Environmental Retrofitting, Fighting Urban Heat Island Toward NEZ Sustainable Smart Cities

Abstract

The discovery of the physical phenomenon of heat island dates back to 1833 when Luke Howard undertakes on the air temperature in London and its surroundings. His research showed how, already at that time, winds get stopped and pulled up by the intensive urbanization, reducing the quality of the outdoor environment while turning cold into warmer areas within the urban settlements (Mills 2008). The last centuries confirmed how intense urbanization can make the temperature rise to several degrees, activating a vicious circle where car and energy use become more necessary, and co-cause for further temperature arise. No doubt, cities are—literally—heat islands if compared to their surroundings. The crescent environmental stress is the main challenge while targeting quality of life in today’s urbanization, in order to enable the public space to welcome citizens and encourage their outdoor activities. It requires a consistent commitment to the built environment and the awareness of the main role of public space as an interactive platform for a sustainable and human-centered smart city. As matter of fact, it is the public space where social life shapes and grows. The public space was often defined as the smart city's interface. So, smart cities need to generate and maintain a welcoming, healthy, livable, vibrant public space to have a reason to be. The scope of smart cities is to create a space first, the infrastructure to strengthen the connections among people and between people to the place. (Petrucci 2022) Cities worldwide must respond to a growing and diverse population, ever-shifting economic conditions, new technologies, and a changing climate. The task becomes especially challenging in extreme climate environments, such as in the Far East, in the African continent, or in the GCCs, which are also the countries where the most extensive urbanizations are taking place. Both intense urbanization growth and extreme weather conditions make here mandatory, more than everywhere else, an integrated strategy to achieve livable and sustainable cities through the fight against the urban heat island.

1 Intro: Heat Island and Urban Livability

The discovery of the physical phenomenon of heat islands dates back to 1833 when Luke Howard studied the air temperature in London and its surroundings. His research showed how, already at that time, winds were stopped and pulled up by intensive urbanization, reducing the quality of the outdoor environment while turning cold into warmer areas within urban settlements (Mills, 2008). The last centuries have confirmed how intense urbanization can make the temperature rise to several degrees, activating a vicious circle where car and energy use become more necessary and causing further temperature to rise. Undoubtedly, cities are—literally—heat islands if compared to their surroundings. Crescent environmental stress is the main challenge when targeting quality of life in today’s urbanization to enable the public space to welcome citizens and encourage their outdoor activities. It requires a consistent commitment to the built environment and awareness of the main role of public space as an interactive platform for a sustainable and human-centered smart city. In fact, it is the public space where social life shapes and grows. The public space was often defined as the smart city’s interface. Therefore, smart cities need to generate and maintain a welcoming, healthy, livable, vibrant public space to have a reason to be. The scope of smart cities is to create a space first and the infrastructure to strengthen the connections among people and between people and the place (Petrucci, 2022). Cities worldwide must respond to a growing and diverse population, ever-shifting economic conditions, new technologies, and a changing climate. The task becomes especially challenging in extreme climate environments, such as in the Far East, in the African continent, or in the GCCs, which are also the countries where the most extensive urbanizations are taking place. Both intense urbanization growth and extreme weather conditions make here mandatory, more than everywhere else, an integrated strategy to achieve livable and sustainable cities through the fight against the urban heat island.

2 Factors to the Urban Heat Island

The formation of urban heat islands (UHIs) in urban areas depends on numerous factors, and the assessment of the effect of each on temperature, both real and perceived, requires complex exponential calculations and numerical models. The complexity of the equation tells us that the issue can only be approached as a comprehensive process that takes into consideration the multiplicity of interactions among all factors. This is probably also the reason why the issue has not been properly addressed thus far, showing very few successful case studies worldwide. As demonstrated by Howard two centuries ago, the first cause of UHIs is urbanization itself at the density of the built masses. The increased density of human-made structures, surface materials drier than their surroundings and radiation of sensible heat, such as waste heat from vehicles and buildings, requires a complete rethinking of urbanism as a cooperative system and considering it as the only tool available to address and possibly solve the island heat effect; greater urban density means an increase in the anthropogenic sources of heat. Thus, the city’s massiveness is the first cause of urban heat islands, and its growing congestion requires addressing the problem with drastic solutions to reverse it and apply CO₂ negative solutions.

