Blending Nature with Technology: Integrating NBSs with RESs to Foster Carbon-Neutral Cities

Abstract

Nature-based solutions (NBSs) offer a promising framework for addressing urban environmental challenges while also enhancing social and economic resilience. As cities seek to achieve carbon neutrality, the integration of NBSs with renewable energy sources (RESs) presents both an opportunity and a challenge, requiring an interdisciplinary approach and an innovative planning strategy. This study aims to explore potential ways of achieving synergies between NBSs and RESs to contribute to urban resilience and climate neutrality. Focusing on the railway station district in western Thessaloniki (Greece), this research is situated within the ReGenWest project, part of the EU Cities Mission. This study develops a comprehensive, well-structured framework for integrating NBSs and RESs, drawing on principles of urban planning and energy systems to address the area’s specific spatial and ecological characteristics. Using the diverse typologies of open spaces in the district as a foundation, this research analyzes the potential for combining NBSs with RESs, such as green roofs with photovoltaic panels, solar-powered lighting, and solar parking shaders, while assessing the resulting impacts on ecosystem services. The findings reveal consistent benefits for cultural and regulatory services across all interventions, with provisioning and supporting services varying according to the specific solution applied. In addition, this study identifies larger-scale opportunities for integration, including the incorporation of NBSs and RESs into green and blue corridors and metropolitan mobility infrastructures and the development of virtual power plants to enable smart, decentralized energy management. A critical component of the proposed strategy is the implementation of an environmental monitoring system that combines hardware installation, real-time data collection and visualization, and citizen participation. Aligning NBS–RES integration with Positive Energy Districts is another aspect that is stressed in this paper, as achieving carbon neutrality demands broader systemic transformations. This approach supports iterative, adaptive planning processes that enhance the efficiency and responsiveness of NBS–RES integration in urban regeneration efforts.

1. Introduction

Integrating renewable energy sources (RESs) into urban environments is a key strategy for achieving the goals of energy transition and climate neutrality [1]. Cities consume the largest proportion of global energy, and, consequently, cities are responsible for 70% of global CO₂ emissions related to energy consumption [2,3]. Therefore, the deployment of RESs in cities is critical to reducing carbon emissions and improving energy autonomy. However, the application of RESs in densely built environments presents challenges, such as limited installation space, interaction with the built environment, and socio-economic parameters.

At the same time, nature-based solutions (NBSs) are increasingly being adopted in urban design to deliver multiple benefits and reduce climate risk in cities. NBS are tools that can enhance the resilience of cities by improving aspects like thermal comfort (urban heat island reduction), air quality, and water and flood management [4]. They are also increasingly promoted to enhance urban sustainability, as they can positively contribute to achieving various Sustainable Development Goals (SDGs), such as tackling poverty, good health and well-being, clean water, life on land, and reducing inequalities (e.g., in access to green spaces) [5,6].

As cities strive to transition toward more sustainable and resilient futures, the integration of NBSs with RESs is emerging as a promising strategy [7,8]. While NBSs primarily focus on enhancing ecosystem services and urban livability, RESs provide a critical pathway for reducing carbon emissions and fostering energy independence. Individually, both approaches contribute to urban sustainability; however, their combined implementation offers untapped potential for maximizing environmental, social, and economic benefits.

Recent literature and empirical applications highlight that combining NBSs with RESs can enhance ecosystem services and contribute to energy efficiency and pollution reduction [9,10]. Specifically, this can be achieved through (i) the creation of synergetic environmental benefits deriving from their combination, such as mutually reinforcing systems where NBSs can enhance RES efficiency; (ii) a comprehensive approach to tackling climate-related challenges; (iii) land use optimization and efficiency; and (iv) the creation of self-sufficient cities that rely less on external energy sources and costly climate-adaptation measures. Therefore, integrating NBSs with RESs offers a holistic sustainability approach, ensuring environmental protection, economic feasibility, and social well-being. However, the challenge remains regarding how we can achieve efficient NBS–RES integration at the urban level.

This study explores planning practices and strategies for integrating NBSs with RESs, emphasizing the need to consider and set interdisciplinary targets/criteria, such as urban environmental enhancement, social well-being, and spatial heterogeneity. Key design interventions analyzed in this paper include permeable pavements, tree canopies, green roofs, and blue infrastructure for water management, alongside RES solutions that are either embedded within NBSs or co-located in shared public spaces. A structured methodology—incorporating best-practice analysis, literature review, and spatial data analysis—guides the development of the proposed integrated NBS–RES framework. This study identifies innovative design typologies that merge NBSs with RESs while ensuring alignment with the unique characteristics of the local urban environment.

The western center of Thessaloniki, Greece, and particularly the railway station district, serves as the pilot area of this study. This district faces escalating challenges related to climate change, including heat island effects, air pollution, and flash flooding. It is also affected by mobility disruptions linked to the prolonged construction of the metro and sociο-economic disparities. Given its ongoing transformation, the area seems to be an ideal case for exploring how integrated NBSs (i.e., green infrastructure and green networks) and RES solutions (i.e., photovoltaic panels and solar shading) can enhance urban neutrality. Key areas of intervention include the transformation of public open spaces and the reconfiguration of neglected urban pockets, some of which contain underutilized green spaces.

The structure of this paper is as follows. Section 2 provides an extended literature review on the role of NBS and the ways in which they can be combined with RESs in urban settings. Section 3 describes in detail the case study of the railway station district of Thessaloniki, showing how an integrated NBS–RES framework can be developed. Finally, Section 4 discusses some key findings of this exercise in relation to potential urban projects that could be developed in the intervention area, whereas Section 5 presents a connection to the broader discussion of Positive Energy Districts (PEDS). This paper concludes by discussing how NBS–RES integration could be further developed and implemented to foster carbon-neutral cities, considering potential barriers, limitations, and spatial specificities (Section 6).

2. Literature Review

2.1. Designing NBSs in Urban Settings

According to the European Commission, NBSs are defined as “solutions that are inspired and supported by nature, which are cost-effective, simultaneously provide environmental, social and economic benefits and help build resilience” (https://research-and-innovation.ec.europa.eu/research-area/environment/nature-based-solutions_en (accessed on 23 April 2025)). Moreover, the International Union for Conservation of Nature stresses that these are “actions aimed at protecting, sustainably managing and restoring natural or modified ecosystems and which effectively and adaptively address societal challenges, while providing benefits for human well-being and biodiversity” [11]. Hence, NBSs offer an interdisciplinary and innovative approach to address social, environmental, and economic challenges faced by modern cities.

Their design and implementation integrate the principles of sustainability, biodiversity protection, and enhancement of ecosystem services, making them particularly important, especially in urban areas where adaptation to climate change is essential [12,13]. Specifically, NBSs promote biodiversity by enhancing food security, the health of citizens and ecosystems, and the protection and availability of clean water, as outlined in the Kunming–Montreal Global Biodiversity Framework, which attempts to cease and reverse nature loss by 2030 [14]. Moreover, NBSs encompass urban greenery, such as parks and street trees, which help mitigate high temperatures, regulate air quality, and manage water flows. They also include the allocation of natural habitat spaces in floodplains, which can play a crucial role in controlling and reducing the impacts of flooding [4]. Urban greening through NBSs is considered vital for improving life in cities and climate adaptation efforts, as it contributes to well-being, health, water quality, microclimate, and overall sustainability [15,16].

