• Developed method of separating optical and thermal property calculations
• Used MoWiTT to develop "heated layer" method of determining thermal properties
• MoWiTT measured system SHGC to check method
  

The transmission of solar energy through a complicated system such as a window with a venetian blind and/or other shading device is a well-defined optical problem. Although it requires a complex calculation, there are a number of methods of accomplishing it by computer. The key unknown is the fate of energy that is optically absorbed in the window or shading system, a fraction of which is thermally transported into the space. There is presently no reliable way of calculating this fraction. However, if one can measure it, the same measurement can be applied to fenestration systems with a wide range of optical properties, so long as their geometry and thermal properties are the same. This is a considerable simplification.

In the figures, the MoWiTT calorimeter is used to determine the fraction of the energy absorbed in a venetian blind that ultimately flows into the adjacent space. The absorption is simulated by heating the blind slats with an electric current. Prior to application of the current the shape of the energy flow is fit with a theoretical function containing adjustable constants, and the values of the constants (which relate to the optical and thermal properties of the fenestration system) are determined. When the electric current is switched on to dissipate a known amount of power in the blind, the measured curve is displaced upward by the amount of this power that flows into the calorimeter chamber. The ratio of this displacement power to the applied power gives the "inward-flowing fraction" of the absorbed energy for the blind.