Category Archives: The Experimental Analyze Of The Solar Energy Collector

Coatings with paint

Figure shows a summary of the work done to produce selective paint. It is possible to observe high solar absorption values, but undesirably also high thermal emissivity. The paints obtained until the moment are not selective.

In the initial work with paints, the objective was to get good optical properties for paint with the organic pigment C6o/ C70. High solar absorption (95% and 96%) was reached. The problem was the emissivity, which is strongly dependent on coating thickness. With the coil method adopted for coating, the lower thickness achieve was 7pm, with 80% of emissivity and 95% of solar absorption. To reduce the thickness and consequently the emissivity, spray technique was tested and it was possible to achieve 4pm of thickness and emissivity of 74%, with 96% absorption.


Fig. 6. Absorption variation with wavelength for different paint samples.

Without the possibility to reduce thickness to lower values with methods of easy application, it was also tested the incorporation of metallic pigments in the paint with 16% CVP of C60/ C70 pigment, considering that the thermal conductivity of metallic pigments would lower the emissivity values. Both copper pigment with average grain size between 63 and 90pm and stainless steel with average grain size of 3 pm were tested. The mix was done adding 16% of metallic pigment weight to the already prepared paint with C60/ C70 pigment. Figure 6 shows that this did not improve the paint behaviour in relation to emissivity.

Adding higher quantity of metallic pigment, about 50%, to the base of paint, without use of organic pigments, hoping to increment thermal conductivity of the coating and obtain lower emissivity,

independently of thickness coating, also did not improve the emissivity and, without organic pigment, the absorption decreased to 36%. The fact that the metallic pigment used, was stored for a long time (surface highly oxidized) could cause the observed behaviour. Also the surface shape of used pigments could explain the observed behaviour, since the surface contact area between metallic particles and the metallic substrate was not adequate to increment conductivity. These aspects will be explored in near future.

Topography of paint with organic pigment obtained by SEM (Fig.7.a) allows us to identify a granular morphology, with grains agglutinated by resin. It is visible agglomerates of small grains; which rough surface that can improve absorption.

Fig. 7. a) SEM (30000x) surface micrograph of paint with organic pigment. b) Surface
photography by optic microscope (45x) of paint with organic and Cu grains.

4- Conclusions

Optical properties of titanium oxide are strongly dependents of deposition parameters, and some of these are interrelated, which become very difficult to relate optical properties with change of each parameter, but it is possible to conclude that best values of absorber selectivity were obtained in dc mode and in pulsed dc mode with 200kHz, with oxygen flow rate changing between 0 and 2.5ml/min with adequate slope. Adequate slope depends of deposition rate which depends of deposition power, total pressure, oxygen partial pressure and pulsed frequency and all of these parameters are important, once that for solar absorber selectivity the final thickness and oxygen gradient concentration along of the film thickness are determinants. Best optical properties for oxide titanium sputtered films were 88% for solar absorption, with 7% of emissivity for deposition parameters of: pulsed frequency 200kHz, reverse time of 0.4ps, discharge current of 0.7A, argon flow rate of 50ml/min and oxygen flow rate changing from 0 to 2.5ml/min. The morphology of oxide titanium films is columnar, with columns oriented in direction of growing film, which seem to be continuous from the substrate to the top of the film. Subsequent immersion in solution with antocyanin didn’t show to improve solar absorption.

For paints, the results obtained until the moment weren’t satisfactory. The best couple values for solar absorption and emissivity were respectively 94%, and 74%. Emissivity is dependent on thickness of coatings and with the used application techniques, the minimum thickness reached was 4pm, not low enough to obtain infrared transparency. The effort to reduce emissivity of paints adding metallic particles were unfruitful, at least using for the shapes and sizes of metallic particles used. Surface topography shows grains agglutinated with binder.

Aknowledgements -To Fundagao para a Ciencia e Technologia by the financial support through the

referred research project POCTI/ENR/62660/2004 “Development of new spectrally selective coatings with

organic pigments for absorbers of solar collectors.”


