Innovative Strategies for Thermal Energy Optimization and Renewable Energy Integration in Net-Zero-Energy Buildings: A Comprehensive Review

Abstract

The thermal performance and energy efficiency of buildings are critical factors in achieving sustainable energy systems as energy needs for heating and cooling are expected to represent more than 50% of global final energy consumption. This study analyzes conventional renewable energy systems for heating and cooling in buildings, focusing on strategies for developing net-zero-energy buildings. This review covers the integration of renewable energy, the use of intelligent energy management systems, and the optimization of thermal processes. It also compares various systems based on their advantages and limitations and analyzes emerging trends in the thermal management of buildings in different climate zones. The synthesis of recent literature highlights practical recommendations for achieving high thermal performance in buildings, including the importance of selecting appropriate energy systems based on local climatic conditions, optimizing system efficiency, and taking advantage of new materials and advanced technologies. This review aims to contribute to promoting sustainable construction practices with the integration of renewable energy sources and improving the energy efficiency of buildings.

1. Introduction

Thermal comfort of buildings is one of the most important human needs. Fifty percent of the energy consumed in residential buildings worldwide is used for this purpose [1]. Figure 1 represents an example of thermal energy (heating/cooling) demand in Switzerland, where heating energy demand is plotted in red and cooling demand in blue. The lighter red and blue areas are due to population growth. While the “Full” use of cooling scenario assumes that every building is equipped with cooling systems, the “High”, “Medium”, and “Low” scenarios assume that cooling systems are used according to the development of cooling degree-days and socioeconomic factors [2]. To improve the thermal processes in buildings, with high thermal performance and high energy saving, important factors should be considered, such as climate change, and the choice of energetic systems depending on environmental conditions [3,4,5,6].

Influencing the thermal mass of buildings can also be economically valuable [7,8]. Many researchers have highlighted the large differences in thermal comfort and energy savings that can be achieved by focusing on a specific design and building materials [9,10,11,12,13,14,15,16,17,18]. The latest technology of data-driven models shows a good reduction in energy consumption, which can be up to 80% [19,20,21,22,23].

The percentage of energy saved in building heating and water heating depends on the system used and its suitability for long life. Solar thermal and geothermal systems have shown high efficiency in residential heating applications, offering significant energy savings and reduced CO₂ emissions through innovative configurations such as geothermal heat exchangers and integrated heat pumps [24,25,26]. Daylighting systems and phase change materials (PCMs) significantly contribute to reducing building energy loads by optimizing natural light use and enhancing thermal storage capacity, especially for seasonal energy management [27,28,29]. Meanwhile, these advanced thermal storage solutions, such as phase change materials or other systems for capturing industrial waste heat, offer promising methods to reduce heating demand, although challenges in storage efficiency and heat transfer remain [30,31]. The same is true for the percentage of energy saved in building cooling. Each system has better thermal performance, depending on the climatic conditions, the space in the building, and the performance of the passive technology [32,33,34,35,36,37,38].

To build integrated thermal–solar systems, it is recommended to meet the heat demand in buildings in a sustainable way [39,40]. Another way to improve the thermal comfort of a building is to design and construct a suitable building envelope that also saves energy and has a low environmental impact [41]. Different thermal processes in buildings have been studied individually or in groups [39,40,41,42]. Systematic reviews of cooling/heating systems and thermal processes in buildings are usually combined with renewable energy and electricity generation studies to achieve net-zero buildings, making the study information more general [43].

The main objectives of this study were to highlight cooling/heating systems and the improvement of thermal processes in buildings through the integration of renewable and conventional energy systems, to improve energy savings and thermal performance. Thus, this review focuses on a hybrid approach that combines various systems, such as geothermal heat pumps and photovoltaic systems, to improve energy efficiency and reduce carbon emissions rather than the separate study of each technology [4]. Details are given on the effect of other factors that can contribute to improving the energy efficiency of buildings, such as the materials chosen and even the use of automatic learning algorithms to predict consumption to optimize the overall energy performance of the building [9,19]. This study emphasizes climate adaptability and mentions information on specific solutions in different climatic regions [5].

Despite significant advancements in optimizing thermal energy systems for buildings, there remains a clear research gap in fully understanding and comparing the performance of individual and hybrid systems across diverse climatic conditions and building types. Most studies focus on isolated systems, lacking a comprehensive perspective on integrated approaches that combine multiple renewable and conventional energy sources to achieve net-zero-energy goals effectively. Additionally, while various thermal processes have been examined separately, the interactions and overall impact of combined systems on building energy efficiency and sustainability remain underexplored. This review addresses these gaps by providing a systematic comparison of individual and hybrid thermal systems for heating and cooling, focusing on energy optimization for net-zero-energy buildings. This manuscript contains a structure that starts with Section 2 outlining the methodological approach used in selecting and analyzing the relevant literature. Then, Section 3 explores individual and hybrid thermal systems, discussing their applicability and efficiency in various climates. On the other hand, Section 4 examines improvements in thermal processes, highlighting improvements in building thermal mass and data-driven control models. Section 5 addresses space heating and domestic hot water processes, emphasizing energy-saving solutions and innovative system designs, while Section 6 explores passive and active cooling techniques, focusing on technologies that enhance energy efficiency. Section 7 reviews various energy sources and systems for thermal processes, comparing renewable and conventional sources. Another feature is analyzed in Section 8, which discusses domotics and smart energy management technologies that support sustainable building operations. Lastly, Section 9 covers recent innovations and developments in related technologies, with a focus on the materials and methods that promote energy efficiency. Through this structured approach, this study offers a consolidated analysis of innovative strategies that promote the design and operation of net-zero-energy buildings.

2. Methods

This review followed a systematic approach to identify and evaluate the relevant literature on energy-efficient heating and cooling systems, focusing on technologies that contribute to zero-energy buildings. The selection process followed specific inclusion and exclusion criteria to ensure the relevance and quality of the studies analyzed.

To fulfill the inclusion criteria, only articles published between 2014 and 2023 were considered to ensure that recent advances in the field were covered. Only articles from peer-reviewed journals from reputable databases, including Science Direct, Scopus, and Web of Science, were selected. Priority was given to studies dealing with renewable energy systems, energy-efficient thermal processes, and technologies aimed at achieving zero-energy buildings. Finally, all research examining the performance of individual and hybrid systems for heating and cooling of buildings, as well as thermal energy storage methods, was included.

The exclusion criteria adopted made it possible to exclude from this study all work focused solely on energy generation (e.g., photovoltaic systems) without addressing thermal processes. Articles that did not contain quantitative data on energy savings or the thermal efficiency of heating and cooling systems were also excluded. The exclusion extended to research not related to residential or mixed-use buildings or that did not address thermal processes specific to buildings. Figure 2 presents the annual distribution of the scientific articles used in this review, illustrating the methodological approach in selecting recent and relevant literature across the stated time span. This distribution reflects the progressive developments in the field, ensuring comprehensive coverage of innovations and trends in thermal energy optimization and renewable energy integration for net-zero-energy buildings. This approach enables a robust synthesis of both historical and current research, providing insights that span nearly a decade.

