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Phase Change Materials in Hot Water Generation Systems: A Review
The utilization of phase change materials (PCMs) in solar water heating systems (SWHS) has undergone notable advancements, driven by a rising demand for systems delivering superior performance and efficiency. Extensive research suggests that enhancing heat transfer (HTE) in storage systems is crucial for achieving these improvements. This review employs a bibliometric analysis to track the evolution of HTE methods within this field. While current literature underscores the necessity for further exploration into hot water generation applications, several methodologies exhibit significant promise. Particularly, strategies such as fins, encapsulation, and porous media emerge as prominent HTE techniques, alongside nanofluids, which hold the potential for augmenting solar water heating systems. This review also identifies numerous unexplored techniques awaiting investigation, aiming to pave new paths in research and application within the field of hot water generation. It highlights methods that could be used independently or alongside predominantly used techniques.
Thermal energy storage (TES) using phase change materials (PCM) has been widely investigated for various applications from very low to very high temperatures due to its flexible operating temperature range, high energy storage density, and long-life cycle at a reasonable cost. The use of PCM in building components and hot water production can reduce the building energy demand, indoor temperature fluctuation, and better demand-side management by utilising available renewable energy and off-peak electricity. This paper presents a state-of-the-art review of the application of PCM domestic thermal heating. The classifications of TES systems, advantages of PCM over other TES systems, and the methods to overcome shortcomings of PCM are discussed in brief. Then the various novel techniques employed in underfloor heating, wall heating, PCM integration in domestic hot water tanks, and developing latent heat thermal energy storage units are extensively reviewed and the major findings of the research works reviewed are tabulated. Based on the extensive review conducted, the important factors to be considered for selecting a suitable PCM for these applications are summarised, and the commercially available PCM for the above applications are listed with their major thermo-physical properties and supplier details in the appendix.
The use of phase change materials in solar thermal collectors improves their thermal performance significantly. In this paper, a comparative study is conducted systematically between two solar receivers. The first receiver contains paraffin wax, while the other does not. The goal was to find out to which degree paraffin wax can enhance the energy storage and thermal efficiency of evacuated tubes solar collectors. Measurements of water temperature and solar radiation were recorded on a few days during August of 2021. The experimental analysis depended on two stages. The first stage had a flow rate of 7 L/hr, and the second stage had no flow rate. A flow rate of 7 L/hr gave an efficiency of 47.7% of the first receiver with phase-change material, while the second conventional receiver had an efficiency rate of 40.6%. The thermal efficiency of the first receiver during the day at which no flow rate was applied was 41.6%, while the second one had an efficiency rate of 35.2%. The study's significant results indicated that using paraffin wax in solar evacuated tube water-in-glass thermal collectors can enhance their thermal energy storage by about 8.6% and efficiency by about 7%. Moreover, the results revealed that the solar thermal collector containing paraffin wax had an annual cost of 211 USD/year. At the same time, the receiver's yearly fuel cost was 45 USD. Compared to an electrical geyser, the annual cost reached 327 USD, with an annual fuel cost equaled 269 USD. The first receiver's payback period was 5.35 years.
Phase change materials (PCM) have been widely used in Thermal Energy Storage (TES) Systems. Considering the energy efficiency and the use of domestic hot water, the melting temperature range of phase change materials is considered to be optimal in the range of 50–60 °C. The most commonly used is sodium acetate trihydrate-based phase change material, which has the advantages of high latent heat and low price, but its high supercooling, low thermal conductivity, and phase separation affect its application. Therefore, this paper used sodium acetate trihydrate, disodium hydrogen phosphate dodecahydrate (DSP), and expanded graphite (EG) as raw materials to prepare composite phase change materials (CPCM) and used physical disturbance to further improve their properties. Firstly, their thermophysical properties were investigated by the step cooling curve method, differential scanning calorimetry (DSC), and x-ray diffraction (XRD). Secondly, in order to further evaluate the effect of physical disturbance on CPCM crystallization, further experimental studies were carried out by adjusting the rotor mass and rotational speed. The experimental results showed that when 1.5 wt.% DSP, 1.5 wt.% EG and physical perturbation work together, the CPCM phase transition temperature is 56.7 °C, and the latent heat is as high as 258.98 kJ/kg. At this time, its thermal conductivity increased from 0.62 w/m·k to 1.1625 w/m·k, and its subcooling degree decreased from above 20 °C to less than 0.5 °C, and no phase separation occurred. The greater the disturbance momentum (the greater the rotor mass or the greater the rotational speed), the shorter the induction time, which is more conducive to the crystallization of CPCM. The results obtained in this paper are instructive for the preparation of efficient new CPCMs.
The present study proposes the phase change material (PCM) as a thermal energy storage unit to ensure the stability and flexibility of solar-energy-based heating and cooling systems. A mathematical model is developed to evaluate the PCM melting process, considering the effect of nanoparticles on heat transfer. We evaluate the role of nanoparticles (Al2O3-, copper- and graphene-based nanofluids) in enhancing the performance of the melting process of phase change materials. The results show that natural convection due to the buoyancy effect dominates the flow behaviour even in the initial stage of the PCM melting process. High natural convection at the bottom of the annular tube moves the melted PCM upward from the lateral, which pushes the liquid–solid interface downward. The addition of 3% vol Al2O3 nanoparticles boosts PCM melting performance by decreasing the melting time of PCM by approximately 15%. The comparison of Al2O3, copper and graphene nanoparticles demonstrates that higher thermal conductivity, ranging from 36 to 5000 W m−1 K−1, does not contribute to a significant improvement in the melting performance of PCMs.
Due to its large latent heat and high energy storage capacity, paraffin as one of the phase change materials (PCMs) has been widely applied in many energy-related applications in recent years. The current applications of paraffin, however, are limited by the low thermal conductivity and the leakage problem. To address these issues, we designed and fabricated form-stable composite PCMs by impregnating organic paraffin within graphite-coated copper foams. The graphite-coated copper foam was prepared by sintering multilayer copper meshes, and graphite nanoparticles were deposited on the surface of the porous copper foam. Graphite nanoparticles could directly absorb and convert solar energy into thermal energy, and the converted thermal energy was stored in the paraffin PCMs through phase change heat transfer. The graphite-coated copper foam not only effectively enhanced the thermal conductivity of paraffin PCMs, but also its porous structure and superhydrophobic surface prevented the paraffin leakage during the charging process. The experimental results showed that the composite PCMs had a thermal conductivity of 2.97 W/(m·K), and no leakage occurred during the charging and discharging process. Finally, we demonstrated the composite PCMs can be readily integrated with solar thermoelectric systems to serve as the energy sources for generating electricity by using abundant clean solar-thermal energy.