[1] Project POCTI/ENR/62660/2004 “Development of new spectrally selective coatings with organic pigments for absorbers of solar collectors”, Fundagao para a Ciencia e Tecnologia.

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[3] A. J. Martins, C. Nunes, M. J. Brites, M. Lopes Prates, V. Teixeira, M. J. Carvalho, Journal of Nanoscience an Nanotechnology, Vol. 8, 1-5, 2008.

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[13] J. Y. Kim, EBarnat, E. J. Rymaszewski, T. M. Lu, J. Vac. Sci. Technology A 19(2), 429-434, Mar/Apr 2001.

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[16] P. S. Henderson, P. J.Kelly, R. D.Arnell, H. Backer, J. W. Bradley, Surface and Coating Technology 174­175 (2003) 779-783.

[17] R. D. Arnell, P. J. Kelly, J. W. Bradley, Surface and Coating Technology 188-189 (2004) 158-163.

[18] N. J. Cherepy, G. P. Smestad, M. Gratzel, J. Z. Zhang, J. Phys. Chem B 1997, 101, 9342-9351.

[19] H. Backer, P. S. Henderson, J. W.Bradley, P. J.Kelly, Surface and Coating Technology 174-175 (2003) 909-913.

[20] A. Belking, Z. Zhao, D. Carter, L. Mahoney, G. McDonough, G. Roche, R. Scholl, H. Walde, Society of Vacuum Coaters, 43rd Annual Techn. Conf. Proc.-Denver, April 15-20, 2000.

[21] Jindrich Musil, Jan Lestina, Jaroslav Vlcek, Tomas Tolg, J. Vac. Sci. Technology A 19(2), 420-424, Mar/Apr 2001.

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[24] Sang-Wong Park, Jae-Eun Heo, Separation and Purification Technology 58 (2007) 200-205.

Technical improvement of a small modular parabolic trough collector

Klemens Schwarzer, Jan Kroker, Markus Rusack

Fachhochschule Aachen / Campus Julich
Solar-Institut Julich (SIJ)

Heinrich-Mufimann-Str. 5, D-52428 Julich, Germany
Corresponding Author: schwarzer@sij. fh-aachen. de


Throughout an earlier research project the Solar-Institut Julich (SIJ) developed a small sin­gle axis tracking modular parabolic trough collector with an evacuated absorber tube, a high — reflective aluminium mirror, an anti-reflective solar glass cover and a step motor drive with worm gear and tracking system. In consequence of the small aperture area of 2 m2 the collec­tor is lightweight and can be used for roof mounting. The aspired operating temperature level is 120 to 200 °C. Measurements and regression analysis have shown an overall effi­ciency of approximately 63 % at a working temperature of 160 °C (relating to 800 W/m2 of solar direct radiation).

In March 2007 the SIJ has started a further research project in association with four partners of the German industry in apparatus engineering, absorber technology and drive engineering. The main aim of the project is the improvement of the collectors’ technical characteristics and its thermodynamic performance. In addition the production costs are to be reduced. Fur­ther the project aims at a series production of the collector.

The project is state-aided by the German Federal Ministry of Education and Research. Keywords: small parabolic trough collector, collector deep drawing, collector improvement

1. Background

Throughout an earlier research project from 2003 to 2005 the Solar-Institut Julich (SIJ) developed a small single axis tracking modular parabolic trough collector, named PTC 1000. Possible appli­cations are the supply of process heat for hotels and hospitals, for industry applications and for the supply of cooling energy. With the end of the project the construction of the collector was finished and three prototypes were built.

Since March 2007 the SIJ tends to optimize the PTC 1000 prototype in a continuative project since weak points not only of constructional background had been detected throughout the testing phase. The main aim is to achieve a series-production readiness. The project is accomplished in collabora­tion with four partners of the German industry in apparatus engineering, absorber technology and drive engineering, which are Wallstein Ingenieur-Gesellschaft mbH, NARVA Lichtquellen GmbH + Co. KG, Ingenieurburo Annas & Partner GmbH and SMF Spanlose Metall Formung GmbH & Co. KG.