Even though urban planning has long recognized the importance of green spaces for city residents, the integration of NBSs and their ecosystem services into public policy is a more recent development. Usually, the term “green infrastructure” describes the spatial structure of ecosystems (green spaces) in the urban environment, while the term “nature-based solutions” describes human interventions that utilize the functions of these ecosystems (green infrastructure) to achieve environmental, social, and economic goals. These policies focus on evaluating potential outcomes, analyzing their direct and indirect contributions to human well-being, and assessing the benefits derived from specific ecosystem services [17,18,19]. However, there is often a lack of focus on how NBSs can effectively address specific vulnerabilities and challenges within urban environments characterized by high spatial heterogeneity, thus limiting the full exploitation of their potential for adaptation and resilience [20]. To achieve a successful implementation, it is essential to consider the local specificities and socio-economic conditions of the area. This ensures that solutions are not only tailored to local needs and characteristics but also embedded within a comprehensive planning framework that fosters sustainable and resilient development [6].

The design challenges of NBSs in urban environments are multidimensional and require an integrated approach. These include, inter alia, the creation of a strong and balanced evidence base capable of assessing the effectiveness of NBSs, especially in terms of their competitive and/or complementary relationship with other more traditional or conventional solutions that are usually technology-based. To the authors’ knowledge, even though spatial competition between NBSs and other urban elements has been identified as a potential drawback for their implementation, it has not been investigated in depth by previous studies. In addition, it is necessary to assess the long-term potential impacts of possible alternatives, the choice between which involves a series of trade-offs, such as the scale of application, the ecosystem services selected to be prioritized, and inequalities in benefit/cost distribution [21,22,23]. Emphasis should, therefore, be placed on effective NBS planning and management to prevent unintended yet foreseeable spatial consequences, such as gentrification.

The complexity of the design and implementation of NBSs calls for an integrated systemic approach and coordinated action. In this context, Dumitru et al. [24] underline that collaboration between scientific, political, and social actors is critical to their success. Adopting an integrated systemic approach means that NBSs should not be designed in isolation but rather complement and strengthen existing local policies related to resilience, sustainability, climate neutrality, and risk management strategies. In terms of its long-term effects, the design of NBSs requires systematic evaluation, both in terms of results (benefits) and the achievement of their objectives, as well as in terms of their ability to adapt to future climate and social changes [25,26].

On the neighborhood scale, challenges related to resilience (e.g., flooding, urban heat island effects, the reduction of air pollution) and climate change mitigation (e.g., achieving a climate-neutral city/neighborhood) should be addressed at the local level [27,28]. These may include measures and solutions in buildings, streets, and open public spaces that should be as extensive as possible to improve the quality of life of residents and to be accepted by the local community [29]. On this scale, it is very important to consider spatial heterogeneity and individual neighborhood characteristics to derive the most appropriate NBSs. To achieve this, a thorough analysis of the study area is essential for identifying a typology of available spaces suitable for green and energy interventions. This typology will serve as a foundation for selecting appropriate NBS categories based on their suitability and effectiveness.

2.2. Integrating NBSs with RES Technologies

As stated at the beginning of this study, integrating RESs in urban environments is critical for achieving energy transition and climate neutrality [1]. The most common RES applications in cities include photovoltaic (PV) systems, urban wind energy, and geothermal energy. Photovoltaics, particularly building-integrated PV (BIPV), are widely used due to their ease of installation and low maintenance requirements [30,31]. Small vertical wind turbines, despite challenges such as noise pollution and visual disturbance, have also emerged as feasible solutions for dense urban areas [32]. However, the efficient integration of these technologies into urban environments still remains complex due to space limitations, regulatory barriers, and socio-economic considerations [33].

Integrating NBSs with RES technologies presents an innovative approach to creating resilient and sustainable urban ecosystems. This integration can contribute significantly to achieving climate neutrality in cities while simultaneously improving the microclimate and enhancing biodiversity. Recent academic literature highlights the potential for creating synergies between these two domains, suggesting that their integration can provide multifaceted benefits, including improved microclimate conditions, enhanced biodiversity, and increased energy efficiency [7,8]. For example, green roofs or green corridors with photovoltaic panels can be used to generate renewable energy but also to improve the visibility and accessibility of green spaces, thus promoting their use by urban residents [8]. Similarly, solar urban gardens incorporate smart solar trees and solar-powered lighting to enhance public spaces while producing renewable energy [8,34]. Another emerging solution is the biomimetic design of wind turbines in the form of trees, which blend aesthetically with urban landscapes while harnessing wind energy with minimal visual disturbance [35]. A study conducted in Zurich, Switzerland, found that green roofs can lead to a higher annual energy yield from PV systems compared to conventional roofs [34]. Additional evidence highlights the potential of community solar gardens to provide equitable access to renewable energy, particularly for low-income communities, by utilizing existing green spaces in urban areas [36]. Similarly, Hakimizad et al. [37] show that solar panels in urban green spaces offer year-round benefits, such as winter heating, summer cooling, and nighttime lighting. Likewise, the use of biomass and geothermal energy in parks and green infrastructure can reduce reliance on fossil fuels, as demonstrated by projects such as the Eden Project in the UK [38].

Below, we present some key applications of NBS and RES integration, indicating a general description, the RES solution, and the NBS applied in each case (

Table 1. Examples of combining RES and NBS in different cities around the world.

City	Description	RES Solution	NBS Solution
Gardens by the Bay, Singapore	A 101-hectare urban green space with iconic structures like Supertree Grove and Flower Dome, integrating nature into the city.	Supertrees use solar energy and biomass for power; greenhouses have energy-efficient cooling.	Features 1.5 million plants, vertical gardens, and water management systems for biodiversity and climate resilience.
The Eden Project, Cornwall, UK	An environmental and educational site with massive biomes replicating different climates.	Uses geothermal energy, solar panels, wind turbines, and biomass boilers for power.	Recreates ecosystems, supports biodiversity, and employs rainwater harvesting and wetlands for water management.
The Green Spine, Melbourne, Australia	A sustainable skyscraper development with two towers covered in vertical gardens.	Solar panels, wind power, and smart shading optimize energy use.	Vertical gardens insulate, support biodiversity, and utilize rainwater and greywater for irrigation.
Bosco Verticale, Milan, Italy	Twin residential towers integrating extensive vegetation for urban reforestation.	Solar panels and geothermal heating improve energy efficiency.	Houses 900+ trees and 20,000+ plants for air purification, noise reduction, and biodiversity.
Masdar City, UAE	A low-carbon, zero-waste eco-city near Abu Dhabi, focusing on sustainability research.	Solar farms, rooftop photovoltaics, wind cooling, and waste-to-energy tech.	Green roofs, urban agriculture, and shaded walkways enhance biodiversity and cooling.
Shanghai, China	A city integrating natural elements to boost sustainability and resilience.	Expanding renewable energy and energy-efficient technologies.	Green corridors, planted roofs, and vertical forests improve air quality and provide green spaces.
Freiburg, Germany	A model sustainable city with a focus on renewable energy and eco-friendly architecture.	Photovoltaic roofs, with many energy-positive buildings.	Large green belts, parks, and rainwater collection for irrigation and cooling.
Hammarby Sjöstad, Sweden	A former industrial suburb transformed into a low-impact, sustainable district.	Solar, biogas, and lake water for district heating and cooling.	Canals and green corridors enhance biodiversity and ecosystem connections.
Kalundborg Symbiosis, Denmark	A leading industrial symbiosis model where waste becomes a resource.	Uses biomass and wind power for energy.	Wetlands naturally treat waste and support local biodiversity.

