Energy Efficiency in Buildings: Toward Climate Neutrality

Abstract

The pursuit of climate neutrality requires global systemic actions involving the use of solutions aimed at reducing emissions. Changes must be introduced in all sectors affecting climate change, namely power engineering and district heating, construction, transport, and industry, as well as agriculture and forestry. Analyzing the structure of final energy consumption in the EU by sector, it can be stated that households account for 27% of the total energy consumption. Comprehensive actions are needed to increase the energy efficiency of buildings. The aim of this paper was to indicate aspects of improving energy efficiency in buildings and their equipment, taking into account the striving for climate neutrality. Analyzed possibilities and conditions of using various solutions of energy-efficient systems aimed at increasing energy resilience and security and preventing environmental degradation. Particular attention was paid to construction and material solutions, as well as installation solutions, which increased the accumulation and energy efficiency of the building. These activities are closely related to the conditions and dynamics of the heat exchange process in the applied solutions and are also related to the factors influencing thermal comfort and energy consumption in buildings. Due to the growing popularity of modern information technologies and artificial intelligence in energy management in recent years, this article reviews the latest research in this area. One of the directions of future research indicated by scientists is autonomous building control in real time, adapting to the momentary needs of users. The analysis of the possibilities of using modern energy efficiency solutions in buildings conducted in this work may be useful for optimizing heat and energy management models and models of society’s consumption as an element of energy transformation towards climate neutrality and counteracting the deepening of energy poverty.

1. Introduction

In accordance with the European Climate Law, the European Union is committed to achieving climate neutrality by 2050 [1,2]. The aim of the European Union (EU) climate policy is not only to reduce greenhouse gases but also to strive to eliminate the development of the economy based on fossil fuels, with the aim of transforming the economy into a zero-emission one, drawing energy from renewable sources. The directions of changes in the energy sector have a decisive impact on the economic state of all sectors of the economy and on the need for technological transformation. EU policy relating to climate and the environment can be distinguished as the development of technologies that substitute for fossil fuels, which are to ensure energy security and energy independence on a European Union scale, and as counteracting climate change that may have drastic effects on the economy and quality of life of citizens. The EU strategy for achieving climate neutrality by 2050 is the European Green Deal. It is a package of policy initiatives that aim to set the EU on the path to a green transition, with the ultimate goal of reaching climate neutrality by 2050 [3]. The European Green Deal was launched by the Commission in December 2019, and it underlines the need for a holistic and cross-sectoral approach in which all relevant policy areas contribute to the ultimate climate-related goal. The package includes initiatives covering the climate, the environment, energy, transport, industry, agriculture, and sustainable finance—all of which are strongly interlinked. One of the Green Deal initiatives is the fit for 55 package, which aims to translate the climate ambitions of the Green Deal into law. The package is a set of proposals aimed at reviewing climate, energy, and transport legislation and introducing new legislative initiatives to align EU regulations with the EU climate goals [3]. Striving for climate neutrality requires global systemic action consisting of a range of solutions aimed at reducing emissions. Broadly understood changes should be introduced simultaneously in all sectors that affect climate change, i.e., in energy and heating, construction, transport, and industry, as well as agriculture and forestry. A broadly understood energy transformation needs to be implemented in individual sectors, taking into account their specificity. Comprehensive actions are necessary to increase energy efficiency, use modern technologies and low-emission materials, and use renewable energy sources, waste heat recovery, circular economy, etc., In the European Union, 85% of buildings were built before 2000, and 75% of them have poor energy performance. Buildings are responsible for 42% of the energy consumed in the EU in 2021 and for 36% of direct and indirect greenhouse gas emissions related to energy [4,5]. The combustion of fossil fuels also causes air pollution, which has a negative impact on human health both directly, penetrating the body, causing allergies and lung diseases, and indirectly, being a carrier of heavy metals, microorganisms, and bacteria. European Union Directive 2008/50/EC [6] determines the permissible average daily concentration of air pollution.

These activities are closely related to the conditions and dynamics of the heat exchange process in the applied solutions. Analysis of heat exchange process models allows for the optimization of solutions and cost reduction.

The use of modern heat exchange solutions in various sectors of the economy leads to improved energy efficiency, increased energy security and prevention of environmental degradation. Changes in heat and energy management models and consumption models of society are also necessary as an element of the energy transformation towards climate neutrality and counteracting the deepening of energy poverty.

2. Climate Neutrality Background

Climate neutrality, in other words, carbon neutrality, means maintaining a balance between carbon emissions and their absorption from the atmosphere in carbon sinks. Removing carbon monoxide from the atmosphere and then storing it is called carbon sequestration. To achieve net zero emissions, all global greenhouse gas (GHG) emissions will need to be balanced by carbon sequestration. The reason for the drive to reduce emissions lies in the changes that are occurring in the climate, along with its instability. In the last century, human activity has caused significant changes in the Earth’s surface temperature (Figure 1). According to the definition of the NASA Earth Observatory, “global warming is an extremely rapid increase in the Earth’s average surface temperature over the last century, primarily due to the release of greenhouse gases from human combustion of fossil fuels. The average global surface temperature of the Earth increased between 1906 and 2005 by 0.6 to 0.9 degrees Celsius, and the rate of temperature increase almost doubled in 50 years” [7]. Maintaining a temperature favorable for life is possible thanks to the natural greenhouse effect, but the currently occurring increased greenhouse effect causes it to increase. Greenhouse gases prevent infrared radiation from escaping from the Earth, resulting in an increase in the temperature on its surface. The increase in temperature is caused by an increased number of greenhouse gas molecules, which in turn cause a larger amount of infrared radiation emitted by the Earth to be absorbed by the atmosphere and then radiated back to its surface, resulting in an increase in temperature.

In parallel to the increase in the Earth’s temperature (Figure 1), there has been an increase in carbon dioxide in the atmosphere (Figure 2). Human activity during the Industrial Revolution started to increase the concentration of greenhouse gases in the atmosphere through the burning of fossil fuels and the cutting down of forests that absorb carbon dioxide. According to NASA data, from 1750 to 2009, the level of carbon dioxide increased by 38% [7].

Human activity during the Industrial Revolution started to increase the concentration of greenhouse gases in the atmosphere through the combustion of fossil fuels and the cutting down of forests that absorb carbon dioxide. According to NASA data, from 1750 to 2009, the level of carbon dioxide increased by 38% [7]. The composition of greenhouse gases is carbon dioxide CO₂, methane CH₄, nitrous oxide N₂O, and industrial gases such as hydrofluorocarbons HFC, perfluorocarbons PFC, sulfur hexafluoride SF₆, and nitrogen trifluoride. Carbon dioxide CO₂ is also classified as a gaseous pollutant along with sulfur dioxide SO₂, nitrogen oxide NO_x, carbon monoxide CO, and hydrocarbons C_nH_m. Air pollution generated in the process of heat generation is associated with negative effects on water and soil and is associated with a threat to human health, life, and safety. The effects of the substances produced are presented in

Table 1. Harmful substances—stage of formation and effects (prepared on the basis of [9,10]).

Pollution	Stage of Creation	Impact on Human Health
Sulfur dioxide—SO2	Emitted during fuel combustion, it creates sulfuric acid in the atmosphere in reactions	It can exacerbate asthma symptoms, limit airway and lung function, and cause headaches and general malaise.
Nitrogen oxides—NOx	Emitted directly during combustion, they form nitric acids in the atmosphere through reactions	It may have a negative effect on the liver and lungs, exacerbate the symptoms of respiratory infections and increase susceptibility to respiratory infections.
Fine dust—PM2.5, PM10	Primary—emitted in the combustion process of hydrocarbon fuels and secondary	It may cause an increase in the incidence of respiratory and circulatory diseases, arrhythmia; it may cause asthma attacks, chronic cough
Non-metal volatile organic compounds—NMVOC	A very large group of organic compounds that play an important role in the formation of ozone (photochemical) smog	Reduced life expectancy due to short- and long-term exposure, increased risk of cancer, osteoporosis, kidney dysfunction
Ozone—O3	It is produced in the atmosphere by reactions of NOx and other pollutants, including NMVOC, in the presence of sunlight	It has a negative impact on the respiratory system and may worsen asthma symptoms
Heavy metals—Hg, As, Cd, Ni, Pb	Natural components of coal emitted during combustion	They can cause cancer, hereditary defects
Radioactive elements	Radiation risk from the migration of radioactive elements contained in coal during its use	They have a global impact on premature mortality and morbidity in humans and have a carcinogenic effect

[9,10].

Structural changes in the economy and energy consumption are needed to affect energy security, climate protection, and lower energy demand. Climate change affects energy demand. Climate models for assessing how climate change impacts supply–demand match, quantified by the fraction of demand met by local wind or solar supply, are presented in [11]. The World Energy Outlook 2024, prepared by the International Energy Agency [12], provides in-depth analysis and strategic insights into every aspect of the global energy system. Against a backdrop of geopolitical tensions and fragile energy markets, the report explores how structural shifts in economies and in energy use are shifting the way that the world meets rising demand for energy. The uncertainty, speed, and shape of the energy transformation that must be implemented as part of the pursuit of climate neutrality are presented in three scenarios that allow us to examine the surrounding reality by 2050 (Figure 3). Accelerated and net zero are broadly consistent with the IPCC’s “coherent Paris” scenarios. In all three scenarios, final energy demand peaks as energy efficiency gains accelerate [13].

The scenarios are based on existing technologies and do not take into account the impact of completely new or unknown technologies, but they illustrate key trends and uncertainties related to the possible development of energy markets by 2050 [13].

