The simulations described above were carried out for a mesh of cylinders with infinite conductivity. Real low-e coatings normally use layers of silver or gold to establish the low emissivity in the IR spectral range. These metals have a high but not an infinite conductivity. Nevertheless, to get a first impression of the properties of meshes made out of real metals, it is possible to transfer the results of the numerical simulations to conventional low-e coatings.

A layer of metal with infinite conductivity would be opaque regardless of the thickness of the layer. The transmittance of a mesh of cylinders made of the metal calculated in the numerical simulations derives from radiation passing through the gaps between the metal bridges of the mesh. With the increase in transmittance due to this radiation it is possible to calculate the increase in transmittance due to micro-structuring of a real low-e coating. This means that the transmittance Treal of a mesh made out of a real low-e coating on a glass substrate will be between the transmittance Tcoat of the conventional low-e coated pane and the transmittance Tfloat of an uncoated pane of float glass, depending on the transmittance TmeShof the infinite conductive mesh:

Teal(2) = Tco„(2) + Tmesh(2) ■ (^,(2) — ^(2))

The same applies to the low-e properties: as there is no additional absorption, the emissivity Sreai of the mesh made of a conventional low-e coating will be between the emissivity £,coat of the conventional coated pane and the emissivity £float of an uncoated pane of float glass, depending on the transmittance Tmeshof the infinite conductive mesh:

^real ^"coat (2) + ^"coat (2))

The solar transmittance and low-e properties are then calculated by averaging these spectral values over the solar spectrum and the spectrum of heat radiation respectively. Through micro-structuring, the solar transmittance of the Pilkington Optitherm SN coating seen in Figure 2 could be increased by nearly 10 percent points from 0.62 to 0.72. The emissivity would increase from 0.048 to 0.060. The transmittance of this micro-structured coating is shown in Figure 9.

Conclusion

Numerical simulations using the FDTD method have shown that the solar transmittance of low-e coatings can be considerably increased. The penalty is a slightly increased IR emissivity.

Micro-structured low-e coatings are currently being processed. Measurements will follow accordingly.

Literature

[1] R. E. Bird, R. L. Hustrom, L. J. Lewis, “Terrestrial Solar Spectral Data Sets”. Solar Energy Vol. 30 (6), pp. 563-573 (1983)

[2] “International Glazing Database”, National Fenestration Rating Council (2003) http://www. nfrc. org

[3] K. S. Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media”. IEEE Transactions on Antennas and Propagation Vol. 14(3), pp. 302-307 (1966)

[4] J. C. Maxwell, “ATreatise on Electricity and Magnetism” (1873)

[5] D. M. Sullivan, “Electromagnetic simulation using the FDTD method”. IEEE Press, Piscataway (2000)

[6] Advanced Systems Analysis Program (ASAP), Breault Research Organization, Tucson, Arizona

http://www. breault. com/html/soft_asap. html