Developing analytical model for nighttime cooling of internal thermal mass

Abstract

Nighttime mechanical ventilation of internal building thermal mass has the potential to create energy flexibility by shifting peak cooling demand. The research on nighttime cooling of internal thermal mass is limited due to lacking simplified models by considering mass dimension and real-world discrete climate data like hourly outdoor air temperatures. This study develops an analytical model for nighttime cooling of internal thermal mass with a constant air change rate and hourly varied air temperatures. The analytical model is verified by both numerical, conduction transfer function methods, and ANSI/ASHRAE Standard 140-Case 600. The analytical model is applied to quantify the free cooling energy storage in 48 selected U.S. cities in different climate zones and in the 16 climate zones of California. Among the 48 cities, the maximum free cooling energy storage is reported in Santa Fe, NM with a total free cooling energy storage of 19.1 kWh m-2 a-1 and a net free cooling energy storage of 3.88 kWh m-2 a-1. Coastal regions in California are not suitable for nighttime ventilation of internal thermal mass. The maximum total free cooling energy storage in California achieves 27.5 kWh m-2 a-1, while the maximum net free cooling energy storage is 6.11 kWh m-2 a-1. The analytical model has a potential to be integrated into whole building energy simulation software to improve the calculation of the effect of internal thermal mass.