Bioclimatic and Vernacular Approaches in Hot and Aride Climate at Urban Scale: A Comparison on M’Zab and Yazd Valleys

Review on integrated photovoltaic-green roof solutions on urban and energy-efficient buildings in hot climate

Urban development is intensifying significantly in cities, mainly due to rapid population growth. However, this development is accompanied by increased vulnerability to climate change, leading to higher energy consumption. This energy demand is even more pronounced in hot climates, where temperatures are very high during the summer season. The Algerian Sahara is no exception, and is experiencing very accelerated development, which has given rise to the appearance of new urban districts, and different urban characteristics compared to the original core, which constitutes the urban form most adapted to the desert climate and in perfect harmony with the immediate environment. This study attempts to assess the effect of the different urban forms that make up the fabric of the city of Ouargla in south-east Algeria, taken as a case study, on the energy consumption of buildings in a hot climate. This research adopts a mixed approach to evaluate the energy consumption of three buildings located in three neighbourhoods with different urban characteristics. A quantitative estimate of the energy consumed for cooling during the summer season for the three typical buildings selected was simulated using DesignBuilder software, taking into account the city’s specific climatic data, as well as the urban characteristics of the three neighbourhoods selected for the study. As a result, this evaluation covers both the architectural and urban scales. For the qualitative assessment, a field survey was carried out using questionnaires to assess the quality of thermal comfort outside neighbourhoods and its impact on building interiors, and consequently on energy consumption. The results indicate the significant effect of urban form on energy consumption, confirming that the traditional form is the most appropriate to achieve better energy efficiency in a hot climate. Therefore, the research recommends taking into account various influential factors and applying design strategies, regarding urban form, from the first stage of the design process.

The increasing frequency and severity of weather extremes caused by climate change evidence the need to assess buildings beyond their typical thermal and energy performance under normal operation. It is also essential to evaluate thermal resilience to safeguard occupants' health during extreme events and power outages. This study proposes a simulation framework to evaluate and enhance the thermal resilience of buildings against indoor overheating using an integrated set of performance metrics. This work also addresses how to aggregate resilience profiles of single buildings into the urban scale, supporting the evaluation of thermally resilient communities. This is the first step to connecting building and urban scales in a resilience analysis, seeking to further address other stakeholders' needs in the future. The application of the framework is exemplified through a case study considering three different climates in Brazil. This analysis allowed identifying cases with poor thermal resilience and essential dependence on air conditioning to guarantee the survivability of occupants during extreme hot weather. Nonetheless, by only changing the envelope's thermal transmittance and thermal mass, buildings' thermal autonomy increased up to 65% points and cooling loads were reduced by up to 61% in the hottest climate, São Luís. However, additional strategies are necessary to mitigate remaining indoor extreme thermal conditions, such as solar shading and increased air movement.

This study adopts a multi-scale, simulation-driven approach to evaluate the performance of different passive roof types in mitigating Urban Heat Island (UHI) in hot arid climate. A comparative analysis was performed for selected roof types; green, pond, cool, and dark roofs. At the urban scale, ENVI-met v5.7.1 was employed to simulate microclimatic impacts, including Mean Radiant Temperature (MRT) at the pedestrian street level (1.4 m) and above building canopy level (25 m). The results revealed that green roofs were the most effective in mitigating UHI on the urban scale, reducing MRT by 1.83 °C at the pedestrian level and by 3.5 °C at the above canopy level. Surprisingly, dark roofs also performed well, with MRT reductions of 1.81 °C and 3.5 °C, respectively, outperforming pond roofs, which showed reductions of 1.80 °C and 0.31 °C. While cool roofs effectively reduced MRT at the pedestrian level by 1.80 °C, they had adverse effect at the canopy level, increasing MRT by 15.58 °C. At the building scale, Design Builder v7.3.1, coupled with Energy Plus, was used to assess indoor thermal and energy performance. Pond and cool roofs reduced operative temperature by 0.08 °C and 0.07 °C, respectively, followed by green roofs, with a 0.05 °C reduction, while dark roofs increased it by 0.07 °C. In terms of energy performance, green roofs yielded the greatest benefit, reducing cooling load by 3.3%, followed by pond roofs, with a 1.32% reduction; cool roofs showed negligible reduction, while dark roofs increased it by 1.2%. Finally, a Python-based Multi criteria Decision Making (MCDM) analytical framework integrated these findings with additional factors to optimize thermal comfort, environmental impact, sustainability, and feasibility and rank strategies accordingly. The analysis identified green roofs as the optimal solution, followed by pond roofs and then cool roofs tied with the base case, leaving dark roofs as the least favorable strategy. This study’s key contribution lies in its integrated simulation–decision analysis methodology, which bridges urban climatology and building performance to provide actionable insights for sustainable urban design. By validating green roofs as the most effective passive strategy in hot arid regions, this work aids policymakers and planners in prioritizing interventions that support climate-resilient urbanization.

