Rational selection of near-extreme coincident weather data with solar irradiation for risk-based air-conditioning design

Abstract

The selection of design solar irradiance in the current ASHRAE and CIBSE design handbooks is independent on design dry-bulb and wet-bulb temperatures. The probability that the load would not exceed the system capacity determined on this basis may not match the reliability level that the design weather data were meant to safeguard. Hence, a statistic method was developed for the rational selection of coincident solar irradiance, dry-bulb and wet-bulb temperatures. The new method combines, based on the principle of heat transfer, the three weather parameters and building characteristics into a single impact factor, which may be called as equivalent temperature. This ensures that the joint probability of occurrence of the three coincident weather parameters can match the capacity reliability level of air-conditioning systems, which may be required by building owners. The method was applied to historic weather records of 25 years in Hong Kong to generate coincident design weather data. These new design weather data were compared with those design weather data produced by the traditional method. Results show that traditional design solar irradiance, dry-bulb and wet-bulb temperatures may be significantly overestimated in many conditions. Moreover, the new method allows HVAC engineers to determine the peak cooling load directly without the need for calculating 24 h cooling loads on one design day for every month of the year. This greatly simplifies the design cooling load calculation. Although the new design weather data are only for buildings whose thermal lag is less than 1 h, the method provides a basis for further generating design weather data for buildings with a large thermal lag.