UDC8: Buildings climate and energy consumption I : tropical, continental and arid climate cities
Impact of increasing the depth of urban street canyons on building heating and cooling loads: Case study of Tel Aviv, Israel
Ben-Gurion University of the Negev, Israel
Many of the existing buildings in Tel Aviv’s older neighbourhoods suffer from structural weaknesses and might collapse in the event of a major earthquake, of which there is a high probability. Israel’s National Guideline Plan 38 (NGP 38) seeks to address this problem, facilitating the renovation of unsafe buildings by allowing construction of additional floors in existing buildings undergoing reinforcement, thus providing a financial incentive and a suitable regulatory framework. However, the increase in urban density, although desirable from other aspects, may also be expected to exacerbate the urban heat island.
The study examines the potential effects of increasing building height by means of computer simulation. The Canyon Air Temperature model is used to generate site-specific weather data from time-series measured at a reference weather station, accounting for urban geometry, materials and surrounding land cover. These data are used as inputs for assessing:
a) The ‘climatic cooling potential’, a metric that estimates the potential for cooling by ventilation in a non-air conditioned building whose temperature is assumed to oscillate harmonically in response to the diurnal cycle of external air temperature, with a time lag and decrement factor that are due to the presence of thermal mass.
b) Heating and cooling requirements in a fully air conditioned building, using the EnergyPlus building thermal simulation software.
Both indicators were calculated first for the reference weather station (Bet Dagan); then for conditions in typical existing streets in southern Tel Aviv; and finally for different scenarios of increased building height, up to a total of 8 floors. The simulations confirm that deeper streets resulting from implementation of NGP 38 are likely to create more intense nocturnal heat islands and to reduce wind speed.
The buildings modelled are typical of those found in the older quarters of Tel Aviv, with concrete block walls and flat concrete roofs with minimal thermal insulation. Windows are small, single-glazed and display no preferred orientation. Although any major retrofit may include a thermal upgrade, the study assumed that additional floors would be identical to existing ones, to focus on the effects of possible modifications to microclimate. It was found that the expected increase in cooling needs from higher air temperature was tempered by the effect of mutual shading by adjacent buildings. Annual consumption was further reduced by a decrease in heating. And, because intermediate floors are less exposed to the environment than a low-rise house, they require less heating and cooling. The compound effect, in this case, is that despite causing a more intense UHI, adding floors to low-rise buildings may reduce annual specific energy consumption, from about 34 kWh/m2 for a 1-floor building to about 26 kWh/m2 for a 7-floor building.
The effect of elevated night time air temperature in Tel Aviv is reflected in the values of the Climate cooling Potential. In reference weather station the total cooling potential for the month of July is 947 degree-hours; the comparable value for an exposed area near the sea is only 744 degree-hours, because nocturnal cooling is moderated by proximity to the sea. When the effect of existing 2-story buildings on air temperature is included, the cooling potential is reduced slightly to 696 degree-hours. Increasing building height to 4, 6 or 8 floors results in further reduction of the potential to 521, 374 and 178 degree-hours for the month of July, respectively.
Thus, the finding that the impact of the proposed construction on building energy consumption may be minor should be qualified by noting that assumptions regarding occupant behavior, including thermal preferences and the thermostat set points for heating and cooling, may in fact have a greater impact on energy consumption than changes in meteorological conditions. Furthermore, as the reduction in the Climate Cooling Potential indicates, occupants of non-air conditioned buildings would suffer disproportionately from elevated nocturnal temperatures and reduced wind speed on warm summer nights – encouraging use of air conditioning that may otherwise have been avoided.
Evaluation of Smart Shading Structures in Mitigating Urban Heat Island in a districts of Hot Arid Climate City (Abu Dhabi)
1Polytechnic of Milan, Italy; 2Masdar Institute of Science and Technology, UAE; 3Masdar Institute of Science and Technology, UAE; 4Masdar Institute of Science and Technology, UAE
More and more the world population is concentrating in the cities converting natural areas into urbanized areas by changing the thermal properties also. As the cities evolve the local climate changes as well. And this change is shown perfectly in the Urban Heat Island (UHI) phenomenon. Indirectly the UHI increases the energy consumption used for the cooling systems inside the buildings. This is translated in additional cost and one step back into the main target of having a sustainable city.
This paper provides an overview on how with the help of different tools such as UMI, Energyplus and Ecotect we can have the results of the energy consumption of 5 different districts in Abu Dhabi, a city with hot arid climate. The energy simulations are divided in two groups. The first group includes the current energy consumption of the different typologies of buildings placed in an urban district and the results are taking into consideration the surrounding environment. The second group of energy simulation analyses the same districts taking into consideration smart shading devices spread into the different districts according to the shading butterfly provided from Ecotect. By proposing this intervention of the necessary shading in the different districts, there is a possibility to moderate the temperatures inside the buildings and as a result the energy consumption by improving in the same time the outdoor quality.
The innovation stands in the application of this smart shading structures in this type of city and the measurement tools used for such proposal. Part of the work for the energy simulation is the preparation of different templates for the selected building categories, the weather data for the hot arid climate, the use of the shoebox model and experimenting UMI as a new tool that makes this kind of simulation possible. The simulation results will be then compared with the field observation data for the different building typologies taken into consideration. This comparison helps keeping the results near to the real energy consumption conditions.
Climate-responsive residential buildings in India. Just a drop in the ocean?