Strategies for achieving a carbon-neutral society are the subject of [14]. Decarbonization technologies and initiatives and negative emission technologies are discussed. The authors propose plans for achieving carbon neutrality, such as switching from fossil fuels to renewable energy, developing low-emission technologies, developing low-emission agriculture, changing dietary habits, and increasing the value of food and agricultural waste.

Global CO₂ emissions for 2023 increased by only 0.1% relative to 2022 (following increases of 5.4% and 1.9% in 2021 and 2022, respectively), reaching 35.8 Gt CO₂. These 2023 emissions consumed 10–66.7% of the remaining carbon budget to limit warming to 1.5 °C, suggesting that permissible emissions could be depleted within 0.5–6 years (67% likelihood) [15]. The awareness of the need to implement rapid actions to reduce the negative impact of the mining and energy industries on the environment by 2050 is obvious. The banking sector in Poland is open to financing socially responsible investments that support activities aimed at climate neutrality [16].

Given the problem of global warming, it is necessary to take measures to reduce energy consumption in every field concerning the economic sector and human life.

According to Eurostat Statistics Explained [17], in the structure of final energy consumption in 2022 (Figure 4), the share of households was about 27%, second only to the transportation sector, whose share was about 31% (excluding international air transport and maritime bunker transport).

Therefore, in addition to measures in the transportation sector, which include increasing the share of bio-components in liquid fuels, the development of hydrogen technologies, the introduction of more energy-efficient drives (hybrid and electric), and plans to ban the registration after 2035 of new cars with internal combustion engines powered by non-renewable fuels, technologies that reduce energy consumption in the household sector are essential.

Some research highlights the technical and physical constraints on deploying renewables to mitigate CO₂ emissions, the importance of scaling up investments to accelerate energy transition to PV and wind power and the optimal route to achieve carbon neutrality in the long run [18]. It is undoubtedly necessary to develop resilient and safe cities with modern energy-efficient construction, industry, and transport [19]. Modeling results presented in [20] show that battery electric vehicles are the key enablers of climate neutrality, representing 60–89% of trucks and 79–96% of buses by 2050, but modern automobile technology has become integral to human society [21]. Progress, policy gaps and opportunities towards climate neutrality are included in the report [1]. The report provides a first general assessment of progress and policy consistency in different sectors: energy supply, industry, transport, buildings, agriculture, and land use, land use change, and forestry (LULUCF).

In the pursuit of climate neutrality, an energy transformation, which entails the problem of energy poverty, is therefore necessary. An example is China, which has initiated a rapid energy transformation since 2013, consisting of replacing traditional solid fuels with modern clean energy. Despite the huge success of the energy transformation, its impact on household energy costs and the related energy inequality remains largely unexplored. Studies show that between 2013 and 2017, about two-fifths (43.0%) of surveyed households switched from traditional solid fuels to clean energy [22]. However, 56.1% to ~61.0% of them came from extremely poor or poor households, which raises deep concerns about the growing energy burden on households. As a result, the share of surveyed households in energy poverty increased from 30.1% to 34.2%.

3. Energy Poverty Issue in Households

Precise determination of the building’s thermal power, depending on its function, technical condition, size, equipment, etc., requires determining the thermal insulation parameters, taking into account climatic conditions. Adjusting power to current needs is key to optimally supplying buildings with energy. Poorly insulated buildings cause heat to escape quickly, which leads to increased heating costs, and residents have to pay more to keep their homes warm. It is one of the main causes of energy poverty. This also results in low thermal comfort and difficulties in maintaining the right temperature, especially in winter. An additional aspect related to improper heat exchange in a building is health problems. Living in too cold, too hot, or humid conditions can lead to health problems.

The demand for energy for heating and cooling is influenced by the structure and insulation of the building, the technology of its construction, and climatic conditions [23,24,25,26]. Specific and dynamic heat transfer is an important issue from the point of view of assessing the energy efficiency of buildings [27,28]. It is important at the design and construction stage, as well as during the operation of buildings or their parts. The heat transfer process in a building is a fundamental factor in the pursuit of climate neutrality. Thermal management within a building, appropriate quality of construction and building design, materials used, and installation solutions can significantly affect heat consumption and greenhouse gas emissions.

The requirements for the insulation of buildings in Poland are specified in the Regulation of the Minister of Infrastructure on the technical requirements that buildings and their location should meet [29]. The methodology for determining the energy performance of a building results from the regulations [30]. Various heat storage technologies using latent heat-storing materials, as well as phase-change materials [31], increase the accumulation capacity of the building, which has an impact on increasing energy security. The absorption of light in conductive materials was analyzed in [32].

The influence of geometric characteristics of the building’s facades on the heat transfer is analyzed in [33]. There is the impact of the changes in the required thermal insulation of the building envelope on energy demand and heating costs [34]. The influence of the thermal insulation thicknesses of external walls on heating cost from the ecological and economic assessment is analyzed in [35].

Issues of heat transfer in heat exchangers were emphasized in the work [36]. There is a correlation between some parameters of the isolation of buildings and the wind-free stream velocity and wind-to-surface angle. In the work [37], it was shown that the convective heat transfer coefficient value strongly depends on the wind velocity.

Heat transfer and its dynamics can be analyzed using mathematical and simulation models [27]. Methods based on the coupling of three different types of simulation models, namely spectral optical model, computational fluid dynamics model, and building energy simulation, are presented in [38]. Physical phenomena, notably optical, thermodynamic, and fluid dynamic processes, have been analyzed for commercial buildings with double-skin façades. The modeling of heat transfer taking advantage of heat energy accumulation in building walls is the goal of the work [39]. The paper focused on the future optimization of a control strategy. The intensity of heat transfer is greater in thermal bridges [26]. The issue of simulating heat transfer through point thermal bridges is the subject of the paper [40].

The principles of development of state energy policy, principles, and terms of supply and use of fuels and energy, including heat and operation of energy enterprises, determine the organs in charge of fuel and energy economy and define The Act of 10 April 1997 on the Energy Law as amended [41]. According to the Energy Law Act, energy companies involved in the transmission or distribution of energy are obliged to maintain the capacity of devices, installations, and networks to provide energy supply in a continuous and reliable manner while maintaining quality requirements. Proper management of heat and energy supply is important, taking into account the current political situation of the country and energy security, which was confirmed in the works [42,43]. A research and innovation agenda for energy resilience that was presented in the work [44] shows that energy resilience will require careful coordination between grid, mini-grid, and smaller off-grid energy products, but limited technical capacity and weak governance in some jurisdictions can hamper implementation. Energy resilience was defined as the “ability to reduce the impact of shocks and stresses, including the capacity to anticipate, absorb, adapt to, and rapidly recover from such events and to transform where necessary” [45]. The inclusion of energy resilience in policy and program design can safeguard and accelerate the transition to clean and affordable energy for all [44].

The specification of research methods commonly encountered in the study of heat transfer processes in mini-channels of heat exchangers is presented in the paper [46].

Two groups of heat transfer intensification methods are known: passive, which does not require an external energy supply, and active, which requires additional energy from outside the system [47]. A review of recent passive heat transfer enhancement methods is the subject of the work [48].

3.1. The Problem and Determinants of Energy Poverty

The concept of energy poverty was first introduced into EU law by the Directive on common rules for the internal market in electricity (2009/72/EC) [49]. It has since been expanded in the narrative on a just and fair energy transition, and over the last decade, the EU has stepped up its efforts to make energy poverty a key concept [50].

Energy poverty occurs when a household must reduce its energy consumption to a degree that negatively impacts the inhabitants’ health and well-being. It is mainly driven by three underlying root causes [50]:

• A high proportion of household expenditure spent on energy;
• Low income;
• Low energy performance of buildings and appliances.

3.2. Energy Poverty Indicators

Various indicators are used to assess the level of energy poverty, including the following:

• Energy price index, household expenditure;
• Housing cost overload index;
• At-risk-of-poverty and social exclusion index;
• Unemployment index;
• High cost–low income index.

3.2.1. Household Expenditures—Energy Price Index

Energy poverty undoubtedly depends on energy prices in a given country. Electricity prices, an indicator describing the phenomenon of energy poverty, are divided into two groups, which are energy prices in households and in other entities consuming energy. Additionally, a very important element of price formation is taxes and other fees, which significantly affect the price values of energy [51]. Energy prices in European households are variable over the year and half a year; these changes are illustrated in Figure 5. In a large part of countries, an increase in prices was noted in 2023. Rising bills are the cause of the omnipresent inflation, which was intensified by the war in Ukraine.

Cutting off from certain sources also in highly developed countries, which previously were profitable solutions, resulted in the need to implement greater diversification of raw material and energy supplies.

Figure 6 presents a half-year graph of changes in the electricity price index without taxes for consumers other than households in individual European countries for 2023. The data presented illustrate an unfavorable, larger, or smaller increase in prices in almost every European country after the first half of 2023 due to inflation.

In terms of the housing cost overload indicator, the state of energy poverty is influenced and undoubtedly plays an important role by the rental value indicator, the value of which is defined in Euro/m². It more precisely defines the average value of rent paid for a given energy class and type of building. In this way, it can be seen that a high value of this indicator also generates problems with fees, which results in a deterioration of the financial situation of vulnerable groups. The measurable value of the state of energy poverty is also the average value of rent, the unit of which is defined in euro or zloty per month. It is worth adding that the unit of measurement of this indicator refers to a square meter of the premises’ area. Changes in the value of the rental cost overload indicator depending on the year and country of occurrence in Europe are presented in Figure 7 [53].

