MicroV Covering Materials for Solar Collectors with High Light Transmittance

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1 MicroV Covering Materials for Solar Collectors with High Light Transmittance P.J. Sonneveld 1, P. Kratz 2 and G.L.A.M. Swinkels 1 1 ) Wageningen UR, A&F, P.O. Box 17, 67 AA Wageningen, The Netherlands, tel , fax , piet.sonneveld@wur.nl 2 ) ZEN Solar, De Run 5421, 554 DG Veldhoven, The Netherlands, Introduction A maximal transmittance of the covering material for solar collectors is important for a good yield of the system. Furthermore it gives the possibility to apply a second sheet material to increase thermal insulation. The light transmittance is predicted and optimised with a ray tracing computer program, which have resulted in the optimal geometry of the material. The light transmission, thermal insulation, structural performance and yield aspects of solar collectors are subject of this research. The basic material of the sheets is glass. Methods Intensive research is carried out to develop the special geometry of the sheet material with maximal light transmission. When a beam of light is incident to a flat transparent sheet, part of this light will be transmitted, according to Fresnell law, and another part will be reflected. With ray-tracing software an increase in light transmission of structured materials was discovered. This surface structure of the material will result in a decrease of light reflections. This will result in extra light transmission over the complete solar spectrum with increasing angles of the microstructure. This investigations w When a beam of light is incident to a flat transparent sheet, part of this light will be transmitted, according to Fresnell law, and another part will be reflected (Fig. 1a above). At lower angles of incidence reflection increases. For a flat sheet the reflected part of the light does not enter the greenhouse and therefore is not available for plant growth. For a zigzag-surface the primary reflected part of the light hits again another part of the sheet surface and after transmission then partly enters the greenhouse after all (Fig. 1a below). This is especially effective at low angles of incidence. By this effect the transmittance for diffuse light of a zigzag-shaped single sheet increases about 5% compared to a flat single sheet of PC. A&F developed the idea and the optimal shape, thickness and grid of such a zigzag-sheet. Calculations and measurements of single and double sheets with different pigment additives and with and without coatings were performed. For the calculations of the light transmission a ray tracing computer program was developed. In Fig. 1b a sample of the zigzag-structure material is seen, which is manufactured with a vacuum form technique. In Figure 2 the calculated and measured data of light transmission are presented for a single PC-sheet and a double-wall sheet with and without additives. The horizontal axis shows the inclination of the zigzag-shape, the vertical axis shows the diffuse light transmittance. The diagram shows that without the colour additives (pigments), normally applied for cosmetic reasons only, a considerable improvement of the light transmittance is realised. Without pigments the light transmittance of these sheets increases with 4% (single layer) to 6 % (double layer) as can be seen in Figure 2. It can also be seen in Figure 2 that the local optimum for the inclination of the zigzagshape is When regarding other criteria like insulation value, material consumed and material strength (see below) an inclination of 48 is ideal. The transmittance for

2 diffuse light of a double zigzag-sheet with an inclination of 48 without colour pigments is 78.8%. The measurements were carried out on the integrating sphere of IMAG with a sample size of 5 x 5 cm. Fig. 1a:The principle of the transmittance and reflection of light beams hitting a flat sheet (above) and a zigzag-sheet (below). Fig. 1b:Example of a double layer zigzag-structure manufactured with vacuum form techniques. The final material can be made of glass, polymer sheet material or film but due to the high stagnation temperatures of solar collector in this case glass material is chosen. Fig.2 Overview of the light transmission of a 1mm thick Zigzag-designs as a function of the zigzag angle (zero is planar material) for different absorption coefficients of the materials

3 The efficiency of a solar collector is calculated as a function of the radiation normalized temperature difference for single and double sheet material and compared with single and double glass covering. With Dutch climate data the yearly yield is calculated with a simulation program. FLAT SHEET MICRO-V SHEET Fig. 1a:The principle of the transmittance and reflection of light beams hitting a flat zigzag-structure manufactured with sheet (above) and a zigzag-sheet (below). With a collector made with the new microv material the yield be tested in practice. ZEN SOLAR develops this collector in co-operation with A&F Innovations. Fig.2 Overview of the light transmission of a 1mm thick Micro-V-designs as a function of the structure angle (zero is planar material) for different absorption coefficients of the materials

4 Results The increase of light transmittance as a function of the angle of the microv structure is depicted in Fig. 1. Compared with the glass covering the calculated efficiency will by increased with 5 % compared with single glass covering and 9 % compared with double glass covering. For the double sheet material the efficiency can be doubled compared with single glass covering at high radiation normalized temperature differences. Het collector rendement is allereerst afhankelijk van het product (invalshoekafhankelijke) transmissie van de collector afdekking en de (invalshoekafhankelijke) absorptiecoëfficiënt (, ), a(, ) van de absorber. Dit rendement verminder met de warmteverliezen die afhankelijk zijn van de warmtedoorganscoëfficiënt U voor enkellaags afdekking is deze waarde 6 W/m2K en voor dubbellaagsafdekking bij benadering 3 W/m2K. Het collector rendement is dan te bepalen met: (, ), a(, ) waarbij U T * U T G * 2 * T een stralingsgenormeerde temperatuursverschil is volgens * T T T met G de globale straling (in Nederland maximaal 8 W/m2). Met de in Tabel G 2-3 aangegeven waardes voor de lichttransmissie en een absorptie coëfficiënt van,96 voor de collector is het rendement van de collector te berekenen. Het resultaat is weergegeven in Fig. 2 en 3 voor respectievelijk loodrecht invallende straling en diffuse straling. Tabel 2- Eigenschappen van de materialen volgens EN 42 en NEN 2675 Nr. Materiaal Standaard Transmissie enkel (%) Transmissie dubbel (%) direct diffuus direct diffuus 1 Glas a. PAR vlg. NEN 2675 b. Solar vlg. EN 42 2 Micro-V structuur a. PAR vlg. NEN 2675 b. Solar vlg. EN Voor enkel microv afdek materiaal ontstaat 4-5 % rendementsverhoging t.o.v. enkelglas. Bij een dubbeldek ontstaat een rendementsverhoging van 8-1 %. Het resultaat is dan dat zowel bij grotere als bij kleinere temperatuursverschillen met dubbel microv afdekking een meeropbrengst ontstaat. Bij grotere temperatuursverschillen neemt het rendement en dus de opbrengst toe tot ca. het dubbele van enkel glasafdekking.

5 1 1 Rendement l oodr echt i nval l ende st r al i ng.8 1Gi 2Gi.6 1Vi 2Vi Ts i.25 Genor mal i seer d t emper at uur sver schi l Fig. 3 Het rendement van een zonnecollector bij loodrecht opvallende straling voor verschillende afdekkingmaterialen, + enkellaags glas, dubbellaags glas, X enkellaags microv en dubbellaags microv,

6 1 1 Rendement di f f use st r al i ng.8 1dGi 2dGi 1dVi 2dVi Ts i.25 Genor mal i seer d t emper at uur sver schi l Fig. 4 Rendement van een zonnecollector voor diffuus opvallende straling voor verschillende afdekkingmaterialen, + enkellaags glas, dubbellaags glas, X enkellaags microv en dubbellaags microv Conclusions A new covering material for solar collectors is developed with 5 % enhanced transmittance. A typical solar collector covered with the new material with give 5 % more yield. Extra yield increase can be obtained with a double covering. Keywords: transparent material, light transmission, thermal insulation