The Uncontrolled urban sprawl and the unadopted building to the environment are the main causes that generate the urban heat island (UHI) phenomenon. This phenomenon contributes to the degradation of the environmental quality of the interior and exterior spaces and increases the energy demand of buildings. In hot and arid regions, the cooling needs of space during the hot season are much greater. This situation imposes the use of electrical air-conditioning equipment for longer periods, which leads to a drastic consumption of electrical energy, implicitly, rise the greenhouse gas emissions, which in turn accentuate the phenomenon of the urban heat island. In this context, one of the most effective strategies to moderate heat stress at the urban scale is the adoption of the cooling roof technique "cool roof”. In fact, solar reflectivity (SR) and high emissivity (IE) of cool materials allow them to absorb less solar energy than conventional concrete roofs, reducing the amount of heat transferred to the atmosphere. This research aims to evaluate the impact of cool roof on the mitigation of the urban heat island phenomenon and to investigate its effect on reduction of energy consumption. Experimental work was carried out in Biskra hot and arid climate.

Energy savings in buildings or UHI mitigation? Comparison between green roofs and cool roofs

Thermal comfort plays a main role in encouraging people to use outdoor spaces, specifically in hot arid and humid climates. The reconciliation of climatic aspects during the urban design phase is limited in implementation, due to the need for multidisciplinary collaboration between desperate scientific fields of climatology, urban planning, and urban environmental modelling. This paper aims to create an integrated interface between the microclimate, outdoor thermal comfort, and design guidelines. The investigation combines subjective and objective approaches, including on-site field measurements, a structured questionnaire using the seven-point American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE 55) thermal sensation votes, and a correlation study of these votes and the microclimatic parameters. Pedestrian thermal comfort was then examined under six shading scenarios, addressing the form and opening of shading devices using computational fluid dynamics. Modelling is based on four dependent variables: wind velocity, ventilation flow rate, air temperature, and the physiological equivalent temperature (PET) index. Findings indicate that the form and location of apertures of the shading devices were the dominant factors in achieving thermal comfort on the urban scale, and led to a reduction in air temperature and a physiological equivalent temperature of 2.3–2.4 °C. Subjective votes indicate that people who live in hot arid climates have a wider range of adaptation and tolerance to local climatic conditions Accordingly, a psychometric chart, for the case study outdoor thermal comfort was developed.

This study calls for the integration of context-based socio-cultural habits and learning from local practices in providing outdoor thermal comfort in conservation areas. These parameters have direct impacts on outdoor activities, especially in hot arid climates. The study took place in two nearby locations one renovated and all external shadings removed to provide visual vistas to monuments while on the same street, no more than 1500 m apart, local shading practices were left in places. Sun-exposed as opposed to shaded sites were compared for subjective thermal comfort and outdoor activity, via structured interviews, observations, and wide-ranging micrometeorological measurements. The aim was to investigate psychological factors, including overall thermal comfort and perception, in addition to environmental parameters, such as solar radiation intensity and thermal adaptation. The analysis illustrates the importance of shading as a dominant factor in achieving thermal comfort on the urban scale, with a neutral temperature in summer of 29.9 °C and 29.2 °C for shaded and sun-exposed locations, respectively. The results suggest people may be more willing to tolerate higher temperatures in shaded rather than sun-exposed locations. Moreover, cultural constraints and context-based behaviour proved to have some influences on people’s levels of adaptation and their thermal behaviour.