Heat is given off by buildings, transport systems, and industry by dense surface materials in the ground and buildings, which massing limits the wind flow and reduces its potential cooling effect, while the sealed soil facilitates fast evaporation. These last points alone are extremely important if considering that wind velocity can double over open water and wet surfaces. Sun exposure and direct and diffuse radiation are also important factors, where diffuse radiation is directly related to air pollution. Albedo, as the measure of absorbed heat during the day by each urban surface and its heat emission overnight, plays a crucial role in generating UHIs. This means that streets, sidewalks, roofs, and buildings’ walls release the accumulated heat and contribute to the formation of urban heat island days and nights. Even color can play an important role here, considering that black surfaces exposed to the sun can become hotter than white surfaces, on the order of 7–21 °C.

The abovementioned factors can be defined as passive factors to albedo rise. To the already extreme complexity in measuring the interaction among passive factors, the active factors to albedo rise must be added, such as the presence of heat waves related to district heating, traffic and industrial activity, and the high use of air conditioning. Peaks in temperature within the districts create peaks in heat islands through both passive and active factors, and the temperature increase makes it difficult for any outdoor activities, requiring more intensive use of cars and more intensive use of air conditioning. That is how the vicious circle is activated and further implemented.

To break the vicious circle of UHI generation and reverse it, it is essential to analyze the urban structure as a holistic body where passive buildings and a more extensive passive urbanism are aligned with the reduction of resource use through compactness. Unless the whole circle is taken into consideration and addressed through informed policies by the decision-makers, any solution will not lead closer to a Near Zero Energy (NEZ) city, only partially reducing the urban temperature and not offering outdoor livability in extremely hot climates.

3 Buildings’ Contribution to the Urban Heat Island

Considering the extreme case of southern countries, which are experiencing one-third of the years at a temperature above 40 °C, buildings are completely dependent on air conditioning to protect inhabitants from heat stress. In the UAE, for example, buildings consume more than 80% of the total electrical generation, where the cooling systems are responsible for approximately 70% of the building’s peak electrical load despite the government of Dubai initiating several efforts to improve building efficiency and move toward a more sustainable city (Biggart, 2021). The heat discarded by air conditioners in the air, on the other hand, contributes to dramatically increasing the street temperature. It can be locally measured in a range of temperature improvement from 1 to 4 °C, depending on the type of implant (Tremeac et al., 2012). This extra warmth gets piled on top of the urban heat island effect coming from other factors. A city of one million people can be as much as 3 °C hotter than the area immediately around it due to extra heat generated by air conditioning, and in fact, downtown buildings would require 25% more air conditioning than the same buildings in a rural area (Hyde, 2007). This means that the actual use of air conditioning generates a temperature rise in the districts and an exponential increase in energy consumption while offering a lower benefit to the inhabitants. It is truly a vicious cycle, a vicious circle of increasing street temperature and, therefore, increasing the air cooling demand. Furthermore, it has been demonstrated that in hot and dry cities, air conditioning use at night plays an important role in contributing to the nocturnal urban heat island and increasing with its cooling demands; having urban heat during the night means not allowing the natural cooling of the city due to a lack of thermal gap, making natural ventilation systems less efficient. Several studies have shown that the implementation of a comprehensive sustainable mitigation strategy for air conditioning use would achieve several objectives: a successful reduction in the urban heat island temperature by an average of 2 °C and a reduction in electricity consumption on a city scale while mitigating the urban climate and enhancing the environmental quality. The economic impact would also be significant: an estimation made for the Phoenix metropolitan area shows that successfully reducing the urban heat island temperature would bring at least 1200–1300 MWh of direct energy savings per day alone (Olson, 2014). The solution is not simple and requires some creative thinking, including research about reusing the heat and water wasted from HVAC (Salamanca et al., 2014).

