Balghouthi et al. / Energy Conversion and Management 86 (2014) 11341146

The PTSCs were installed in series and oriented in the directionof the eastwest axis. The absorption chiller then uses thermalenergy converted from sun coming from the collector to generatechilled water. The installation contains also a gas-fired boiler usedas an auxiliary heating to boost the collector array outlet temper-ature when solar energy is insufficient.

3. The parabolic trough solar collector

The parabolic trough solar collectors plant constitutes the mainheat driving source of the air conditioning installation. It is com-posed essentially by a parabolic reflective mirror, receiver tubes,a steel support structure and a single-axis drive mechanism asshowed in Fig. 2. Solar radiations are reflected by the mirrors and

Fig. 2. The parabolic trough solar collector.

focused on the absorber tubes where they are converted into ther-mal energy and transferred to the circulating heat transfer fluid.

The converted solar thermal energy is used to drive the absorp-tion chiller. The driving devices can track the sun, making sunlightfocusing on the receiver all the time. The reflector is parabolic alu-minum polished mirror, with dust-proof coating on the surface.The installed PTSCs comprise three module assembled in series.Each module has a 5.8 m by 2.3 m aperture opening. This13.34 m2 aperture area and 0.68 m2 receiver area correspond to a19.6 geometric concentration ratio. The reflector mirror has a0.89 typical reflectance. The mirror is mounted on a steel supportstructure articulated to steel supports tightly fixed to the groundthrough concrete foundations. The receiver is an absorber tubelocated along the focal line of the parabolic mirror, whose surfaceis treated and covered by an aluminumnitrogen/aluminum selec-tive absorptive layer to enhance the heat collecting efficiency. Theabsorber tube is contained within an evacuated glass envelope tominimize conduction, convection and radiation losses. The collec-tor is orientated with its axis in the EastWest direction. Theadvantages of this tracking mode are that very little collectoradjustment is required during the day and the full aperture alwaysfaces the sun at noon.

4. Optical performance evaluations

The performance of the concentrating collectors is sensitive tothe optical errors. The optical quality in particular the geometricprecision of the reflectors, the support structure and the receiverplacement have a significant impact on the efficiency and thuson the overall performance of the collectors plant. Any deviationfrom the optimum shape can lead to optical losses, therefore it isimportant to have a tool that can measure surface slope errors withadequate precision [12]. In practice, there are many techniques ofoptical measurement used to analyze the optical errors and hencedetermining the intercept factor. Of which we can cite the photo-grammetry, the video scanning Hartmann optical test and the


deflectometry. In this study the shelf photogrammetry was usedfor the examination of the parabolic trough solar collectors. Theprocedure and results are exposed hereafter.

4.1. Photogrammetry quality tests

The shelf photogrammetry method was used to assess the opti-cal quality of the parabolic trough collectors. The method is basedon the registration of object points in several images taken fromdifferent viewing positions [17]. 3D coordinates of these pointsare then reconstructed from their coordinates on images usingthe software AICON 3D Studio. It is a valuable tool for the geomet-ric analysis of large solar trough concentrators. This method is use-ful for determining the surfaces deformations and the slopedeviations from the ideal shape of the collector.

4.1.1. Camera adjustment and calibrationA Nikon D300 camera was used with a Nikkor MF 20 mm f/2.8D

lens and a Sunpak Ring Flash. The camera and lens specificationscan be seen in Table 1.

In order to determine accurate camera model parameters (focallength, principal point position and lens distortion parameters),the camera must be calibrated. This is could be done by taking sev-eral photos convergent to a target grid specially designed for thatpurpose. Different distances and camera format positions are usedto avoid coupling between internal parameters [14]. In our case,scaled bares containing coded targets with known, precise andfixed separating interval were used. Four scaled bares disposedtogether and forming a 3D fixed shape were used. This processwas conducted in the dark so that the only significant light sourcewas from the flash of the camera. Additional adjustments wereconducted using software called DPA CHECK to improve imagesharpness and precision. By running a set of 50 images of thescaled bares through AICON 3D studio, the calibration parametersof the camera were determined. However, the measurement sys-tem is operated at changing temperatures, usually between 10and 40 C. Under these conditions, the distortion parameters ofthe camera and lens system change. Therefore, one and the samecamera is used to take all pictures instead of using various fixedcameras for the photogrammetric measurement. With thisapproach, the distortion parameters can be calibrated within eachmeasurement leading to reliable high measurement precisions.The coefficients describing radial-symmetric distortion, tangentialdistortion, affinity and shear were determined too, to be used laterfor image corrections. The parameters that were used during thisstudy are listed in Table 2.