3. Individual and Hybrid Systems of Thermal Processes

Thermal energy (heating and cooling) is expected to account for more than 50% of global final energy consumption in the next few years, with cooling accounting for 50% of local peak electricity demand in many places [1]. Also, according to the dynamic bottom-up building stock model, which combines dynamic material flow analysis with building energy modeling, in the Netherlands, by 2050, space heating demand will decrease by about two-thirds, and energy demand for hot water will increase to 92% of space heating demand [5]. Climate change will increase ambient temperatures, leading to the increased cooling demand in buildings and potentially reducing the effectiveness of the individual passive solutions that could be used to limit this increased demand. According to the study conducted in Portugal, a hybrid scenario that includes a geothermal heat pump, natural ventilation, and personal comfort systems is a robust solution for heating and cooling in a warming climate, as it can reduce final energy consumption by more than 90% compared to a non-hybrid system [3]. In addition, it is important to note that simulations of individual systems and hybrid systems show that individual use of a geothermal heat pump results in a 20 °C increase in ground temperature over 50 years, while the use of the hybrid system mentioned above results in a 6 °C increase in ground temperature over the same number of years [4]. In the Netherlands, the greenhouse gas emissions from operating energy are reduced by about 60–90%, depending on the hybrid energy sources [5].

4. Thermal Processes Improvements

In this section, improvements that control the thermal processes are mentioned.

4.1. Building Thermal Mass

An analysis of five residential buildings in different climatic zones (Seville, Malaga, Madrid, and Canary Islands) has shown that by using the thermal mass of the building as an efficient energy storage (preheating, precooling, and night ventilation), maximum economic savings of 3.2% in heating and 8.5% in cooling can be achieved. If the buildings have been previously renovated, the economic savings can even be doubled [7]. In addition, using data from Chicago neighborhoods in a computational fluid dynamic simulation, researchers were able to demonstrate the importance of building orientation and the number of floors. The results suggest that orientation and number of floors are critical to the intensity of building loads and that neighborhood shape can affect residential building heat loads by up to +27.1% and −18.6%, and office building heat loads by +17.2% and −7.7%. The proposed method and framework by researchers can provide design guidelines for the optimal energy efficiency of the buildings in the neighborhood [8]. Also, the Energy Efficiency Directive requires individual heat balancing in buildings where central heating/cooling systems are present, where this is technically feasible and cost-effective. The different approaches of EU member states to heat balancing and related issues raise several technical, legal, and consumer protection issues. The Directive could provide policymakers with several proposals to improve the transparency and reliability of distribution rules of the building’s thermal mass [44]. One of the main barriers to obtaining the building’s thermal mass for small and medium-sized buildings or low-income households is the high cost of sub-metering and maintenance. The researchers proposed approaches that could reasonably estimate heating and cooling consumption from meter data and show that they can be complementary [45]. However, it is important to combine physically based urban building energy models with uncertainty and sensitivity analyses to obtain reliable urban simulations. In an Italian city as an example, the uncertainty and sensitivity analyses lead to a reduction in the overestimation of peak load in residential buildings from 80% to 25%, and the deviation in the calculation of energy demand decreases from 18% to 10% [22].

4.2. Control Data-Driven Models in Buildings

To control data-driven models that estimate building energy consumption using machine learning algorithms, different types and quantities of sensors can be used to improve the accuracy of the model and minimize the cost of energy consumption for heating, cooling, hot water, and ventilation in residential buildings during heating and cooling days [19]. Applying such an approach with specific characteristics in a case study of a commercial building during the heating and cooling season could result in energy savings of up to 12%, with a mean error of 8% in performance prediction [20]. In another case, a multi-energy system on the campus of the University of Parma allows savings in operating costs of up to 80% compared to the baseline by allowing a variation of ±2 °C around the indoor temperature set point and optimizing the water supply temperature [21].

4.3. Building Design and Material

To optimize the thermal performance of a building, it is important to quantify the relevance of passive house measures in the early stages of building design [9]. It is recommended to use the environmental performance analysis to build an energy efficiency model for residential buildings, which includes the planning and design strategy, the form design strategy, the construction strategy, the system design strategy, and the equipment strategy, especially the sub-items of form design and space allocation in the formal design strategy. After applying these strategies to a Chinese university building, the requirements of the energy efficiency standard for public buildings (GB 50189-2015 [46]) of 65% energy saving were met [10]. Figure 3 shows the comparison of the indoor temperature of typical dormitory buildings in the south of China in summer. A multilayer perceptron neural network using various optimization methods was used to predict the heating and cooling of energy-efficient buildings. There are eight important and independent factors of the dataset: area, wall area, roof area, relative compactness, total height, orientation, glazing area, and glazing area distribution [23]. Also, an analysis conducted in an Australian school auditorium found that architectural design parameters such as cooling set point temperatures and roof design reduced operating temperatures by up to 14.2% and 20.0%, respectively. This resulted in a significant reduction in hours of thermal discomfort and a 43.7% and 41.0% reduction in energy consumption, respectively [47]. In addition, the building materials and the construction method of the building have a great influence on the energy consumption for heating/cooling. Experience shows that energy consumption can be reduced by up to 3% if the building materials have a higher thermal mass. Furthermore, additional wall insulation contributes to a significant reduction in energy consumption and reduces the impact of non-insulated thermal mass in the building [11].

The thermal processes in a building are many and varied. One important component that affects the building climate is the walls. Therefore, it is important to keep the temperature fluctuation of the walls close to zero compared to the temperature set according to the season (comfort temperature). The use of water as a thermal barrier system installed in the walls of a multi-family house has proven the effectiveness of such systems in stabilizing the indoor climate, with the fluctuation not exceeding 0.8 °C [15]. Another comparative study of a building constructed with straw bales shows satisfactory thermal performance with very low energy consumption and minimal gas emissions [12]. Moreover, a low-density sub-soil and fiber mixture was optimized to maintain the low-carbon materials and thermal insulation of the building walls. The authors suggest using the mixture in two layers for better results [13]. Also, 3D-printed concrete enjoys considerable attention due to its potential to drive the development of the construction industry. Moreover, the thermal behavior of the 3D-printed concrete prototype can be used as a simulator for the thermal behavior of a real building [14]. The measurements showed that the highest temperatures in the perimeter zones were found on the west façade of a building in Korea. In addition, the highest values were measured on the inside of the windows in this façade. However, an air barrier system was used to reduce the cooling loads in the perimeter zones, which helped to keep the temperature stable in the perimeter and core zones [16]. On the other hand, the analysis of both shading and the use of different building envelopes according to typical and future weather scenarios showed a 3.6% reduction in annual overheating hours with a reduction of more than 2.4 °C on the hottest day [17]. Also, one of the most important parameters for heating energy consumption in the building models studied is the U-value of the opaque building envelope, the effective shading, and the most important parameter for cooling energy consumption is the ratio of windows to floors [9]. Moreover, the useful internal heat gain increases slightly when the window-to-wall ratio increases from 0.1 to 0.8, while the useful solar heat gain decreases significantly. For multi-family buildings with a high-performance thermal envelope, minimizing solar heat gain is also important in winter in cold climates [18].

5. Space Heating and Domestic Hot Water

The space heating and domestic hot water processes in buildings vary depending on the type of energy supply (fossil fuel or renewable), and the installation (individual or integrated) [25]. Integrated solar thermal energy systems have high efficiency in heating residential buildings [24]. Also, to optimize building heating, a geothermal heat exchanger is integrated with the heat pumps in a single-family house in Zurich, Switzerland. The results show that the ground temperature is significantly reduced to almost 5 °C in the short term compared to the initial temperature of 11.5 °C. In addition, the temperature rises again above the initial temperature due to solar regeneration because of the excess heat in summer. However, due to insufficient regeneration in long-term operation, the ground temperature continuously decreases to almost 4 °C after 20 years of operation [26]. Also, for space heat demand, the windows retrofit results in a 27.3% reduction in total CO₂ emissions at almost no additional cost. Retrofitting the entire building with windows, walls, roofs, and floors is a CO₂-optimal reduction solution, but performs worst in terms of cost optimization [26]. Concentrator photovoltaic window systems are also recommended for use in the building, where they can not only significantly improve the uniformity of daylight and active daylight area, but can also significantly improve the use of solar energy and well meet the thermal energy needs of the building [27].