The project is divided into four work packages: In the first stage, which ended end of Octo­ber 2007, the weak points of the prototypes such as thermodynamic and technical deficiencies have been identified and a specification sheet for the new collector design has been elaborated. In the second stage concepts for the individual components have been worked out in detail and the new design concept of the collector has been decided at the end of June 2008. Currently, a detailed planning takes place, so that the production can start in September 2009. In the last work package the newly developed collector prototypes will be set up at the test facility of the SIJ and perform­ance tests will be carried out to evaluate the new collector design.

To show the demand of optimization regarding the collectors’ construction and thermodynamics, the features and the determined deficiencies of the prototype are described in the following.

Experimental set-up

Solar collector efficiency has been measured with the indoor test rig shown in Fig. 1. It consists of a 12kW mercury lamp array, an integral thermostat plant, fluid temperature and air temperature probes, a pyranometer, an anemometer, a precision flow meter and a wind generator.


Fig. 1. Test rig scheme.

This installation meets the requirements established in the European Standard EN 12975 to measure the efficiency of solar collectors.

Two prototypes of solar collectors have been manufactured using tabulators. In the first one, we have used a continuous twisted copper tape. The tape was placed all along the riser tubes of the harp. It had

0. 2 mm thickness and 5 mm width. The length of the 180° twist was 20 mm approximately. The second one was made using a steel chain as an insert. The chain had 5 mm width. It was placed in the same way as the first one, in the riser tubes. Both prototypes were constructed using the basis of a commercial collector of Isofoton, risers having an inner diameter of 7 mm. This way, the only difference between the prototypes and the standard design was the addition of tabulators.

The experimental sequence was as follows: i) efficiency test of the commercial collector, ii) efficiency tests of prototype 1 and iii) efficiency tests of prototype 2. In order to analyze the influence of water flow, we measured the efficiency of the prototypes at three different flows: 160 kg/h, 320 kg/h and 500 kg/h.

Each test was made during a whole day, and all tests were carried out in consecutive days. All of them were completed according to EN 12975.

2. Results

Table 1 shows a comparison between the efficiency of the standard collector and the turbulators prototypes (mass flow of 160 kg/h).

Table 1. Efficiency coefficients


Prototype 1 (twisted tape)

Prototype 2 (chain)













The main result is the 3% increase of q0 in the first prototype. However, there is also an opposite growth of the loss coefficients, ai y a2. In order to make the analysis, all three efficiency curves have been plotted in Fig. 2.


Fig. 2. Efficiency curves.

The 3% gain in the left side of the curve seems to be reduced in the right side to 2%. Although prototype 1 is better, the second prototype equals its efficiency at non dimensional temperature T*=

0.07. We can confirm that there is a consistent efficiency increase along the curve when using tabulators.

Furthermore, we have made three different tests for both prototypes, at three different mass flows: 160 kg/h, 320 kg/h and 500 kg/h. The results of these tests are shown in Tables 2 and 3.

Table 2. Prototype 1. Efficiency coefficients at different mass flows

160 kg/h

320 kg/h

500 kg/h













Table 3. Prototype 2. Efficiency coefficients at different mass flows

160 kg/h

320 kg/h

500 kg/h













It is observed that there is no significant variation in the efficiency in terms of mass flow, in any case. For both prototypes, the efficiency remains at approximately the same value.

The uncertainty of the efficiency curves has been estimated according to EN 12975 [4], and its value is ± 1.9%.

3. Conclusions

Experimental tests have demonstrated the suitability of using tabulators to improve solar collectors’ efficiency. A 2-3% efficiency increase can be obtained. Moreover, the insertion of twisted tapes has been reported to be a better option than the use of a chain.

There is no significant variation of the efficiency depending on mass flow when the two types of turbulators described are used.

