Modern Thermal Energy Storage Systems Dedicated to Autonomous Buildings

Abstract

This paper presents a detailed analysis of the research into modern thermal energy storage systems dedicated to autonomous buildings. The paper systematises the current state of knowledge concerning thermal energy storage systems and their use of either phase change materials or sorption systems; it notes their benefits, drawbacks, application options, and potential directions for future development. The rapid proliferation of studies on installation systems, new composites, and phase change materials requires a systematisation of the subject related to short- and long-term thermal energy storage in building structures. This paper focuses on assessing the validity of the current improved thermal energy storage solutions for buildings with very high energy efficiency standards and buildings that are energy-independent. The paper presents the current results of the energy and economic analyses of the use of heat storage systems in buildings. This paper shows the optimal heat storage systems for autonomous buildings. Moreover, it also shows other potential ways to develop systems and composites capable of storing heat in autonomous buildings.

1. Introduction

The development of modern energy-efficient construction engineering has been determined by changes occurring during building design. A logical choice is to increase the use of thermal energy from renewable sources for building heating, to reduce the heat losses, and, ultimately, to reduce the building’s demand for heat or cold. In this context, a satisfactory solution in terms of architectural and construction design of a building is to achieve a state where the direct and indirect solar radiation gains and internal gains from the building’s operation make up for the heat losses due to penetration and building ventilation. Nevertheless, when striving to achieve low-emission buildings, their functioning across their entire life needs to be considered.

In addition to their obvious energy efficiency, autonomous buildings are powered with energy from renewable sources and, characteristically, are as independent as possible from an outside energy and matter supply throughout their lives. Such buildings are also analysed from the perspective of the construction materials used and the way that they relate to closed-loop systems; these analyses use the life cycle cost (LCC), life cycle assessment (LCA), and life sustainability cost analysis (LSCA) methods [1,2,3,4,5,6]. In this context, autonomous buildings fulfil the criteria of sustainable growth [7,8,9,10,11] with regard to reductions in the consumption of non-renewable resources and their replacement with renewable resources, reductions in waste production and pollution, and the consumption of renewable resources no faster than they can be replenished.

An intuitive response to the criteria of sustainable growth in construction is to increase the share of renewable energy generation, primarily solar energy, in passive systems which require no additional power supply. Such systems can be successful if highly efficient heat and cold short- and medium-term storage systems are applied. Heat and cold storage systems used within building components are a solution frequently described in scientific papers; these studies enable autonomous construction objectives to be achieved in accordance with sustainable growth principles. Due to the dynamic development of energy-efficient construction, as well as the numerous scientific studies on new materials, composites, and heat and cold storage technologies for building structures, along with their installation systems, the authors of this paper see a need to systematise the current state of knowledge on this subject. To facilitate the use of the information included in this paper, a graphical diagram of the subject is shown in Figure 1.

1.1. Thermal Energy Acquisition and Distribution Methods from Renewable Sources

Renewable energy sources are naturally renewed during their use. These are supplies that are replenished as quickly as they are consumed. By definition, renewable energy (electricity or heat) is produced from solar, wind, water, geothermal energy, biomass, biofuels, biogas, or hydrogen obtained from renewable sources.

• Solar energy. Most renewable sources are directly or indirectly dependent on the sun. Most of the direct gains are absorbed at latitudes around the equator, but this energy is then dispersed across the planet in the form of winds and ocean currents.
• Wind energy. Air currents can be captured and used to drive wind turbines. Wind energy shows the fastest growth among all renewable sources.
• Hydropower. We can also obtain energy from water, based on either its movement or its temperature differences.
• Geothermal energy. This is obtained by capturing the heat of the earth itself, usually from depths of up to several kilometres below its surface. It is an expensive source of renewable energy.
• Biomass. We know different forms of solid biomass: wood fuel, organic components of municipal waste, or unused parts of agricultural crops. Most types of biomass contain usable energy.
• Biofuels. Liquid biofuels are generally bioalcohols (e.g., bioethanol) or biooils (biodiesel or pure vegetable oils). Their biggest advantage is the lower emissions.
• Biogas. This can easily be produced from biologically active waste substances that arise, for example, from the production of paper or sugar and from sewage, animal waste, and other substances. These various wastes must be allowed to settle together and to undergo natural fermentation to produce methane.

1.2. ZeroEnergy and Autonomous Buildings

Zeroenergy buildings (ZBs) are designed and built to use as little energy as possible. When renewable energy is added to these buildings, they are able to produce enough energy to meet or exceed their operating requirements. The idea of zeroenergy buildings is based on ensuring that the heat gains are equal to the heat losses generated during their operation. This is achieved by reducing heat losses through the thermal enclosure and ventilation. This should be accompanied by maximum heat gains from solar radiation and the profits from building equipment and users.

Autonomous buildings (ABs) are designed to be energy self-sufficient. They require the unique modelling and engineering of a forward-looking building with renewable technology integration, clean energy storage, and demand reduction as focal points. Moreover, the AB is not only a zero carbon and zero energy building, but also has zero grid connection and zero energy bills (ZZZZ)—engineering zero. These are original and innovative approaches, leading to practical solutions to national environmental challenges, such as climate change, air quality, green energy production, and local management. The study found that ABs are more expensive than traditional buildings because they embody the integration of renewable technologies and highly energy-efficient materials; yet, they offer the best engineering services and products, which can raise the bar for Kuwaiti villas and provide multiple solutions: increased housing, increases in green energy production, increases in air quality, and significant reductions in CO₂ emissions, along with the saving of money and the built environment.

An extension of the idea of zeroenergy buildings is the concept of netzeroenergy buildings (NZEB). This is based on the aim of increasing the energy self-sufficiency of a building. The NZEB concept includes the achievement of a zero-heat balance for the building as well as the full renewability of the energy sources required to power the auxiliary equipment for the heating system and everyday appliances. The construction of such buildings relies on an extensivephotovoltaic (PV) system and small wind turbines. Legal acts specifying the energy performance building design (EPBD) take into consideration only the energy demand for the heating, cooling, and, sometimes, lighting of buildings. Such acts omit the energy needed to power home appliances and the aspects related to the temporary thermal comfort of building users. Unfortunately, the temporary sense of satisfaction with the thermal conditions inside the building determines the actual demand for electricity, heat, and cold, such as by opening the window when the ventilation system is temporarily inefficient. Furthermore, an important factor that is currently not considered when establishing the building energy performance is the formingof building heat balances on an average monthly, not hourly, basis, which neglects the effects of the demand for heat and cooling that vary over the day. The above deficiencies in the procedures for assessing the energy efficiency of buildings lead to major discrepancies between the theoretical and the actual performance. Therefore, heat storage in the building structure can reduce the temporary demand for heat and cooling by the building users. Such actions significantly contribute to reducing the actual energy demand of the building by improving the thermal comfort of its users.

2. Conventional Possibilities, Advantages, and Disadvantages of Heat Storage in Building Elements

The main factors determining the efficiency of heat storage systems are their cost, the efficiency of their heat storage and distribution, and, indirectly, their environmental impact. The important factors determining the efficiency of the heat accumulator are enthalpy of the phase change, heat transferability, chemical neutrality, low flammability, and low toxicity.