The elaboration of a techno-economic analysis of the hybrid heating system combining solar energy with heat storage (phase change material) allows the evaluation of the thermal efficiency, which was 51.3% for a three-story office building in Tianjin, China. The cost was 2.12 EUR/m²/season. This is significantly lower than the centralized heating rate for non-residential areas in Tianjin, which, as of 2022, was approximately 5.62 USD/m²/season, representing a 61% cost reduction compared to conventional heating [28]. Also, the phase change material was installed in a hut to perform a comparative study with a reference hut. The measurements show the next percentages over the months of use: heating energy savings of 40% in May and 10.3% in June/July 2019 were achieved. The use of phase change materials for space cooling also resulted in cumulative energy savings of 30% in March/April and 10% in January [29]. Moreover, the high melting points of salt hydrates are valuable for heating needs in central buildings. Salt hydrates are used as heat accumulators for the short-term storage of thermal energy. Despite the narrow application scope, imperfect performance evaluation, inefficient heat transfer enhancement, and unclear market prospects, this technology can be optimized from the aspects of material development, performance evaluation, heat transfer enhancement, and application feasibility [31]. Additionally, as an energy source, using waste to heat industrial buildings is an effective solution. One example is a district heating plant in Norway, 90% of its annual heat production comes from recovering heat from the waste gases of a ferrosilicon plant [30].

Table 1. Examples of the energy supply, installation, thermal storage, and the efficiency of building heating methods.

Reference	Energy Supply	Installation	Efficiency
Ravi Kumar, Krishna Chaitanya [24]	(Fossil fuel and renewable energies)	Integrated solar thermal energy systems	have high efficiency in heating residential buildings
Miglani, Orehounig [26]	(Fossil fuel and renewable energies)	A geothermal heat exchanger is integrated with heat pumps	27.3% reduction in total CO2 emissions at almost no additional cost
Xuan, Li [27]	(Renewable energies)	Concentrator photovoltaic window systems	improve the use of solar energy and well meet the thermal energy needs of the building
Wang, Guo [28]	(Renewable energies)	A hybrid heating system combining solar energy with heat storage (phase change material)	thermal efficiency, which was 51.3%
Gholamibozanjani and Farid [29]	//	Phase change material	heating energy savings of 40% in May and 10.3% in June/July 2019 were achieved.
Knudsen, Rohde [30]	//	Heat exchange (annual heat production comes from recovering heat from the waste gases of a ferrosilicon plant)	district heating 90%
Zhao and Wang [31]	//	Salt hydrates (short-term storage of thermal energy)	imperfect performance evaluation, inefficient heat transfer enhancement, and unclear market prospects

summarizes the above part.

6. Space Cooling

There are several passive cooling techniques. Each system has different thermal performance, depending on the specific climatic conditions, the space in the building, and the performance of the passive technique [32]. The ground-coupled multiple cooling systems achieve an average energy efficiency of 6.516, which is 53.2% higher than the reference air-cooled system and 6.5% higher than the conventional water-cooled system. In addition, a hybrid system combining the geothermal heat exchanger with a cooling tower achieves a 19.5% reduction in make-up water consumption [33]. As an example, to improve thermal conditions in livestock buildings, passive cooling systems are highly recommended, especially if the needs of livestock are taken into account, taking into account environmental conditions and the specific properties of building materials [34]. Moreover, radiant cooling systems (wall and ceiling cooling) are low-energy systems that can help efficiently meet occupant comfort needs [35].

The next section mentions several new trending methods, and more systems will be mentioned later. The use of phase change materials is considered an effective way to improve thermal comfort in buildings and reduce energy consumption. The most important step in selecting a phase change material is to choose the one that offers the best temperature range for optimizing thermal comfort, depending on the climatic conditions [36]. Nonetheless, many critical problems associated with inorganic phase change materials that limit their performance in modern construction applications are identified and discussed. Various proposed solutions to mitigate these problems are discussed in detail [37]. Also, a 3D numerical model simulating the influence of phase change material and nano-phase change material in the soil near the geothermal heat pump. The simulation shows that the addition of phase change materials and nano-phase change materials to the soil reduces the increase in soil temperature from 9.78 °C to 8.72 °C when the system is operated continuously for five years [38].

Earth Pipe Cooling (EPC) is a passive cooling system that uses stable underground temperatures to cool ambient air. This is an approach that requires minimal operational energy, does not require refrigerants, and thus does not promote greenhouse gas emissions. Recent studies highlight the efficiency of EPC systems under various climatic conditions. For instance, Mahdi-Ul-Ishtiaque et al. [48] demonstrate that the inclusion of inlet turbulators can enhance airflow dispersion, which, in turn, optimizes cooling efficiency by maintaining a more uniform temperature distribution. This addition can reduce the temperature by approximately 0.8 °C, resulting in daily energy savings of about 0.84 kWh in a controlled environment. Singh et al. [49] evaluate Earth Air Heat Exchangers (EAHEs) with concrete pipes, demonstrating that EAHE systems provide cooling capacities tailored for hot–dry and hot–humid climates. They found that the cooling potential varied significantly based on parameters such as pipe length and depth, with the highest performance observed at longer lengths and greater depths in hot–humid conditions. Specifically, cooling potentials of up to 24,080 kWh were achievable with 45 m pipes in these climates. These findings reinforce EPC as a feasible alternative for sustainable building cooling, particularly for climates with high cooling demands.

Table 2. Examples of the specific climatic conditions, type of the building, and the performance of the passive technique for cooling methods.

Reference	Specific Climatic Conditions	Type of the Building	The Performance of the Passive Technique
Buscemi, Catrini [33]	Located in Southern Italy	An office building	The ground-coupled multiple cooling systems achieve an average energy efficiency of 6.516, which is 53.2% higher than the reference air-cooled system and 6.5% higher than the conventional water-cooled system.
Szabó and Kalmár [35]	//	//	Radiant cooling systems are low-energy systems that can help efficiently meet occupant comfort needs.
Park, Lee [36]	//	Building walls	The use of phase change materials is considered an effective way to improve thermal comfort in buildings and reduce energy consumption.
Junaid, Rehman [37]	//	Building walls	Inorganic phase change materials that limit their performance in modern construction applications.
Daneshazarian, Bayomy [38]	//	Soil	Phase change material and nano-phase change material reduce the increase in soil temperature near the geothermal heat pump from 9.78 °C to 8.72 °C
Mahdi-Ul-Ishtiaque et al. [48]	Hot and humid climates	Generalized study, applicable to confined spaces and residential buildings	The study highlights that incorporating aerofoil-shaped turbulators near the inlet improves air circulation and cooling performance. This modification results in a temperature reduction of approximately 0.8 °C, leading to an additional daily energy saving of about 0.84 kWh for the cooling space.
Sing et al. [49]	Hot–dry and hot–humid climates	Various buildings (residential, commercial)	EAHE systems with concrete pipes show strong cooling potential, with up to 24,080 kWh achieved in hot–humid climates, especially effective in extreme climates due to the high cooling capacity of concrete-based EAHE systems.

summarizes the articles mentioned above.

