Effect of Glazing Thermal Conductivity on Cabin Soak Temperature 2012-01-1207

Previous papers by the present authors described use of computational fluid dynamics (CFD) to quantify the effect of glazing thermal conductivity on steady-state heating, ventilation and air-conditioning (HVAC) load under wide-ranging climate and state of motion scenarios, and to estimate the significance of this effect for electric battery performance. The CFD simulations yielded the total heat transfer between the ambient and the cabin of a model car, including radiative and convective heat transfer. The five-fold lower inherent thermal conductivity of polycarbonate relative to glass was found to reduce steady-state HVAC load by several percent in all scenarios, leading to reduced greenhouse gas emission or increased electric range, according to the type of vehicle.

This paper complements the previous study by quantifying through simulation the effect of glazing thermal conductivity on cabin soak temperature, the latter reflecting a balance between radiative and convective heat transfer in a closed, unventilated, parked car with HVAC off, in a hot, sunny environment. Soak temperature has been a focus in the regulatory arena as an initial condition in proposed tests of air-conditioning contribution to tailpipe emissions. Two glazing configurations are simulated: a baseline configuration with glass at all locations, and a reduced thermal conductivity configuration with polycarbonate glazing substituted at the backlite and rooflite. A difference in soak temperature of less than 1°C is found for the two glazing configurations under the conditions of relatively high solar radiation and ambient temperature in Phoenix, Arizona at mid-day in June. This difference is small compared to both the soak temperature relative to the ambient and the variation in soak temperature found for a range of optical and heat transfer parameters for non-glazing elements of the cabin. The insignificance of glazing thermal conductivity for soak temperature is reconciled with its significance for steady state HVAC load in terms of air movement at the inside and outside glazing surfaces.