The most important property is the heat capacity of the substance, which directly affects the amount of accumulated heat in the substance in relation to the volume. The higher the heat capacity, the smaller the volume of the substance needed to accumulate a certain amount of heat. Thermal conductivity is also an important property; it affects the transfer of heat at the interface between the substance that accumulates heat and the heat-carrying substance that distributes the heat from the accumulation system to the point of consumption. Higher thermal conductivity ensures better heat transfer and increased efficiency. Last, but not least, the reversibility of the substance is also important, i.e., the ability to heat/cool repeatedly without degrading the material. This property is extremely important for substances that change states in the heating or cooling process. In the accumulation of sensible heat, thermal energy is stored during the heating of a substance that has suitable properties for these purposes. Most often, water, which has a high heat capacity (4.18 kJ/(kg·K)), is used to accumulate thermal energy in the form of sensible heat.

The advantages of this system mainly include low investment costs, the non-toxicity of the heat storage substance, and the available range of hot water storage tanks. The fact that the thermal capacity of the accumulated sensible heat is limited can be considered a negative. In order to store a large amount of energy, a large storage volume is theoretically required, which reduces the efficiency in terms of heat loss. It is also true that increasing the temperature of the substance in order to store more heat increases the heat loss at the interface of the material with the surrounding environment, which reduces the efficiency of the process.

Heat storage using phase changes is a method frequently discussed in the scientific literature. The advantages of this method are high heat storage density, almost isothermal storage, and the capability to enable wide application in the building structure. Nevertheless, the most commonly used groups of phase change materials are often characterised by a limited heat transfer capacity when they are in the solid state, with a thermal conductivity of 0.2–0.7 W/m·K. This problem has been addressed by adding thermal conductors, such as copper and aluminium alloys or carbon fibres with a thermal conductivity of 200, 370, or 470 W/m·K, respectively. There are various heat storage materials. With each method of heat accumulation, it is advisable to use different materials and to consider their physical and chemical properties. Water, oils (with the ability to accumulate higher temperatures), and solid substances (aggregates and concrete) are mainly used to accumulate sensible heat. The advantages of using solid substances are the elimination of the risk of liquid substance leakage and their affordability, although their heat capacity is significantly lower compared to that of liquids.

The accumulation of latent heat mainly involves the use of organic (paraffins, fatty acids) and inorganic substances (inorganic salts). The melting point of paraffins is in the range of 12–71 °C, while inorganic salts are in the range of 30–120 °C. The disadvantages of organic substances are their flammability and low thermal conductivity, which limits their use. For the accumulation of thermal energy bound in a chemical reaction, many organic and inorganic substances can be used which primarily meet the perfect reversibility of the chemical reaction. Currently, there are many systems that use accumulated solar energy. Its use finds application in heating applications (hot water heating, radiant heating), but also for technological purposes and electricity production. In practice, the most widespread is the heating of drinking water and the accumulation of energy in the form of sensible heat in hot water tanks. However, for the needs of heating and seasonal accumulation, the systems are still limited in terms of efficiency and financial return. Inorganic salts, which are able to accumulate thermal energy, appear to be an advantageous solution, but their use is limited due to their low thermal conductivity. To optimize this method, it is necessary to use measures that increase the thermal conductivity and thus the overall efficiency of the heat accumulation system. Currently, the accumulation of heat in the form of group heat can mainly be solved experimentally, although putting it into practice still requires further research. Similarly, the method of heat accumulation in the form of a chemical reaction is currently the subject of experiments, and so far, it appears to be a highly advantageous process in terms of the efficient storage of a large amount of heat in a small volume with minimal heat loss.

The literature describes methods to better systematise the options for thermal energy storage in the structures of different materials; Figure 2 shows a diagram of examples.

Another group of systems that enable thermal energy to be stored comprises sorption systems. Sorption-based heat and cold storage systems utilise the bond energy of the molecules that form hydrated salts or metal hydrates. In construction, they are used as bothheat and cold storage systems, often on a large scale, and as substances that improve heat pump performance. Fan et al. [12] presents a study on a three-phase sorption thermal energy storage system that improves heat pump performance. The results the study presents show that a coefficient of performance (COP) of 7.53 was achieved with a top and bottom heat source temperature difference of 20K. Similar methods of using sorbents to improve heat pump performance are discussed elsewhere [13,14,15,16,17,18]. Zhang et al. [19] indicate that solid–gas sorption thermal energy storage systems are currently the most effective for obtaining thermal energy from solar plants. The advantages of such systems include high thermal energy storage density, low heat losses during storage, the ability to store thermal energy for prolonged periods of time, and flexible operation modes. As an example of such compounds, SrCl₂, with an evaporation/condensation heat of 1630 J/g at −5 °C, was studied. Similarly, Yan et al. [20] noted that salt sorbents were a leading solution that enabled waste heat or renewable energy to be reclaimed. It was also demonstrated that the primary limitations in using this technology, as with PCM, are the ensuring of the necessary kinetics for the thermal energy storage process and the selection of the sorbent’s parameters to suit its application.

The drawbacks of using sorption systems for storing thermal energy include a frequently encountered non-linear relation between the temperature increase and the actual amount of heat released. This issue was investigated by Fumey et al. [21], who demonstrated that the thermal energy storage potential of the sorbent was not fully utilised. A sample solution for this issue was presented by Lin et al. [22], where an Al₂O₃ and LiCl composite absorbent was used. The resulting sorbent with a 16.44% salt content showed an absorption of 0.39 g/g and a thermal energy storage density of 345.88 kWh/m³.

Another solution with which to improve sorption cold storage system performance is to use a sorption thermal battery (STB), which contains a zeolite and MgCl₂, as described by Choa et al. [23]. The battery is dedicated to high-cooling power storage systems, providing a 15.1% increased thermal storage density, compared to a pure zeolite sorbent, and allows storage of up to 686.86 kJ/kg. On the other hand, Nguyen et al. [24] discuss the results of the experimental testing of an activated carbon and MgSO₄ sorption composite. Thanks to the high specific area of activated carbon, the test composite showed an improved water sorption efficiency and a hydration heat of 920 J/g.

Another use for sorption systems is the new, efficient method of generating hydrogen from biomass gasification and decarbonisation, as described by Zhang et al. [25]. The method improves the hydrogen generation efficiency by 17.4% compared to conventional methods.

Heat and cold storage capabilities by salt or metal sorption have a high potential in theory, but due to the lower actual ability to fix water, their applicability is limited. The papers [20,23,24,25,26] discuss methods for reducing this issue under certain specific conditions, such as a particular reaction environment, the addition of a new component, or a changed place where they are applied. Sorption systems provide an important potential for heat and cold storage at high volumes and high power, which has found practical application in industry.

3. Possibilities, Advantages, and Disadvantages of Heat Storage in Systems with Phase Change Materials

Due to the possibility of stabilising daily temperature changes in building components through the use of substances and materials in their structures which are capable of isothermal heat storage, the thermal comfort for the users of buildings modified in this manner has begun to improve. Scientific studies have begun testing phase change materials in different forms, such as microgranulate, or directly as mortar additives and elements of walls or ceilings and as autonomously applied micro- or mini-batteries of thermal energy in transparent and opaque building partitions, as well as in ventilation, air conditioning, and heating systems. Using the same phase change material, but in different construction partition types, can result in diametrically different effects, which is why this paper separately analyses the use of PCMs in transparent and opaque partitions as well as complex thermal energy storage systems.