This paper presents the development of a computational model of latent thermal energy storage (LTES) in a shell and tube configuration with longitudinal fins. The model describes the physical process of transient heat transfer between the heat transfer fluid (HTF) and the phase change material (PCM) in LTES. For modeling the phase change of the PCM, the enthalpy formulation was used. Based on a one-dimensional computational model, a new Trnsys type was developed and written in Fortran. Validation of the LTES model was performed by comparing numerically and experimentally obtained data for the melting and solidification of paraffin RT 25 as the PCM and water as the HTF. Numerical investigations of the effect of HTF inlet temperature and HTF flow rate on heat transfer in LTES confirmed that significant improvement in heat transfer between the HTF and PCM could be achieved by increasing the HTF inlet temperature during charging or decreasing the HTF inlet temperature during discharging. Increasing the HTF flow rate did not significantly improve the heat transfer between the HTF and PCM, both during charging and discharging. The presented, experimentally validated LTES model could be used to analyze the feasibility of integrating LTES into various thermal systems and ultimately help define the specific benefits of implementing LTES systems.
A numerical study was carried out to investigate charging and discharging processes of different phase change materials (PCMs) used for thermal storage in an innovative solar collector, targeting domestic hot water (DHW) requirements. The aim was to study PCMs that meet all application requirements, considering their thermal performance in terms of stored and retrieved energy, outlet temperatures, and water flow rate. Work was carried out for three flat-plate solar panels of different sizes. For each panel, a PCM tank with a heat exchanger was attached on the back plate. Simulations were conducted on a 2D domain using the enthalpy–porosity technique. Three paraffin-based PCMs were studied, two (A53, P53) with phase-change temperatures of approximately 53 °C and one of approximately 58 °C (A58H). Results showed that, during charging, A58H can store the most energy and A53 the least (12.30 kWh and 10.54 kWh, respectively, for the biggest unit). However, the biggest unit, A58H, takes the most time to be fully charged, i.e., 6.43 h for the fastest feed rate, while the A53 unit charges the fastest, at 4.25 h. The behavior of P53 lies in between A53 and A58H, considering stored energy and charging time. During discharging, all PCMs could provide an adequate DHW amount, even in the worst case, that is, a small unit with a high hot water consumption rate. The A58H unit provides hot water above 40 °C for 10 min, P53 for 11 min, and A53 for 12 min. The DHW production duration increased if a bigger unit was used or if the consumption rate was lower.
In recent years, phase change materials (PCMs) have been presented as a suitable alternative for thermal energy storage (TES) systems for solar water heater (SWH) applications. However, PCMs' low thermal conductivity and the high dependence on external conditions are the main challenges during the design of TES systems with PCMs. Design actions to improve the performance of the TES systems are crucial to achieve the necessary stored/released thermal energy and guarantee the all-day operation of SWHs under specific system requirements. In this study, a TES with PCM in the configuration of a heat exchanger was redesigned, focused on achieving two main targets: an outlet water temperature over 43 °C during discharging time (15 h) and efficiency over 60% to supply the hot water demand of two families (400 L). A four-step redesign methodology was proposed and implemented through numerical simulations to address this aim. It was concluded that the type, encapsulation shape, and amount of PCM slightly impacted the system's performance; however, selecting a suitable sensible heat storage material had the highest impact on meeting the system's targets. The redesigned TES reached 15 operating hours with a minimum outlet water temperature of 45.30 °C and efficiency of 76.08%.
Thermophysical properties such as latent heat, viscosity and melting temperature could be changed for different physical properties of dispersed nanoparticle such as size, shape, and concentration. In this study, Nanocomposites-Enhanced Phase Change Materials NePCM are formed by dispersing Aluminium (Al) and Copper (Cu) nanoparticles into paraffin wax in various mass fractions (0.1, 0.3, 0.6, 1, 2.5 and 5%). The impact on the thermophysical properties of paraffin wax by the nanoparticles is also investigated. Heat conduction and differential scanning calorimeter experiments are used to investigate the effects of different nanoparticle concentrations on the melting point, solidification point, and latent capacity of nanocomposites. Experimental results show that the dispersion of nanoparticles of Al and Cu can decrease the melting temperature and increase the solidification temperature of PCM. this dispersion could also be limited due to increase in dynamic viscosity of the NePCM. Furthermore, Al and Cu nanocomposites with mass fractions of 2% and 1%, respectively, show better enhancements in the thermal storage characteristics of the paraffin compared to the next higher mass fraction. Ó 2021 The Authors. Published by Elsevier B.V. on behalf of Faculty of Engineering, Alexandria University This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: R. Elarem et al., Experimental investigations on thermophysical properties of nano-enhanced phase change materials for thermal energy storage applications, Alexandria Eng. J. (2021), https://doi.
Thermal energy in the solar thermal energy storage system cannot be stored for a long time during the evening hours as well as days that have minimal sunlight due to heat transfer to the surrounding atmosphere. PCMs are being implemented into these storage systems to enhance the thermal storage capacity of solar energy. PCMs based latent heat storage technique can extend thermal energy storage over long periods due to its ability to store energy. Several different types of PCMs with various chemical compounds and melting temperatures can suit various thermal storage applications in real life. However, the concept of combining these PCMs types with different ratios is an area within this field that has not yet been investigated as it is time-consuming and a matter of numerous combinations and trials. A model solar thermal energy storage system with the flexibility to test different PCM's blends can omit the problem. This research work intended to propose a design as testing apparatus that can successfully operate and investigate the different PCM's blends to find the optimum ratios for a highly efficient solar thermal water heating system. An appropriate laboratory-sized prototype design has described that can replicate the functions of a solar thermal water heating system. The necessary assumptions and mathematical relations are also presented for the proposed design.
Usage of phase change materials’ (PCMs) latent heat has been investigated as a promising method for thermal energy storage applications. However, one of the most common disadvantages of using latent heat thermal energy storage (LHTES) is the low thermal conductivity of PCMs. This issue affects the rate of energy storage (charging/discharging) in PCMs. Many researchers have proposed different methods to cope with this problem in thermal energy storage. In this paper, a tubular heat pipe as a super heat conductor to increase the charging/discharging rate was investigated. The temperature of PCM, liquid fraction observations, and charging and discharging rates are reported. Heat pipe effectiveness was defined and used to quantify the relative performance of heat pipe-assisted PCM storage systems. Both experimental and numerical investigations were performed to determine the efficiency of the system in thermal storage enhancement. The proposed system in the charging/discharging process significantly improved the energy transfer between a water bath and the PCM in the working temperature range of 50 °C to 70 °C.
