Performance analysis of solar collectors with nano-enhanced phase change materials during transitional periods between cold and warm seasons in the continental temperate climates

1. Introduction

Solar energy contributes to SDG 7 by providing abundant, renewable, and carbon-neutral power. Despite its advantages, solar energy has drawbacks such as intermittent and inefficient storage systems, which limit its widespread adoption. Thus, advancements in energy storage technologies are necessary for enhancing the reliability and usability of solar power. Thermal energy storage systems, particularly those integrating advanced phase change materials (PCMs), offer a solution for stabilizing energy supply and maximizing the utility of solar energy systems.

Basically, paraffin was selected as the phase change material (PCM) due to its favorable thermal properties and low cost, making it one of the most commonly used materials for thermal energy storage. With a phase change temperature suitable for low-temperature applications and a high latent heat storage capacity, paraffin provides good thermal efficiency for systems like solar collectors. However, one of its main limitations is its low thermal conductivity, which can hinder the rapid transfer of heat. To address this, aluminum oxide (Al₂O₃) nanoparticles were introduced, known for their superior thermal conductivity at the nanoscale. The addition of Al₂O₃ into the PCM composition aims to accelerate heat transfer and enhance the overall efficiency of the system. Moreover, using a low concentration of 0.5 % by mass strikes a balance between improving thermal performance and controlling associated costs.

While previous studies primarily focused on integrating nePCMs into flat plate solar collectors, PVT systems, and desalination units, the current research introduces a unique approach by incorporating nePCMs into transpired solar collectors (TSCs). Unlike flat or evacuated tube collectors, TSCs require innovative thermal storage solutions to address heat loss and airflow dynamics. This study advances existing knowledge by: innovative system design, real-world application and comprehensive validation.

Although the use of nanoparticles to enhance the thermal conductivity of phase change materials (PCMs) is well-established, this study focuses on the practical integration of nano-enhanced PCMs (nePCMs) within a transpired solar collector (TSC) system, tested under real-world conditions during transitional seasonal periods. The novelty of this research lies in its experimental approach, specifically the use of 1000 nePCM-filled plastic spheres within the TSC for improved thermal energy storage and management. By applying this solution in a realistic setup, the study aims to evaluate its effectiveness in addressing the intermittency of solar energy, providing valuable insights for future applications in the building sector.

This study focuses on improving energy efficiency and addressing the challenges of intermittent solar energy. It explores the integration of nano-enhanced phase change materials (nePCMs) into transpired solar collectors (TSCs) as a way to enhance thermal storage. Using 1000 spheres filled with aluminum oxide-enhanced paraffin, the research evaluates the system’s performance during transitional seasons in temperate climates. The results aim to advance energy storage methods and provide practical solutions for increasing efficiency in the building sector.

This research connects directly to SDG 7 by exploring innovative methods to improve the efficiency and thermal storage capacity of solar energy systems. By integrating nano-enhanced phase change materials (nePCMs) into transpired solar collectors (TSCs), the study aims to develop a practical and efficient solution for addressing solar energy fluctuation. This aligns with the broader objectives of promoting sustainable and clean energy access while contributing to SDG 13—Climate Action—by reducing reliance on fossil fuels and lowering carbon emissions. Thus, the study not only advances scientific understanding but also supports global efforts toward a sustainable and energy-secure future.

2. Experimental setup

The primary objective of this study is to evaluate the thermal performance and energy storage capabilities of a transpired solar collector (TSC) integrated with nano-enhanced phase change materials (nePCMs). The research focuses on examining the impact of aluminum oxide-enhanced paraffin-based PCMs on heat storage and release efficiency while developing a layered configuration of nePCM-filled spheres to optimize thermal energy management within the collector. Additionally, the study aims to validate the system’s performance under real-world conditions during transitional seasons, using detailed experimental measurements and mathematical modeling. By comparing the thermal performance of the nePCM-integrated TSC with traditional systems, this research seeks to assess its scalability and potential applications in energy-efficient building systems.

