## Conclusions and Further Works

The „Ray-Tracer” software uses the Ray-Tracing method to study some physical phenomena and is useful to study and simulate the mechanisms that take place in the solar radiation concentration process from the non-imaging photovoltaic concentrators. The paper presents results of simulated experiments regarding the concentration of the radiation by a paraboloidal photovoltaic concentrator installed on the roof of a house in the insolation conditions of a clear sky day.

 Fig.3 Fig.4
 Fig. 7 Fig.8

The geometric characteristics of the paraboloidal photovoltaic concentrator are: parameter p = 200 mm, input aperture radius R = 50 mm, height h = 62.5 mm, focal distance f = H0 = 10 mm, geometric concentration factor Cgeom = 6.25. The photovoltaic cell is placed in the focal plane of the paraboloid and has the radius r = 20 mm. The photovoltaic concentrators are placed on the roof oriented South, у = 0, and the inclination is equal with the latitude of the place 5 = 45 deg. The maximum value of the incidence angle on the input aperture to which the concentration produces is 6max = 30.06 deg. For the given case, the minimum value of the incidence angle is reached at noon in September, 6min = 1.507 deg. The theoretical factor of optical concentration is Coptic, teoretic = 3.98. For the given situation, the maximum optical concentration factor is Cmax = 3.57 in September at noon. The optical concentration factor, in the described experiments, is higher than 1 between 1030 o’clock and 14 o’clock. The maximum density of the solar radiant flow on the photovoltaic cell is reached in March, at noon, Brec = 2837 W/m2. The efficiency of the concentration varies between 12.31%(June) and 26.10% (September). A photovoltaic installation with the collecting area of 12.50 m2, with 5000 paraboloidal concentrators, provides monthly the electric energy quantity variyng from 71 kWh, in December, to 383 kWh, in March. The electric energy production satisfies the the need of a family and allows the monthly delivery, in spring, summer, autumn and winter of approximately 125 kWh to the national energetic system.

The following papers will refer to installations with paraboloidal concentrators at which the distance between the parabola’s peak and the photovoltaic cell is variable. The purpose of these papers is to determine the optimum position of the cell depending on the paraboloid’s peak so that the optical efficiency to be higher that 1 for a longer period of time.

Acknowledgments. This work was supported by the grant CEEX-247 References

[1] Swanson, R. M., Photovoltaic concentrators in Photovoltaic science and engineering edited by Luque, A. and Hegedus, S., Wiley, pp. 449 — 505, 2002.

[2] Bowden, S. B.E., A High efficiency photovoltaic roof tile, a thesis of University of New South Wales, April 1996.

[3] Brogren, M., Optical efficiency of low-concentratingsolar. Energy systems with parabolic reflectors, Uppsalo University, Sweden, 2004.

[4] Winston, R., Principles of Solar Concentrators, Solar Energy 16, pp. 89-95, 1974.

[5] McIntosh, K. R., Swanson, R. M., Cotter, J. E., A simple ray tracer to compute the optical concentration of photovoltaic modules, Progres In Photovoltaics: Research And Applications, 14, pp 167 — 177, 2006.

[6] Gray, J. L. , The physics of the solar cell concentrators in Photovoltaic science and engineering edited by Luque, A. and Hegedus, S., pp. 61 — 113, 2002.

[7] Glassner, A. , ed. An introduction to ray tracing, Academic Press, San Francisco, pp. 263—294, 2002.

[8] Emery, K., Measurement and characterization of solar cells and modules concentrators in Photovoltaic science and engineering edited by A Luque, A. and Hegedus, S., Wiley, pp. 701-753, 2002.

[9] Nilsson, J., Optical design and caracterization of solar concentrators for photovoltaics, Lund University, Licentiate Thesis, 2005.

[10] Danescu, Al., Bucurenciu, S., Petrescu, St., The utilisation of solar energy (in Romanian), Ed. Tehnica, 1982.

[11] Fara, L., Tulcan-Paulescu, E., Paulescu, M., Photovoltaic systems (in Romanian), Matrix, 2005.

[12] Paulescu, M., Schlett, Z., Practical aspects in the photovltaic conversion of the solar energy (in Romanian), Mirton, 2002.

