A Literature Review on the European Legislative Framework for Energy Efficiency, Nearly Zero-Energy Buildings (nZEB), and the Promotion of Renewable Electricity Generation

Abstract

Improving the energy efficiency of buildings is a major priority within the context of the European objectives to reduce greenhouse gas emissions by 55% by 2030 and to achieve climate neutrality by 2050. Nearly Zero-Energy Buildings (nZEBs) offer a promising solution to significantly reduce energy consumption and promote the use of renewable energy sources. There is a significant gap in the scholarly literature regarding systematic reviews focused on the advancements in European legislation related to energy efficiency. Consequently, this paper aims to provide a comprehensive synthesis of the key legislative norms targeting the energy efficiency of buildings and the necessity of utilizing renewable energy sources for electricity generation, with a particular focus on the forecasts for the year 2030. The objective is to offer valuable reference resources and to support the global expansion of nZEB implementation in a sustainable and resilient manner. This research thoroughly evaluates the development of nZEBs, emphasizing design concepts, technological innovations, and their impact on energy efficiency. An analysis of the main barriers to implementation highlights high costs, limited technological feasibility, regulatory constraints, and insufficient stakeholder engagement. The purpose of this paper is to review the literature on building energy efficiency and the European trajectory from passive to zero-energy buildings.

1. Introduction

Sustainable development is a paradigm that acknowledges the complexity and interdependence of economic, social, technological, and ecological dimensions. This approach aims to promote a dynamic balance among these components by identifying and leveraging positive synergies while avoiding long-term trade-offs [1]. Economic growth, particularly within linear development models based on excessive resource extraction and consumption, exerts considerable pressure on the environment, leading to pollution, the depletion of natural resources, and ecosystem degradation [2]. Greenhouse gas emissions, inadequate waste management, and excessive energy consumption are manifestations of the complex interdependence between economic activity and environmental health [3]. Although GDP is a key indicator of economic activity, its growth is often accompanied by environmental deterioration, driven by intensified industrial activities, the increased consumption of natural resources, and waste generation. This negative correlation between economic growth and ecological impact highlights the need for alternative development models that reconcile economic prosperity with environmental protection [4]. Paradoxically, at a time when we understand the impact of human activities on the planet better than ever, we are witnessing an acceleration of environmental degradation processes. Despite scientific advancements providing solutions for building more sustainable societies, the transition to a sustainable economic model remains slow and fragmented, undermining efforts to ensure a prosperous future for future generations [5]. The current context, marked by accelerated climate change and the depletion of natural resources, underscores the importance of scientific research in finding sustainable solutions. The European Union (EU)’s objectives to ensure energy security and reduce greenhouse gas emissions require a comprehensive approach that considers economic, social, environmental, and technological aspects. Interdisciplinary research is essential for identifying innovative solutions and building a sustainable future for Europe [6].

The widespread and justified interest in the literature on housing reflects a significant concern for this subject. However, a frequently overlooked aspect in historical analyses is the contribution of the family in defining housing programs. Consequently, the prevailing perspective on this subject tends to be external. Theoretical discussions in this field cover a wide range of economic, political, social, sustainable, and technical themes, each with their own undeniable importance. Without negating their significance, this paper aims to go beyond this spectrum and focus on factors aimed at maximizing energy efficiency and minimizing energy consumption for heating, cooling, and ventilation in the context of housing. Through the implementation of energy improvement measures, a transition toward the concept of “passive house” can be achieved, aiming to provide a comfortable living environment and significantly reduce the energy required for heating and cooling, with a lower environmental impact and reduced operating costs for the owners of such buildings.

A significant challenge facing Europe is the high level of energy consumption associated with buildings. In 2024, the construction and building sector was responsible for a substantial share of global energy consumption, accounting for nearly 40% of total energy demand and generating 36% of global energy-related CO₂ emissions [7]. While in the past, housing choices were limited to a few existing options, Europe has now reached a historical moment where people are beginning to choose where and how they live [8]. Although this shift is still tentative and sometimes awkward, housing selection is becoming increasingly important, significantly impacting how homes are designed [9]. In Romania, although budget remains the predominant factor preselecting residential options, alongside notions of square meters and price per square meter [10], housing must begin to realign with the current needs of families, as they are now defined, and the impact of our existence on the environment.

To achieve the European objectives in energy efficiency, member states must implement specific measures for building renovation. This approach brings multiple benefits, such as enhancing energy security, stimulating the economy, and improving the quality of life for citizens. European directives, including the Energy Efficiency Directive (EED) [11], the Energy Performance of Buildings Directive (EPBD) [12], and the Renewable Energy Directive (RED) [13], serve as fundamental tools for promoting energy renovation and the use of renewable energy to achieve the Nearly Zero-Energy Building (nZEB) standard for both existing and new buildings. The European Green Deal represents the latest initiative in sustainable development, serving as the primary instrument for implementing the United Nations’ 2030 Agenda, including the Sustainable Development Goals (SDGs) [14]. This innovative approach integrates climate and environmental policies into a cohesive framework for the first time, thus promoting a comprehensive vision across sectors such as climate, environment, energy, transport, industry, agriculture, and sustainable finance [15]. Key objectives include reducing net greenhouse gas (GHG) emissions in the EU by at least 55% by 2030 compared to 1990 levels [16], alongside the goal of achieving climate neutrality by 2050. This strategy not only addresses current climate challenges but also opens new avenues for sustainable economic development in Europe [17].

The EU has adopted a set of ambitious objectives to transform the building sector into a low-carbon industry, emphasizing the nZEB standard, focusing on the extensive renovation of the existing building stock, and promoting the nZEB energy standard [18]. Despite the significance of the nZEB objective, its large-scale implementation is hindered by a series of interconnected obstacles, including high renovation costs, limited access to financing, and deficiencies in professional skills [16]. In this context, the action plans formulated under the Renovation Wave aim to renovate approximately 35 million buildings by 2030, prioritizing energy efficiency improvements and atmospheric emissions reduction [19]. The adoption of strict energy performance standards at the level of each member state is essential to create a harmonized and coherent framework across the EU, facilitating the achievement of common climate objectives and fostering innovation and competitiveness in the construction sector [20].

The transition to a low-carbon economy offers the energy sector new opportunities for development and for strengthening its position on the international stage [21]. Global economic expansion has placed increasing pressure on energy resources, driving innovation in the energy field and highlighting the necessity of transitioning to cleaner and more sustainable energy sources [22]. The ecological challenges associated with traditional energy production and consumption have prompted a strategic reorientation of the energy sector, focusing on the development and implementation of renewable technologies [23]. Energy is a crucial catalyst in achieving sustainable development goals, essential for economic growth, improving quality of life, and eradicating poverty [24]. However, to ensure sustainable progress, it is imperative to transition to efficient, low-carbon energy systems [25].

Decarbonizing the economy represents a major strategic challenge for the European Union, driven by the need to limit the impact of climate change and ensure a sustainable future for future generations. Through instruments such as the Emissions Trading System (ETS) and the Energy Union, the EU aims to accelerate the adoption of renewable energy and reduce dependence on fossil fuels [26]. While there is broad consensus on the necessity of this transition, its implementation is complex and requires coordination at both the European and national levels [27]. Although the consumption of renewable energy is increasing, its impact on total CO₂ emissions is partially offset by economic growth and the persistence of energy-intensive consumption patterns [28].

The main contributions and novelties of this research lie in providing an overview of the evolution of energy efficiency in Europe over the course of more than three decades, from 1988 to the present. The study also examines Romania’s national perspective on the implementation of European directives and the promotion of increased energy efficiency in buildings. The paper details the key elements defining the concept of nZEBs, offering an in-depth analysis of the most relevant studies and works in the field of energy efficiency in construction, with a particular focus on the design phase. Another essential aspect addressed is the need for the integration of electricity generation from renewable sources at the local level, in the context of the objectives for reducing greenhouse gas emissions and achieving climate neutrality. The paper emphasizes projections for renewable energy by 2030 and the importance of implementing innovative and sustainable solutions through the energy policies developed by the EU to build a resilient and sustainable future.

The paper is organized as follows: Section 2 provides a comprehensive retrospective analysis of the evolution of energy efficiency in Europe over more than three decades and the issue of reducing energy consumption in buildings in Romania. Section 3 highlights the need for the gradual transformation of the built environment, both existing buildings and new ones, towards the nZEB concept and presents a systematic review of the literature to identify and analyze the most significant scientific contributions in the field of energy efficiency in buildings. Section 4 discusses how to enhance energy security and reduce dependence on fossil fuels through the promotion of renewable energy sources. Section 5 presents discussions on the chosen research topics. Finally, the conclusions of this paper are summarized in Section 6.

2. The Policy Framework for Energy Efficiency

2.1. European Policies in the Construction Sector

The paper provides an overview of the development of energy efficiency in Europe, covering a period of over three decades, from 1988 to the present:

European policies in the construction sector primarily aim to create a sustainable built environment with a low environmental impact. A central aspect of these policies is the promotion of energy efficiency in buildings through the establishment of high energy performance standards and the encouragement of the use of eco-friendly materials and technologies [29]. These measures contribute to the reduction of greenhouse gas emissions, decrease dependence on fossil fuels, and increase comfort in residential and public spaces.