The solar energy is absorbed by the perforated plate (6) which preheats the fresh air from outside before it is introduced into the living space. During the day in the cold season, the outside air releases excess heat to the accumulation mass, consisting of paraffin and nanomaterials (nePCM) stored in plastic spheres (7). The spheres are positioned in a metal mesh structure (4) supported at the bottom and top (3, 8). The metal mesh was chosen primarily to prevent displacement of the spheres within the frame, while also allowing efficient heat transfer due to its high thermal conductivity.

The hot air is brought into the interior living space by means of a fan located at the outlet of the collector. The operating flow rate of the fan was determined according to the interior space of the test container according to the national indoor environmental regulations in force and has the value of 50 m³/h.

At night, the dynamic insulation represented by an aerogel blanket move on a system of movable belts (5) on rollers (2), driven by a mechanism controlled by the mechanical system (9) outwards, to limit heat loss to the environment, protecting nePCM materials. Therefore, the stored heat will be released into the fresh air taken in from the outside. The insulation system comes as a complementary part of the insulation given by the system made of sandwich panels.

In the case of the warm season, the process is the opposite, the heat being taken from the inside and being accumulated in the non-PCM. Fresh, preheated air is introduced into the interior during the night when the outside temperature is below the comfort value.

The technical sketches of the collector were conceived in SolidWorks and rendered in SolidWorks Visualize (Fig. 3).

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Fig. 3. Solar collector a. mounted on the container’s exterior frame, b. general view without the perforrated plate; c. solar collector-lateral view; d. solar collector-back view; e. accumulation mass-before assembling.

With the use of nanoparticles, an increase of 11 % in thermal conductivity can be achieved.

In order to experimentally characterize the operation of the solar collector, the main parameters monitored were the plate temperature (at 3 different heights), the exterior temperature, the temperature at the inlet and outlet from the solar collector, the solar radiation intensity, wind speed and air flow. For temperature measurements, NiCr-Ni thermocouples, class 2, featuring PVC insulation, with an operational temperature range from −10 to +105 °C, and an accuracy of ±0.5 °C were utilized. The thermocouples were connected to an ALMEMO® connector and interfaced with an Ahlborn 710 data logger for data acquisition. The Star Pyranometer, model FLA628S, is used to measure global solar radiation independently of ambient temperature. It has an uncertainty of ±3 % and a resolution of 0.1 W/m², with a measurement range of 0 to 1500 W/m². Instrumentation for wind velocity and direction includes a Wind Velocity Sensor (Type FVA615-2) and a Wind Direction Sensor (Type FVA614). The wind velocity sensor measures horizontal wind speed within a range of 0.5 to 50 m/s, with an accuracy of ±0.5 m/s and ±3 % of the measured value, a resolution of 0.1 m/s, and operates in temperatures from −30 to +70 °C. The wind direction sensor measures across a 0° to 360° range, with an accuracy of ±5°, a resolution of 11.25°, and an operational temperature range of −30 to +70 °C.

The solar collector was installed on the exterior of a container constructed from sandwich panels, designed to simulate a living space with an interior volume of 50 m³.

The distribution on different types of PCMs was performed from bottom to top in the following way: 400 spheres with phase change temperature between 18 and 21 °C; 300 spheres 21–28 °C and 300 spheres 28–31 °C. This particular configuration is based on prior research conducted by the authors, aimed at optimizing performance under a range of climatic conditions. Experimental data were then compared with a mathematical model to validate the model and enable future simulations of the system’s performance under varying conditions.

By incorporating aluminum oxide nanoparticles into paraffin-based PCMs, this study enhances the thermal conductivity, enabling a more effective energy storage and release. The use of nePCM-filled plastic spheres in a layered configuration further optimizes energy management, ensuring that heat is captured during peak solar periods and released during cooler periods. From a practical perspective, the outcomes of this research are directly applicable to energy-efficient building design. The study provides a solution for reducing heating demands in temperate climates, particularly during transitional seasons when energy demands fluctuate. By increasing the efficiency of TSCs, this research contributes to the broader goal of decarbonizing the building sector, which is one of the largest contributors to global greenhouse gas emissions. The findings also have implications for energy policy and urban planning, supporting the transition to low-carbon cities.

