Comparative Analysis of CIBSE Admittance and ASHRAE Radiant Time Series Cooling Load Models

Abstract

Due to the impacts of carbon emissions on climate change and the expected dramatic increase in global cooling demand by 2050, it is of a paramount importance that the required energy to cool buildings is accurately predicted. This ensures that equipment is appropriately sized, which ultimately reduces energy consumption and global carbon emissions. CIBSE and ASHRAE standards are both widely adopted for cooling load predictions, but they adopt different calculation methods, with CIBSE adopting admittance and ASHRAE adopting radiant time series (RTS), which produce significantly different results in cooling load. This study comparatively and qualitatively evaluates the CIBSE admittance and ASHRAE RTS cooling load models by analysing their structures and key input parameters for a mock-up building to identify inconsistencies between the two methods. There were flaws within both models that resulted in the CIBSE method underpredicting the cooling load, whereas the ASHRAE method typically overpredicting it. This resulted in a maximal average difference of over 60%. The substantial predicted cooling load difference was mainly caused by the ASHRAE RTS model, which was highly receptive to solar gains, and it consequently led to overprediction in cooling load when compared to the CIBSE admittance model.