Source: Authors’ elaboration.

). We can see that there is a diversification when applying synergies between NBSs and RESs in different urban settings, considering the available spaces for interventions as well as the needs for and types of climate mitigation aspects.

Despite their potential, integrated NBS and RES solutions face several challenges. The spatial and temporal mismatch between energy production and consumption necessitates efficient energy storage and grid management strategies [39]. Advancements in technology continue to offer innovative solutions, but they also bring new challenges and limitations that need to be addressed for optimal efficiency [40]. For example, Tasneem et al. [41] examine the suitability and efficiency of different urban wind farm models and conclude that commercial wind farms are difficult to replace with urban wind farms due to the chaotic and turbulent nature of city winds, which limits their performance. Additionally, balancing aesthetic considerations with technological advancements remains an ongoing challenge, particularly in densely populated urban areas.

3. Methodological Approach and Case Study Analysis: The Railway Station District of Thessaloniki

The ReGenWest methodology is guided by two contemporary design paradigms. On the one hand, urban sustainability and climate-sensitive planning, as articulated in LEED v4.1 and the forthcoming LEED v5 standards (https://www.usgbc.org/leed/v5), which emphasize the coherence and interdependence of sustainability measures across city, district, and building scales, support an integrated approach that aligns environmental performance with broader urban planning objectives. On the other hand, the smart city and design thinking approach promotes rapid, iterative design cycles and continuous stakeholder and user engagement, which foster continuous feedback and improvement, ensuring that solutions remain adaptable and responsive to both environmental goals and community needs.

The design process of the ReGenWest project includes three subsequent iterations, each building upon and improving the previous one. The first iteration focuses on problem definition, mapping the renovation area, identifying spatial opportunities and potential types of intervention, and conducting an extensive literature review of relevant cases. This phase concludes in the development of a renovation model tailored to the local context. The second iteration presents the selected model to stakeholders, users, and investors, aiming to incorporate considerations of diversity and inclusion, as well as to identify financial opportunities, barriers, and viable business models. The third iteration optimizes the proposed solutions and projects with respect to cost and carbon emissions, codifies the renovation master plan, and estimates the direct environmental impact using an environmental monitoring system.

In the following sections, we present the outcomes of the first iteration process focusing on the mapping of the intervention area, the identification of open space elements, and the potential types of NBS and RES interventions.

3.1. Analysis of the Intervention Area and Open Space Typologies

The intervention area, located in the western part of Thessaloniki (Figure 1), retains a neighborhood character with old high-rise social housing and mid-rise apartment buildings (3–4 floors) mixed with commercial and city-center land uses. It is marked by underutilized open spaces, many of which serve as informal parking due to the lack of private parking and the expansion of the metro and the national railway facilities (OSE). Despite the presence of permeable soil, the area lacks well-designed public spaces that meet residents’ needs, leaving lower-income communities, often living in cramped housing, to extend private activities into shared spaces. Existing urban squares, parks, and roadside areas require modifications to improve resilience against intensifying climate challenges such as heat waves, floods, and strong winds. Similarly, schoolyards—currently dominated by concrete and minimal greenery—need improvements for climate adaptation. Vulnerable groups, including children, the elderly, a small Chinese community, women, parents, and people with mobility restrictions, face exclusion due to poor accessibility, inadequate amenities, and safety concerns. Those experiencing energy or food poverty would benefit from spaces for socialization, urban agriculture, and renewable energy generation. Additionally, the many unused rooftops and open spaces present opportunities to enhance ecosystem services, create community spaces, and develop sustainable infrastructure. The ongoing design process aims to reclaim and reimagine these neglected spaces, fostering a more inclusive, livable, and climate-resilient urban environment.

Our analysis identifies 11 typologies of open spaces suitable for green and renewable energy interventions. These typologies are classified and correspond to specific locations, numbered according to Figure 2. Apart from the identified typologies, throughout the whole intervention area, there are also roadside trees requiring densification and the widening of the natural/perforated roofs of apartment buildings and uncovered apartment buildings/plots.

3.2. Identification of Potential NBS Solutions

As a next step, we identified specific NBS categories that are suitable for this intervention area. More specifically, we explored solutions tailored to the urban environment, with a particular focus on the dense urban fabric of the study area. For this purpose, the typology used by the European Commission [4] was chosen as a basis of our analysis, comprising six main general categories of NBSs, each containing several solutions. The six main categories are (i) green solutions in buildings; (ii) urban green areas connected to grey/man-made infrastructure; (iii) parks and (semi-)natural urban green areas; (iv) community land and community gardens; (v) agricultural land; (vi) green areas for water management; and (vii) blue infrastructure/water areas [42,43,44]. Given the urban nature of our case study, we excluded category (v), which pertains to agricultural land, as it does not apply to the intervention area. Additionally, the last two categories (vi and vii) have been combined into a single category (“Blue and Green Areas for Water Management”) with water as a central component. Finally, several subcategories of solutions, primarily related to blue infrastructure, have been removed, as they are not relevant to the study area, which lacks natural water bodies such as lakes or streams.

Beyond identifying the categories mentioned above, another important step was to connect them with specific subcategories acting as solutions tailored to the needs and characteristics of the study area. To perform this correspondence, the World Bank’s “NBS Directory for Urban Resilience” [45] and the “Technical guide to NBS” of the UNaLab—Urban Nature Labs research project [46] have been used as key sources. The identified NBS solutions and their correspondence to the general NBS categories are presented in

Table 2. Identification of potential NBS solutions adapted to the intervention area needs and specificities.

