Comparative study of using periodic daily and long-term weather data for cooling system sizing and impact of thermal mass

Abstract

The energy efficiency of buildings is increasing due to energy performance requirements, and the basis for reaching high energy performance is a well-designed and insulated building envelope. Therefore, office buildings' cooling needs depend primarily on solar and internal heat gains, whereas outdoor temperature has a significantly smaller effect. Furthermore, the highest cooling capacities may occur in spring or autumn when the solar angles are smaller. For that reason, the cooling systems of office buildings are required to be sized based on dynamic building performance simulations. Most of such designs in Northern Europe are performed using IDA ICE simulation software, which uses the ASHRAE Fundamentals heat balance method by default. The design calculations are carried out using a periodic steady-state method which consists of repetitive simulations of selected hot days until the building is not heated up from day to day using the final designed cooling capacity. The process of heating the space by thermal loads in buildings with high thermal mass and well-controlled solar heat gains takes a longer time than in traditional buildings. Thus, the effect of building thermal mass on reducing the design cooling loads might be underestimated. In this paper, we analyze to what extent the ASHRAE Fundamentals method underestimates the effect of the building thermal mass. For this purpose, the cooling system sizing with a focus on a zonal level according to the ASHRAE handbook is compared to the system sizing results of a 30-year simulation using IDA ICE simulation software. A hypothetical office building with four offices toward North, West, South, and East is developed and used for the simulations. The building body comprises four alternatives A to D, which can also be called: very light, light, heavy, and very heavy. The study showed that the current method of cooling design did not significantly underestimate the thermal mass effect in buildings with heavy construction. The thermal mass impact was at its maximum in the southern office, resulting in 5 W/m2 or approximately 20% difference between structures A and D' s cooling capacities using both simulation methods. The difference between results from simulation methods is negligible. However, the simulations for more accurate cooling system sizing with criteria related to the operative temperature need to be done using specific weather files developed for simulations in longer periods.