High density of population and vertical buildings seems to be the only aspect fit to the concept for city in the developing country. The vertical housings then become a matter of necessity in high density area, in which the building groups themselves significantly contribute to microclimate at urban scales. This study is going to give descriptions of outdoor thermal comfort of public housing in Bandung by means its correlation between urban forms and mean radiant temperature. A number of simulations have used ENVI-met to reveal a better urban form which addresses the role of urban physics in the study of outdoor thermal comfort in a hot humid climate area.

Evaluation of the effects of courtyard building shapes on solar heat gains and energy efficiency according to different climatic regions

Solar reflective ‘cool roofs’ absorb less sunlight than traditional dark roofs, reducing solar heat gain, and decreasing the amount of heat transferred to the atmosphere. Widespread adoption of cool roofs could therefore reduce temperatures in urban areas, partially mitigating the urban heat island effect, and contributing to reversing the local impacts of global climate change. The impacts of cool roofs on global climate remain debated by past research and are uncertain. Using a sophisticated Earth system model, the impacts of cool roofs on climate are investigated at urban, continental, and global scales. We find that global adoption of cool roofs in urban areas reduces urban heat islands everywhere, with an annual- and global-mean decrease from 1.6 to 1.2 K. Decreases are statistically significant, except for some areas in Africa and Mexico where urban fraction is low, and some high-latitude areas during wintertime. Analysis of the surface and TOA energy budget in urban regions at continental-scale shows cool roofs causing increases in solar radiation leaving the Earth–atmosphere system in most regions around the globe, though the presence of aerosols and clouds are found to partially offset increases in upward radiation. Aerosols dampen cool roof-induced increases in upward solar radiation, ranging from 4% in the United States to 18% in more polluted China. Adoption of cool roofs also causes statistically significant reductions in surface air temperatures in urbanized regions of China (−0.11 ± 0.10 K) and the United States (−0.14 ± 0.12 K); India and Europe show statistically insignificant changes. Though past research has disagreed on whether widespread adoption of cool roofs would cool or warm global climate, these studies have lacked analysis on the statistical significance of global temperature changes. The research presented here indicates that adoption of cool roofs around the globe would lead to statistically insignificant reductions in global mean air temperature (−0.0021 ± 0.026 K). Thus, we suggest that while cool roofs are an effective tool for reducing building energy use in hot climates, urban heat islands, and regional air temperatures, their influence on global climate is likely negligible.

Numerical evaluation of thermal comfort in traditional courtyards to develop new microclimate design in a hot and dry climate

The impact of urban morphology and building's height diversity on energy consumption at urban scale. The case study of Dubai

Annual cycle parameters (ACP) provide a global climatology of annual land surface temperature (LST) based on daily 1 km MODIS observations. These are based on a simple model of the annual temperature cycle and allow estimating LST patterns under largely cloud-free conditions for every day of year. Further, they deliver measures for the LST variability and the frequency of cloud occurrence. It has been demonstrated, that they reproduce important surface climate characteristics at global and urban scale but their ability to reproduce topo-climates has yet to be studied in detail. In this paper their suitability to investigate climatic variability at km scale were studied at the case of the Canary Islands (Spain). This Archipelago, has a very stable climate dominated by the Azores high and the trade wind belt, but shows a large number of micro-climates ranging from arid hot climates to cold climates. It was found that ACPs are a relevant source of climatic information at km scale in complex orography. Specifically, known features such as subsidence inversion, the resulting sea of clouds, the strong differentiation in precipitation between the flat and high islands, as well as the northern and southern slopes at the latter were clearly visible in the parameters.

This chapter addresses energy solutions at the urban scale, principally though not uniquely for cooling. Whilst district heating (DH) systems are widespread, district cooling (DC) is less well known, although DC systems have existed for some years in both hot-dry and in hot-humid climates. There are major efforts today to spread awareness of DC, not least in Asia. After a brief overview of the development worldwide and scope of district energy systems, we focus on Malaysia, which is a leading example. We also highlight a little discussed conflict between the level of individual buildings versus that of urban energy planning. Which level should be prioritised and under what conditions?