General NBS Category	NBS Solutions
Green buildings: Include practices and elements designed to create or integrate green features in buildings.	⮚ Green walls: Green facades or planted structures on exterior walls, which offer aesthetic value, reduce heat, and improve air quality. These are divided as follows: Ground-based green walls: Vertical plantings rooted in the ground are often used to shade or reduce heat on exterior walls. Facade-bound green wall: Plantings directly supported by the building wall, improving thermal insulation and aesthetics. ⮚ Green roofs: Surfaces covered with vegetation that help improve thermal insulation, rainwater absorption, and biodiversity. These are divided as follows: Facade-bound green walls: Lightweight roofs with low-maintenance plants, ideal for large surfaces. Intensive green roofs: Roofs with the ability to support large plants or even trees, offering greater biodiversity. Smart roofs: Green roofs that integrate technologies such as sensors, automation, and water management systems to optimize energy efficiency, rainwater management, and temperature. ⮚ Green balconies: Plantings on balconies that contribute to the aesthetics and cooling of the building while improving air quality. ⮚ Atriums: Interior space with natural light that often includes plantings, creating a green environment within the building. ⮚ Green pavements and green parking pavements: Outdoor surfaces of buildings that allow water infiltration, often covered with grass or other plants, contributing to rainwater management. ⮚ Green fences and noise barriers: Constructions with plantings that are used as plots/property boundaries or to reduce noise while enhancing aesthetics and biodiversity.
Urban green areas connected to grey infrastructure: Green areas within the urban environment that are directly connected to grey infrastructure, such as roads, railway lines, schools, or parking lots. These areas combine functionality with environmental benefits.	⮚ Tree alleys and street trees/hedges: Trees or tall shrubs along roads that provide shading, reduce heat, and improve air quality. In other studies/surveys and classifications, this subcategory is also referred to as street tree canopies or group of trees. ⮚ Street green and green verge: Green corridors or small green belts along roads, usually for beautification or environmental management. In other studies/surveys and classifications, this subcategory is also referred to as green avenues, boulevard trees, and urban green corridors. ⮚ House gardens: Private gardens around houses offering green spaces. ⮚ Green spaces near railway tracks (Railroad banks): Plantings along railway tracks that reduce erosion, enhance biodiversity, and absorb noise. ⮚ Green playgrounds/school grounds: Green spaces that enhance children’s well-being and contact with nature in their everyday environment. ⮚ Green parking lots: Parking spaces designed with green elements, such as permeable surfaces or plantings, to reduce heat and better manage rainwater.
Parks and (semi-)natural urban green areas including urban forests: This category refers to large or small natural areas within cities that include parks, forests, and other green infrastructure, promoting biodiversity, recreation, and environmental management. These areas are critical for enhancing climate resilience and quality of life.	⮚ Large urban park: Extensive green areas within cities. ⮚ Neighborhood green space: Smaller-scale parks or green areas that serve the needs of local communities/neighborhoods. ⮚ Small and pocket parks: Small green areas within densely populated and space-restricted urban centers. ⮚ Botanical gardens: Specially designed areas for the conservation and presentation of plants and trees, aiming for education and recreation. ⮚ Institutional green space: Green areas belonging to educational or public institutions (beyond school yards). ⮚ Church yards. ⮚ Sports facilities (Green sport facility): Areas that combine sports activities (e.g., stadiums, fitness equipment, running tartans) with nature and urban greenery. ⮚ Shrublands: Natural areas dominated by shrubland. ⮚ Abandoned and derelict areas with patches of wilderness: Areas that gradually return to nature, often a refuge for wildlife.
Allotments and community gardens: Describes solutions that focus on participatory cultivation and creation of green spaces, promoting social cohesion, sustainability, and food production.	⮚ Community vegetable gardens: Areas for collective cultivation and food production, with actions that promote social participation and sustainable management. ⮚ Community (urban) gardens (Horticulture): Describes the systematic cultivation (not necessarily of edible species) for biodiversity, education, and recreation within urban areas.
Blue and Green areas for water management: This category includes nature-based solutions intended for stormwater management (sustainable urban drainage systems) and flood prevention. They combine elements of water (blue infrastructure) and green (green infrastructure). In literature they often appear as sustainable urban drainage systems or as infiltration, filtration and biofiltration structures.	⮚ Rain gardens: Landscaped (usually small) green spaces that collect and absorb rainwater, reducing flooding and improving water quality. They are usually planted with aquatic or hydrophilic plants, which withstand wet conditions and contribute to the biological degradation of pollutants. ⮚ Swales/Filter strips: The (planted) channels (grooves) collect and guide rainwater, allowing it to be absorbed into the soil and reduce erosion. Similarly, filtration zones are planted areas that are placed along runoff surfaces (e.g., roads, parking lots) to reduce rainwater flow and absorb pollutants before they reach water bodies. They are often referred to as bioswales, i.e., as biological grooves/channels. Soil infiltration and rainwater purification (World Bank, 2021). ⮚ Small (artificial) lakes/ponds: Small man-made (or sometimes natural) lakes used to store and manage water while also providing an environment for biodiversity. They contribute to the collection and storage of rainwater, the improvement of local ecology, and the provision of neighborhood recreational areas. They are often divided as follows: (a) Detention ponds capture and temporarily store rainwater during periods of heavy rainfall [47] and evacuate it at a specified time. These lakes can be attractive graphic elements in public spaces, e.g., around playgrounds and sports grounds. (b) Retention ponds are are permanently full of water and have planted ends. They collect rainwater from surrounding areas, add storage capacity, and relieve pressure on surface water treatment and sewage systems while providing urban habitats and enriching the diversity of public green spaces. ⮚ Permeable pavements: Surfaces that allow rainwater to penetrate and be absorbed into the soil, reducing surface water concentration and the likelihood of flooding.

, each defined by distinct characteristics and functions. This approach enables the exploration and integration of diverse solutions tailored to the specific needs and characteristics of the Thessaloniki railway station district’s urban fabric.

3.3. Matching Open Space Typologies, NBS Solutions, and Ecosystem Services

As a final step of this exercise, we performed a matching between the identified open space typologies (Figure 2), NBS solutions (Table 2), and the ecosystem services relevant to the railway station district of Thessaloniki. Ecosystem services refer to the ecological characteristics, functions, and processes that contribute directly or indirectly to human well-being through various pathways [48]. The ecosystem services approach highlights nature’s role as an active participant in shaping our health and quality of life [49]. In this context, NBS solutions are expected to provide different ecosystem services depending on the characteristics of the urban environment where they are applied [50].

The identified ecosystem services that apply in our case study include cultural, regulatory, provisioning, and habitat and supporting services. More specifically, cultural services relate to entertainment, aesthetic pleasure, cultural heritage, and spiritual and inspirational value. Regulatory services incorporate aspects related to the management of air quality; urban microclimate regulation; the mitigation of environmental hazards, such as flooding and heat stress; and the control of invasive or parasitic species that affect the area’s ecological resilience. These services are increasingly supported through NBSs, such as green corridors and permeable surfaces, which not only enhance ecological resilience but also contribute to carbon neutrality when integrated with RESs like solar-powered infrastructure and energy-efficient transit systems. The provisioning type of ecosystem services includes food, water, medicines, and other resources derived directly from nature. In this case, NBSs in urban environments mainly concern community gardens, green terraces, and other micro-farming. Finally, habitat and supporting services include those of habitat provision, biodiversity conservation, nutrient cycle, and soil protection and conservation. The matching results are given in

Table 3. Matching intervention site open space typologies, NBS solutions, and ecosystem services.