Latent heat thermal energy storage systems (LHTES) are useful for solar energy storage and many other applications, but there is an issue with phase change materials (PCMs) having low thermal conductivity. This can be enhanced with fins, metal foam, heat pipes, multiple PCMs, and nanoparticles (NPs). This paper reviews nano-enhanced PCM (NePCM) alone and with additional enhancements. Low, middle, and high temperature PCM are classified, and the achievements and limitations of works are assessed. The review is categorized based upon enhancements: solely NPs, NPs and fins, NPs and heat pipes, NPs with highly conductive porous materials, NPs and multiple PCMs, and nano-encapsulated PCMs. Both experimental and numerical methods are considered, focusing on how well NPs enhanced the system. Generally, NPs have been proven to enhance PCM, with some types more effective than others. Middle and high temperatures are lacking compared to low temperature, as well as combined enhancement studies. Al2O3, copper, and carbon are some of the most studied NP materials, and paraffin PCM is the most common by far. Some studies found NPs to be insignificant in comparison to other enhancements, but many others found them to be beneficial. This article also suggests future work for NePCM and LHTES systems.
Phase Change Materials (PCM) can absorb energy while heating as it undergoes a change in phase and emits the absorbed energy to the environment in a reverse cooling process. Organic PCMs have been extensively used for thermal energy storage in building applications due to its phase transition temperature within the human comfort temperature zone. It has a tendency to leak which limits its application. Encapsulation of PCM in a core-shell pattern have efficiently resolved the issue. This paper is focused on Bio-based PCMs and the enhancement of thermal conductivity, encapsulation techniques of Organic PCMs. It also highlights its versatile application in food, textile, solar energy systems, buildings and paints.
Latent heat storage is the best possible ways of storing thermal energy. This provides higher storage density with very small temperature difference between storing and releasing heat energy. This review paper provides a detailed classification of Phase change materials (PCMs) along with their varied applications. To the best of author’s knowledge till now, nearly all the applications of PCMs are listed in this short review. The gap between demand and supply of energy can be bridged by using PCMs and thereby it has become a very attractive technology. The use of a PCMs in thermal insulation, thermal comfort and energy storage have been tested in many applications.
Entire world is struggling to explore newer methods to overcome the energy crisis. With limited and exhausting energy resources researchers around the world are finding efficient ways to store thermal energy. One of the efficient ways is to store thermal energy in the form of latent heat energy using phase change materials (PCMs). Latent heat storage (LHS) units have been widely adopted owing to superior energy storage density and constant operating temperature. But the thermal performance of these systems is limited due to low thermal conductivity of the PCMs. Researchers across the globe working on approaches to improve the thermal performance of these systems. The present review paper is concerned with a review on PCMs, methods to improve performance of LHS system using PCMs with special emphasis on use of various additives such as nano particles and porous materials to increase the thermal performance of PCMs and types of PCMs for specific applications. The factual data presented in this paper would be helpful in selection of appropriate PCM for specific application, and to adopt suitable performance enhancement technique to obtain most optimal LHS system
One of the major drawbacks of solar water heating systems is unable to supply hot water during night time or off sunshine hours. The integration of phase change material with solar water heating systems is cost effective and efficient solution to overcome this major problem associated with solar water heating systems. The phase change material integrated with solar water heating system stores thermal energy during sun shine hours and this stored energy can be recovered during off shine hours or night time to produce hot water. The phase change material can be integrated with water tank of collector, evacuated tubes, external water tank for solar collector and flat plate collector by adding layers at the bottom of absorber plate. The integration of phase change material with SWH system not only overcome the drawbacks of SWH system but also enhance the efficiency of conventional SWH system. Many investigations for the application of TES materials integrated SWH system have been carried out and found a significant enhancement in the performance. This paper presents a comprehensive review of recent advances in the applications of PCM with SWH system for TES.
The aim of this paper is providing a detailed review for the applications of phase change materials (PCMs) in residential heating. The study focuses mainly on its use in domestic water heating systems. Different studies accounting for structural characterization, research methodology, and long term performance were carefully assessed. The gaps in the literature have been highlighted and recommendations for future studies have been presented. The technical gaps urge the need for research to be directed towards enhancing PCM properties in the domestic system, novel integration of PCM within the system, and optimization of system performance based on real-time accurate weather data. The economic opportunities for such systems were presented in different locations of the world by investigating different energy efficiency measures. The work presented herein identifies potential energy management opportunities that can significantly reduce our reliance on fossil fuels. Consequently, promoting a green future and mitigating greenhouse gas emissions. It is structured to provide guidance for researchers and engineers working in inclusion of PCMs in residential heating applications.
In this work, technologies related to the storage of solar energy, utilizing the latent heat content of phase change materials, for the production of domestic hot water are reviewed. Many researchers have been involved in this field, in order to accomplish the targets of environmentally friendly solutions and higher efficiency. For domestic use, materials with melting temperature between 40 and 80 °C are commonly studied, with paraffins, fatty acids, salt hydrates and alcohols being the most popular. For harvesting the solar radiation, usually flat plate or evacuated tubes solar collectors are used, either commercial ones or modified. The storage unit may include only phase change material or it can have a hybrid form combined with water. The outcome of the most studies, is that the addition of phase change materials in comparison to systems without latent storage, increases the duration of heat release towards the domestic water at the end of the day and also increases the solar collector's efficiency because it does not experience large temperature fluctuations. However, difficulties emerge during the selection of the appropriate storage material as this must have a high melting temperature in order to provide hot enough domestic water, but not higher than the temperature that the solar collector can produce, in order to fully melt the material. Moreover, investigation is necessary for the selection of the optimum water flow rate that minimizes the charging duration and maximizes the system efficiency and hot water amount. Another challenge that researchers face is the low thermal conductivity of many phase change materials. For this purpose, methods of improving the performance of system have been examined and they are also reported.
Thermal energy storage (TES) is extensively applied in production and daily life. As a basic work, we designed a single tank phase change TES domestic hot water system using night valley power. Based on the single tank TES experimental platform, a 2D numerical model of the system was established. To optimize the performance of a single tank TES system, the effect of heat transfer fluid (HTF) inlet velocity and temperature on the heat release of the storage tank was analyzed based on simulation. The enhanced heat transfer effect of the radial cascade system and axial cascade system was also compared. The results showed that 0.8m/s is the optimal HTF inlet velocity in the calculation range. The rule of the heat release about the HTF inlet temperature just meets the different hot water requirements in winter and summer. The heat release capacity of the radial cascade system and axial cascade system is higher than that of the conventional single-stage system, and the proportion of latent heat release in radial cascade is larger. This study will provide theoretical guidance for the optimal design of the single tank TES domestic hot water system in the future.
