Refurbishment of apartment buildings in the Mediterranean Region for natural ventilation: implications for building design
thesisposted on 02.09.2015, 13:24 by Eftychia Spentzou
With the emergence of climate change, the increasing figure of energy consumption for cooling in buildings expresses an urgent need for energy conscious design of new and existing buildings, and there is a significant opportunity for implementation of natural ventilation strategies. The high-energy consumption of the Greek domestic sector, the number of existing multi-storey apartment buildings, the small rate of building retrofitting in Greece and the warm, dry climate of Greece, indicate the potential to achieve significant energy reductions for cooling via natural ventilation. The aim of this research was to evaluate the energy saving potential of natural ventilation solutions for domestic buildings in the Mediterranean climate to deliver summer comfort, and to propose a low-energy refurbishment design guide. The natural ventilation efficiency of an urban multi-storey apartment building in Athens and the potential implementation of advanced natural ventilation strategies, were evaluated using modelling tools. This would provide the knowledge for future energy refurbishments. The building was a representative example of over 4 million buildings in Greece. Several ventilation strategies were implemented in a single apartment (51.4m2) and evaluated in order to enhance the existing single-sided ventilation strategy of the building, including: daytime and nighttime ventilation; cross ventilation strategies; use of a wind-catcher; lightweight dynamic façade with shading system; new internal openings; and passive downdraught evaporative cooling strategies. The ventilation performance of the strategies was investigated over the full cooling period using DTM simulations. Controlled natural ventilation strategies, in response to internal and external air properties, delivered: occupants comfort; ventilation rates increase; and reductions in air temperatures and in CO2 levels. Natural day and night ventilation contributed to significant temperature reductions (up to 7oC) relative to the base-case ventilation strategy. The proposed strategies marginally reduced the hours during the cooling period for which the CO2 levels exceeded the upper acceptable limit for comfort. The strategies also achieved air change rates above the minimum acceptable values for comfort were provided; and therefore occupants comfort was achieved. De-coupled internal-external steady state CFD airflow simulations were performed to predict wind pressures across the building openings, and to predict detailed ventilation rates for a number of climate scenarios. Using CFD it was possible to overcome the limitation of DTM and predict average pressures at the location of the openings, considering the location of the building within its surroundings (both external and internal flow simulations were performed), leading to accurate results. It was predicted that the ventilation performance of the wind catcher was significant relative to the simple single or cross-ventilation strategies. The downdraft evaporative cooling performed best at low ventilation rates providing up to 4oC further temperature reductions. Indoor comfort was provided during windless hours for specific strategies (buoyancy driven); this is significant considering that low wind speeds (below 1m/s) were predicted for 14% of the cooling period. The performance of the strategies varies considerably with regard to both wind speed and direction; these should be considered when retrofitting natural ventilation strategies in existing buildings. The proposed strategies delivered natural cooling and adequate ventilation rates, relative the base-case strategy. The combined wind catcher and dynamic façade strategy performed the best; this combined strategy would be recommended for the Mediterranean sub-climate, and for buildings comparable to the type studied. This should be combined with evaporative cooling strategies particularly during windless hours, and mechanical cooling only when these strategies do not provide sufficient performance. For both the CFD and DTM results, empirical relationships were established with statistical methods between indoor air properties and climate characteristics, which can be used to predict behaviours under conditions that have not been examined using simulations. This assists extrapolation of patterns in ventilation performance, to facilitate design guidance of the natural ventilation strategies for implementation in similar buildings. The established performance of the natural ventilation strategies in the case study building assisted the development of a prototype scenario for similar building designs with comparable climatic context. A low-energy refurbishment design guide for natural ventilation was proposed that provides guidelines and design recommendations. Retrofitting such natural ventilation strategies in existing apartment buildings in similar climates presents a significant opportunity to achieve significant energy consumption reductions.
- Architecture, Building and Civil Engineering