[13] De Sabata, C., Luminosu, I., De Sabata, A., Palea, A., On the Design of a Solar, Partially Energetically Independent House in the Region of Banat, Bul. St. Univ. "Politehnica" din Timisoara, Transactions on Mechanics, 52(66),4, 2007, pp. 82-87, ISSN 1224-6077, .

[14] Luminosu, I., De Sabata, C., De Sabata, A., Theoretical and experimental researches over the posibility of realizing a solar house that is partially independent thermoenergetically (in Romanian), Buletinul AGIR, fondat 1918, ISSN 1224 — 7928, XII, nr. 3, iulie — septembrie, 2007, pp.31 — 44.

[15] Paulescu, M. , Algorithms for estimating the value of the solar energy (in Romanian), Matrixrom, Bucuresti, 2005.

[16] Luminosu, I., Zaharie, I., Costache, M., Damian, I., Optical concentrators in photovoltaics installations, Conferinta Nationala ”Instalatiile pentru Constructii §i Confortul Ambiental”,183 — 190, Timisoara, martie 2007.

[17] Luminosu, I., Zaharie, I., Costache, M., Damian, I., Ray-tracing — an analysis method of optical concentrators, Conferinta Nationala ’Instalatiile pentru Constructii §i Confortul Ambiental”, pp191 — 199 , Timisoara, martie 2007.

[18] Luminosu I., Nagy M., Mirrors and thermo-solar devices with radiation concentrators

produced at the Polytechnics University in Timisoara, Bulletins for Applied and Computer Mathematics, (PAMM), Budapest University of Technology and Economics, BAM Nr. 2173, pp. 135 — 146, 2004.

[19] **** Tehnical Report of CEEX-247, dec. 2007

## Effect of thermotropic layers on collector efficiency

In Fig. 1 the efficiency factor is plotted as a function of the absorber temperature for a collector without and with thermotropic overheating protection at a solar irradiation of 1200 W/m2 and ambient air temperatures of 0 and 30°C. The thermotropic layer exhibits a solar transmittance of 0.90 in clear state and of 0.10 in opaque state. The collector without a thermotropic layer reaches maximum

absorber temperatures of ~160°C and ~175°C at ambient temperatures of 0 and 30°C, respectively. It is observable that the stagnation temperatures can be controlled by the use of thermotropic layers. To achieve maximum absorber temperatures of 90°C, switching temperatures of the thermotropic film between 55 and 60°C are required. A slight effect of the ambient air temperature on the efficient working temperature range is discernible. At elevated ambient air temperatures of 30°C the efficiency drop is shifted to lower temperatures. However, the efficient working temperature exceeds 60°C even at an ambient temperature of 30°C. Thus the overheating protected collector is appropriate for domestic hot water and space heating applications.

 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 absorber temperature [°C]

Fig. 1. Collector efficiency versus absorber temperature of a solar collector with twin-wall sheet glazing and
black absorber (a=0.95, є=0.90) at a solar irradiation of 1200W/m2 and ambient air temperatures of 0°C and
30°C; solid lines: collector without overheating protection (no TTL); dashed lines: collectors with thermotropic

glazing (switching temperature: 55-60°C).

In Fig. 2 the effect of the thermotropic layers switching range on maximum absorber temperatures at a solar irradiation of 1000W/m2 and an ambient air temperature of 20°C is shown. The residual solar transmittance of the thermotropic layer in opaque state is varied between 0.20 and 0.60. The transmission of the layer in clear state remains constant at 0.85. Compared to layers exhibiting a solar transmittance of 0.90 in the clear state (Fig. 1), the collector efficiency is shifted to slightly lower values. As to layer design, this indicates that the solar transmittance should exceed 0.85 in the clear state. The collector without thermotropic overheating protection reaches absorber temperatures of about 160°C. It is observable that by applying thermotropic layers exhibiting a residual transmittance of 0.30 to 0.35 the absorber temperatures do not exceed 90°C. Layers exhibiting transmittance values above 0.35 lead to a successive increase of maximum absorber temperatures up to 130°C for a solar transmittance of 0.60 in the scattering state.