Regarding the novelty of the study, this research moves beyond theoretical models and laboratory setups by testing a TSC integrated with nePCM-filled plastic spheres under actual environmental conditions. The layered arrangement of spheres, optimized based on phase change temperatures, enables the system to effectively capture and release heat during daily and seasonal temperature variations. This practical approach creates a connection between experimental findings and their applicability to real-world energy systems.

3. Methodology

The methodology of this study involves the experimental evaluation of a transpired solar collector (TSC) integrated with nano-enhanced phase change materials (nePCMs) under real-world conditions. Aluminum oxide-enhanced paraffin-based PCM spheres were arranged in a layered configuration within the collector to optimize heat storage and release. The experimental setup included thermocouples and pyranometers to measure temperature profiles, solar irradiance, ensuring precise data acquisition. Mathematical modeling was employed to validate the system’s performance, with metrics such as mean bias error (MBE) and root mean square error (RMSE) used to assess model accuracy. Comparative analysis was conducted between the nePCM-integrated TSC and traditional systems to evaluate improvements in thermal efficiency, energy storage capacity, and scalability for practical applications. This comprehensive approach provides a robust framework for analyzing the effectiveness of nePCM in enhancing solar energy systems.

The mathematical model developed for this study aims to predict the thermal performance of the solar collector and the nano-enhanced phase change material (nePCM) storage system. By accounting for the key heat transfer mechanisms, the model provides insights into the energy dynamics between the metal absorber, airflow, and the thermal storage mass, allowing for a comprehensive understanding of the system’s efficiency under real-world conditions.

The mathematical model employed in this study considers the overall energy balance between the elements of the assembly.

The thermal balance equation for the metal absorber is given by the energy balance:(1)

The air absorbs the heat collected by the metal plate exposed to the sun, through forced convection heat transfer. The equation of convection heat exchange between the metal absorber and the air is:(7)where: [W/m² K] is the coefficient of forced convection and [K] is the mean logarithmic temperature difference.

The equation of convection heat exchange between the air and the back plate is:(8)where: [W/m²K] is the coefficient of forced convection between the air and the back plate and [K] is the mean logarithmic temperature difference.

The temperature of the back plate was also determined using the following energy balance equation:(9)

The terms that account for the heat losses to the exterior depend on the surroundings and exterior environment of the back plate.

The convection heat transfer coefficient was determined as:(10)where: Nu [–] is the Nusselt number that depends on the type of convection heat transfer and the geometry of the heat transfer surface, λ [W/mK] is the coefficient of heat conduction of the air film, and l [m] is the characteristic length that depends on the heat transfer surface.

The characteristic length: l = d_perf [m].

The heat losses are determined by the wind and by natural convection.

The characteristic length: l = H_a [m] — the height of the absorber.

The coefficient of convection accounts for the mixt effect of heat losses determined by the wind and by the natural flow of the air:(14)where N = 3.

The heat losses of the back plate to the exterior were determined similarly as for the metal absorber, considering the exterior environment and the surroundings.

After modeling the solar collector’s heat transfer processes, the next section focuses on the LHTES, where thermal energy is stored in nePCM-filled spheres. The mathematical modeling of the LHTES is based on the enthalpy method, which simulates the phase change process and tracks the energy storage and release within the nePCM.

While the solar collector transfers heat to the airflow through convection, the latent heat thermal energy storage (LHTES) plays an important role in storing this energy for later use because, basically the nePCM spheres in the LHTES act as thermal batteries, absorbing excess heat during the day and releasing it during cooler periods. The following mathematical model focuses on simulating the heat transfer within the LHTES system, with special consideration for the phase change process of the nePCM.

3.1. Mathematical modeling of the PCM sphere

The following heat transfer mechanisms were considered and modelled:

• –

  forced convection between the air and PCM.

• –

  thermal conduction within the solid phase.

• –

  natural convection within the liquid phase, assimilated with thermal conduction by employing the correlation of effective thermal conductivity proposed by Liao et al. [27].

The enthalpy method was employed to predict the thermal behavior of the LHTES. Thus, the governing equation of the mathematical model reads:(16)where ρ_nePCM [kg/m³], H_nePCM [J/kg] and λ_nePCM [W/m/K] are the nePCM’s density, enthalpy, and thermal conductivity, respectively, and r [m] is the radius.

