Long-term measurements of the solar transmittance of the two glasses were carried out in the outdoor test facility mentioned in section 2. Measurements were carried out in the test period April 30, 2008 — August 8, 2008. Due to problems with the measuring equipment measurements were not carried out in the period May 23 — June 2, 2008. Measured daily values for the total and diffuse radiation on the glasses are shown in figure 5. The daily rain amounts are shown as well. Figures 6 and 7 show daily transmittances for the normal glass and for the glass with the antireflection treated surfaces. The daily transmittances are the ratios between the daily radiation transmitted through the glass and the daily radiation on the glass surface.

The measured daily transmittance for the normal glass is varied between 0.82 and 0.88, and the measured daily transmittance for the antireflection treated glass is varied between 0.89 and 0.93. For the whole test period the transmittances are 0.84 for the normal glass and 0.91 for the glass

with the antireflection treated surfaces. The transmittance is for the whole test period increased by 8%, corresponding to 7 % points, by the antireflection treatment. The daily transmittance for both glasses is relatively low in sunny days without rain. This might be caused by dirt attached to the glass surfaces. The daily transmittance for both glasses is relatively high in rainy days and in days after rainy days. This might be caused by the fact that the glasses are washed clean during rainy periods.

The measured transmittances are compared to calculated transmittances based on the measurements for the clean glasses, that are the indoor measurements of the transmittances for direct radiation and the outdoor measurements of the transmittances for diffuse radiation.

The calculated hemispherical-hemispherical transmittances at a specific time are found by: Thh = ((Ey 0.85 )+((E-Ed) 0.904- (1 — tan43 jj-^j ))/E for the normal glass

Thh = ((Ey 0.93 )+((E-Ed) 0.960- (1 — tan5’1 j 1 ))/E for the glass with antireflection surfaces where E is the total irradiance on the glass, W/m2

Ed is the diffuse irradiance on the glass, W/m2 0 is the incidence angle, °

Measured and calculated daily transmittances for the two glasses are seen in figure 8. The measured daily transmittances are up to 4% lower than the calculated daily transmittances for both glasses. The difference between the measured and calculated daily transmittances is relatively large in sunny periods and relatively small in periods with and after rain. The reason is as mentioned, most likely that the glasses are washed clean during rain showers.

The ratio between the measured solar radiation transmitted through the glass and the calculated solar radiation transmitted through the glass for the whole test period is 0.98 for the normal glass and 0.97 for the glass with the antireflection treated surfaces. That is: The transmitted solar radiation is reduced by 2% for the normal glass and by 3% for the glass with the antireflection treated surfaces due to dirt and water on the glass surfaces.

Considering the measuring accuracy it is concluded that the antireflection treatment has no significant influence on how much dirt and water attached to the glass surfaces reduce the solar transmittance during a Danish summer period.

The weather might influence the conditions and therefore the measurements will be continued during the autumn 2008 and winter 2008-2009.

Fig. 8. Measured and calculated daily transmittances for the normal glass and for the glass with the

antireflection treated surfaces.

3. Conclusion

Long-term side-by-side measurements of the solar transmittance for a normal glass and a glass with antireflection treated surfaces show that the solar transmittance is increased by 8% by antireflection treatment in a Danish summer period.

The measured transmittances in the summer period is 2-3% lower than the calculated transmittances based on measurements for the clean glasses. The antireflection treatment has, in the Danish summer no significant influence, negative or positive on the transmittance reduction caused by dirt and water attached to the glass surfaces.

Nomenclature

Tgh is the directional-hemispherical transmittance at the incidence angle 0, —

rhh is the hemispherical-hemispherical transmittance, —

E is the total irradiance on the glass, W/m2 Ed is the diffuse irradiance on the glass, W/m2 0 is the incidence angle, °

References

[1] S. Furbo & L. J. Shah (2003). Thermal advantages for solar heating systems with a glass cover with antireflection surfaces. Solar Energy 74, pp 513-523.

[2] J. Birck Laustsen & K. Johnsen (2008). Udvikling af vsrktojer til at fremme energieffektiv anvendelse af solafsksrmninger. Slutrapport for ELFORSK-projekt 337-094. Department of Civil Engineering, Technical University of Denmark, report no. R-187.