Table 1Technical specifications of the camera and lens used.

Camera model name Nikon D300: NikonD2X-nikkorMF-20 mm

Resolution 4288 2848 (Pixel)Sensor size 23.584 15.664 (mm)Pixel size 5.5 5.5 lmLens type Nikkor 20 mmLens focal distance 19.980 mmMaximum aperture f/2.8Lens construction 12 elements in 9 groupsPicture angle 94Distance scale Graduated in meters and feet from 0.25 m

(0.85 ft) to infinity (1)Aperture scale f/2.8f/22Diaphragm Fully automaticMount Nikon bayonet mountAttachment size 62 mm (P = 0.75 mm)Filters 62 mm screw-inDimensions Approx. 65 mm 42.5 mm extension from

flange; approx. 54 mm long (overall)

4.1.2. Collector targetingThe concentrator mirror surfaces are problematic because of

their reflective behavior. The absence of natural points must besolved using targets arranged on a collector surface. These targetswill be detected automatically during the photogrammetric work.The usual procedure is to attach a sheet of adhesive vinyl on thesurface, with a printed array of targets with appropriate size andshape [19].

Retro reflective targets are fixed on the mirror surface. Some ofthese targets are coded to be directly distinguished by the software(Fig. 3).

The targets are distributed uniformly throughout the surface toensure greater precision. A reference cross is also fixed to establisha reference point for measurements (Fig. 4). In addition, a numberof coded targets are distributed over the surface to allow the iden-tification by the software. Even, the absorber tube, the ends of therotating axis and the reflector plane of symmetry are marked.

4.1.3. Image capturing and 3D processingThe images were taken from different positions and different

elevations in order to cover the entire collector area and to assurehigh level of recognition of the non-coded targets by the software.The angular separations between the camera positions of less than15 or greater than 165 should be avoided. An angular separationof 90 is optimal for minimizing errors of angular sensitivity of thecamera, but a half-angle of this size is acceptable. Calculatingthe coordinates in three dimensions requires that each point ofthe object must appear in at least two or more images taken atangles converging [20]. More than 100 pictures were taken toensure greater accuracy during processing. The scale of a picturecannot be determined from photographs that do not containobjects of known size, and so the scaling rods (bars of knownlength) are included in all scenes (Fig. 5).

The locations and orientations of the camera are automaticallydetected when photogrammetric measurements are processed,and it is not necessary to measure and record these sites while tak-ing photographs.

Digital processing of the captured images was done using a pro-cessing software, AICON 3D. The software has interfaces in whichwe introduced the camera settings, cross reference, scaling rodsand coded targets parameters (Fig. 6).

A 3D coordinate system is used to restore the digital pictures.Each target coded and non-coded, identified by a unique numberis assigned by three absolute coordinates (Xa, Ya, Za). A changeof reference is needed to duplicate the representation of the actualshape of the reflector. Thus, it is necessary to identify and readjust

Table 2Metric calibration parameters of the camera and lens used.

Value Standarderror

Focal distance 19.9820 mm 0.0002 mmPrincipal point offset in x-image

coordinateXH = 0.2097 0.00029

Principal point offset in x-imagecoordinate

YH = 0.2428 0.00034

3rd order term of radial distortioncorrection

A1 = 2.8421e004 2.11e007

5th order term of radial distortioncorrection

A2 = 6.6295e007 2.622e009

7th order term of radial distortioncorrection

A3 = 5.9024e010 1.0396e011

Coefficient of decentering distortion B1 = 1.8631e005 2.048 e007Coefficient of decentering distortion B2 = 1.1269e005 1.922 e007Differential scaling between x and y C1 = 1.257e004 1.82 e006Non-orthogonality between x and y

axesC2 = 3.9660e005 1.84 e006

Fig. 3. Collector targeting.

Fig. 4. Reference cross and scaling rods used as reference points for measurements.

