Cooling of the Building Structure by Night-time Ventilation

In modern, extensively glazed office buildings, due to high solar and internal loads and increased comfort expectations, air conditioning is increasingly applied even in moderate and cold climates, like in Central and Northern Europe. Particularly in these cases, night-time ventilation is often seen as a promising passive cooling concept. Many successful examples of passively cooled buildings demonstrate the possibility of providing good thermal comfort conditions without the need for energy-intensive air conditioning systems. However, due to uncertainties in the prediction of thermal comfort, architects and engineers are still hesitant to apply passive cooling techniques.

The basic concept of night-time ventilation involves cooling the building structure overnight in order to provide a heat sink during the occupancy period. As this requires a sufficiently high temperature difference between the ambient air and the building structure, the efficiency of night cooling is highly sensitive to climatic conditions and hence also to climate warming. In the first part of this PhD study, the potential for passive cooling of buildings by night-time ventilation was evaluated by analysing climatic data, without considering any building-specific parameters. A method for quantifying the climatic cooling potential (CCP) was developed based on degree-hours of the difference between building and external air temperature. Applying this method to climatic data of 259 stations shows very high night cooling potential over the whole of Northern Europe and still significant potential in Central, Eastern and even some regions of Southern Europe. However, due to the inherent stochastic properties of weather patterns, series of warmer nights can occur at some locations, where passive cooling by night-time ventilation alone might not be sufficient to guarantee thermal comfort.

Possible time-dependent changes in CCP were assessed for the period 1990-2100, with particular emphasis on the Intergovernmental Panel on Climate Change (IPCC) "A2" and "B2" scenarios for future emissions of greenhouse gases and aerosols. The study was based on 30 Regional Climate Model (RCM) simulated datasets, as obtained from the European PRUDENCE project. Under both emissions scenarios and across all locations and seasons, CCP was found to decrease substantially by the end of the 21st century, so that night-time cooling will cease to be sufficient to assure thermal comfort in many Southern and Central European buildings. In Northern Europe, a significant passive cooling potential is likely to remain, at least for the next few decades.

Because heat gains and night ventilation periods typically do not coincide in time, heat storage is essential for effective night cooling, and thus a sufficient amount of thermal mass is needed in the building. In order to assess the impact of different parameters, such as slab thickness, material properties and the surface heat transfer, the dynamic heat storage capacity of building elements was quantified based on an analytical solution of one-dimensional heat conduction in a slab with convective boundary condition. The potential of increasing thermal mass by using phase change materials (PCM) was also estimated. The results show a significant impact of the heat transfer coefficient on heat storage capacity, especially for thick, thermally heavy elements. For thin, light elements a significant increase in heat capacity due to the use of PCMs was found to be possible.

In order to identify the most important parameters affecting night ventilation performance, a typical office room was modelled using a building energy simulation program (HELIOS), and the effect of different parameters such as building construction, heat gains, air change rates, heat transfer coefficients and climatic conditions on the number of overheating degree hours (operative room temperature >26 °C) was evaluated. Besides climatic conditions, the air flow rate during night-time ventilation was found to have the largest effect. However, thermal mass and internal heat gains also have a significant impact on the achievable level of thermal comfort. A significant sensitivity to the surface heat transfer was found for total heat transfer coefficients below about 4 W/m2K.

The convective heat transfer at internal room surfaces is highly affected by the indoor air temperature distribution and the near-surface velocities both of which can vary significantly depending on the air flow pattern in the room. Increased convection is expected due to high air flow rates and the possibility of a cold air jet flowing along the ceiling, but the magnitude of these effects is hard to predict. Heat transfer during night-time ventilation in case of mixing and displacement ventilation has been investigated in a full scale test room. The performance of night time cooling was evaluated based on the temperature efficiency of the ventilation. The results show that for low air flow rates displacement ventilation is more efficient than mixing ventilation. For higher airflow rates the air jet flowing along the ceiling has a significant effect, and mixing ventilation becomes more efficient.

Combining the results of the previous steps, a practicable method for the estimation of the potential for cooling by night-time ventilation during an early stage of design is proposed. In order to assure thermal comfort two criteria need to be satisfied, i.e. (i) the thermal capacity of the building needs to be sufficient to accumulate the daily heat gains within an acceptable temperature variation and (ii) the climatic cooling potential and the effective air flow rate need to be sufficient to discharge the stored heat during the night. The estimation of the necessary amount of thermal mass in the building is based on the dynamic heat storage capacity. The air flow rate needed to discharge the stored heat at a certain climatic cooling potential is assessed based on the temperature efficiency of the ventilation.