7. Energy Sources and Systems for Thermal Processes in Buildings

The energy sources for thermal processes in buildings are diverse. Among the systems used for thermal processes, there are usually three to four types: conventional, renewable, conventional integrated with renewable, and renewable integrated with other renewable systems.

7.1. Renewable Energy Sources and Energetic Systems

7.1.1. Improving the Thermal Process Using Energetic Systems and Thermodynamic Methods

A variety of renewable energy sources and energetic systems and thermodynamic methods are available for cooling/heating buildings, some of which are mentioned below.

• Humidification/Dehumidification (HDH) Processes

The hybrid desiccant wheel system can simultaneously meet the building’s cooling, and heating needs [50]. The system is capable of providing a heating capacity of 176.9 kW with a thermal coefficient of performance of 2.223 and a heat recovery efficiency of 87.1%. In addition, this system can recover 115.2 kg/h of water in the form of condensate with a recovery efficiency of 76.36%, and with the design data of the system, a cooling capacity of 9.873 kW can be achieved [51]. Moreover, by modeling the air humidification and dehumidification system in MATLAB, where the inlet relative humidity was assumed to be a constant value of, for example, 45%, while the inlet temperature was assumed to be a variable value from 37 °C to 29 °C, the coefficient of performance has several values from 1.8 to 4.4, where the magnitude of the coefficient of performance depends on the air inlet values of temperature, relative humidity, and recovered energy from the discharge process [1].

Solar chimneys can be used to power humidification/dehumidification (HDH) processes. They leverage solar radiation to create a pressure-driven flow of air that can be used for various purposes, including powering HDH systems. In the context of HDH, solar chimneys provide a natural and sustainable way to induce airflow, improving the circulation of air through the system and thereby increasing the overall water production efficiency [52]. By combining solar chimneys with HDH systems, studies have demonstrated improvements in energy efficiency, as the chimney aids in the natural ventilation process, thus reducing the need for mechanical ventilation [53]. Additionally, the solar chimney setup can reduce operational costs, particularly when applied in arid or semi-arid regions with high solar radiation, as demonstrated in experimental setups and simulation models [54].

Air heaters play a crucial role in HDH processes by preheating the air entering the system. The preheated air has higher moisture-holding capacity, which enhances evaporation rates and subsequently increases the productivity of freshwater output. Different configurations of solar air heaters—flat-plate, parabolic, and evacuated tube designs—have been developed to maximize heat transfer efficiency [55].

In HDH systems, air heaters often employ solar energy as the primary heat source, which aligns with sustainability goals and reduces dependency on fossil fuels [56]. Experimental studies have shown that incorporating solar air heaters significantly boosts the performance of HDH systems [57]. Abdel-Dayem et al. [58] found that improvement of the evaporation process can significantly improve water production and the overall efficiency of a solar humidification–dehumidification (HDH) system and that higher feed temperatures improve system’s performance but increase the salt scaling. The efficiency of solar air heaters can be further enhanced through innovative designs such as fins, baffles, and selective coating materials, which improve thermal conductivity and heat retention [55,59]. These design improvements have been demonstrated to increase the temperature of the preheated air while minimizing heat losses, thus boosting the overall thermal performance of the HDH system.

• Thermal Insulation and Storage Processes

In this part of the work, various main processes are considered: thermal storage (high temperature [31], low temperature [36]) and thermal insulation [13]. The use of thermal storage in heating and cooling systems accounts for better energy efficiency and system operation [60]. Even in Arctic settlements, the transition to heating/cooling systems based on renewable energy sources is feasible, taking into account thermal energy storage [61].

For a case study in western Switzerland, it was shown that a single geothermal heat pump can meet up to 63% of cooling and 55% of heating needs in residential buildings by 2050, increasing to 87% and 85%, respectively, when district heating/cooling systems are used [62]. In addition, literature shows that lowering the temperature of geothermal water from 120 °C to 108 °C leads to a reduction in the heating and cooling capacity of a combined cooling–heating power plant by 28.87% and 14.72%, respectively [63]. Even though the unbalanced heating and cooling load in winter and summer, especially in regions with a heating- and cooling-dominant climate, may affect the performance of the geothermal heat pump system in long-term operation, the hybrid system (integrated with solar collectors or cooling towers), according to the simulation developed using a model in TRNSYS with data from Akita city (northern Japan), plays an effective role in both climates to balance the load on the geothermal heat exchanger, keep the ground temperature constant, and consequently improve the long-term performance [64].

Also, the thermal caisson system is a combination of geothermal cycles and a special phase change material. Researchers connected a high-efficiency heat pump to it. The thermal caisson system is expected to reduce overall costs by up to 49% compared to geothermal heat pump systems. In addition, this system could increase the coefficient of performance by up to 16% compared to geothermal heat pump technology (with a mean change from 3.7 to 4.3 in January) [65]. Moreover, the use of a thermal energy storage (TES) system in combination with heat pumps in an industrial building results in increased energy demand. In addition, by integrating this combination with renewable energy generation sources, such as PV panels, the installation of TES becomes thermally and economically attractive regardless of the prevailing electricity tariff [66].

Cold–warm aquifers are natural thermal energy storage systems that can be used as precoolers/preheaters to optimize the heating and cooling process of buildings by integrating them into the conventional system to reduce energy consumption and gas emissions [67]. Also, soil at a shallow depth has a nearly constant temperature throughout the year. The soil at this depth could be used as a heat sink/source. For five years, the soil temperature of a large office building in Hamburg was recorded, and it was found that manual adjustments to operating procedures could eventually bring the soil back to its original temperature level. Long-term, sustainable, and energy-efficient use of the floor temperature is, therefore, possible in principle in this case [68]. The results from the transient model indicate a minimum performance increase of 11% compared to other combined cooling–heating systems that integrate solar thermal and thermal storage components. This improvement applies across typical days in summer, winter, and transitional seasons, demonstrating a consistently higher efficiency level relative to baseline combined cooling, heating, and power (CCHP) systems operating in either the electric-load-following or thermal-load-following modes [69].

7.1.2. Renewable Energy Sources

• Solar Radiation and Radiant Cooling

Solar thermal is an important renewable source of thermal energy that meets space heating, water heating, process heating, and cooling needs. This energy source can also be used effectively in Nordic countries [70]. Solar energy can be used to improve the thermal performance of another system or even provide it with electricity. One of the systems of interest under the UK’s net-zero target for new and renovated residential buildings by 2050 is a photovoltaic-assisted solar heat pump [71]. A solar-assisted building with a thermal energy system has been proposed to reduce the heating costs of the system and to motivate building owners to adopt this solution as a cost-effective thermal energy system for their buildings. Simulations were performed based on different scenarios for the integration of a district heating system. The comparative simulation analysis of the performance developed using TRNSYS software version 7.1 shows that the maximum overall efficiencies of 74.51%, 62.35%, and 52.35% are achieved for the ultra-low, low, and third-generation district heating models, respectively [72]. The results of a comprehensive analysis of the energy, economic, and environmental potential of hybrid photovoltaic–thermal and conventional solar cogeneration systems show that the hybrid photovoltaic–thermal and solar cogeneration systems outperform the other systems in terms of total energy yield, with annual electrical and thermal energy yields reaching 82.3% and 51.3%, respectively, of the space heating, swimming pool heating, and hot water needs of the University Sports Centre of Bari, Italy [73].