A more detailed study to optimize the design of the tabulators will be done. However, the simplicity of the materials used and the efficiency enhancement obtained in this work, demonstrate that this solution is an adequate and suitable way of improving solar collectors.


[1] Duffie, Beckman. Solar engineering of thermal processes. Wiley-Interscience, 1980.

[2] P. Promvonge, S. Eiamsa-ard. Heat transfer behaviors in a tube with combined conical-ring and twisted-tape insert. ScienceDirect, Elsevier, 2007.

[3] S. Ray, A. W. Date. Friction and heat transfer characteristics of flow through square duct with twisted tape insert. ScienceDirect, Elsevier, 2002.

[4] UNE EN 12975. Sistemas solares termicos y componentes. Captadores solares. AENOR, 2006.

Overheat protection

Thermal stagnation and risk for overheating is generally aimed to be avoided in any collector sys­tems if these have metal-based — or polymeric collectors. The intention with a built-in overheating mechanism for polymeric collectors is to be able to use low-cost commodity plastics in glazed col­lectors.

High operational temperatures in polymeric collectors can be avoided by suitable hydraulic system design and dimensioning. Especially for solar combisystems with large collector areas the integra­tion of the collectors into the facade reduces the risk for thermal stagnation during summer time.

At the same time it improves the performance during the heating season.

Natural or forced ventilation of the collector between absorber/glazing or absorber/thermal insula­tion can be used for the overheat protection of polymeric collectors. As illustrated in Fig. 11 (a) a flap is triggered by a temperature sensitive mechanism and opens when a critical temperature in the collector is reached, so that ambient air can ventilate and cool the collector [11, 12].

Functional materials / thermotropic coatings are a central topic in ‘Subtask C: Materials’ of IEA — SHC Task 39 and considerable R&D has been done, e. g. [13, 14, 15]: The principle is that the thermotropic coating switches from transparent to opaque at a critical temperature for the absorber material Tc. The coating can be applied on the glazing and reduces the transmittance (Fig. 11 (b)).or on the absorber and reduce the absorptance for temperatures above Tc (Fig. 11 (c)).

Подпись: Fig. 11 Various approaches to prevent overheating in (polymeric) solar collectors


Another principle for the overheat protection is proposed in the patent by Griessen and Slaman [16]. The refraction index of the collector glaz­ing is changed by a simple mechanism and re­duces the transmittance for solar radiation. The glazing is a prismatic structured optical layer, which is hollow inside. “The glazing is air-filled and transparent under normal operation but dur­ing stagnation filled with an appropriate fluid being totally reflective above the boiling point of the heat carrier in the absorber” [16].

Except for the first examples, the mechanisms for overheating protection are not commercial yet, but the R&D reveals the effort for making polymers with lower temperature resistance available for the use in glazed collectors.

Reduction of stagnation temperature

1.1. Stagnation temperatures in reference — and ventilated collector (set-up A)

The measurements of the maximum temperatures in the reference — (Tr) and the ventilated collector (Tv) are shown in Fig. 2 for different days and tilt angles. The difference between these temperatures gives the reduction of the maximum temperature, which can be obtained by ventilating the solar collector. The maximum temperature reduction occurs during the warmest period of the day and lies in the present cases between approximately 20-30 K. Fig. 2 (a) and (b) display data from days with high solar irradiance. The collector tilt angle в was 45° for (a) and (b). The bottom slit aperture was 10 mm for (a). The maximum temperature of the ventilated collector, Tv, was slightly below 130 °C.

a) June 9, 2006 b) June 10, 2006 c) August 3, 2006


Tilt angle: 45°; slit aperture: 10 mm Tilt angle: 45°; slit aperture: 20 mm Tilt angle: 90°; slit aperture: 20 mm

Fig. 2 Maximum temperatures in the ventilated (Tv) and reference collector (Tr) for different days and tilt angles; Im is the global solar irradiance, Ta the ambient temperature [set-up A];

For (b) the bottom slit aperture was extended to 20 mm and the maximum temperature Tv was slightly below 120 °C. In (c) the collector tilt angle was changed to 90° with a bottom slit aperture of 20 mm. Here a maximum temperature of Tv ~ 100 °C of the ventilated collector was measured.