The boundary conditions are:

At the external boundary r = r_int:(17)

At the centre r = 0:(18)where K [W/m²K] is the coefficient of global heat transfer, and T_air [°C] is the air temperature.

The enthalpy profile within the spherical PCM was determined by solving Eq. (18) using the explicit finite difference method.

The evolution of the enthalpy during phase change is given in Fig. 7, along with the characteristic temperatures. For a melting process, the inferior temperature T_inf [°C] represents the start temperature of the phase change process, and the superior temperature T_sup [°C] is the stop temperature. In the case of solidification, phase change starts at T_sup and ends at T_inf. The inferior and superior limits of the phase change enthalpies are designated as H_inf [J/kg] and H_sup [J/kg], respectively.

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Fig. 7. Characteristic temperatures and enthalpies.

The corresponding temperature of each enthalpy value was determined, based on the phase of the nePCM, as follows:(19)where: L_PCM [J/kg] is the latent heat and γ [–] is the liquid mass fraction of the melted PCM:(20)

The spherical PCM was divided based on a mesh consisting of 7 nodes, and the time step was set at 5 s.

The temperature at the inlet of the LHTES was determined using the mathematical model for the TSC, taking into account the heat transfer that occurs between the exterior air and the TSC. Considering that the air temperature variation within the cross-sectional area was neglected, after heat exchange with a generic row of spheres sp,i, the temperature was determined as:(21)where: K [W/m²K] is the global heat transfer coefficient, determined as:(22)where: α [W/m²K] is the coefficient that accounts for forced convection between the air and the PCM, determined with the Nusselt correlation: Nu = 0.33Re^0.6, valid for Reynolds numbers between 20 and 150,000 [–] [28], and λ_e [W/mK] is the encapsulation’s thermal conductivity coefficient.

4. Results and discussions

4.1. Experimental investigation

To evaluate the performance of the solar collector, it was installed on the exterior wall of a container designed to simulate a living space. The collector’s position was determined based on prior research conducted at the Technical University of Civil Engineering Bucharest, which identified the optimal orientation to maximize thermal radiation. This specific position ensures maximum solar radiation during transitional periods, particularly in the temperate Romanian climate. In this context, the solar collector demonstrates the potential to significantly reduce the heating system’s load, especially during the transitional periods between warm and cold seasons. To monitor its performance, data was collected from April 12th to May 9th, 2024. Key parameters, including solar radiation, outdoor temperature, and plate temperature, were analyzed, as illustrated in Fig. 9, Fig. 10, Fig. 11, to provide a comprehensive overview of the system’s operation. For a more detailed analysis and to validate the mathematical model, May 8th was selected as a representative day within the monitoring period. This approach allows for a deeper understanding of the collector’s performance and its contribution to energy efficiency during the transitional climatic conditions.

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Fig. 9. Intensity of solar radiation in the intervals 12.04–22.04 and 22.04–09.05.

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Fig. 10. Outdoor temperature vs. plate average temperature in the range 12.04–22.04 and 22.04–09.05.

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Fig. 11. Outdoor and outlet temperature from the collector in the intervals 12.04–22.04 and 22.04–09.05.

The experimental evaluation of the experimental stand was carried out by monitoring at 1-min intervals the following parameters: outdoor air temperature, metal plate temperature, temperature at the outlet of the collector, solar radiation intensity, wind speed and relative humidity. The monitoring involved the use of Almemo NiCr-Ni sensors mounted on the perforated plate (at the bottom, middle and top), at the outlet of the collector (2 pieces) as well as in a shaded area of the external environment, connected to a data acquisition unit. A star pyranometer is used to measure the total radiation in the collector plane, i.e. the sum of the direct radiation from the sun and the diffuse radiation from the sources that participate in the heat transfer by radiation from the environment outside the collector.

