Special Issue “Urban Sustainability and Resilience of the Built Environments”

Urbanization and the development of urban areas are profoundly altering the relationship between society and the environment. The study of urban sustainability and resilience aims to understand existing urban dynamics and respond to the challenges of creating livable urban futures. The built environment includes human-made building and infrastructure stocks that constitute forms of physical, natural, economic, social and cultural capital. Even with elevated scholarly attention, strategies for bridging the gap between research and practice remain elusive, and efforts to understand and create more sustainable and resilient urban centers in the built environment have often fallen short. This Special Issue seeks to showcase recent developments, disruptive new concepts, validated simulations and creative applications of these twin concepts: urban sustainability and the resilience of the built environment. Submissions focus on urban governance, urban planning, sustainable development and resilience, environmental and hazard governance, climate risk adaptation and mitigation, including energy-efficient solutions, and the built environment in general.

Scholars contributed a total number of 19 articles, including 1 review, in various fields related to urban sustainability and the built environment to this Special Issue. Several of the included papers are dedicated to the integration of solar energy in the new, efficient buildings. New and innovative configurations of both solar collectors and photovoltaic panels have been addressed. Teodosiu et al. [1] assessed the performance of a glazed transpired solar collector prototype under real long-term climatic conditions. The results show that the GTC configuration, featuring a 30 mm air gap between the absorber and the glazing, leads to improved heat transfer efficiency and superior global effectiveness regardless of airflow rates through the solar air collector. This optimized GTC configuration was further studied via integration within the façade of a full-scale experimental building (container-type, light structure). Comparative experimental studies were then carried out concerning the heating energy consumption and ventilation load of the experimental building, both without with GTC having been implemented in the ventilation system, under real Bucharest weather conditions. The data achieved indicate that the GTC prototype is capable of substantially reducing the ventilation load: up to 25% for low solar radiation (below 200 W/m²) and over 50% (achieving even 90%) for moderate solar radiation (between 250 and 380 W/m²). Finally, for high solar radiation (over 400 W/m²), the GTC outlet air temperature exceeded the interior temperature set-point (22 °C) of the experimental building. In the same direction, Berville et al. [2] proposed a new variant of double skin transpired solar collector (DSTSC) which they analyzed in their paper, thus providing guide values and a technical point of view for engineers, architects, and constructors when designing this kind of transpired solar collector. Three important parameters were addressed in this study through numerical simulation: the influence of a separation plate introduced in a TSC, turning it into a DSTSC; the air layer thickness influence on the performance of the collector; and the influence of the used absorber materials on the separation plate material. Greater levels of heat exchange efficiency were noticed for the DSTSC at every imposed airflow rate compared with the TSC. The efficiency the solar collector gradually increased with increasing thickness until it reached a value of 20 cm, and there was no significant variation at a thickness of 30 cm.

In the context of energy conservation and sustainable development, building design should account for the energy efficiency criteria by using renewable energy sources. Double-skin facades (DSF) represent innovative energy-efficient techniques that have received increasing interest worldwide. The study of Chereches et al. [3] reports the results of an experimental campaign, performed on a full-scale double-skin façade, using the in situ measurement methodology. The thermodynamic behavior of the façade is studied under real exterior climatic conditions in Romania in hot and cold seasons, and performance indicators in terms of pre-heating efficiency and dynamic insulation efficiency were determined. Three summer periods are analyzed, each corresponding to the outdoor air curtain scenario for three ventilation modes in naturally or mechanically ventilated single-story DSF buildings. Results revealed that the third ventilation scenario, which combines horizontal and vertical openings, gives the best efficiency of 71.3% in the double skin façade functioning. During the cold season, the channel façade behaved like a thermal buffer between the building and the exterior air, ensuring the requisite thermal energy for partial or integral heating of the building.