3.1. Phase Change Materials in Transparent Partitions

Numerous examples of using phase change materials combined with transparent partitions were presented by Soares et al. [27]. The main objectives of using PCMs in this partition class was to substantially increase their thermal inertia and to better adapt the absorbed solar radiation energy to the heat gains and demand profile. In this context, it is justified to use phase change materials for modifying windows, the shading elements working in conjunction with windows, or the window woodwork. To this end, PCMs were used as filling for inter-pane spaces [28,29] in window packages or as light-permeable coatings [30]. Furthermore, phase change materials are used as the filling in blinds and roller shades [31,32,33,34,35] or as the filling for thermal energy batteries installed within double facades [36]. Additionally, PCM thermal energy storage performance has also been tested by applying it inside glass bricks [37]. In the above examples, phase change materials were used in the form of granulate or mini-packages by coating pure PCMs [38,39,40,41]. An example of an organic phase change mixture in the form of microcapsules and stable capsules is shown in Figure 3.

Another PCM form frequently described in scientific papers is shape-stabilised composites, SSPCM [42,43,44,45]. They take the shape of capsules or packages covered with a coating that keeps the PCM in place and prevent its controlled unsealing [42,46]. Examples of using transparent materials directly in windows or window blinds are described elsewhere [31,32,47]. An image of the use of phase change materials in the form of stable stalled polycarbonate composite capsules, as well as directly, is shown in Figure 4.

3.2. Phase Change Materials in Opaque Partitions

As with opaque partitions, increasing the thermal accumulation capacity of opaque construction partitions is the object of numerous scientific studies. The daily temperature fluctuation reduction effect of PCMs will be particularly visible in buildings with a lightweight frame construction. Even if provided with sufficient thermal insulating power for the partitions forming the external envelope, the thermal conditions in these buildings are sensitive to changing conditions in the external environment.

Studies on solving this issue are described by Soares et al. [48], who investigated the use of granulated PCM-containing drywall. The tests and their analyses were conducted for the Mediterranean climate in Coimbra, Portugal, and concerned the functioning of lightweight steel-framed (LSF) buildings. The test results demonstrated that building cooling and heating performance improved by 62%. Similar analyses and results in using PCM granulate are described by Saffarin et al. [49]. Similar conclusions concerning the reduction in the cold demand in buildings located in a hot climate were made by Zahir et al. [50]. Significant potential was shown for solar radiation energy absorption using thermal energy storage (TES) systems, as well as for solar radiation energy use for the needs of the building’s users. However, due to the limited ability to regulate heat accumulation and distribution, these solutions are not widespread. There is an example [51] of a large-scale study on buildings with PCM-containing partitions; the study holistically discusses their impact on thermal load levelling reduction (TLLR), the reduction in CO₂ emissions (CO₂ ES), the average indoor temperature reduction (AITR), the average heat gain reduction (AHGR), and the energy cost savings (ECS). The experiment was conducted in the hot climate of Egypt, and the results demonstrated that head load was reduced by 8.71%, supplied electric power by 56 W, and CO₂ ES by 1.35 kg per day. It was also demonstrated that the PCM performance was the highest when used as part of the roof structure.

On the other hand, Al-Absi et al. [52] discussed an external wall envelope made of foamed concrete (FC) and a PCM. The tests were performed under real climate conditions in Malaysia, achieving a 6.75 °C temperature reduction inside the test chamber and leading to a reduced temperature amplitude attenuation decrement and a 32.1% reduced structure heating rate.

Another example of using PCMs in opaque partitions is the use of foamed cement with a PCM, as described by Li et al. [53]. The effects of the test composite were verified in five climate types in China, and the results demonstrated that PCMs performed the most effectively in a temperate, warm climate.

Phase change materials are also used as additions to brick ceramic elements. Agarwal et al. [54] present the results of testing a PCM composite made of n-Eicosane and OM35, applied to ceramic bricks. The analysis was performed in the Indian climate, where a room was tested made of bricks modified in the above manner, equipped with a gravity ventilation system. The results showed a 32% reduction in cold demand, but the solution was deemed economically non-viable due to the estimated payback period of 181 years. An example of using PCMs in ceramic hollow bricks is shown in Figure 5.

An interesting solution to the problem of temperature regulation limitations in buildings using PCMs is described by Leitzke et al. [56]. Based on the example of a building in the climate of Brazil, a system was proposed involving separate areas designed with different internal air temperatures. This enabled the streamlining of the heat distribution system and the improvement of the efficiency of the heat and cold management within the building. A system designed in this manner showed a 65% reduction in cold demand. Another solution to the issue of heat distribution in rooms with PCMs was described by Wang et al. [57], where an adaptive dynamic building envelope integrated with PCMs (ADBEIPCM) was proposed. An analysis of multiple previously collected cases demonstrated a significant improvement in heat distribution within the modified building components.

An awareness of the performance variability of PCM solutions when applied in different climate conditions is fundamental. Imafidion et al. [58] discusses the results of numerical simulations of the effects of using retrofitted phase change prefabricated walls for the thermal modernisation of rooms in the Canadian climate and in the tropical climate. It was shown that the payback period in the Canadian climate was 35 years, while in the tropical climate it was 7.6 years.

The use of phase change materials in the form of passive composites and heat and cold storage systems faces major limitations in the distribution of the stored heat and cold, as shown in [48,49,50,51,52,53,54,55,56,57]. For this reason, the solution for some of these limitations is ready-made thermal energy batteries, which often form part of active heat and cold distribution systems. Some examples of PCM use in combination with opaque partitions, as described by Lichołai et al. [59], are shown in Figure 6.

3.3. Phase Change Materials in the Form of Thermal Energy Batteries/Storage Systems

Thermal energy battery storage systems are used both as elements working passively with building elements, such as a thermal envelope [48,49,50,51,52,53,54,55], and as active systems supporting the processed heat and cold distribution and storage installations [60].

Zhu et al. [60] present a high-temperature phase change material (H-PCM) system, which uses the phenomenon of carbon dioxide evaporation and condensation. The solution is characterised by a high thermal energy storage and heat distribution efficiency due to the fact that it maintains the properties of a liquid and convective heat transfer by CO₂ in either state of matter.

Another example of using phase change materials in systems utilising heat exchanges between liquid and gas is the system described by Momeni et al. [61]; this system stores waste and residual heat from electric and hybrid car ventilation systems. The researchers demonstrated that using a PCM with a latent heat of 323 J/g extends the time to heat a space by 6 min after the power is turned off, and greatly accelerates its heating after the power is turned on.

Examples of a passive thermal energy battery dedicated to the transparent partitions used in moderate climates are shown elsewhere [62,63]. It is a ready-made product containing a new phase change material in the form of a eutectic mixture of propyl palmitate and butyl stearate, with a melting/freezing enthalpy of 186 J/g at 18–28 °C [64,65,66]. The external surface of the battery is made of aluminium covered with an absorption coating. The results of numerical analyses and empirical tests show an increased thermal performance of the transparent partitions with thermal energy batteries on sunny days. At the same time, increased heat losses by transparent partitions were noted on cloudy days when compared to an identical reference window. An example of a PCM thermal energy battery applied directly beside the building windows is shown in Figure 7.

A different application for phase change materials in solar plants or electronic systems is described by Li et al. [67]. The ability to isothermally store large amounts of thermal energy was used to reduce the temperature of electronic systems working as thermoelectric energy generators (TEG). The results showed that the peak temperature values for the test electric systems were reduced by 33.5 °C, and the demand for the current to power the test system was lowered by 1/10.

