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Energy Efficiency and Energy Saving in Buildings

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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "G: Energy and Buildings".

Deadline for manuscript submissions: closed (15 January 2026) | Viewed by 14668

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Prof. Dr. José Luis Molina Félix

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Guest Editor

Department of Energy Engineering, University of Seville, 41092 Sevilla, Spain
Interests: thermal modeling and simulation of buildings; building elements and HVAC systems and components; optimization of thermal systems; object-oriented programming of thermal models
Special Issues, Collections and Topics in MDPI journals

Dr. Teresa Rocío Palomo Amores

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Guest Editor

Department of Energy Engineering, University of Seville, 41092 Sevilla, Spain
Interests: outdoor thermal comfort; characterization and integration of passive heating and cooling techniques; energy efficiency
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The investigation of energy efficiency in buildings remains a fundamental aspect of addressing the contemporary climatic, economic and social challenges. In recent decades, research has made significant advances, resulting in the development of strategies that enhance indoor comfort while simultaneously reducing energy consumption. Nevertheless, it is not always the case that these solutions entail a reduction in the use of resources. In light of the ongoing expansion of urban areas and the increasingly evident effects of climate change, the pursuit of more efficient and sustainable measures has assumed a heightened urgency. While advances in architectural design, innovative materials and smart technologies have offered promising solutions, further exploration is required in order to ascertain their implementation, scalability and affordability.

This Special Issue aims to collate research that investigates innovative strategies to enhance the energy efficiency of buildings, striking a balance between comfort and sustainability. It encompasses studies that address challenges such as passive system modeling, real-world applications, and studies that promote practical and scientifically sound solutions that are applicable in different contexts.

Topics of interest for publication include, but are not limited to, the following:

Passive cooling/heating strategies in buildings.
The integration of sustainable and innovative interior conditioning technologies.
Integral solutions to reduce energy consumption in buildings through intelligent systems and/or renewable energies.
Methods for assessing energy efficiency throughout the life cycle of buildings.
The relationship between the reduction in indoor consumption and its impact on outdoor comfort.
Active thermal storage systems in buildings.
An evaluation of sustainable cooling or heating measures in different climates.
Indoor comfort implications of measures applied to a building.

Prof. Dr. José Luis Molina Félix
Dr. Teresa Rocío Palomo Amores
Guest Editors

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Keywords

energy efficiency in buildings
passives strategies
indoor comfort
sustainable technologies in buildings

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Published Papers (10 papers)

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Research

22 pages, 2785 KB

Open AccessArticle

Intelligent Optimization of Ground-Source Heat Pump Systems Based on Gray-Box Modeling

by Kui Wang, Zijian Shuai and Ye Yao

Energies 2026, 19(3), 608; https://doi.org/10.3390/en19030608 - 24 Jan 2026

Cited by 1 | Viewed by 510

Abstract

Ground-source heat pump (GSHP) systems are widely regarded as an energy-efficient solution for building heating and cooling. However, their actual performance in large commercial buildings is often limited by rigid control strategies, insufficient equipment coordination, and suboptimal load matching. In the Liuzhou Fengqing Port commercial complex, the seasonal coefficient of performance (SCOP) of the GSHP system remains at a relatively low level of 3.0–3.5 under conventional operation. To address these challenges, this study proposes a gray-box-model-based cooperative optimization and group control strategy for GSHP systems. A hybrid gray-box modeling approach (YFU model), integrating physical-mechanism modeling with data-driven parameter identification, is developed to characterize the energy consumption behavior of GSHP units and variable-frequency pumps. On this basis, a multi-equipment cooperative optimization framework is established to coordinate GSHP unit on/off scheduling, load allocation, and pump staging. In addition, continuous operational variables (e.g., chilled-water supply temperature and circulation flow rate) are globally optimized within a hierarchical control structure. The proposed strategy is validated through both simulation analysis and on-site field implementation, demonstrating significant improvements in system energy efficiency, with annual electricity savings of no less than 3.6 × 10⁵ kWh and an increase in SCOP from approximately 3.2 to above 4.0. The results indicate that the proposed framework offers strong interpretability, robustness, and engineering applicability. It also provides a reusable technical paradigm for intelligent energy-saving retrofits of GSHP systems in large commercial buildings. Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