Energy-efficient buildings require an integrated design approach from the very beginning of the design process and good integration of mechanical and electrical services with passive systems to avoid extra costs. Studies on energy consumption conducted in Europe found that a glass-to-wall ratio of less than 20% gives the minimum life-cycle cost of buildings (IEA, 2010). In the Middle East, it has been tested how building orientation and thermal insulation can save up to 20% in residential buildings and how the use of appropriate window glazing and orientation in high-rise office buildings saves up to 55% energy. Moreover, integrating natural ventilation can reduce energy consumption by up to 30% in villas to up to 79% in high-rise office buildings (Friess, et al., 2017). Long-term forecasts worldwide show that the heating energy demand is projected to decrease by 34% by 2100, while the cooling demand is estimated to increase by 72% over the same period, making air conditioning a necessity to maintain acceptable indoor comfort levels in wider geographical regions. The resulting electric load attributable to HVAC equipment accounts for 40% of the total annual average electrical load and up to 60% of the summer peak load (Krarti et al., 2018). The implementation of evaporative systems displays significant energy savings, and using mixed-mode ventilation in conjunction with a cooling tower for slab cooling makes night use of HVAC no longer needed, resulting in energy savings from 55% to 73% over the global variable air volume and up to 25% over the simple daytime mixed-mode system (Ezzeldin et al., 2013). The above results indicate that natural ventilation is an effective passive cooling strategy that should receive more attention to reduce the environmental footprint. Passive cooling is a large part of the vernacular architecture and the urban solutions of ancient civilizations; it requires expanding the design process as part of a broader sustainability agenda, including identity scouting of the so-called “cool vernacular” by linking the themes of social progress, technological and industrial transformation within a discussion of global and local trends, climate types, solution sets, and relevant low-resource utilization technologies. This type of holistic assessment of the environmental impacts of buildings includes not only the form and fabric of the building but also a wider set of parameters, such as the lifestyle of the occupant (Brophy et al., 2011). New solutions in more efficient air conditioning or electrical systems, such as controls and building energy management systems, are an important part of the ongoing research; however, these should be considered only after a comprehensive evaluation of passive solutions to eliminate energy leaking from the building. Building plans and forms emerge in a complex process where functional, technical, and aesthetic considerations all contribute to a synthesis. Wind, solar availability and direction, shelter and exposure, air quality, and noise conditions will inform the relationship of the building to its external environment and affect the form and design of the envelope. Bioclimatic heating, cooling, daylighting, and energy strategies should mesh at an early stage with the architect’s other priorities. The support of governments through incentives and other strategies is crucial, together with a strategic plan of developing an upgrade of the national industry to start a self-production of high-end and sustainable building construction elements.

Natural ventilation strategies and indirect lighting can be implemented, arriving at fully integrated building systems through high technology envelopes, sensors, and smart devices to support energy management. Finally, buildings can be upgraded through active systems for energy production by renewable sources: geothermal, natural air precooling/preheating, thermal mass, heat exchange systems, and photovoltaic. Governments must control this kind of strategic approach to avoid the propaganda effect of running into photovoltaic systems and any other manifesto approach without first respecting the right protocol for saving energy leakage. In contrast, it could have a boomerang effect on investments, sustainability, and social awareness, discouraging the public from obtaining the scarcest results despite a large investment (Petrucci et al., 2022). Measurement and assessment tools also play a significant role in defining the quality and not just the quantity in balancing inputs and outputs within the complexity of a holistic approach looking at the building as a part of the interconnected system, a fractal approach to the overall planning, and an objective within the city-planning and building regulatory framework, allowing policy changes to facilitate the process in both new and existing districts.

In the process of designing sustainable solutions for new buildings, the approach to the existing real estate offers a major challenge. Nevertheless, a massive intervention in existing real estate is essential to contribute to reducing the UHI effect and requires a combined short- to mid- to long-term approach strategy. Implementing the 4Rs of the Circular Carbon Economy Reduce, Reuse, Recycle, and Remove existing real estate addresses the existing gaps in the urban challenge toward Near Zero Emission and CCE Index (KAPSARC, 2020), which is true, especially in the assessment of the construction value chain. The reducing, reusing, and recycling approach to new and existing buildings must include the implementation of passive environmental solutions as the starting point in eliminating the source of excessive heat in the buildings and any cause of energy consumption. Otherwise, the implementation of the technological solution will only generate extra costs without an effective reduction in the environmental level. Passive solutions for existing buildings are the main part of their environmental retrofitting and can be applied visibly or invisibly. In the first case, these contribute to a reshaping of the building and eventually enhancing its volume or livable spaces; in the second case, the building gets an extra coating of insulating materials to reduce the heat transfer from indoor/outdoor and a replacement of windows and doors. Those actions would better follow a synergic plan and eventually concur to a completely new appeal from the building by encapsulating it into a new façade or adding extra volumes as an immediate return for the investment. In sustainable architecture, the link between building performance and the design of the envelope is critical while balancing more expensive building solutions and the improved balance between heat gain and heat loss. A life cycle cost analysis should be used to evaluate the contribution of ventilation openings, thermal functions of thermal mass, acoustic and energy protection, orientation and functional reasons of glazed elements self-shading, screening, and integration of photovoltaic technology. Once the causes of energy leakage in buildings are removed, the minimization of energy consumption comes from the Rs of recycling and reuse: starting from graywater recycling and optimizing materials and moving forward, the question links the construction of the built environment with a wider industrial and economic strategy. The prevailing economic model is linear and translates into raw materials being mined to manufacture components that are subsequently used and ultimately end as waste at the end of their lifecycle. The demand for raw materials is predicted to double by 2050. The reuse is based on the fact that not necessarily changing means that everything existing deserves to be trashed or cannot be repurposed through an incentive of locally based business in handcrafting. This might be the case with furniture but also with doors and windows. Recycling requires a bigger scale program and an industrial plan toward systemic recycling of construction materials, making urban communities a leading model in developing circular economy models.