One example of natural, environmentallyfriendly thermal energy batteries involves the salt gradient solar ponds described by Rghif et al. [68]. These are salted natural or artificial water bodies of various sizes which, due to the dissolved salts, have an increased heat storage capacity compared to pure water. These reservoirs can be used as natural stabilisers for the local microclimate or as a source of low-temperature heat. A similar solution for phase change systems was discussed by Gu et al. [69], who demonstrated that combining low-temperature heat pump (HP) systems with PCMs that improve the thermal capacity of a low-temperature source reduces the power needed to supply the heat pump. It is particularly important today, when reducing building energy demand and heat emission levels is called for.

In a critical approach to modern applications, when considering the results so far achieved and the realities of using phase change materials in buildings and components, one needs to compare the limitations of using the PCMs themselves and the possibilities of applying them effectively.

It is obvious that increasing a partition or building element’s thermal capacity alone, without first investigating the thermal conditions at the location where a phase change material is to be applied, will not only not bring the assumed results but, as shown by Musiał et al. [62], may in fact degrade the thermal parameters of the partition. This is a consequence of, for example, improperly selecting the location to apply a PCM with a particular phase change enthalpy and temperature range [38,64]. Additionally, a frequently encountered problem is the selection of the form and geometry of the PCM capsules or mini-packages, which limit heat distribution. A consequence is incomplete PCM melting and an additional reduction in stored heat removal during, for example, its solidification. The above issue is discussed elsewhere [38,64].

Another phenomenon affecting phase change performance is the material’s behaviour under overheating and overcooling conditions. Excessive heating or cooling of the PCM may lead to a situation where, despite increasing its temperature above the melting point or reducing it below the solidification point, the phase change does not occur or is noticeably delayed. At best it leads to the incomplete utilisation of the heat and cold storage capacity by the PCM, while at worst it causes irreversible changes in its structure, preventing it from functioning as designed. A comprehensive description of this phenomenon is given elsewhere [38].

Due to PCM phase change reversibility within the melting and solidification range and the ensured immutability of its physicochemical parameters, even after many thousands or tens of thousands of phase change cycles, the most commonly used PCMs are organic or derivatives. According to different papers [70,71], the stable capabilities and thermal energy storage performance are necessary conditions to make PCM application economically viable in composites with resin-, cement-, gypsum-, or epoxy-based matrices, etc. In this respect, a significant proportion of the inorganic phase change materials, including light metal salt hydrates, are inferior to organic PCMs. As described by Vogel [71], this is a consequence of the phenomenon of congruence, which is an irreversible separation of crystallisation water from the solid part of the salt as a result of its sedimentation. There are ways, as described by Smolec [72], to counteract this phenomenon by adding emulsifying agents. Nevertheless, despite the available methods of preventing the congruence of inorganic salt hydrates and their much higher phase change enthalpy values than organic PCMs, they are infrequently used in construction and scientific research.

Another limitation in using organic phase change materials is their relatively low self-ignition temperature (about 150 °C), which limits their use in installation systems, structurally important building elements, or electric cars. Example comparisons of the physical properties of organic and inorganic PCMs can be found elsewhere [38,64,71]. Furthermore, an extremely important issue that applies to all groups of phase change materials is the ensuring of the sealing of the capsule, microcapsule, and larger PCM package coatings. In addition to the obvious need to ensure the chemical inertness of the PCM and matrix, it is important to ensure similar thermal expansion coefficient values and the ability of the coating to withstand significant plastic deformation. This is necessary due to the phenomenon of PCM volume change by up to 20% when they change their state of matter during melting or solidification [38,40,41,71]. The aspect of ensuring PCM capsule sealing is also important due to the undesirable PCM reactions that may occur with conventional construction materials should their protective coating lose sealing.

The above considerations summarise the limitations and scientific and technical problems most frequently described in the scientific papers which investigate the use of phase change materials in construction. For better clarity on this, a detailed summary of the places and methods of phase change material application in construction materials and building components is provided in

Table 1. Methods of PCM use in construction and methods of combining them with traditional construction materials.

		Place of Application of Phase Change Materials in Buildings (Bibliography Numbers)
Method of combining PCM with conventional building materials		Walls	Floors	Windows and blinds	Heat storage	Active solar system
	Combining capsules, microcapsules containing PCM with cement or gypsum	53–55, 59	53	27–29, 38–41	67
	Impregnation of porous materials, e.g., aerated concrete, ceramic bricks	55, 58–59	73		87
	Direct mixing of PCM with cement or gypsum	38	38		86
	Production of stable composites containing up to 80% of pure PCM with a polymer matrix (HDPE—shape-stabilized PCM)		64–66	42–66	20	94
	Boards laminated with an inner layer of PCM	57	56		42–45	78, 94
	Heat accumulators in the form of cylinders or cuboids with dimensions of a few to several centimetres covered with a polymer coating, placed in the free spaces of hollow elements	12		31–31, 47	60–61	67, 72

.

4. Examples of Modelling the Functioning of Heat Accumulators Using Phase Change Materials

Modelling the thermal functioning of phase change materials is the subject of numerous deliberations and scientific studies. Attempts to represent the physical and chemical processes of the state of matter changes in substances, mixtures, or endo- and exergonic chemical reactions as a function of heat flux density over time have utilised several principal methods:

• Finite elements method, using splines and specific boundary conditions.
• Finite differences method, using splines and specific boundary conditions and meeting the thermal diffusivity condition.
• Finite volumes method, using splines and specific boundary conditions.
• Statistical methods linked to the experiment plan and multi-variable function response planes.
• Methods using fuzzy sets, e.g., Mamdani–Assilian models, used when a large pool of empirical results for verifying the model is not available.
• Methods using artificial neural networks.

The methods listed above, except the methods using fuzzy sets [73,74] and the artificial neural network methods [75,76,77], are modified solutions of the J. Stefan problem [78]. The problem involves determining the position of a variable relative to time and a coordinate system of a discrete boundary grid between a liquid and solid substance.

The majority of scientific studies use finite element methods to model the prediction of the thermal functioning of phase change materials or buildings and composites containing PCMs [78,79,80,81]; some use the finite differences method [82,83,84,85] and, more rarely, the finite volumes method [86,87].

The finite elements method and finite volumes method are characterised by a high calculation accuracy, although preparing the model is often time-consuming and requires knowledge of the boundary condition characteristics at the model construction stage. On the other hand, the finite differences method is more intuitive and allows the obtaining of explicit analysis results and variables over time for the entire time section studied. Unfortunately, it is severely limited by the need to meet the condition of thermal diffusiveness and the need to properly select the time step value, which requires validating the model and the discrete grid.