Open Space Typology	Relevant NBS Solutions	Ecosystem Services
1-Empty spaces/plots and spaces used as parking	Green playgrounds and yards, green parking Neighborhood parks, pocket parks Permeable pavements	Cultural
		Regulatory
		Provisioning
		Supporting
2-Primary and secondary education schools	Green gardens, extensive and smart green roofs, green walls Community urban gardens Permeable pavements	Cultural
		Regulatory
		Provisioning
		Supporting
3-Higher education facilities	Green walls Rows of trees As the area is not open to visitors, the solutions proposed concern the boundary with the public space	Cultural
		The current picture of abandonment of the old building of the veterinary school creates an impression of abandonment and insecurity which could be reversed with some green interventions.
4-Urban squares that require modifications for better performance in extreme weather events	Rows of trees and individual trees near roads, green spaces near roads Permeable pavements	Cultural
		Regulatory
		Provisioning
		Supporting
5-Small/pocket and activity parks	Neighborhood parks and green spaces, small and pocket parks, sports facilities	Cultural
		Regulatory
		Provisioning
		Supporting
6-Uncovered areas around old social housing complexes	Community urban gardens and vegetable gardens	Cultural
		Regulatory
		Provisioning
		Supporting
7-Roadside tree plantings	Urban green areas connected to grey (artificial) infrastructure Permeable pavements, channels, and filtration zones	Cultural
		Regulatory
		Provisioning
		Supporting
8-Roofs of apartment buildings	Extensive and smart green roofs	Cultural
		Regulatory
		Provisioning
		Supporting
9-Uncovered apartment buildings/plots	Green patios Private gardens, green parking areas	Cultural
		Regulatory
		Provisioning
		Supporting
10-OSE Station	Rows of trees and individual trees near roads; green spaces near roads; plantings along railway lines that reduce erosion, enhance biodiversity, and absorb noise; green parking spaces Small and pocket parks Permeable pavements	Cultural
		Regulatory
		Provisioning
		Supporting
11-Churches with courtyards	N/A (Churches will not be included in the scope of this project)	N/A

Source: Authors’ elaborations.

below. As we can see, all the identified ecosystem services are affected by the application of NBS solutions, indicating a different mixture each time, depending on the open space typology and its characteristics.

4. Integrating NBSs with RES Technologies: Replicable Solutions for Urban Districts

Taking stock of the interventions in the pilot area, we further specify a concrete set of integrated urban projects for combining NBSs with RESs to advance carbon neutrality. A detailed description of the identified project types that can serve as a replicable model for other city districts, thereby increasing urban sustainability and achieving carbon neutrality goals, is provided below.

4.1. Developing Green and Blue Corridors

A holistic approach is necessary to maximize the impact of urban greening. Rather than treating green spaces as isolated features, connecting them through corridors ensures that ecosystem services extend beyond individual parks and squares. These interconnected spaces provide recreational, ecological, and climate resilience benefits [51]. Well-designed green corridors enhance biodiversity, improve air quality, and promote healthier urban environments. They also play a critical role in stormwater management and flood resilience. Green and blue infrastructure, including permeable pavements, bioswales, rain gardens, urban tree cover, small ponds, and wetlands, offers an effective alternative to conventional stormwater systems [52]. These features help retain stormwater, reducing the risk of flooding while simultaneously improving water quality and groundwater recharge.

Another significant benefit is their ability to mitigate urban heat. When designed with adequate length and width and strategically positioned within the urban fabric, green and blue corridors create cooling effects [53]. They provide shade, reduce the urban heat island effect, and promote air circulation, leading to more comfortable urban environments. In addition to climate resilience, green corridors promote sustainable mobility and public health. They encourage walking, cycling, and other forms of micromobility, fostering low-emission transport options [54,55,56]. These spaces enhance urban livability by creating safe, pleasant routes for pedestrians and cyclists. Improved accessibility and safety contribute to healthier lifestyles and increased social interaction [55,57].

Furthermore, urban green corridors serve as vital habitats for flora and fauna. Research indicates that these spaces support bird populations, which contribute to residents’ well-being as part of cultural ecosystem services [58]. The protection of pollinators, such as bees, has also been recognized in urban planning initiatives [59]. Studies further show that fragmented green spaces negatively impact biodiversity, whereas ecological corridors help mitigate habitat loss and species decline [60,61]. Additionally, maintaining healthy ecosystems within cities strengthens resilience against invasive species and pests.

Beyond the environmental benefits, green and blue corridors can enhance economic and social dynamics by linking green spaces with cultural landmarks, commercial areas, and community hubs. Integrating natural elements into urban planning creates multifunctional spaces that improve quality of life, stimulate local economies, and foster social cohesion [55]. Additional research highlights the strengths and challenges of green corridors across Europe and the United States, while recent studies have started to explore their potential in regions such as Malaysia and Saudi Arabia [56].

The ReGenWest intervention area highlights opportunities to implement practices for climate adaptation in cities. The existing network of pedestrian pathways, parks, yards, squares, and vacant plots makes this area particularly well suited for the implementation of green and blue corridors. These corridors will be effectively integrated into the local landscape, enhancing cooling, managing stormwater, and improving urban resilience while also delivering broader environmental and social benefits to the city.

4.2. Enhancing Urban Greenery at the Eye Level

The concept of the “city at the eye level” emphasizes the experiential quality of urban environments from a pedestrian perspective, focusing on the creation of sustainable, visually engaging, and interactive public spaces that foster resilience, social interaction, and comfort [62]. In this context, densifying urban greenery is not simply decorative but also a fundamental aspect of human-centered urban design that enhances sensory richness, psychological well-being, and environmental resilience. This need becomes particularly pressing in densely populated urban environments, like the study area, where creating greener walkable spaces is crucial. Effective urban design methods—such as integrating greenery through street trees, vertical systems, pocket parks, and green street furniture—can transform the urban fabric, making it more pedestrian-friendly and stimulating. These strategies are especially relevant in Thessaloniki’s railway station district, where current sidewalk tree conditions, land cover constraints, and urban heat stress phenomena highlight the urgent need for context-sensitive greenery solutions that promote safety, resilience, and quality of life.

In the case study area, sidewalk trees are severely constrained by cement, which limits root growth and makes them vulnerable to collapse during extreme weather events. The removal of numerous trees without proper replacement exacerbates climate-related risks, including urban heat, flooding, and wind damage, and moves the area further away from its climate neutrality goals. This was highlighted in late 2024, when several trees were uprooted by strong winds, posing a danger to pedestrians and causing property damage. Additionally, the lack of exposed soil prevents proper water absorption, contributing to frequent flooding: 208 incidents were recorded between 2012 and 2018, with an increasing frequency in areas like the railway station. Beyond providing cooling and shading, tree roots and permeable surfaces are essential for pedestrian safety and climate resilience. In Thessaloniki, sidewalks are often rendered hazardous by dislodged cement and tiles, a direct result of constrained root systems.

Densifying street trees is, thus, a cornerstone of this strategy by offering shade, aesthetic appeal, and ecological benefits [63] but also encouraging outdoor activity, supporting physical and mental health, enhancing life satisfaction, and generating positive economic impacts on the economy of the nearby streets and blocks [64]. To maximize the benefits of this approach, it is essential to select climate-resilient species that provide dense, effective canopies while also addressing the key challenges associated with implementing street trees as NBSs in dense urban environments. These challenges include the potential obstruction of wind flow, damage to public property from root-induced pavement cracking (a significant issue in the study area that also creates tripping hazards for pedestrians), and the risk of triggering allergies from tree pollen [65]. Thoughtful species selection, strategic placement, and innovative design solutions are, therefore, critical to overcoming these obstacles and ensuring long-term success.