 20 30 40 50 60 70 80 90 100 110 120 130 140 150 absorber temperature [°C]

Fig. 2. Collector efficiency versus absorber temperature of solar collectors with twin-wall sheet glazing and black absorber (a=0.95, є=0.90), at a solar irradiation of 1000W/m2 and an ambient air temperature of 20°C; variation of switching performance of the thermotropic layers (TTL) (solar transmittance: 0.85 in clear state and 0.20 to

0.60 in opaque state).

## 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.”

References

[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.

[2] M. J. Brites, C. Nunes, S. Vieira, D. Quintino, Lopes Prates, J. Alexandre, V. Teixeira, M. J. Carvalho, Proceedings of CIES 2006-XIII Iberic Congress, and VIII Iber American Congress of Solar Energy, Lisboa, Portugal, 9-10 November 2006.

[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.

[4] C. Nunes, V. T eixeira, M. L.Prates, N. P.Barradas, and A. D. Sequeira, Thin Solid Films 442, 173 (2003).

[5] M. J.Brites, C. Santos, B. Gigante, S. Nascimento, H. Luftmann, A. Fedoro v, and M. Berberan-Santos, New J. Chem. 30, 969 (2006).

[6] S. Nascimento, M. J.Brites, C. Santos, B. Gigante, A. Fedoro v, and M. Berberan-Santos, J. Fluoresc. 16,

245 (2006).

[7] K. Hara, Y. T achibana, Y. Ohga, A. Shinpo, S. Suga, K. Sayama, H. Sugihara, and H. Arakawa, Sol. Energy Mater. and Sol. Cells 77, 89 (2003).

[8] M. K.Gunde, Z. C.Orel, and M. G.Hutchins, Sol. Energy Mater Sol. Cells 80, 239 (2003).

[9] Z. C.Orel, Sol. Energy Mater. Sol. Cells 68, 131 (2000).

[10] Z. C.Orel, Sol. Energy Mater. and Sol. Cells 68, 337 (2001).

[11] K. Zakrzewska, M. Radecka, A. Kruk, W. Osuch, Solid State Ionics,8404 (2002).

[12] S. G.Springer, PE. Schmid, R. Sanjines, F. Levy, Surface and Coating Technology 151-152 (2002) 51-54.

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

[14] V. Vancoppenolle, P. Y.Juan, M. Wautelet, J. P.Douchot, M. Hecq, Surface and coating technology 116­119 (1999) 933-937.

[15] S. G.Springer, PE. Schmid, R. Sanjines, F. Levy, Surface and Coating Technology 151-152 (2002) 51-54.

[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.

[22] I. Safi, Surface and Coatings Technology, 127 (2000) 203-219.

[23] P. J. Kelly, C. F. Beevers, P. S. Henderson, R. D. Arnell, J. W. Bradley, H. Backer, Surface and Coating Technology, 174-175, 795-800, 2003.

[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

Abstract

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

 Standard Prototype 1 (twisted tape) Prototype 2 (chain) П0 0.753 0.782 0.773 a1 3.184 3.052 3.363 a2 0.0138 0.0176 0.0073

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 П0 0.782 0.785 0.782 a1 3.052 3.45 3.16 a2 0.0176 0.012 0.013

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

 160 kg/h 320 kg/h 500 kg/h П0 0.773 0.779 0.78 a1 3.363 3.085 3.35 a2 0.0073 0.0219 0.014

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.

References

[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)).

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

D. 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

Abstract

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.

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.

Nomenclature

 A Area of absorber (m2) T Temperature (K) Angle arctan(x/y) (°) x Cavity width (perpendicular to tubes) (m) d distance (m) y Cavity length (along tubes) (m) E Elasticity modulus (Pa) z Height of cavity (m) P Pressure (Pa) 0 Diameter (outer) (m) t thickness (m) о Stress (Pa) Subscripts a Amplitude, Ambient p0.2 The limit where a rest of 0.2 % deformation will persist allow Allowable p Absorber plate e Endurance t Tube in absorber g Cover glass T500 When T=500K max Maximum UTS Ultimate strength mean Mean value weakest The weakest part of the construction

min Minimum

References

[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

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

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

Abstract

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