Currently, the starting point in modelling non-stationary, complex heat flow in explicit systems is to determine the temperature or energy level values at consecutive points of the assumed discrete grid for moment t+1 (the next time step). Examples of a general equation in the finite differences method for a two-dimensional discrete grid [62,70] are expressed by Formulas (1) and (2).

$${\mathrm{T}}_{\mathrm{i},\mathrm{j}}^{\mathrm{t}+1}=\frac{∆\mathrm{t}}{{\mathrm{C}}_{\mathrm{w}\mathrm{i},\mathrm{j}}·{\mathsf{\rho }}_{\mathrm{i},\mathrm{j}}}·\left(\begin{array}{c}\frac{{\mathrm{T}}_{\mathrm{i},\mathrm{j}-1}^{\mathrm{t}}-{\mathrm{T}}_{\mathrm{i},\mathrm{j}}^{\mathrm{t}}}{{\mathrm{R}}_{\mathrm{i},\mathrm{j}-1\leftrightarrow \mathrm{i},\mathrm{j}}}+\frac{{\mathrm{T}}_{\mathrm{i},\mathrm{j}+1}^{\mathrm{t}}-{\mathrm{T}}_{\mathrm{i},\mathrm{j}}^{\mathrm{t}}}{{\mathrm{R}}_{\mathrm{i},\mathrm{j}\leftrightarrow \mathrm{i},\mathrm{j}+1}}+\\ +\frac{{\mathrm{T}}_{\mathrm{i}-1,\mathrm{j}}^{\mathrm{t}}-{\mathrm{T}}_{\mathrm{i},\mathrm{j}}^{\mathrm{t}}}{{\mathrm{R}}_{\mathrm{i}-1,\mathrm{j}\leftrightarrow \mathrm{i},\mathrm{j}}}+\frac{{\mathrm{T}}_{\mathrm{i}+1,\mathrm{j}}^{\mathrm{t}}-{\mathrm{T}}_{\mathrm{i},\mathrm{j}+1}^{\mathrm{t}}}{{\mathrm{R}}_{\mathrm{i}+1,\mathrm{j}\leftrightarrow \mathrm{i},\mathrm{j}}}\end{array}\right)+{\mathrm{T}}_{\mathrm{i},\mathrm{j}}^{\mathrm{t}}$$

(1)

$$∆\mathrm{t}\le \mathrm{m}\mathrm{i}\mathrm{n}\left[\frac{{∆\mathrm{x}}_{\mathrm{i},\mathrm{j}}^2}{2·\frac{{\mathsf{\lambda }}_{\mathrm{i},\mathrm{j}}}{{\mathsf{\rho }}_{\mathrm{i},\mathrm{j}}·{\mathrm{C}}_{\mathrm{w},\mathrm{i},\mathrm{j}}}}\right]$$

(2)

where R(_i−1,j)—heat resistance between points i − 1 and j; ϱ_i,j—density of points i,j; C_wi,j—specific heat of the material at points i,j; T_ti,j−t—temperature of points i,j at time t; Δt—time step; ${∆\mathrm{x}}_{\mathrm{i},\mathrm{j}}^2$—square of the thickness of the elements i,j; λ_i,j—thermal conductivity coefficient i,j.

On the other hand, representing the proper thermal functioning of a PCM during its phase change is performed by modifying Formula (1) and changing the scalar specific heat with a spline. This function describes the rate of PCM heating over time and over a discrete grid; the function is used separately when the PCM is solid orliquid or during the phase change. In theory [63], the relationship in question can be described using Formula (3).

$${\mathrm{C}}_{\mathrm{w}\mathrm{i},\mathrm{j}.\mathrm{P}\mathrm{C}\mathrm{M}}=\left\{\begin{array}{cc}{\mathrm{m}}_{\mathrm{s}}·{\mathrm{C}}_{\mathrm{W}.\mathrm{S}}·({\mathrm{T}}_{\mathrm{T}}-{\mathrm{T}}_0)& \mathrm{i}\mathrm{f}{\mathrm{T}}_{\mathrm{P}\mathrm{C}\mathrm{M}}>{\mathrm{T}}_{\mathrm{T}}\\ {\mathrm{m}}_{\mathrm{T}}·{∆\mathrm{H}}_{\mathrm{T}}& \mathrm{i}\mathrm{f}{\mathrm{T}}_{\mathrm{P}\mathrm{C}\mathrm{M}}={\mathrm{T}}_{\mathrm{T}}\\ {\mathrm{m}}_{\mathrm{l}}·{\mathrm{C}}_{\mathrm{W}.\mathrm{l}}·({\mathrm{T}}_{\mathrm{l}}-{\mathrm{T}}_{\mathrm{T}})& \mathrm{i}\mathrm{f}{\mathrm{T}}_{\mathrm{P}\mathrm{C}\mathrm{M}}<{\mathrm{T}}_{\mathrm{T}}\end{array}\right.$$

(3)

where T_T—phase change temperature of the PCM; C_w—specific heat of the PCM; ΔH_T—PCM phase change enthalpy.

In this case, changing the PCM enthalpy value during its phase change is determined as the difference between the enthalpy of a specific point in the discrete grid at the next and current time step, as in Formula (4).

$${∆\mathrm{H}}_{\mathrm{T}}={\mathrm{H}}_{\mathrm{i},\mathrm{j}}^{\mathrm{t}+1}-{\mathrm{H}}_{\mathrm{i},\mathrm{j}}^{\mathrm{t}}$$

(4)

The enthalpy values of any points in the discrete grid, for consecutive time steps, is determined using Formula (5) [62], by using the temperatures at these points to determine whether or not the phase change temperature has been reached.

$${∆\mathrm{H}}_{\mathrm{T}}^{\mathrm{t}+1}=\left\{\begin{array}{cc}\underset{\mathrm{T}=0}{\overset{{\mathrm{T}}^{\mathrm{t}+1}}{\int }}{\mathrm{h}\left(\mathrm{T}\right)}_1\mathrm{d}\mathrm{T}-\underset{\mathrm{T}=0}{\overset{{\mathrm{T}}^{\mathrm{t}}}{\int }}{\mathrm{h}\left(\mathrm{T}\right)}_1\mathrm{d}\mathrm{T}& \left|\begin{array}{c}{\mathrm{h}\left(\mathrm{T}\right)}_1\to \mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{l}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}\\ \mathrm{w}\mathrm{h}\mathrm{e}\mathrm{n}{\mathrm{T}}^{\mathrm{t}+1}>{\mathrm{T}}^{\mathrm{t}}\end{array}\right.\\ \underset{\mathrm{T}=0}{\overset{{\mathrm{T}}^{\mathrm{t}+1}}{\int }}{\mathrm{h}\left(\mathrm{T}\right)}_2\mathrm{d}\mathrm{T}-\underset{\mathrm{T}=0}{\overset{{\mathrm{T}}^{\mathrm{t}}}{\int }}{\mathrm{h}\left(\mathrm{T}\right)}_2\mathrm{d}\mathrm{T}& \left|\begin{array}{c}{\mathrm{h}\left(\mathrm{T}\right)}_2\to \mathrm{f}\mathrm{o}\mathrm{r}\mathrm{c}\mathrm{l}\mathrm{o}\mathrm{t}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}\\ \mathrm{w}\mathrm{h}\mathrm{e}\mathrm{n}{\mathrm{T}}^{\mathrm{t}+1}\le {\mathrm{T}}^{\mathrm{t}}\end{array}\right.\end{array}\right\}$$

(5)

The above equations are dedicated to pure phase change materials or their isotropic composites. On the other hand, the approach described by Musiał et al. [87] allows the determination of the heat flux flowing through an anisotropic composite using Formula (6).