A number of papers [4,5,6,7] were dedicated to the optimization of PV systems. In [4], Hudisteanu et al. present a numerical model regarding the passive cooling of PV panels through perforated and non-perforated heat sinks. A typical PV panel was studied in a fixed position and tilted at 45 degrees from the horizontal, with the wind direction towards its backside. A challenging approach was used in order to calibrate the base case of the numerical model according to the NOCT conditions. Further validation of the accuracy of the numerical simulation consisted of a comparison between the results obtained for the base case, or indeed the heat sink, with horizontal non-perforated fins and the experiments presented in the literature. Six types of heat sink, each attached to the backside of the PV panel, were numerically studied. The analyzed configurations focused on heat sinks with both perforated and non-perforated fins that were distributed horizontally and vertically. The CFD simulation was also conducted by modeling the air volume around the PV panel in real wind conditions. The main output parameters were the average temperature and the convective heat transfer coefficient on the front and back of the PV panel. The most important effect of cooling was achieved in low-wind conditions and high levels of solar radiation. For vair = 1 m/s, G = 1000 W/m² and ambient temperature tair = 35 °C, the percentage of maximum power production achieved was 83.33% for the base case, while in the best cooling scenario it reached 88.74%, assuring a rise in the power production of 6.49%. Paper [5] presents a numerical model for the passive cooling of PV panels through perforated and non-perforated heat sinks. A typical PV panel was studied in a fixed position, tilted at 45 degrees from the horizontal with the wind direction towards its backside. A challenging approach was used in order to calibrate the base case of the numerical model according to the NOCT conditions. Further validation of the accuracy of the numerical simulation consisted of a comparison between the results obtained for the base case or heat sink with horizontal non-perforated fins and the experiments presented in the literature. Six types of heat sink attached to the backside of the PV panel were numerically studied. The analyzed configurations focused on heat sinks with both perforated and non-perforated fins that were distributed horizontally and vertically. The CFD simulation was also conducted by modeling the air volume around the PV panel in real wind conditions. The main output parameters were the average temperature and the convective heat transfer coefficient on the front and back of the PV panel. The most important effect of cooling was achieved in low-wind conditions and high levels of solar radiation. For vair = 1 m/s, G = 1000 W/m² and ambient temperature tair = 35 °C, the percentage of maximum power production achieved 83.33% for the base case, while in the best cooling scenario it reached 88.74%, assuring a rise in the power production of 6.49%.

In their article, Baouche et al. [6] describe a methodology for the simulation and the design of a solar tracker system, making use of the advantages that the orientation and efficiency of the PV panel offer due to the latitude and the number of hours of sunshine in the testing area. This proposed methodology was experimentally validated through the implementation of a single-axis solar tracker at a specific location (36.261° latitude), allowing the incorporation of a high-availability, low-precision, and low-cost tracking mechanism. Based on the results, the feasibility of this type of solar tracker for latitudes close to 36° was demonstrated by the lower cost of this tracking system compared to traditional commercial systems. Furthermore, this system contributed increased collection efficiency compared to a fixed device. Our results provide an excellent platform for engineering technology researchers and students to study the design theory of a sun-tracking solar system.

Photovoltaic/thermal (PV/T) systems are innovative cogeneration systems that ensure the cooling of photovoltaic (PV) backsides, as well as the simultaneous production of electricity and heat. However, producing an effective means of cooling the PV back is still a challenge that affects the electrical and thermal performance of the PV/T system. In their work [7] El Fouas et al., present a PV/T numerical model to simulate the heat flux based on the energy balance implemented in MATLAB software. The numerical model is validated through a comparison of the three-layer PV model with the NOCT model and tested under the operation conditions of a continental temperate climate. Moreover, the effect of velocity and water film thickness as important flow parameters on heat exchange and PV/T production is numerically investigated. The results revealed that the PV model strongly agrees with the NOCT variant. An efficient heat transfer was obtained while increasing the velocity and water film thickness, with optimal values of 0.035 m/s and 7 mm, respectively, at an inlet temperature of 20 °C. The PV/T system ensures a maximum thermal power of 1334.5 W and electrical power of 316.56 W (258.8 W for the PV). Finally, the comparison between the PV and PV/T system under real weather conditions showed the advantage of using the PV/T.