Fig. 5. Samples of targeted collector photos taken from different locations for photogrammetry processing.


targets previously glued to mark the axes coordinates. Usually, oneof the axes is selected to be the rotation axis of the parabolictrough (in our case it is the Y axis), the origin is chosen on one ofthe reflector ends symmetry axis and a second axis (Z-axis) willbe marked with targets attached to the absorber tube. Consideringthat the tracking axis (Y axis) of the parabola is the eastwest axis,the Z-axis was oriented to the south and the positive X coordinatescorresponded to the lower part of the reflector. After the acquisi-tion and processing of images, the software will generate a graphas a scatter plot.

4.2. Analysis and readjustment

4.2.1. Real reflector surface representationThe real surface of the reflector generated by the AICON 3D

processing software was provided as set of points recopying the

real shape of the parabola and related to a 3D referential (x,y,z)(Fig. 7).

In order to identify the reflector surface errors, it must be com-pared to an ideal surface of the parabola defined by the followingEq. (1):

Z X2

2 p X2

4 f 1

To distinguish the surface errors, a comparison of two criteria isneeded: the height deviations regarding the ideal collector DZ andthe slope deviations Da (Fig. 8).

The slope errors are the deviations of the surface normaldirections away from the ideal normal directions. They could bedetermined using Eqs. (2) and (3):

a tan1 X1 X2Z1 Z2

2

Fig. 6. Interfaces of the digital images processing software, AICON 3D.

Fig. 7. Representation of the real surface of the reflector generated by the AICON 3D processing software as set of points recopying the real shape of the parabola.


Da areal aideal 3

where a is the surface curvature and Da is the slope deviation.

4.2.2. Height deviationsThe knowledge of Z coordinate deviations or the height devia-

tions provide information on the magnitude and location of theconcentrator surface differences in respect to the ideal parabolicshape, and can be useful to improve the manufacturing processand the collectors mounting and adjusting [16].

The height deviations are obtained using a Matlab code thatshows the difference between the actual and the ideal surface areaof the reflector (Fig. 9). By examining figure, we noticed that theheight distribution has almost two areas with an antisymmetricshape, this observation allowed to identify the problem whichwas an error in the identification during the photogrammetricanalysis. There was a shift in the marking of the focal plan relativeto the correct position which caused an angular reposition aroundthe Y axis.

After readjusting of the referential by a rotation of an angleU = 0.019 around the Y axis and analyzing the new coordinates,a representation of the distributions of the surface height errorsin 3D was generated by a MATLAB code (Fig. 10). The graph showsthat the surface deformations increase going from the center to theedges of the reflector.

These deformations are probably caused by the wind impactsand the weight of the metallic structure. The most intense defor-mations were located in the edges of the trough, with a maximumvalue of 7.5 mm. Although a bigger part of the surface has a defor-mation value with an average of 2.5 mm.

4.2.3. Slope errorsThe optical performance of a trough concentrator depends on

the slope errors, which tend to deviate the reflected rays awayfrom the ideal focal line. Slight variation of the position where lightis reflected is less important than the change in the direction of thereflected rays. The coordinates of the targets were used to estimatethe differences of local slopes. The slopes (Sx)m, given by Eq. (4) arecalculated along the x-axis using the coordinates of two adjacenttargets (Pi and Pj).

Sxm Zjm ZimXjm Xim

4

The subscript m here designates measured data while the sub-script d designates design data.

The slope calculations were performed in the plane (X, Z). Theideal slopes (Sx)d were calculated with Eqs. (5) and (6) using thecoordinates Zi of the Z-axis and Xi along the X-axis and the focallength f.

Fig. 8. Scheme of the height deviations regarding the ideal collector DZ and theslope deviations Da.


Sxd Zjd ZidXjm Xim

5

Zi Xi2d

4f6

Fig. 9. Distribution of the height deviations of the reflector

Fig. 10. Distribution of the height deviations of the reflecto

The slope error DSx was defined as the deviation of the mea-sured slope from the design slope (Eq. (7)):

DSx Sxd Sxm 7

The standard deviation rslope of the zero-mean Gaussian distri-bution of the local slope errors was calculated using equation (Eq.(8)) by where n is the number of calculated slopes.

rslope

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXn1

DSx2

n

s8

The slope errors were performed using a separate post process-ing step in MATLAB based on the Delaunay triangulation [21]. Theresults yielded a maximum absolute slope error of 27.8 milliradian,and a standard deviation rslope = 11.9 milliradian.