By monitoring the values recorded for the intensity of solar radiation during the period 12.04–09.05.2024 (grouped in 2 successive graphs in Fig. 9) it is possible to easily identify the days when the sky was clear, and the Sun acted with high intensity on the perforated plate of the collector (e.g. 13–15.04) as well as the days when the sky was cloudy, possibly accompanied by rain (29.04 for example). Strictly for the monitored period of time, the maximum utility of the collector is obtained during periods when the outside temperature and the intensity of solar radiation are high during the day, thus ensuring the release of a high amount of heat to the storage area, and the temperature during the night is low, thus allowing the heating of the fresh air taken in from the outside.

By monitoring the outdoor ambient temperature and the plate temperature—calculated as the average of three sensor readings positioned on the plate—during the period from April 12th to May 9th, 2024 (presented in two consecutive graphs in Fig. 10), a clear trend was identified. The results indicate that the plate temperature consistently exceeds the ambient temperature during periods of peak solar radiation intensity, with temperature differences reaching up to 20 °C. Even on overcast days, the plate temperature remains marginally higher than the ambient temperature, likely due to the contribution of diffuse solar radiation. During nighttime, the temperature difference between the plate and the ambient environment diminishes, with occasional instances where the plate temperature falls below that of the ambient air. This phenomenon is attributed to radiative cooling, wherein the exposed surface of the plate loses heat to the sky through radiation. These findings highlight the thermal dynamics of the collector and its capacity to capture solar energy under varying atmospheric conditions, offering insights into its efficiency in transitional climatic periods.

One of the most important analyses to perform is the comparison between the ambient temperature and the air temperature at the outlet of the solar collector. This analysis is particularly important during nighttime, as observed in the two graphs presented in Fig. 11.

In most cases, the solar collector successfully achieves its objective by preheating the fresh air drawn from the outside. During the monitoring period, the outlet air temperature remained 2–3 °C higher than the ambient temperature, even during the night. Fig. 11 highlights this difference, showcasing that the nePCM’s ability to store and release heat contributed significantly to this thermal gain. The temperature of the nePCM spheres varied between 18 °C and 31 °C, depending on their layer and proximity to the heat source. This layering ensures that the thermal energy is distributed effectively to maintain a comfortable indoor air temperature. If, after heat transfer to the storage medium, the temperature of the incoming fresh air exceeds the comfort thresholds for indoor spaces (as per current regulations, a minimum temperature of 20 °C during the cold season and 24 °C during the warm season is recommended), a constructive modification of the experimental setup may be implemented to bypass the air after heat release to the nePCM storage unit.

Thermographic images complement the temperature data monitored by the sensors and are shown in Fig. 12. A noticeable color difference can be observed in the areas where the sensors are mounted. Thermographic readings in these areas should be avoided, as the values would not be accurate due to differences in emissivity between materials. Thermographic inspection should only be conducted on materials with similar or closely matching emissivity values. Therefore, in Fig. 12, only the metallic region is relevant for analysis, as it can reach temperatures of 50 °C or higher on sunny days during the transition between winter and summer.

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Fig. 12. Thermographic inspection of perforated plate.

As the experimental research was conducted during temperatures approaching those of the warm season, the solar collector proved significant in preheating the fresh air drawn from outside during the night, helping to maintain a comfortable indoor temperature within the range of 20–24 °C. Thermographic inspection revealed that approximately two-thirds of the collector’s surface exhibited higher temperatures, which also varied depending on the angle of incidence of solar radiation on the surface. For a comprehensive assessment of all energy efficiency parameters, May 8th was selected from the experimental dataset for detailed analysis. In the next stage, weather data corresponding to this day will be incorporated into the mathematical model to validate the experimental results, following the evaluation and interpretation of the collected data.

The analysis of Fig. 13 shows that during the day, the temperature at the outlet of the solar collector falls between the outside air temperature and the plate temperature. This is expected, as the plate is directly heated by solar radiation and consequently has the highest temperature in the system. A key advantage of the energy storage system is that it allows the stored energy to raise the outdoor air temperature by 2–3 °C, with a slight decrease of 1–2 °C in the early morning. Initially, in the morning, the temperatures remain close to 17 °C until around 8:00 h, when the solar radiation starts to increase. The plate temperature rises rapidly, peaking above 37 °C around midday, while the outlet air reaches about 32 °C. Between 12:00 h and 17:00 h, the plate and outlet air temperatures fluctuate but remain significantly higher than the ambient, indicating effective heat transfer. The plate remains consistently warmer than the outlet air, demonstrating the system’s efficiency in absorbing and storing heat. Notably, even after the solar radiation decreases around 17:00 h, the system continues to release heat for several hours, as seen by the outlet air and plate temperatures, which only begin to approach the ambient temperature after 20:00 h. This extended heat release highlights the system’s thermal storage capacity, maintaining heat transfer even in the absence of direct sunlight.