Tenant overload with rent costs, and consequently also with energy purchase costs, is varied in Europe. This situation is reflected in the current economic situation in relation to the analyzed period. A large part of countries struggling with too high inflation and economic crisis have a high percentage of rent overload indicators, including Greece, Luxembourg, and Romania.

3.2.2. Poverty and Social Exclusion Risk Indicator

An important indicator when it comes to the situation of people at risk of energy poverty is the social exclusion and poverty indicator itself because these two causes show some of the most noticeable consequences in relation to the social and economic situation in households and other entities at risk. The values of the poverty and social exclusion risk indicators in European countries are presented graphically in Figure 8 [54].

The data cited show that there are quantitative differences in the percentages for specific countries. Therefore, specifying the area of risk and identifying vulnerable groups plays an important role in actions aimed at improving the situational conditions of entities at risk of energy poverty. This will allow for further actions that can be taken by the authorities of individual countries, as well as by formulating specific actions and implementing them by the authorities of the European Union throughout Europe.

3.2.3. Unemployment Rate

Lack of employment affects the financial situation of people of different ages and thus increases energy poverty. The unemployment rate by country in 2022 is presented in Figure 9 [55].

The countries with the lowest percentage include the Czech Republic, Germany, Malta, and Poland, while the highest values of the indicator occur in Greece, Spain, Italy and Sweden.

3.2.4. High Costs–Low Income Indicator

The high costs–low income indicator takes into account the disposable income of households, which is reduced by the value of building maintenance costs. The fixed charges in question are independent of recipients, which is why they are sensitive to their changes, especially since the financial situation of the household is uncertain. In this way, it is possible to more precisely specify the situation of energy poverty not only on the basis of the income of individuals [56,57].

3.3. Possible Solutions to the Problem of Energy Poverty

The level of energy poverty depends, to a large extent, on the technical condition of the building, the way in which thermal comfort requirements are met, the efficiency of heat exchange, and energy prices. Therefore, the main aspect necessary to reduce it is to take care of the good technical condition of the building and its equipment, the use of modern, effective installation solutions, and proper operation. Increasing awareness of the methods of saving energy and optimizing consumption is also important. This will consequently translate into good energy efficiency of the building, as well as an appropriate economic effect through proper energy management.

Modern solutions in the field of improving energy efficiency and saving energy in buildings are currently a major challenge for governments and societies. The use of thermal energy storage (TES) technology in buildings reduces peak loads and more effective management of energy used in buildings [58]. Storing thermal energy can be divided into three categories through the use of [59]:

-

Specific heat, which is the simplest and cheapest way of storing both in the form of liquids and solids;

-

Phase change energy, using phase change materials (PCMs), which are able to absorb, accumulate, and release energy in the phase change temperature range. During the phase change, significant amounts of heat can be absorbed or released at a practically constant bed temperature;

-

Heat of chemical changes—a process that occurs under the influence of a chemical reaction, during which heat can be released (exothermic reactions) or heat must be supplied for its operation (endothermic reactions). The obtained energy is released in an exothermic reaction. An example of this type of reaction is obtaining heat using hydrogen. Energy storage technologies based on the use of phase change materials (PCMs) have gained much attention in the construction sector among architects and engineers over the last four decades [60,61]. PCM materials used for thermal energy storage fill the gap between energy supply and demand by absorbing excess energy in buildings, which makes them a future technology. Currently, the application of PCM materials covers several areas in a wide temperature range from −20 °C to 200 °C for heating, cooling and hybrid systems combining heating and cooling [62].

One of the main factors influencing energy poverty is the low energy efficiency of buildings and inefficient use of energy. Energy consumption in a building depends on external factors (related to climatic conditions), structural and architectural solutions, and internal installations and equipment, which together affect the thermal balance of the building.

An important aspect of improving the energy efficiency of a building is the insulation of partitions. Each building partition should have appropriate thermal insulation specified in national regulations through the heat transfer coefficient (U_max). Depending on the type of partition and the function it performs, they have different construction and technology of construction. Depending on the construction solution and the adopted calculation dimensions, the values of the transfer coefficients may take different values: negative, zero, or positive [26]. From the point of view of heat transfer, the most advantageous solution is to use a partition construction whose heat transfer coefficient value will be close to zero.

The reduction in energy demand and, thus, the reduction in heating and cooling costs in a building while maintaining thermal comfort conditions of rooms is influenced by the thickness of thermal insulation of building partitions [63]. It is worth emphasizing that in order to maintain high insulation, it is necessary to continuously use building partitions in dry conditions so that mold does not form on their surface and condensation does not occur in the volume of the partition [64]. In addition to the insulation of external partitions, the construction of the partition itself plays an important role by increasing the heat accumulation capacity (e.g., in the wall layer) [65]. An interesting solution in terms of reducing heat losses in a building is the use of passive solar systems that are integrated with the building envelope. One example of such solutions is the Trombe wall [66,67]. This type of wall combines the function of collecting solar energy (through glazing) and accumulating thermal energy (in the form of a storage wall). The heat storage capacity depends on the properties of the materials from which the partition was made. The effect of increased heat storage, which is then transferred to the adjacent space, can be achieved by using a phase change material in the construction of the Trombe wall, which is then transferred to the adjacent space [68,69].

When designing windows in a building, it is worth using the “effective window selection method”, which allows you to select windows with the highest thermal efficiency appropriate for a specific surface and structure. At the stage of window selection, the following parameters are taken into account: glazing coefficient, window profile, type of insulating glass, and type of spacer [70,71].

While newly designed buildings are currently energy efficient, older buildings are a problem. The solution to this problem is their thermal modernization through the insulation of walls, roofs, and foundations, replacement of outdated installations, application of heat recovery, and use of renewable energy sources.

One of the basic solutions when it comes to saving thermal energy is rational management. Educating society is extremely important in this matter, for example, by teaching appropriate habits and behaviors, paying attention not to overheat buildings, and reducing the heating temperature when a room is not used for a longer period of time. A good solution to the problem of energy poverty is the actions of governments. In countries where the percentage of energy poverty is high, households have low incomes. Assistance in providing appropriate installations and devices that meet the requirements of energy saving can play a very important role. An appropriate practice in this area is also all kinds of subsidies from public funds. These funds, especially when purchasing devices and with the thermal modernization of buildings, could significantly contribute to reducing the level of energy poverty and prevent its recurrence in the following years. It is worth emphasizing that specifying the conditions and causes of the occurrence of energy poverty allows for the consistent implementation of appropriate aid measures to combat this phenomenon.

4. Heat Transfer in HVAC Systems

Developing civilization, increasing levels of urbanization, and improving living standards have significantly increased energy consumption in buildings. People spend most of their lives indoors [72], so it is particularly important to maintain indoor thermal comfort, which translates into improved quality of life, satisfaction, and productivity at work.

Modern construction should be both comfortable and economically and environmentally friendly. A key area in building and energy engineering to maintain thermal comfort, adequate air quality and energy efficiency in buildings are HVAC (heating, ventilation, and air conditioning) systems. Modern solutions in this area are based on advanced technologies, which enable effective control of temperature, humidity, air cleanliness and air circulation in rooms, and, most importantly, their cooperation to ensure optimal conditions in a given interior [73]. The operation of HVAC systems, however, is characterized by high energy demand, accounting for almost half of a building’s total energy consumption [73,74]. In this context, improving the energy efficiency of buildings becomes crucial not only for reducing building utility costs but also for reducing greenhouse gas emissions and, thus, the carbon footprint.

A comprehensive HVAC system consists of many components that work together to provide the expected level of comfort in buildings. The basic processes carried out by HVAC systems include the heating, cooling, exchange, purification, humidification, and dehumidification of air. These processes are carried out through the use of appropriate installations and equipment, such as the heating system, air conditioning system, and mechanical ventilation. In addition, the various systems differ in the type of heating or cooling medium and the method of its distribution.

The main classification of HVAC systems distinguishes between central systems and local systems [75]. The criterion for selecting the appropriate system depends on a number of factors, as described in

Table 2. Characteristics of central and local HVAC systems.

Criteria	Central Systems	Local Systems
Special requirements	Special requirements	Special requirements
Special requirements	The main devices are located in a special room outside the cooling/heating zone. A heating/cooling medium distribution system is necessary	No additional room for devices. Devices can be installed directly in the cooled/heated space and on the roof, on the casing or next to the building
Application	New buildings	New, existing, termodernized buildings
Investment cost	High investment cost	Affordable investment costs
Operating cost	Energy-efficient main appliances	Less energy-efficient appliances
Conservation	Easy access to the main equipment, located in one separate room	Difficult access to equipment that is installed in different parts of the building
Reliability	Long life of equipment. Possibility of installing main backup equipment	Reliable system, but estimated lifespan of equipment is shorter. Possibility to install more devices in different localizations

.

The appropriate selection of HVAC systems for a particular building depends on the climate, technical parameters of the building, individual user preferences, and budget.

4.1. Central HVAC Systems

Central HVAC systems can be designed to create thermal comfort for one or more zones with different thermal and humidity parameters. Its main components are located outside the zones to be served in a suitably selected central location, either inside or outside the building. Since central systems are designed to maintain the required thermal comfort conditions for zones with different thermal loads, they must be equipped with a number of sensors monitoring the parameters of each zone.

The heat transfer medium can be air, water, or mixed systems. The following diagram (Figure 10) shows the basic types of central HVAC systems.

The central air HVAC system is based on an air handling unit built of interconnected devices, as shown in Figure 11. These devices include supply and exhaust air fans, humidifiers, heaters, coolers, exchangers, and air filters.

