Abstract
Phase change material in conjunction with a passive latent heat thermal energy storage approach is a potentially effective way to solve the growing concerns about building energy usage. This research examines the thermal performance of building envelopes in Miskolc, Hungary. The factors impacting the thermal performance of phase change material integrated into the building envelope were assessed. The findings indicate that the enthalpy and melting temperature of the phase change material significantly impact the effectiveness of phase change material-based walls. It was determined the optimal location of the phase change material layer is possible. This determination is intricately related to the thermal properties of the phase change material and the prevailing environmental conditions.
1 Introduction
The rise of global energy consumption in buildings has become a pressing issue in many countries. According to the International Energy Agency (IEA), there has been a recent 40% increase in energy demand for commercial buildings and a 61% increase for residential buildings worldwide. The predominant energy usage in these buildings is attributed to Heating, Ventilation, and Air Conditioning (HVAC) systems. If this trajectory continues, energy consumption in these buildings is expected to escalate by as much as 37% by the year 2050 [1]. Ensuring a comfortable environment remains a primary consideration for commercial and residential buildings, irrespective of weather conditions, placing distinct requirements on the energy needed for heating and cooling. The energy consumption of a building contributes to 20–40% of the total energy demand [2–5]. Hence, enhancing system efficiency would yield significant advantages in managing energy consumption. Effective thermal management is crucial for ensuring system efficiency. Thermal Energy Storage (TES) systems represent a recent advancement in thermal management technology, allowing the storage of thermal energy rather than its wasteful dissipation. This technology can be implemented through various methods, including storing energy through thermochemical processes and sensible and latent heat storage [6, 7]. The utilization of Phase Change Materials (PCMs) is observed in systems employing Latent Heat Storage (LHS) [8]. PCMs can absorb and release heat throughout phase transitions characterized by constant temperature. These materials exhibit high heat absorption and release capacities within a compact unit volume. Heat is stored during the melting and charging phases and released during the solidification and discharge phases. In numerous applications, PCMs effectively regulate energy, allowing precise control of the required amount of energy. Moreover, PCMs shift peak energy demand to off-peak hours, improving building efficiency [9–11]. PCMs can transform solid and liquid states, absorbing the latent heat of fusion and its reverse process. Similarly, they can transition from liquid to gaseous states by absorbing the latent heat of vaporization and its reverse process. Additionally, PCMs can shift from solid to gaseous states by absorbing latent sublimation heat and its reverse process while maintaining a constant temperature. The solid-to-liquid phase change method is commonly employed in LHS [12]. Figure 1 visually represents the phase change phenomena from solid to liquid. The shaded area in the figure represents the cumulative heat energy stored when a substance transforms from solid to liquid. The calculation includes sensible heat in the solid state, latent heat generated by phase changes at a constant temperature, and sensible heat in the liquid state to determine the overall stored energy.
Heat retained as a substance changes phases (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
PCMs are commonly classified into organic, inorganic, and eutectic categories based on their chemical composition, as depicted in Fig. 2. Each category exhibits a spectrum of melting temperatures and thermo-physical properties, rendering some more suitable for specific applications than others [13, 14].
Categorization of PCMs (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
Integrating PCM into the building envelope can decrease peak temperatures by as much as 4 °C, ensuring consistent thermal comfort in summer daytime conditions [15]. The efficacy of PCM thermal performance is influenced by various parameters (positions, thicknesses, and melting temperatures), some of which may adversely affect its functionality. It is advisable to address these parameters to optimize PCM performance and efficiently harness its full potential.
This study investigates the impact of incorporating PCMs into the building envelope in Miskolc, Hungary. The building envelope in Miskolc comprises cement 3 cm, brick 15 cm, and plaster 2 cm. The core question is: to what extent do these three parameters play a role in minimizing heat transfer? This study is dedicated to examining the thermophysical properties of PCMs and explaining the crucial parameters that have an impact.
2 Model characteristics
A visual representation of the simulated external wall is illustrated in Fig. 3. This depiction mirrors the construction of walls commonly found in buildings in Miskolc, the location of this study. The simulated wall is composed of layers of cement 3 cm, brick 15 cm, and plaster 2 cm. As a result, the total thickness of the wall, excluding the consideration of the PCM, is 20 cm, and its height is 20 cm. Table 1 outlines the properties of the construction materials used in the simulation.
Wall schematic without PCM (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
Properties of building envelope materials [16]
Property | Cement | Brick | Plaster | |
Density (kg m−3) | ρ | 2,300 | 880 | 1,680 |
Specific heat (J kg−1∙K) | Cp | 2,000 | 880 | 1,085 |
Thermal conductivity (W m−1∙K) | k | 1.40 | 1.30 | 0.22 |
To model the PCM-based wall, the following assumptions were considered:
Adequate coverage and isolation for the upper and lower surfaces of the wall;
Uniform layers, including plaster, brick, concrete, and PCM;
Convection for the liquid phase of PCM should be included within the solid phase;
Neglect the PCM's volumetric expansion.