The application of thermal storage materials in solar systems involves materials that utilize its sensible heat energy, thermo-chemical reactions or phase change materials, such as hydrated salts, fatty acids paraffin and non-paraffin like glycerol. This article reviews the various exergy approaches that were applied for several solar systems including hybrid solar water heating, solar still, solar space heating, solar dryers/heaters and solar cooking systems. In fact, exergy balance was applied for the different components of the studied system with a particular atten-tion given to the determination of the exergy efficiency and the calculation of the exergy during charging and discharging periods. The influence of the system configuration and heat transfer fluid was also emphasized. This review shows that not always the second law of thermodynam-ics was applied appropriately during modeling, such as how to consider heat charging and dis-charging periods of the tested phase change material. Accordingly, the possibility of providing with inappropriate or not complete results, was pointed.
In this study, the thermal performance of latent heat thermal energy storage system (LHTESS) prototype to be used in a range of thermal systems (e.g., solar water heating systems, space heating/domestic hot water applications) is designed, fabricated, and experimentally investigated. The thermal store comprised a novel horizontally oriented multitube heat exchanger in a rectangular tank (forming the shell) filled with 37.8 kg of phase change material (PCM) RT62HC with water as the working fluid. The assessment of thermal performance during charging (melting) and discharging (solidification) was conducted under controlled several operational conditions comprising the heat transfer fluid (HTF) volume flow rates and inlet temperatures. The experimental investigations reported are focused on evaluating the transient PCM average temperature distribution at different heights within the storage unit, charging/discharging time, instantaneous transient charging/discharging power, and the total cumulative thermal energy stored/released. From the experimental results, it is noticed that both melting/solidification time significantly decreased with increase HTF volume flow rate and that changing the HTF inlet temperature shows large impacts on charging time compared to changing the HTF volume flow rate. During the discharging process, the maximum power output was initially 4.48 kW for HTF volume flow rate of 1.7 L/min, decreasing to 1.0 kW after 52.3 min with 2.67 kWh of heat delivered. Based on application heat demand characteristics, required power levels and heat demand can be fulfilled by employing several stores in parallel or series.
It is an essential and most abundant energy resource on earth. Solar energy can be caught and utilized in a few different ways and as a sustainable energy source is a significant part of our virtuous energy future. Its technology can be passive solar or active solar depending on how it can store excess energy and converting into solar power the storage of solar energy require a storage medium or accumulator. It can help to store excess solar energy for future use. One of the best methods to store heat energy from the sun is by making use of phase change material (PCMs) due to a huge ton of captured latent heat and isothermal kind of PCM during heat expansion and dismissal during phase change. PCM becomes an ideal candidate for a thermal energy storage medium in building applications. Now a day PCM gained special attention in building applications like water and air heating application, building-integrated system (ceilings, walls, and floor), H.V.A.C/solar absorption system for building, and other useful applications PCM concrete, PCM shutters, solar cooker, and solar dryer. In this paper, an attempt has been made to provide an overview of the progress and application of PCM in solar thermal energy.
Thermal energy storage (TES) is an essential part of a solar thermal/hot water system. It was shown that TES significantly enhances the efficiency and cost effectiveness of solar thermal systems by fulfilling the gap/mismatch between the solar radiation supply during the day and peak demand/load when sun is not available. In the present paper, a three-dimensional numerical model of a water-based thermal storage tank to provide domestic hot water demand is conducted. Phase change material (PCM) was used in the tank as a thermal storage medium and was connected to a photovoltaic thermal collector. The present paper shows the effectiveness of utilizing PCMs in a commercial 30-gallon domestic hot water tank used in buildings. The storage efficiency and the outlet water temperature were predicted to evaluate the storage system performance for different charging flow rates and different numbers of families demands. The results revealed that increases in the hot water supply coming from the solar collector caused increases in the outlet water temperature during the discharge period for one family demand. In such a case, it was observed that the storage efficiency was relatively low. Due to low demand (only one family), the PCMs were not completely crystallized at the end of the discharge period. The results showed that the increases in the family’s demand improve the thermal storage efficiency due to the increases in the portion of the energy that is recovered during the nighttime.
Solar energy is the most promising heat source for meeting energy demand without having negative impact on the environment. Solar energy is, however, intermittent in nature and time dependent energy source. In order to mitigate the intermittent supply of solar energy for water heating, the use of phase change material (PCM) comes into play. The PCM acts as a heat source for the solar heating system when the intensity of the solar radiation is low or no longer active. Knowledge about the thermal effectiveness of solar collector with PCM is paramount.So, this study developed a mathematical model to evaluate the thermal behavior of a flat plate solar collector integrated with a phase change material (PCM). This mathematical model developed for the flat-plate with PCM was based upon the conservation and heat transfer equations and used to predict the thermal behavior of integrated phase change material in solar collector during thermal storage. The energy balance equations for the flat-plate heating components of the collector and PCM were formulated numerically. The model was used to investigate the effect of inlet water temperature, water mass flow rate, outlet water temperature and the melt fraction during charging and discharging modes at each of the respective nodes. A comparison was made with a collector with and without PCM. The results show that charging and discharging processes of PCM have multiple stages. The addition of PCM in the first stage causes a decrease in temperature during charging and an increase during discharging. The highest water temperatures reached for the collector without and with a phase change material were approximately 51 and 74°C respectively Comparisons were also made between the simulated and experimental data for the solar water heater without and with PCM. Minimum inlet water temperatures of 41.54 and 36°C were observed for the simulated and experimental model while, 42.69 and 74°C were both recorded for outlet temperatures respectively, for solar water heater without PCM. For the solar water heater with PCM, the inlet temperatures for the simulation and experimental model were found to be 42°C and 56°C respectively. A maximum outlet water temperature of 108°C was obtained from the experimental model compared to 45°C obtained from simulation. The temperature of the hot water obtained was remarkable and sufficiently enough for many domestic and industrial applications. Finally, the solar water heating system with phase change materials finds useful application and acts as a renewable energy resource in regions where there is inconsistent or poor sunlight.
The thermal energy storage (TES) system is used to store the heat energy for longer periods and retrieve the heat energy as and when required. Experiments were conducted on the TES system with stearic acid (SA) as phase change materials (PCM) with and without iron scrap additives (IS) filled in spherical capsules. The PCM was filled in high-density polyethylene (HDPE) capsules of spherical shape. The process of charging, discharging, and the heat energy retrieved for the aforementioned PCMs were investigated and compared with various heat sources. The TES tank performance was studied with a variable/constant heat source at different flow rates, i.e. 2, 4, and 6 LPM. The results showed that the TES tank is charged to 70 °C in 204 min with 6 LPM flow rate, whereas for 2 LPM flow rate, the TES tank was charged to 70 °C in 254 min for the variable heat source. In the case of a constant heat source, to reach 70 °C, it took 54 min, 43 min, and 33 min for 2 LPM, 4 LPM, and 6 LPM flow rates, respectively. The total heat capacity of the TES tank at 70 °C was around 10,400 kJ. The output hot water at an average of 45 °C was found to be around 164 litres which means that the heat energy recovered from the TES tank was around 32%. The system with IS along PCM filled in spherical capsules was able to give 25% of hot water in extra than the same capacity of the sensible heat storage system. The results obtained reveal that heating and cooling processes were taking place at a faster rate of 13% with the addition of IS particles to the PCM when compared to pure PCM in the spherical capsules.
