Reduced Steady State Heating and Air Conditioning Loads via Reduced Glazing Thermal Conductivity 2011-01-0126

Heat transfer between the ambient and the air in a vehicle cabin determines the nominal steady state load on the vehicle's heating, ventilation and air conditioning (HVAC) system, a significant factor for vehicle efficiency and greenhouse gas emissions. This paper highlights the effect of glazing (i.e. window) thermal conductivity on steady state heat transfer, with high and low thermal conductivities represented respectively by monolithic glass and standard polycarbonate. Computational fluid dynamics simulations are summarized for a model car cabin including HVAC vents, interior seating, and a rooflite. Passenger and moisture effects are not included. Monthly temperature and radiation data for Phoenix, Arizona and Minneapolis, Minnesota are used to define hot and cold climate scenarios. The simulations yield the heat transfer and its sensitivity to glazing thermal conductivity and ambient conditions, for both stationary and moving cars, when cabin air is maintained at a comfortable temperature. The results show that the inherently lower thermal conductivity of polycarbonate relative to glass, with common optical transmission parameters, affords reduced steady state heat transfer, and thus reduced nominal HVAC load, in all of the climate and state of motion scenarios. Summarized for selected scenarios and glazing locations are breakdowns of the heat transfer into radiation and convection components, and the average inside glazing surface temperature. The latter is found to be closer to the steady state cabin air temperature for polycarbonate than for glass, in both hot and cold climates.