Category Archives: The Experimental Analyze Of The Solar Energy Collector

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.

On the Efficiency of the Paraboloidal Static Optical Concentrator

I. Zaharie*, I. Luminosu and D. Ignea

Departmental de Fizica, Universitatea “POLITEHNICA” din Timisoara
Bulevardul Vasile Parvan No.2, 300233, Timisoara, Romania7
*Corresponding Author: ioan. zaharie@fiz. upt. ro, izaharie@gmail. com

Abstract

The paper presents a numerical study, using its own the Ray — Tracer software, about optical efficiency, during equinoxes and solstices, of a static paraboloidal concentrator installed on the roof of a building. The roof is inclined with 45 deg and is oriented South.

The optical efficiency is maximum during equinoxes and minimum during solstices.

All these results are in concordance with the results reported in literature illustrating the fact that the proposed software tool is competitive.

Keywords: paraboloid, flow’s density, aperture, optical efficiency

1. Introduction

In the competition between classically obtained electric energy and photovoltaically obtained electric energy, the solar generated energy price is prohibitive because the materials are expensive and the efficiency is low. A solution consists in the concentration of solar light. Though, the using of concentrators that follow the sun is not a realist solution because the installation’s price is high and non-conveniable for users. As a consequence, there are used more cheap static concentrators that are also easy to use [1, 2, 3].

The Ray-Tracer software is using the Ray — Tracing method and is used to study the paraboloidal concentrator. The concentrator is of non-imaging type. The small dimensions paraboloidal concentrators are considered as architectural elements mounted on the houses roofs. The paraboloids walls are made out of very cheap plastic materials covered in void with nanometric film made of aluminium or silver. The reflexion coefficient, R, does not depend on the incidence angle and we consider for R the value, R=0.96. The concentrated radiation falls on the nanostructured photovoltaic cells located in the focal plan of the paraboloid. The efficiency of the photovoltaic cells increases from 14-16 % up to 24 — 30% [4, 5, 6, 7].

Effect of thermotropic layer on maximum absorber temperatures

The crucial factor concerning the application of thermotropic glazing is the switching range. For thermotropic hydrogels, thermotropic polymer blends and thermotropic systems with fixed domains maximum switching ranges of 77%, 52% and 25% are achieved, respectively. According to this for an all polymeric flat-plate collector with twin-wall sheet glazing and black absorber thermotropic hydrogels, thermotropic polymer blends and thermotropic systems with fixed domains would limit maximum absorber temperatures to 75, 90 and 125°C, respectively (assuming 85% solar transmittance in clear state) [1]. This allows for the application of cost-efficient plastics with more or less advanced engineering properties and temperature-stability as absorber materials for solar collectors. However, no product is commercially available today, which exhibits switching temperatures, the switching performance and the long-term stability needed for solar thermal applications. Thus further research and development on thermotropic materials is required to make the systems ready for the solar thermal market.

References

[1] G. M. Wallner, K. Resch, R. Hausner, Solar Energy Materials and Solar Cells, 92 (2008) 614-620.

[2] P. Nitz, H. Hartwig, Solar Energy, 79 (2005) 573-582.

[3] A. Seeboth, J. Schneider, A. Patzak, Solar Energy Materials and Solar Cells, 60 (2000) 263-277.

[4] H. Watanabe, Solar Energy Materials and Solar Cells, 54 (1998) 203-211.

[5] P. Nitz, H. R. Wilson (2008). In Proceedings of 2nd Leobner Symposium Polymeric Solar Materials, Leoben, Austria, pp. XIII-1-XIII-6.

[6] K. Resch, G. M. Wallner, R. W. Lang, Macromolecular Symposia, 265 (2008) 49-60.

[7] A. Beck, T. Hoffmann, W. Korner, J. Fricke, Solar Energy, 50 (1993) 407-414.

[8] A. Beck, W. Korner, H. Scheller, J. Fricke, W. J. Platzer, V. Wittwer, Solar Energy Materials and Solar Cells, 36 (1995) 339-347.

[9] D. Chahroudi, (1995). US 5404245.

[10] D. Chahroudi, (1983). US 4389452.

[11] L. M. Geever, D. M. Devine, M. J.D. Nugent, J. E. Kennedy, J. G. Lyons, A. Hanley, C. L. Higginbotham, European Polymer Journal, 42 (2006) 2540-2548.

[12] K. Yamamoto, T. Serizawa, Y. Murakoa, M. Akashi, Journal of Polymer Science: Part A: Polymer Chemistry, 38 (2000), 3674-3681.

[13] G. M. Campese, E. M.G. Rodrigues, E. B. Tambourgi, A. Pessoa, Brazil Journal of Chemical Engineering, 20 (2003), no. 3.

[14] J. H. Lee, D. G. Bucknall, Journal of Polymer Science: Part B: Polymer Physics, 46 (2008) 1450-1462.

