Category Archives: The Experimental Analyze Of The Solar Energy Collector

Development of solar collectors with FracTherm® aluminium roll-bond absorber

M. Hermann

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany
Tel.: +49 7 61 / 45 88 — 54 09, Fax: +49 7 61 / 45 88 — 94 09
michael. hermann@,ise. fraunhofer. de

Abstract

Aluminium becomes more and more interesting as a solar absorber material. One possibility to produce an absorber entirely made of aluminium is given by roll-bond technology which is well established for large-scale series production of e. g. evaporators for refrigerators. This technology offers the possibility to build solar absorbers with high efficiency, since the channel design can be varied without additional costs. At Fraunhofer ISE a computer algorithm called FracTherm® was developed which is capable of generating a fractal-like, multiply branched channel design on a given area (also non-rectangular), similar to natural hydraulic networks. The aim of this bionic approach was to obtain a high efficiency by means of a uniform flow distribution as well as a low pressure drop. A small solar collector with FracTherm® roll-bond absorber (0.59 m x 1.0 m) had already been built and compared with absorbers featuring serial (meander absorber) and parallel (harp absorber) channel arrangements, respectively, within the research work of a doctoral thesis. Since the test absorber was relatively small, the focus of current research and development is on investigations of larger absorbers with sizes typical for solar collectors. This work, which is still at the beginning, will be done together with industry partners within the European project BIONICOL. This paper describes the challenges coming up with the development of large aluminium roll-bond absorbers with FracTherm® channel design including issues such as inside and outside corrosion, application of a selective coating, the connection of several roll-bond absorbers, fluid flow investigations and stagnation behaviour.

Keywords: FracTherm®, roll-bond, bionic, aluminium absorber

1. Introduction

State-of-the-art solar collectors with fin-and-tube absorbers mostly feature serial (serpentine or meander absorber) or parallel (harp absorber) channel arrangements. However, serial arrangements lead to a high pressure drop due to the channel length, whereas the parallel arrangements can feature non-uniform flow distributions (depending on diameters and lengths of riser and header tubes) [1]. The bionic so-called FracTherm® approach developed at Fraunhofer ISE (Fig. 1) allows creating fractal­like, multiply branched channel structures on a given area which lead to a low pressure drop as well as a uniform flow distribution. The resulting complex structures are not suitable for being built as a fin- and-tube construction, but they can easily be realized as aluminium roll-bond absorbers. Small

FracTherm® test absorbers (0.59 m x 1.0 m) had already been designed, built and compared with meander and harp absorbers within the frame of a doctoral thesis. These investigations had already been published in detail in [2], [3] and [4]. The thermal efficiency had been very high for all test absorbers. It could have been shown that the flow distribution of the FracTherm® absorber was much more uniform and the pressure drop for high volume flows was up to 8 % less than in the roll-bond harp absorber. However, both the results of the experiments carried out so far and the construction of the collector are not directly scalable to a standard size collector (1.0 m x 2.0 m). Fig. 2 shows the small test absorber investigated earlier and a standard size collector with a possible FracTherm® channel structure. The questions arising with the development of a large aluminium absorber with FracTherm® channel design are to be answered within the European research project BIONICOL.

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Fig. 1. The FracTherm® algorithm

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Fig. 2. Test absorber (left) and standard size absorber to be developed (right)

MEGASOL: a new technology for building big, cheap solar water heating collectors on site

Kerr MacGregor*

MacGregor Solar, 31 Temple Village, Edinburgh, Scotland EH23 4SQ
tel +1875 830 271

* Corresponding Author: kerr @macgregorsolar. com

Abstract

This paper describes a novel method of building large, cheap solar water heating collectors on site. It is based on using synthetic rubber pipes with enhanced thermal conductivity which are squeezed against the underside of an aluminium solar absorber sheet. The collector can easily be built on site using unskilled labour at a relatively low cost. In addition the collectors are freeze-tolerant and can handle corrosive fluids.

1. Introduction

Several attempts have been made (Bartelsen et al, 1999) to use polymers pipes for solar heat collectors. However, these require specially formed absorber plates to encase the pipes. This method uses plain, flat absorber sheets which are cheaper and more readily available.