In this work, a thermal energy storage tank using Phase Change Materials (PCM) is experimentally investigated. It is part of a thermal storage technology based on solar collectors and efficient heat pumps for heating, cooling and Domestic Hot Water (DHW) production. It comprises a rectangular tank filled with PCM and a staggered finned heat exchanger (HE). The tank is designed for DHW production according to the EU Commission Regulation No 814/2013 requirements. Stored energy density and heat transfer rate during the melting and solidification stages are used to evaluate the adequacy of produced hot water amount and the storage efficiency of the tank. Two organic PCMs were tested, A53 and A58H, having nominal melting temperatures of 53 °C and 58 °C respectively. With the defined operating conditions, the tanks can be charged either by the sun or by a heat pump in less than 2 h, with a heat transfer rate above 5 kW for the first half of the storage capacity. During discharging, the system can produce instantly 106 lt of DHW with temperature above 40 °C. Experimental results confirmed the ability of the tank to meet the requirements of a DHW installation and to increase the efficiency of the coupled solar collector or geothermal heat pump.
Enhancing the reliability and acceptability of solar base thermal energy system requires efficient thermal storage to enable storing the surplus energy collected during the day time for later use during non-day light hours. However, it is well known that most phase change materials (PCMs) employed with thermal storages suffer from low thermal conductivity. The geometry of fins has a major impact on the heat transfer rates of thermal storage. For this purpose, comparative thermal performance assessment during charging is achieved for latent heat storage unit (LHSU) using different fins geometry having identical added volume per PCM. An experimental analysis is conducted on a three geometries of LHSU: non-finned LHSU, longitudinal finned LHSU (LF-LHSU) and circular finned LHSU (CF-LHSU). Besides, a visual observation of liquid fraction fronts is applied to confirm the completion of phase change cycle. Experimental results showed that the total charging time observed to reduce up to 69% and 55% using CF-LHSU and LF-LHSU respectively. It is also found that the highest cumulative energy stored enhancement using CF-LHSU was about 52% as compared with LF-LHSU. The experimental comparative assessment suggests that CF-LHSU gives better charged thermal load operations by factor of 2.1 as compared to LF-LHSU.
The energy storage is the capture of energy at one time to utilize the same for another time. This review article deals with thermal energy storing methods and its application in the vicinity of solar water heating systems as well as solar air heating system, solar cooker, green house building, cold storage, refrigeration and air conditioning, solar thermal power plant, defence applications, and the materials used to store that thermal energy effectively. There are large numbers of phase change materials which are used to trap the useful thermal energy to utilize in future for minutes, hours, days, months or even years. This paper provides the information of desirable and undesirable properties of different PCMs.
The future of renewable energy lies in the efficiency of energy storage technology used for storing energy produced by the renewables. The sporadic nature of solar energy has a demand for energy storage and efficient storage materials and devices. Therefore, energy storage technologies are gaining a wide range of attention from researchers. This paper mainly focuses on the development of fatty acid/metal ion composite by incorporating sodium ions into the lauric acid to enhance its thermophysical properties. Lauric acid is doped with 0.2, 0.5, and 1 wt% of the sodium metal to form a fatty acid/metal ion composite. Fabrication of the composite without any sophisticated methods or materials is the advantage of the present work. DSC, TGA, thermal conductivity, thermal diffusivity, and FTIR characterization have been conducted to understand the thermal and structural properties of the synthesized fatty acid/metal ion composite. Morphology of the composite was studied using scanning electron microscopy imaging to study the porous nature of the composite. Enthalpy of fusion of the composite was found to be ~ 153, ~ 157, and 161 J/g by adding 0.2, 0.5, and 1 wt% of sodium metal into lauric acid, due to which the enthalpy of phase change was found to be enhanced by 5.3, 7.9, and 10.6%, respectively, in comparison with the enthalpy of pure lauric acid. Besides, the composite exhibited a small reduction in melting point with the increase in wt% of sodium metal in the composite. FTIR spectra of the prepared composite showed that there is no reaction taking place between lauric acid and sodium metal, making it a more stable composite. TGA analysis revealed that the decomposition temperature was enhanced by 30% by the addition of sodium metal into lauric acid, making it shaped-stable and suitable for thermal energy storage application.
Phase change materials (PCM) are materials with the ability to store a large amount of energy (latent heat) during their change from solid to a liquid phase. This takes place at certain melting temperature. Amid the global energy crisis, multiple applications of these materials have been studied at the theoretical, numerical and experimental approach, obtaining promising results in terms of an increase in the efficiency of these systems. However, the application of these materials is being studied since there are no rules or predictions of the feasibility of its application in diverse weather conditions. The tropical climate condition is one of the least studied in this context. In this work, a review of the main findings of recent studies conducted in tropical climate conditions is presented. Additionally, an analysis of the main challenges and opportunities of the application of PCM in the climate of Panama is performed. It was concluded that some applications in passive cooling and solar water heating systems might have the potential for their implementation. However, further studies are required to take into account other applications.
Solar energy is a renewable energy source that can be utilized for different applications in today's world. The effective use of solar energy requires a storage medium that can facilitate the storage of excess energy, and then supply this stored energy when it is needed. An effective method of storing thermal energy from solar is through the use of phase change materials (PCMs). PCMs are isothermal in nature, and thus offer higher density energy storage and the ability to operate in a variable range of temperature conditions. This article provides a comprehensive review of the application of PCMs for solar energy use and storage such as for solar power generation, water heating systems, solar cookers, and solar dryers. This paper will benefit the researcher in conducting further research on solar power generation, water heating system, solar cookers, and solar dryers using PCMs for commercial development.
In present study, the efficient parameters on thermal energy storage in a double-wall tank with phase-change materials have been investigated. At first, the effect of using fins in distribution of phase-change materials has been studied. Inside the tank where the inlet-heated water is there, the inlet temperature and Reynolds number have been investigated. Also, on tank walls where the phase-change materials are placed, the effect of using of fins and the type of phase-change materials has been investigated. By mounting fins on areas with phase-change materials, the melting time reduces significantly. Therefore, after mounting fins, the melting of phase-change materials has been reduced from 60 to 80% in approximately 8 h. In water zone, by increasing inlet temperature from 340 to 360 K, the melting time of phase-change material reduces significantly, in a way that, after approximately 8 h, the amount of melted materials changed from 67 to 87%; however, the change in Reynolds number does not have any considerable influence. In final section, the effect of thermophysical properties of phase-change materials on melting process has been studied. The obtained results reveal that using materials with lower specific heat and melting temperature causes the reduction of melting time, and hence, for melting 90% of phase-change materials with wax, 14 h is needed, while, by using SavEHS34, these materials change their phase in 5.5 h.