[15] H. Okamura, T. Maruyama, S. Masuda, K. Minagawa, T. Mori, Journal of Polymer Research, 9 (2002) 17­21.

[16] H. Okamura, S. Masuda, K. Minagawa, T. Mori, M. Tanaka, European Polymer Journal, 38 (2002) 639-644.

[17] WT. Wu, Y. Wang, L. Shi, Q. Zhu, W. Pang, G. Xu, F. Lu, Chemical Physics Letters, 421 (2006) 367-372.

[18] H. Uyama, S. Kobayashi, Chemistry Letters, 9 (1992) 1643-1646.

[19] A. Georg, W. Graf, D. Schweiger, V. Wittwer, P. Nitz, H. R. Wilson, Solar Energy, 62 (1998) 215-228.

[20] H. R. Wilson, SPIE, 2255 (1994) 214-225.

[21] H. R. Wilson, in Grassie K. et al. (Eds) Functional Materials — EUROMAT, vol. 13, Wiley VCH,

Weinheim, pp 221 — 233.

[22] J. Schneider, A. Seeboth, Materialwissenschaft und Werkstofftechnik, 32 (2001) 231-237.

[23] W. Siol, H. J. Otto, U. Terbrack, (1993). EP 0181485.

[24] W. Eck, H. J. Cantow, V. Wittwer, (1993). EP 0559113.

[25] E. Jahns, H. Kroner, W. Schrof, U. Klowdig, (1995). EP 0749465.

[26] A. Goetzberger, M. Muller, M. Goller, Solar Energy, 69 (2000) 45-57.

[27] H. R. Wilson, A. Raicu, P. Nitz, (1996). In Proceedings of Eurosun 1996, Freiburg, Germany, pp. 534-539.

[28] F. S. Buehler, M. Hewel, (1999). EP 0985709.

[29] C. Schwitalla, H. Godeke, H. Konig, (2002). EP 1258504.

[30] K. Resch, G. M. Wallner (2007). In Proceedings of ISES SWC 2007, Beijing, China, pp. 541-545.

[31] Informationsdienst BINE — Schaltbare und regelbare Verglasungen (2002), http://www. bine. info/.

New application fields

• Applications with a great changing energy demand like weekend break in industrial processes (e. g. industrial laundry).

• Useful temperature levels at low radiation and low ambient temperatures (space heating).

• Need of air with high temperature level e. g. bakerys, burning in of coatings.

• Avoidance of humidity problems in buildings and condensation occurences in sensitive electrical applications. Also through the regeneration of sorption materials at daytimes.

• Applications with lightweight, water critical, or optical flexible requirements.

• Drying of sorption materials with high temperature.

• Drying applications (e. g. wood, food etc.).

• Heating of industrial or high rise buildings with large collector fields.

• Solar cooling: open-cycle desiccant evaporative cooling (DEC).

• Solar domestic hot water and space heating with a fin heat exchanger.

3.2 System Components

• Collector

• Fan

• Solar controller with temperature sensors.

Optional

• Heat exchanger, air — fluid (fluids are for e. g. water, oil, steam, chemical products).

• Storage tank e. g. with water, pebble beds or sorption materials.

• Solar radiation sensor to check the system.

2. Conclusions

At the Kollektorfabrik the series production of the solar air collector has started.

A new collector for the requirements of applications with high solar fraction without the risks of a stagnation state is available.

For private usage the collector is suitable for space heating and domestic hot water preparation. Especially holiday residences can be dehumidified while not occupied in order to avoid moisture while there is no risk of stagnation problems. During occupation the system can additionally provide domestic hot water.

For commercial and industrial usage the collector fits best for drying applications, solar cooling and large scale heat demand.

The first stage of the experiment

Part

Temperature(°C)

1

55

2

68

3

59

4

76

5

67

6

76

7

61

8

65

9

56

10

48

Table 1. The surface temperature values of the collector

Подпись: Figure 7.Measurements from the holes of the collector. Figure 8.Measurements from the air entry points of the collector.

As seen in Figure 4 and 6, in case of the collector surface receives radiation from infrared radiation lamps, air entry is realized from the hole 25 but air exit from the holes 23,24,26 and 27 isn’t realized. Incoming air from bottom is advanced in a way in the air chambers of the collector but air exit (holes 23,24,26 and 27 in figure 6) from the top of the collector is unrealized. (Figure 7 and 8) The surface temperature values of the collector are given in Table 1. In such a case, the system isn’t constituted the stack effect.

Results and discussion

The tested solar collector is a commercial product with a covered gross surface of 2.541m2. The test was carried out in the Test Station of IEE, CAS, located in Beijing, China, at 37.5° N latitude and at 116.7° E longitude. The test period reported was realized from May 17th to October 19th, 2007.