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18 pages, 3642 KB

Open AccessArticle

Analysis of Useful Energy Demand for Heating Purposes in a Building with a Self-Supporting Polystyrene Structure in a Temperate Climate

by Krzysztof Wąs, Grzegorz Nawalany and Miroslav Žitňák

Energies 2025, 18(24), 6514; https://doi.org/10.3390/en18246514 - 12 Dec 2025

Viewed by 622

Abstract

This article presents an analysis of the useful energy demand for heating purposes in a single-family, single-storey building with a self-supporting polystyrene structure, which is a relatively niche solution, in relation to a traditional masonry structure with similar partition thickness. The structures considered met the requirements for passive buildings. The analysis was performed on three locations in Europe with a temperate climate, i.e., Kołobrzeg in Poland, Vienna in Austria, and Essen in Germany. The research showed significant savings in the energy demand of the polystyrene structure compared to the masonry structure for each location, ranging from 38% to 52%. Similarly, the heating period was 21% to 38% shorter in individual locations. This shows that polystyrene construction allows for a significant reduction in building energy demand, leading to lower operating costs. Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

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28 pages, 4700 KB

Open AccessFeature PaperArticle

From Data to Action: A Methodological Approach to Address Energy Poverty in Private Multi-Family Buildings

by Alberto Lodovico Ghiberti, Giorgio Dutto, Maria Ferrara and Enrico Fabrizio

Energies 2025, 18(23), 6194; https://doi.org/10.3390/en18236194 - 26 Nov 2025

Cited by 1 | Viewed by 660

Abstract

Achieving the decarbonization of the building stock by 2050 requires not only technological innovation but also strategies capable of addressing energy poverty, which threatens to exclude millions of households from a fair transition. Measuring this phenomenon remains challenging: at the European level, monitoring systems rely mainly on aggregated statistics, useful for territorial comparisons but often too approximate to describe the conditions of individual households and dwellings. This paper proposes a building-scale methodology that integrates socio-economic and technical data collected directly through surveys, interviews, and utility bills. The approach was applied to a private multi-family building built in the early twentieth century in Turin (Italy), involving 16 households. Results indicate that 31% of households exceed the 10% energy expenditure threshold, with heating emerging as the main cost driver. Correlation analyses suggest that single parameters such as income or dwelling size are not sufficient on their own to explain vulnerability, whereas the integration of socio-technical factors provides a more detailed picture of household conditions. Based on this evidence, four intervention strategies were developed, ranging from the insulation of the envelope to the installation of photovoltaics, conceived to be implemented progressively according to real technical and economic constraints. The novelty of this study lies in linking building-scale evidence with concrete design solutions, bridging the gap between measurement and action. Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

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24 pages, 3203 KB

Open AccessArticle

Optimising Behavioural Control Based on Actual HVAC Use in Naturally Ventilated Buildings