The energy consumption of domestic hot water accounts for 10-20% of building energy consumption. Such high energy consumption heightened the need for utilizing energy-efficient technologies in domestic hot water systems. Among existing technology, integrating phase change materials (PCM) into domestic hot water tanks is very attractive due to the fact that adding PCM in tanks would enhance the storage density of the system. Most of these studies encapsulated PCM and then put it on the top of the tank. These studies highlighted the positive aspects of the integration of PCM. However, they suffer from the fact that the capacity of hot water in tanks will be reduced. To address this issue, in this study a new structure hot water tank was proposed with PCM placed on the side of the tank. In order to evaluate the thermal performance of this new tank, experiments were performed based on a cylindrical hot water tank of 150 L capacity. Results show that the integration of PCM can significantly improve the thermal performance of the hot water tank. Compared to a tank without PCM, the time length for water temperature drop from 65 to 40 ºC at the top of the tank with PCM is over 8.5% longer. Moreover, the change of the amount and the location of PCM results in different water stratification in the tank.
Solar heating system has raised much attention due to the global energy shortage and environment pollution. The dynamic heat release performance of water tank (WT) with phase change materials (PCM) plays an important role in the operation of solar heating system, while seldom available literatures focused on this research topic. In this paper, an experimental system for heat release of the WT-PCM was established. The temperature variations of the water and the PCM during heat release of the WT-PCM and the traditional water tank were measured respectively, and their dynamic heat release performance were also analyzed based on the measured data. The results are as follows: the heat release of the WT-PCM can be divided into three phases: sensible heat before phase change, latent heat during phase change and sensible heat after phase change, and the water temperature will be temporarily heated after the sudden drop; the water supply temperature decline rate of both tanks will increase dramatically at the initial stage of heat release, when the PCM temperature is reduced to 50℃, the water supply decline rate of the WT-PCM is less than that of the traditional water tank, while when PCM is completely solidified, the water supply temperature decline rate of the WT-PCM is more than that of the traditional water tank. The heat release performance of the WT-PCM is much better than that of the traditional water tank. The research results can provide useful reference for engineering application of the WT-PCM.
Increasing energy consumption in residential and public buildings requires development of new technologies for thermal energy production and storage. One of possibilities for the second listed need is the use of phase change materials (PCMs). This work is focused on solutions in this area and consists of two parts. First one is focused on different designs of thermal energy storage (TES) tanks based on the phase change materials. The second part is the analysis of tests results for TES tank containing shelf and tube heat exchanger and filled with phase change material. Thermal energy storage tank is analyzed in order to use it in domestic heating and hot utility water installations. The aim of this research was to check the applicability of phase change material for mentioned purpose. Results show that using phase change materials for thermal energy storage can increase amount of stored heat. The use of properly selected PCM and heat exchanger enables the process of thermal energy storing and releasing to become more efficient.
Thermal storage system (TES) with phase change material (PCM) is an important device to store thermal energy. It works as a thermal buffer to reconcile the supply energy with the energy demand. It has a wide application field, especially for solar thermal energy storage. The main drawback is the low value of thermal conductivity of the PCM making the system useless for thermal engineering applications. A way to resolve this problem is to combine the PCM with a highly conductive material like metal foam and/or nanoparticles. In this paper a numerical investigation on the metal foam effects in a latent heat thermal energy storage system, based on a phase change material with nanoparticles (nano-PCM), is accomplished. The modelled TES is a typical 70 L water tank filled with nano-PCM with pipes to transfer thermal energy from a fluid to the nano-PCM. The PCM is a pure paraffin wax and the nanoparticles are in aluminum oxide. The metal foam is made of aluminum with assigned values of porosity. The enthalpy-porosity theory is employed to simulate the phase change of the nano-PCM and the metal foam is modelled as a porous media. Numerical simulations are carried out using the Ansys Fluent code. The results are shown in terms of melting time, temperature at varying of time, and total amount of stored energy.
This paper presents numerical investigations on a heat storage utilizing sodium acetate trihydrate (SAT) as phase change material (PCM). The heat storage can be used both in short-term and in long-term by utilizing stable supercooling of SAT. The store contains 137.8 kg PCM and 75 L water. Based on a validated CFD model, the flow conditions of the heat storage was analyzed. Uneven flow distribution inside the heat storage was revealed. Three design optimization methods were investigated to eliminate the uneven flow distribution. The results were analyzed using key performance indicators inclusive charging time, charged heat, degree of thermal stratification and the mixing of the heat storage. The influence of flow direction, inlet size and addition of a porous plate on the thermal performance of the heat storage were elucidated. Concerning flow direction, after moving the inlet from the bottom to the top of the storage, the time needed to completely melt the PCM was shortened by 50%. The best storage design was identified: For charge of the storage, the top inlet should be used, while for discharge, the bottom inlet should be used. Concerning the size of inlet opening, the charging time of the heat storage was reduced from 75 min to 51 min by using 3.0 mm radius instead of a 11.2 mm inlet. The small inlet size (3.0–8.0 mm) was suggested to make a uniform temperature distribution inside the heat storage. Short circuit was completely eliminated by adding a porous plate with 10% porosity. The charging time of the heat storage was shortened 28% by adding the porous plate. Finally, recommendations were proposed for different applications of the heat storage. The findings of the paper serve as a good basis for designers and manufacturers of PCM heat storages.
U-type evacuated tube collector (ETC) is one of the most popular types of solar collectors for solar thermal applications. However, ETCs suffer from drawbacks caused by the intermittency of solar radiation. The addition of phase change materials (PCMs) inside ETC can reduce the energy fluctuations by storing the excess thermal energy during the daytime and releasing it to meet the supply deficit in the absence of sunlight. Therefore, a numerical model was conducted to assess the thermal performance of a PCM-filled U-type ETC with fins and experimentally validated. The design of different energy storage solutions, including the melting point of the PCM, the flow rate of heat transfer fluid (HTF) and the diameter of the inner glass tube were optimized. A two-fold beneficial effect of extending the hot water supply period and a reduction of the peak outlet temperature of HTF is achieved by filling PCM inside U-type ETC with fins. When the melting temperature of the PCM is 323 K, the reduction of the HTF peak outlet temperature is 7.4 K and the liquid fraction of the PCM approaches to a relatively high value, while the thermal efficiency and the thermal storage efficiency of the ETC are 50.72% and 19.20%, respectively. Meanwhile, the hot water supply time over 308 K extends by 160 min, compared to the case without PCM. It is also found that a high rate of the HTF inlet flow can shorten the hot water supply time with a smaller liquid fraction of the PCM. Furthermore, enlarging the glass tube leads to a higher thermal efficiency and thermal storage efficiency. This work shows that filling PCM with appropriate designs in the ETC with fins can promote the performance of ETC .