Another prominent strategy is the implementation of vertical green facades and green walls, which are particularly effective in high-density urban environments where horizontal space is constrained. These systems can be integrated into existing building exteriors, maximizing the use of available vertical space and contributing to urban cooling, noise reduction, and air purification while also enhancing the visual appeal of building facades and enriching the sensory experience for pedestrians [66]. Additionally, they provide valuable support for urban biodiversity, fostering a more sustainable urban ecosystem. A similar and very innovative solution to the challenge of limited ground-level green space in high-density areas is green shades. Green shades are a system of tensile sails covered with vegetation, offering an optimal solution on shopping streets, terraces, and squares where, due to a lack of space or the difficulty of intervention, trees or other types of vegetation cannot be installed. Finally, incorporating greenery into street furniture, such as benches and lighting columns, enables the flexible and adaptive greening of public spaces. All these NBS solutions are highly relevant to our case study, as they are specifically tailored to fit highly urban environments with limited space availability.

The densification of trees along urban roads plays a critical role not only in enhancing the pedestrian experience but also in advancing climate neutrality goals through carbon sequestration. Trees act as natural carbon sinks, absorbing CO₂ from the atmosphere and storing it in their biomass and surrounding soil. When strategically planted in dense formations along streets, they increase the cumulative carbon uptake potential of the urban landscape [67]. This process contributes directly to reducing atmospheric CO₂ concentrations, thereby complementing emission-reduction measures. In dense city areas like Thessaloniki’s railway station district, where available land for large green spaces is limited, roadside tree densification offers a highly effective and space-efficient solution. Furthermore, long-term studies show that even modest increases in urban tree canopy coverage can significantly enhance the city’s overall carbon storage capacity while mitigating the urban heat island effect [68].

Enhancing and densifying urban greenery at the eye level requires a multifaceted approach that considers various factors, including context-specific design, a multi-solution approach, technological advancements, and pedestrian-centric planning. A context-specific design approach emphasizes the need to tailor urban greening strategies to local urban morphology and climate conditions [69]. For example, factors such as urban morphology, shading, and local wind patterns are critical for ensuring that greenery not only thrives but also serves its intended purpose in the urban environment. Incorporating advanced technologies, such as remote sensing and deep learning, can further optimize the placement and performance of greenery interventions [69]. These technologies allow for a more precise understanding of factors like sunlight, shade, temperature, and wind, enabling planners to design spaces that maximize the benefits of greenery.

A multi-solution approach is also essential for densifying greenery at the eye level in dense urban areas, combining various categories of ground-level plantings and vertical greening systems. This strategy maximizes the available space while creating a more engaging urban environment for pedestrians. Ground-level greenery, such as street trees and shrubs, offers direct interaction with nature, while building-integrated green systems like green walls, vertical gardens, and green shades bring nature into architecture and provide more ecosystem services per unit of available space (e.g., contributing to temperature regulation, air quality, aesthetics, etc.). Pedestrian-centric planning is also a crucial element in enhancing urban greenery at the eye level. Hence, by placing plants and green elements where they can be easily interacted with—such as along sidewalks, on building facades, or integrated into street furniture—encourages people to connect with nature in their daily lives as well as promotes movement throughout the city (local neighborhoods), especially during hot summer days.

4.3. Integrating NBSs and Ecosystem Services into Metropolitan Mobility Infrastructures

Metropolitan mobility infrastructures, including central train and bus stations, airports, and other transportation hubs, are critical components of urban development and economic activity. However, these infrastructures often generate significant environmental pressures, such as air and noise pollution, habitat fragmentation, and increased land consumption [70]. As cities face escalating challenges from climate change, biodiversity loss, and urban sprawl, the integration of NBSs and ecosystem services into metropolitan mobility infrastructure presents an innovative and sustainable approach to urban planning and resilience [71]. Rethinking mobility infrastructure in terms of ecological and environmental performance is essential for reducing the negative impacts of urban mobility systems while enhancing the resilience and ecological health of metropolitan areas [72].

Mobility infrastructures in metropolitan areas are closely linked to several environmental and ecological challenges. These include air and noise pollution, land consumption and urban sprawl, habitat fragmentation and biodiversity loss, and a high level of climate vulnerability. Transportation hubs are major sources of greenhouse gas emissions, contributing to climate change and deteriorating urban air quality [73]. The physical footprint of transport infrastructures often results in the fragmentation of natural habitats, reducing landscape connectivity and impeding species migration [74]. This, in connection with land consumption, can disrupt local ecosystems, both natural and human, by creating discontinuous and unfamiliar places. Mobility infrastructures are vulnerable to climate-related events such as heat waves, flooding, and storms.

Integrating NBSs and ecosystem services into metropolitan mobility infrastructures requires a shift from traditional grey infrastructure (e.g., concrete and steel) to hybrid or green infrastructure solutions that incorporate natural elements and processes under sophisticated landscaping strategies. One effective strategy for integrating NBSs into mobility infrastructure is to rethink and redesign them in strong connection with the preexisting landscape. Linear parks, vegetated embankments, and green roofs on transport stations can enhance urban biodiversity while mitigating air pollution and noise. For example, the Thessaloniki Railway Station could be redesigned to include green roofs and vertical gardens, which would enhance biodiversity, provide insulation to reduce energy consumption, and improve air quality. Moreover, planting native vegetation outside the station building or along railway lines and roadways could restore ecological connectivity and create microhabitats for urban natural life.

There are many pioneer examples that follow this design process, such as the Singapore Rail Corridor by MVRDV or the most recent design project for the expansion of Athens International Airport (AIA) by the Anemos Design Consortium, consisting of Grimshaw, Haptic, K-Studio, Arup, Leslie Jones, Triagonal, and Plan A. The Singapore Rail Corridor is more than a park connector or a trail; it is an inspired community space offering the extraordinary experience of the reinvention of hidden spaces within Singapore to inspire movement and connection, intuitive play, and new ways of experiencing the environment [75]. On the other hand, the design approach for AIA’s expansion, rooted in a deep respect for the generous Athenian–Mediterranean landscape, prioritizes sustainability and operational excellence, building on AIA’s commitment to net-zero operations and low-carbon principles. With a rich landscaping strategy and an infusion of daylight in all parts of the terminal building, the sense of place is always dominant [76]. Other solutions may include integrating bioswales and rain gardens around the Thessaloniki Railway Station, which could reduce surface runoff, improve water quality, and create green spaces for public use. Additionally, restoring natural floodplains along transport corridors would enhance water retention and reduce the vulnerability of transport systems to extreme rainfall events.

Transforming metropolitan mobility infrastructures into multifunctional spaces that integrate ecosystem services can increase urban resilience and adaptability to climate change and provide social and economic benefits. This shift toward ecological urbanism in transport infrastructure requires adaptive governance, innovative financing mechanisms, and stakeholder collaboration.

4.4. Establishing Virtual Power Plants for Smart, Distributed Energy Management

The ReGenWest renovation area presents multiple opportunities for local, distributed renewable energy production, primarily through the installation of solar panels. The most promising locations for integrating RESs are the flat rooftops of apartment buildings. These rooftops offer a valuable opportunity not only to generate clean energy but also to create shared green spaces within a dense urban environment. Their direct exposure to sunlight makes them ideal for capturing solar radiation, while incorporating green roof systems helps mitigate urban heat island effects and improve air quality.