Since 1988, the EU has placed special emphasis on reducing greenhouse gas emissions and minimizing energy consumption, with the main objective of limiting carbon dioxide emissions through the promotion of energy efficiency in the construction sector. Directive 1989/106/EEC of 21 December 1988, on the harmonization of the laws and regulations of member states concerning construction products, stipulates that construction works, including heating, cooling, and ventilation systems, must be designed and executed in such a way as to minimize energy consumption during their use, taking into account local climatic conditions and ensuring user comfort [30].

Directive 1993/76/EEC of 13 September 1993, with the objective of limiting carbon dioxide emissions through increased energy efficiency, obliges member states to develop and implement specific programs for improving energy performance in the construction sector, as well as to submit reports on the measures adopted. Although this directive has started to generate notable beneficial effects, there is a clear need for a complementary legal framework capable of establishing more concrete measures aimed at significantly tapping into the still underutilized potential of energy savings. At the same time, such an instrument would contribute to reducing the major discrepancies between member states in terms of the outcomes achieved in this area [31].

The EU’s concern regarding the reduction of emissions and energy consumption continued in 2002, when Directive 2002/358/EC was proposed [32]. The document set out the commitments of industrialized countries to reduce greenhouse gas emissions, which contribute to global warming. Developed countries were required to reduce total emissions by at least 5% between 2008 and 2012, compared to 1990 levels. The EU revisited the directive on 31 May 2002 and agreed to fulfill their collective commitments for the first commitment period. At the 2012 Doha Climate Change Conference, a second commitment period was established (starting 1 January 2013, and ending 31 December 2020). The Doha Amendment added new legally binding commitments for the EU to meet during the second commitment period.

The first major initiative to establish a European framework for the energy performance of buildings took place in 2002 with the adoption of Directive 2002/91/EC (EPBD) [12], which must be assessed based on a methodology adaptable to each region, taking into account not only thermal insulation but also important factors such as heating and cooling systems, the use of renewable energy sources, and building design. A uniform approach, carried out by qualified specialists with guaranteed independence based on objective criteria, will contribute to the standardization of energy-saving regulations in the construction sector and provide future property owners or users with a clear understanding of energy performance in the European real estate market [33]. This directive imposed a series of requirements on member states, including the implementation of building certification schemes (energy performance certificates—EPCs), inspection regimes for large heating and cooling systems, as well as the integration of performance standards in construction. In many respects, the EPBD raised the standard of regulations across all EU member states, aligning them with the best existing practices.

Directive 2006/32/EC primarily aims to promote energy efficiency in the EU member states. It includes measures and objectives to reduce energy consumption and greenhouse gas emissions [34].

Directive 2009/406/EC, dated 23 April 2009, concerning the efforts of member states to reduce greenhouse gas emissions in order to meet the community commitments for their reduction by 2020, establishes binding national targets for the reduction of carbon dioxide emissions, within which energy efficiency in the building sector will play a crucial role [35]. At the same time, Directive 2009/28/EC (RED) promotes the use of energy from renewable sources and supports energy efficiency, within the context of a binding target for integrating renewable energy, which should constitute 20% of the total energy consumption of the Union by 2020 [13].

The reduced energy consumption and increased use of energy from renewable sources also play an important role in promoting energy supply security, technological development, and creating employment opportunities and regional development, particularly in rural areas. Thus, Directive 2010/31/EU establishes that from January 2019, all new buildings used by public authorities must have near-zero energy consumption (nZEB certification). The same rule applies to private buildings starting from January 2021 [36].

Figure 1 tracks the evolution of the European legislative framework (2012–2020) in the field of energy efficiency in buildings, highlighting the key milestones that led to the adoption of the requirement for all new buildings to meet the nZEB standard.

Directive 2012/27/EU (EED), on energy efficiency [11], represents an important legislative tool within the EU framework, aimed at promoting energy efficiency and sustainability through eco-design of products.

Under Regulation (EU) 2018/1999 [37] on the governance of the Energy Union, the European Commission assessed the draft national integrated energy and climate plans. The assessment included the level of ambition of the objectives, targets, and contributions set to collectively meet the EU’s goals. Specifically, the analysis focused on the EU’s 2030 targets for renewable energy, energy efficiency, and the level of electricity grid interconnection aimed for by member states.

Directive 2023/1791/EU reaffirms the primary objective of transforming the EU’s building stock into passive buildings (with nZEB certification), in alignment with the provisions of the Paris Agreement [17], ratified by the EU on 11 December 2019. With the publication of the Union’s Energy Strategy, under the United Nations Framework Convention on Climate Change (UNFCCC), the signatory parties committed to keeping the global average temperature well below 2 °C above pre-industrial levels, with ongoing efforts to limit the temperature increase to 1.5 °C. The EU has assumed a key role in addressing climate change through five main dimensions: energy security, decarbonization, energy efficiency, the internal energy market, and research, innovation, and competitiveness. Thus, the EU is committed to leading the global energy transition by fulfilling the objectives outlined in the Paris Agreement on climate change, aiming to provide clean energy across the EU. To meet this commitment, the EU has pledged to reduce net greenhouse gas emissions across the economy by at least 55% by 2030 and achieving climate neutrality by 2050 [38], compared to 1990 levels, in accordance with the updated nationally determined contribution submitted to the UNFCCC Secretariat on 17 December 2020.

In line with the announcements made in the European Green Deal, the European Commission presented its strategy for a renovation wave in the Communication of 14 October 2020, titled “A Renovation Wave for Europe—Greening Buildings, Creating Jobs, Improving Living Conditions”. This strategy includes an action plan consisting of specific regulatory, financial, and facilitation measures, aiming to at least double the annual rate of building energy renovations by 2030. Additionally, the strategy seeks to promote deep renovations, targeting the renovation of 35 million buildings by 2030 and the creation of jobs in the construction sector.

The European Commission has already outlined a clear vision to achieve climate neutrality by 2050. The EU has begun modernizing and transforming its economy to reach this goal. Between 1990 and 2018, greenhouse gas emissions were reduced by 23%, while the economy grew by 61%. However, current policies will only reduce emissions by 60% by 2050, indicating that more ambitious climate measures are required in the coming decade. Although the EU, through Directive 2018/844/EU, encouraged its member states to advance towards transforming existing buildings into nZEBs [39], there is currently no explicit regulation requiring all residential building renovations to meet nZEB standards [40].

In this context, the most recent Directive 2024/1275/EU [12] on energy efficiency mandates that member states prepare a general report on buildings that do not meet nZEB requirements and take measures to improve the energy efficiency of buildings [41]. This initiative is particularly important, considering that 75% of buildings in the EU have low energy performance, the global urban population is expected to reach 68% by 2050, and current renovation rates are insufficient to meet these challenges [40]. In general, reducing energy consumption in buildings is a crucial step towards a cleaner environment, the sustainable use of resources, and the reduction of costs associated with energy and daily living [42].

2.2. A National Perspective on Energy Efficiency

In Romania, the total built area is 493,000,000 m², of which 86% is dedicated to residential buildings. Of the 8.1 million housing units, single-family homes are predominant, accounting for 61% of the total. Nearly 47.5% of residences are located in rural areas, where 95% of the units are individual houses. In urban areas, 72% of homes are located in apartment buildings, with each building having an average of approximately 40 apartments. Over 60% of these buildings have a height of P + 4 floors, while 16% have P + 10 floors [43].

Romania possesses a significant heritage of buildings predominantly constructed between 1960 and 1990, characterized by a low level of thermal insulation. This is due to the lack of regulations concerning the thermal protection of buildings before the energy crisis of 1973. The final energy consumption for these buildings ranges between 150 and 400 kWh/m² per year [44]. Considering the aging and energy inefficiency of the built stock, Romania has identified the development of the RES sector as a key lever for transitioning to a low-carbon economy and ensuring long-term energy security.

Starting from a significant technical energy potential, Romania has implemented a legal and institutional framework to support renewable energy sources (RES), in alignment with European directives, to stimulate the development of this sector. The “Strategy for the Utilization of Renewable Energy Resources” [45], alongside the transposition of European directives into national legislation, includes the provisions of Directive 2001/77/EC [46], which were transposed into national law [47,48] on promoting electricity production from renewable energy sources, establishing the system for promoting electricity production from renewable energy sources. These decisions laid the foundation for a RES promotion system based on support mechanisms such as green certificates. Romania set an ambitious target to increase the share of energy from renewable sources in gross final consumption to 24% by 2020, representing a 6.2% increase compared to the reference year of 2005 (the reference value for 2005 is 17.8%).