2. Construction of absorber

The starting point is a foundation board which can be of timber or chipboard. A layer of thin insulation e. g. reflective bubble polythene is then laid on the foundation board. A grid of the polymer pipes is then laid on the insulation, spaced at about 150mmcentres. The pipes can be temporarily held in place by adhesive tape. Then a sheet of highly conductive material, e. g. aluminium, is laid on top of the pipes and is fastened using screws to the underlying foundation board. The thickness of aluminium and pipe spacing are chosen so that the polymer pipes are squeezed against the underside of the aluminium, but are not closed to the passage of fluid. The aluminium sheet is then painted with a solar absorbing paint. Alternatively, aluminium which has a selective surface can be used. Finally, glazing such as twin wall polycarbonate is located over the absorber and held by glazing bars. A diagram showing a cross-section of the collector is shown in Fig 1 below.

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Fig 1: A cross-section of the absorber 3. Suitable piping material

Obviously the pipe material should have good thermal conductivity and long life. EPDM has been previously used. Its thermal conductivity can be enhanced by including materials such as carbon black or graphite in its composition. That can also increase the mechanical strength of the polymer. For example, the addition of 40phr of carbon black to EPDM can increase thermal conductivity from 0.2 to 0.5 W/mK and tensile strength from 3Mpa to 17Mpa. It should also increase resistance to UV degradation, though in this application the tubing is shielded from solar radiation.

Acceptance angle and mirror deformations

Our second study considers the interactions between three different quantities: acceptance angle, vertical edge displacement Delta Z and collection efficiency maximum Emax. The results of these energetic and angular analyses are summarised in Fig. 7. The configuration examined in this study includes a linear parabolic mirror of focal length f=780mm. The absorber is a metal pipe of diameter D=50mm, enclosed into a glass tube of diameter G=70mm and thickness T=2mm.

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The acceptance angle is estimated examining plots of collection efficiency versus angular misalignment (tilt angle), like the curve in Fig. 6. The solar trough collector is tilted for assessing the acceptance angle and, at the same time, the mirror is deformed, as described in Sections 2-3. The parameter chosen to indicate the deformation of mirror surface is again the vertical edge displacement Delta Z. Starting from the results of the first study, the range considered for Delta Z is (-2.5 mm; +2.5 mm).

The critical optical characteristic in this second study is the acceptance angle obtained for every solar trough with deformed mirror. It represents the maximum tilt angle for which the collection efficiency approximately maintains its maximum value Emax, presenting only very little energy losses (<1% of Emax). The acceptance angle basically depends on the respective positions of mirror surface and absorber, as well as, on other characteristics not examined in this paper.

Figure 7 reports acceptance angle and collection efficiency maximum Emax as a function of the vertical edge displacement Delta Z. The collection efficiency has been calculated for all examined cases and its maximum value is reported in the labels of Fig. 7; these Emax values are obtained without angular misalignment.

As it could be supposed, Figure 7 evidences that the acceptance angle reaches its maximum value for Delta Z = 0, corresponding to the parabolic profile (see Table 1). Then the acceptance angle decreases when the absolute value of Delta Z improves, but the curve is asymmetrical. In analogy to the different results obtained in the first study for the two cases of elliptic or hyperbolic deformations, the behaviour of the curve in Fig. 7 depends on the deformation type.

The previous study individuated two different limiting values for Delta Z: 2.5 mm for the elliptic case and -3.0 mm for the hyperbolic case. For edge deformations included in this range of Delta Z values (-3 mm; +2.5 mm), the collection efficiency maintains its maximum value, so these mirror deformations do not introduce energetic losses. The first study does not consider any angular misalignment of absorber and deformed mirror.

Whereas the second study simulates a more realistic situation, where misalignment errors interact with mirror deformations, and the consequences are assessed analysing the variations of acceptance angle and collection efficiency.

The maxima of collection efficiency present only minor variations, but their behaviour is not symmetrical with respect to Delta Z = 0. The optimum position corresponds to vertical edge displacement Delta Z = 1, for elliptic deformation of the mirror.

1. Conclusions

A solar trough collector has been analysed using ray tracing simulations. The main optical components of the system are linear parabolic mirror and absorber, composed of a metal pipe surrounded by a glass tube. The reference layout for the solar trough has focal length f=780mm and absorber dimensions D=50mm, G=70mm, T=2mm.

The extensive research has investigated several possibilities for the optical configuration, varying most of trough geometrical features. For the parabolic mirror, it has considered the dependence on mirror width, length and focal length; the effect of mirror deformations and errors in surface finishing. For the absorber, it has examined metallic pipe diameter and shape, glass tube diameter and thickness. Beside these geometrical features the research has studied angular misalignment (of solar trough) and absorber displacement with respect to parabolic mirror. All mentioned effects have been analysed taking into account the sun tracking. The main aspects considered in this latter analysis are solar trough positioning, with respect to Earth rotation axis, and errors in daily and monthly tracking.

