Study On The Selection Method Of Weather Parameter For Air-conditioning System Design

Air-conditioning design weather conditions, including dry-bulb temperature, wet-bulb temperature and solar irradiation, are essential and fundamental data for the design of building air-conditioning systems. They simultaneously act on a building, and are driving potentials of heat transfer through building envelope and direct mass exchange by infiltration and ventilation. Coincident design weather conditions are required in determining the peak cooling load for sizing air-conditioning systems. Improper design weather conditions may lead to oversized or undersized HVAC system, which will result in unnecessary extra capital cost and low part-load efficiency or frequent failures in providing sufficient cooling. Even a small difference in the design temperature and solar irradiance may have significant economic implications. Hence, coincident solar irradiance, dry-bulb and wet-bulb temperatures should be properly selected.However, the selection of design solar irradiance in the current ASHRAE and CIBSE design handbooks has nothing to do with 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. Moreover, the two building factors, a longer thermal lag of more than one hour for building envelops and a lag of indoor radiation to convert into cooling load, impact the instantaneous heat input and instantaneous cooling load, and the selection of weather parameters for air-conditioning system design. Hence, both factors should be taken into account in the selection of weather parameters for air-conditioning system design.Based on the principle of heat transfer, by adopting two concepts of conduction time series and radiant time series, the paper developed a new statistic method for the rational selection of coincident solar irradiance, dry-bulb and wet-bulb temperatures. The method, which combines the three weather parameters and building characteristics into a single impact factor called as equivalent temperature, 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. 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.