Solar irradiation and coincident dry-bulb temperature selection for load calculations

Traditionally in the determination of heat loads in summer, internal solar heat gains are calculated using near-extreme solar irradiation data. The latter are typically from the ASHRAE Handbook-Fundamentals clear sky model in the fenestration chapter. Using these and other gains, the plant is then sized based on the synoptic weather data (dry-bulb, wetbulb, etc.) near-extreme values. These are based on the values in the ASHRAE Handbook-Fundamentals using 0.4%, 1.0%, and 2% exceeded values. Combining independent near-extreme values and modeled data can lead to more severe design conditions than are likely in reality. A method of determining these solar and synoptic near-extreme values from measured values is proposed based on examination of the cumulative distribution functions. Examination shows the method to be robust and sensitive to changes in weather parameters. Such a method is also found to provide more realistic design conditions.