This paper summarises the results concerning two studies developed in the framework of the extensive research on solar trough collectors. The first study introduces an original methodology to reproduce rigid deformations of the linear parabolic mirror. Then it examines the consequences of mirror deformations on the collection efficiency of solar trough. The second study analyses the interactions between mirror deformations, misalignment and tracking errors.

In both studies the light concentrated by the parabolic mirror and received by the absorber is expressed as collection efficiency, corresponding to the ratio between focused light and entering light. The other fundamental parameter introduced is the vertical displacement of deformed mirror extreme Delta Z, chosen to indicate the amount of mirror deformation.

The methodology to simulate the deformations is based on the introduction of conic constant K and conic equation to represent the mirror profiles. The deformation can be of two types: elliptic, for -1 < K < 0 or hyperbolic for K < -1. While for K = -1 the conic equation represents a parabolic profile. The vertical edge displacement Delta Z in our convention is positive in the elliptic case and negative in the hyperbolic case, while Delta Z = 0 for the parabola.

This procedure to replicate the deformations of a parabolic mirror is simple and efficient. But the most interesting result is that it seems to reproduce the imperfect rigidity and the flexibility of a real solar collector.

The result of the first study is the identification of two different limits of Delta Z for the elliptic case (2.5 mm) and for the hyperbolic case (-3.0 mm). For mirror deformations with Delta Zin the interval (-3 mm; +2.5 mm), the collection efficiency keeps its maximum value, indicating that the corresponding mirror deformations do not cause losses in the collected energy.

In the second study the deformation effects are combined with the angular misalignment, obtained by a rigid tilt of mirror and absorber. Here the fundamental optical characteristic is the acceptance angle, representing the maximum misalignment angle for which the collection efficiency maintains its maximum value.

The results of the second study is a plot summarising the interaction between collection efficiency, acceptance angle and mirror deformations. The collection efficiency is the most important quantity in the application to solar light exploitation, because it indicates the level of performance in energy collection of the solar system. While for the aspects of alignment and sun tracking the crucial parameter to be taken into account is the acceptance angle of solar collector.

Acknowledgments

The research has been developed in the framework of the S. A.L. T.O. project. S. A.L. T.O. (Solar assisted cooling Toscana) is a research integrated project POR Ob. 3 Toscana 2000/2006 Misura D4, partially financed by the REGIONE TOSCANA-settore Promozione e sostegno della Ricerca (Tuscany Region). Thanks are due to the industrial partners FAIT group and CEVIT for their support to our activities in developing Solar Cooling Systems.

References

[1] C. Ciamberlini, F. Francini, G. Longobardi, M. Piattelli, P. Sansoni Solar system for the exploitation of the whole collected energy Optics and Laser in Engineering 39/2, 233-246 (2003).

[2] F. Francini, D. Fontani, D. Jafrancesco, L. Mercatelli, P. Sansoni Solar internal lighting using optical collectors and fibres, 6338-22, Proceedings of SPIE Vol. #6338, Optics & Photonics SPIE Conference San Diego — USA 13-17 Aug. 2006.

[3] F. Francini, D. Fontani, D. Jafrancesco, L. Mercatelli, P. Sansoni Optical control of sunlight concentrators, 6339-08, Proceeding of SPIE Vol. #6339, Optics & Photonics SPIE Conference San Diego — USA 13-17 Aug. 2006.

[4] F. Francini, D. Fontani, D. Jafrancesco, L. Mercatelli, P. Sansoni Designing solar collectors and optical fibers for daylighting. A novel system exploits solar energy by collecting and channeling sunlight to illuminate interior spaces — Illumination & Displays — Science and Technology: SPIE Newsroom DOI: 10.1117/2.1200612.0487 (2006).

[5] D. Fontani, F. Francini, D. Jafrancesco, G. Longobardi, P. Sansoni Optical design and development of fibre coupled compact solar collectors Lighting Research & Technology 39, 1, 17-30 (2007).

[6] D. Fontani, F. Francini, P. Sansoni Optical characterisation of solar collectors Optics and Laser in Engineering 45, 351-359 (2007).

[7] P. Sansoni, D. Fontani, F. Francini, L. Mercatelli, D. Jafrancesco Optics for concentration on PV cells T3-3.2-08, ISES Solar World Congress 2007, Beijing — China, 18-21 Sept. 2007.

