Solar Heat Load on the Vehicle Occupants

Vehicle occupants, unlike building occupants, are exposed to continuously varying, non-uniform solar heat load. Automotive manufacturers use photovoltaic cells based solar sensor to measure intensity and direction of the direct-beam solar radiation. Use of the time of the day and the position - latitude and longitude - of a vehicle is also common to calculate direction of the direct-beam solar radiation. Two angles - azimuth and elevation - are used to completely define the direction of solar radiation with respect to the vehicle coordinate system. Although the use of solar sensor is common in today’s vehicles, the solar heat load on the occupants, because of their exposure to the direct-beam solar radiation remains the area of in-car subjective evaluation and tuning. Since the solar rays travel in parallel paths, application of the ray tracing method to determine solar insolation of the vehicle occupants is possible. Calculating the solar exposure however requires the knowledge of geometry of the passenger compartment of a vehicle in addition to the direction of the direct-beam solar radiation. Geometry information includes 3D coordinates of the vehicle glasses and the passenger seating location. Planar surface approximation is used to represent both the glasses and the seats. Further the seat coordinates are shifted to obtain exposure of the occupants than the seats. Shifting of the seat coordinates is performed according to the thickness of chest and lap of an average adult. The solar exposure calculation also requires correction to the measured solar intensity. The correction is to account for attenuation of the solar radiation by the transmittance of the vehicle glasses. The solar heat load is then obtained by multiplying the occupant’s solar exposure area in m² and the transmitted solar intensity in W/m² for the given solar angles and vehicle geometry. The results of solar heat load so obtained are compared with the CFD Fluent data for a compact SUV.