by Jiajing Wu, Rongxin Qiu, Xiaoyu Ying, Shuqin Chen and Xueyuan Zhao

Energies 2025, 18(23), 6130; https://doi.org/10.3390/en18236130 - 23 Nov 2025

Cited by 2 | Viewed by 579

Abstract

Multi-objective optimisation is essential for balancing building energy efficiency and thermal comfort. Existing research primarily focuses on passive optimisation strategies that assume fixed behavioural patterns of a ‘rational occupant’. However, these studies often overlook the impact of stochastic occupant behaviour on building energy efficiency and thermal comfort. Furthermore, they fail to consider the inherent randomness, variability, dynamic nature, and feedback mechanisms of individual actions. As a result, this oversight can lead to suboptimal energy efficiency, insufficient thermal comfort, and a poor user experience. This study examines a naturally ventilated research building equipped with split-type air conditioning in China’s hot summer and cold winter climate zone. The research develops a rapid prediction model for air conditioning (AC) energy consumption and thermal comfort based on actual HVAC behaviours, incorporating the AC and natural ventilation (NV) operation schedules. The model utilises Artificial Neural Networks (ANNs), importance analysis, and batch simulation. Furthermore, a multi-objective optimisation decision-making model is developed to balance building AC energy consumption and indoor environmental thermal comfort, using the NSGA-II algorithm. The results indicate that when building design parameters comply with the current energy-saving design standards, behavioural optimisation can lead to a 31.4% reduction in energy use for building AC systems while enhancing thermal comfort by 37.5%. Furthermore, by implementing integrated optimisation strategies, comfort can be improved by as much as 92.6% without raising energy consumption. Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

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37 pages, 364 KB

Open AccessArticle

Comparative Framework for Climate-Responsive Selection of Phase Change Materials in Energy-Efficient Buildings

by Javier Martínez-Gómez

Energies 2025, 18(22), 5982; https://doi.org/10.3390/en18225982 - 14 Nov 2025

Viewed by 955

Abstract

Integrating phase change materials (PCMs) into buildings and HVAC systems improves thermal comfort and energy efficiency. This study presents a climate-responsive methodology for selecting optimal PCMs using a multi-criteria decision-making (MCDM) framework. AHP was employed to determine the relative importance of key thermophysical properties, including melting point (47.5%), latent heat of fusion (25.7%), volumetric latent heat (13.5%), thermal conductivity (6.8%), specific heat capacity (3.3%), and density (3.3%). These weights were applied across five MCDM techniques—COPRAS, VIKOR, TOPSIS, MOORA, and PROMETHEE II—to evaluate 16 PCM alternatives for three representative climate zones: temperate (18 °C), subtropical (23 °C), and tropical hot/desert (28 °C). The results consistently identified n-Heptadecane (C17) as the most suitable PCM for temperate and subtropical climates, while n-Octadecane (C18) and hydrated salts such as CaCl₂·6H₂O and Na₂CO₃·10H₂O were optimal for tropical zones. Results show that n-Heptadecane (C17) is optimal for temperate and subtropical zones (COPRAS K = 1.00; TOPSIS C = 0.79–0.82; PROMETHEE φ = 0.21–0.22), while n-Octadecane (C18) and hydrated salts such as CaCl₂·6H₂O and Na₂CO₃·10H₂O perform best in tropical climates (TOPSIS C = 0.85; PROMETHEE φ = 0.26). These PCMs offer high latent heat (up to 254 kJ·kg⁻¹) and volumetric storage (up to 381 MJ·m⁻³), enabling significant reductions in HVAC loads and improved indoor temperature stability. The convergence of rankings across methods and alignment with existing literature validate the robustness of the proposed approach. This framework supports informed material selection for sustainable building design and can be adapted to other climate-sensitive engineering applications. The framework introduces methodological innovations by explicitly mapping PCM melting points to climate-specific comfort bands, incorporating volumetric latent heat, and validating rankings through cross-method convergence (Spearman ρ > 0.99). Sensitivity analysis confirms robustness against weight perturbations. The approach supports practical PCM selection for both new and retrofit buildings, contributing to EU and US energy goals (e.g., 40% building energy use, DOE’s 50% reduction target). Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