A single-zone system consists of an air handling unit, a heat and cooling source, distribution ducts, and supply equipment. The main advantage of single-zone systems is the simplicity of design, construction, and maintenance, as well as the low investment cost compared with other systems. The main disadvantage, however, is the operation of only one zone.

A multi-zone air HVAC system provides for each zone in a building separate supply ducts of cool and warm (or return) air, which are mixed in the air handling unit for the needs of a specific zone. A multi-zone air system consists of an air handling unit and heating and cooling water piping, transporting the medium to heaters and chillers and working for a single thermal zone.

The advantage of a multi-zone system is the ability to create thermal comfort simultaneously for several zones with different requirements without the energy losses associated with the final reheating or cooling system. The main disadvantage is the need for a large number of air ducts supplied to each zone.

For rooms that require varying airflow due to changes in heat loads, variable air volume (VAV) HVAC systems are an appropriate solution. A VAV system is built with a central air handling unit that delivers supply air to a VAV flow control device located in each zone. The supply air temperature in each zone is controlled by varying the supply air flow rate. The main disadvantage is that adjusting the airflow rate can adversely affect other neighboring zones with different or similar airflow rates and required temperatures.

In an all-water HVAC system, heating or cooling water is transported from a central system to the rooms served. This type of HVAC system is relatively small compared with air systems because water has a higher heat capacity than air, and the use of liquid as a heating or cooling medium requires a small volume to transfer heat. Heat receivers in water heating systems can be, for example, surface radiators, convection heaters, or convectors. Examples of cooling receivers, on the other hand, can be chilled beams, fan cooling apparatuses, or surface cooling. The most common heat and cooling receiver used in buildings is the fan coil.

Air-water HVAC systems are hybrid systems that combine the advantages of all-air and all-water systems. This gains a small installation size by using water system components, while air system components work only for ventilation requirements. The heating or cooling medium, which is water, is responsible for carrying 80–90% of the building’s heat load, with the air medium responsible for the remainder of the heating or cooling load. There are two main types: fan coils and induction units [75].

One type of HVAC system is the installation of surface heating or cooling in the form of heating and cooling panels placed on floors, walls, or ceilings. The process of heat exchange is based on the principle of radiation between the human body and the heating or cooling surface and using natural convection between the air and the heating or cooling plane. For cooling, the surface temperature should be higher than the dew point temperature of the air to avoid condensation of moisture on the surface. On the other hand, the maximum surface temperature in the case of ceiling heating should not cause excessive heat over the heads of users.

The installation of surface systems is often characterized by higher investment with respect to the systems discussed but shows lower operating costs. The advantage of surface systems is their small size and the possibility of “hiding” in the fixed elements of the building, which is beneficial from an aesthetic and hygienic point of view.

4.2. Local Systems HVAC

Depending on the design of the building, as well as the individual needs of the users, there may be a need to install an HVAC system directly in a heated or cooled zone. This type of system is considered a local HVAC system because each unit serves its own zone without crossing boundaries with other neighboring zones. In this situation, a single zone requires a single control point connected to the thermostat to activate the local HVAC system. Some facilities have several local HVAC systems as the appropriate units to serve specific single zones. Local HVAC systems are not connected and integrated with central systems but are still part of a large, comprehensive HVAC system [76]. There are many types of local HVAC systems, as shown in Figure 12.

Local cooling systems include active and passive systems. Active systems provide not only cool but also adequate air distribution in a specific zone and humidification control. An example of such a device is an evaporative air conditioner. In passive systems, the cooling effect is obtained on the basis of convection or evaporation, such as in the case of chilled beams, which are mainly natural systems, such as convection cooling in an open window and evaporative cooling in fountains [75].

Local ventilation systems are devices responsible for forcing air movement and its proper circulation within a room without changing air parameters, such as fans. These devices improve the thermal comfort of rooms by allowing heat transfer through the process of convection.

The local air-conditioning system is a complete system with heating and cooling functions equipped with a fan and control automation. An example of such a system can be a window air conditioner installed in window openings without air ducts or monoblock portable air conditioning units.

Local HVAC systems also include split-type units built from two units: a condenser located outside the building and an evaporator installed indoors. The two units are connected by a refrigerant-filled duct and wiring. A modification of split systems is a multi-split system built with one condenser unit (outdoor unit) and multiple evaporator units (indoor units) to serve multiple zones with different heat and humidity requirements.

4.3. Heat Recovery and Storage in HVAC Systems

Of significant importance in the aspect of heat exchange in buildings is the recovery of heat standing in HVAC systems. The use of a variety of heat or cold recovery techniques makes it possible to significantly reduce primary energy consumption and thus lower the carbon footprint. Thanks to the very good insulation standards in modern construction, the energy demand for heating fresh ventilation air is increasing. In the total energy demand for heating, it reaches values in the range of 40 ÷ 60% for single-family buildings and even more than 80% for special buildings [77]. The solution to the increasing share of ventilation heat demand in the total heating energy demand of a building is the use of heat recovery from exhaust air.

The most popular method of heat recovery in mechanical ventilation systems is the use of heat exchangers. There are various types of heat exchangers used in units: plate, cross, counterflow, and enthalpy exchangers. In the case of a single-family residential building, the use of mechanical ventilation with heat recovery with an efficiency of 50% reduces the demand for non-renewable primary energy for heating purposes by 13–31% in relation to a building with gravity ventilation and by 29–47% in the case of using a heat recovery exchanger with an efficiency of 70% [78]. High heat recovery efficiency can be achieved in ventilation units with plate heat exchangers as a result of reducing the plate spacing and thus increasing the flow resistance and the temperature at which the surface of the exchanger plates frosts in winter. The phenomenon of frosting is practically eliminated by installing pre-heaters. These devices are usually electric, which eliminates the risk of the freezing of the heating medium. This solution, however, is not advantageous in terms of the use of primary energy in the case of using energy from the power grid. A more advantageous solution from the point of view of the demand for primary energy is the use of an air-ground heat exchanger (GWC). The principle of their operation is based on the use of the heat accumulation of the ground, which, at a depth of about 2 m, has a constant temperature throughout the year in the range of 2–6 °C in winter and 8–12 °C in summer [78]. The air flowing through the ground exchanger is heated in winter and cooled in summer. In addition, ground heat exchangers are very good heat storage for heating systems. This is because thermal energy can be stored in groundwater resources [79].

An interesting way to use renewable energy sources is to use a warm air intake or a solar chimney. These are rare elements of the ventilation system that allow the use of solar radiation energy. Outside air, before being directed to the ventilation unit, can, for example, flow through a transparent chamber with a storage filling, undergoing preheating in winter. In turn, solar chimneys use the difference in air density caused by the heating of its layers, leading to the creation of a chimney draft forcing the flow (e.g., through a GWC) without the need to use electricity for this purpose to drive fans. Accurate design and energy balancing of this type of system is complicated, which is why simplified solutions of warm air intakes are used in practice, while solar chimneys are very rarely used [78].

The potential of solar radiation energy can be used not only to produce electricity, which is necessary for the operation of HVAC devices, but also as a heat source in heating systems. An interesting solution is to use heat to produce cooling by using absorption and adsorption units [80].

A novel solution is the use of low-temperature waste heat to heat buildings. The source of such waste heat, in addition to air removed from rooms in the mechanical ventilation system, can also be gray wastewater used to heat cold water. Standard heat recovery solutions are aimed at systems operating independently of each other. Modern solutions in this regard are based on hybrid systems [81,82].

4.4. Optimization of HVAC System Operations

Modeling the temporal and spatial weather changes that affect the energy demand in buildings is crucial for the decarbonization of energy systems. The energy demand for heating and cooling depends on the weather, the building structure, and its equipment, as well as the number of people in the building and their behaviors and habits. There are many tools in the literature for modeling the spatial and temporal effects of weather on the variability of the energy demand for HVAC systems, but most of them have a limited territorial scope of their application. Few tools can be used to study the heating and cooling demand with the same global scope and consistency [60]. In the paper [83], a method for optimizing the operation of HVAC systems that takes into account the weather forecast error based on probable information about subintervals of outdoor temperature in order to plan the energy consumption of HVAC systems is proposed. Compared with the method that considers the weather forecast error based on the mean and variance of historical data, the simulation results show that the proposed method [83] effectively reduces the electricity cost with a shorter computation time, and the electricity cost is lower compared with the traditional method.

Currently, HVAC systems are increasingly becoming elements of central building management systems (BMSs). Manufacturers equip devices with solutions that enable their integration with BMS. Control of building installations can take place at several levels of their integration. The simplest method is to independently control each installation operating in the facility. A more advanced solution is the communication of HVAC systems with other building elements (e.g., air conditioning devices cooperating with the window opening and closing system, hotel cards, or fire protection system). A building covered by the BMS is equipped with a huge number of sensors that allow for ongoing monitoring of its condition (required parameters, their changes and fluctuations, changes in external conditions). The basic ones are temperature and humidity sensors that record not only the condition in the rooms or outside but also the parameters of the media—circulating air, heating water, or chilled water. The role of the central management system is to automatically transfer tasks to executive elements based on data collected from all sensors. These data are collected using signal concentrators and control and monitoring switchboards, thanks to which the system can react in real time.

In addition, detecting faults in buildings and HVAC systems is an important aspect that guarantees optimal operation of buildings, but it is still the subject of research and rarely found in real applications [84].