Fig. 11 shows the distributions of the slope errors on the reflec-tor surface. The sign convention for the slope errors has beendefined positive for reflected rays passing above the focal lineand negative for reflected rays passing below the focal line. It canbe drawn that the lower edge along the trough length is deviatedfrom the ideal surface with negative deviations; the vertex of theconcentrator is slightly positive-deviated. There is a zone in rightside of the lower part of the collector showing the largest devia-tions from the ideal slope of up to 27 milliradian and there is also

surface before readjustment of the focal plan marking.

r surface after readjustment of the focal plan targeting.


a small zone in the top-right corner with important deviations. Theslope errors go from 3 to +27 milliradian.

Using these results, the concentrator geometry could beimproved by readjusting the reflector assembly on the structure.

4.3. Uncertainty analysis

The characteristics of the camera and the calibrations parame-ters presented in Tables 1 and 2 were used by AICON 3D Studiofor image corrections. The software calculates the three-dimensional coordinate values defining the positions of the targetsin space and the relative uncertainties. The precision of each targetcoordinate were provided during the digital processing of thecaptured images. Fig. 12 gives a sample of the photogrammetricmeasurement uncertainties of the target positions. The largestuncertainty of all single distance measurements is below0.4 mm while the standard deviation is 0.12 mm.

Similarly to the measurement of slope deviation, the calculationof the slope deviation uncertainties were carried out using an algo-rithm implemented in MATLAB. A detailed uncertainty analysiswas performed according to GUM 08 [20]. The maximum measure-ment uncertainty in the surface slope is 1.0 mrad with a standarddeviation of 0.6 mrad.

4.4. Total optical errors

In a real collector there will be several statistically independentsources of optical error: lack of perfect specularity, macroscopicsurface deviations in position and slope, displacement of the recei-ver, and tracking errors. Averaged over time and over the entirecollector or array of collectors, all of these errors can be assumedto be approximately Gaussian [22]. The standard deviationaccounts for all optical errors and could be calculated in Eq. (9):

r2optical 4r2slope r2specular r2displacement r2tracking 9

In our study we consider the case of normal incidence, thetracking errors can be overlooked. Even the reflector material isthe polished aluminum which has a low diffusive reflectance, con-sequently, the specular errors can be neglected in the calculation ofthe total errors. Accordingly, Eq. (9) is reduced to Eq. (10):

r2optical 4r2slope r2displacement 10

The receiver displacement or misalignment error rdisplacement isdefined as the angular deviation of the location of the receiver

Fig. 11. Distributions of the slope e

centerline from the design focus of the parabola. The average of thiserror was estimated to 6.8 milliradian. Considering rslope of11.9 milliradian, we get an optical errors roptical of 24.75 milliradian.

The standard deviation of the total errors (combined optical andsun shape) of the collector is given by Eq. (11):

r2tot r2optical r2sun 11

A Gaussian distribution approximation for the errors caused bythe sun shape rsun was adopted to be 2.6 milliradian as expectedby Rabel [22]. Therefore the total error rtot was 24.9 milliradian.

5. Intercept factor

The geometric accuracy of parabolic trough collectors isdescribed by the intercept factor, which includes the optical effectsof reflector shape and receiver absorber alignment among others.The intercept factor is defined as the fraction of the rays incidenton the aperture surface of the reflector that are intercepted bythe receiver. It refers to the question if the rays hit the absorberor not. Two optical analysis methods have been used to character-ize the intercept factor of the concentrator: the sum-of-squaresapproximation or the total errors and the camera-target-method.

5.1. Method of the total optical errors

This method uses the plot of Bent et al. [23] giving the interceptfactor at normal incidence angle cn versus the product of the totaloptical error rtot and the geometric concentrating factor C for dif-ferent rim angles W (Fig. 13).

In our case the rim angle or the opening angle of the parabolictrough is about 75, the geometric concentrating factor C is 19 andthe product rtotC is 473.1 milliradian. Accordingly the interceptfactor determined is cn = 0.62.

5.2. The camera-target-method CTM

Another method to estimate the intercept factor of parabolictroughs is based on the indirect CAMERA-TARGET-methodCTM. This method uses a flat, diffuse reflecting target placed per-pendicular to the absorber tube. A cut in the target allows to enve-lope the receiver tube almost entirely. The solar rays around thereceiver are visualized on the objective, and the rays which missthe absorber tube are identified. Many pictures at different receiverlocations are captured (Fig. 14).

rrors on the reflector surface.