One of the biggest challenges the world is facing these days is to reduce the greenhouse gases emissions in order to prevent global warming and limit its effects. Since a significant quantity of CO₂ emissions result from the energy production process required in industrial applications or the use of buildings, waste heat recovery is an important aspect in the fight to preserve the planet. In paper [8], an innovative waste heat recovery system, capable of recovering waste heat energy from a variety of cooling liquids used in industry or in different processes, was designed and subjected to experimental investigations. The equipment uses heat pipes to capture thermal energy from the residual fluids transiting the evaporator zone and transfer it to the cold water transiting the condenser zone. The efficiency of the heat exchanger was tested in 9 scenarios by varying the temperature of the primary agent to 60, 65 and 70 °C and the volume flow rate of the secondary agent to 1, 2 and 3 L/min. The temperature of the secondary agent and the volume flow rate of the primary agent were kept constant at 10 °C and 24 L/min, respectively. The results were later validated through numerical simulations. This process confirmed that the equipment can easily recover waste thermal energy from used water with low and medium temperatures at very low costs compared to the traditional heat exchangers. The results were promising, revealing an efficiency of the equipment of up to 76.7%.

On the same topic, the increasing demand for energy due to comfort requirements in the built environment, coupled with the development of road networks and the amplification of the heat island effect call for a comprehensive approach that can answer both issues. The lifespan of an asphalt layer is affected by surface temperature. The study of Dogeanu et al. [9] showcases the feasibility of heat recovery and its effects in terms of energy harvesting efficiency and asphalt surface temperature by creating a numerical model and validating the model based on onsite measurements at a laboratory scale. The experimental setup was developed at the Technical University of Civil Engineering in Bucharest, and measurements were monitored during the summer. The heat recovery system used for this study was made of copper pipes, and its material cost and layout optimization need to be addressed in future studies. The numerical model was validated using measured data. During this study, the authors obtained favorable results in terms of heat recovery, reducing both surface temperature and the selection of system materials.

Heat transfer and its practical implications are of the utmost importance when storing electrical energy. In this context, supercapacitors (SCs) are electrical energy storage devices which have the peculiarity of storing more electrical energy than capacitors and supply it at higher power outputs than batteries. This, together with the fact that SCs are endowed with high cyclability and long-term stability, makes them very attractive devices for electrical energy storage. Thermal transfer around a novel arrangement of a module of five rows of SCs is discussed in the study of Victor et al. [10]. A mixed aligned/staggered configuration is studied with the aim of exploring a new possibility that can improve heat transfer more than the other configurations previously studied in the literature. The maximum SC current rate current is 84 A, and the maximum temperature is 65 °C. The module undergoes both charge and discharge cycles. The current tests are performed up to 50 A for natural convection and up to 70 A in instances of forced convection. For the natural convection case, the SC located in the center of the module is the most critical from the temperature point of view, and the temperature evolution shows the necessity of a cooling system. The relative temperature reaches 27 °C for 50 A, and the permanent regime cannot be reached with a current greater than 50 A. Thereafter, the impact of position and current on the temperature of SCs in forced convection is examined. The airflow mean air velocity is 0.69 m/s. The temperature of the SCs located on the third and fourth row is very close. However, the last row is the least cooled. This low temperature rise can be explained by the change from an aligned to a staggered arrangement between these rows. Compared to the natural convection case, a significant decrease is observed for the relative temperatures. The difference between the highest and lowest temperature augmentation also decreases, but still remains high. The temperature difference becomes greater than 5 °C if the continuous current exceeds 39 A. A CFD numerical simulation is performed for steady state conditions at the maximum experimental current rate in order to better understand the thermal and flow behavior. Overall, the numerical and experimental results are in good agreement, with a temperature deviation of less than 10%.

Energy efficiency also applies to cold storage. In their article [11], Girip et al. present a technical and economic analysis of sandwich panels used in cold stores’ construction, equipped with a polyurethane insulation layer (PUR). The authors determine the optimal thickness of the insulating layer (OIT), corresponding to the 5 climatic zones in Romania. The operating and investment costs for cold and frozen storage in these 5 climatic zones have been assessed. The results obtained from the analysis show that, regardless of the climatic zone, the OIT for cold storage is 150 mm and for frozen storage is 180 mm. The investment cost increases by 41% and the expenditure on operating energy decreases by 8.3% for 180 mm for cold storage in comparison to OIT. Moreover, this tendency is also maintained in the other case of frozen storage, where, by increasing the thickness above OIT at 200 mm, the investment cost is increased by 20% and the expenditure on operating energy is decreased by 6.7%. The SEC has an average value of 54.83 kWh·m³/year for cold storage and 74.55 kWh·m³/year for frozen storage, respectively. The average values obtained in the paper were compared with those presented in the literature and resulted in deviations of about 1.58% for refrigeration, and hence 4.1% for freezing.