For cold, temperate, and hot climates, four building-integrated photovoltaic systems controlled by solar radiation through windows were presented for energy performance evaluation. The simulation results of the system showed that the energy savings of the building were up to 26% in the Mediterranean climate (Athens) and up to 40% in the hot desert climate (Dubai). While in the cold climate (Prague), there is no significant change in the net energy use of the building, where its savings were 1.7%. The system maintained indoor thermal comfort for up to 54% of the time and ensured visual comfort for up to 83% of the time, for typical usage scenarios without auxiliary heating or cooling. These metrics represent the effectiveness of the system in providing a comfortable indoor environment under varying seasonal conditions [74]. On the other hand, the solar collector could be a good renewable energy system, used as a preheater for domestic water. The combination of a heat pump and solar collector in building heating optimizes the reduction in energy consumption and gas emissions, and experience shows that the renewable system can take 30% of the total work of the heat pump in temperate climates; the energy saved is usually more valuable in a warmer climate than in a colder one [75].

Solar radiation is the main reason for increasing the temperature of the building envelope (windows, roof, walls, etc.), which affects the thermal performance of the building interior and increases the energy demand for air conditioning to ensure satisfactory indoor thermal comfort. Therefore, passive cooling methods are highly recommended, especially in hot climates. Thermochromic smart windows, daylight coolers, and reflective paints are three important technologies for reducing solar gain on the building envelope [76]. Recent research shows that radiant cooling offers high cooling potential in many applications because it is a great low-temperature heat sink. The application of radiant cooling in buildings is more related to its passive integration into the building envelope, which limits its overall effectiveness. The study shows that the maximum temperature drop of the roof exterior varies between 10 and 20 °C almost throughout the year. However, the indoor temperature drop varies between 0 and 3 °C in the closed environment for personnel activities, so this study needs to be further investigated in the future [77].

• Renewable Energy System Combinations in Buildings

It is recommended to use integrated renewable energy systems as they are sustainable and contribute to the achievement of countries’ energy supply goals [78]. The combination of renewable energy systems, such as photovoltaic/solar thermal and geothermal, results in meeting the building’s cooling, heating, and electricity needs with energy savings of 30.10%, annual cost savings of 26.58%, and overall performance of 39.52% [79]. Renewable energy systems can also help in providing the conventional systems’ need for fuel gas. For example, power-to-gas technology is a very interesting hybrid process for generating gas from renewable sources. In this process, electricity from renewable energy sources is used to produce H₂ through water electrolysis, which is combined with carbon to form CH4, with the burnt oxyfuel from the oxyfuel boiler used as the carbon source [80].

The first important step before installing integrated renewable energy systems is to select suitable locations on the map according to the largest number of available renewable energy sources (solar, wind, etc.) to integrate them with conventional systems (powered by diesel, batteries, etc.). In Kenya, for example, the best hybrid and sustainable systems are solar/wind/diesel/battery compared to other feasible alternatives [78]. Also, to further improve the exergy efficiency of the system, it is recommended to use an energy system with multiple temperatures. For example, in geothermal heat pump systems coupled with thermally activated building systems, the higher-temperature chilled water can be supplied to the thermally activated building systems, while the lower-temperature water can be supplied to the air handlers for dehumidification [81].

In some hybrid heating systems with geothermal heat pumps and district heating and cooling, annual operation costs can be reduced by 64 tEUR and CO₂ emissions by 92 tons, while the ground temperature remains stable [82]. The importance of introducing improvements in the new requirements for the contribution of renewable energy to water heating, limiting energy demand, and limiting energy consumption will lead to reduce the energy consumption of the residential building stock and promote energy retrofits in buildings to achieve the European targets for 2020 and 2030 [83]. As an example, the integration of fully passive cooling and heating systems with daylighting functions in the hot summer and cold winter zones results in energy savings of 23% compared to conventional cooling and heating systems for courtyard buildings in the Jiangnan region of China [84].

A dynamic simulation model shows that optimizing energy storage capacity and optimizing the strategy of combined cooling, heating, and power systems with photovoltaics increase the reduction in gas emissions by 74.86%, energy savings by 36.83%, and annual cost savings by 13.04% [85]. Also, a hybrid integrated wind-solar energy system with a heat and electricity storage system in a near-zero-energy-consumption building in St. Petersburg, Russia, was evaluated. The dynamic study shows that the system can meet up to 61% of the building’s annual heating load. The system saves 13% of energy, with nearly 69% of the electricity generated by the wind turbine sold to the grid. CO₂ emissions are reduced by 13,859 kg/year. The maximum monthly energy and exergy efficiencies are 41% and 11%, respectively. The net present value is positive after 12, 14, and 17 years, assuming interest rates of 1%, 3%, and 5%, respectively [86], while the results of the transient analysis of the hybrid system optimization show an improvement in heat demand of up to 57.9% [87].

An investigation of nearly 90,000 renovated dwellings in the Netherlands with pre- and post-renovation data of actual and calculated energy consumption shows that deep renovations result more often in lower-than-expected energy savings than single renovation measures, but they result in the highest average energy savings compared to other thermal renovation measures [88]. Also, in a residential and commercial building in Krakow, southern Poland, three of four heating systems were fed with renewable energy sources. The largest amounts of heat were supplied by a biomass boiler, with a share of 30% to 50% [89]. Moreover, in the district considered prototypical, advanced district heating systems combined with low-energy building heating and cooling systems achieved 49% lower source energy consumption intensity than conventional systems [90].

Based on future climate changes, centralized district heating and cooling systems, ultra-low-temperature district heating, and fifth-generation bidirectional district heating and cooling systems were evaluated from technical, economic, and environmental perspectives, as well as considering the use of thermal energy storage. It was found that the use of a bidirectional fifth-generation district heating and cooling system is the best economic choice. However, electricity prices are 50% dependent on wind energy, which determines the cost of ultra-low-temperature district heating and fifth-generation bidirectional district heating and cooling systems, while the cost of centralized district heating and cooling systems is determined by nuclear power plants. In addition, the thermal energy storage limitation is inversely proportional to the heat-to-power ratio of the system [91]. The integration of energy geostructures into bidirectional fifth-generation district heating and cooling systems offers a greater share of low-cost geothermal energy. However, it is still in its infancy, and further research is recommended to better evaluate and develop the systems [92].

Even if the share of renewable energy systems in the environment is lower in mild and Mediterranean climates, in the Republic of Croatia, for example, up to 73% of the total electricity demand for heating/cooling could be met by variable renewable energy sources such as wind and photovoltaics [93]. To provide air conditioning and hot water to a small residential building in cold climates (Helsinki, Berlin, and Strasbourg), the use of hybrid energy systems connected to the power grid (high-efficiency geothermal heat pump, rooftop photovoltaic panels, thermopane) is recommended. TRNSYS version 18 and NeMo were used to create detailed dynamic models for the long-term operation of the energy system. The simulation shows a high coefficient of performance, reaching values of 5.4, with an electrical self-consumption of 71% at the coldest site [94,95].

