P. J. Sonneveld and G. L.A. M. Swinkels

Wageningen UR, A&F, P. O. Box 17, 6700 AA Wageningen, The Netherlands, tel. +31.317 476 438, fax. +31.317 4 75 347, E-mail: piet. sonneveld@wur. nl

Introduction

Maximal light transmittance of the covering material is important for solar collectors maximising yield of the system. Furthermore a second sheet of covering material can be applied with low light loss to increase thermal insulation. Therefore research is aimed at improving light transmission. Ray tracing method has been applied to design the optimal geometry of the material. Light transmission, thermal insulation, structural performance and yield aspects of solar collectors are combined in this research with glass as basic covering material

Methods

Intensive research is aimed at developing the special geometry of the sheet material with maximal light transmission. With ray-tracing software light transmission of structured materials was optimised. These investigations were started to increase light transmittance of double insulated greenhouses, the approach is promising for solar collectors too. A beam of light incident to a flat transparent sheet will be partly transmitted, according to Fresnell law, and partly 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 system and therefore is lost for the solar collector. For a zigzag surface, the primary reflected light hits the other part of the sheet surface with favourable angle if incidence and after transmission will (partly) enter the system after all (Fig. 1a below). This is especially effective at low angles of incidence of the primary light. By this effect the transmittance for direct and diffuse light of a zigzag-shaped single sheet of PC increases with about 5% compared to a flat single sheet. 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 prototype sample of the zigzag-structure material is shown. In Figure 2 the calculated data of direct perpendicular light transmission is presented for a single PC — sheet.

In Figure 3 the calculated and measured data of diffuse light transmission are presented for a single PC-sheet and a double-wall sheet with and without additives. The measurements were carried out at A&F, applying an integrating sphere allowing a sample size of 50 x 50 cm.

The diagram shows a good agreement between calculations and measurements. A considerable improvement of the light transmittance can be realised. The diffuse light transmittance of the sheets increases with 4% (single layer) to 6 % (double layer).

It can also be observed that the local optimum for the inclination of the zigzag-shape is 45- 60o. When regarding other criteria like insulation value, material consumed and material strength an inclination of 48o is ideal. The transmittance for diffuse light of a double zigzag sheet with an inclination of 48o is 78.8%.

Fig. 1a:The principle of the transmittance and reflection of light beams hitting a flat sheet (above) and a zigzag-sheet (below).

The developed zigzag-sheet is manufactured of polycarbonate (lexan) by GE-Plastics and applied in new types of greenhouses (sonneveld et al. 2003). The principle can also be applied for solar application but a difficulty for this application is the rather high stagnation temperatures with sometimes occur in solar collectors. Therefore the typical covering material for solar collectors should be glass. However the ZigZag shape is difficult to be made of a glass sheet. Therefore an adaptation to very small surface V-structure (Fig.4) is studied.

0 10 23304)50 60 70 80 90

angle [deg

Fig.3 Overview of the diffuse light transmission of a 1mm thick Zigzag-designs as a function of the zigzag angle (zero is planar material) for different absorption coefficients o the materials

This surface V-structure of the material will result in a decrease of light reflections in the same way as the ZigZag structure. This will result in extra light transmission over the complete solar spectrum with increasing angles of the microstructure.

01

Results

Light transmittance is depicted in Fig. 5 as function of the angle of the Micro-V structure. It shows that zigzag angle near 45 degrees decreases transmission considerably and that the optimum angle is at about 50 degrees. This increase of about 5 % is comparable with other antireflection methods (Furbo 2003)

The collector efficiency is the most important factor. It can be calculated as a function of the radiation normalized temperature difference and the heat loss coefficient U.

Both transmission and absorption are dependent on the angle of incidence. The efficiency decreases due to heat losses, which depend on the heat transfer coefficient U of the cover for well-insulated collectors. For single layer covering U value is 3.5 W/m2K and for double covering approximately 2.3 W/m2K. The collector efficiency can be determined with:

V = Vo[r(<P, в),а(<р, в)]- U ■ T*

with р0[т(ф, в),а(ф,0) the reference efficiency dependent on the product of the transmission т of the collector covering and the absorption coefficient a of the absorber dependent on the angle of incidence ф and the tilt angle Ф of the north-south oriented solar collector, T* a radiation normalized temperature difference according to

and G the global radiation (in the Netherlands maximum at 800 W/m2 in this

case 400 W/m2 is chosen).

With the transmission values given in Table 1 and a typical absorption coefficient of 0,96 for the collector the efficiency of the collector can be calculated. The results are presented in Fig. 6 and 7 for respectively direct perpendicular radiation and diffuse radiation.

For single Micro-V covering material an increase of 4-5 % can be observed compared with single glass. For a double sheet covering this increase in efficiency is 8 %. The total result for double layer Micro-V is extra yield at lower and higher temperature differences. At higher temperature differences the efficiency can be doubles compared with a single glass covering.

Fig. 7 Efficiency of diffuse perpendicular radiation for different types of transparent

covering materials,…………….. single layer glass, …………….. double layer glass,————- single layer

Micro-V and ———— double layer Micro-V

With Dutch climate data (Reference Year van weather station De Bilt, The Netherlands) the yearly yield is calculated with a simulation program for Solar Boilers VABI for a ZEN Solar system with 2.75 m2 collector surface area and a boiler of 90 dm3. The tap water demand is 110 dm3 per day with an input water temperature of 15 oC and an end temperature of 65 oC. This corresponds with an energy load of 8391 MJ per year. The optical efficiency of the absorber is 0,867. In Table 2 the yield of the systems with different cover materials are summarized. Changing from single glass to Micro-V glass will result in 4 % extra yield. An extra yield of 9 % is possible with a double Micro-V covering.

Table 2 Yield with the different covering materials Transparent cover Single normal glass Single glass with Micro-V Double normal glass Double glass with Micro-V Transmission 0,905 0,955 0,824 0,90 Optical efficiency 0,784 0,828 0,714 0,780 Thermal loss factor 3,5 3,5 2,3 2,3 Yield per year [MJ] Yield compared with 3820 3975 3917 4164 reference system [%] 100,0 104,0 102,5 109,0

Conclusions

A new covering material for solar collectors is in development with 5 % enhanced transmission over the whole solar spectrum. For high temperature applications of solar collectors the efficiency can increase up to 50 % with a double covering due to the higher insulation value of the double sheet material with a good transmission. An yield increase of 4 % is seen by changing the standard glass by micro-V. With double Micro-V glass the extra yield is 9 % as a result of the higher insulation value.

Literature

Sonneveld, P. J., G. L.A. M. Swinkels and D. Waaijenberg, 2002, Greenhouse design for the future, which combines high insulation roof material with high light transmittance, Paper no. 02SE013, International Conference on Agricultural Engineering (AgEng), Budapest, Hungary, 30 June — 4 Juli 2002, pp. 8 Sonneveld, P. J, Adriaanse F., 2002, New Energy Saving Greenhouse Roof with a High Light Transmittance — Zigzag greenhouse glazing, Paper no. 02SE004, International Conference on Agricultural Engineering (AgEng), Budapest, Hungary, 30 June — 4 Juli

2002, pp. 102

Furbo S., Shah L. J., Thermal advantages for solar heating system with a glass cover with antireflection surfaces, Solar Energy, 75, pp. 513-523

Keywords: transparent material, light transmission, thermal insulation