$${\mathrm{Q}}_{\mathrm{A}.\mathrm{E}}=\left\{\begin{array}{c}\begin{array}{ccc}\mathrm{i}\mathrm{f}{\mathrm{T}}_{\mathrm{p}}<{\mathrm{T}}_{\mathrm{l}}& \to & {\mathrm{A}}_{\mathrm{O}}\underset{\mathrm{t}=1}{\overset{\mathrm{t}=\mathrm{O}}{\int }}{\mathrm{q}}_{\mathrm{r}}\mathrm{d}\mathrm{t}\end{array}\\ \begin{array}{c}\begin{array}{ccc}\mathrm{i}\mathrm{f}{\mathrm{T}}_{\mathrm{p}}={\mathrm{T}}_{\mathrm{l}}& \to & {\mathrm{m}}_{\mathrm{L}}\underset{\mathrm{t}=\mathrm{O}}{\overset{\mathrm{t}=\mathrm{n}}{\int }}∆{\mathrm{H}}_{\mathrm{E}.\mathrm{r}}\mathrm{d}\mathrm{t}\end{array}\\ \begin{array}{ccc}\mathrm{i}\mathrm{f}{\mathrm{T}}_{\mathrm{p}}>{\mathrm{T}}_{\mathrm{l}}& \to & {\mathrm{A}}_{\mathrm{O}}\underset{\mathrm{t}=1}{\overset{\mathrm{t}=\mathrm{O}}{\int }}{\mathrm{q}}_{\mathrm{r}}\mathrm{d}\mathrm{t}\end{array}\end{array}\end{array}\right.$$

(6)

where T_l—phase change temperature; T_p—sample temperature; A_O—external surface of the PCM sample; q_r—heat flux density flowing through the sample from the PCM; m_L—mass of the PCM being melted; ΔH_E.r—PCM melting/solidification heat.

When modelling more complex problems concerning PCM systems, where it is required to consider the heat transfer between a thermal energy battery and the environment inside a building or another building component by convection and radiation, then the finite elements method equations are used. Such calculations are often performed using MATLAB [63], Adina [59] Energy Plus [86], or Ansys fluent [88] software. A different approach is presented for using the fuzzy inference method and artificial neural networks [73,74,75,76,77]. In this case, it is enough to have a pool of empirical results from experiments to obtain a result at an acceptable error level.

Selecting the method with which to model the functioning of thermal energy battering with phase change materials is dependent on the available empirical data and knowledge of the experimental boundary conditions. An additional factor determining whether numerical method simplification can be used is the need to meet additional solution stability conditions.

5. Examples of New Phase Change Composites

Attempts to solve the technical and scientific issues with the use, application, and functioning of phase change materials, as described in the previous section, have resulted in the design of new phase change composites. A large portion of the scientific articles published on the subject concern composite phase change materials with improved heat distribution properties.

In this context, a logical and scientifically valid method of improving the thermal conductivity of solid phase change materials is to ensure they work together with metals that conduct heat well, as described by Xu et al. [89]. The metals applied directly with PCMs may be used in various forms, as the steel fibres presented by Cui et al. [90] or as the distributed nanoparticles discussed by Liu et al. [91]. One needs to be aware that improving PCM thermal conductivity using highly processed materials with a large carbon footprint, i.e., metal alloys, may not be the most effective solution. Another method of applying metals to intensify heat distribution in PCMs is to use them in a foamed form [92,93] and to investigate the relationship between a foamed copper content of 0.43 to 2.15% and the PCM melting time. The results showed that the most beneficial copper content in the composite was 0.86%, which allowed the conductive heat exchange to be intensified, while not restricting the heat exchange by the free convection of the liquid PCM.

Improving heat permeation by using spatial structure conductors is described by Zhao et al. [94]. The article presents a method of producing metal foams as a framework for PCM composites obtained using Kelvin cells.

An unusual and interesting method of intensifying PCM heat exchange is the use of hypergravity. Zhang et al. [95] demonstrated that by using a high centrifugal force to change gravity from 1 g to 9 g the total melting time of the test PCM could be reduced by 60.24%. An additional factor affecting PCM heat transfer performance was its method of application and its arrangement. Liu et al. [96] investigated changes in the melting time of a mixture of aliphatic RT27 alkanes as affected by their position relative to the vertical and the heat-conducting rods inserted in them. An increase in the intensity of PCM heat transfer can be obtained by increasing the speed of its convection. Ho et al. [97] proved that by increasing the flow rate of liquid PCM to 18.4·10⁻³ m/s, its melting time could be reduced 2.83 times, down to 6 min.

Another option to improve heat distribution within a PCM, as described in the scientific articles, is forced convection. Often, as in studies [94,98,99,100,101,102], it is performed by applying the PCM in a twin pipe system for the convective transfer of heat. In this context, the use of nano-liquids as a medium that intensifies heat and liquid flow in a unit of time is viable. Some studies [103] used a nano-liquid (1% Al₂O₃ solution) as the heat transfer medium in a twin pipe system containing a PCM. The effect of the above changes was the achievement of a heat transfer that increased by 32% compared to pure water.

Another category of phase change material modifications intended to create new composites is the inclusion of conductor materials. It is a compromise between increasing the conductivity of the composite and maintaining its thermal capacity. A good example is the new, metallic, wood-derivative phase change material presented by Lin et al. [102]. Coated microcapsules with a PCM, covered with a copper layer, were used to improve the thermal capacity of porous wooden elements. The results confirmed an improvement in the composite’s thermal conductivity of 362%, compared to pure wood, while maintaining an average melting/solidification enthalpy of 92 J/g.

Ryms et al. [103] discuss the use of a free PCM with an addition of post-pyrolytic carbon obtained from tyre recycling. The resulting composite was used as a concrete additive. The composite contained the PCM RT21; free concrete and the recycled material enabled a PCM content of up to 32% to be maintained within the structure of the concrete elements. Other experimental results on a composite of PCM and expanded glass aggregate are described by Yousefi et al. [104].

These tests resulted in a composite of PCM and glass fibres with a saturation capacity of up to 80% and a thermal conductivity of concrete that was reduced by 47%.

Another example of the improvement of PCM thermal conductivity is the PCM and biocarbon composite, as discussed elsewhere [105]. The biocarbon was obtained through a pyrolysis of invasive aquatic plants. The testing of the newly obtained composite showed a 13.82-fold increase in thermal conductivity compared to a pure PCM. An important technical issue is soaking, with the application of the phase change materials to fibrous, porous structures. Zhang et al. [106] show an interesting way to solve this problem. The soaking of a pipe matrix made of carbon fibres with a liquid PCM was performed using centrifugal force while rotating the matrix. The test results of the organic PCM and carbon fibre composite produced in this manner showed that a yield point of 19 MPa and a phase change enthalpy of 77.83 J/g were achieved. An example of the versatility of phase change materials is provided by Vennapusa et al. [107], with a PCM and nanographene composite used as a component for clothing materials. The study showed an increased thermal accumulation capacity and improved user thermal comfort. Example images of PCM and conductor material thermal energy mini-batteries and their components are shown in Figure 8.

Another group of phase change composites consists of those where the direct application of a pure PCM is possible thanks to the use of a cement, silica, or polymer matrix. While the matrices improve heat distribution within the composite, they also provide the composite with a framework and a suitable strength and keep the PCM where it was applied even when it melts. Examples of PCM composites with a cement matrix that are used in construction and have a sufficient load-bearing capacity are described [108,109]. The papers present the strength test results for composites of an organic PCM with a slag aggregate and silicon carbide matrix, which confirm their higher compressive and bending strength compared to a PCM composite containing a natural aggregate. Zhao et al. [94] present a wood and plastic phase change composite. The tests showed that the initial strength properties of the material were maintained after the PCM was applied, while its thermal conductivity was increased by 26.7%, which improved the heat exchange efficiency between the environment and the PCM.

