A comprehensive study on the complete charging-discharging cycle of a phase change material using intermediate boiling fluid to control energy flow

https://doi.org/10.1016/j.est.2021.102235Get rights and content

Highlights

  • The low thermal conductivity of PCMs is a significant disadvantage which has restricted their utilization to meet success.

  • In the present study, an innovative technique named IBF is introduced to accelerate the melting and solidification processes in PCMs.

  • A non-solvable intermediate fluid is employed to either take or lose the heat from or to the PCM indirectly, via boiling within PCM.

  • Paraffin and acetone are used as phase change material and intermediate boiling fluid, respectively.

  • This technique is an applicable method to control energy flow in which, by adjusting the container pressure and using different amounts of intermediate boiling fluid, the freezing and melting rates of phase change materials can be controlled.

Abstract

The low melting and solidification rates of phase change materials (PCM), which traces back to their low thermal conductivity coefficient, has led the application of these materials to face limitations. This paper aims to explore the effectiveness of a novel method called intermediate boiling fluid (IBF) in speeding up the energy storage and transfer processes in PCMs during a complete charging-discharging cycle. Throughout this novel technique, paraffin and acetone are utilized as PCM and IBF, respectively. In the solidification process, there is no direct contact between the cold source and the molten paraffin, while acetone, as an intermediate fluid, is being boiled via absorbing paraffin's heat and ultimately causing paraffin to be cooled down and solidified. The melting and solidification experiments were run in a test cell with and without acetone. The experimental results indicate that utilizing this technique dramatically enhances the solidification rate and improves the melting rate to a moderate level. It is illustrated that by using this method under the optimum condition the solidification time, melting time, and the total melting and solidification time decrease by 77 times, 22 percent, and 80 percent, respectively, compared to the conventional method. It is also concluded that by adjusting the container pressure and using different amounts of intermediate boiling fluid (IBF), the freezing and melting rates of phase change materials can be controlled.

Introduction

The proliferation of human demands for energy, in addition to the depletion of fossil fuels energy sources and hazardous environmental side effects of these sources, have forced humanity to seek renewable energy sources as alternatives [1]. Despite all the benefits, renewable energy sources, these types of energy come with a significant drawback. Periodic accessibility to these sources, in some cases, restricted permanent utilization. Hence, the storage of renewable energy sources for permanent consumption is of high importance. Using thermal energy storage (TES) systems can be one of the best approaches to supply energy on demand. TES can be carried out via three different approaches including, sorting as 1-sensible heat, 2-thermochemical heat, and 3-latent heat [2], [3], [4]. Since phase change materials (PCMs) have high energy-storage density at a relatively constant temperature, the interests in the latent heat storage method have dramatically increased.

Since PCMs absorb and lose energy under constant temperature, they can be utilized as temperature controllers. For instance, Zhou et al. [5] stated that PCMs with a temperature range of 18–30 °C, having chemical compatibility with materials used in buildings, can be used for construction purposes. They declared that the application of PCMs in walls, ceilings, and floor can play a substantial role in preventing temperature fluctuation during day and night. This objective can be fulfilled by storing energy throughout the day and using the stored energy at night for heating purposes. Moreover, PCMs can help to keep the temperature of battery cells constant by absorbing the produced heat. Chunjing et al. [6] proved that the PCM composite could reduce the temperature in LiFePO4 batteries by almost 32% and 37% for 40 A and 80 A currents, respectively. Temperature regulating fabrics are of another PCMs' applications in controlling temperature. Sarier et al. [7] Stated that phase change materials with a melting temperature range of 18–35 °C, can be used in temperature-regulating fabrics for comfort and relaxation of the body. Another application of PCMs can be cited as their high energy-storage densities. This feature is employed for several different purposes. For instance, it can be used to lower down the energy consumption, simultaneously increasing the efficiency in domestic freezers. Yusufoglu et al. [8] investigated the effects of four distinct PCMs types on two different refrigerators. The results revealed that the on-off timing of the refrigerator's compressor was optimized using PCM. It was also found that using 0.95 kg of PCM can store the wasted energies and improve the energy-saving up to 9.2%. Also, these materials can be used to recover industrial heat loss. Pandiyarajan et al. [9] used a shell and finned tube heat exchanger, which was integrated with a PCM tank, to recover the waste heat from a diesel engine exhaust. The performance of the engine was evaluated with and without the presence of the heat exchanger. The outcomes showed that PCM usage could recover 10–15% of the consumed fuel energy in the form of heat that otherwise would have been wasted through the exhaust. This percentage was noticeably 33–50% of the whole wasted heat of the exhaust. Furthermore, since the climate condition, business, industrial and urban activities fluctuate throughout days, the HVAC systems must be compatible with these variable circumstances. In other words, the peak of electricity consumption occurs during the day and is significantly different from its values at night. However, it is possible to transfer this peak (peak shaving) by using PCMs in HVAC systems. Besides, applying this method can lead to better stability of systems as well as smaller system sizes [10]. Wang et al. [11] presented a novel HVAC system using a storage tank filled with microcapsule PCM (MPCM) for storing energy in Hong Kong. They studied the system performance at different hours. They studied the system performance at different hours. Their results showed that using a low amount of PCM was able to shift the peak hours of the consumption rate of ventilation systems from daytime to nighttime as well as provide an energy consumption reduction. Nowadays, solar water heaters are becoming more widespread and being introduced as an alternative to electrical and gas water heaters. A solar water heater can prevent the release of 50 tons of CO2 into the atmosphere throughout 20 years [12]. PCMs can be used in these systems to store energy as latent heat. Regarding the latent heat storage features of PCM, the amount of energy that can be stored in solar water heaters integrated with PCM is much higher than the conventional solar water heaters due to the reduction of heat loss to the environment. On the other hand, their temperature fluctuation is lower than the conventional types [12]. Faegh et al. [13] incorporated phase change material in the condenser of a solar still to absorb the latent heat of vapor during the condensation process so that it could be used during the night for more freshwater production. They used some thermosyphon heat pipes to transfer heat from and to the PCM, indirectly. Using this configuration, the desalination system was able to continue producing freshwater after sunset. Dispersing encapsulated PCMs in a coolant fluid to enhance its effective heat capacity is another application of PCMs, in which PCM can enhance the heat storage capacity of the base fluid, due to its high latent heat [14]. There are other applications for PCMs including greenhouses, thermoelectric generators (TEG), heat pumps, electronic cooling and etc. [15], [16], [17].