7.2. Conventional Energy Sources and Systems

It is important to highlight that, in a large complex building, the heating, ventilation, and air conditioning (HVAC) systems can meet energy performance regulations, achieving a high energy efficiency of 122.6% due to effective energy recovery processes. However, despite this high energy efficiency, the system’s exergetic efficiency remains very low at only 3.68%. Thus, while the system uses energy efficiently from a quantity perspective, it does so with minimal alignment to the quality and utility of energy for specific processes, which results in significant losses in energy quality relative to ideal thermodynamic conditions [94]. This condition highlights the importance of exergy analysis in buildings to improve the quality of matching between energy supply and demand and consequently increase the sustainability of the building energy system. From the exergy point of view, the reason could be the energy conversion systems, energy distribution level, and energy released to the environment, which accounted for 54, 21, and 16% of the total exergy outflow consumed, respectively. Another reason is probably that the energy efficiency of the system decreases when the exergy demand decreases but the same energy sources are used to meet the demand [95]. For that, the use of geothermal heat pumps offers only limited system benefits, since their advantages are less than the peak demand and the fluctuations of the electric heating load are reduced [96]. Another conventional energy source is natural gas, which is used in a boiler for domestic heat supply [97].

According to researchers, some conventional energy sources used to heat/cool the building are economically better than others. For example, a comparison of the profitability of fuel cells and internal combustion engines as cogeneration systems in supermarkets shows that the payback period for installing a fuel cell system can be 5.9 years, compared to 5.6 years for internal combustion engines, when policies are considered. If no incentives are considered, the payback period for fuel cells as cogeneration systems could be 10 years, compared to 8.5 years for internal combustion engine systems [98]. In addition, it was explained that heating and cooling systems that use fossil fuels are very efficient [99].

Traditional HVAC systems in buildings typically operate based on fixed schedules and setpoints, providing heating or cooling at a constant rate regardless of actual occupancy levels or real-time environmental conditions. This approach, although functional, often leads to inefficiencies and increased energy costs. HVAC systems generally use time-based controls that are not responsive to variations in occupancy, indoor air quality, or external climate conditions, which can result in overuse of energy. Additionally, conventional systems rely heavily on fossil fuels and are not optimized for integration with renewable energy sources, such as solar or wind power, which can lead to missed opportunities for reducing carbon emissions. Improving traditional systems can involve the integration of smart technologies, such as sensors and Internet of Things (IoT) solutions, to enable demand-based control. This could allow HVAC systems to respond dynamically to real-time data from occupancy sensors, indoor temperature, and air quality monitors. For instance, smart sensors can adjust heating or cooling output based on the number of occupants and their specific location within a building, optimizing thermal comfort while conserving energy. Additionally, using predictive algorithms powered by machine learning can help anticipate changes in temperature or occupancy, further reducing energy waste. Furthermore, integrating renewable energy sources directly into HVAC systems through hybrid solutions, such as combining ground-source heat pumps with solar energy, can significantly reduce dependency on fossil fuels and enhance the overall sustainability of building operations.

8. Domotics Technological Solutions

Home automation, often referred to as smart home or building automation systems, offers a range of technologies that can integrate renewable energy sources, optimize energy use, and improve the overall energy efficiency of buildings. These technologies go beyond basic energy management and can enable real-time control, monitoring, and automation of building systems, including heating, cooling, lighting, and appliances. As energy demand grows and the drive to integrate renewable energies intensifies, home automation can make an essential contribution to achieving sustainable and energy-efficient buildings.

8.1. Advanced Smart Energy Management Systems (SEMS)

Powered by AI and IoT, Smart Energy Management Systems (SEMSs) represent a key point of home automation. These systems analyze data from various sources, including weather forecasts, building occupancy rates, and energy tariffs, to adjust energy consumption in real time. SEMSs can accommodate renewable energy systems, such as photovoltaic (PV) panels and wind turbines, with traditional energy sources to create hybrid energy systems that optimize energy flow.

By using advanced algorithms and automatic learning models, SEMSs allow the prediction of energy consumption patterns and enable dynamic management of energy storage and consumption components. Combining SEMSs with renewable energy systems in commercial buildings can reduce energy consumption by 25%, improving thermal comfort by continuously adjusting HVAC systems based on real-time occupancy data [2,100]. Additionally, IoT-enabled predictive maintenance within SEMSs can further cut energy usage by approximately 15% by ensuring systems operate at peak efficiency and proactively identifying maintenance needs to avoid energy-intensive repairs [5]. Another critical advantage of SEMSs is their capacity for peak load management, where excess energy is stored during off-peak times and used during peak demand hours, stabilizing grid loads and yielding substantial cost savings for building operations [1]. These energy management capabilities make SEMSs an essential component for achieving net-zero-energy goals in modern sustainable buildings.

8.2. IoT-Enabled Predictive Maintenance and Energy Efficiency

The IoT plays a key role in predictive maintenance and energy optimization in home automation. IoT sensors monitor the performance of HVAC, thermal, and photovoltaic systems, as well as other appliances, allowing the building manager to predict when maintenance is required before equipment failure occurs. This predictive maintenance approach reduces downtime and extends the useful life of energy systems. For example, Yang et al. [5] demonstrated that IoT-based predictive maintenance of energy systems, integrated with SEMSs, reduced overall energy consumption by 15% in a residential complex. The system achieved this result by ensuring that equipment operates at optimum efficiency and reducing the need for energy-intensive repairs. The IoT also enables energy conservation by accurately monitoring building conditions and adjusting the energy supply accordingly, ensuring that energy is only used when it is needed.

8.3. Integration of Renewable Energy and Home Energy Storage Systems (HESSs)

Home automation can increase the efficiency of renewable energy sources by integrating home energy storage systems (HESSs). These systems store excess renewable energy produced during peak production periods (e.g., sunny days for solar thermal and photovoltaic panels) and release it during periods of high demand or when renewable energy production is low (e.g., during the night or cloudy weather). As well as improving energy reliability, HESSs can be programmed to store energy during low-cost periods and release it when energy prices are higher, further reducing energy costs for homeowners. Ahmad and Ding [1] pointed out that integrating HESSs with smart home systems makes it possible to increase energy self-sufficiency by up to 60% in residential buildings with photovoltaic systems, ensuring that buildings rely less on grid electricity and more on locally stored renewable energy.

8.4. Automated Smart Climate Control and HVAC Systems

Control systems are important for optimizing energy consumption in buildings, and smart thermostats and HVAC systems are examples of the most common home automation technologies in this area. These systems can learn user behavior, monitor indoor environmental conditions, and adjust heating and cooling to maintain comfort and minimize energy consumption. Integrating SEMSs with smart thermostats ensures that energy is only used when necessary and that renewable energies are prioritized.

According to Mutschler et al. [2] the automation of climate control through SEMSs in combination with a ground source heat pump and photovoltaic panels can reduce energy consumption for heating and cooling by up to 44%. What is more, intelligent HVAC systems can also adjust their operations based on energy tariffs, maximizing cost savings by shifting energy use to off-peak periods.

In addition, HVAC systems with predictive climate control algorithms can precool or preheat spaces based on weather forecasts or occupancy schedules, further reducing energy consumption and dependence on grid electricity. Zero-energy buildings in particular benefit from these intelligent climate control technologies, as they can manage energy in such a way as to ensure that renewable sources are fully used, reducing carbon footprints and energy bills.

8.5. Smart Lighting and Appliance Control Systems

Intelligent or smart lighting systems are an important component of home automation, and make it possible to adjust the intensity of lighting based on the building’s occupancy rate, daylight levels, and time of day. By incorporating sensors that detect movement and ambient light, these systems can automatically switch off lights in unoccupied rooms or reduce them when natural light is available, leading to substantial energy savings.

