NATURAL VENTILATION OF HIGH-RISE BUILDINGS - A Methodology for Planning With Different Analysis Tools and Case-Study Integration

Natural ventilation of buildings has the potential to significantly reduce energy consumption related to cooling and fanning. This can be achieved by providing good indoor air quality without any electricity demand and improving thermal comfort in the summer through increased daytime airspeed and high night ventilation rates. In high-rise buildings, however, natural ventilation is still not a widely preferred means of ventilation. The main reason is the lack of information on the required system design. Evaluation tools and instruments are not suitable for complex flow path design. Only few results are available on the performance of naturally ventilated high-rise buildings, especially where energy conservation is considered. The current thesis is predicated on this research gap. The existing barriers for implementing passive technologies can be lowered by creating a quantifiable framework that accounts for all the relevant input parameters in the design process. In order to reach this goal, a planning and simulation approach is developed. Simulations results are compared to those of a reference case-study. The 28-floor ‘Kanyon’ office tower, situated in Istanbul, is selected to demonstrate the applicability. From the energy metering, it is concluded that mechanical cooling and ventilation result in significant electricity consumption. Detailed information on the building and its operation has been made available by the building management. In addition, the impact of different moderate climates is analysed. The primary objectives of the thesis can be stated as the development of a design approach, and the investigation of the feasibility of the proposed design, based on an existing case-study building virtually adapted. The approach is developed in three steps, including conceptual design considerations, the development of a preliminary design tool, and a detailed design development. In the first step, an architectural concept is developed for wide-shaped high-rise buildings where it is impossible to realise simple cross or single-sided ventilation. Conceptual adaptations addressing the flow-path design are a central chimney strategy in respect to the building width, isolated, modular segments in respect to the building height and opposed, wind adapting openings. Other solutions proposed for passive cooling are improved shading devices and activation of the structural mass for night-time ventilation. In the second step, the originally developed ‘HighVent’ planning tool is introduced. Simple electrical circuit analogies, for both ventilation and thermal models, are found to be suitable in supporting the passive system planning. As classic design day conditions are too strict for passive system design, meaningful boundary conditions are provided. Openings can be sized automatically including an optimization process. The program first calculates the flow-path for a given airflow rate with unchanging boundary conditions. These values are then provided to the thermal module, which calculates the dynamic thermal comfort. The procedure is repeated till the system size is sufficient for passive cooling. In the third step, the annual performance is exemplarily modelled with EnergyPlus building energy simulations including airflow networks and controls. This includes the ‘HighVent’ tool preliminary design outputs, the conceptual adaptations made, and the remaining features of the as-built Kanyon building. The design approach is then further evaluated by comparing mechanical operation with an operation based on passive and hybrid control. Indicators proposed to evaluate the functionality are the energy consumption compared to that of mechanical ventilation and cooling systems, and compliance with the thermal comfort limits; additional aspects are the ventilation rates and the indoor air quality reached. Simulation results indicate that properly designed and controlled natural ventilation shows a good functionality. Control over the openings is crucial, as otherwise ventilation rates can get too high and the rooms tend to cool down too much even during summer. It is shown that the ‘Adaptive Temperature Amplifier’ control algorithm developed is very robust. Differences in climate have a varying impact. For example, in the climate of Stuttgart, further adaptations to the preliminary design are not necessary, whereas in Istanbul adaptations might be reasonable. However, to satisfy the comfort expectations in Turin, there is a necessity for adaptations or a hybrid cooling concept. That humidity values meet comfort expectations must be discussed and accepted by all project stakeholders, else hybrid operation might be a good alternative. To systematically study the possible energy conservation while maintaining thermal comfort, the energy consumption of identical buildings with different variants (passive/hybrid/active) is benchmarked against the as-built scenario. Results show that the Kanyon’s primary energy input can be reduced by approximately 30% to 40% for passive operation and by 28% to 34% for hybrid operation. This verifies the initial assumption that energy conservation of passively cooled and ventilated office spaces is significant, especially when compared to highly energy consuming state-of-the-art office towers. The results of this research work are intended, on the one hand, to support building planners in better understanding and implementing passive cooling measures and, on the other hand, to contribute to further development of sustainable building practices.