One of the mainstream directions for CO₂ emissions reductions, both from household heating and hot water-producing facilities, is to lower the levels of hydrocarbons in combustibles by replacing them with hydrogen. The authors of the article [12] analyze condensing boilers which operate when hydrogen is mixed with standard gaseous fuel (CH₄). The hydrogen (H₂) volumetric participation in the mixture is considered to vary in the range of 0 to 20%. The operation of the condensing boilers will be numerically modeled by computational programs and prior validated by experimental studies concluded in a European certified laboratory. The study concluded that an increase in the combustible flow by 16%, together with a decrease in CO₂ emissions by approximately 7%, will compensate the maximum H₂ concentration situation without further implications for the boiler’s thermal efficiency. By assuming 0.9 (to/year/boiler) as the value of CO₂ emissions reduction for the condensing boiler determined in the paper, and extrapolating it for the estimated number of boilers to be sold for the period 2019–2024, a 254,700-ton CO₂/year reduction resulted.

The resilience of the built environment is reflected in its capacity to provide a healthy and safe ecosystem for its occupants. In this context, two papers were dedicated to fire safety. Indeed, statistics show that most fires occur in civil residential buildings. Most casualties in such circumstances are due to the inhalation of hot air, loaded with smoke, leading to intoxication with substances harmful to the human body. Paper [13] aimed to develop a CFD model that relates the operation of the sprinkler system to the operation of the ventilation system through the air temperature in a specific point close to the sprinkler position. A real-scale experiment was carried out, and a CDF model was developed. Several parameters of the CFD model (thermal conductivity of the experimental test room walls, numerical grid elements’ dimensions, burner heat release rate variation) were imposed onto the model, so that the resulting entire time variation of the temperature next to the sprinkler location corresponds to the real measured variation. Two other experiments were used to validate the numerical model. Besides the air temperature, at this point, additional essential parameters were determined for the entire experimental space: indoor air temperature, visibility, oxygen concentration, and carbon dioxide concentration. It has been found that, if the ventilation rate increases, the indoor temperatures at that specific point decrease, and the sprinkler is activated later. Indeed, in some cases, it might never be activated. However, this conclusion is not valid for the entire analyzed space, as the ventilation system alongside the natural air movement imposes specific air speed and specific temperature distribution inside the analyzed space.

Teodosiu and Kubinyecz [14] propose an analysis method to assess the efficiency of the planned emergency strategy (coupled operation, ventilation systems–PSDs system) in the instance of a train, on fire, stopping at the platform of a subway station retrofitted with platform screen doors. The approach is based on computational fluid dynamics (CFD) full-scale simulations to predict the airflow, temperature, and pollutant (carbon monoxide—CO; carbon dioxide—CO₂) concentrations caused by the fire. The results show the evident contribution of PSDs in stopping the dispersion of hot and polluted air in the subway station during the entire simulation time (20 min from the arrival of the train on fire). Consequently, the investigated emergency strategy (exhausting air both through the “over track system” and the “under platform system”, simultaneously with the opening of the PSDs on the side with the train on fire) assures the safe evacuation of passengers as soon as they have left the subway train. The results indicate that access to the platform is not perturbed by high temperatures or dangerous concentrations of CO and CO₂.