5. Energy Efficiency of Buildings

Savings and efficient use of energy are becoming a key strategy in the world related to climate change and increasing demand for energy. According to data from the European Commission, buildings in the European Union (EU) are responsible for 40% of energy consumption and 36% of greenhouse gas emissions [85]. The majority of the current building stock in the EU was built without taking into account energy performance requirements. A total of 35% of buildings in the EU are over 50 years old, and over 40% of buildings were built before 1960 [86,87]. By 2001, over 220 million buildings were built in the European Union, which translates into 85% of the EU building stock [88]. The improvement of the energy quality of buildings can be achieved by introducing regulations and legal acts that require modernization and various types of improvements, as well as by educating the public about climate threats. If appropriate actions are taken in this direction, energy savings of about 20–40% can be expected [89].

The 2023 amendment to the Energy Performance Directive increases the final energy consumption reduction rate by at least 11.7% by 2030 compared with the forecast for 2030. The annual energy savings rate has been raised to 1.49% and will be gradually increased [4].

The revised Energy Performance of Buildings Directive, approved by the European Parliament in March 2024, aims to reduce the environmental impact of the European building stock, with a view to achieving zero-emission new residential buildings by 2030 and climate neutrality for all buildings by 2050. The Directive highlights the importance of financing large-scale renovations, encouraging Member States to allocate funds to initiatives that guarantee minimum energy savings. In this context, EU Member States must introduce measures to achieve a reduction of at least 16% in average primary energy consumption by 2030 and a reduction of at least 20–22% by 2035 in residential buildings [90].

A total of 75% of buildings in the European Union are energy inefficient, and the annual renovation rate of buildings is low, ranging from 0.4% to 1.2%. Only 11% of existing buildings in the European Union Member States undergo partial renovation each year, which does not always lead to an improvement in the energy performance of buildings. In contrast, deep renovations that reduce energy consumption by at least 60% are carried out in only 0.2% of the building stock per year [88].

The Energy Efficiency Directive (EED) obliges all EU Member States to renovate at least 3% of the usable floor area of buildings per year, in line with nearly zero-energy building nZEB standards [4].

According to a study by the Buildings Performance Institute Europe (BPIE), the current weighted annual renovation rate of 1% needs to be at least doubled by 2030, reaching 3% by 2035 and 4% by 2040. It is estimated that a full renovation of residential buildings in the European Union would reduce the energy demand for heating buildings by 44%, or 777 TWh. In addition, investments related to building renovation can result in 46% gas savings, 44% heating oil savings, and 48% coal savings [91].

Therefore, to achieve higher energy efficiency of buildings, the goal has been set to increase the renovation rate to 2% by 2030 and to support deep renovations. It is essential to renovate buildings to reduce energy consumption, reduce pollutant emissions, use renewable energy sources, reduce energy-related operating costs, and reduce the rate of fuel poverty.

Germany and other EU countries have shown a strong political commitment to renovate their building stock to high energy efficiency and carbon neutrality standards using two main types of instruments: regulation and subsidies. In Germany, buildings are responsible for around 30% of CO₂ emissions. This is mainly because 75% of the building stock is from before 1979. Currently, it is required for new buildings to achieve an energy efficiency standard equivalent to 50 kWh/(m²·year). The pace of energy-efficient building renovation needs to be accelerated. For example, renovating an old house with an average floor area of 174 m², which currently uses an average of 250 kWh/(m²·year) for heating, to a minimum renovation standard of 100 kWh/(m²·year) would save 26,100 kWh/year and reduce CO₂ emissions by 4.75 tCO₂/year. Building heating systems are predicted to produce more than 5.2 million tonnes of CO₂ equivalent (mtCO₂e) above the 2022 limit set for Germany [92].

5.1. Energy Consumption in the Buildings Sector

Globally, total energy consumption in the construction sector has increased by an average of 1% per year over the past decade, reaching 133 EJ in 2022. Natural gas provided 23% of the energy demand in buildings worldwide, and electricity provided more than one-third of the energy demand in the construction sector. In recent years, the share of natural gas has been decreasing, and the share of electricity has been increasing with the increasing ownership of appliances and air conditioners and the electrification of heating and cooking. Modern bioenergy provides 4% of the energy demand in buildings, and other direct uses of renewable energy sources in the form of solar energy and geothermal heating have more than doubled over the past decade and now account for 2% of demand [93].

In 2021, final energy consumption in the EU Member States (EU-27) reached 940 Mtoe. The largest recipient was the transport sector (31.1%), followed by households (residential 29.6%), industry (27.2%), services (14.6%), and agriculture (3.4%). The share of final energy consumption in residential buildings in individual EU Member States is presented in Figure 13 [93].

According to data from 2022, final energy consumption in EU households accounted for 25.8% [94].

The heating, ventilation, and air conditioning (HVAC) system has the greatest impact on household energy consumption, accounting for approximately 70% of consumption [95]. The structure of energy consumption in EU households is presented in Figure 14. The highest energy demand is still related to space heating (63.5%) and hot water preparation (14.85%). In EU countries, the most commonly used energy carrier in heating systems remains natural gas, which accounted for 30.9% of final energy consumption in 2022. The share of natural gas in the total fuel balance shows a downward trend; a decade earlier, its share was 45%. The situation is similar with the use of coal products, the share of which decreased from 4% to 2.3%. In the case of electricity consumption in the buildings sector, the situation is different, with its share increasing from 9% to 25.1% and the share of renewable energy sources and waste increasing from 14% to 22.6% [96]. Figure 15 presents the share of individual energy carrier or sources in EU households, according to Eurostat statistics [94]; the term heat in Figure 15 denotes the use of district heating.

In the US, the residential sector accounts for 22.2% of total energy consumption, while the commercial building sector accounts for 18% [97]. In China, in 2016, the energy consumption of buildings was 3.63 × 10¹¹ kWh, accounting for 20.62% of the total national energy consumption, and the total carbon dioxide emissions from the buildings sector were 1.96 billion tons of CO₂ equivalent, accounting for 19.0% of the total national emissions [98]. India ranks third in the world after China and the United States in terms of energy consumption, with 45% of the energy supplied to buildings for cooling, 28% for lighting, and 12% for heating. The residential energy consumption in India is 0.63 MWh per capita, which is equivalent to a total of 1710.3 million metric tonnes of CO₂ emissions [99]. The residential sector in Nigeria consumes about 65% of the total annual energy consumption in the country. In high-income households, ventilation and cooling account for 29% of total energy consumption, lighting 18%, refrigeration 12%, cooking 9%, and hot water preparation 5%. In middle-income households, ventilation and cooling account for 10% of total energy consumption, lighting 27%, refrigeration 21%, cooking 19%, and hot water heating 8% [100].

In order to decarbonize the building sector, it is particularly important to phase out fossil fuels, as two-thirds of the energy used for heating and cooling buildings in EU Member States still comes from fossil fuels [92], and to increase the energy efficiency of existing buildings [101]. The combustion of raw materials such as hard coal, lignite, or natural gas generates about 36% of the total carbon dioxide emissions in the European Union [102]. According to a 2006 study by Nishio and Hoshino, it is possible to reduce carbon dioxide emissions by 52% if a gas or oil boiler is replaced with a heat pump [96]. Replacing a heat source using fossil fuels or wood with a ground-source heat pump with a seasonal coefficient of performance of 3.5 reduces CO₂ emissions by 30% [103].

The planned reduction of greenhouse gas emissions by 55% by 2030 is an extremely ambitious goal, which requires a simultaneous reduction of greenhouse gas emissions from buildings by 60% by 2030, a reduction of final energy consumption in buildings by 14%, and a reduction of energy consumption for heating and cooling by 18%. It is expected that the renovation of existing buildings will reduce total energy consumption by 5–6% and reduce carbon dioxide emissions by about 5% [88]. Compared with traditional buildings, the passive house standard contributes to achieving 80–90% energy savings. It is the most effective way to reduce the energy consumption of a building, reduce carbon dioxide emissions, and improve thermal comfort in rooms [98].

5.2. The Impact of Climate Change on the Energy Efficiency of Buildings

Assessment and improvement of energy efficiency of existing buildings are becoming priority research areas due to the worsening climate and changing energy prices. Climate change due to global warming has a direct impact on the energy demand for heating, ventilation, and air conditioning of spaces, its amount, and the ability to maintain a comfortable indoor temperature. Therefore, considering the lifespan of a building, there is a need to include future climate predictions in the modeling of building design and compliance with regulations [104].

Studies conducted in Canadian office buildings clearly indicate the impact of climate change on the construction sector. Based on long-term climate forecasts covering 50 years, it has been shown that climate change will significantly increase the cooling loads of buildings while reducing heating loads. The size of these changes will be determined by the climate zone and temporary external conditions. In the case of colder climate zones, energy consumption will decrease in the winter, which will become increasingly shorter, but in addition, the demand for energy needed to cool rooms will significantly increase. As a result, the consumption of natural gas, which is the main energy carrier in Canada, will be reduced. At the same time, an increase in the consumption of electricity needed to cool rooms has been noted [104].

The increase in energy consumption is not only predicted in Canada. Such a phenomenon is also predicted in warmer regions of the world, for example, in India. Studies conducted on the basis of observations of buildings in Indian cities indicate that the rate of growth of primary energy consumption (58.73%) in India is much higher than the world average (21.04%). This phenomenon is caused primarily by the increased demand for energy needed to cool spaces. The increase in energy consumption is a consequence of the overall increase in global temperature but also of local temperature increases occurring in cities. The most common phenomenon is the urban heat island effect, which is characterized by an increase in ambient temperature in highly urbanized areas compared with rural areas [105].

