Building energy simulations, especially those concerning pre-cooling strategies and cooling/heating peak demand management, require careful analysis and detailed understanding of building characteristics. Accurate modeling of the building's thermal response and material properties for thermally massive walls or advanced materials like phase change materials (PCMs) are critically important. However, one input variable that many times is not correctly assessed is time-step dependency. This study is part of an ongoing research effort that has led to the verification and validation of the EnergyPlus PCM and Conduction Finite Difference (CondFD) algorithms in EnergyPlus as well as the simplification of enthalpy linearization for PCMs. This paper demonstrates the importance of using the correct time step depending on the simulation objectives and technology analysed. Using field data of different pre-cooling strategies, this analysis shows and explains the time-step dependency shown in some cases. It also suggests appropriate time-step values depending on the application and HVAC controls.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings XII, December 2013