4 A Holistic and Local-Based Approach to Fight the Urban Heat Island

Field research and data analysis essentially demonstrate the urban paradox: urban density is at the same time a cause and solution to sustainable living. Cities are economic powerhouses, resource consumption centers, and significant producers of greenhouse gas emissions. On the one hand, compact cities are the most sustainable living model in terms of soil and energy consumption, and density makes cities catalysts for economic and social opportunities; on the other hand, urban density is the first cause of generating urban heat islands.

It means we need density, and in the meanwhile, we urgently need to rethink the way we densify. Instead of recurring to partial solutions, it is crucial to work on synergic strategies, addressing consistent remedies to the urban form, as density’s generator, and the result of the complex interaction of interdependent pressures and influences: climatic, economic, social, and political, strategic, aesthetic, technical, and regulatory. Energy efficiency is part of an integrated search for sustainable development recognizing the local, regional, and global impact of cities on people and the environment. Planning decisions have a pervasive and long-lasting impact on social cohesion and the quality of life of the individual and the collective, locally and globally. A wider comprehensive approach to cities is needed, organically including a multiplicity of aspects concurring to it at the different scales of intervention; a comprehensive strategy from macro to micro scale, involving planning, urban design, architecture, and all their subdisciplines.

UHI is mainly researched and treated by urban planners and misses the importance of retrofitting the existing built environment to achieve a reduction in energy consumption and CO₂ emissions. The targeting of livable and walkable districts, as a matter of fact, requires a comprehensive approach, including macro- and microscale interventions that cooperate to minimize energetic consumption and the related CO₂ footprint for new and existing built environments. There are several critical relationships: among the buildings themselves; among the buildings and the topography of the site; and the overall harmony of behavior between buildings, vegetation, and natural and artificial landforms. When internal and external spaces are designed with bioclimatic aims, buildings and the space surrounding them react together to regulate the internal and external environment to enhance and protect the site, local ecosystems, and biodiversity. Aspects such as land use, density, transportation, green space, water and waste, energy, microclimate, site selection, site planning, building form, urban fabric, orientation, and facades are all components of the same macro project. Guidelines for an integrated effort of environmental requirements such as energy performance, heating, cooling, ventilation, lighting, indoor air quality, energy production elements, and the opaque transparent ratio of elevations must facilitate a holistic approach. It can be achieved in new constructions through the already consolidated and shared environmental labels and protocols such as SDGs and New Urban Agenda, LEED, BREAMS, and others, and—in the case of the existing districts—it can be further enhanced into a wider sustainable strategy, including a retrofitting of the built environment. Moreover, it contributes to a circular economy system that involves paradigm shifts in how urban environments are designed and built.

5 Conclusions and Recommendations

Density is a crucial factor in sustainability and UHI management, and cities are expanding faster than the urban population, with an urbanization growth rate of 1.8 versus the 1.2 rates of population growth. This phenomenon has generated urban sprawl, soil consumption, exploitation of natural resources and raw materials, and increasing production of CO₂. Peripheral districts load the environment and public administration with up to 10–100 higher costs in infrastructures than urban retrofitting. The extension of urbanized areas not only increases the environmental, construction, and management costs but also generates an expansion of the urban heat island, moving it from an urban to a regional level. If not properly addressed, the increase in urbanization up to 90% of the world population expected for the year 2050 will make UHIs a phenomenon not only limited to downtowns but also extended to peripheral and regional urbanized or semiurbanized areas.