In the perspective of climate forecasts in the time horizon of 2050–2100, taking into account the increase in the average air temperature by 4.4 K compared with the normal climate conditions of the years 1961–1990, considerations were made regarding the energy needs of buildings on the Swiss Central Plateau. The studies showed a decrease in the annual demand for heat energy for residential buildings by 40% in the considered period of time. On the other hand, for office buildings with internal heat gains of 20–30 W/m², an increase in the demand for cooling energy by up to 1050% and a decrease in the demand for heat energy by 36–58% were shown. It was also observed that the heating season is shortened by up to 53 days [106].

In Vienna, studies on the impact of climate variability on the demand for thermal energy were conducted on nine representative types of office buildings. The authors concluded that by 2050, the demand for heat could decrease by 11–30% (depending on the age of the building) [107].

5.3. Analysis of Factors Influencing Energy Consumption in Buildings

The energy efficiency of a building refers to the degree of preparation of the building in order to ensure the comfort of use in accordance with its intended purpose while ensuring the lowest possible energy consumption by the building. According to the given definition, a building with high energy efficiency should be characterized by low energy consumption. On the other hand, an energy efficiency assessment is an assessment of a set of building features that affect the consumption of energy required for its use, including an assessment of the thermal insulation of building partitions and the overall efficiency of the technical equipment used in the building.

The energy quality of a residential building is characterized by the annual demand for non-renewable primary energy EP.

In Figure 16, the dynamics of changes in the maximum values of the EPH+W index for heating, ventilation, and hot water preparation in multi-family buildings located in Poland over the years are shown [108]. From 1 January 2021, the EPH+W index in Poland should not exceed 70 kWh/(m²·a) for single-family buildings and 65 kWh/(m²·a) for multi-family buildings [28].

Energy consumption in buildings depends on, among other things, the following:

• The building’s location relative to the cardinal directions;
• The climate zone in which the building is located;
• The building’s geometry;
• The building materials used in the building’s construction;
• The method of operation and management;
• The parameters of the internal environment;
• The behavior of users;
• The devices.

A summary of important external and internal factors influencing the energy consumption of buildings is presented in Figure 17 [89,109]. Factors related to insulation materials and heating systems play an important role in determining the energy efficiency of a building [90].

5.3.1. Weather Conditions

Among weather conditions, the most important factor influencing the demand for heating and cooling energy is the outdoor temperature [110]. High outdoor air temperature reduces the demand for energy used for heating purposes while increasing the demand for cooling purposes [111]. In turn, relative humidity has a significant effect only during cooling. The value of the energy performance coefficient of the heating, ventilation, and air conditioning (HVAC) system is influenced by both outdoor temperature and relative humidity. Based on the conducted studies, it has been shown that the urban heat island phenomenon can reduce the efficiency of HVAC installations by about 25% and that, for outdoor relative humidity below 40%, the efficiency of cooling towers increases [89].

In the case of solar radiation, heat gains from insolation through transparent partitions cause an increase in the internal air temperature, which is beneficial in winter [112]. In summer, solar heat gain should be minimized by using, for example, sun protection devices. Solar radiation also causes the heating of external walls, which reduces the heat flow from the interior of the building to the outside through the building envelope [113]. However, when the building is located in an urbanized area exposed to air pollution, as well as in shaded places, the impact of this factor is reduced.

Another important aspect influencing the assessment of the energy efficiency of a building is the study of solar radiation in the location of a given building [114]. The optimal use of daylight reduces the need to use artificial lighting, which leads to increased user comfort and energy savings [115]. Buildings with large windows and a properly designed interior layout can make the most of natural lighting. However, in the summer months, too much sunlight can lead to overheating of the building interior and, therefore, an increased demand for cooling. Data characterizing solar radiation is essential during the energy optimization of buildings. They enable the precise design of HVAC systems, implementation of effective insulation strategies and long-term energy planning, which leads to significant energy savings and improved comfort of residents.

The least significant effect on building energy consumption is wind speed [89]. Wind increases the heat demand due to infiltration, where warm indoor air is replaced by lower-temperature air that requires heating [113].

5.3.2. Building Characteristics

Another important factor influencing the energy demand is the characteristics of the building, including, among others, the shape of the building, the orientation to the cardinal directions, the type of window glazing, the window-to-wall ratio (WWR), and the thickness of partition insulation [89].

The impact of building shape on its energy demand is greatest in countries exposed to extreme weather conditions [89]. It is also important to note that building shape can cause self-shading, which limits the absorption of heat from solar radiation [116].

There are many possibilities for creating a building structure with complex shapes, but in such cases, it is crucial to design large glazing on the southern external walls to reduce the probable heat losses. Studies conducted in Helsinki, which is located in a cold climate, show that the increasing shape ratio A/V of the building increases the total annual energy demand for heating and cooling. On the other hand, this factor is less significant in Ljubljana, in conditions with a higher average annual temperature, taking into account the location and size of the glazing [117].

The shape of the building has the greatest impact on smaller buildings, such as single-family houses. For this reason, it is common to design buildings with a compact shape, with an A/V coefficient in the range of 0.7–0.8 m⁻¹. In buildings with a larger cubic capacity, such as multi-family buildings, the shape is not as important [118].

Based on the research, the optimal shape of the building located in Boulder, Colorado, USA, was selected in the form of a trapezoid facing south. The best location for windows, taking into account the annual energy demand, turned out to be the south-eastern and south-western walls. In relation to cooling energy, the optimal location of windows was on the north side to limit heat gains from solar radiation [119].

The energy demand of a building is largely determined by its envelope, the type of building materials used, and the composition of external building partitions, which affect the climatic conditions inside the building [120]. The thermal insulation of walls is of the greatest importance due to their large surface area [89].

Considering the ratio of the window-to-wall area (WWR coefficient), it is important to maintain a balance between excess heat gain from solar radiation and beneficial radiation. The optimal value of the WWR coefficient, recommended for design, is 10–25%, depending on the type of glazing [89]. In residential buildings with a low WWR coefficient, the use of optimal thermal insulation in external walls reduces energy demand primarily for heating purposes. This procedure is not as effective in buildings characterized by significant glazing, which are often office buildings. In such buildings, the type of glazing, which allows for the collection of heat from solar radiation, has a greater impact than improving the heat transfer coefficient of partitions [120].

Improving the energy performance of buildings and reducing greenhouse gas emissions from buildings in the European Union aims to achieve a zero-emission building stock by 2050, taking into account outdoor and local climatic conditions, as well as indoor climate requirements and economic viability [121].

5.3.3. Technical Building Systems

Improving the energy efficiency of buildings is necessary for many reasons, including economic, environmental, and social aspects. Current building resources consume a significant amount of energy for cooling, heating, hot water heating, or powering electrical devices, including lighting. All these factors translate into the energy efficiency of a building. Improving efficiency allows for a significant reduction in operating costs, which is particularly important in the context of rising energy prices. From an economic point of view, well-designed investments using energy-saving technologies pay off relatively quickly in the form of reduced energy costs.

In order to reduce heat consumption for heating residential buildings, special attention should be paid to the following:

• Analysis of the shape of the building and its location (building shape factor);
• Meeting the requirements for thermal insulation of building partitions and glazed surfaces;
• Use of heat gains from sunlight and internal sources;
• Thermal efficiency of heat sources and heating system elements;
• Method of settling heating costs, proper measurement of the installation;

In the context of energy savings in the building and reducing operating costs, the following factors should be considered:

• Tightness of window and door joinery;
• Thermal bridges and tightness of building partitions;
• Type of ventilation;
• Heating systems,
• Cooling systems;
• Energy management and control system in the building;
• Use of renewable energy sources.

Heating installations should be equipped with devices that enable the settlement of the costs of the heat supplied and with devices that automatically regulate the temperature separately in individual rooms [28]. Based on field studies, energy savings of approx. 30%, obtained thanks to the installation of individual heat cost allocators in a multi-family building, with a simple SPBT payback time of this modernization shorter than one heating season, were demonstrated [122]. The installation of heat cost allocators and thermostatic radiator valves is associated with achieving heat savings in the range of 8–40% (on average 20%) [123]. The installation of thermostatic valves on radiators with simultaneous hydraulic regulation of the heating installation by setting the appropriate initial settings of these valves allows for achieving savings in the consumption of heat supplied to a multi-family building in the range of 15.4–19.8%. Lower savings in heat consumption, ranging from 9.1% to 10.7%, occur when using only thermostatic valves [124]. The use of differential pressure riser valves in the heating system eliminates overflows, reduces noise and vibrations, and allows for improving the energy efficiency of the system and achieving a reduction in heat consumption ranging from 12.9% to 17% [125].

The improvement of energy efficiency should also include domestic hot water heating systems and, in particular, those that provide for the circulation of hot water to ensure greater comfort for users. This is due to the fact that heat consumption for circulation can constitute up to 70% of the total heat consumption in the scope of the hot water heating system [126,127]. For example, in this scope, the use of temperature control valves (TCVs) under each circulation riser in the hot water installation reduces the heat consumption for the preparation of 1 m³ of hot water from 8.5 to 27.0% (on average by 13%), with a simple payback time SPBT of less than 3 years [126].

The reconstruction of a group heating node supplying a group of 13 multi-family buildings to individual heating nodes reduced heat consumption for hot water by an average of 29% and for heating needs by an average of 24% [128].

Detailed actions to increase the energy efficiency of HVAC systems have been presented in REHVA Guidebook 32 [101].

According to [121], zero-emission buildings should be equipped with measurement and control devices to monitor and regulate indoor air quality. Currently, the mandatory installation of automation and control systems takes place in non-residential buildings with an effective nominal power demand above 290 kW. From 2030, this obligation will be extended to non-residential buildings with a power demand above 70 kW. However, new residential buildings and residential buildings undergoing major renovation must be equipped with selected monitoring and control system functions in order to improve and optimize their management and operation.