The work of Liu et al. [101] points out that buildings equipped with intelligent lighting control systems and integrated with renewable energy systems have reduced energy consumption for lighting by up to 30%. Integration with photovoltaic systems also allowed buildings to use stored energy for lighting during periods when grid power was expensive, further increasing energy savings.

In addition, smart appliances connected to home automation systems can be programmed to operate during periods of low energy demand or when renewable energy is available. This form of demand management is particularly effective in reducing peak demand and ensuring that renewable energy is fully used.

8.6. Integration of Radiative Sky Cooling in Domotics Systems

Radiative sky cooling is an emerging technology that allows buildings to be cooled passively by radiating heat into the cold sky at night. When integrated into smart home systems, radiative cooling panels can be optimized to cool buildings without resorting to active systems such as energy-intensive HVAC systems. These systems can automatically open or close the cooling panels based on outside temperatures and the building’s thermal loads. Wu et al. [77] showed that integrating radiative cooling panels into SEMSs enables a 20% reduction in cooling energy demand in buildings located in hot climates. By allowing the intelligent system to decide when to activate passive cooling, buildings can maintain comfortable temperatures without relying on active cooling systems, thus reducing energy consumption.

Home automation technological solutions are essential tools for achieving energy efficiency and optimizing the integration of renewable energies in modern buildings.

Through intelligent energy management, the integration of IoT, the possibility of using energy storage, automatic climate control, and radiative cooling, buildings can significantly reduce their energy consumption, take greater advantage of renewable energy sources, and reduce their dependence on the energy grid. Future research should focus on improving the adaptability of these technologies to different climatic conditions, advancing AI-driven predictive maintenance, and exploiting renewable hybrid systems to improve the energy sustainability of buildings.

9. Advancements in Related Technologies

The thermal process developments intended to reach high thermal performance with energy consumption reduction in buildings, where the new trend is to achieve net-zero buildings by using various renewable energy sources in the thermal processes. This leads to reductions in (a) energy consumption, (b) gas emissions, (c) global warming, and (d) the costs of space heating/cooling and water heating [43]. Table 3 shows the energy saving in buildings using different cooling/heating systems and thermal processes in different countries, from published studies since 2017.

The valuation or quantification of energy saving in thermal processes of net-zero building systems depends on the calculation method, type of renewable energy, building characteristics, and energy conversion, where the valuation methods are important in understanding and improving valuable systems [102]. By applying the recommendations of the emergent-based optimization model to the design of the building’s energy systems, energy, economic, and environmental benefits are achieved [103]. In addition, better-insulated buildings equipped with efficient heating and ventilation technologies have lower energy requirements for space heating and cooling [104]. Moreover, the energy storage of the phase change material saves about 30% of the consumed energy in thermal processes when the volumetric thermal energy storage density is up to 430 MJ/m³. In contrast, the thermochemical energy storage density is up to 1510 MJ/m³, which promises higher energy savings. However, further research efforts are needed to optimize the operating conditions, efficiency, cost, and system design [105].

To achieve sustainable thermal processes in buildings, various combinations of energetic systems are used. The following are some examples of thermal processes in net-zero buildings. In British Columbia, Canada, the optimal combination of geothermal heat pumps and photovoltaic systems covers 44% of the building’s heating needs [106]. Also, in Seoul, Korea, 30% of the total thermal energy demand in buildings is met by integrated renewable energy. The optimal renewable energy system varies. In various residential buildings, the percentage of thermal energy can exceed 70% for photovoltaic systems or geothermal heat pumps. Consequently, electricity and gas deliveries to all buildings in the area decreased by a total of 17% [107]. Moreover, for a virtually energy-free building in Benevento (southern Italy, Mediterranean climate), the use of model predictive control of the room cooling system enabled optimal values to be determined for the set point temperatures, which in this case were 26 °C, resulting in a cost saving of around 28% [108].

Table 3. Saving energy using different cooling/heating systems and thermal processes in different countries, from published studies since 2017.

Reference	Cooling/Heating System or Thermal Process	City or Country	Energy and Cost Saving
Frank et al. [104]	better-insulated buildings	//	lower energy requirements for space heating and cooling (1.4% reduction in total energy consumption and an approximate 62% reduction in energy consumption for appliance operation)
Lizana, Chacartegui [105]	phase change material with up to 430 MJ/m3 of volumetric thermal energy storage density	//	saves about 30%
Karunathilake, Hewage [106]	geothermal heat pumps and photovoltaic systems	British Columbia, Canada	covers 44% of the building’s heating needs
Song, Oh [107]	photovoltaic systems or geothermal heat pumps	Seoul, Korea	electricity and gas deliveries to all buildings in the area decreased by a total of 17%
Ascione, De Masi [108]	model predictive control of the room cooling system for the set point temperatures, which in this case were 26 °C	Benevento (southern Italy, Mediterranean climate)	cost saving of around 28%
Du, Li [109]	The heating/cooling flexibility of a smart residential energy management system	the hot summer and cold winter zones of China	energy savings of 34.4% in Nanjing
Shin, Baltazar [110]	high-efficiency heating, ventilation, and air conditioning systems, a high-performance building envelope, energy-efficient lighting, and a solar system	the hot and humid climate of the United States	the renovated portion has 37–50% higher energy savings
Reda and Fatima [111]	the development of thermal solar technologies is necessary for thermal processes in net-zero buildings	from Finland and other northern European countries such as Sweden, Norway, and Estonia	on-site thermal energy production by conventional solar technologies is not sufficient to achieve the net-zero-energy target, and thermal performance is also not satisfactory

Table 3. Saving energy using different cooling/heating systems and thermal processes in different countries, from published studies since 2017.

Reference	Cooling/Heating System or Thermal Process	City or Country	Energy and Cost Saving
Frank et al. [104]	better-insulated buildings	//	lower energy requirements for space heating and cooling (1.4% reduction in total energy consumption and an approximate 62% reduction in energy consumption for appliance operation)
Lizana, Chacartegui [105]	phase change material with up to 430 MJ/m3 of volumetric thermal energy storage density	//	saves about 30%
Karunathilake, Hewage [106]	geothermal heat pumps and photovoltaic systems	British Columbia, Canada	covers 44% of the building’s heating needs
Song, Oh [107]	photovoltaic systems or geothermal heat pumps	Seoul, Korea	electricity and gas deliveries to all buildings in the area decreased by a total of 17%
Ascione, De Masi [108]	model predictive control of the room cooling system for the set point temperatures, which in this case were 26 °C	Benevento (southern Italy, Mediterranean climate)	cost saving of around 28%
Du, Li [109]	The heating/cooling flexibility of a smart residential energy management system	the hot summer and cold winter zones of China	energy savings of 34.4% in Nanjing
Shin, Baltazar [110]	high-efficiency heating, ventilation, and air conditioning systems, a high-performance building envelope, energy-efficient lighting, and a solar system	the hot and humid climate of the United States	the renovated portion has 37–50% higher energy savings
Reda and Fatima [111]	the development of thermal solar technologies is necessary for thermal processes in net-zero buildings	from Finland and other northern European countries such as Sweden, Norway, and Estonia	on-site thermal energy production by conventional solar technologies is not sufficient to achieve the net-zero-energy target, and thermal performance is also not satisfactory

The heating/cooling flexibility of a smart residential energy management system was investigated using 13,005 datasets from a nationwide field study in the hot summer and cold winter zones of China. The flexibility was demonstrated where a large energy savings potential was suspected (e.g., 34.4% in Nanjing), and it was found to be reliable in avoiding excessive cooling/heating demand [109]. Also, for the hot and humid climate of the United States, the army has designed and built a net-zero-energy building at Army Base Fort Hood in Texas. Several energy-efficient building technologies have been developed to reduce the energy consumption of the entire installation by 30% by 2030. The technologies used are high-efficiency heating, ventilation, and air conditioning systems, a high-performance building envelope, energy-efficient lighting, and a solar system. A comparative analysis between the renovated portion and the non-renovated portion shows that the renovated portion has 37–50% higher energy savings [110]. In addition, detailed analyses from Finland and other northern European countries such as Sweden, Norway, and Estonia show that the development of thermal solar technologies is necessary for thermal processes in net-zero buildings. However, in northern latitudes, on-site thermal energy production by conventional solar technologies is not sufficient to achieve the net-zero-energy target, and thermal performance is also not satisfactory [111].