An important step taken by national authorities towards increasing energy efficiency was the adoption of Law no. 121/2014 [49], which represents the basic legal framework for implementing energy efficiency policies in Romania. The law aims to reduce energy consumption in all economic sectors, improve the energy performance of buildings, promote renewable energy sources, and reduce greenhouse gas emissions. Emergency Ordinance no. 130/2022 [50] creates the necessary legal framework to facilitate access to innovative financial instruments, such as European grants, for implementing projects aimed at improving the energy performance of buildings. Emergency Ordinance no. 119/2023 [51] concerns the regulation of standards for financial instruments dedicated to supporting measures for improving energy efficiency in industry. Through this legal framework, the aim is to mobilize private and public investments to reduce energy consumption in the industrial sector through specific financial instruments, called “green financial instruments”. This emergency ordinance aims to approve Romania’s National Recovery and Resilience Plan (PNRR) [52] and contribute to achieving Romania’s and the EU’s climate objectives.

In order to meet the objectives set by the EU, each member state was required to submit a draft of the National Integrated Energy and Climate Plan (NIECP) for the period 2021–2030 to the European Commission. European states have the freedom to develop their own national plans, tailored to specific local conditions, including specific definitions for nZEBs and energy indicators. The NIECPs set national objectives and contributions towards achieving the EU’s climate goals. Romania submitted its own NIECP draft in 2018, and the National Integrated Energy and Climate Plan 2021–2030 was adopted in April 2020 [53].

On the other hand, the European Commission emphasized that Romania will need to set a more ambitious target for reducing primary and final energy consumption by 2030 in order to meet the EU’s energy efficiency target. Thus, Romania aims for primary energy consumption of 32.3 Mtoe and final energy consumption of 25.7 Mtoe, which will lead to energy savings of 45.1% compared to the estimated primary energy consumption for 2030 and 40.4% for final energy consumption, compared to the PRIMES 2007 reference scenario. Additionally, to comply with the requirements of Article 7 of Directive 2018/2002/EC [54], Romania must achieve new energy savings equivalent to 10.12 Mtoe during the period of 2021–2030. Following a detailed analysis, Romania decided to adopt alternative measures and policies to support energy savings. Moreover, a draft related to the Long-Term Renovation Strategy was presented for public consultation, with the current renovation scenario proposing increased energy efficiency and significant CO₂ savings, as well as new facilities for the production of RES-E, mostly in the form of photovoltaic panels for existing buildings [53].

The promotion of measures to enhance the energy performance of buildings is provided for in Law no. 372/2005 [55]. The law establishes the legal framework for improving the energy performance of both new and existing buildings, thus contributing to the reduction of energy consumption, the mitigation of greenhouse gas emissions, and the creation of a more sustainable built environment. The Law on the Energy Performance of Buildings has been successively updated by Law no. 101/2020 [56]. Another update was made with the adoption of Law no. 238/2024 [57], aimed at promoting measures to improve the energy performance of buildings, taking into account external and location-specific climatic conditions, interior comfort requirements, optimal cost levels, and energy performance requirements, as well as enhancing the urban appearance of localities.

One of the main objectives of the republished and updated Law no. 372/2005 is to establish a methodology for calculating the energy performance of buildings. Thus, through the adoption of the Thermal Performance Calculation Methodology for Buildings, guideline MC 001/1-2006 [58], Romania has made an important step towards reducing energy consumption in the construction sector and achieving national energy efficiency objectives. The regulation aims to establish a coherent method for evaluating and certifying the energy performance of both new and existing buildings, with various functions, transposing the provisions of European Parliament and Council Directive 2002/91/EC on the energy performance of buildings into Romanian law through Law no. 372/2005 on the energy performance of buildings.

Part I establishes the methodology for determining the hygrothermal-energy characteristics of the building envelope components, for their intended use. Part II focuses on the characterization of the other subsystems of the building product, which are the building’s installations and equipment, while Part III addresses the method for conducting the building’s energy audit and the issuance of the building’s energy performance certificate. The new methodology for calculating the thermal performance of buildings, guideline MC 001-2022, introduces stricter requirements regarding the energy performance of buildings, thus stimulating the development of innovative and efficient construction solutions [59], transposing the provisions of European Parliament and Council Directive 2010/31/EU on the energy performance of buildings into Romanian law.

To support the renovation of the national stock of residential and non-residential buildings, both public and private, and its gradual transformation into a real estate portfolio with high energy efficiency and low carbon emissions by 2050, the Ministry of Public Works, Development and Administration (MLPDA) is developing a Long-Term Renovation Strategy [60], which will be approved by government decree. The Renovation Strategy was published on 17 December 2020 and will be updated every 10 years. It was submitted to the European Commission as part of the National Integrated Energy and Climate Plan, prepared by the Ministry of Economy, Energy, and Business Environment. To facilitate the mobilization of investments in renovations, Law No. 101/2020 provides access to accessible and transparent advisory tools, such as one-stop-shops for consumers and energy consultancy services, in order to support energy-efficient renovations and the use of relevant financial instruments.

Romania aims to develop prosumers in residential, industrial, and agricultural sectors while modernizing electricity networks and implementing smart meters. Integrating distributed generation and prosumers into the national grid is a strategic priority. In the coming years, photovoltaic capacity is expected to grow through medium-sized solar parks on degraded land and small installations by energy consumers. The adoption of Law No. 184/2018 [61], regulating renewable energy promotion, clarified the status of prosumers in Romania.

3. Nearly Zero-Energy Buildings (nZEBs)

3.1. From Passive to Nearly Zero: The Evolution of the Energy-Efficient Building Concept

3.1.1. The Concept of a Passive Building

Although three decades have passed since the construction of the first passive house in Darmstadt, Germany, in 1990 [62], these buildings have demonstrated benefits not only for the environment but also for the economy and human health [63]. However, numerous myths surrounding these homes can lead to confusion among both insufficiently informed professionals in this field and the general public. Before delving into specifics, it is essential to clarify some of the most widespread myths.

Contrary to popular belief, the specific energy consumption of a passive house is not zero but very low, approximately 15 [kWh/m² per year]. Passive buildings are not restricted to a particular architectural style. They can be designed in traditional, modernist, or contemporary styles [64]. The exterior appearance of a passive house is shaped by the preferences of the client and the architect [65]. There are no restrictions on the construction materials used; thus, passive houses can be built using insulation made from straw bales as well as the most sophisticated and advanced materials available on the market, such as aerogel, vacuum insulation panels, or phase-change materials [66].

Passive houses represent an energy efficiency standard developed by German physicist Wolfgang Feist in the late 1980s [62]. Initially designed to minimize heating and cooling costs for residential buildings, the concept has proven equally effective for buildings serving functions other than residential purposes [67].

Calculating energy losses and savings for a passive building through energy modeling and thermal simulation using specialized software provides a detailed and efficient approach to evaluating the energy performance of the structure [68]. This analysis can be conducted at various stages of the building’s construction. In the design phase of a passive building, significant emphasis is placed on the building envelope (insulation). However, there is often insufficient attention given to the appropriate integration of passive design strategies during the planning phase for a building in a specific climate and geographic location [69].

Due to the increasing demand for energy in buildings, there is growing concern among the research community and governments to explore and implement viable methods for minimizing building energy consumption [70]. Among the various approaches proposed thus far, passive design has proven to be highly promising from both technical and economic perspectives, as detailed in [71]. However, the energy performance of a specific passive design option depends on various parameters [72], such as the following:

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climatic/geographical location;

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orientation;

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solar shading;

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geometry (shape);

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window-to-wall ratio (WWR);

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number of building floors.

Although the concept of a passive house represents a significant step toward energy efficiency and sustainability in construction, it is not without certain limitations and challenges, such as the need for detailed design and precise construction, which can result in significantly higher initial costs compared to conventional buildings [73]. The use of high-performance materials and complex technical systems may substantially increase the budget, while the certification process for a passive house can be expensive, potentially discouraging some investors. Additionally, the technical systems of a passive house must be adapted to the specific climatic conditions of the region where it is built [74]. Despite the undeniable advantages of passive houses, it is important to remain aware of these limitations. However, with the continuous development of technologies and standards, as well as the growing demand for energy-efficient buildings, these gaps can gradually be overcome, making the passive house concept more accessible and widespread [75].

3.1.2. The Concept of an nZEBs

The Energy Performance of Buildings Directive (EPBD) [12] has driven a revolution in the construction sector, with nZEBs among the most cutting-edge solutions. By integrating local renewable energy sources, these buildings are at the forefront of sustainable construction, delivering both energy efficiency and a reduced environmental impact [76].

In recent years, the concept of nZEBs has emerged as a priority on the European political agenda and has sparked extensive academic debate. As successors to passive buildings, nZEBs have taken the concept of energy efficiency to a new level, harmoniously integrating the principles of sustainable construction with modern renewable energy technologies [77]. However, despite significant progress, several international challenges remain, including the precise definition of the concept, the establishment of robust calculation methodologies, and the evaluation of the actual impact of nZEBs on energy consumption and greenhouse gas emissions [78].