Developing a dynamic urban growth strategy is the first step to densifying the city and achieving the expanded targets while also keeping the city from additional urban sprawl infringing on sensitive environmental ecosystems that have been there for thousands of years. The challenge of densification currently would be to curb urban sprawl. It would limit further land consumption and seal and generate controlled sprawl, with regular reviews of the defined borders according to expansion needs. We need smart growing cities based on multicentered and polycentric models targeting density, accessibility, and connectivity, planning along with the public transport system, and generating a strategic distribution of facilities and attractors (for citizens and investors) within the city. Implementation policies are required to encourage infill and urban regeneration, making the housing stock newly available within the city while preserving the historical flair of the existing heritage. New urban codes must acknowledge urban sprawl and restructuring the urban texture in terms of economic growth, sustainability, and quality of life. Being linked to measurable development outcomes, such as the enhancement of quality of life, the improvement of local economic opportunities, and growth with the reduction city’s carbon footprint, are crucial. It implicates the recognition of the importance of a form-based approach to urbanization and a systemic approach from macro to micro scale. Moreover, density upgrades require a conjunct effort between the public, providing the legal infrastructure and proper incentives, and private investors through public private projects (PPPs) to finance costly infill development and urban regeneration projects. Private-sector investment can be a good source of capital if the local government can ensure that private investments meet public needs. Moving further from PPP to the wider collaborative form of Participatory Public–Private Projects, governments can induce added value and capture the opportunity to build up urban culture and a sense of belonging from developers and stakeholder engagement to ensure effective locally based strategies toward a circular carbon economy and maximize the benefit from environmental resources targeting the NEZ. It would further increase small and medium businesses and activate a circular economy process, strengthening the collectivity and linking it with stakeholders and decision-makers. An urban approach including retrofitting strategies indirectly contributes to the other main parameters of NEZ urbanism.

The fight against UHIs brings up disruptive considerations in the design and management of cities and requires a reassessment of planning policies. To win the challenge, cities must develop tailored and locally based strategies for regulating the built environment through multicriteria evaluation systems facilitating real changes in public and private behaviors and moving their future users toward active and collective awareness and actions. To embrace the challenge, here is a list of recommendations, surely not exhaustive, that can possibly be further implemented:

• 1. Urban interventions to be handled at each phase of their planning as a complex system considering deep interactions between macro and micro scales within a comprehensive sustainable system around its circular economy.

• 2. Cities to apply the first R-Reduce as the first step toward Near to Zero Emission (NEZ) and Circular Carbon Economy (CCE) Index toward a holistic approach to managing emissions across energy systems and economies and to achieving carbon circularity.

• 3. Cities and their governments to move forward to the next 3R-Recycle Reuse Remove with integrated industrial and strategic planning on construction materials and their comprehensive life cycle management.

• 4. Cities and their governments should include air conditioning as a part of the climate challenge, as well as its possible solution, as a major contributor to greenhouse gas emissions, requiring more effective and widely used methods for passively cooling buildings.

• 5. Cities and their government to endorse and implement informed research on the so-called “cool vernacular” targeting a revival of indigenous knowledge in human-centered urbanism and architecture.

• 6. Environmental concepts to be expanded include macro- and microclimatic conditions to influence bioclimatic strategies as a greater synthesis between building elements and local climate conditions.

• 7. Nature-based solutions should be adopted as the main intervention in urban design and architecture for passive environmental strategies and implementing technologies at the final stage of the project.

• 8. Passive and active cooling/heating strategies to be coded generate a range of opportunities to improve the effectiveness of design outcomes.

• 9. Public space and smart public transport must be enhanced as mutual factors to reduce urban heat island effects and generate vibrant livable cities.

• 10. Cities’ reforestation should be encouraged throughout the city to contribute to human well-being and health and to environmental refurbishment.

• 11. Displacement economization should be made, allowing the diminution of particular vehicles and the reduction of gaseous emissions of pollutants.

• 12. Public and private sectors are co-creators, along with the people, in public–private projects and participatory public–private projects to activate a long-lasting urban culture and awareness.