Refurbishment of apartment buildings in the Mediterranean Region for natural ventilation: implications for building design

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

Enhancing indoor comfort in existing apartment buildings in Athens using natural ventilation

Computer simulation and field studies were conducted to investigate the implementation of natural cooling strategies in existing apartment buildings in Athens; the most typical urban domestic building type of Greece. Thermal performance analysis and airflow modelling in a specific apartment were conducted for the summer period using dynamic building energy simulation tools. The indoor thermal comfort was evaluated with reference to the adaptive thermal comfort theory. Changes to the fenestration and the utilisation of a light shaft, assist the natural cooling of the building and improve the previous single-sided ventilation strategy. Results indicate indoor air quality in the spaces being significantly enhanced, whilst the percentage comfort hours were increased, suggesting a significant reduction of the buildings' cooling demand

Assessing natural cooling strategies in apartment buildings using de-coupled internal-external airflow simulations

Computational fluid dynamics simulations were conducted to investigate the performance of enhanced natural ventilation strategies in an existing multi-storey apartment building in Athens, a typical urban Greek domestic building type. De-coupled airflow modelling was employed to predict the airflow patterns around the case study building at the neighbourhood scale, along with the prediction of the internal airflow patterns and indoor air temperatures at the scale of a single apartment. Implementation of a wind-catcher and a second façade layer have been investigated to enhance the natural ventilation of the building and improve the original single-sided ventilation strategy

Natural ventilation assessment of an existing apartment building in the Mediterranean using time-dependent CFD

The benefits and limitations of time-dependent and steady state computational fluid dynamics simulations when evaluating natural ventilation were explored in a naturally ventilated case study apartment in the Mediterranean. For wind driven flows, indoor air properties responded quickly (i.e. within 1-min) to changing outdoor conditions, except indoor air temperatures (up to 30-min). The outdoor air temperature variations could reverse the flow direction during buoyancy-driven ventilation. Strong correlations between the steady state and transient simulation results were predicted (<1% error). Comparable indoor temperatures were found from simulations with both coarse and fine time steps, and up to 2% difference was found for indoor velocities

Evaluating energy savings retrofits for residential buildings in China

Building retrofit plays an important role in reducing energy consumption and carbon dioxide emissions whilst increasing occupant thermal comfort. This study used DesignBuilder to predict the energy saved by retrofitting a typical flat in Chongqing, a city in the hot summer, cold winter region of China. To increase the reliability of predictions, the model was verified by measured indoor air temperature for a one-week period in April. Five retrofit measures were evaluated, external wall insulation, new windows, increased air tightness, external shading, and higher efficiency of air conditioning. Three types of households with different AC operating schedule were assumed, high, medium and low. The variance in the model predictions due to the uncertainty in the model input parameters was calculated. The results showed that the energy saved depended on the use that was made of the AC system. For high energy users, 40 to 68% of the annual space-conditioning energy could be saved depending on the retrofit, whereas for low energy users the savings were 30 to 58%. Thermal comfort has improved in winter for low and medium energy users, but no improvement in summer

Priority school building programme: an investigation into predicted occupant comfort during the heating season in naturally ventilated classrooms

Natural ventilation solutions can provide sufficient outside air to maintain adequate indoor air quality (IAQ), which can improve occupants’ performance in classrooms and provide reductions in energy consumption for space conditioning. In this study, the effect of cool outside air and the vent opening configurations on IAQ and occupant thermal comfort in naturally ventilated classrooms during the heating season was examined. Dynamic and steady state computer simulations were performed to investigate the internal conditions of a naturally ventilated classroom, designed to meet the requirements of the Priority Schools Building Programme (PSBP) Output Specification. The modelled designs considered natural cross ventilation airflow through high-level top hung-out or bottom hung-in openings, and a stack (atrium). Dynamic thermal modelling results indicate that adequate IAQ and occupant thermal comfort could be achieved using natural ventilation. However, the CFD simulation results predicted occupant discomfort due to draughts in the regions close to the openings. Bottom hung-in vents reduced draught impact and the study also suggests moving occupants away from the draught zones to minimise the effect of discomfort draughts on occupant comfort. The air velocity and airflow patterns in the classrooms were influenced by the shape, size, location of internal openings, and the flowrate through the openings. This could be controlled by introduction of new openings with lower airflow rates through each opening