In recent years, the nZEB concept has been extensively discussed and analyzed, particularly within the European Union, but it also remains a topic of international interest concerning the establishment of boundaries and calculation methodologies for nZEBs. The United States Department of Energy (DOE) [79] published the document “A Common Definition for Zero Energy Buildings, Campuses, and Communities”, which provides clarifications regarding the concept. The term “nearly” is defined through a cost-optimal analysis, in accordance with the calculation methodology outlined in Article 3 and Annex I of the relevant legislation.

The Federation of European Heating, Ventilation, and Air Conditioning Associations (REHVA) [80] has developed a detailed definition of nZEBs, focusing on the energy flows that must be included in primary energy calculations. In compliance with the requirements of the Energy Performance of Buildings Directive (EPBD) [12], the system boundary defined in Standard EN 15603:2008 [81], “Energy Performance of Buildings—Overall Energy Use and Definition of Energy Ratings”, was adjusted to include on-site renewable energy production.

The debate surrounding nZEBs has intensified over the past decade, particularly focusing on certain aspects that still require clear definition. Among the key elements contributing to the definition of the nZEB concept, as illustrated in Figure 2, are energy performance criteria, calculation methods, the integration of renewable energy sources, and the establishment of energy consumption limits.

Physical boundary

The level of the physical boundary is one of the most debated aspects, closely linked to the integration of renewable energy sources into the energy balance. The boundary of a system can encompass either a single building or a group of buildings. In the case of building groups, it is not mandatory for each individual building to achieve an almost zero energy balance; however, the combined energy balance of the entire group must meet this requirement. The integration of renewable energy into a district heating system is typically carried out at the neighborhood or infrastructure level, while photovoltaic systems are primarily considered at the level of a single building or a building complex. If a photovoltaic installation is located near a building and the system boundary is restricted to the building, it is considered off-site; however, it is considered on-site if the photovoltaic installation is connected to the same grid as the building.

Period of balance

The reference period used for calculating the energy balance can vary significantly. While the evaluation interval can be hourly, daily, monthly, or seasonal, the total duration of the assessment may cover one year, the entire life cycle of the building, or its operational period.

Connection to the energy infrastructure

Most definitions of nZEBs implicitly involve connection to one or more utility networks, such as the electricity grid, centralized heating and cooling systems, gas networks, or biomass and biofuel distribution networks. As such, these buildings can both import and export energy from these networks, thereby avoiding the need for on-site energy storage. Grid-connected nZEBs use the energy infrastructure both as a source and as an absorber of electrical energy. On the other hand, off-grid nZEBs require an energy storage system to cover peak consumption periods or times when renewable energy sources are unavailable. Additionally, requirements regarding energy performance, indoor air quality, comfort levels, and monitoring are mandatory.

Metric of balance

Various units can be used in the definition or calculation methodology of nZEBs. The most commonly applied unit is primary energy, while final or supplied energy is the simplest to implement. Other options include final energy (also known as delivered energy, end-use energy, or unweighted energy), CO₂ equivalent emissions, exergy, and energy cost. It is also essential to specify the conversion factors within the definitions.

Type of energy use

Many definitions of nZEB focus exclusively on operational energy (heating, cooling, lighting, ventilation, domestic hot water) and exclude other energy uses, such as cooking, appliance usage, or embodied energy. However, the energy required for the manufacture, maintenance, and decommissioning of construction materials can have a significant impact. According to the EN 15603/2008 standard [81], energy evaluation should include energy consumption that does not depend on occupant behavior, actual weather conditions, or other internal and environmental variables, such as heating, cooling, ventilation, domestic hot water, and lighting (in the case of non-residential buildings). Other options for inclusion in the calculation are household appliances, central services, and electric vehicles.

Renewable supply

Renewable energy sources can be used both on-site and off-site, depending on the availability of local resources, such as sunlight or wind, or the possibility of transporting resources, such as biomass, to the consumption site.

In conclusion, the design of Nearly Zero-Energy Buildings largely depends on the definition implemented at the national level. Although nZEB definitions can vary significantly between countries, there are a few essential common criteria:

• sensitivity to location—climatic zone;
• the values depend on the architectural program;
• differentiation of energy efficiency levels between new buildings and renovated ones;
• the use or non-use of the reference building method for calculation;
• the type of energy included in the calculation may cover heating, cooling, domestic hot water, air conditioning, lighting, and household appliances;
• the type of renewable energy used (biomass, solar, wind, geothermal, etc.) and the relationship between the building and the energy source are important factors;
• energy performance indicators include the maximum allowable value for primary energy;
• carbon emissions can be a primary indicator of energy performance (in countries such as England, Norway, or Spain) or a complementary indicator to primary energy (in countries like Austria or Romania).

3.2. Literature Review

3.2.1. The nZEBs in Europe

According to the study [82], nZEBs are structures designed to integrate commercially available renewable energy technologies alongside advanced energy-efficient construction strategies. The primary goal of these buildings is to minimize or completely eliminate the use of fossil fuels, thus contributing to the reduction of greenhouse gas emissions and promoting energy sustainability.

The EU aims to significantly reduce the energy consumption of buildings by implementing stricter energy standards and promoting the nZEB concept. Research in this area has evaluated the cost-effectiveness of various renovation measures for residential and non-residential buildings from the 1960s and 1970s, identifying optimal solutions to achieve the nZEB standard [83]. The conclusions of the study indicate that to ensure an accurate assessment of renovation costs, the harmonization of data collection methodologies at the European level is necessary. The European Commission should develop a framework in this regard. A detailed case study on the renovation of existing buildings in Europe [84] thoroughly examines several practical cases of renovating buildings to the nZEB standard in various European countries, presenting a wide range of data on energy consumption, costs, and implemented technical measures. The data shows that a variety of innovative technologies can significantly contribute to reducing energy consumption in the building sector. In order to create a space for dialogue and cooperation in the field of energy efficiency, the European Commission has provided the European Energy Efficiency Platform (E3P) [85] as an essential tool for collecting and disseminating information on building renovation towards the nZEB standard. Through a centralized database, the platform provides an overview of renovation projects implemented in member states, thus facilitating the exchange of knowledge and best practices in the field.

Another study emphasizes that efforts to develop residential nZEBs focus on designing these types of buildings by optimizing the interface between the building and the energy infrastructure, integrating renewable energy sources, and implementing innovative solutions to enhance energy efficiency [86]. A case study conducted on an existing residential building [87] demonstrated that modernization could lead to a significant reduction in greenhouse gas emissions, exceeding 60%, with a potential for further optimization, up to 96%, through the implementation of additional measures. The analysis revealed a significant correlation between economically optimal renovation solutions and those with the greatest emission reduction potential, such as thermally insulating the building envelope, replacing windows with double-glazed units, and installing heat pumps. These measures resulted in a reduction of greenhouse gas emissions by over 60%. By implementing advanced measures, such as enhanced thermal insulation, highly energy-efficient heating and cooling systems, and integrated solar technologies, a significant emission reduction of up to 96% was achieved.

The global market for products and services associated with the construction and renovation of nZEBs is projected to register a compound annual growth rate (CAGR) of 44.5% from 2014 to 2035, surpassing a value of $1.4 trillion in earlier years. This significant growth reflects the increasing adoption of the nZEB concept worldwide, driven by stricter energy efficiency requirements and growing concerns regarding climate change and sustainability [77]. The research conducted [84] proposes a paradigm for the design of nZEBs optimized from a cost perspective. The framework analysis includes factors such as initial and operating costs, energy prices, and local climate data. A notable result of the study indicates that the most common optimal configuration for nZEBs involves the use of superior insulation, ensuring the airtightness of the building, implementing A++ class energy efficiency devices, using efficient lighting systems, integrating energy management systems at the residential level, and installing photovoltaic panels. Depending on the climatic context, priority should be given either to airtightness and insulation in cold climate areas or to the efficiency of household appliances and minimizing solar gains in warm climate regions.

3.2.2. The nZEB Design Strategies

According to a previous study, savings of up to 76.57% can be achieved through the efficient implementation of nZEB design strategies [88]. For example, in cold temperate climates, for buildings with a Window-to-Wall Ratio (WWR) greater than 50%, adopting an appropriate solar shading strategy would be more energy-efficient and cost-effective than providing high-performance insulation for the building envelope (walls and windows) without shading. Moreover, in cold temperate climates, significant variations in the energy performance of a given shading configuration (in a specific building) can occur depending on the geographical location. Many studies have been reported on the energy performance of solar shading devices and high-performance insulation in buildings in various climatic locations, and a review of some of these exemplary recent studies is presented here.