20 pages, 2077 KB

Open AccessArticle

Assessing the Thermal Storage Potential of Timber and Hybrid Activated Slabs: A Simulation-Based Comparison of Different Construction Types

by Andrea Agner and Doris Österreicher

Energies 2025, 18(21), 5691; https://doi.org/10.3390/en18215691 - 29 Oct 2025

Cited by 1 | Viewed by 922

Abstract

Thermally activated building systems (TABS) rely on high thermal mass materials, such as concrete, which perform well thermally but have a high carbon footprint. This study systematically investigates the thermal behavior of bio-based materials—spruce, pine, beech, and oak—in TABS using numerical simulations, comparing them with conventional and hybrid materials like concrete and clay. A total of 120 variants were simulated with different pipe diameters, spacing, embedment depths, and inlet temperatures. Thermal properties, particularly thermal conductivity and specific heat capacity, significantly influenced component activation efficiency. Concrete exhibited a characteristic cooling time of 71 h at an inlet temperature of 26 °C (pipe diameter 16 mm), while pine reached 80 h under the same conditions. The use of capillary tube mats extended the cooling times to 75 h for concrete and 92 h for pine. Although concrete provides the best thermal performance, certain bio-based materials achieve comparable results under optimized conditions. Hybrid systems with mineral components offer additional potential for improvement. These findings demonstrate that ecologically sustainable component activation using bio-based materials is feasible with only moderate efficiency losses compared to mineral-based systems, provided system parameters are appropriately adapted. Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

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34 pages, 2388 KB

Open AccessFeature PaperArticle

Safe Reinforcement Learning for Buildings: Minimizing Energy Use While Maximizing Occupant Comfort

by Mohammad Esmaeili, Sascha Hammes, Samuele Tosatto, David Geisler-Moroder and Philipp Zech

Energies 2025, 18(19), 5313; https://doi.org/10.3390/en18195313 - 9 Oct 2025

Cited by 6 | Viewed by 3728

Abstract

With buildings accounting for 40% of global energy consumption, heating, ventilation, and air conditioning (HVAC) systems represent the single largest opportunity for emissions reduction, consuming up to 60% of commercial building energy while maintaining occupant comfort. This critical balance between energy efficiency and human comfort has traditionally relied on rule-based and model predictive control strategies. Given the multi-objective nature and complexity of modern HVAC systems, these approaches fall short in satisfying both objectives. Recently, reinforcement learning (RL) has emerged as a method capable of learning optimal control policies directly from system interactions without requiring explicit models. However, standard RL approaches frequently violate comfort constraints during exploration, making them unsuitable for real-world deployment where occupant comfort cannot be compromised. This paper addresses two fundamental challenges in HVAC control: the difficulty of constrained optimization in RL and the challenge of defining appropriate comfort constraints across diverse conditions. We adopt a safe RL with a neural barrier certificate framework that (1) transforms the constrained HVAC problem into an unconstrained optimization and (2) constructs these certificates in a data-driven manner using neural networks, adapting to building-specific comfort patterns without manual threshold setting. This approach enables the agent to almost guarantee solutions that improve energy efficiency and ensure defined comfort limits. We validate our approach through seven experiments spanning residential and commercial buildings, from single-zone heat pump control to five-zone variable air volume (VAV) systems. Our safe RL framework achieves energy reduction compared to baseline operation while maintaining higher comfort compliance than unconstrained RL. The data-driven barrier construction discovers building-specific comfort patterns, enabling context-aware optimization impossible with fixed thresholds. While neural approximation prevents absolute safety guarantees, reducing catastrophic safety failures compared to unconstrained RL while maintaining adaptability positions this approach as a developmental bridge between RL theory and real-world building automation, though the considerable gap in both safety and energy performance relative to rule-based control indicates the method requires substantial improvement for practical deployment. Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

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25 pages, 4102 KB

Open AccessFeature PaperArticle

Theoretical and Simulation-Based Approach to BIPV Systems Integrated with Modular Building

by Julia Brenk, Barbara Ksit and Bożena Orlik-Kożdoń

Energies 2025, 18(16), 4457; https://doi.org/10.3390/en18164457 - 21 Aug 2025

Viewed by 1835

Abstract

This study presents a simulation-based analysis of a steel modular building that integrates technologies that support the energy transition in the built environment. The focus is placed on the implementation of building-integrated photovoltaics (BIPVs), with photovoltaic modules incorporated into the façade and balcony railings. Several modern photovoltaic façade systems were examined. In addition, the study considers the application of photovoltaic glazing enhanced with active quantum coatings. Seven distinct BIPV modules were analysed, each characterised by unique features, with particular emphasis on the influence of colour in tinted variants. A performance degradation analysis was conducted for railing-mounted modules with varying glass tints. The simulation results were correlated with the building’s electricity demand. Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