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 500 1000 1500 2000

Dev

iatio

n (m

m)

target distance (mm)

Fig. 12. Photogrammetric measurement uncertainties relative to the targetspositions.


These pictures were used to determine an approximate inter-cept factor of the collector at the target location. By integratingover two half circles; one in front of the absorber tube for theincoming rays and one behind the absorber tube for the passingrays [24]. The intercept can be calculated as the ratio of the raysthat hit the absorber to the incoming rays. Fig. 15 shows the esti-mations of the intercept factor in different locations of the receiver.The obtained intercept factor varies from 60% to 87% with an aver-age value of 70%.

6. Optical efficiency

The optical efficiency gO is defined as the amount of radiationabsorbed by the absorber tube divided by the amount of direct nor-mal radiation incident on the aperture area. The optical efficiencywhen the incident radiation is normal to the aperture is given inEq. (12):

gO qmsaccn 12

Accordingly, considering the optical properties of our collectorqm = 0.89 and the effective product transmittance absorbancesac = 0.85, the optical efficiency of the collector can be calculatedby substituting cn by their values obtained by the two methods.

Fig. 13. Determination of the Intercept factor using the plot of Bent et al. [21].

The resulting optical efficiencies at normal incidence obtainedusing the camera-target-method and the method of the total opti-cal errors are respectively 0.514 and 0.48.

7. Thermal performances

The thermal performance of solar collectors can be determinedby experimental performance testing under controlled conditions.In general, experimental verification of the collector characteristicsis necessary in order to determine the thermal efficiency of the col-lector. There are a number of standards, which describe the testingprocedures for the thermal performance of solar collectors. Themost well known are the ISO 9806-1:1994 and the ANSI/ASHRAEStandard 93:2003 [25].

Since the collector will be optimized based on either instanta-neous or all-day average efficiency, a steady-state thermal analysisof the receiver will suffice for the design studies. The performanceof the PTSC according to the previously mentioned tests is deter-mined by obtaining values of the useful heat gain, the collectorinstantaneous thermal efficiency, the energy gained by the storagetank and the overall efficiency, for different parameters of opera-tion; incident radiation, ambient temperature and inlet HTFtemperature.

7.1. Performance testing of the parabolic trough collector

The solar collectors plant was tested during the summer. Anacquisition data system HP Agilent Data Logger was used torecord solar radiation intensities, temperatures at different loca-tions of the receiver and the reflector, the HTF mass flow ratesand the wind velocity. 30 thermocouples K, 4 Platinum resistancethermometers Pt100, two Pyranometer CM 21, two PyrheliometersCHP1 and two flow meter were connected to two Agilent HP34901A 20-Channel Multiplexer for data acquisition andprocessing.

The used instrumentation devices, their specifications and rela-tives precisions are presented in Table 3.

The used heating fluid HTF in the presented experiments is thepressurized water. Actually and due to problems of overpressure,we used thermal oil instead of water to assure more stability ofthe installation pressure.

A summer day measurements are presented in Fig.16. The heat-ing fluid HTF flow rate were 1.2 m3/h. The direct solar normalradiation varied from 400 to 950 W/m2 and the maximum HTFtemperature at the collector outlet was 164 C.

Experimental data shows that the solar thermal system nor-mally takes about three to 4 h to heat the system to 160 C, thenominal operating temperature of the absorption chiller. Theoverall system has a high heat capacity. This heat capacityconsumes much useful solar energy and prolongs the warm upperiod before solar energy is available to be used by the absorp-tion chiller.

There were thermal losses occurring at the receiver due to thedifference between the glass cover temperature and the ambienttemperature whose values are 49 C and 37 C respectively. Even,in addition of the losses of vacuum in the annular space betweenthe absorber and the glass cover, heat losses are due to heat con-duction throughout the steel supports, which are in contact withboth absorber tubes and the glass cover.

This constitutes a weak point in the design of the collector. Infact, the temperature of supports exceeded 60 C. Besides, thereflector surface temperature is higher than the ambient tempera-ture and is equal to 42 C. Accordingly, there were energy losses onthe surface and the reflection was not perfect. This could be due todust deposit on the reflector surface.

Fig. 14. Photos of the CAMERA-TARGET-method CTM procedure.

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80%

90%

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

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tor

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Fig. 15. Determination of the intercept factor using the CAMERA-TARGET-method.

Fig. 16. Evolutions of the temperatures at different locations of the parabolictrough solar collector as well as the solar radiation and the ambient temperature.