5.3.4. User Behavior

Energy consumption in buildings depends on the habits and level of thermal comfort to which residents are accustomed [107].

Educating residents on energy-saving activities is one of the proven ways to reduce the consumption of heat supplied to buildings, allowing for energy savings of 4 to 30% [101,129]. Educational approaches can be divided into three main streams: traditional education (development and delivery of information leaflets), the feedback approach, and the installation of smart technologies (smart grids and plug load) [122,129].

Feedback and comparisons of energy consumption with neighbors are the most effective educational methods for promoting energy-saving behaviors among residents [130]. The online feedback approach has been found to be more rational and efficient than traditional energy-saving education because it can help building occupants: (i) assess their daily energy consumption profile; (ii) compare different energy consumptions; and (iii) identify inappropriate behaviors and provide suggestions for improving energy efficiency [129,131].

Individual metering and settlement of the costs of supplied energy on this basis have a direct impact on the behavior of users and on changes in their habits and contribute to reducing energy consumption in buildings. This is confirmed by studies conducted in multi-family buildings in Bilbao (Spain), where a 15–20% reduction in energy consumption was achieved in the first two years after installing energy meters, with the simple payback time of this modernization being about 10 years [132].

The installation of smart meters in buildings contributes to energy savings and, thus, to the improvement of energy efficiency. Measuring systems inform users primarily about the actual energy consumption and the actual time of its consumption and, at the same time, induce energy-saving behaviors that lead to a reduction of energy consumption by 4–12% [133] or even by 30–40% [134].

5.4. Decarbonisation of Buildings through the Use of Renewable Energy Sources

The overarching goal of European energy policy is the use of renewable energy sources in the building sector. Different forms of financial support are proposed by the countries, for example, in the form of public subsidies for heating buildings. A multi-faceted comparative assessment of the impact of different policy approaches on the energy efficiency of buildings was carried out in Austria (AT) and Switzerland (CH). In 2017, around 48% of the heat used in residential buildings in Austria came from renewable sources, while in Switzerland, this share was 30%. It was found that between 2000 and 2017, there was an increase in the supply of heat from renewable sources in AT and CH, which amounted to 31 PJ and 20 PJ, respectively, and energy savings related to improved energy efficiency amounted to 83 PJ and 97 PJ, respectively. It was estimated that the introduction of public subsidies for heating in 2014–2017 contributed to around half of the energy savings in Austria and around 36% in Switzerland. It is forecasted that in AT, by 2040, the share of heat supplied from renewable sources in residential buildings should be increased by 131% (149 PJ) compared with 2017 and, in CH, by 210% (120 PJ) by 2050 [135].

According to [121], all new buildings have to be designed in a way that optimizes the use of solar radiation energy. This particularly concerns the principles of the arrangement of appropriate solar energy installations, which are as follows:

• From 2027, solar installations must be installed on all new public and non-residential buildings with a usable area of over 250 m²;
• From 2028, they must be installed on all existing non-residential buildings with a usable area of over 500 m² after major or deeper renovation;
• From 2030, they must be installed on all new residential buildings and covered car parks adjacent to buildings.

The electrification of heating combined with the decarbonization of electricity is one way to reduce the energy intensity of buildings [136].

Photovoltaic conversion of solar radiation energy, especially using building-integrated photovoltaic (BIPV) systems, is increasingly used for electricity production. BIPV systems use photovoltaic panels embedded in the external envelope of a building to produce electricity for the building. They replace the standard external building envelope, contributing to the improvement of its energy performance. The proposed solution allows for reducing the energy consumption of the building, reducing CO₂ emissions, generating green energy, in particular through glazing integrated photovoltaic GIPV, and reducing thermal discomfort in rooms. Based on modeling studies and an environmental and economic assessment, an improved BIPV system has been proposed for buildings located in different regions of Egypt with a hot climate (Aswan, Cairo, and Alexandria). The results obtained for the considered climates and models showed CO₂ emission reductions ranging from 9% to 31% and reductions in discomfort hours ranging from 10% to 25%, depending on the model specifications [137].

Electrification in heating can be carried out by changing the heating technology. The optimal technology is considered to be one that is characterized by, among other things, high energy efficiency during the production of heat energy, limited impact on the natural environment and climate, high development potential and market absorption, and the potential for innovation and cooperation with the national power system. Modern heat pumps are one of the devices that provide such possibilities [136].

Ground-source heat pumps are becoming increasingly popular due to their potential to reduce primary energy consumption and thus reduce greenhouse gas emissions [138].

Ground-source heat pumps are characterized by high efficiency and reliability of operation, but the cost of their installation is relatively high. Therefore, air-compressor heat pumps are more popular than ground-source heat pumps in single-family buildings. By combining an air-source heat pump system with a solar electricity production system, a heat source with low operating costs for generating heat energy and heating domestic hot water can be obtained. New technologies allow for the connection of solar systems with heat pump systems. By using solar radiation and ambient energy, solar heat pump water heaters are environmentally friendly. In addition, the implementation of energy-saving technologies is a key strategy for reducing pollution and greenhouse gas emissions, which contributes to mitigating climate change and improving public health worldwide [139].

Solar heat pump systems that integrate the thermal power of solar and air-source heat pump systems can operate more reliably and offer a better coefficient of performance than standard solar heating systems. Studies have shown that the solar photovoltaic panel system powering the air-source heat pump achieves the best technical and economic performance, which achieves the best results on average with a coefficient of performance (COP) of about 3.75, but with moderate costs and payback time. Solar thermal systems and solar photovoltaic–thermal hybrid systems combined with an air-source heat pump system show lower performance with an average COP of 2.90 and 3.03, respectively. Additionally, the hybrid system has the highest cost and the longest payback time, while the solar thermal system has the lowest costs and the shortest payback time [140].

Most heat pump installations are combined with one or more heating technologies to ensure system reliability and security of supply. Large-scale heat pumps are often accompanied by auxiliary systems such as a heat storage tank or a boiler. On the other hand, on small scales, it is more common to combine a heat pump with a gas-condensing boiler, known as a hybrid heat pump. This innovative solution allows for the decarbonization of the heating system while increasing the efficiency of the heat source. Combined with intelligent control systems and storage, hybrid heat pumps can provide large energy savings. Furthermore, with advances in the decarbonization of the gas network and the generation of energy from renewable sources, hybrid heat pumps have the potential to provide fully decarbonized domestic heat [141].

Research conducted in the UK shows that a hybrid heat pump can potentially reduce greenhouse gas emissions by up to 45% compared with a gas-condensing boiler [142].

It is predicted that the use of hybrid heating devices can reduce peak energy loads by up to 24% while reducing carbon dioxide emissions by up to 75% [102].

Another very important aspect of using heat pumps as heating devices is the reduction of carbon dioxide emissions and PM10 and PM2.5 particulate matter emissions. It is predicted that in 2030, heat pumps will emit about 40% less CO₂ compared with heating using a coal boiler or electric heating [143].

As studies conducted in Great Britain show, if heat pumps were used by 20% of residential buildings, the peak load could increase by as much as 14% (7.5 GW) [136]. In order to avoid such a large increase in energy demand, it is important to change the heat pump operation profile. In order to maintain thermal comfort while reducing the load, it is necessary to use energy accumulators. These can be both smaller daily heat stores and larger seasonal heat accumulators that are able to provide heat for a period longer than one day. Additionally, the walls of thermally efficient buildings equipped with a domestic hot water tank can be treated as a daily heat store.

In 2018, around 60% of single-family homes in Sweden used heat pumps as their main heat source. Thanks to intelligent energy demand management and energy storage, it was possible to provide the right amount of energy when the heat pump was unable to produce the necessary amount of energy due to changing weather conditions. Heat storage technologies used in buildings are commonly classified as sensible, latent, and thermochemical energy storage. Studies show that storing energy contained in hot water is about 100 times cheaper than storing electricity. Therefore, water tanks are most often used, while storage tanks filled with phase change material are less common [144].

Studies conducted on the basis of the Swedish district heating system, taking into account and comparing the efficiency and economic benefits of typical thermal energy storage technologies, have shown several important issues related to the phenomenon of heat accumulation. It was found that the least cost-effective option for heat storage is the use of a water tank compared with the use of a tank filled with a phase change material or building insulation. This is due to high heat losses and low storage density in the low-temperature range. However, taking into account the discharge efficiency of the water-filled tank itself, it has been shown to be a better storage material than in the case of a tank filled with a phase change material. The lower discharge power of this tank is caused by the low thermal conductivity of the material [145].

A case study of a smart building energy system in the UK has shown the potential benefits of a phase change material-based heat storage tank to reduce end-user electricity bills by up to 20% through active demand management. Although the phase change material-based tank has limitations, such as material fatigue and limited discharge power, it may become a potential heat storage technology option in the future as technology advances and manufacturing costs decrease [146].

Considering the different heat storage methods, i.e., building envelope insulation, water storage, or phase change material storage, and the life-cycle costs, the condition with a temperature deviation of 1 K can generate the greatest savings for building insulation. Increased thermal insulation of buildings and lower heat losses reduce the charging and discharging power of energy storage. This phenomenon results in reduced energy dissipation and cost savings compared with poorly insulated buildings. However, the cost-saving ratio remains basically the same because the life-cycle costs of the devices are also lower in renovated buildings. The main economic benefit of installing any thermal energy storage is the reduction of peak power utilization. It has been found that buildings with variable temperature setpoint schedules and daily peak load profiles have a life-cycle cost savings potential of 2% to 5% below the current electricity price, while buildings with a stable schedule have almost no benefits [147].

