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.