1Lab'Urba-GU, UPEM, France; 2DENERG, Politecnico of Turin, Italy
Nowadays, the building sector in India is characterized by a ferocious rhythm of construction and qualitatively poor and inappropriate to climate architectural solutions. Residential buildings mostly share the same typology, with thin walls, lack of shading systems, and lack of insulation, especially on the roof. They are vulnerable to high tropical temperatures and to extreme climate events, harboring at the same time a rising demand of better levels of thermal comfort coming from the middle-classes. For that reason, they are quickly shifting towards a total dependency on air conditioning, with consistent effects on energy consumption, health and urban climate (Heat Island Effect).
Our study proposes low-cost retrofit strategies to improve the quality of the existing stock of residential buildings in Koltata. The term “quality” makes reference here to buildings’ capacity to control the indoor environment and ensure thermal comfort for the occupants with moderate energy consumption. The suggested solutions are equally applicable to new constructions. The choice was made to privilege architectural and technical solutions and behavioral adaptations in order to avoid total dependency on air conditioning.
To this purpose, we drew on a study based on the analysis of one typical 3-4 stories building in Kolkata. The identified low-cost and technically simple interventions (such as insulation, double-glazed windows, shading overhangs, improved night ventilation) were tested through simulations and the results compared to a baseline case study (whose model was validated thanks to in situ measures recorded with data loggers). We adopted different methods to identify the indoor comfort temperature used as a reference for our simulations. We identified the most effective solutions combining reduction of indoor temperature, economic feasibility and decrease of energy consumption.
The 3-4 stories building in Kolkata represent about 85% of the existing constructions in the city; the residential building sector in India appears to be the third energy consumer, but its importance is expected to rise due to the growing rate of urbanization and equipment. In a climate-changing scenario, residential buildings will play an important role at the urban scale. This study tries to go beyond the architectural scale to consider a possible generalization of its results at the urban scale of the city of Kolkata.
Shading effect of Alley Trees and Their Impact on Indoor Comfor
1Budapest University of Technology and Economics, Hungary; 2University of Szeged, Hungary
The tendency of the last decades showed that energy-efficiency in architecture means a well-insulated and airtight building shell, however these features although provide a good indoor thermal comfort in the wintertime, also increase the risk of overheating during the summer. This leads to the more frequent use of air conditioning devices providing a self-generating process from the urban heat island point of view – what’s more it also increases the energy-consumption during the summer season. In order to achieve a high energy performance architects have made efforts for shading and evaporative cooling. The best tool for that is the use of deciduous plants as to improve the microclimate around the building so that the solar access is obstructed in summer. The use of Green Infrastructure in different levels of planning processes, which would provide sustainable solutions for urban management, is also prescribed in the EU Biodiversity Strategy 2020.
The significance of vegetal shading is that it can decrease the risk of overheating and also the negative effects of urban heat island. Although there are some previous data about the effect of vegetation, there are still questions in the scope of the microclimatic and energetic effect of vegetation planted in front of the façades. That is why we aimed to analyse more precisely the shading effect of alley trees, and their impact on indoor comfort. Our preliminary studies have shown that trees can effectively mitigate the heating up of building envelope due to shading. If shaded the temperature of wall surface can be ~5-6°C less opposite the non-shaded state. In addition we also showed that – depending on species – a tree in front of the façade can decrease the solar gain on internal horizontal surface up to ~43-47 per cents. As the tree obstructs the solar access of the wall and that of transparent surfaces, a difference in indoor comfort is to be observed too.
The shading efficiency of trees is a species-specific attribute, because of the varying crown structure and leaf density. Our analyses aimed at the quantification of the transmissivity of characteristic individuals of three frequently planted species (Celtis occidentalis, Sophora japonica, Tilia cordata). The measured data were the amount of transmitted shortwave radiation, compared with a measurement point under unobstructed sunlight. The highest transmissivity values (worst shading potential) was observed in case of Sophora species, the two other trees can be characterized with a bit lower transmissivity values, similar to each other. These attributes highly affect the trees’ potential to improve the indoor thermal comfort and facilitate energy saving. On base of our measured data the cooling load of the buildings and the risk of summer overheating is calculated. These types of analyses can form a base for targeted model development and adaptation (e.g. i-Tree).
Energy and Comfort in School Buildings in the South of Portugal
University of Algarve, Portugal
This work presents software that simulates the thermal response of buildings with complex topology and evaluates the indoor building climates, namely the thermal comfort and air quality in indoor environments. In this study the implementing of heating, ventilation and air conditioning systems with intelligent control, based on the PMV index, using geothermal and solar radiation energy. Instead of the traditional control based on the air temperature, the implemented system is based on the PMV index, which is based on the values of air temperature, air velocity, relative humidity, mean radiant temperature, the clothing level and the physical activity level. This methodology ensures acceptable levels of comfort, for low levels of power consumption.
The program used in this study, developed by the authors, calculates the values of air temperature inside the compartment and conduits, the temperature of opaque and transparent bodies of the building, the mass of water vapor and other gases inside the compartments and pipes, the water vapor on the surface of the building bodies, the water vapor and other gases in the solid matrix of opaque and inner bodies, the relative humidity of the air inside the compartments, the air velocity and the mean radiant temperature inside the compartments.
The university school building analyzed in this work, with three floors and 110 spaces, 122 transparent bodies (windows) and 1516 opaque bodies (interior and exterior walls, floors, roof and doors). The study is carried out both in summer and winter conditions, and the cycle of occupation and the air renewal rate are considered. The heating, ventilation and air conditioning work only when there are occupants in the compartments.