Efficiency of a linear parabolic mirror for geometrical deformations

Подпись: 2D. Fontani 1*, P. Sansoni 1, F. Francini 1, D. Jafrancesco 1, G. Chiani 2, M. De Lucia

1 CNR-INOA Istituto Nazionale di Ottica Applicata, Largo E. Fermi 6 — 50125 Firenze — Italy
2 Dip. Energetica — CREAR, Univ. di Firenze, Via Santa Marta, 3 — 50139 Firenze — Italy
* Corresponding Author, daniela. fontani@inoa. it


A linear parabolic mirror for sunlight concentration has been analysed and simulated. The study examines the geometrical deformations of the parabolic profile and their effects on solar light collection. The application is a solar trough, whose parabolic mirror has been optically designed to concentrate the sunlight on a cylindrical receiver.

The analysis procedure is based on the use of a mathematical representation for parabolic and deformed profiles. The mathematical approach consists in introducing conic constant and conic equation to represent the mirror profiles. This methodology to replicate the deformations of a parabolic mirror is simple and efficient. But the most interesting result is that it seems to reproduce the flexibility of a real solar collector and its imperfect rigidity.

The optical simulations allow controlling all the optical parameters; nevertheless collection efficiency and acceptance angle are probably the most important for our application. The optical characteristics have been monitored to evidence how much they are affected by geometrical deformations of the mirror profile. Finally the mirror deformation effect has been combined to alignment and tracking errors.

Keywords: optical project, ray tracing, deformations.

Over all performance

For making it easier to see any general trends about what are a good and a less good construction, a compilation of the safety factors and the varied parameters were made, which can be seen in table 4.

In the upper part of the table each row shows the safety factors of one solar collector. The x-marks describe the configuration. In the lower part of the table, the mean values for each column is presented. For the columns describing configuration, the mean values are the averages of the SFweakest of the x — marked rows. The last row shows the quotients between each column’s mean value in relation to the mean value of SFweakest.

image103 image104

Two parameters show significant importance to the safety factor. The important factors are area and angle. for setting figures of the impact of the parameter a trend analysis was done. The formula of the mean safety factors dependence of the area is expressed in (6)

N. B. the matrix of input data for the areas and angles were centered in order to keep a low condition number. They were also divided with maximum deviation in order to make them easier comparable.

There are no corresponding formulas describing the safety factor dependency of thickness of glass since there are no really general agreements.

Table 4. Safety factors and varied parameters.



4. Discussion

The purpose of this work was using commonly used dimensions and materials when building collectors. However, there are variations in almost all dimensions between different commercial collectors, and when trying to keep down the analysis data some of the factors where set constant.

The shape and design of the connection between glass and absorber has influences on the stresses in the materials, especially in the connections. In the absorber there is no maximum of stresses in the connection points. This makes the maximum stresses trustworthy also in other designs, at least when the stresses can be distributed in a less concentrating way at the connection point. In the glass the case is the opposite since the maximum of stresses is in the connection to the absorber. This makes probably the maximum of stress in the glass in this article dependant on the choice of connection. And our speculation is that the maximum stress in the glass always will be a question of how the connection is designed, i. e. the maximum of stresses in the absorber is more or less universal while it is not that for the glass.

5. Conclusion

There are numerous parameters influencing the strength of a gas filled solar collector. The most obvious parameters are the distance between glass and absorber, the area of the absorber and the angle build by arctan(width/height). The safety factor increases with low angles (long tubes), big areas and small distances.