Thermotropic systems with fixed domains

In thermotropic systems with fixed domains scattering particles, which exhibit a sudden change of refractive index with temperature, are statically embedded in a matrix material. At low temperatures the layer is translucent, as the refractive indices of matrix and domain are almost equal. The differing temperature dependence of the refractive index for the components above the switching threshold causes the thermotropic film to turn opaque [2].

One way to manufacture thermotropic systems with fixed domains is the incorporation of thermotropic core-shell polymer particles in a thermoplastic matrix. Such a material is commercially available from EMS Chemie AG (Switzerland) [28]. However, the gradual transition from clear to cloudy of this material is not optimal for solar collector applications.

Other thermotropic systems with fixed domains are based on the dispersion of paraffin in the matrix of a curable resin [6,29,30]. Such thermotropic systems possess a high potential for active solar thermal systems. The materials show a steep and rapid switching process within a small temperature range and an extraordinary high reversibility at low hysteresis [6,30]. The switching threshold is easily adjustable between 25 and 100°C [6,29,30]. A solar transmittance below 85% in clear state is reported. At present a moderate change in transmittance by 25% is achieved [31]. The material production is environmental friendly and cost-efficient. For thermotropic systems with fixed domains no comprehensive research on long-term stability and ageing was reported yet. However, as the additive is statically embedded in the matrix, the materials may posses a high durability. Further research should focus on the adjustment of appropriate switching temperatures, an improvement of the switching range and on the guarantee of sufficient long-term stability even under demanding conditions.

Fields of applications

In general there are two types of applications for solar thermal air collectors. Open systems in which outdoor air can be heated up directly and circulation systems with a closed loop. In a closed loop application the heat can be used directly or if needed transferred to another medium by means of a heat exchanger like shown in figure 3.

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The Experimental Analyze Of The Solar Energy Collector

Подпись: Figure 3. View of the experiment system

The collector profiles are tested using infrared radiation lamps in the laboratory. (Figure 2,3) The collector reaction to change in the value of heat transfer is measured. Air temperatures and velocities are measured at the bottom (air entry) and top (air exit) of the collector profiles.

The selective coated collector is divided four regions receive radiation from four infrared radiation lamps and ten measurement points are defined from bottom to top. (Figure 4) The bottom and top air temperatures and velocities measurement points of the collector are shown in Figure 5. Air velocity measurement holes on the front face of the collector are given in Figure 6.

Test Procedure

1.2. Previous test procedures

All different sensors to register the climatic and operational parameters should be calibrated and installed. The collector would be mounted according to the manufacturer’s installation instructions.

The whole recirculation system must be verified in test conditions; the temperature of supply water to the collector from the storage tank must be keep in a stable level; and the automatic acquisition data system must be installed and tested.

1.3. Test

The test is firstly carried out in steady-state conditions according to ISO 9806. The collector will be tested over its operating temperature range under clear sky conditions in order to determine its efficiency characteristic. Data points will be obtained for at least four water inlet temperatures spaced evenly over the operating temperature range of the collector. Especially, one inlet temperature shall be selected such that the mean temperature in the collector lies within ± 3K of the ambient air temperature, in order to obtain an accurate determination of n0. At least four independent data points

shall be obtained for each inlet temperature, to give a total of 16 data points. After this, in order to evaluate the thermal performance of solar collectors during the actual operation, solar collectors will be continuously tested during the day to obtain the characterization under different weather and working conditions.

Building integration

Подпись: Fig. 4. Examples of integrated polymeric collectors with additional function as building element Source: puren GmbH, Stale Skogstad, Solarcentury GmbH, Solkav, Solarflex;

The application of polymeric materials opens for new production techniques, allows new types of shapes and e. g. smart snap-designs. Many examples exists where polymeric collectors due to shape, design or simply due to the material have an added value as building part, replace conven­tional materials and produce thermal energy. Fig. 4 illustrates some examples, e. g. glazed flat plate collectors having modular building standard, replace conventional roof — or faqade cladding (a, b) and have the additional function of a sound shield towards a heavy-traffic road (b). The collector in (c) is shaped just like conventional roof tiles, allows easy retrofit and no transitions between the collector — and tile-roof are needed. Pool absorber pipes provide shading as roof of a carport (d). An innovative application integrates EPDM absorber pipes into the floor of outdoor, PU-based and walk-able tartan tracks in sport arenas or swimming pool surroundings (e).

Summary and Outlook

With roof mounted linear Fresnel collectors it is possible to provide industrial process heat of up to 200°C, which makes them well suited to power efficient absorption chillers.