A study [89] focused on the combined effect of glazing area, shading device properties, and shading control and demonstrated that an appropriate combination of these elements could bring significant savings in cooling and lighting demands for a building. In a residential building in Seoul, Korea, the research [90] observed that placing horizontal shading devices and vertical shading panels together could save 20% of the cooling energy demand. In a study on a typical house with a terrace roof in the Netherlands, another study [91] showed that solar shading could reduce cooling energy demand by up to 74%. The effect of the Window-to-Wall Ratio (WWR) on the energy performance of solar shading for office buildings in China was reported in the work [92], which found that for large WWRs, solar shading devices during the summer can bring significant savings in cooling demand and overall energy demand for the building. In paper [93], an automated shading system with a control strategy was proposed for commercial buildings, and its impact on cooling and lighting energy demand was studied, considering six different locations in the United States. They stated that the proposed shading system could save cooling energy by up to 40% and lighting energy by up to 25%. In study [94], optimal passive design strategies for buildings in heating-dominated, cooling-dominated, and mixed climates were proposed, and it was found that an optimal combination of passive solutions such as shading, insulation, and natural ventilation could save up to 54% of cooling energy demand in hot climates. Twenty-five different climate types were simulated to produce the best practices for minimizing building energy demands (cooling and heating), in addition to life-cycle costs. The adaptable thermal comfort of occupants was examined to obtain more practical and detailed passive design solutions. According to research [95], which explored the energy performance of insulation, shading, and glazing options in Spanish school buildings in a temperate continental climate, it was shown that a combination of these options could bring savings of 15.9% for heating and 17.7% for cooling. While the majority of studies on solar shading have focused on windows or glazing surfaces, there is growing interest in shading opaque wall surfaces (non-window areas), as demonstrated in a study on residential buildings in Hong Kong. It was also shown [96] that shading opaque walls can bring energy savings of up to 8%.

Polyurethane insulation and triple-glazed high-energy-efficiency windows are recognized for their superior energy performance; however, their effectiveness depends on many factors, such as the Window-to-Wall Ratio (WWR), the external climate where the building is located, and the geographical location. Researchers [97] studied the thermal and energy performance of polyurethane insulation enhanced with paper mixture, applied to buildings in various regions of Turkey. Polyurethane with a 3% paper mixture and a thickness of 0.0245 m demonstrated the best thermal resistance, achieving a maximum heat reduction of 14.6%. On the other hand, ref. [98] studied an insulation system with four types aimed at reducing energy consumption and environmental impact, applied to a multi-story building in a hot summer and cold winter climate in Iran. Among the various options, they identified eight solutions that could generally save over 70% of energy consumption, with the maximum savings (76.89%) achieved with polyurethane-based insulation. In study [99], a thin-glass triple-glazed window was proposed as a better alternative to standard double-glazed windows with thermal coatings and tested in three different climatic locations in the United States. Compared to regular glass, the energy-saving potential of this thin triple-glazed glass was 16% in Minneapolis (heating-dominated climate), 12% in Washington DC (mixed climate), and 7% in Houston (cooling-dominated climate).

Several studies have been conducted regarding the influence of climatic/geographical location on the energy efficiency of shading devices. For example, study [100] investigated the impact of the climatic location on the optimal Solar Heat Gain Coefficient (SHGC) of glazing surfaces and their energy performance. They considered eight cities each from the USA and China (16 in total), representing typical climate zones. They observed that the energy-saving potential of shading devices significantly depends on the climatic location, with the optimal SHGC potentially saving up to 37.8% energy in US cities and 24.8% in Chinese cities. Furthermore, Ref. [101] examined the effect of perforated exterior shutters on solar heat gain and natural light in an office building in the typical climate of China. They showed that the performance of perforated shutters depended on the perforation rate, shutter spacing, and the climate in which the building was located. The concept of a self-shading envelope was proposed in [102] as an excellent passive design option for controlling heat gain in a building, which was demonstrated in [103] in an office building in Malaysia. However, the literature is scarce regarding the appropriate combination of shading devices and self-shading in different types of buildings in various climatic and geographical locations. Additionally, there is the possibility of a comparative analysis of the effectiveness of shading and high-performance insulation for different types of buildings in various locations. Consequently, paper [104] presented a shading strategy that combines shading devices and a self-shading envelope for a multi-story hotel building in the hot-humid climate of Dhahran, Saudi Arabia (KSA), which could achieve a 20.5% reduction in annual energy consumption with a low additional cost. This saving was significantly higher than that achievable through improving the envelope with high-performance insulation (polyurethane insulation for walls and roofs and triple-glazed windows with thermal coatings) without shading. Study [105] aimed to conduct an extensive survey of over 60 residential buildings in the Al-Amarah region, Iraq, to investigate the use of the most common building materials in the building envelope structure. For each combination of construction elements, the collected data were tabulated and compared to identify trends and optimal alternatives. The results indicated that the use of reflective glass was the most effective option for exterior windows due to its properties in reducing solar gains and improving the energy efficiency of buildings. The study on the energy performance of passive facades [106] opens interesting perspectives for the development of nZEBs. By combining innovative materials, intelligent energy management systems, and efficient construction solutions, it is possible to create buildings that not only significantly reduce energy consumption but also generate renewable energy.

3.2.3. Smart Materials for nZEBs

Phase change materials (PCMs) are considered innovative in the field of architecture due to their ability to absorb significant amounts of heat at high temperatures while occupying relatively small volumes and release it at lower temperatures. This results in the suppression of interior temperature instability, providing a major advantage for their use in nZEBs [107]. In study [108], stabilized PCMs in form were integrated into the walls of light buildings (LBW), and it was found that PCMs can efficiently manage fluctuations and increases in indoor air temperature during the summer, reducing the maximum indoor temperature by 8.5 °C. Paper [109] discusses the implementation of stabilized PCMs in passive heating buildings, observing a reduction in radiant temperature nonuniformity by 20%. To assess the thermal inertia of light buildings integrated with PCMs, study [110] presents the development of a simplified method for calculating the PCM heat storage coefficient, based on dimensional analysis and numerical simulation. Using this method, Ref. [111] concluded that the thermal inertia index of light buildings can be improved by 60.3% when phase change materials are used. Additionally, paper [112] evaluated the impact of PCMs applied to light buildings in various climatic conditions using EnergyPlus 8.7. According to the studies, phase change materials can effectively minimize indoor discomfort, but the energy savings rate depends on the climatic conditions and the thermal insulation characteristics of the envelope. It can be deduced from these studies that PCMs applied to LBW can enhance thermal performance and maintain indoor temperature fluctuations within a comfort range. However, the effectiveness of thermal performance improvement mainly depends on the heat storage and release capacity of the PCMs, which in turn are highly influenced by the heat exchange between the wall surface and the ambient thermal environment for LBW integrated with PCMs.

Many researchers have evaluated the effects of PCMs on buildings in various external thermal environments. For example, in study [113], it was indicated that PCMs combined with shading devices under natural conditions are more effective in improving indoor thermal comfort in most climate zones in Algeria, with an energy savings potential ranging from 44.13% to 59.11%. Another study [114] presented an investigation of the influence of phase change materials on energy consumption in humid environments, finding that the energy savings rate decreased with increasing humidity from 1.64% to 1.32%, but the effect was not significant during winter. The effects of solar radiation were investigated in the paper [115], and it was concluded that appropriate PCMs can save up to 75% of heating requirements. Study [116] highlighted the high dependency of PCM performance on weather conditions and emphasized the need for the selection of different PCMs in various climatic regions. Regarding the PCM itself, several parameters [117] can significantly affect PCM performance, including PCM location, phase transition temperature, thickness, and latent heat. Other researchers [118] studied the effect of various PCM parameters on the time delay and the attenuation coefficient of the phase change stabilized-form wall panel based on the enthalpy method model and revealed that the phase transition temperature is an important factor affecting the evaluation index, with relatively optimal values of latent heat and thickness of PCMs under certain external heat disturbance. In study [119], a single-zone software solution from the EnergyPlus 8.7 program was used to study the thermal performance of PCM wall panels in light buildings in Shanghai from the perspectives of phase transition temperature, location, and thickness. It was found that the appropriate phase transition temperature varies for different room locations and exhibits seasonal differences due to varying solar radiation intensity.

PCMs demonstrate their effectiveness in constructing nZEBs both in the short term (daily) and long term (seasonal), using a variety of application techniques and materials. For example, study [120] presents the incorporation of microencapsulated phase change materials (e.g., paraffin wax) into drywall or plaster, which can significantly increase the thermal absorption capacity of lightweight construction walls (drywall). Other applications for active cooling systems involve the use of macroencapsulated eutectic salts [121] that melt at an appropriate temperature. Eutectic salts have a precise and well-defined melting point, making them ideal for applications that require accurate temperature control. The results of study [122] demonstrate that cogeneration systems with thermal storage represent a promising research and development direction in the energy sector. By optimizing the operating parameters of these systems and exploring new storage technologies, it is possible to significantly improve energy efficiency and expand their applications in the field of nZEBs.

3.2.4. Efficient Equipment for nZEBs

A case study of a single-family residential nZEBs [123] demonstrates how careful and integrated design of the building’s exterior and energy systems can not only meet almost the entire energy demand from renewable sources but also generate an energy surplus. Paper [124] proposed an innovative energy balance strategy for nZEBs, considering the actual energy demand of these buildings. The study highlighted the increasing demand for grid electricity, emphasizing the importance of ensuring that no non-renewable energy is used to power a home. The study also presented a summary of energy consumption measurement methodologies for nZEBs [125], proposed by organizations from eight different countries: Germany, Austria, Canada, Denmark, Italy, Norway, the United States, and Switzerland. Additionally, some research has previously been conducted on thermal satisfaction within NZEBs. One of the main objectives of the study was to make a clear distinction between the thermal parameters relevant to evaluating the thermal comfort of occupants in a net-zero energy building, thus providing a more precise framework for analyzing comfort conditions in such types of buildings.

