Category Archives: The Experimental Analyze Of The Solar Energy Collector

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.

Prediction of the steam volume

Подпись: vG = Max Подпись: V* Подпись: -SPP - Подпись: (4)

For a reliable dimensioning of the expansion vessel, knowledge of the steam volume (SV) emerging during the stagnation process is necessary. Unlike SPP, the steam volume is not a fixed parameter of the collector array, but is additionally dependent on the diameter and heat losses of the collector circuit pipework. The emerging steam volume during stagnation per collector aperture area vG can be calculated as follows:

with

vg

Steam volume in the collector loop per collector aperture area

liters/m2

v*

pipe

Internal volume of the collector loop pipes per meter

liters/m

Q*

loss, pipe

Heat losses per meter of pipe during stagnation

W/m

vG, coll

Steam volume in the collector per collector aperture area

liters/m2

vcoll

Total fluid volume of the collector per aperture area

liters/m2

У Qloss, pipe

‘ G, coll; vcoll

At the moment of maximum SPP and SV, the specific steam volume in the collector array vG, coll has been determined to approx. 0.5 liters/m2 for all the types of collector arrays examined. The specific thermal heat losses of the collector circuit pipes (PHL) during stagnation Q* ; depend

on the steam temperature, the ambient temperature, the pipe diameter da and the thickness of pipe insulation dHI. A list of the calculated heat losses of the pipes is shown in Tab. 1.

Tab 1. Internal volume and heat losses (X = 0.05 W/mK) under stagnation conditions (AT = 115 K) per
meter of pipe. The number-combination of the pipe dimensions indicates the external diameter da and the
wall thickness sR in millimeters. dHI denotes the level of insulation thickness: 50% means, that the
insulation thickness is 0.5 times the external tube diameter.

Pipe dimension (daxsR)

18×1

22×1

28×1.5

35×1.5

42×1.5

54×2

88.9×2

108×2.5

Unit

V*

pipe

0.20

0.31

0.49

0.80

1.19

1.96

5.66

8.33

Liters/m

QUipe (dHI = 50%)

36.5

38.4

40.5

42.3

43.6

45.1

47.5

48.3

W/m

QUipe (dHI = 100%)

27.6

28.3

29.2

29.8

30.3

30.8

31.6

31.8

W/m

Main challenges of the BIONICOL project

The two main differences of the new collector to be developed compared with state-of-the-art collectors are the absorber material (aluminium directly in contact with the heat transfer fluid) and the production method (generating the channels out of the absorber instead of attaching tubes to the absorber). Moreover, the channel design is more flexible and the channels do not have a circular cross­section. All these aspects lead to some consequences with respect to construction and production and thus to the main project aims:

• Further development and improvement of the FracTherm® computer program

• Adaptation of the roll-bond process for the production of solar absorbers

• Adaptation of a glass batch coating plant for the selective coating of solar absorbers

• Development and testing of appropriate heat transfer fluids

• Development and field testing of solar collectors with the new absorbers

Suitable absorber material

Aluminium has the best ratio of thermal conductivity to cost and is the best material for this application. A thickness of approx 1mm gives reasonable collection efficiency (with 150mm pipe spacing) and also sufficient flexibility to avoid closing up the pipe completely.

4. Suitable surface finish for absorber plate

The two options are either non-selective paint or a selective surface. Comparative tests have shown an advantage in collection efficiency of 10-15% for selective over non-selective surfaces. For this low cost application, a non-selective absorber is probably optimal.

5. Suitable back insulation material

Since the collector is built on top of a material e. g. wood which already has quite a low thermal conductivity, then the additional insulation requirements are modest. For this application, a reflective bubble polythene sheet was used. It is easily available

6. Suitable glazing

Twin wall polycarbonate is cheaper and lighter than glass. It is also tough and virtually unbreakable. Its optical transmission is reasonable and its thermal resistance is better than single glass. Modern polycarbonate with UV resistant coating should last for at least 15 years before replacement. Therefore twin — wall polycarbonate (10mm thick) was chosen for this application. Standard glazing bars for the polycarbonate were used to locate the sheet.