Two prototypes and two commercial units of the PSE linear Fresnel process heat collector were installed so far. The tests and measurements at the pilot solar cooling system in Bergamo show a reliable automatic operation and a typical efficiency of the PSE Fresnel collector of approx. 40% with respect to DNI at 180°C. The system is continuously operated since late summer 2006.

In late 2007 and early 2008 the first commercial systems were commissioned in Spain and Tunisia. Both will be evaluated by the customers in the frame of research/demonstration projects.

We are step by step upgrading our production facilities in Freiburg, Germany and expect more projects for solar cooling and for the generation of process heat for industrial processes in the near future.

Acknowledgement

The development of the PSE Fresnel process heat collector was partially funded by Deutsche Bundesstiftung Umwelt.

References

[1] A. Haberle, M. Berger, F. Luginsland, C. Zahler, M. Baitsch, H.-M. Henning, M. Rommel: Linear Concentrating Fresnel Collector for Process Heat Applications, 13th International SolarPACES Symposium on Solar Thermal Concentrating Technologies, Seville, 2006

[2] Lupfert, E.: Test Report PTR Parabolic Trough Receiver 2005 — Modelling Parameters from Test Results, DLR, 2005

[3] A. Haberle, M. Berger, F. Luginsland, C. Zahler, M. Baitsch, H.-M. Henning, M. Rommel: Linear Concentrating Fresnel Collector for Process Heat Applications, ESTEC, 3rd European Solar Thermal Energy Conference, Freiburg, 2007

Glass cover and terminal strip

The prototype is covered by a glass plate. Thus the sensible parts of the collector like the reflector sheet and the absorber system are protected against mechanical impacts like hail and sandy dust. Additionally, the glass cover increases the torsion stiffness of the trough and simplifies the clean­ing. Around the outer border of the trough, a flange strip with a crimping is screwed. The glass cover, surrounded by an EPDM-sealing, and the flange strip are clamped together with a terminal strip.

The flange strip is mounted with a numerous number of screws, so that the assembly is complex and very time consuming. Moreover, it is not possible to dissemble the terminal strip without de­stroying it.

Planned optimization: The new parabolic body will be designed with a border, so that an addi­tional flange strip won’t be necessary anymore. The deep drawing process allows adding such a border without large additional effort. With a special designed terminal strip, the glass sheet will be fixed on the troughs’ body. In difference to the former concept the terminal strip will be remov-

able. Thus, changing of reflector material and/or absorber system parts can easily be done by re­moving the glass cover. All parts are accessible and removable then, without disassembling of the whole collector. This provides an economic profit not only concerning dis-/assembly time but also concerning operational costs.

1.2 Reflector

The prototypes’ reflector comprises a 0,5 mm thick sheet, which is coated with a high reflective surface. According to the manufacturers’ description the degree of reflection is 95 %. The sheet is inserted into the trough and pressed into the parabolic shape by the glass cover respectively the sealing.

Since the sheet is not connected to the trough, the surface is wavy, especially at the borders directly under the glass cover. The resulting decrease of efficiency could be determined with the above mentioned photogrammetry (see Fig. 3). Due to the non-removable terminal strip, the access to the reflector sheet is problematic.

Planned optimization: In the optimized concept a very similar reflector with a slightly lower re­flection rate of 92 % will be used. In difference to the prototypes design it will be glued to the sur­face of the parabolic trough by a self-adhesive backside of the reflector sheet, to achieve a consis­tent shape without any waves. The surface of the reflector is scratch resistant. Thus, it is not neces­sary to change the reflective material too often which lowers the systems’ operational costs, despite the fact, that the surface of the reflector sheet is additionally protected against mechanical impacts from the ambience by the covering glass plate.

2. Economical aspects

The material costs of the prototype were around 400 €/m2. Caused by increased material costs, this level will not be reducible with the new collector prototypes. Material costs of 250 €/m2 are aspired for the parabolic body, bearings, the reflector, the glass cover and the absorber system. Approxi­mately 150 €/m2 are planned for the trestle, the tracking system, the actuation and the control sys­tem.

The costs in a later on series production strongly depend on the produced units. Thus, it is difficult to predict the costs but it is anticipated to lower the costs for the all over concept in future.

3. Further steps

As mentioned above, the optimization and the further development towards a series production will go on until the end of June 2009. In this time the further constructive optimization and the fabrica­tion of the collectors will be finished. The tests will start in the following at the collector test facil­ity of the SIJ. The series production is planned to start in the year 2010.