It is not clear to the same extent about selection of material, thicknesses etc. If you take a look again at table 4 and make a comparison between the 3 and 4 mm glass respectively on the 2 m2, 45°, absorber plate in aluminium, you can see that the one with a thicker glass has a smaller safety factor. By choosing a thicker glass the safety factor of glass raise, but at the same time the safety factor for the tubes goes down and will be the weakest safety factor. Therefore, a strengthening of one part can punish itself if it is too big.

There are no special restrictions of either using Copper or Aluminum in the collector. Here we only examined 0.5 mm Aluminum and 0.25 mm Copper, due to thermal demands.

The stresses in the glass are affected by the connection to the absorber. When using other connections the calculations have to be redone.



Area of absorber (m2)


Temperature (K)


arctan(x/y) (°)


Cavity width (perpendicular to tubes) (m)


distance (m)


Cavity length (along tubes) (m)


Elasticity modulus (Pa)


Height of cavity (m)


Pressure (Pa)


Diameter (outer) (m)


thickness (m)


Stress (Pa)


a Amplitude, Ambient


The limit where a rest of 0.2 % deformation will persist




Absorber plate




Tube in absorber


Cover glass


When T=500K




Ultimate strength


Mean value


The weakest part of the construction

min Minimum


[1] M. F. Ashby, and D. R.H. Jones, (1998), Engineering Materials 1, 2 edn, 0 7506 3081 7, Butterworth — Heinemann.

[2] B. Sundstrom, (1998), Handbok och formelsamling Hallfasthetslara (Hand book and formula collection in mechanics of materials), 2 edn, Institutionen for hallfasthetslara KTH, Sodertalje.

[3] N. Khorasani, Design Principles For Glass Used Structurally, (Lunds universitet), pp 79, ISSN 1103-4467, 2004

[4] S. Sunnersjo, (1992), FEM i praktiken (FEM in practice), 2 edn, 91-7548-541-9, Industrilitteratur AB, Uddevalla.

[5] J. A. Duffie, and W. A. Beckman, (1991), Solar engineering of thermal processes, First edn, 0-471-51056-4, John Wiley & Sons, Inc.

Introduction of a new Solar Air Collector for processes with high solar fraction

B. Eng. Kurt SchOle, (FH) Thorsten Siems,

(FH) Yan Schmitt

K. Schule & T. Siems GbR
WaidmattenstraBe 6
Germany, 79232 March-Buchheim
E-Mail: info@kollektorfabrik. de

Phone: +49 (7665) 9471521 Fax: +49 (7665) 9329796


There are several good reasons to cover the future energy demand as much as possible with renewables. In future, energy (oil, gas, wood etc.) will become more and more expensive. More than ever, it is important to have a safe supply of the energy for cooling / heating or for industrial processes. Therefore also more solar thermal collectors will be necessary in the future. Solar thermal systems need a solar collector that guarantees an intrinsically safe operation. Solar air collectors can guarantee that. Solar thermal air systems are particularly important for solar cooling, since many countries in the world do have electricity shortages in summer. The solar thermal industry is striving to cope with the fast-growing energy demand for heating. The sector has to deal with the rules of the open market. Their solutions have to be visible, sustainable, reliable and at the end, they have to be cost efficient. Kollektorfabrik has developed a new solar air collector, which meets these requirements.

Keywords: solar air collector, solar fraction, stagnation, process heat

Reduction of Losses for Different Collector Technologies 3.1 Improvement of Flat-Plate Collectors

The main challenge while improving flat-plate collectors is to reduce their heat losses without sacrificing too much of the optical properties. Possible measures are:

• double glazed collectors with anti-reflective (AR) glasses

• hermetically sealed collectors with inert gas fillings

• additional glass or foil covers with low emission coating

Подпись: Transparent covers Inert gas filling

The first two approaches were applied e. g. by the SCHUCO company.

Figure 1: Construction of the SCHUCO double glazed AR flat-plate collector
(Source of left picture: SCHUCO)

The aim o f the development was to create a flat-plate collector with high efficiency at temperatures up to 120°C for process heating and solar air-conditioning purposes. Since most of the heat lost in a flat plate collector is lost via the front side, it is necessary to reduce heat losses here. This is achieved by double-glazing.