The integration of RESs with NBSs on rooftops yields multiple synergies. Green roofs naturally cool solar panels, enhancing their efficiency, while the vegetation helps filter air pollutants and reduces dust accumulation on the solar cells, further improving performance. When used for micro-agriculture, such as community allotments, these rooftops can also contribute to food security and urban self-sufficiency. Their semi-private nature enables residents to extend their living environments and encourages community interaction. With coordinated rooftop management, apartment buildings can effectively combine photovoltaic panels, solar water heaters, and small-scale gardens or allotments—transforming underused surfaces into resilient, productive urban assets.

Additional opportunities for distributed RESs exist in currently vacant or underutilized plots in the area, such as asphalt-covered car parks, which intensify heat island effects during summer. These spaces can be redesigned to incorporate RESs and NBSs, such as shaded pedestrian paths, solar panel canopies, and EV charging infrastructure. Educational facilities, both primary and secondary, also offer significant potential for rooftop RES installations, which can be paired with appropriate NBSs. Similarly, open areas around social housing complexes could accommodate shaded solar structures for scooters and vehicles, while solar-powered lighting can be deployed using ground-mounted, wall-mounted, or tree-mounted fixtures. This local potential for renewable energy generation provides a strong foundation for a more integrated initiative: the implementation of a virtual power plant (VPP) at the city district level. Such a system would combine solar energy production, smart grid infrastructure, and energy storage solutions—supporting the transition toward self-sufficient net-zero energy districts (S-NZEDs).

To integrate distributed RESs into carbon-neutral smart cities and enhance transparency and control, virtual power plant systems serve as an essential mechanism. A VPP aggregates numerous small-scale Distributed Energy Resources (DERs) into a coordinated virtual unit that functions like a conventional power plant, offering both observability and manageability [77]. Typically, a VPP integrates various technologies such as combined heat and power units, fuel cells, photovoltaic panels, heat pumps, solar thermal collectors, and other thermal or electrical energy systems, all operated via a central control platform [78]. This architecture allows for a blend of generation assets, including both dispatchable units—capable of adjusting output on demand—and intermittent sources dependent on environmental variability, as well as flexible loads and energy storage systems [79].

A notable example of a district-level VPP is the GridFriends initiative, part of the Schoonschip project launched by Amsterdam Smart City [80]. Schoonschip is a sustainable residential community of 46 households located in northern Amsterdam. One of the project’s defining features is its intelligent microgrid, which enables residents to act as both energy producers and grid operators. This setup was made possible through a special regulatory exemption granted by the Dutch Ministry of Economic Affairs. The community has a single shared connection to the public electricity grid, while all infrastructure beyond the meter is privately owned and managed by the residents. The smart grid facilitates peer-to-peer energy exchange, with algorithms optimizing energy flows. During summer, the system maximizes local solar energy use, whereas in winter, the priority shifts to minimizing peak loads on the main public grid connection. By coordinating energy storage and distribution, the VPP allows the community to balance supply and demand internally, resulting in an energy-neutral neighborhood powered through platform-based energy management [80]. Schoonschip illustrates how a prosumer-based energy culture can drive sustainable transitions. Since its launch in 2018, the smart grid has expanded across multiple new projects and remains one of the most advanced residential smart grids in the world [81].

The added value of district-level VPPs in the transition toward carbon-neutral smart cities is considerable. Their benefits fall into three primary areas. First, VPPs facilitate a shift in energy culture and behavior—an essential condition for carbon-neutral smart cities. Participants in a VPP, understood as complementors within a platform ecosystem, are no longer passive energy consumers. Instead, they become prosumers, actively influencing both energy supply and demand. As small-scale DERs become more widespread, this cultural shift gains importance. While today’s energy production remains largely centralized, the rise of renewable sources is driving a steady move toward decentralization, with VPPs playing a central role in efficiently managing these distributed resources [82].

Second, VPPs help reduce stress on the electricity network. By enabling local energy generation and consumption, they reduce the need for long-distance, high-voltage transmission, thereby minimizing energy losses. More critically, VPPs help overcome network bottlenecks that arise when the capacity of RESs exceeds the distribution system’s ability to transfer them. At peak production times, RES units are often curtailed to preserve grid stability. VPPs address this challenge by coordinating generation, storage, and consumption at the local level, thereby increasing the overall efficiency and utilization of renewable energy.

Third, a model we developed to evaluate the transition toward self-sufficient net-zero energy districts in European cities shows that such a transition is feasible through a combination of two energy technologies: energy savings via building refurbishment and smart systems on the one hand, and localized renewable energy generation through photovoltaic panels installed on public and private buildings on the other [83]. In Southern European cities, this combination is sufficient to achieve S-NZEDs. However, in Central and Northern Europe—where energy demand is higher and solar yields are lower—achieving net-zero status requires additional renewable energy imported from external sources.

4.5. Designing an Environmental Monitoring System for Improving Efficiency

The aforementioned analysis has shown that urban planning and design is a critical aspect in this process, as it can optimize the effectiveness of combining theoretical principles and practical applications. However, alongside traditional design practices, the literature also highlights the importance of applying continuous environmental monitoring to maximize the potential outcomes of NBS and RES integration in urban settings [17,84,85].

Environmental monitoring systems (EMSs), by utilizing real-time data collection through remote sensing, IoT sensors, and GIS mapping, provide insights into air quality, temperature regulation, water management, and energy efficiency. Continuous monitoring helps identify potential inefficiencies and areas for improvement, ensuring that implemented solutions adapt to changing environmental conditions [86]. In this context, data-driven approaches facilitate evidence-based decision-making, allowing urban planners to optimize the design, placement, and functionality of NBSs and RESs for enhanced urban resilience and sustainability [84,85].

For the railway station district in Thessaloniki, an initial EMS design has been developed (Figure 3). The key components of the proposed system include a data collection network, consisting of CO₂ sensors, meteorological stations, air quality monitors, and photosynthesis measurement stations, strategically placed in parks and on streets and rooftops. A data management platform connects these sensors through an IoT network, storing data in the cloud for real-time access and historical analysis. Advanced machine learning algorithms help detect trends and anomalies and predict future CO₂ absorption, while GIS mapping provides dynamic visualizations of intervention impacts. Additionally, the EMS incorporates external data sources such as satellite imagery (e.g., Sentinel-2, Landsat), citizen science contributions via mobile apps, and municipal or academic datasets to enrich environmental assessments. To quantify impact, CO₂ absorption indicators compare baseline values with post-intervention data, aligning findings with climate neutrality goals. Finally, interactive digital tools like real-time dashboards, GIS mapping, and automated alerts enhance decision-making and public engagement. Citizen participation is further encouraged through crowdsourced environmental reporting, open data access, and participatory design platforms, ensuring the continuous refinement and optimization of NBS and RES integration in urban settings.

Building on this framework, EMS data will enable the assessment of various configurations of NBSs and RESs, evaluating their influence on environmental performance and progress toward carbon neutrality. Agent-Based Modeling can simulate how different public policy interventions in NBS and RES types, as well as actions initiated by citizens and organizations within the district, contribute to advancing closer to or achieving a net-zero state. This iterative modeling supports adaptive design, allowing city planners to test hypotheses, explore trade-offs, and co-create solutions with stakeholders that optimize carbon neutrality impact. Ultimately, the synergy between real-time data, predictive analytics, and community input establishes a dynamic feedback loop, fostering more informed, inclusive, and responsive urban design practices that advance the district toward climate neutrality.