7.2. Thermal efficiency and heat losses

The performance of the parabolic trough concentrator was eval-uated according to the thermal tests by obtaining values of the use-ful heat gain, the available solar energy on the collector and thecollector instantaneous thermal efficiency for different parametersof operation such as the incident beam solar radiation, ambienttemperature and inlet HTF temperature.

The useful heat gain was determined from the measurements ofthe inlet and outlet HTF temperatures and mass flow rate. Theavailable solar power was the product of apparent concentratorarea by the incident beam solar radiation at normal incidence.

Fig. 17 presents the evolutions of the available solar power andthe useful power transferred to the thermal fluid. We can see thatthe available solar energy reached maximum values between13:00 and 15:00 with a peak of 30 kW at 14:20 local time. The use-ful heat gain first increases, reach a peak value of 12 kW around14:00 then decreases. Data for the available solar energy and usefulheat gain by the collector was used to determine the instantaneousefficiency.

As expected in Fig. 18, the instantaneous efficiency of the para-bolic trough solar collector increases, reaches a maximum of 0.43then declines. Moreover, when the reflector becomes dirty by dustdeposition, the efficiency decreases notably and does not exceedsthe value of 0.30.

The thermal efficiency of the PTSC was also performed understeady state conditions as proposed by ASHRAE Standard93(1986) [26]. The periods of tests were chosen when the operat-ing conditions are almost in steady states such as the direct solarradiations, the HTF flow rate, the wind speed and the ambient tem-perature. The collector efficiency was plotted against the differencebetween average HTF temperature in the receiver and the sur-roundings temperature divided by the direct normal radiation(Fig. 19). The obtained curve was approximated to a straight line.The highest efficiency of the system is realized when the meanHTF temperature is equal to the ambient temperature (no thermallosses).

This efficiency represents the optical efficiency at the normalincidence, it is about 0.58. This value is comparative to the valuesof the optical efficiency determined in Section 6 using the camera-target-method and the method of the total optical errors. The pro-prieties of the receiver such as the absorption and the emissivityhave also an important effect on the thermal performances.

Beside, the thermal efficiency decreases with the increase of thetemperature difference between receiver fluid and the surround-ings (Tm Ta) or the decrease of the incident normal solar radiationDNI.

The thermal efficiency is affected by heat losses caused by theconductive, convective and radiative exchanges with the surround-ing. When the temperatures of the receiver and the glass coverincrease, the radiative losses increase. Even the convective lossesincrease due to the temperature gradient between the receiverand surrounding and wind velocity. Even, the thermal tests haveshown that there were considerable heat losses in the receiverdue to heat conduction throughout the steel supports, which arein contact with both absorber tubes and the glass cover. Moreover,the heat losses were extremely high during the night from the solarcollector tubes. This reduces the HTF temperature overnight; lead-ing to a morning temperature close to ambient. These losses wereessentially due to radiant exchanges between the absorber tubesand the night sky. These night-time heat losses were responsiblefor a start-up delay time of the absorption chiller. In fact, the par-abolic trough solar collector needs about 4 h to heat up the HTF to a

Table 3Specifications and precisions of the used instrument devices.

Instrumentation device Measured physical parameter Precision

Thermocouple type K Temperature (C) 0.2 CPlatinum resistance thermometers Pt100 Temperature (C) 0.2 CPyranometer CM 21 Kipp and Zonen Global solar radiation (W/m2) 5 W/m2

Pyrheliometers CHP1 Kipp and Zonen Incident normal solar radiation (W/m2) 1% of the measured quantity (W/m2)Flow meter TME/UMC-3 Water flow rate (kg/h) 0.3% of the measured quantity (kg/h)Flow meter OMG Oil flow rate (kg/h) 0.3% of the measured quantity (kg/h)Anemometer NRG #40C Wind speed (m/s) 0.1 m/s

Table 4Comparison with similar designs.