Although PCMs have many advantages, their low thermal conductivity, which is considered a significant disadvantage, has restricted their successful utilization. This significant drawback slows down the solidification process. Given that natural convection is the main mode of heat transfer in the melting process, the drawback mentioned above is not as significant as it is for the solidification process. Since conduction is the main mode of heat transfer during the solidification process, this downside is more noticeable. During solidification, the closer parts to the cold source get solidified at first. Considering the low thermal conductivity of PCM in addition to the dominance of conduction heat transfer in the solid part, the frozen section plays the role of an insulator for the external parts, which are still in liquid form. This prevents the heat transfer from the liquid part into the cold source and consequently brings about the inhibition of a proper discharge. Thereupon, the staggering potential of these materials in various industries, particularly the TES field, has picked up researchers' intrigue to seek for improvement. The majority of these attempts are mainly categorized as 1-The addition of some other materials with high thermal conductivity to PCMs, 2- modifying the storage tank structure of PCMs to improve their thermal performance.

Considering their high conduction heat transfer coefficients, metallic nanoparticles and their oxides can be suitable choices to enhance the performance of PCMs. Safaee et al. [18] employed Fe3O4 magnetic nanoparticles along with a magnetic field to improve the melting time of paraffin as a PCM. They dispersed nanoparticles at the concentrations of 0.5%, 1%, and 2% wt. Into the paraffin and found that due to the increment of viscosity as well as thermal conductivity with the increment of concentration, the optimum concentration to be used is 1% wt. They also applied a magnetic field with the intensities of 0.01 T and 0.02 T and observed that the melting time related to the 0.01 T magnetic field is less than that of the higher intensity which was attributed to the increment of viscosity, as a negative parameter, with the increment of magnetic intensity. A 12% improvement in melting time was achieved using the optimum concentration of nanoparticles and the proper applied magnetic field. Cui et al. [19] used copper nanoparticles to improve sodium acetate trihydrate performance as the PCM. Copper nanoparticles at the mass concentrations of 0.4%, 0.5%, 0.6%, 0.7% and 0.8% were added to the PCM. It was found that when the optimum concentration (0.5%) of nanoparticles was added, the rate of heat transfer increased by 20 percent due to higher thermal conductivity. Meanwhile, it was found that though the increment of concentration is very effective on the thermal conductivity, as a positive parameter, however it raises the viscosity as well and consequently weakens the heat transfer. Therefore, the best-reported concentration in many studies was not the highest one. The outcomes of Wu et al. [20] studies revealed that utilizing the copper nanoparticle with a mass concentration of 0.1% can improve the melting and solidification times up to 30.3% and 28.2%, respectively. Also, according to the DSC tests with 100 cycles, there was little variation in the composition properties, demonstrating the high stability of composition. Ho et al. [21] investigated the effects of the addition of Al2O3 nanoparticles to paraffin. Their results clarified that the thermal conductivity of paraffin-nanoparticle composition nonlinearly increases by adding Al2O3. On the other hand, the melting latent heat of the composition reduced approximately by 7% and 12.6% for the concentrations of 5% and 10%, respectively. Moreover, Carbonic nanoparticles are another category that is being used to enhance the thermal features of PCMs. These materials usually have a lower density, better stability, and superior dispersity than metallic nanoparticles [22]. Wang et al. [23] used multiwall carbon nanotubes (MWCNT) to enhance palmitic acid properties. It was illustrated that the thermal conductivity of matrix composed of palmitic acid and MWCNT intensifies at higher nanoparticle concentration. The tests results verified the capability of MWCNT with a mass concentration of 1% in increasing the matrix thermal conductivity up to 30% compared to the pure palmitic acid. İnce et al. [24] employed graphite nanoplates to boost the thermal properties of myristic acid (as a PCM). The Fourier Transform Infrared Spectroscopy (FTIR) test results showed that graphite nanoplates with concentrations of 0.5%, 1%, and 2%, increased the myristic acid thermal conductivity up to 8%, 18%, and 38%, respectively. Additionally, the melting and solidification cycle of the produced composite was repeated 100 times, and the outcomes revealed that the thermal properties of the composite did not change, which confirmed the satisfying stability of the prepared composite.

