Energy Storage and Energy Efficiency in Buildings and Cities

Abstract

The primary challenge for European society today is to strike a balance between maximizing energy efficiency and environmental care, while also ensuring an accessible and safe living environment. The research presented in this Special Issue addressed various aspects of energy storage methods and covered advances in the energy efficiency of buildings and cities in light of the climate change awareness and the need to reduce energy consumption and the carbon footprint from the built environment. Results of empirical and modelling research were compared to advanced simulations and measurements rooted in real-world case studies performed with the purpose of extending the knowledge on holistic sustainable design towards efficient energy use. Key aspects enabling improvements in the energy performance of buildings and contributing to the achievement of climate goals cover thermal comfort and overheating in buildings and cities, including district heating, hydrogen energy storage, renewable energy source integration, carbon emissions, and the economic benefits of building deep renovation. The research findings help us to understand the critical importance of transforming the built environment into renewable energy sources while supporting the energy efficiency of buildings, cities, and neighbourhoods.

1. Introduction

Despite several efforts to reduce the environmental impact of the construction sector, in 2025, buildings remain the largest energy consumer, as they account for approximately 32% of global energy consumption and around 34% of greenhouse gas (GHG) emissions, primarily due to heating, cooling, and domestic hot water use [1]. The effective utilization of renewable energy sources (RESs) in buildings and their energy efficiency are critical challenges in the global climate mitigation agenda. The Energy Performance of Buildings Directive has been one of the primary laws in the European Union governing the sustainability of buildings, with the aim of achieving the full decarbonization of the building sector by 2050 [2]. The building sector is therefore crucial to achieving the EU’s energy and climate goals. The Special Issue “Energy Storage and Energy Efficiency in Buildings and Cities” in Energies brings forward recent empirical and modelling studies rooted in real-world case studies and advanced simulations, advancing knowledge on holistic sustainable design. The results of thorough studies presented by seven research groups have been chosen for this Special Issue. The selected studies address thermal comfort and overheating on both the building [3] and city scale [4], which is followed by a district heating configuration comparative analysis [5]. Energy and carbon emissions in the context of adaptation to climate change have been considered by Widera [6] and Yang, Hamza, and Gilroy [7]. Building-Integrated Photovoltaics in building renovation has been proposed by Del Hierro López et al. [8], while Sadowski [9] analyzed the payback periods for external wall insulation.

2. Thermal Comfort and Overheating Assessment

In order to increase thermal comfort and reduce heat loss in cold climates, one of the basic strategies for improving the energy efficiency of buildings is increasing airtightness via thermal insulation. In some cases, however, these actions may increase the risk of overheating in warmer seasons, especially in the context of global temperature increases [10]. For instance, in Ireland, warmer summers are predicted to result in higher temperatures inside residential buildings, which poses a challenge to the thermal comfort and well-being of residents [11]. Sajadirad, O’Hegarty, and Kinnane’s [3] analysis of 1100 social housing units in Dublin contrasts adaptive and static thermal comfort metrics. Results show low actual overheating risk but identify critical methodological gaps in static evaluation frameworks, underscoring the need for refinement in thermal comfort assessment.

2.1. Urban Heat Island and Energy Demand

The Urban Heat Island (UHI) effect develops due to locally elevated higher temperatures across urban areas compared to their rural surroundings. The atmosphere across urban areas undergoes significant changes due to surface heat exchange, which gives rise to distinct radiative, thermal, and aerodynamic properties [12]. Hashemi et al. [4] model the Urban Heat Island (UHI) effect, showing its amplification of cooling demand in Des Moines, USA. Their study highlights that resilient energy planning must integrate climate projections, UHI mitigation, and enhanced building standards to protect vulnerable urban populations. An applied UWG model enables the simplified representation of urban climate dynamics, incorporating building typologies, vegetation, and urban morphology. The authors indicate that integrating high-resolution land use datasets, traffic-based heat emission models, and computational fluid dynamics simulations could enhance model accuracy while incorporating socioeconomic vulnerability assessments, providing a more comprehensive understanding of how energy demand shifts may disproportionately impact disadvantaged communities. The issue of Urban Heat Island mitigation was also addressed by Kubilay et al. [13], while Yang et al. [14] analyzed the influence of buildings’ retrofitting on heat reduction in cities.

Extreme heat resulting from climate change and exacerbated by the UHI effect poses a serious threat to the living conditions of rapidly growing urban populations and is responsible for the loss of vegetation and biodiversity in cities [15,16]. In order to limit these negative effects, strategies are currently being developed to reduce high temperatures in cities, such as the use of reflective or permeable materials, as well as solutions that allow water to be retained in the city structure. This is achieved by elements of blue–green infrastructure such as urban greenery and water reservoirs, but also by optimizing urban geometry [17]. Shi et al. performed modelling of temperature reduction in urban areas with green–blue infrastructures [18], and Stern et al. proposed a methodological approach for their design [19].