However, short-term storage is not the only type of storage currently available on the market. Another way to store energy is to use seasonal heat storage. They are more difficult to make, mainly because they require larger areas and complex construction work. However, they can store significantly larger amounts of energy. They store energy obtained from solar farms, large-scale heat pumps, or cogeneration units. The role of seasonal heat storage is usually played by underground tanks or deep wells that cooperate with the heating network [143].

Seasonal thermal storage can provide flexibility to smart energy systems and is characterized by low cost per unit energy capacity and application in various geographical and geological locations. Depending on the heat storage mechanisms, seasonal thermal storage concepts can be divided into three main types: sensible heat storage (SHS), latent heat storage (LHS), and thermochemical heat storage (THS). Thermal energy storage in boreholes and aquifers are technologies that can be used for both heating and cooling purposes and provide large storage capacity, while the energy density and thermal conductivity are relatively low. Thermal energy storage in aquifers is increasingly used in district heating systems but on the condition that a subsurface aquifer is available at an appropriate depth. On the other hand, latent heat storage with high energy density can provide heat at an almost constant temperature. Thermochemical heat storage has the highest energy density and low heat loss because the storage medium is usually stored separately but requires a more complex system. Compared with the reference heating alternatives, i.e., natural gas heating and solar energy for decentralized systems, only thermal energy storage in the excavation pit and low-temperature aquifer is economically competitive. In the system with seasonal heat storage, thermal energy generated primarily from sustainable sources is accumulated and used in winter. It serves as a supplement and regulation of the heat supply system and shows the potential to coordinate the seasonal mismatch between heat supply and demand. Additionally, it improves the overall efficiency of the heating system [148].

Heat storage technologies are constantly developing, and new applications are being found for them. In 2019, the implementation of thermal energy storage in heating and cooling systems was investigated. Thermal energy storage is considered to increase the thermal potential in combination with district heating. The studies analyzed the combinations of sensible heat, latent heat, and thermochemical heat storage, taking into account each type of technology. The transition of current energy systems to next-generation district heating and the balancing of energy networks are considered [149].

In the UK, the main socio-technical factors that influence the implementation of new thermal storage technologies have been analyzed. Thermal storage can support a fully decarbonized energy system in three key ways: by providing network benefits, cost benefits, and facilitating the integration of renewable energy sources. Thermal storage systems have been developed that utilize waste energy and provide links between urban systems including electricity, heat, sewage, waste, and transport. The analysis suggests that thermal storage currently remains a relatively niche technology in the UK in a stable system based on the existing natural gas network. Given the continuing importance of financial support mechanisms for the implementation of thermal storage in the UK, investment in thermal storage is becoming increasingly popular. However, in the medium to long term, the natural gas home heating network needs to be decommissioned or repurposed for the UK to meet its mandatory carbon dioxide reduction targets, and this drive supports the transition, particularly for home heating [150].

Renewable energy sources play a significant role in improving the energy efficiency of multi-family residential buildings, allowing for a reduction in the annual demand for non-renewable primary energy EP. The use of technologies such as photovoltaic modules, heat pumps, and solar water heating systems contribute to a significant reduction in the demand for energy from conventional sources, which in turn leads to a reduction in greenhouse gas emissions into the atmosphere. Photovoltaic panels generate electricity that can be used to power lighting in common areas, elevators and other electrical systems, which reduces the operating costs of buildings. Heat pumps using energy from the air, water or ground can effectively heat buildings in winter and cool them in summer. Additionally, solar water heating installations allow for a significant reduction in the energy consumption needed to prepare hot utility water, which is an important element in the energy balance of a building. Integration of renewable energy sources with building management systems enables optimization of energy consumption, monitoring of its efficiency, and automatic adjustment of operating parameters of devices to current needs, which leads to additional energy savings. These technologies not only reduce operating costs but also increase the value of the property, benefiting both owners and residents [151].

6. Modern Trends of the Energy Management in Buildings

The enormous technological progress significantly facilitates actions aimed at improving the energy efficiency of buildings, reducing energy consumption and CO₂ emissions. Digital technologies support both efficient building operation and renovation planning by collecting data from different sources for simulation and automation [152]. The development of the IT industry makes buildings smarter, which allows for more efficient operation of HVAC systems while improving the health and safety of occupants. Currently, there is an improvement in individual systems in buildings, such as fire protection, security, and lighting systems. However, these solutions often work independently and do not take into account the mutual interactions within a given facility. New solutions in the field of IT information technologies enable the integration of all building systems in such a way that HVAC systems are optimized for the operation of other building systems.

One of the tools used to improve the energy efficiency of buildings is building information modeling (BIM) [153,154,155]. BIM software is used to model and optimize projects by planning, designing, building, and maintaining BIM models.

The concept of digital twin (DT) has gained great popularity among researchers in recent years and has been adopted as an important aspect of the digital transformation of energy systems [152,156]. Although the concept of digital twins is not new, its adoption in the energy sector is recent and aims to increase operational efficiency. Figure 18 graphically presents the basic elements of digital twin technology.

Digital twins (DTs) are virtual representations of physical objects or systems created based on real-time data. They use simulation, so-called machine learning (ML), and reasoning to support decision-making [157,158,159]. In the context of the construction industry, it is a tool that integrates different types of sensors and collects and processes data that are used to gain new operational insights, such as real-time visibility into a building’s energy consumption, demand, and energy management patterns. DT technology helps in monitoring the energy efficiency of a building, optimizing energy consumption and overall building performance. As a virtual replica of a physical element, DTs also enable modeling the future behavior of a building and predicting its response to changes using forecasts based on current and historical data [160,161,162].

In order for a digital twin to fulfill its role, it must have access to a large amount of data from different sources. The sources of these data include embedded sensors, wireless sensor networks, building information modeling (BIM) data, and outdoor weather data. A digital twin can combine multiple performance data across a building—including location, events, assets, and people—and then integrate with predefined building scenarios.

Despite significant efforts by most countries to promote building decarbonization, global implementation of energy efficiency measures in existing buildings is still insufficient to achieve net zero carbon emissions by 2050. Existing buildings account for the vast majority (over 97%) of the total building stock [163]. Most of these buildings are energy inefficient, rely on fossil fuels for heating and cooling, and use outdated technologies and equipment. Therefore, it is recommended to focus on the existing building stock by implementing comprehensive energy efficiency improvements through well-planned renovation and retrofit processes [164].

In the context of accelerating the pace of modernization of existing buildings and achieving energy and environmental goals, it should be mentioned that most of the current tools supporting energy modernization are based on pre-simulated data, which do not reflect the real dynamics of the operation of building components and systems. This results in a large discrepancy between the assumed effects before modernization and the actual results obtained after modernization [164]. Due to the lack of precise calculations and incorrect decisions, most modernized buildings will have faults and inefficient systems. Therefore, there is a need for systematic commissioning of modernized building systems and their monitoring and performance after the modernization process.

While digitalization and use of data from various building sensors, smart meters, and IT devices show a positive effect on improving building performance, such measuring devices are generally not available in existing buildings, and their installation is expensive and complicated. Therefore, there is a need for an alternative and temporary collection and integration of information and data in such buildings. In response to this challenge, Danish researchers [164] have proposed a novel digital twin solution called “DanRETwin”. This technology is based on data from inexpensive sensors and meters that are seamlessly installed in an existing building. Their installation allows users to monitor power, flow, heating, and cooling by simply attaching the sensor units to the outside of cables and pipes without the need for technical expertise. This is achieved by implementing advanced mathematical AI models, eliminating the need for expensive and complex hardware. Data from different IT devices are stored and used to develop adaptive energy models for different building systems and components. This optimization approach is integrated with digital twin technology to select the most efficient, cost-effective solution for the retrofit of a specific building [164].

7. Conclusions and Directions for Future Research

Fulfilling the EU’s climate neutrality goals requires energy transformation that takes into account the use of renewable energy sources, the application of various energy-efficient system solutions, and the capabilities of local recipients and energy markets. The transformation must be implemented in individual sectors with attention to energy security and the resilience of systems to threats.

The use of modern heat transfer solutions in various sectors of the economy leads to improved energy efficiency, increased energy resilience and security, and prevention of environmental degradation.

Changes are also necessary in the models of heat and energy management, energy market management, and social consumption models as an element of energy transformation in the pursuit of climate neutrality.

Ensuring energy resilience is a central pillar of energy policy in the region, but innovative approaches are needed to meet these urgent challenges.

In the face of current climate change, energy savings and efficient use are becoming a key strategy in the world.

The analysis of the conditions and dynamics of the heat transfer process in the modern construction, material, and installation solutions used in buildings is necessary for the optimization of heat and energy management models and society’s consumption models as an element of the energy transformation toward climate neutrality and counteracting the deepening of energy poverty.

Although the results of previous work [155,165,166,167] are promising, further research is needed to optimize energy consumption and heat transfer in buildings. One future research direction is autonomous building control that adapts to the needs of users in near real time so that the physical conditions adapt to the current and expected behavior of users, taking into account, among others, room occupancy, type of activity, lighting needs, and weather forecasts [167]. Future research should also address the development of methods that do not rely solely on BIM data. Further research on artificial intelligence and machine learning is warranted to improve digital twin applications, solve the complexity of integration, and reduce the costs and time associated with their implementation [155]. At the same time, new research on digital twins for buildings should pay special attention to the measurable benefits obtained from the digital twin, providing a reliable assessment of their value.