The thermal energy efficiency of buildings depends on the thermal energy policy factors, building definition, regional policy, standard, technology, and the relationship between these factors. Based on more than 400 cases in cold regions, the United States is closer to achieving zero-energy buildings than the European Union and China. In general, the achievement of zero-energy buildings needs to be further advanced [112].

The use of combined cooling, heating, and power systems to save energy may lead to an imbalance between energy supply and demand in the future. For this, it is important to provide an optimization method that is characterized by good economics, stability, and accuracy and supports the preparation of operating plans for individual buildings [113].

Despite significant advances in the energy efficiency of buildings and the optimization of thermal processes, there are still several areas where further research is needed. Below are some key areas that could serve as a focus for future research:

• Thermochemical Energy Storage Optimization

Thermochemical energy storage (TES) systems offer high energy densities, but their efficiency and cost-effectiveness need further improvement. Future research could focus on optimizing the operating conditions, material development, and system design to increase their thermal energy storage capacity and reduce system costs. Investigating novel materials and improving heat transfer efficiency will also be crucial for scaling TES systems for widespread use [105].

• Integration of Smart Energy Management Systems

While smart energy management systems have demonstrated significant potential for reducing energy consumption, future work could explore integrating these systems with advanced data analytics, artificial intelligence (AI), and Internet of Things (IoT) technologies. This would allow for real-time optimization of energy use in buildings, predicting energy demand, and dynamically adjusting thermal processes to reduce wastage. Expanding this concept to large-scale buildings or entire urban areas could provide more comprehensive energy savings [109].

• Improved Model Predictive Control (MPC)

Model predictive control (MPC) systems have shown promising results in optimizing thermal comfort and energy savings in buildings. However, future work could address the limitations in predictive accuracy by incorporating more sophisticated climate modeling and user behavior analytics. MPC could be further refined to better adapt to extreme weather conditions, especially in areas experiencing rapid climate change, making it more robust for global applications [108].

• Hybrid Renewable Systems for Net-Zero Buildings

While hybrid systems that combine photovoltaic (PV), solar thermal, and geothermal systems have proven effective, future research can explore the potential of integrating other renewable energy sources, such as wind power and energy storage systems, for achieving net-zero buildings. The optimization of energy storage in these hybrid systems, particularly in urban settings where space is limited, would be an area of significant interest. Research could also focus on the economic feasibility and lifecycle costs of these hybrid systems [79,106].

• Building Envelope and Material Innovation

Future work can explore the development of advanced building materials that provide better thermal insulation and lower carbon footprints. For instance, research into materials with phase change properties, nano-materials, and bio-based materials could enhance the thermal performance of building envelopes. The potential for 3D-printed concrete and other modern construction techniques to revolutionize thermal performance is also a promising field of study [92].

• Radiative Sky Cooling Technologies

Although radiative cooling technologies have been increasingly studied, their practical application in buildings remains limited. Future research could focus on enhancing the cooling efficiency of radiative materials and their integration into existing buildings. In particular, passive cooling strategies in tropical and desert climates require further investigation to determine how best to deploy these technologies to reduce cooling demand [77].

10. Conclusions

This review provides a detailed analysis of hybrid energy systems, smart management solutions, and advanced thermal materials, to improve the energy efficiency of buildings. By integrating renewable energy sources with advanced thermal management systems such as geothermal heat pumps and intelligent energy management systems (SEMSs), significant improvements in energy consumption and energy performance have been observed globally. Key findings include the benefits of combining multiple renewable energy sources, which can achieve better thermal performance and greater efficiency compared to using stand-alone systems. This work highlights several key areas relating to thermal processes and energy efficiency in buildings, particularly in the context of integrating renewable energy systems to achieve net-zero-energy targets:

• With the increase in global temperatures as a result of climate change, a greater demand for cooling in buildings and also a reduction in the effectiveness of passive thermal solutions, especially in regions experiencing extreme heat, are expected. Consequently, it will be necessary to use more advanced, building-integrated renewable energy systems, such as geothermal heat pumps or photovoltaic (PV) systems, to maintain good energy efficiency while meeting the growing demand for cooling.
• It is necessary to develop comprehensive energy efficiency models that incorporate diverse strategies, including design, construction, and adaptation strategies for systems and equipment. In this way, energy consumption savings of more than 60% can be achieved compared to the current situation, particularly in residential buildings where configuration and design are critical to overall energy performance.
• Taking advantage of the dynamic behavior of the thermal mass of buildings constitutes an effective strategy for improving energy efficiency. By utilizing the mass of the building as a thermal energy storage mechanism, particularly during preheating or precooling cycles, buildings can achieve significant energy savings. This strategy can be optimized by combining it with advanced energy management systems that use real-time data from multiple sensors to regulate the temperatures of indoor spaces.
• The comparison between conventional energy systems and systems based on renewable energy sources indicates that although conventional systems are efficient and may have lower installation costs, they are dependent on fossil fuels and produce greenhouse gases; consequently, they are less environmentally sustainable. Combining multiple renewable systems produces better thermal performance than relying on a single solution, although the integration process can increase initial costs.
• The potential for using energy storage systems, in particular thermochemical energy storage, that offer higher energy densities is dependent on their optimization. It is necessary to continue research work on this topic, focusing on improving operational efficiency, reducing costs, and refining the system design. TES systems unlock significant energy savings, especially in dense urban areas where energy storage capabilities are crucial to balancing energy demand with renewable energy supply.
• Adapting energy systems to regional climates, such as selecting efficient heating methods in colder climates or adopting passive cooling strategies in hot, dry regions, ensures maximum energy efficiency. Insulating buildings and implementing advanced heating and ventilation technologies also contribute to significantly reducing energy demand for either heating or cooling.

In summary, data-driven optimization models and IoT-based sensor integration improve the flexibility and adaptability of these systems, especially in predicting energy demand and optimizing its use in real time. Additionally, advanced building materials, such as those incorporating phase change materials, as well as the use of thermal mass for energy storage, present an opportunity to further reduce energy consumption in heating and cooling processes.

Finally, the integration of renewable energy systems with smart energy management and advanced thermal processes presents the most viable path to achieving net-zero-energy buildings. However, future research should focus on making these technologies more profitable and scalable, particularly through advances in thermochemical energy storage and hybrid renewable energy systems. Furthermore, climate-specific adaptations and early-stage energy planning will be key to ensuring the success of these systems on a global scale.