2.2. System-Level Modelling for District Energy

Kiss, Horváth, and Szánthó applied MATLAB Simulink to compare three district heating substation configurations [5]. They demonstrated that a triple-heat-exchanger setup yields the lowest return temperatures, enabling higher renewable heat integration—validating the importance of system-level thermal control. Their evaluation of the simulation results revealed that none of the examined configurations fully utilized the available 300 L of domestic hot water (DHW) storage capacity and that, in all cases, only approximately two-thirds of the total volume was effectively discharged. Additionally, the simulations highlighted a potential risk of overheating in two out of three examined configurations due to the absence of effective control on the DHW preheater and a high return temperature from DHW circulation [20]. However, when correctly designed, the optimized district heating can reduce return temperatures, while lower return temperatures increase the efficiency of the system [21]. It has also been confirmed that the adequate storage methods in the district scale support the integration of renewable energy sources [22,23].

2.3. Rural Buildings, User Behaviour, and Energy Monitoring

Energy modelling and monitoring is critical not only for urban, but also for rural areas. Yang, Hamza, and Gilroy [7] investigated rural Chinese housing behaviour, revealing persistently high energy intensity and discomfort. Their mixed-methods analysis underscores that installing sensors and long-term monitoring across seasons can inform more targeted interventions for extreme climates. Ozarisoy & Elsharkawy [24] highlighted that the correct identification of occupants’ needs in terms of their thermal comfort requires a better understanding of the impact of energy efficient technologies on occupants’ well-being and health. Widera [6] identified a shift in environmental attitudes and highlighted the imperative to support GHG-reducing lifestyles through prosumer ecosystems and sustainable mobility, stressing that policies must incentivize green community engagement for lasting transformation. Chen et al. [25] presented decarbonization technologies and initiatives, analyzing negative emissions technologies, and discussed carbon trading and carbon tax approaches. They also underscored the role of the resilience of contemporary buildings and cities in the development of decentralized energy systems.

3. Innovative Technologies for Renewable Energy in Buildings

3.1. Integrated Photovoltaics (BIPVs) in Renovation

Building-Integrated Photovoltaics (BIPVs) is an innovative technology that enables the combination of renewable energy production with a variety of visually interesting building envelope solutions. BIPVs can be applied to a range of different materials and colours, which can be implemented on almost any surface or part of a building [26]. BIPVs supports the transformation of buildings from energy consumers to energy producers, thus playing a significant role in the decarbonization process [27] and in building energy independence for cities, reducing energy consumption and greenhouse gas emissions [28]. Del Hierro López et al. [8] introduce a typology and design catalogue for integrating Building-Integrated Photovoltaics (BIPVs) into low-income housing in Spain, addressing both architectural integration and economic vulnerability to energy costs, a reflection of growing cooling demand amid climate shifts. At the same time, a lot of effort is dedicated to the thermal modernization of existing building stocks [29,30]. Sadowski’s [9] study on thermal retrofits finds that payback periods for external wall insulation (CPP) are generally short, varying by wall type, insulation material, and region, making such investments both environmentally and economically sound.

3.2. Advances in Energy Storage in Buildings

A growing body of work illustrates that energy storage systems (ESSs) are an effective solution to intermittent RES coupling and spatiotemporal mismatch. Coccato et al. [31] categorized battery applications at the individual, shared, and district levels, highlighting hierarchical optimization frameworks and state-of-charge/degradation modelling. Walichnowska et al. [32] find that ESSs enhance photovoltaic (PV) systems’ operational efficiency, reducing losses by up to 6% and boosting energy self-sufficiency in residential units. Several reviews stress the critical role of intelligent control systems in energy flexibility. Li et al. [33] underscore how smart building management, using data analytics and AI, enables dynamic demand responses, increasing flexibility and resilience.

Seasonal thermal storage received attention from Hiris at al. [34], who outlined the deployment of large-scale storage media (up to 100,000 m³) to decouple renewable supply and demand across seasons within district energy systems, while Li et al. [33] also demonstrated that co-optimizing thermal and battery storage with model-predictive control reduces operational cost and CO₂ output in university buildings. Hydrogen-based energy storage methods currently seem to be some of the most promising. Fangqin et al. [35] indicate magnesium hydride’s potential in hydrogen storage because of its high hydrogen storage capacity, good cyclic durability, and reasonable cost. He et al. demonstrated that the embedding of CdS on MOFs could significantly increase the photocatalytic efficiency of CdS for visible light-driven hydrogen production [36]. Green hydrogen production using a proton exchange membrane (PEM) electrolyzer provides a scalable solution for energy storage [37,38,39]. Al-Ghussain et al. explored the feasibility of green hydrogen production using excess energy from a country-scale 100% solar–wind renewable energy system [40]. Liu et al. analyzed an electric–thermal–hydrogen generation system comprising photovoltaics, wind power, solar thermal power generation, electrolytic cells, a hydrogen storage tank, and proton exchange membrane fuel cells [41].