Phase change material (PCM) development for solar water heating systems is a promising utilization area as thermal energy storage and management, as abundant solar energy is available in the universe. However, the main huddle of PCM utilization is less thermal conductivity value, which produces inconvenience in the heat transfer rate. This study has fabricated beeswax-expanded graphite PCM composite via the melt-mixing method and examined it for thermal energy storage. In the developed PCM composite, the expanded graphite worked as a thermal conductivity promoter. Chemical properties were determined using an X-ray diffractometer, and it was found that no chemical reaction occurred within the materials. Scanning electron microscope is used for microstructural analysis and showed that the interconnection of the structure caused the better thermal conductivity. The photography tracking exhibited adequate physical stability beyond the melting temperature. Thermal conductivity of PCM composite enhanced 3.4 times from 0.3 to 1.02 W/mK due to in-plane high-thermal-conductivity structure of expanded graphite. Phase transition temperature and latent heat were analyzed using differential scanning calorimetry (DSC). Based on the result of DSC, the phase transformation temperature of the PCM composite has not changed significantly. Still, the latent heat of the sample has been decreased 14.4% in melting and 15.06% in solidification. According to thermogravimetric analysis, the fabricated composite is thermally stable for utilizing a temperature range below 100 °C. Numerical analysis of composite was performed for the efficiency of heat storage. Numerical and experimental results disclosed that heat storage efficiency increased with the increment of thermal conductivity. The phase alteration temperature and heat storing capacity showed potential PCM composite material for solar water heating systems.
In the present study, nanoparticles of MgO, TiO2, and Graphite with mass ratios of 3, 5, 7, and 10% were added to the structure of the synthesized Lauric acid (LA) microcapsules, and the effect of nanoparticles on the thermal properties of microencapsulated Lauric acid as non-toxic phase change material for energy storage applications was investigated. The microcapsules containing nanoparticles were prepared by emulsion polymerization of Styrene as the shell. The microencapsulation ratio (E.R) of LA increases with the amount of MgO and TiO2. However, the microencapsulation ratio was reduced by increasing the mass ratio of nano graphite. The highest microencapsulation ratio (69.90%) belonged to the microcapsules containing 10% of titanium oxide nanoparticles. Scanning electron microscopy (SEM) images showed that microcapsules obtained containing TiO2 were spherical with a smooth surface and narrow particle size distribution. The thermal stability and thermal conductivity coefficient for the pure LA, microencapsulated LA with/without nanoparticles were examined. The thermal stability improved with the increasing mass ratio of the nanoparticles, no considerably. The microcapsules with 10% of TiO2 nanoparticles had higher thermal stability. The weight loss temperatures in the first and second steps are 287 °C and 435 °C, respectively. The thermal conductivity of the lauric acid was increased by microencapsulation from 0.146 W/m.K to 0.149 W/m.K. The thermal conductivity coefficient of microcapsules increased by adding nanoparticles. Finally, the thermal energy storage performance of the obtained samples was evaluated in a designed experimental setup. The decrement percentage of the onset of the melting process time for lauric acid microcapsules and the microcapsules containing graphite nanoparticles, titanium oxide, and magnesium oxide were 1.2, 4.7, 8.5, and 16.7%, respectively.
Nowadays, thermal energy storage using Phase Change Materials (PCMs) receives a great interest due to its high energy storage density especially for low and medium temperature storage applications. Nevertheless, PCM suffers from the low thermal conductivity during the charging and discharging of heat. In this study, the multiple PCM technique has been investigated as a thermal enhancement technique in a shell-and-tube heat exchanger. The investigations have been conducted experimentally and numerically using different PCM arrangements and working conditions for Paraffin wax. The experimental work shows higher average temperature of PCM in the multiple PCM arrangement than the other two arrangements of low and high PCM arrangement. The conducted numerical analysis focuses on the geometrical parametric study by varying both the radial and the axial dimensions of the cylindrical storages. The results show that longer PCM storage enhances the melting time compared to the wider PCM storage for the same amount of PCM.
A combination of heat transfer augmentation techniques is highly necessary to enhance the performance of Thermal Energy Storage (TES) systems employed in a wide range of applications. The major issue is that many of the Phase Change Materials (PCMs) possess low thermal conductivity (0.2 W/m K), resulting in an inefficient melting process. Triplex Tube Heat Exchanger (TTHX) based TES system is both numerically and experimentally studied using Paraffin (RT82) with Alumina (Al2O3) nanoparticles that has a charging temperature in the range of 78.15–82.15 °C . The experimental findings indicate that the Paraffin is not completely melted within the required time of four hours for the inside heating method at 97 °C. The Paraffin is successfully melted for both sides heating at 90 °C in lesser time and average temperature than the outside heating. With different charging temperatures, the Paraffin melting was consumed a short time for the non steady state at the mass flow rate of 29.4 kg/min, compared with the 16.2 and 37.5 kg/min for inner and outer tubes. Other outcomes were that with the fins–nanoparticle combination, an improved performance for melting the Paraffin, compared with those that occurred without nanoparticle. Furthermore, in the numerical study, compared with the pure Paraffin case, the melting time was minimized for TTHX with longitudinal fins (12%) and TTHX with triangular fins (22%) for the PCM having 10% nanoparticle, respectively. Close agreement is found between the numerical and experimental findings.
Latent heat thermal energy storage (LHTES) uses phase change materials (PCMs) to store and release heat, and can effectively address the mismatch between energy supply and demand. However, it suffers from low thermal conductivity and the leakage problem. One of the solutions is integrating porous supports and PCMs to fabricate shape-stabilized phase change materials (ss-PCMs). The phase change heat transfer in porous ss-PCMs is of fundamental importance for determining thermal-fluidic behaviours and evaluating LHTES system performance. This paper reviews the recent experimental and numerical investigations on phase change heat transfer in porous ss-PCMs. Materials, methods, apparatuses and significant outcomes are included in the section of experimental studies and it is found that paraffin and metal foam are the most used PCM and porous support respectively in the current researches. Numerical advances are reviewed from the aspect of different simulation methods. Compared to representative elementary volume (REV)-scale simulation, the pore-scale simulation can provide extra flow and heat transfer characteristics in pores, exhibiting great potential for the simulation of mesoporous, microporous and hierarchical porous materials. Moreover, there exists a research gap between phase change heat transfer and material preparation. Finally, this review outlooks the future research topics of phase change heat transfer in porous ss-PCMs.