Energy saving potential of different setpoint control algorithms in mixed-mode buildings

Mixed-mode buildings can combine the use of natural and mechanical systems to achieve the desirable internal conditions. However, it is essential to effectively control a mixed-mode building to minimize the energy consumption without compromising the thermal comfort of the occupants. The aim of this research is to develop different setpoint control algorithms for mixed-mode buildings, by using a variety of adaptive methodologies such as ASHRAE Standard 55, IMAC model and EN15251, and evaluate their energy saving potential for Bangalore and Mumbai, India and Gatwick, UK. Cosimulations were used for this research. EnergyPlus was used to develop the building geometry and coupled with Modelica, where the control algorithms were developed. This is a novel simulation approach to assess control algorithms in buildings and provides great flexibility for future use of the control algorithms. The results showed that the effective control of mixed-mode building can result approximately in 40% energy saving in Indian cities compared to fully mechanical conditioned buildings whilst maintaining comfortable internal conditions for 90% of the year

Assessing the impact of control algorithms in direct evaporative cooling systems in mixed-mode buildings

Direct evaporative cooling (DEC) is one of the most commonly used cooling systems in many parts of the world with mainly hot and dry climatic conditions. Various simulation-based studies have been conducted to explore the potential of direct evaporative cooling in buildings. However, current dynamic thermal simulation tools use a simplified on/off control approach and do not allow modelling of situations where advanced algorithms are used in controlling DEC units. This paper couples EnergyPlus with Dymola® to simulate and assess the benefits of sophisticated control strategies for DEC units in mixed-mode buildings. This is a novel simulation approach for investigating control of DEC units in buildings that provides great flexibility for investigating future advanced control algorithms. The simulated results suggested that using the proposed sophisticated control algorithms for DEC units it is possible to achieve energy savings up to 35% compared to the base-case scenario and achieve up to 92% comfort hours for Ahmedabad, India. Similar results were predicted for Gatwick, UK

Sensitivity analysis of proposed natural ventilation IEQ designs for archetypal open-plan office layouts in a temperate climate

Designing naturally ventilated deep, open-plan offices could improve occupants’ thermal comfort and productivity and ensure energy reductions; however, this can be challenging when relying on façade only openings. This research examines the ventilation performance sensitivity of atria, innovative façade openings and interior layouts of open-plan offices, in order to identify optimal typologies. Different building typologies are developed through a combination of various atria designs and configurations, with the effective use of highaspect-ratio (HAR) openings with a similar dimension to that of the floor-to-ceiling height, in either a mid-level vertical (MLV) or high-level horizontal (HLH) orientation. Steady-state computational fluid dynamics (CFD) simulations are performed to predict internal air flow and temperature distribution in a moderate climate and water-bath modelling (WBM) experiments to validate the computational models. Results showed that MLV provide superior cooling potential (up to 2.5oC reductions) and higher ventilations rates; despite, increasing thermal gradients. Unobstructed atria with a horizontal profile similar to that of the building footprint also performed well. Overall, façade opening design was shown to be the most influential design parameter. This research has presented guidance based on reliable results to better equip building designers and architects in the design of successful naturally ventilated deep, open-plan offices

Low-energy cooling and ventilation refurbishments for buildings in a Mediterranean climate

In view of the ageing domestic building stock and increasing reliance on fossil fuels for cooling and ventilation of buildings, there is an urgent need for improved design knowledge and sustainable measures such as natural ventilation and passive cooling to mitigate climate change and future proof the built environment. This paper forms an appraisal of a range of low-energy refurbishment measures, i.e. building design alterations and passive systems, which were employed and evaluated in an apartment building in Greece. The applicability of these in domestic buildings in hot climates is assessed and their design implications evaluated. Implementation of wind-catchers, dynamic façades, and evaporative cooling had the highest ventilation and cooling potential, while improvements of the interior layout to allow for new airflow paths could provide further cooling to spaces and solutions to safety