The influence of thermal mass on the predicted climate cooling potential in low energy buildings

Even in Northern European climates, overheating in many Nearly Zero Energy Buildings is a barrier to year round occupant satisfaction with the indoor thermal environment. Improved energy performance and enhanced thermal comfort should not be perceived as a rigid dichotomy of concepts. However, an acceptable thermal environment, during extended cooling periods now present in NZEB’s, can come at a high energy cost if mechanical cooling is used. Passive cooling through the integration of natural ventilation principles with the building morphology and materials has long been championed as a viable alternative to mechanical cooling. This paper presents findings from an investigation into the effect incorporating energy storage can have on the assessment of climate cooling potential for low energy buildings. A method to include the energy accumulation in the steady state energy balance equation for predicting the balance point temperature used to assess the ventilative cooling state is proposed. The energy accumulation in the thermal mass is restricted to an amount that raises the temperature of the material by less than 1K thereby supporting an assumption that the approach is steady state. To achieve this the thermal mass is estimated based on the dimensions of the structure but the depth is assumed as sufficiently large to facilitate a low utilisation of the available capacitance in the structure thus resulting in a temperature change of less than 1K.   The approach is applied to a case study in Cork Ireland. Results show that even a modest energy accumulation significantly influences the prediction of ventilative cooling hours in winter months. Required cooling airflow rates are also modified significantly. Simplified approaches to incorporating thermal mass in early stage assessment of climate cooling potential should be considered further and can influence whether or not a passive strategy may be adopted for a building design.