The temperature stabilisation effect of PCMs is also used when designing food packaging materials. One study [110] examined several composites of porous materials and PCMs, investigating how they affected the heat and cold storage capabilities of the test packaging.

The tests have proven the formation of stable composites of organic PCM (caprylic acid) with porous materials, such as molecular sieves, clay soils, or perlite.

6. Methods of Improving Heat Distribution and the Limitations in the Use of Phase Change Thermal Energy Batteries

Improvements in heat distribution within a phase change material and between the material and the element of a building or installation are principally achieved in one of two ways. One is to increase the thermal conductivity of the PCM itself by modifying it with high heat conductivity materials, such as copper or aluminium. The other method involves intensifying the heat transfer through free convection or forced convection, and it depends on the character of the PCM used and on the location where the material is applied. Between the two main solutions to limited PCM heat distribution, active systems (which require additional power supply) are used to improve heat transfer through convection and apply mainly to modifying PCMs in transparent partitions, building installation systems, and larger thermal energy storage systems [111,112,113,114,115]. On the other hand, the intensification of PCM heat conduction is usually used in passive solutions (which do not require an additional power supply) and dedicated modifications for opaque building partitions, as well as for small, compact thermal energy batteries. [52,59,116,117,118,119,120]. An example of an active system for a mobile indoor blind with a PCM is shown in Figure 9.

Phase change materials in the form of coated microcapsules are frequently used in many scientific studies [38,41], and they are an element of several finished products in the form of additives for cement matrix-based mortars or drywall systems. PCM microcapsules are characterised by a satisfactory thermal stability, but unfortunately, they are also characterised by an insufficient effectiveness in transferring the heat they absorb. Zhao et al. [121] analysed microcapsule compositions in the context of a polymer matrix content to phase change core content ratio. Their results showed that a 2:1 weight ratio of the matrix (styrene-divinylbenzene) to the PCM core (n-octadecane) enabled themaintenance of the necessary thermal stability while increasing the microcapsule mass loss temperature from 115.5 °C to 160.5 °C when compared to conventional matrices.

Kant et al. [122] present an overview of the fundamental research on the possibilities to improve heat distribution. The article notes the necessity to achieve long-term functioning for thermal energy storage systems in the context of their widespread use in construction.

Chao et al. [123] present a system for the passive absorption of solar radiation and the accumulation of this energy by phase change polyethylene glycol (PEG) packages placed in a concrete composite. The analysis results demonstrated an improvement in the test building’s thermal energy accumulation system by 23.4% with a payback period estimated at 4.2 years.

Another group of studies showing improvements in PCM heat conduction are those that used allotropic forms of carbon, such as graphene, fullerenes, or recycled waste materials from metalworking or refining processes [116].

The use of phase change materials to improve the effectiveness of heat distribution in ventilation installation systems or in convective heat exchangers is the subject of numerous scientific papers [73,74,75,116]. One of the more frequently referenced technical issues in scientific studies on PCM applications in these systems concerns the type, geometry, and shape of the conductor exchanger (usually made of copper of an aluminium alloy). Wang et al. [124] demonstrated that among the flow-based heat exchangers, the most effective in terms of distributing the heat stored in the PCM are conductor exchangers with a concentric design and radially arranged heat conducting elements.

Wang et al. [57] presented options for adapting thermally dynamic building envelopes containing phase change materials in sealed packages. Empirical and simulation calculation results were analysed, showing frequent limitations in the PCM applications in construction, such as their thermal capacity not being fully utilised, limited heat distribution ability within the PCM, or limited ability to control the PCM.

An interesting solution is to use the cascade heat storage system (CLHS), as described by Shen et al. [125], in ventilation systems with phase change materials. This partially solves the problem of the thermal overload of conventional thermal energy storage systems containing a single PCM type. The test system allowed the risk of overloading the thermal energy batteries to be reduced by 73.21%.

7. Development of Heat and Cold Storage Systems in Different Types of Climates

The intended purposes of using phase change materials, such as by adapting the heat or cold demand curves to fit their reservoirs in thermal energy storage systems, are factors that decide the effectiveness of the thermal energy storage systems. Depending on the outdoor and indoor climate conditions or the need to acquire and store energy from renewable sources, as in moderate climate conditions, or the need to prevent room overheating in a subtropical climate, PCMs can be placed either on the indoor or the outdoor side of the partition. The above relations are discussed by Lu et al. [126].

Due to reasons related to technology, the PCM ability to isothermally store heat is usually used for short-term thermal energy storage systems, which limits the viability of their use. The following papers provide examples of solutions to the issues of overcooled or overheated rooms in different climate types. A method for preventing window overheating is to place vertical grids of window packages between the glass panes, as described by Lai et al. [127]. The empirical results obtained in China and the USA showed a reduction in the cooling load of air-conditioned buildings by 37.8% and 24.8%, respectively.

Samandi et al. [128] propose the use of horizontal mobile roller blinds, which simultaneously provide better natural lighting in rooms and reduce costs. In addition to the ability to store heat, the tested roller blinds can reflect part of the solar radiation to the interior thanks to the PCM they contain.

Improved building window shading performance in warm climates has been achieved with the use of controlled shading. The system described in [129,130] is based on changing the blind position depending on the time of day, season, latitude, and light demand. Chi et al. [131] show an example from India, demonstrating that the use of such a system leads to a reduction in room cooling costs by 7.57%. A similar fixed outdoor blind solution was tested in cold Chinese climate conditions [132]. The test results showed that the most effective angle for outdoor window blinds was 90°. The result of the tests was an energy saving of 21.77% compared to the reference rooms.

The proper use of transparent partition shading systems with PCMs [31,32,133] can reduce the costs of room heating or cooling. There are also modifications and modernisation options that improve the thermal performance of transparent partitions with PCMs that use electrochromic installations [134], solutions supported with air flow systems [135], and greenery systems that stabilise the temperature [136].Vinga et al. [137] considered a modification of the vertical greenery surfaces with a mobile window shutter system. The results showed that windows combined with greenery systems enabled an 11.5% reduction in the need for room cooling during the summer.

The essence of window and roller blind modernisations in buildings is the viability of the modernisation across the entire building life cycle. Li et al. [108] provide an empirical and numerical analysis of the viability of window modernisation with phase change materials under Brazilian climate conditions. The analysis was performed using the Energy Plus software, and the results demonstrated a significant reduction in the air conditioning costs of the adjoining rooms when using external wooden shading systems.

Due to their purpose of providing transparency and light permeability, transparent partitions are characterised by low thermal accumulation capacity. For this reason, their temperature is sensitive to momentary changes in atmospheric conditions, external climate, and room microclimate. Due to the solar radiation gains, they also exhibit high instantaneous efficiency, suggesting broad opportunities for successful modifications with phase change materials. Furthermore, for the above reasons, PCM modifications of transparent partitions may bring more palpable effects than the modification of opaque partitions.