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Fig. 13. Outdoor temperature vs. plate temperature vs. collector outlet temperature (for the day under review, 08.05).

Additionally, it is noteworthy that during the summer, the stored heat can be repurposed for drying fruits and vegetables, while in the cold season, it can be used to preheat the air, thereby reducing the thermal load on the heating system.

The importance of air velocity monitoring, presented in Fig. 14, is justified by the intensification of heat transfer by forced convection, between the perforated plate heated by the Sun and the outside air. At high air speeds, some of the heat accumulated by the plate is dissipated to the outside environment. This is not the case for the day under review, as the air velocity values are low, mostly below 3 m/s.

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Fig. 14. Wind speed on the monitored day.

For the evaluation of the heat released by the sun, the accumulated heat, the efficiency of the solar radiation conversion and the efficiency of the collector, the following correlations were used:

• 1.

  The heat flux that reaches the plate (depending on the intensity of solar radiation and the surface of the collector of 2 m²)

(23)

• 2.

  The recovered heat flow (depending on the air flow set at the fan 50 m³/h, the average air density 1.15 kg/m³, the specific heat of the air at constant pressure 1005 J/(kg·K). The formula for calculating the heat flux comes from the calculation of the sensible heat released, coming from the temperature difference, related to time

(24)

• 3.

  The efficiency of heat recovery, a quantity that makes sense during the day, when the intensity of solar radiation is different from zero, is calculated as the ratio between the heat flux recovered and the heat flux that reaches the plate from the Sun. The calculation is made only for the time interval in which the Sun is in the sky, since during the night the value of the term from the denominator will be zero, and the division would not make sense mathematically

(25)

• 4.

  The heat exchange effectiveness, similar, calculated for the day because at night the temperature of the plate reaches equilibrium with the temperature of the external environment and is related to the way in which the heat transfer to the storage mass is made

(26)

The graph presented in Fig. 15 illustrates the efficiency of the solar collector utilizing a storage mass made from PCM enhanced with nanomaterials (nePCM). The efficiency shows a dynamic variation throughout the day, with several notable peaks. From 8:00 to 9:00, the system exhibits a sharp rise in efficiency, quickly reaching around 20 %. This increase corresponds to the onset of significant solar irradiance, indicating the collector’s rapid response to sunlight as the nePCM material begins storing and transferring heat effectively. The early peak values, some reaching as high as 25 %, could be attributed to the initial absorption of solar radiation and the strong thermal storage capacity of the nanomaterial-enhanced PCM.

Between 9:00 and 12:00, efficiency fluctuates significantly, with multiple spikes that exceed 30 %, likely due to changes in solar intensity and potential shading effects. These fluctuations suggest that the collector is highly sensitive to variations in sunlight exposure yet maintains its ability to transfer heat efficiently. The nePCM may also contribute to these peaks by allowing faster heat absorption and release, especially when irradiance is uneven, resulting in short-term efficiency boosts. From 12:00 to 14:00, the graph shows continued variability in efficiency, with the system maintaining moderate values between 10 % and 20 %. These levels are lower than the earlier peak but indicate consistent heat transfer despite possible midday reductions in sunlight due to clouds or changing solar angles. The presence of nePCM helps sustain a reasonable level of efficiency during this period by regulating the release of stored thermal energy. After 14:00, efficiency decreases steadily but remains fluctuating between 5 % and 15 %, which could be related to the diminished solar input in the afternoon. However, the system continues to demonstrate its capability to release stored heat from the PCM, as reflected in occasional spikes nearing 20 %. By the end of the day, around 16:00, efficiency approaches near-zero values, aligning with the reduction of solar radiation. The estimated average efficiency throughout the day is approximately 15 %, with peak values exceeding 30 % during periods of high solar intensity.