Figs. 2-6 present the main experimental results. Fig.2. shows lineal fit to test data for instantaneous efficiency curve and equation is provided. Fig.3. shows second order fit to test data for instantaneous efficiency curve and equation is also provided. All of these experimental data can be obtained under the steady-state measurement conditions according to ISO 9806-1. The instantaneous efficiency data is based on the gross area of solar collector and fluid flow rate used for the tests is 0.02 kg/m2.s. With the experimental data obtained in Fig.2. and Fig.3. and using the proposed equations (1)-(7), the following parameters were calculated for this day test: the highest nis 0.608 and the lowest nis 0.528.

Obviously when the inlet temperature is close to the ambient temperature, the heat loss of solar collectors will be less than its heat loss as the difference between the inlet temperature and the ambient temperature is very large. On the other side, because of the all-glass evacuated tube, the heat loss can be effectively reduced under the high temperature operation mode.

image065

Fig.2. Lineal fit to data for instantaneous efficiency curve

image066

image067

Fig.4. Instantaneous efficiency in different month

In Fig.4, different instantaneous efficiency curve in different months can be provided. Through this figure, the common situation of solar collectors operated during a long period can be predicted. In the past 5 months the ratio of the actual useful energy extracted to the solar energy intercepted by the collector can reach a comparatively high point. It means that this kind of solar collectors can work in a good condition and supply actual useful energy on a stable level. With the experimental results of several months, it is possible to obtain common sense and characteristic curves of solar collectors.

image068

Fig.5. Instantaneous efficiency and solar irradiance profiles during the day test period

image069

Fig.6. Difference of temperature and solar irradiance profiles during the day test period

Fig.5 and Fig. 6 shows in a certain day the variation of instantaneous efficiency, the difference between inlet temperature and ambient temperature of solar collectors and solar irradiance. In Fig. 5 the whole variation trend to some extent which instantaneous efficiency match solar irradiance is similar, but in the afternoon there exits opposite direction between two factors. Because all-glass evacuated tube can reduce heat transfer from inside to ambient, the energy can be reserved effectively. Meanwhile, if the direct radiation from solar energy is decreasing, for example, the clouds shadow the direct radiation, the ratio will be increased in an opposite direction to the solar irradiance. This result can be demonstrated by the same variation trend between the temperature difference and solar irradiance from Fig. 6. In the day test all-glass evacuated tube solar collector shows the reliable stability and high quality to supply the actual useful energy, and it can keep a good performance. The average instantaneous efficiency can arrive at 0.74 during the day test from 9:45 in the morning to 17:25 in the afternoon.

2. Conclusion

In this paper, a comprehensible test method to determine the thermal behaviour of solar collectors has been carried out. Through this kind of test an overall thermal performance can be provided for the product, such as the fit equation of the instantaneous efficiency considering the influence of inlet temperature, ambient temperature and solar irradiance, actual operation condition in a long period and the relationship among efficiency, difference of temperature and solar irradiance. And then, the manufacturer will obtain the information in order to improve the thermal efficiency of solar collectors. At the same time, the information is also useful to design the ideal solar water heating system with all­glass evacuated tube collectors for engineers. This procedure test also allows comparing, under the same test conditions, systems with some changes, for example with different materials, absorber selective surfaces, as well as thermal insulation, in order to analyze the influence of these parameters and together with an economical study can offer to the manufacturer the convenience or not to implement this modification.

References

[1] J. A.Duffie, W. A.Beckman, (1991). Solar Engineering of Thermal Processes, Wiley, New York.

[2] ISO9806-1, (1994). Test Methods for Solar Collectors — Part 1: Thermal performance of glazed liquid heating collectors including pressure drop.

[3] EN 12975-2, (2001). Thermal Solar Systems and Components Solar Collectors Part 2: Test methods.

[4] GB/T 4271, (2000). Test Methods for the Thermal Performance of Flat Plate Solar Collectors.

[5] Perers B, Dynamic Method for Solar Collector Array Testing and Evaluation with Standard Database and Simulation Programs, Solar Energy 1993, 50:517-26.

[6] Zeroual A, A New Method for Testing the Performance of Flat-plate Solar Collectors, Renewable Energy 1994, 4:825-32M.

Polymeric materials in collectors

The polymer performance pyramid (Fig. 5) is ranking the various polymers according to price and temperature performance. Most polymers in the following collector examples are of commodity and engineering plastics. Generally, commodity plastics are used in high volume and for a broad range

Подпись: Fig. 5. Polymer performance pyramid of applications, e. g. packaging and house­hold products where mechanical properties and service environments are not critical. Such plastics exhibit relatively low me­chanical properties and low cost. Engineer­ing plastics are a group of plastic materials that exhibit good mechanical and thermal properties in a wide range of conditions. High-performance plastics have temperature resistance, strength, dimensional stability and chemical resistance even in demanding applications.