5. Aligning NBS–RES Integration with Positive Energy Districts

The integrated approach of combining nature-based solutions with renewable energy sources, as proposed in this study, provides a critical pathway that must be combined with the development of Positive Energy Districts (PEDs). PEDs represent an advanced urban sustainability model, aiming not only to achieve a net-zero energy balance but also to generate a surplus of renewable energy within clearly defined urban boundaries. While both carbon-neutral city districts and PEDs pursue environmental sustainability, their complementarity lies in the ambition of PEDs to actively contribute renewable energy to the broader grid, thereby reinforcing regional energy resilience and accelerating decarbonization.

The integration of NBSs and RESs supports this transition through three interrelated mechanisms: (1) reducing energy demand via microclimate regulation and enhanced thermal comfort; (2) enabling localized renewable energy generation by embedding energy infrastructure in green urban fabrics and buildings; and (3) offsetting residual emissions by leveraging the carbon sink capacities of urban ecosystems. NBSs—such as tree canopies, vegetated corridors, and green roofs—mitigate the urban heat island effect, lower ambient temperatures, and enhance thermal comfort in both public and private spaces. This localized cooling effect significantly reduces building energy demand, particularly for air conditioning during peak summer months. When co-deployed with RES technologies—such as building-integrated photovoltaics, solar-shaded urban furniture, or hybrid green roofs with PV panels—these multifunctional infrastructures enable on-site renewable energy production while preserving urban biodiversity and spatial quality. In parallel, the ecological services provided by NBSs—such as carbon sequestration through dense vegetation, restored soils, and urban rewilding—play a vital role in compensating for residual emissions, including those embedded in construction materials or resulting from unavoidable transport-related activities.

However, achieving carbon neutrality demands broader systemic transformations. These include the deep energy retrofitting of existing buildings to drastically reduce energy use; smart energy management systems that dynamically balance real-time supply and demand; demand-side flexibility through digital platforms and active community participation; and electrified and integrated mobility solutions, such as Mobility as a Service (MaaS), to reduce transport-related emissions. Smart city technologies further reinforce this transition by enabling adaptive infrastructure governance, real-time monitoring, and participatory planning mechanisms aligned with both environmental goals and community needs.

Crucially, carbon neutrality practices alone are insufficient to address the seasonal mismatch between renewable energy generation and energy consumption. Given the limited capacity for urban energy storage at the city level and the pronounced seasonal variability—characterized by energy surpluses in summer and deficits in winter—districts must function as seasonally positive energy producers. This means exporting surplus renewable energy during periods of high generation (typically summer) and importing equivalent amounts during periods of high demand (typically winter) while maintaining an annual positive energy balance.

The synergetic application of NBSs and RESs, when embedded within broader carbon neutrality strategies, aligns closely with the PED framework’s ambitions. The railway station district pilot in Thessaloniki exemplifies how underutilized and typologically diverse urban spaces—such as schoolyards, rooftops, road medians, and vacant plots—can be transformed into multifunctional assets. These interventions simultaneously provide localized climate regulation, generate renewable energy, and function as urban carbon sinks. By combining these design strategies with PED planning, the district can achieve a net-positive energy performance on an annual basis without the need for large-scale seasonal energy storage, thereby accelerating the transition toward low-carbon, regenerative urban futures.

6. Conclusions

As urban areas strive for more sustainable and resilient futures, new approaches to enhancing urban sustainability are being explored. This paper argues that integrating NBSs with RESs presents a promising strategy, as NBSs primarily focus on improving ecosystem services and urban livability, while RESs are essential for reducing carbon emissions and promoting energy independence. Although each approach individually contributes to urban sustainability, their combined implementation holds significant potential to create new opportunities and maximize environmental, social, and economic benefits. To achieve this, it is crucial to develop a comprehensive, well-structured framework for integrating NBSs and RESs that incorporates urban planning and energy principles to effectively address local specificities in the design process.

The railway station district of Thessaloniki has been used as our case study for analyzing how NBSs can be combined with RES solutions, considering the various typologies of the available open spaces as well as the ecosystem services being impacted through NBS solutions. Our results suggest that there are potential windows of opportunity for combining NBSs with RESs, including solutions such as green roofs with photovoltaic panels, solar lighting, and solar parking shaders. The detailed analysis of how the integration of NBSs and RESs can enhance ecosystem services has shown that cultural and regulatory services are consistently impacted across all cases, while provisioning and supporting services are more closely tied to specific interventions and experience a lesser impact from the integration of NBSs and RESs.

We go beyond the immediate context of Thessaloniki by proposing a typology of five major urban project types that offer clear potential for replication and scaling in other city districts. More specifically, replication and scale-up are made possible by developing green and blue corridors to restore ecological connectivity and manage urban microclimates as well as enhancing urban greenery at the eye level to improve walkability, thermal comfort, and social interaction. Moreover, integrating NBSs and ecosystem services into metropolitan mobility infrastructures to support multifunctionality and reduce environmental impacts and establishing virtual power plants for smart, distributed energy management that combines renewable generation with demand-side flexibility offer two additional replicable solutions for boosting NBS–RES integration. In addition, developing an environmental monitoring system (EMS) to guide data-driven decision-making and optimize the efficiency of interventions can be considered a critical factor complementing the previous typologies.

These project typologies provide a flexible yet structured framework that can be adapted to any specific urban morphology, governance structures, and climate priorities of other cities. This strengthens the generalizability of the proposed design approach and supports knowledge transfer for wider implementation in European and international urban contexts. Moreover, by aligning NBSs and RESs within the PED framework, cities can not only enhance energy self-sufficiency and climate resilience but also create replicable models for regenerative urban transformation.

However, the integration of NBSs with RESs faces several limitations, particularly in densely built urban areas, such as the one examined here. These challenges include spatial constraints due to limited available land, socio-economic factors that may hinder adoption and maintenance, and technical difficulties posed by uneven terrain and infrastructure. Moreover, regulatory and governance issues can complicate the coordination of NBS and RES objectives, while the ecological suitability of certain areas, reflected in features such as the lack of natural water bodies or adequate green spaces, limits the types of interventions that can be implemented.

At the same time, transitioning toward integrated NBS and RES solutions requires not only technological advancements but also public awareness and stakeholder engagement. Aligning urban sustainability strategies with financial incentives, participatory governance, and community involvement can further enhance the effectiveness of integrated solutions [87,88,89]. Energy communities, for instance, foster collective ownership of local energy resources and contribute to urban energy self-sufficiency [33]. Ensuring the long-term sustainability of these solutions requires consistent maintenance and public commitment, as there is a risk of unintended social impacts, such as gentrification, that could displace lower-income residents.

Future research should continue to evaluate the long-term viability of hybrid systems, assess their contribution to urban resilience, and develop standardized metrics for measuring their impact on biodiversity and sustainability [35]. It should also explore context-specific frameworks for optimizing NBS–RES integration across varying urban environments, considering factors such as spatial configuration, energy demands, and local climate conditions. Finally, it should focus on engaging stakeholders—including local authorities, researchers, urban planners, and community representatives –in a co-design process of NBS and RES technologies to allow for adaptive and context-specific planning. This inclusive approach not only enhances social acceptability but also improves the long-term sustainability of implemented solutions by ensuring that they align with community needs.