The integration of renewable energy sources (RESs), energy storage systems (ESSs), and energy efficiency (EE) in buildings is pivotal for reducing greenhouse gas emissions and advancing climate-neutral built environments [42]. Recent studies have provided a classification of advanced storage-control systems that balance energy flexibility and financial resilience [43], as well as propose innovative methods of building energy performance evaluation within urban environments and communities [44]. Research on building-level ESS coordination also shows enhanced PV efficiency, reduced losses, and greater self-consumption. AI-driven HVAC optimization confirms increased energy savings with maintained comfort while relying on high-quality operational data [45].

The global transition to sustainable energy sources has resulted in a rapid increase in the integration of renewable energy sources (RESs) into existing power grids [46,47]. Artificial intelligence (AI) methods have been a promising avenue for integrating multiple energy sources, particularly where data is incomplete, fragmented, or uncertain. They also allow for estimative and predictive methodological approaches [48]. The use of AI has reduced errors and uncertainties in RESs optimization, including resource assessment, energy forecasting, system monitoring, control strategies, and grid integration. Machine learning algorithm rules and models, as well as neural network layers and optimization techniques, are examined for their role in complex datasets, reducing error rates, increasing predictive capabilities, and used in dynamic RESs adaptation [49]. Anticipated advances in AI, such as explainable AI, reinforcement learning, and edge computing, are crucial in the context of RESs optimization, as the potential benefits of overcoming these challenges include increased energy efficiency, reduced operating costs, and improved grid stability [50]. The development of AI applications in the renewable energy sector will foster its integration with smart grids, decentralized and off-grid energy systems, electromobility, and advanced autonomous energy management systems [50].

4. EU Legislative Framework and Its Significance

The Energy Efficiency Directive (EED-EU/2023/1791) [51], effective October 2023, codified “energy efficiency first” principles, targeting near-zero energy renovations, especially of public buildings. The Renewable Energy Directive (RED II) also mandates 42.5% RES shares by 2030, reinforcing the impetus for PV ESS deployment under EPBD. Ecodesign regulation aligns product-level standards (e.g., windows, insulation) with performance goals. The revised Energy Performance of Buildings Directive (EPBD-EU/2024/1275) [52], adopted on 24 April 2024 and in force since 28 May 2024, mandates the following:

• A decarbonized EU building stock by 2050;
• Minimum Energy Performance Standards (MEPSs) for poorly performing buildings;
• Zero-Emission Buildings (ZEBs) by 2028 for public structures and 2030 for all new buildings;
• Integration of solar technologies and digital energy management;
• Lifecycle GWP evaluation and renovation passports.

The European Union Member States must transpose it by May 2026, supported by one-stop shops, financing incentives, and protection for vulnerable populations [53].

5. Conclusions

The convergence of RESs, storage technologies, thermal retrofits, smart controls, user engagement, and strategic policies provides a robust foundation for energy-efficient, low-carbon buildings and districts. The academic literature and EU legislative directives align to prioritize integrated, data-driven, and user-centric methodologies. This Special Issue’s key findings include the following:

• Holistic integration: Combined approaches—optimizing thermal envelope, smart HVAC, PVs, ESSs, and controls—are most effective, as affirmed across multiple case studies and modelling exercises;
• Storage’s enabling role: ESSs smooth PV variability, enhance self-consumption, contribute to grid stability, and unlock cost and emission reductions;
• Intelligent control architectures: AI-assisted management systems and model-predictive control are critical for the real-time balancing of comfort, efficiency, and demand responses;
• Policy alignment: EPBD and EED frameworks create strong regulatory drivers for EE, ZEBs, solar-ready design, and digital building management;
• Climate resilience: Retrofitting and storage strategies significantly guard against overheating and Urban Heat Island effects, especially in at-risk communities.

Considering the above, policymakers should invest in energy renovation schemes targeting MEPS compliance, fund research–practice demonstration projects for combined RESs and ESSs, and encourage capacity building via one-stop shops and training programmes. At the same time, the role of researchers and industry is to continue developing scalable, real-world holistic optimization platforms to systematically include occupant behaviours and long-term field monitoring and investigate large-scale seasonal storage in urban and district energy systems. Only through multi-disciplinary collaboration, engaging engineers, planners, investors, inhabitants, and regulators, can the objectives of net-zero urban environments be achieved by 2050.