Study [126] comprehensively addressed the possible causes and health implications associated with excess heat in nZEBs located in northern climate regions. The analysis identified factors such as high building airtightness, superior thermal insulation, and insufficient natural ventilation as the main contributors to the accumulation of excessive heat. Study [127] analyzed the most effective nZEB retrofit packages for semi-detached and terraced homes heated by gas in Ireland, considering the thermal efficiency of building materials and energy demand. The study examined various strategies for improving energy performance, aiming to identify optimal solutions that reduce energy consumption and enhance thermal comfort in homes. Study [128] compared the energy efficiency and initial costs associated with the installation of photovoltaic systems and HVAC (heating, ventilation, and air conditioning) in residential nZEBs, taking into account their variability across different climatic zones. In case study [129], experiments were conducted to evaluate the essential design variables in the development of an optimal design for nZEBs. The aim of the research was to identify parameter combinations that maximize energy performance and the long-term sustainability of nZEBs.

Solar energy was analyzed in this study as a viable solution for assessing the economic feasibility of implementing nZEBs in the non-residential sector of the United States. Paper [130] examined in detail the economic sustainability of solar-based nZEBs through the precise monitoring of technological advancements and dynamic changes in photovoltaic panels (PV). The results indicated that certain regions in the United States are suitable for implementing nZEBs integrated with PV systems, achieving a payback period of 8.44 years by 2040. Additionally, minor improvements in PV panel efficiency could enable buildings to reach the net-zero emissions target, reducing the payback period to less than 10 years by 2034. In the long term, starting from 2044, PV-integrated systems will have a payback period of approximately seven years in most analyzed regions, even in the absence of federal support for the solar investment tax credit. Thus, the efficiency of federal support for solar energy production could be maximized through a gradual annual reduction of 2.5% in this incentive, beginning in 2033. The study demonstrates the economic feasibility of implementing solar-based nZEBs in the non-residential sector of the United States, highlighting the combined impact of technological progress and cost reductions in the development of integrated photovoltaic systems. Another comparative study analyzed the performance of nZEBs with hybrid microgrids in temperate and tropical climates [131]. Climate data, collected over one year, were used to design a hybrid nZEB composed of photovoltaic (PV) modules and converters. The investment profitability was evaluated using economic indicators such as net present cost (NPC), payback period, and operational costs. The results highlight the economic feasibility of the investment, with a payback period of 1.84 years in Thailand and 2.66 years in Pakistan. Additionally, a reduction in the unit cost of electricity of 31% in Thailand and 27% in Pakistan was observed. Moreover, the proposed hybrid system was 9.5% more cost-effective in Thailand and 7.1% more cost-effective in Pakistan compared to the existing grid-connected system, leading to a decrease in the unit cost of electricity by 0.12 USD/kWh in Pakistan and 0.21 USD/kWh in Thailand.

Another study contributed to the nZEB renovation strategy in Estonia [132], analyzing how commercial property companies address the need for building renovations. The study results indicate that office building renovations have led to only marginal improvements in energy performance, with a performance gap of approximately 40% after market-driven renovations. About one-third of commercial buildings have met nZEB standards, while in the remaining cases, the performance gap was around 50%. The average renovation cost was approximately 10 €/m², whereas major renovations required between 130 and 150 €/m². A significant obstacle in this process is the long investment payback period, estimated at 20–30 years, along with the uncertainty of the measures required for large-scale renovations. This increases investment risk for real estate companies, which typically prefer a payback period of no more than 10 years. Interviewees emphasized that tenants are not motivated to invest, as the anticipated savings of 2–3% in rent and energy costs are perceived as insignificant. To initiate nZEB renovation, building owners suggested implementing a program for granting green loans with reduced interest rates for energy efficiency investments, accompanied by incentives and educational programs on energy-efficient renovations. Lastly, tax deductions for owners who modernize their buildings were proposed to encourage these practices.

4. Promoting Electricity Generated from Renewable Sources

An analysis of energy data at the European level indicates a significant contribution from the building sector to total energy consumption, accounting for nearly 40%, and to greenhouse gas emissions, at 36%, considering that 75% of buildings in the Union [133] are still underperforming in terms of energy efficiency (Figure 3).

Natural gas still plays a crucial role in building heating systems, accounting for approximately 39% of the energy used for space heating in the residential sector. Oil ranks second among fossil fuels used for heating, with a share of 11%, while coal contributes about 3% [135]. Consequently, it is essential to reduce energy consumption, in line with the principle of “energy efficiency first”, as stipulated in Article 3 of Directive 2023/1791/EU [41]. Greenhouse gas emissions generated by buildings are not limited to the operational phase. To fully decarbonize the real estate sector by 2050, a holistic approach is required, encompassing all phases of the building life cycle, with a focus on reducing the embodied emissions in construction materials [136].

The global decline in fossil fuel demand, as illustrated in Figure 4, from 501 exajoules in 2022 to 362 exajoules by 2030, carries profound implications for both the economy and society. This significant reduction can primarily be attributed to the rapid expansion of renewable energy sources, the increasing efficiency of energy use, and the implementation of government policies promoting decarbonization. The shift towards cleaner, more sustainable energy systems is not only reshaping the global energy landscape but also fostering greater energy independence and security for many nations [137]. As governments accelerate their efforts to meet climate goals and implement stricter regulations, fossil fuels’ share of the energy mix is expected to continue shrinking. This decline presents opportunities for industries that are pioneering innovations in renewable energy technologies, energy storage, and carbon capture solutions. The results shown in Figure 4 highlight the substantial potential to reduce reliance on fossil fuels, thereby contributing to mitigating the adverse effects of climate change. Additionally, the decrease in fossil fuel consumption is projected to play a crucial role in curbing greenhouse gas emissions, aiding in the achievement of global sustainability targets.

Promoting the generation of electricity from renewable energy sources is an urgent necessity in the current context [139], primarily justified by the need for environmental protection [140], increasing energy independence through reduced reliance on imports and the diversification of supply sources [141] and, not least, for economic and social cohesion reasons [142]. The Sun, the most important source of light, emits energy to Earth with a maximum intensity of solar radiation at the entrance to the atmosphere of 1367 W/m², known as the solar constant [143]. Solar energy represents the electromagnetic radiation emitted by the Sun, generated through nuclear fusion processes. It is the foundation of all life on Earth and accounts for approximately 1.8 × 10¹¹ MW of solar energy absorbed globally, sufficient to meet global energy needs [144]. The energy generated by the Sun is thousands of times greater than the entire amount of energy used by humanity. Solar light and heat have been harnessed by humans since ancient times, using technologies that have evolved continuously [145]. Solar radiation, along with other secondary energy sources such as wind, wave, hydroelectric, and biomass energy, constitutes the majority of renewable resources available on Earth, although only a small fraction of this energy is exploited [146]. Figure 5 illustrates the evolution of clean energy investments in developing countries and emerging markets, set within a scenario aligned with the global objective of achieving climate neutrality by 2050. The data underscore a remarkable increase in financial allocations for renewable energy projects, energy efficiency, and other green initiatives during the period from 2022 to 2030. Specifically, the graph reveals a threefold surge in investments, from 0.77 trillion USD in 2022 to 2.26 trillion USD in 2030.

This rise in investment reflects a strong commitment to sustainable energy and climate action. The shift in funding underscores the role of clean energy in environmental goals, economic growth, and energy security. Emerging markets are essential, balancing growing energy demands with the chance to bypass carbon-intensive systems for cleaner alternatives.