For the assessment of PCM performance, layers with thicknesses of 1 cm, 3 cm, 5 cm, and 0.5 cm were introduced to the envelope. The PCM was positioned in four locations to examine the impact of its placement. Further details regarding the properties of the PCMs utilized in the simulation are shown in Table 2.
Thermo-physical properties of PCM [17]
Property | Organic PCM | Inorganic PCM | ||||
RT-27 | RT-21 | SP-25E2 | SP-21EK | |||
Density (kg m−3) | ρ | solid phase (°C) | 880 at 15 | 880 at 15 | 1,600 at 15 | 1,600 at 15 |
liquid phase (°C) | 760 at 40 | 770 at 25 | 1,500 at 35 | 1,500 at 35 | ||
Specific heat (J kg−1∙K) | Cp | Solid+liquid | 2,000 | 2,000 | 2,000 | 2,000 |
Thermal conductivity (W m−1∙K) | k | Solid+liquid | 0.2 | 0.2 | 0.5 | 0.5 |
Latent heat (J kg−1) | LH | – | 189,000 | 165,000 | 180,000 | 170,000 |
Solidus temperature (°C) | TS | – | 24.5 | 19 | 24 | 20 |
Liquidus temperature (°C) | TL | – | 26.5 | 24 | 26 | 22 |
3 Mathematical model
The phase transition occurrence is a boundary value issue in which the changing border is represented as a time function. Only scenarios with one-dimensional, infinite, or semi-infinite geometries can have analytical solutions for melting PCM. These scenarios assume constant temperature boundary conditions coupled with the consistent thermal properties of the PCM [18]. The enthalpy-porosity method is employed to approximate the solid-liquid interface. As it can be seen in Fig. 4, this approach does not specifically follow the melting contact. Within the PCM area, each cell is assigned a value known as the liquid fraction, representing the cell volume percentage in a liquid state. Following each iteration the liquid fraction is calculated through an enthalpy balance. This method visualizes the phase transition interface as a mushy zone characterized by a varying liquid fraction from 0 to 1. The mushy zone exhibits a declining porosity from 1 to 0, like a pseudo-porous zone. Both porosity and velocity are zero in the region where material solidification occurs [19].
Enthalpy-porosity technique (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
The convective components in momentum equations are discretized using a second-order upwind interpolation approach. In energy equations, discretization of the convective components is achieved through first-order upwind interpolation. The coupling of pressure and velocity is implemented using the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) method, and pressure interpolation is carried out using PRESTO in ANSYS Fluent code. The energy, continuity, and momentum equations converge when residuals fall below 10−10, 10−8, and 10−5.
This study maintained a constant room temperature of Tin = 22 °C (thermal comfort temperature) [20]. For all wall analyses, the convective heat transfer coefficients for the outdoor and indoor surfaces were set at hout = 25 W m−2∙K and hin = 10 W m−2∙K, respectively [21]. The research assumed that the wall's top and bottom surfaces included in the analysis were isolated q = 0 W m−2. The distribution of outdoor ambient temperature over time in the city of Miskolc, Hungary, during three days in mid-July is depicted in Fig. 5. The mean outdoor temperature for 2020 to 2023 was calculated for this period [22].
Mean temperature variations over time in July for Miskolc, Hungary (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
Selecting an optimal time step (Δt) influences the solution runtime. For transient problems, the appropriateness of the time step can be determined by the problem's convergence rate over a series of iterations. In this context, the simulation utilized a Δt value of 10 s, and the solution consistently converged within 10 iterations for the specified time interval.
4 Validation
The numerical approach described in this study was validated experimentally by A. Pasupathy et al. [24]. They constructed an experimental test room measuring 1.22 × 1.22 × 2.44 m3 to assess the impact of PCM panels on the building's roof. Their experimentation employed inorganic salt hydrate (48% CaCl2 + 4.3% NaCl + 0.4% KCl + 47.3% H2O) as the PCM. All walls, except the ceiling, were insulated with 6 mm thick plywood to isolate the particular impact of the PCM panel on the roof. Throughout the experiment, the room temperature is preserved at 27 °C, with convective heat transfer on both the inside hin = 1 W m−2∙K and outside surfaces hout = 5 W m−2∙K. The temperature fluctuations, both experimentally and numerically, on the underneath surface of the ceiling are depicted in Fig. 7. Additionally, the ambient temperature is included in the figure to enhance comprehension.
Evaluating numerical results compared to prior experimental outcomes under consistent ambient temperature conditions (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
As it is illustrated in the figure, the outcomes derived from the numerical way align well with the experimental findings documented by A. Pasupathy et al. [24]. The good agreement between the experimental data and the numerical method makes it easier to investigate the thermal properties of PCM-based envelopes.
5 Results
5.1 Effect of integrating PCM in the building envelope
The study examines the impact of incorporating PCM (RT-27) with a 1 cm thickness. Figure 8 illustrates the indoor surface temperature for the wall with and without PCM. As evident, the wall incorporated with PCM has a lower temperature fluctuation than the one without PCM. This is explained by the fact that the PCM in the wall acts as a heat sink, lowering the amount of energy that is transported from the outside to the interior surface. Consequently, the increase in the temperature of the inner surface is less pronounced.