Model Solitem PTC 1800 IST PTC NEP Solar Polytrough1200

NEP Solar Polytrough1800

Current study

Max. operating temp.(C) 220 280 220 250 200Aperture width (m) 1.8 2.3 1.2 1.845 2.3Focal length (m) 0.78 0.8 0.65 0.65 0.8Rim angle (degrees) 60 72 50 71 73Geometric concentration ratio 15 14.36 15 17.28 19Absorber tube diameter (mm) 38 51 25.4 34 38Absorber tube material and

coatingStainless steel/Blackchrome

Steel/blackenednickel

Stainless steel/Blackchrome

Stainless steel/Blackchrome

Stainless steel/BlackNickel

Absorptivity 0.94 0.960.98 0.94 0.94 0.96Working fluid Press.water/thermal oil Press.water Press.water/thermal oil Press.water/thermal oil Press.water/thermal oilReflector material Polished aluminum Silvered acrylic Polished aluminum Polished aluminum Polished aluminumCover tube diameter (mm) 65 5 56 56 90Reflectivity 0.89 0.89 0.89 0.89 0.89Mean detected intercept factor 0.72 0.76 _ _ 0.620.7Peak optical efficiency 0.53 0.76 0.62 0.68 0.480.502References Weiss et al. [26] Kalogirou et al. [25] Fernandez et al. [2] SPF [29]

Lokurlu et al. [27] NEP SOLAR [28] NEP SOLAR [28]


temperature of 160 C required by the absorption machine to drivethe cooling process. In a previous work [18], we studied the impor-tance of a drain back night thermal storage system for the para-bolic trough collectors. Drain back might significantly reduce theloss of heat over night from the HTF contained in the system. Aninsulated drain-back tank with a capacity of 400 l was added inthe solar collection loop of the installation. A drain-back systempumped and drained all the HTF, at about 140 C in the late after-noon and pumped this hot fluid back into the system the nextmorning. Fig. 20 shows a comparison between HTF outlet temper-ature with and without the drain back storage. The HTF solutionwas drained to the back storage tank at 19:00 local time at a tem-perature of 142 C, the next day at 8:40, the temperature of thestored HTF became 129 C. After pumping the HTF back into thesystems, the solar collectors started heating from a temperature

02468

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7:55 10:19 12:43 15:07 17:31 19:55

Pow

er (k

W)

Local time (hh:mm)

Collector useful Power output (kW)

Available solar power (kW)

Fig. 17. Daily variation of the available solar powers and the useful power of theparabolic trough solar installation.

of 125 C instead of 30 C without the drain back storage. Usingthe drain back storage has allowed the outlet solar collector tem-perature to attain the value of 160 C, required for the chillerstartup, at 10:40 rather than 14:00 in the case of no drain backstorage.

7.3. Comparison with similar designs

The current study is based on optical and thermal performancesof parabolic trough solar concentrator used to drive a coolinginstallation under Tunisia environment. This concentrator is classi-fied in the category of medium temperature parabolic trough solarcollector. Comparison between previous and current research areanalyzed. Although this kind of research is new in Tunisia, theanalysis was made on installations over the word focusing on

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r co

llect

or e

ffec

ienc

y

Local time

Clean Reflector

Reflector with dust

Fig. 18. Daily parabolic trough solar collector efficiency with clean and dirtymirrors.

y = -1.1777x + 0.5816R = 0.9498

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Fig. 19. The thermal efficiency of the PTSC under steady state conditions.

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Tem

pera

ture

( C)

Local me

Tcollec_out with drain back storage

Tcollec_out

Fig. 20. Parabolic trough solar collector outlet temperatures with and withoutbackup night storage.


medium temperature parabolic trough solar collector (80250 C)used for industrial process heat or solar air conditioning. Table 4summarizes the main characteristics of some commercial and pro-totypes of PTCs designed for medium temperature applications andhaving similar dimensions to the concentrator subject of thisstudy. Some additional informations about these collectors aregiven below:

The IST PTC parabolic trough model, founded in the UnitedStates and recently acquired by the Spanish company, AbengoaSolar. The reflector is silvered acrylic or enhanced-polishedaluminum. The absorber is a steel tube with a black nickelanti-reflective coating. The maximum operating temperaturewas 280 C and the peak optical efficiency was 0.75 [27]. The Solitem PTC-1800 was developed by Solitem Company

(Turkey, Germany) with the help of the DLR in design and eval-uating. It consists of an 1.8 m aperture width polished alumi-num reflector, a selective coated stainless-steel absorberhaving 38 mm in diameter The maximum operating tempera-ture was 280 C and the peak optical efficiency was 0.75 [28,29]. NEP SOLAR Polytrough 1200 and Polytrough 1800 were devel-

oped by the Australian company, New Energy Partners PtyLtd. (NEP) in collaboration with Australias CommonwealthScientific and Industrial Research Organisation (CSIRO). Thetwo models have polished aluminum reflectors and stainlesssteel absorber tubes with selective coating (black chrome).The Polytrough 1200 could attend an operating temperature

of 220 C and an optical efficiency of 0.62. While the Polytrough1800 have a maximum temperature of 220 C and a peak opticalefficiency of 0.68 [2,30,31].