The graph presented in Fig. 16 shows the heat exchange effectiveness of the solar collector between 8:00 and 17:00. Early in the morning, effectiveness rises sharply due to the plate heating up quickly under direct sunlight. Throughout the day, heat transfer effectiveness fluctuates, influenced by solar irradiance and transient clouds. Despite this, the system maintains an average effectiveness around 80 %. After midday, peaks above 140 % occur, likely due to heat concentration on the top plate area, given by the Sun positioning in the sky. Even as solar radiation decreases after 14:00, the system continues releasing heat, keeping effectiveness around 50 %–70 %, supported by heat in the storage mass. The average heat transfer effectiveness value, for the monitored interval is 66.8 % being in line with previous research [24,29].

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Fig. 16. Collector heat transfer effectiveness.

5. Conclusions

The performance analysis of the solar collector system incorporating nano-enhanced phase change materials (nePCMs) as a storage medium shows promising results for use during transitional periods between cold and warm seasons in a continental temperate climate. The system demonstrated effective thermal storage capabilities, with daytime plate temperatures reaching up to 20 °C higher than ambient temperatures, and an increase in outlet air temperature of 2–3 °C during the night. These findings suggest a significant potential to reduce heating loads, particularly during periods when intermittent heating is required. Solar radiation intensity varied from 700 W/m² on clear days to 100 W/m² on overcast days. The mathematical model predicted system performance with a Mean Bias Error (MBE) of 3.13 % and a Root Mean Square Error (RMSE) of 3.86 %, confirming the accuracy of the model under real climatic conditions.

A key novelty in this study lies in the integration of 1000 plastic spheres filled with nePCM, specifically designed to optimize heat storage across different temperature ranges. The distribution of spheres, with phase change temperatures between 18 and 31 °C, allows the system to operate effectively under varying climatic conditions, making it adaptable to both cold and warm seasons. The experimental data were further validated by a mathematical model, which showed close alignment with the real-world results, confirming the system’s performance. This validation provides a solid foundation for future simulations and broader applications of nePCM in thermal energy storage systems.

By addressing the limitations of traditional PCMs and demonstrating the practical application of nePCM in a real-world context, this study offers a novel approach to improving the efficiency and reliability of solar energy systems. It can be seen that by integrating advanced thermal storage systems like nePCMs into building designs, the potential to reduce heating loads and contribute to the decarbonization goals set by the European Union is significant.

However, some limitations were observed in the study. The primary disadvantage of this system is the complexity of ensuring the homogeneity of the nePCM mixture and preventing nanoparticle settling. Additionally, the behavior of the nePCM during repeated melting and solidification cycles still requires further investigation to ensure long-term stability.

Although this study was conducted in a temperate continental climate, this system is applicable to geographic regions with climates characterized by seasonal transitions and moderate solar radiation, such as parts of Southern Europe, North America, Central Asia, and certain regions in South America and Australia. These areas experience similar temperature fluctuations and intermittent solar availability, making them suitable for the application of the system tested in this study.

Future research should focus on optimizing the nePCM composition to further enhance thermal conductivity and avoid nanoparticle settling over time. Additionally, testing the system over an extended period, including during both winter and summer, would provide more comprehensive data on its year-round performance. Expanding the study to explore the potential use of the stored heat for other applications, such as drying or cooling, during different seasons, could open new ways for increasing system efficiency and versatility.

CRediT authorship contribution statement

Răzvan Calotă: Writing – review & editing, Writing – original draft, Visualization, Supervision, Project administration, Methodology, Conceptualization. Octavian Pop: Writing – original draft, Visualization, Investigation, Conceptualization. Cristiana Croitoru: Writing – original draft, Supervision, Methodology, Conceptualization. Florin Bode: Writing – review & editing, Supervision, Methodology. Charles Berville: Writing – original draft, Visualization, Validation, Investigation, Formal analysis. Emanuil Ovadiuc: Validation, Investigation, Data curation.