Indoor surface temperature distribution with and without PCM (RT-27) (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
Heat transmission to the room is decreased when the temperature differential between the interior surface and the room is less. Without PCM, the total heat transmission to the room is 2.25 W, while with PCM incorporated, it is 1.74 W. Calculations show that incorporating PCM into the wall reduces energy transfer into the room by 23%.
5.2 Effect of relocating the PCM in the building envelope
The computations from the previous section were particularly applied to the wall shown in Fig. 9b, where PCM was layered between brick and cement. The PCM was installed in close proximity to the wall's external surface. This section looks at the effects of relocating the PCM in four locations on decreasing or increasing the total heat transmission through the wall, as it is shown in Fig. 9.
Schematic of building envelope with PCM (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
These locations are outlined below:
- a)The outside of the building envelope;
- b)Proximity to the outer surface of the building envelope, incorporated between the cement and bricklayers;
- c)Close to the inner surface of the building envelope, incorporated between the brick and plaster layers;
- d)The inner surface of the building envelope.
To achieve this goal, PCM type RT-27 with a thickness of 1 cm was considered at the specified locations. Figure 10 presents the reduction in heat transfer concerning the incorporation position. It is apparent from Table 3 that as the PCM is placed closer to the exterior; there is a more significant reduction in heat transfer. As a result, the investigations were specifically focused on this position.
Heat transmission into the room when the PCM is relocated (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
Total heat flux to the room when the PCM is relocated
Location | Heat flux to the room (W) | Reduction (%) | |
Building envelope without PCM | Building envelope with PCM | ||
a) | 2.5 | 1.90 | 15 |
b) | 2.5 | 1.74 | 23 |
c) | 2.5 | 1.86 | 17 |
d) | 2.5 | 1.86 | 17 |
5.3 Effect of PCM thickness on the building envelope
The optimal location b) was chosen from the previous section. The thickness of PCM was changed systematically, initially from 1 to 3 cm, later, to 5 cm, as it is shown in Fig. 11.
Schematic of building envelope with multi-thickness PCM (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
The amount of energy stored within the PCM shows a significant increase corresponding to the increase in thickness. As a result, it should take less energy to transfer to the room. Figure 12 displays the temperature distribution over the wall's indoor surface for three different thicknesses. The fluctuations of temperature changes across the indoor surface diminish with increasing PCM thickness.
Indoor surface temperature distribution with different thicknesses of PCM (RT-27) (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
However, as heat flux depends on temperature, the heat transferred into the room reduces as thickness increases. Table 4 presents the calculated total heat transmission to the room for three different thicknesses.
Total amount of heat flux to the room when changing the thickness of the PCM
Thickness [cm] | Heat transfer to the room (W) | Reduction (%) | |
Building envelope without PCM | Building envelope with PCM | ||
1 | 2.5 | 1.74 | 23 |
3 | 2.5 | 1.47 | 34 |
5 | 2.5 | 1.30 | 42 |
5.4 Effect of PCM thermo-physical properties
The impact of using various PCM types is examined in this section. Distinct PCMs have varying thermal characteristics. As a result, thermal property changes cause changes in the quantity of melted PCM and, in turn, the stored energy. Figure 13 illustrates the temperature distribution over the wall's indoor surface for different types of PCM.
Indoor surface temperature distribution with different types of PCM (Source: Author's)
Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01153
Table 5 shows the total quantity of heat transmission to the room for each type. SP-21EK has the least amount of heat transmission of any kind among the materials that are being studied. Consequently, this substance has the biggest impact on lowering the quantity of heat flow into the room.
Total amount of heat flux to the room for different types of PCM
PCM Type | Heat flux to the room (W) | Reduction (%) | |
Building envelope without PCM | Building envelope with PCM | ||
RT-27 | 2.5 | 1.74 | 23 |
RT-21 | 2.5 | 1.82 | 19 |
SP-25E2 | 2.5 | 1.67 | 26 |
SP-21EK | 2.5 | 2.09 | 7 |
6 Conclusion
This investigation employed numerical models to assess the engineering implications of integrating phase change materials into building envelopes in Miskolc, Hungary's urban setting. The examined wall configuration comprised layers of cement 3 cm, brick 15 cm, and plaster 2 cm. The effect of PCM positioning within the wall on heat transfer was systematically analyzed at four designated locations. The first two positions were closer to the outdoor surface, while the last two were closer to the indoor surface, allowing for a comprehensive evaluation of the heat transfer dynamics. The following conclusions were drawn from the numerical analysis of the wall's thermal performance: Incorporating PCMs reduces heat transfer into the room. The magnitude of this reduction is intricately tied to factors such as ambient temperature and the specific characteristics of the PCM employed in the building envelope configuration:
Temperature variations are usually reduced when PCMs are incorporated;
The critical criterion for selecting an appropriate PCM is its thermal conductivity. A lower conductivity coefficient is indicative of reduced heat flux to the room;
Achieving optimal performance of PCM in mitigating heat transfer entails positioning it closer to the outdoor surface;
More energy is stored in the building envelope when the proportion of PCM is higher.
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