8. Recommendations for the enhancement of the PTCperformances

The evaluations of the parabolic trough solar collector haverevealed significant optical and thermal losses. The optical analysisof the collector showed that the relatively low optical efficiencieswere caused by the reflectors surfaces deformations and the slopedeviations from the ideal shape of the collector other than the dis-placement of the receiver in the focal line. The thermal losses werecaused by conduction, convection and radiation from the receiverwhich include the glass cover, absorber tube, bellows, flexible hoseand supports. Beside, considerable heat losses occurs in night-timedue to radiative exchange between the receiver and the night skyleading to a morning HTF temperature close to ambient and caus-ing a start-up delay time of the solar cooling installation.

Based on these assessments, several recommendations weregiven for the improvement of the collectors performances:

The mounting of the reflectors should be revised and some mir-rors should be replaced. Even, the supports of the reflectorshould be readjusted in order to minimize the errors from theideal shape of the parabola. The receiver and the absorber tube should be altered to avoid

the misalignments from the focal line. The absorber tube surfaces should be coated by the right selec-

tive coating to get high solar absorptivity and low thermalemissivity. Moreover, the absorber tube diameter should bedetermined by a tradeoff between solar radiation interceptedby the absorber pipe and its thermal losses. The glass envelope should be designed with as small diameter

as possible with material having high transmittance, lowabsorptivity and small thickness. In addition the dimensionsand the exposed area of the bellow should be reduced and theexpansion piece should be contained inside the glass cover. Using a drain back night storage system will significantly

reduce the loss of heat over night from the HTF and decreasethe start-up delay time of the solar cooling installation.

9. Conclusion

A medium temperature parabolic trough solar collector usedto drive a cooling installation located at the Center of Researchesand Energy Technologies (CRTEn, Bordj-Cedria, Tunisia) was stud-ied. Optical evaluations of the collectors using photogrammetrictechniques were performed. The procedures of images capturingand 3D processing are presented. The analysis and readjustmentsof the optical results were conducted using a Matlab codeleading to the identification of the slope errors and the heightdeviations from the ideal shapes of the parabolic trough collec-tor. Moreover, thermal performance testing of the parabolictrough collector was presented leading to the thermal efficiencyand heat losses evaluations. The main results and relevance arethe followings:

The most intense deformations were located in the edges of thetrough, with a maximum value of 7.5 mm. But, an averagedeformation value of 2.5 mm characterized the larger part ofthe surface. The slope errors ranged from 3 to +27 milliradianwith a maximum absolute slope error of 27.8 milliradian, and astandard deviation rslope = 11.9 milliradian. Accordingly, totalerror rtot was 24.9 milliradian and the intercept factor deter-mined was cn = 0.62.


The resulting optical efficiencies at normal incidence obtainedusing both the camera-target-method and the method of thetotal optical errors are respectively 0.514 and 0.48. The instantaneous efficiency of the parabolic trough solar col-

lector increases, reaching a maximum of 0.43 then declines.However, when the reflector becomes dirty by dust deposition,the efficiency decreases notably and did not exceed the value of0.30. The thermal efficiency of the PTSC determined under steady

state conditions as proposed by ASHRAE Standard 93(1986)yielded a highest value of 0.58 when the mean HTF temperatureis equal to the ambient temperature (no thermal losses). Thisefficiency represents the optical efficiency at the normal inci-dence which is about 0.58. Besides, the thermal efficiencydecreases with the increase of the temperature differencebetween receiver fluid and the surroundings or the decreaseof the incident normal solar radiation. The evaluations of the parabolic trough solar collector revealed

significant optical and thermal losses. Therefore, some recom-mendations for the enhancement of the PTSC performanceswere suggested. They concern the revision and readjustmentof some mirrors, the absorber tube coating and alignment, theglass envelope diameter and thickness and the expansion piecesisolation. Even, a night storage tank was proposed.

Acknowledgement

The authors would like to thank the members of the enerMENAproject and the German Aerospace Center DLR for their scientificand financial supports.

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