Due to anti-reflective coating the optical efficiency of 80 % is nearly equal to a single glazed standard flat-plate collector. The space between the glass panes is filled with an inert gas to reduce the heat conductivity. This collector is already available on the market.

Fundamental Study of the Thermal Performance of a Solar Collector with Evacuated Tube for Solar Heating System

Xing Li12*, Zhifeng Wang1, Bowei Wang1

1 Institute of Electrical Engineering, Chinese Academy of Sciences Beijing 100080, P. R.CHINA 2 Himin Solar Energy Group Co.,Ltd, Dezhou, Shangdong 253090, P. R.CHINA Corresponding Author, [


In this paper, the thermal performance of a solar collector with evacuated tube is investigated. Main application for this collector is water heating for domestic or large building, several parameters may affect the performance of the collector, such as the water flow-rate, the different irradiance, the variable inlet water temperature, the flow configuration (series or parallel connection of tubes) etc. These parameters may all be considered in the effort to optimize the overall design. In this study efficiency curves are presented from experimental measurements. And the fit equation of efficiency is provided. After a long period test, the actual efficiency curve in different months will be compared together. Also, the test results in a certain day are presented in order to analyze the comprehensive relationship among inlet temperature, ambient temperature and solar irradiance. Besides, the results of this procedure test can be very useful, firstly, for the local solar manufacturers’ equipment in order to design and optimize its products and secondly, by the engineers in order to select and design the most suitable system.

Keywords: All-glass evacuated tube collector, thermal performance, and solar heating system

1. Introduction

Solar energy is receiving much more attentions in building energy systems in recent years. Solar thermal utilization should be based on the integration of solar collectors into buildings. In most cases, the main utilization is provided with solar water heating system.

In 2005, most of solar energy systems manufacturing in different countries reported a good growth. In all these countries, the export sector is gaining importance. The expected expanding market for solar water heating system demands a standardized test method. In order to gain the optimistic benefits between the investment of heating system and the final results through operating the system, it is also necessary to know clear about the truly thermal performance of the collector, and further process in prediction of thermal performance in different install locations and long-term operation. Moreover, the knowledge of the thermal performance of solar collectors are essential for the prediction of the energy output of any solar thermal system, especially the overall evaluation and design for a water heating system.

Test methods for the thermal performance characterization of solar collectors are different with each other, mainly difference concluding the duration time of test period, the definition of test condition, and mean temperature of the working fluid. Most well-known test methods, such as ISO9806-1 and EN12975-2, are under steady-state conditions to test the thermal performance of the collector. These

steady-state test methods require the strict testing conditions very much in variable climates. And then the actually lasting test period should be longer than the regulated time that the climate condition is ideal to measure. However, because of the extensively regional and application of the international standards, ISO9806 is promulgated to measure the performance of collectors in in-depth study. At the same time it is clear to identify the definition of solar collectors according to ISO 9806.

There are many kinds of solar collectors, in this paper all-glass evacuated tube collector will be presented and experimented. It is a collector composed of an array of dual-glass evacuated tubes using water as the working fluid. The tubes are either all collected in parallel, or divided into two groups connected in series, within each of which the tubes are collected in parallel. Water is fed into the glass tubes through a brass tube flowing towards the closed end where it is reversed backwards then flows through the tube and is collected in the header from where it may be directed into its final use.

The all-side thermal performance of collector will be analysed to supply ample data to the design. So in order to make a comparison, at identical working conditions, between different products within a reference test condition, in this paper the experiment will be operated under steady conditions according to ISO 9806. Although the performance of solar collector can be approximately predicted based on steady test method, it is also affected by the inlet water temperature, water flow rate, and ambient conditions. Consequently, it is necessary to establish a test that can consider the whole conditions and it can be used to by engineers or manufacturers in order to design solar water heating system and optimize them.