Among the factors that influence the resilience and sustainability of cities in general, and of inhabited spaces in particular, extreme temperatures and, for people in particular, thermal comfort are both elements that should be considered. This problem includes all the aspects of comfort for users of inhabited spaces in both buildings and vehicles. The purpose of the paper [15] is to present the details of a recently designed and created thermal manikin that comprises 79 superficial zones with independent neuro-fuzzy temperature regulation. Both the component parts of the manikin and the acceptance strategy are described. Flexible heating elements were used to control the temperature, on which five digital sensors are positioned. In order to establish the relationship between heat loss and ambient temperature, the thermal manikin was calibrated in a climatic chamber. The thermal manikin was able to predict local sensations through the equivalent temperature concept of the so-called predicted mean vote. The manikin has valuable applications, such as in the case of validating CFD models like the ones dedicated to the dynamics of airflows around the human body. This topic has gained a great deal of attention the pandemic context. Paper [16] is related to this issue. Indeed, in the last decade, there has been an increase in the ease and affordability of air travel in terms of mobility for people globally. Airplane passengers may experience different risks of contracting airborne infectious diseases onboard aircraft, such as influenza or severe acute respiratory syndrome (SARS-CoV-1 and SARS-CoV-2), due to nonuniform airflow patterns inside airplane cabins or potential proximity to an infected person. In this paper, a novel approach for reducing the risk of contracting airborne infectious diseases is presented that uses a low-momentum personalized ventilation system with a protective role against airborne pathogens. Numerical simulations, supported by nonintrusive experimental measurements for validation purposes, were used to demonstrate the effectiveness of the proposed system. Simulation and experimental results of the low-momentum personalized ventilation system showed the formation of a microclimate around each passenger with cleaner and fresher air than produced by the general mixing ventilation systems. Of course, some of these considerations can be applied to any ventilated space.

The structure of the buildings is important when aiming to construct a resilient built environment. The aim of the review article [17] paper is to present the relevant information and development available in the scientific literature regarding the seismic behavior of micropiles (MPs) and micropiled structures (MPed). The seismic behavior of MPs is not very well studied, but MPs are used in the retrofitting of old buildings and for new resilient buildings, and in terms of seismic behavior they have a high potential. Additionally, their association with seismic dampers for improving the seismic behavior of buildings has not yet been fully studied and represents a major topic of interest for both new structures and historical monuments. After the introductory section, the paper describes all relevant information regarding MPs, such as types and technology used, seismic behavior, applications for increasing seismic resilience, and experimental and numerical modeling.

Additionally, this Special Issue contains a paper on maximizing the utility of the infrastructures of the built environment paper [18] for public transport operators and municipalities, a development which should enable the relevant actors to make decisions about and optimize public transport schedules during peak hours. In this study, the authors outline the data and the means necessary for the creation and use of a specific database for a dynamic spatial analysis of the public transportation network. This will facilitate the analysis of public transport vehicle operating programs and the simulation of new transport programs using open source software. This paper delivers the first digital map of public transport in Bucharest. Using the QGIS software and the PostgresSQL database, (i) the authors have analyzed the accessibility of public transport stops for residential areas (5 min isochrones, corresponding to walking distances of 400 m), and (ii) they have determined the correlation of transport vehicle programs with the existing transport network to optimize the headway of vehicles. These two elements were considered for the analysis of public transport performance. The research study was based on the tram network in Bucharest, but it can be easily upscaled for the entire public transport network or replicated in other large cities.

However, the built environment is dependent of the natural environment, and in this way a special attention must be paid to natural hazards. One example is addressed in paper [19]. One Romanian municipality, Tulcea, is frequently exposed to rainfall-generated floods, with its lower downtown area (located in the Danube’ former meadow, now protected by dikes) being flooded two-to-three times per year. In this study, our objective was to understand the generation mechanism of these rainfall-triggered urban floods and to identify tailored mitigation options. Due to the lack of reliable information on the sewage network (diameters, slope, material) and the uncertain sewage outflows during heavy rain events, a rather simplified approach was preferred by the authors of this paper. The data processing was performed using GIS tools, and appropriate accounts were given of the digital terrain modeling, ortho-photos, administrative boundaries of the Tulcea municipality, delineation of the urban catchments, imagery of the frequently flooded areas, and the urban sewage network. Subsequently, a fast hydrological modeling and a volume-based flooding approach were developed in order to identify and evaluate the flooded urban areas under extreme rainfall events. Upon the completion of the calibration and validation processes, numerical simulations were run that considered the design storms of different return periods. Due to the high slopes of the hills, and hence the short concentration time of the pluvial waters, a sponge city approach does not seem easy to implement. A more efficient solution, utilizing large capacity buried urban retention tanks in the lower part of the municipality, was alternatively identified. At a later date, this solution will be supported by a set of green measures.

Although submissions for this Special Issue have been closed, more in-depth research in the field of the sustainable built environment continues to address the challenges we all face today, such as climate change, water shortages, and energy crises.