Consequently, another study [138] analysed nine different methods of applying PCMs in transparent partitions. This was conducted in the inter-pane spaces using aerogels and transparent thermal insulation. When discussing the advantages and drawbacks of each solution, the authors noted the benefits of increasing the thermal inertia of windows and showed the limited efficiency of these solutions during the winter season. On the other hand, Qui et al. [139] present the technology and application limitations as well as the detrimental visual aspects of applying different forms of PCMs to window structures.

When modifying transparent partitions with PCMs, their sensitivity to changing outdoor climatic conditions restricts the viable usefulness of PCMs with specific phase change temperature ranges. In this context, PCM usefulness is decided by the phase change enthalpy value and the temperature range in which the change occurs. According to some papers [63,64,134,139,140,141,142,143], it is viable to use PCMs with melting/solidification temperatures within the 16–28 °C range. Ultimately, they depend on the properties of the building element to be modified, the climate type, and the location of the PCM application.

Ensuring the right heat transfer rate between the PCM and the adjacent building elements is one of the main issues in using them. In one study [144], the authors reduced this problem by using honeycombed aluminium panels. This allowed them to increase the thermal energy adsorption, removal, and storage potential in a PCM structure.

Another option for reducing the costs of thermal functioning of building rooms is to use PCMs in the window blind structure. According to one paper [60], the year-long use of blinds with a phase change material in the South Korean climate extended the thermal comfort period by 34% and reduced the cooling load by 44%.

An intensified heat flow in a ventilated window design with a phase change material was achieved in another study [143]. The tests were performed in the Copenhagen climate, and they concerned an analysis of the permanent thermal energy capacity of 3.19 MJ of a PCM applied as 5 mm, 10 mm, or 20 mm thick boards. The results showed that the most thermally effective were the thinnest PCM boards.

Forced air flow as a mechanism to improve PCM heat distribution is important, especially in warm climates. Under such conditions, transparent partitions with external walls can cause building interior overheating. Li et al. [145], on the other hand, analysed the calculated simulations of liquid flow in the inter-pane space of a window package with a surface area of 9 m². Feeding this system with the heat or cold obtained from renewable sources allowed the stabilisation of the window temperatures throughout the day. The deliberations concerned the renewable energy acquisition from multiple different sources, such as low-temperature geothermal springs and active solar systems. The results confirmed that a zero-heat growth in the test room was achieved. A similar solution is described by Lyu et al. [146], and the results show a reduction of 43% in the amount of heat transferred through the test window during the summer period.

Modifying conventional transparent partitions with phase change materials [63,66,147] may result in the degrading of their thermal insulation properties during the night and on cloudy days. An example described by Musiał [66] is a phase change battery applied in the inter-pane space of double windows, enabling direct and indirect solar radiation gains to be absorbed [66]; see Figure 10.

For this reason, another study [148] investigated a new composite of a PCM and silica aerogel. The results showed that the aerogel and PCM packages were effective only in a cold climate.

8. Economic Assessment of the Use of Heat Storage in the Building

One of the main criteria justifying the use of heat storage in buildings is the reduction in financial costs related to the heating and cooling of the building. In addition to the desire to reduce the costs of thermal functioning, thermal self-sufficiency is also important in autonomous buildings. The thermal self-sufficiency is also important for autonomous buildings. Faraj et al. [149] studied the economic benefits of using PCM in the walls, floor, and roof of a test room. The results showed an economic benefit of USD 225 per year; so, it was possible to determine the payback period of 6.82 years for a PCM room upgrade. This result was obtained by using PCM simultaneously in all three components of the building and applying it to active and passive systems.

Hou et al. [150] presented the test results of test rooms with PCM-applied walls. The results of the financial analysis for the most effective PCM parameters and its location show savings of 21.07%. Due to the cost of the PCM and its passive functioning, the investment may only pay off after 25 years.

The thickness of the layer of PCM applied in the building structure has a major impact on the efficiency of heat storage and the investment payback period. Elefky et al. [151] presented an economic and energy analysis of the use of different thicknesses of PCM. The results demonstrated that the most effective combination of PCM in a building component was 70% PCM and 30% secondary component, which meant savings of 15%. Similar conclusions were presented by Tafuni et al. [152], who showed the possible storage of cold and heat in the stone deposits of greenhouses and glass extensions; the result was a potential savings of 153 USD/MWh. On the other hand, Li et al. [153] presented the results of the analysis of waste heat savings through the installation of a buffer tank in the building. This allowed a 31% reduction in the thermal load and annual savings of 5%, and the investment was paid off after 15 years. An alternative to the examples described above may be the use of a hybrid, multi-source heat recovery and storage system [154,155,156,157]. According to Fu et al. [156], hybrid systems can store heat for up to 3–4 months, resulting in savings of 16.9%. On the other hand, according to Taheri et al. [155], it is not the use of phase change materials but a multi-source system of heat pumps with a pressure buffer storage that allows energy cost reductions of up to 78.96%.

An important economic aspect is the supply of electric energy to active heat storage systems. Shayan et al. [158] proved that the use of flexible photovoltaic panels on various oval-shaped structures increased the return on the investment from 34.8% to 40.5%.

The above papers show great opportunities to reduce the costs of heating and cooling in buildings when using either phase change materials or hybrid, complex systems. The least efficient method is the passive operation of pure phase change batteries, while the most efficient solution is complex hybrid systems, which are not used frequently in single-family residential buildings due to their high initial cost.

9. Discussion

This review of the current state of knowledge concerning thermal energy storage systems dedicated to autonomous buildings highlights the current advantages, drawbacks, and limitations of their applications. It must be noted that current heat and cold storage solutions and technologies exhibit similar limitations in terms of the use of their potential. The primary and not fully solved issue is the insufficient heat and cold removal system, which limits their potential.

In the context of improving the thermal performance of autonomous buildings, there is no sufficiently effective and easily available thermal energy storage and distribution system that would use PCMs without a phase change or sorption. However, the goal of the thermal autonomy of buildings can be achieved more easily by using a combination of sorption and phase change thermal energy storage systems. To this end, high-performance composite sorption systems modified with high specific surface area compounds, such as activated carbon, need to be used in heat pump systems and heat and cold recuperation and distribution systems. Additionally, sorption-based installation systems enable effective, isothermal, medium- and long-term heat and cold storage over a broad range of temperatures.

The use of phase change materials within their melting/solidification temperature range can enable the stabilisation of the daily temperature fluctuations in transparent partitions and can extend the time to consume the solar radiation energy absorbed. Additionally, composite phase change materials with high heat conductivity values successfully increase the thermal capacity of lightweight frame construction partitions.

It needs to be noted that an additional factor affecting the viability of using heat and cold storage stems is the character and type of the outdoor climate and the designed temperature inside the building rooms. In this context, PCMs applied in passive systems achieve better performance in warm climates, noticeably reducing peak temperature values and reducing the demand for cooling power.

However, in economic terms, the simple sorption systems and PCM result in savings of around 20–25% of the heating and cooling costs in buildings.

To summarise the above conclusions, the most effective solution is currently a hybrid combination of sorption and phase change thermal energy storage systems applied in those locations and installations where they exhibit the greatest energy benefits.

At the same time, it needs to be noted that further development and improvement is needed in storage systems for waste heat and the heat obtained from renewable sources; this would enable them to be widely applied in construction.

According to the authors, the solutions that should be developed are multi-source hybrid systems for supplying buildings with heat and cold. Other promising solutions are systems for storing heat and electricity using buffer pressure tanks, as well as energy and cooling storage systems for hydrogen turbines.