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18 pages, 1666 KB

Open AccessArticle

Utilizing Fuel and Energy Sector Waste as Thermal Insulation Materials for Technical Buildings

by Artem Pavlychenko, Dariusz Sala, Michal Pyzalski, Serhii Dybrin, Olena Antoniuk and Roman Dychkovskyi

Energies 2025, 18(9), 2339; https://doi.org/10.3390/en18092339 - 3 May 2025

Cited by 8 | Viewed by 2044

Abstract

The growing demand for sustainable construction materials has prompted intensive research into the potential reuse of waste from the fuel and energy sector as effective thermal insulation materials. This study examines the feasibility of utilizing ash–slag mixtures, fly ash, and aluminosilicates as insulation materials for technical buildings. These materials were selected due to their availability and potential to improve energy efficiency in construction. Practical tests were carried out to determine the thermal conductivity coefficients of various samples, which were produced using different cement mixtures as binders to ensure adequate structural strength. The results demonstrated that the use of industrial waste-derived materials not only provides satisfactory thermal insulation properties but also contributes to environmental sustainability by reducing the challenges associated with the disposal of industrial by-products. The study highlights the crucial role of cement as a binder, enhancing the mechanical strength and durability of the insulation samples. The integration of ash–slag mixtures, fly ash, and aluminosilicates into the construction sector may foster the adoption of more environmentally friendly building practices, thereby supporting a circular economy and mitigating the environmental impact of construction activities. The study showed that the lowest thermal conductivity coefficient (0.24 W/m·K) was achieved for mixtures containing fly ash and cement, while the highest value (0.30 W/m·K) was recorded in samples incorporating aluminosilicates. The obtained results confirm the effectiveness of fly ash as a cost-efficient additive that improves the thermal insulation properties of the material. Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

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23 pages, 720 KB

Open AccessFeature PaperArticle

Global Solutions for Sustainable Heating, Ventilation, Air Conditioning, and Refrigeration Systems and Their Suitability to the New Zealand Market

by Nicholas Andrew Harvey and Eziaku Onyeizu Rasheed

Energies 2025, 18(9), 2190; https://doi.org/10.3390/en18092190 - 25 Apr 2025

Cited by 1 | Viewed by 1498

Abstract

This paper attempts to find alternative ways in which heating, ventilation, air conditioning and refrigeration systems can be made more energy efficient and sustainable at a global level. Eight technologies or solutions that either passively or supplementarily reduce the heating or cooling load required by a structure are detailed. These technologies or solutions were then presented to heating, ventilation, air conditioning and refrigeration industry professionals in New Zealand to determine their viability and further establish market readiness towards integrating new, innovative, and sustainable solutions in New Zealand. A literature review was conducted to establish the performance of the selected solutions and understand their operational principles and the efficiency they provided. Qualitative research and data collected via semi-structured interviews provided the data for assessing the viability of the selected technologies in the New Zealand market. Following a thematic and hybrid-thematic analysis of the data, the technologies were ranked, and suggestions were made to help improve innovation and energy efficiency in the heating, ventilation, air conditioning, and refrigeration industry in New Zealand. Of the technologies selected, airtightness, heat recovery ventilation retrofits, materials and design principles, and photovoltaic hot water heating were identified as the most viable. The New Zealand market was deemed not to be in a good position to adopt new or alternative solutions. The main issues affecting New Zealand’s market readiness to assimilate innovative and energy-efficient solutions are a lack of new technologies, poor standards of education throughout the industry, a lack of regulation, and a lack of government incentives. Full article

(This article belongs to the Special Issue Energy Efficiency and Energy Saving in Buildings)

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