In 1839, Edmund Becquerel was the first to convert sunlight into electricity. In 1873, Willoughby Smith discovered photoconductivity in selenium. In 1883, Charles Fritts created the first design of a photovoltaic cell, using selenium plates. Albert Einstein explained the photoelectric effect in 1905, demonstrating how light releases electrons from the surface of metals, a discovery that earned him the Nobel Prize. In 1918, Jan Czochralski paved the way for silicon solar cells by developing a technique for growing single-crystal silicon [148]. In 1954, the first photovoltaic panel was created with the development of the crystalline silicon solar cell at Bell Labs in the U.S., with an energy conversion efficiency (PCE) of 4.5% [149]. Since then, researchers have actively explored low-cost device structures and new materials that exhibit the photovoltaic effect. This effort led to the emergence of second-generation solar cells, mainly based on III-V device structures. Thus, GaAs, CdTe, InP, and CIGS solar cells were introduced in the field of solar photovoltaics [150]. In the 1990s, the third generation of solar cells based on dye-sensitized structures appeared [151]. In the 2000s, organic photovoltaic (OPV) cells were introduced [152]. With the growing interest in nanomaterials, intensive research is focused on identifying new materials for solar devices that are not only affordable but also require less costly processing conditions. Currently, crystalline silicon solar cells dominate the market, but the high costs of manufacturing processes and raw materials are driving researchers to develop new manufacturing technologies for solar panels. Figure 6 illustrates the rapid expansion of renewable energy installed capacity in emerging markets and developing economies, excluding China, between 2022 and 2030. Both the STEPS scenario (declared policies scenario) and the NZE scenario (net-zero emissions policies scenario) show a significant increase in renewable energy production, highlighting these countries’ commitment to transitioning to more sustainable energy systems. In both scenarios, photovoltaic solar and wind energy lead this growth, benefiting from falling costs and favorable government policies [153]. However, the net-zero scenario indicates an even faster acceleration of renewable energy adoption, especially solar and wind energy, reflecting a more ambitious decarbonization goal for the economy [154]. Compared to the STEPS scenario, the NZE scenario shows a substantial increase in the ambition regarding photovoltaic solar energy development. The doubling of installed capacity from 728 GW to 1465 GW underscores the importance of this technology in achieving climate neutrality targets. This difference can be explained by the global adoption of more ambitious energy policies that will accelerate the energy transition while also considering the declining costs of solar technologies as a key factor [155].

Both the STEPS and NZE scenarios in Figure 7 highlight a significant increase in renewable energy production, underscoring the commitment of these economies to decarbonize their energy sectors. In both scenarios, photovoltaic solar and wind energy lead this growth, benefiting from declining costs and favorable government policies. However, the NZE scenario indicates an even faster acceleration of renewable energy adoption, particularly solar and wind, reflecting a more ambitious climate neutrality target [157]. This trend has significant implications for achieving the objectives of the Paris Agreement, as well as for enhancing energy security and the economic competitiveness of these regions [158].

Figure 8 highlights the significant increase in global renewable energy capacity under a scenario aimed at achieving climate neutrality by 2050. This exponential growth, represented by the tripling of installed capacity between 2022 and 2030, underscores a growing global commitment to the energy transition [160].

At the European level, renewable energy is considered one of the main components of the strategy to meet the growing energy demand [162], with a dynamic evolution of the share of renewable energy in total electricity production. The peaks observed in the graph in Figure 9 correspond to periods with optimal weather conditions for wind and solar energy production, while the declines can be attributed to factors such as reduced solar radiation intensity during the winter or periods of weak winds [163]. These fluctuations highlight the importance of developing efficient energy storage systems to ensure a stable and reliable electricity supply [164].

A strong seasonality in renewable energy production is highlighted, with peaks observed during the spring and summer months. Exceeding the 50% threshold in May 2023 and in the three consecutive months of 2024 underscores the significant potential of renewable sources, especially under favorable weather conditions. This trend in 2024 is supported by a combination of factors, including investments in new production capacities, technological advancements, and policies supporting renewable energy [165]. The current trend is to combine different renewable energy sources (wind, solar) into a single system, an approach known as hybridization, to overcome the individual limitations of each energy source [166].

Practical applications of renewable energy, such as air solar heater cogeneration systems [167], represent a dynamic and continually evolving research area. By analyzing the performance of these systems and the factors influencing their efficiency, we can identify innovative solutions to improve the energy efficiency of buildings. This study [168] explores the potential of off-grid hybrid systems based on wind and solar energy to provide a renewable alternative to traditional power grids. Through detailed experimental analysis, the performance of these systems is evaluated under real operating conditions, identifying the strengths and weaknesses of this technology.

The efficient utilization of renewable energy involves providing artificial heating and cooling of indoor spaces through heat pumps. The ground-source heat pump is one of the high-efficiency systems suitable for reducing energy consumption. Compared to the air-source heat pump, it generally has a higher coefficient of performance (COP) due to the more stable and favorable temperature of the ground compared to the outside air [169].

A study investigates the response of a ground-source heat pump and indoor comfort in an nZEB located in Vorarlberg, Austria, over the course of a year [170]. The optimization of the heat pump is conducted using mixed-integer linear programming (MILP) and a simplified RC model. The simulation controls the heat pump based on optimized power signals, leading to reductions of up to 49% in electricity costs and a 5% decrease in building energy consumption. Optimization enhances photovoltaic self-consumption and self-sufficiency by up to 4% while also reducing grid consumption by 5%. Pre-cooling strategies prevent summer overheating, ensuring indoor comfort in compliance with the EN 16798-1 standard, Class II [171].

Study [172] presents the results of a heat pump system that utilizes renewable solar energy, generated by PV as the primary source, assisted by a shallow geothermal application to meet space heating and domestic hot water (DHW) demands in a residential building in Austria. The system integrates phase change materials (PCMs) in specially designed containers, functioning as thermal storage modules, thereby providing an extended energy storage capacity for space heating and DHW subsystems. The performance of this system was compared to that of a conventional system, resulting in a reduction of primary energy consumption by 84.3%. Additionally, maintenance and operational costs were reduced by 79.7% compared to the conventional system, significantly contributing to the achievement of nZEB objectives. Although the initial installation cost of this system is higher than that of a conventional one, the substantial savings in operational costs led to a payback period estimated at 8.7 years. The study highlights that the combined use of a photovoltaic system and a heat pump serves as an effective means to reduce reliance on fossil fuels while simultaneously preventing the overloading of the electrical grid.

Another study on heat pumps [173] highlights that, unlike conventional systems used for comparison, heat pumps enable the achievement of the NZEB standard by balancing annual energy consumption with energy production derived exclusively from on-site renewable sources. This is accomplished by utilizing the available roof surface for installing energy production systems in both analyzed climatic conditions. Additionally, ground-source heat pumps allow for primary energy consumption equal to or lower than 57 kWh/m².

5. Discussion

Based on the analysis of the nZEB standard at the European level and the results of previous studies on defining nZEB principles, along with other relevant research, a series of essential recommendations can be formulated for creating a roadmap for nZEB implementation. The global policy package should include regulatory, facilitation, and communication aspects, supported by various appropriate tools [174]. It is recommended to conduct targeted communication campaigns, as they are key factors for the success of any implementation scheme. Clear communication is indispensable in providing information to consumers and market actors regarding available incentives and measures for improving energy efficiency. Additionally, engaging the general public and stakeholders in all stages of policy implementation in the building sector is important. The evaluation of the impact of planned policies, accompanied by a monitoring and control mechanism, is essential for obtaining a clear picture of the necessary measures and their effectiveness. Financial support for high energy performance should be encouraged through higher fund allocations or reduced interest rates for loans aimed at this purpose. This type of practice, inspired by international examples, can stimulate the widespread adoption of nZEB solutions [175].

The transition to nZEBs faces a series of significant challenges, the resolution of which is crucial for practical, sustainable, and long-term efficient implementation. Major concerns include achieving an optimal balance between energy efficiency and the integration of renewable energy sources, managing local and temporal energy fluctuations, and establishing appropriate energy performance standards. The complexity is further amplified by the integration of additional energy consumption parameters, including energy use within buildings, the energy impact over the building’s life cycle and during its decommissioning phase, as well as the feasibility of collective energy assessments. By addressing these challenges through rigorous evaluation and strategic planning, policymakers and industry stakeholders can facilitate the successful implementation of nZEBs, thus promoting a more sustainable and energy-efficient built environment [175].

In addition to the concept of nZEBs, minimizing energy consumption through efficient design should be a fundamental requirement in the design process and represent the highest priority for nZEBs. Energy efficiency is typically the most cost-effective approach, offering an optimal investment-to-outcome ratio. Prior to implementing renewable energy utilization strategies, optimizing energy efficiency opportunities can significantly reduce the costs associated with renewable energy integration projects, thereby contributing to lower overall project expenses and improving long-term financial viability [176].

Passive buildings are an example of excellence in terms of energy efficiency and sustainability. They significantly reduce energy losses and the need for heating and cooling through innovative design and high-quality building materials. By integrating renewable sources, such as solar and geothermal energy, passive buildings contribute to reducing carbon emissions and creating a healthier environment. Although they require higher initial investments, they generate long-term savings and serve as a crucial model for the future of sustainable construction.

Long-term programs are essential to providing a stable framework and facilitating the strategic planning of all stakeholders. Policymakers should focus on implementing such programs to ensure continuity and enhanced policy efficiency. Aligning building strategies with national and European energy and environmental strategies is important to avoid conflicts with other major policies and ensure their coherence. Within a member state, different tools must be coordinated to ensure their success. It is important to avoid overlap with other financial support instruments in order to offer simple and coherent market tools. These recommendations provide a solid foundation for developing an effective framework for implementing nZEB, ensuring a sustainable transition to nZEBs [177].

