Joakim Karlsson

Modelling the angular behaviour of the total solar energy transmittance of windows

The angle dependence of the total solar energy transmittance, the g-value, is addressed. Factors that influence the angular behaviour and possible ways to simulate it are discussed. A simple and accurate model to simulate the angle dependence is presented. Besides the data for normal incidence, the model employs two inputs: the number of panes in the window and the type of coating used. Detailed information about the coating composition is not required. The model gives good results and could easily be incorporated in building simulation programs or window selection tools where the angular dependence of the g-value for windows needs to be known.

Evaluation of predictive models for the angle-dependent total solar energy transmittance of glazing materials

The predictions of angle-dependent optical properties of glazings are discussed. A categorisation of windows depending on the type of coating on the glazing is discussed as a way of improving the accuracy in the predictive models. Four approximate ways to predict the angle dependence of the total solar energy transmittance are compared. The impact on the energy performance of windows with different angle dependence is assessed in a heating and a cooling dominated climate, respectively. Results imply that by simply using the clear glass angular profile for all types of windows gives quite low errors in the angle dependence prediction, lower than some other previously proposed models. By using a model with window category as input, the errors in angle dependence prediction can be further reduced. The impact on the energy performance from incorrect angle dependence is considerable in some cases but not necessarily critical.

Angle-resolved optical characterisation of an electrochromic device

An electrochromic prototype with WO3 and NiO as electrochromic layers was analysed in an absolute spectrophotometer. The electrochromic glazing was measured in combination with a clear float glass and a low-e glass in order to simulate a ‘real’ window. Similar measurements were performed on a commercial electrochromic product, i.e., a Gentex Night Vision Safety™ (NVS®) mirror from Gentex Corporation, and the results were compared. The spectral transmittance was measured, in bleached and coloured state, over the solar wavelength range at the angles of incidence, φ=0, 40, 60 and 70°. The direct solar transmittance, Tsol, the visual transmittance, Tvis, and the angular dependence for these parameters were calculated.

Angular-dependent optical properties of coated glazings: validation of two predictive algorithms

The angular dependent optical properties of low-e and solar control glazings have been investigated in a European project, ADOPT, within the Standards Measurements and Testing programme. Two new predictive algorithms have been proposed and tested. One algorithm is based on an empirical formula and the other is based on a hybrid equivalent model, which approximates the actual coated glass. The validation procedure has included measurements as well as Fresnel calculations. The algorithms allow the user to calculate the solar heat gain coefficient versus angle of incidence for any transparent window combination without having to perform time-consuming measurements. For the tested window configurations the precision is mostly within one percentage point as compared to measured results.