Spectral problem for water flow glazings

Water flow glazings are characterized by a water chamber which transports the absorbed solar energy in the glazing. A multilayer water flow glazing comprises glass layers, Polyvinyl Butyral (PVB) layers, coatings, air gaps, and a water layer. Heat absorption from solar radiation depends on the spectral properties of each layer. Energy performance is governed by the optical and radiative properties of the layers. The refractive index and the extinction coefficient define the optical properties of a layer. The optical properties of the water layer are known from the scientific literature [1]. Transmittance and reflectance for every wavelength are measurements supplied by manufacturers and are readily available from the International Glazing Database (IGDB) as a function of wavelength at normal incidence. In this work, a net energy balance radiation model similar to Siegel [2] is used to solve the spectral problem. A matrix difference equation for a vector of dimension two with boundary conditions is stated to obtain the absorptances of each layer and each interface. The inputs of this set of equations are the transmissivity of each layer, and the transmittance and reflectances of each interface which are based on the refractive index and the extinction coefficient. When these values are not known, an inverse problem is solved to obtain the refractive index and the extinction coefficient of each layer for every wavelength from transmittance and reflectance measurements of the IGDB. For coatings, another inverse problem is formulated to obtain the properties of the coated interface using also transmittance and reflectance measurements of the coated glass pane and the substrate. The outputs of this work are the wavelength-averaged absorptances of each layer, which determine the fractions of the impinging radiation which is absorbed in each layer. The results constitute the inputs of the thermal problem of a water flow glazing.