The concept of nZEBs represents a significant advancement in promoting energy sustainability in the EU and Romania. This standard stipulates that all new buildings must have nearly zero energy consumption, with the majority of the required energy being sourced from renewable sources. In the EU, the nZEB efficiency standard is part of a broader strategy to reduce carbon emissions and achieve climate goals. In Romania, the implementation of this standard brings challenges related to technological adaptation and initial costs but also offers valuable opportunities to enhance energy efficiency and reduce reliance on conventional resources. In conclusion, the nZEB efficiency concept is a crucial element for sustainable development and a greener, more energy-responsible future.

Romania’s energy strategy aims to develop the energy sector under conditions of security, sustainability, economic growth, and accessibility. The development of this sector is essential for the overall progress of Romania, ensuring a constant and uninterrupted availability of energy products and services at affordable prices for consumers, with a major priority being the increase in electricity production from renewable sources. In the context of energy security imperatives, the energy sector faces significant challenges due to market fluctuations and regional vulnerabilities. Thus, building a resilient energy sector capable of responding promptly and efficiently to any crisis is necessary, ensuring the continuity and security of energy supply. Romania possesses the natural, financial, and human resources required to modernize the energy sector in line with the European Union’s objectives of achieving climate neutrality by 2050. This sector must be prepared to support the transformation of the economy as a whole and contribute to improving the quality of life. As an EU member state, Romania is firmly committed to achieving greenhouse gas emission reduction targets, increasing the share of renewable energy sources, and improving energy efficiency across all sectors. In the current context of the energy crisis and energy transition, Romania must prioritize identifying the best solutions to ensure the complementarity between energy price accessibility, economic competitiveness, the transition to renewable energy sources, and reduced carbon emissions, using natural gas as a transition fuel, developing storage capacities, improving energy efficiency, extending the lifespan of oil and gas fields, diversifying energy sources from domestic production, strengthening energy transport and distribution networks, including regional interconnections, and creating a competitive and efficient energy market, with a focus on the role of the consumer and promoting prosumers.

6. Conclusions

European policy should include well-defined monitoring and control mechanisms aimed at ensuring compliance with energy performance standards and facilitating the transition to nZEBs. A clear legislative framework, supported by reporting tools and energy audits, could contribute to the efficient and uniform implementation of these standards across all member states. The active involvement of all stakeholders—public authorities, real estate developers, financial institutions, and end users—at all stages of planning, implementation, and monitoring is essential for creating a favorable environment for nZEB development. In this regard, an EU level evaluation system should be established to assess buildings’ readiness for implementing nZEB standards [178]. This system could serve as a basis for developing customized renovation and modernization strategies for existing buildings. Moreover, financial institutions should be encouraged to allocate greater resources to energy renovation projects through a comprehensive framework that enables the voluntary use of financial portfolios to improve building energy performance. Additionally, financial support in the form of subsidies, low-interest loans, and other economic incentives can accelerate the adoption of these standards, making investments in energy efficiency more accessible and attractive for property owners and developers. Adopting an integrated approach that combines clear regulations, flexible financing mechanisms, and awareness campaigns could significantly contribute to achieving European energy efficiency and carbon emission reduction goals.

In the context of nZEBs, the technologies and materials with the most promising development prospects include heat pump systems, which play a crucial role in enhancing the energy efficiency of buildings by extracting heat from the surrounding environment to meet heating and cooling needs. Innovative models, such as high-efficiency ground-source heat pumps, are continuously being developed and can operate optimally even in varied weather conditions. These systems not only reduce dependence on fossil energy sources but also contribute to lowering carbon emissions and promote the efficient use of locally produced renewable energy. Photovoltaic solar panels are important for reducing grid energy consumption by converting solar energy into usable electricity. Recent technologies, such as high-efficiency solar panels and building-integrated photovoltaics (BIPV), provide innovative solutions that not only generate energy but also enhance the aesthetics of buildings, allowing for harmonious integration into architectural design. PCMs have the capacity to store and release heat according to variations in ambient temperature. By utilizing PCMs, the energy efficiency of buildings is improved by maintaining comfortable indoor temperatures and reducing the energy required for heating and cooling. The use of advanced insulation materials, such as polyurethane foam or nanotechnology-based materials, significantly contributes to reducing heat loss. These solutions not only improve the energy efficiency of buildings but also help create more comfortable indoor environments with minimal temperature fluctuations. BEMS play a crucial role in monitoring and controlling energy consumption in buildings, optimizing resource use and enhancing overall energy efficiency. By integrating smart technologies, BEMS enable operations to adapt to user needs, contributing to significant energy savings. Heat recovery ventilation significantly improves indoor air quality by filtering and reheating fresh air while reducing the energy consumption needed for heating or cooling. This ensures a healthy and comfortable indoor environment, further contributing to the energy efficiency of the building. By integrating these technologies and materials, nZEBs become not only sustainable but also comfortable, having a positive impact on the environment and contributing to global carbon emission reduction goals.

The efficiency of renewable energy sources is influenced by several factors, including the type of source used, the technology implemented, and local conditions. For example, photovoltaic solar panels, which convert sunlight into electricity, generally have a conversion efficiency ranging between 15% and 22%. These panels are essential in the transition to a sustainable energy system, and their efficiency can be maximized by placing them in locations with maximum sun exposure. Additionally, integrating energy storage systems, such as batteries, allows for the management of surplus energy generated on sunny days, ensuring a constant energy flow even in the absence of sunlight. Solar thermal panels, which convert solar energy into heat, can achieve significant efficiencies of 70–80%. They are frequently used for heating domestic water and spaces, thereby contributing to the reduction of energy consumption from conventional sources.

In the field of wind energy, modern turbines can reach conversion efficiencies of up to 45%, and under optimal conditions, this percentage can rise to 50–60% of the wind’s kinetic energy. Placing wind turbines in areas with consistent and strong winds is crucial for maximizing energy production. Advanced monitoring technologies allow for the optimization of turbine operation and their integration into electrical grids, thus ensuring the stability of the energy system. Hydroelectric plants stand out with conversion efficiencies that can vary between 70% and 90%, thanks to the efficient use of the kinetic energy of water. The careful management of water resources, along with the use of accumulation systems, enables the maximization of energy production and responsiveness to variable consumption demands. Other renewable sources, such as biomass and geothermal energy, also contribute to diversifying the energy mix. Converting biomass into electricity provides a sustainable solution, while geothermal sources harness the Earth’s natural heat for heating and electricity generation.

To ensure the efficient use of these renewable sources, it is essential to implement a series of strategies. Combining different renewable sources allows for a constant energy supply, reducing dependence on a single source. Investments in storage technologies, such as advanced batteries, are vital for managing fluctuations in production and consumption, ensuring optimal balance in the grid. Implementing energy management solutions plays a significant role in reducing consumption during peak periods and maximizing the use of renewable energy. These solutions not only optimize energy performance but also help reduce costs. Furthermore, supporting research and development of innovative new technologies is essential for improving the efficiency and accessibility of renewable energy sources, thereby facilitating the transition to a more sustainable energy future. This integrated approach will enable the maximum utilization of renewable resources, positively impacting the environment and contributing to global carbon emission reduction goals.

The limitations and technical, economic, and social challenges regarding the development and implementation of nZEBs are varied and complex. One of the biggest obstacles to the adoption of nZEBs is the high initial costs. The investments required for technologies such as solar panels, heat pumps, and the use of smart materials can discourage many building owners. They may perceive such expenses as a financial burden, especially in the context of an unstable economy. To overcome these limitations and challenges, it is essential to implement supportive policies, invest in public education, and develop initiatives that facilitate access to nZEB technologies. A collaborative approach involving governments, the private sector, and communities will be crucial to ensure a successful transition to more energy-efficient and sustainable buildings.

Additionally, integrating modern systems into existing infrastructure can be problematic, particularly in older buildings that are not designed to support current technologies. This may require costly and complex modifications, further increasing the financial burden on developers. The efficiency of renewable technologies also depends on local climatic conditions, such as sunlight exposure or wind speed. This variability may limit the applicability of nZEB solutions in certain regions, making them less attractive for investment.

Limited access to financing for renovations and nZEB constructions can represent another major obstacle, especially for small building owners or disadvantaged communities. Without adequate financial support, the implementation of these solutions becomes a challenge, perpetuating economic inequalities in the energy sector. Furthermore, there is an increasing need for qualified specialists in sustainable construction and energy efficiency technologies. The lack of a well-trained workforce can delay or even block the implementation of nZEB projects.

Beyond technical and economic aspects, there are also significant social challenges. A limited number of consumers are aware of the benefits of nZEBs, which can affect the acceptance and demand for sustainable solutions. Awareness campaigns are essential to educate the public and demonstrate the advantages of these solutions, both in terms of energy savings and environmental impact.

Moreover, there is often resistance to change among consumers and construction professionals, who may be reluctant to adopt new technologies and building methods. This resistance may be fueled by concerns about costs, unfamiliarity with new technologies, or a lack of trust in their effectiveness. Additionally, access to nZEB technologies varies significantly among different socio-economic groups, creating a disparity in economic and environmental benefits. Disadvantaged communities, which need energy efficiency solutions the most, often have limited access to these technologies, thus perpetuating existing inequalities.