The low thermal conductivity of the phase change materials limits the performance of latent heat thermal energy storage systems. The thermal conductivity of the phase change materials can be enhanced by incorporating a highly conductive porous medium. The studies available in the literature are limited to the application of metal foams in different geometries of latent heat thermal energy storage systems. In the present study, experimental investigations have been carried out to study the effect of adding copper wire mesh having two different porosities (i.e., 75% and 87%) and pore density of 16 pores per inch inside a cylindrical latent heat thermal energy storage system subjected to three different isothermal surface temperatures. The effect of adding metal wire mesh and isothermal surface temperatures on the heat flux, Nusselt number, and energy stored during the melting process are analysed. The results indicate that embedding copper wire mesh into the phase change materials leads to a uniform temperature distribution inside the latent heat thermal energy storage system, which is more pronounced at lower porosity. The maximum improvement in the charging times by using copper wire mesh with porosities of 87% and 75% are to be 17% and 24%, respectively. Moreover, it is observed that by using composite phase change materials (i.e., a combination of phase change material and metal wire mesh) the rate of heat transfer and energy stored is increased due to higher effective thermal conductivity of composite phase change materials. A correlation is proposed to predict the Nusselt number for different cases of the experiment. It is anticipated that these prospective results will be valuable for optimizing the energy transport in practical thermal energy storage applications.
The future of renewable energy lies in the efficiency of energy storage technology used for storing energy produced by the renewables. The sporadic nature of solar energy has a demand for energy storage and efficient storage materials and devices. Therefore, energy storage technologies are gaining a wide range of attention from researchers. This paper mainly focuses on the development of fatty acid/metal ion composite by incorporating sodium ions into the lauric acid to enhance its thermophysical properties. Lauric acid is doped with 0.2, 0.5, and 1 wt% of the sodium metal to form a fatty acid/metal ion composite. Fabrication of the composite without any sophisticated methods or materials is the advantage of the present work. DSC, TGA, thermal conductivity, thermal diffusivity, and FTIR characterization have been conducted to understand the thermal and structural properties of the synthesized fatty acid/metal ion composite. Morphology of the composite was studied using scanning electron microscopy imaging to study the porous nature of the composite. Enthalpy of fusion of the composite was found to be ~ 153, ~ 157, and 161 J/g by adding 0.2, 0.5, and 1 wt% of sodium metal into lauric acid, due to which the enthalpy of phase change was found to be enhanced by 5.3, 7.9, and 10.6%, respectively, in comparison with the enthalpy of pure lauric acid. Besides, the composite exhibited a small reduction in melting point with the increase in wt% of sodium metal in the composite. FTIR spectra of the prepared composite showed that there is no reaction taking place between lauric acid and sodium metal, making it a more stable composite. TGA analysis revealed that the decomposition temperature was enhanced by 30% by the addition of sodium metal into lauric acid, making it shaped-stable and suitable for thermal energy storage application.
This present work contributes to the improvement in thermal energy storage capacity of an all-glass evacuated tube solar water heater by integrating it with a phase change material (PCM) and with a nanocomposite phase change material (NCPCM). Paraffin wax as PCM and a nanocomposite of paraffin wax with 1.0 mass% SiO2 nanoparticles as NCPCM had been used during the experiments. The results were acquired through the real-time experimental measurements on the all-glass evacuated tube solar water heater integrated with built-in thermal energy storage, functioning under thermosyphonic flow. Three different cases, namely, without PCM, with PCM, and with NCPCM, were considered. The testing procedure involved the observation of total temperature variation in the tank water from 6.00 a.m. to 6.00 a.m. of next morning. Meanwhile, the water was completely renewed for every 12 h. The system performance was studied using energy efficiency, exergy efficiency, and temperature of hot water supply during the next morning, for all the three cases. The investigation exemplifies that the tank water temperature at 6.00 a.m. after one complete day of operation was notably improved to 37 °C and 39.6 °C, respectively, with PCM and NCPCM, whereas it was 33.1 °C for the case without PCM. The energy efficiencies for the three cases were found to be 58.74%, 69.62%, and 74.79%, respectively, and exergy efficiencies of the system were determined as 19.6%, 22.0%, and 24.6%, respectively, for without PCM, with PCM, and with NCPCM. Also, it was evidenced that the thermal conductivity of paraffin wax was considerably increased to 22.78% through the diffusion of SiO2 nanoparticles. Put together, this indicates that the incorporation of PCM and explicitly the dispersion of SiO2 nanoparticles in NCPCM had been significantly improved the thermal performance of the system.
Numerical and experimental analyses are often used to evaluate the solar thermal system with latent heat thermal energy storage (LHTES). However, the relationship between the numerical simulation and actual heat transfer process is still unclear. This work compares the simulated average temperature of two different phase change materials (PCMs) with experimental result at different operation conditions for the purpose of developing a temperature correction model. A novel empirical heat transfer model is then established to improve the simulation accuracy of PCM-based solar thermal systems. The contributions of this work include that (1) the system performance could be evaluated by the proposed empirical heat transfer model under sunny and cloudy weather conditions; (2) two PCMs (i.e., PCM1 and PMC2) are used to evaluate the performance of the proposed empirical heat transfer model. The analysis results demonstrate that (a) during solar irradiation variations, the temperature of the latent heat storage tank is much more stable than that of the sensible heat storage tank; (b) the heat transfer power and heat storage amount of PCM1 per unit mass are less than that of the heat transfer oil, while the observations on PCM2 are as opposed to these obtained from PCM1; (c) there is a critical time/temperature for switching the operation mode of the heat storage between different weather conditions; (d) under the conditions of low temperature, low solar radiation intensity and short-time heat storage, LHTES may prefer to perform sensible heat storage rather than latent heat storage, and vice versa. The findings of this paper may provide a theoretical basis to determine the amount of the PCMs and the control strategy of operation model for LHTES.
To explore the heat and mass transfer characteristics during phase change, a well-designed test rig with melting front visualization was designed and fabricated. The results show that the different inclination angles have a great influence on the melting process of pure paraffin. Compared with the 60-degree inclination, the full melting time of pure paraffin at 30-degree obliquity decreased by 25.30%. The visualization demonstrated an inclined melting front, which indicated the contribution of local natural convection in the fluid phase. However, there seems to be little influence of inclination angles for the melting process of paraffin embedded in metal foam. Local thermal non-equilibrium between metallic ligaments and the saturating PCM (paraffin) was experimentally observed. This revealed the feasibility of two-equation model for modelling the phase change process in open-cell metal foam.