The main attempts in another group of studies were to improve the heat transfer rate from the hot source to the PCM and from the PCM to the cold source by modifying the structure of the heat storage tank containing the PCM. Mat et al. [25] studied the effects of the inner fin, outer fin, and simultaneous implementation of both on a cavity containing RT82 as the PCM. Throughout their investigations, the influences of several parameters, including the length of the fins, were studied. The results showed that using the optimized fin length can reduce the melting time up to 43.3% compared to the case without fin implementation. Sheikholeslami et al. [26] examined the effects of a novel design of fins in addition to nanoparticles using the Finite Element Method (FEM) in order to enhance the solidification process of PCMs. They studied the energy discharge rate as well as maximum energy storage density. The outcomes showed an improvement in the solidification rate of snowflake shape fins solidification rate up to 4.5 times compared with the no-fin case. Adding nanoparticle with a mass concentration of 5%, enhanced this rate up to 1.2 times. Based on the results obtained, they stated that using fins is superior to employing nanoparticles. Singh et al. [27] also experimentally studied the effect of fins as well as graphene nanoplatelets (GNP) on the melting rate of PCM. They employed annular finned tubes with three different heights and GNP at three volume concentrations of 1%, 2%, and 5%. It was stated that a maximum reduction in the melting time of 47% or 55% could be achieved when only the most extended finned tube was used or only 5% GNP was used, respectively. Besides, it was shown that the combination of finned tube with 5% GNP results in 68% reduction in melting time compared to the case without fin and GNP.

Moreover, metallic or carbonic foams can be used for the sake of boosting PCMs' effective thermal properties, considering their high thermal conductivity coefficient. Ji et al. [28] used Ultra-thin Graphite Foams (UGFs) for improving the thermal performance of paraffin as a PCM. It was concluded that using the optimum porosity of UGFs can enhance the effective thermal conductivity of paraffin up to 18 times. s. Xiao et al. [29] investigated copper and nickel metal foams' impacts on the effective thermal properties of paraffin. The experimental and theoretical outcomes indicated that the thermal conductivity of paraffin/nickel foam and paraffin/copper foam composites are 3 and 15 times that of the pure paraffin, respectively. However, they noted that adding foams influences melting and freezing temperatures of paraffin which must be taken into account when designing a thermal storage system.

Another recent technique utilized by researchers to enhance the melting and solidification processes of PCMs is employing direct contact of an intermediate fluid with PCMs. In this method, a non-solvable intermediate fluid is used to transfer the heat from and to the PCM. The fluid is directly injected into the PCM; afterward, it either takes or loses the heat from or to the PCM, which causes the PCM to freeze or melt. In other words, the heat transfer to and from the PCMs occurs by the variation in intermediate fluid temperature. Martin et al. [30] attempted to store energy using PCMs to enhance the performance of cooling system and reduce the electricity consumption peak. They attempted to find the maximum capacity of cooling systems by providing direct contact between water and PCM. In their proposed setup, water was injected into the PCM straightly from its top to freeze the PCM by taking its heat and was collected through a valve located under the container. It was concluded that a cooling capacity of 30–80 kW per cubic meter could be achieved by using this technique. Ramazani et al. [31] presented a novel method to enhance phase change materials' solidification process. They used the intermediate boiling fluid's latent heat (IBF) instead of the sensible heat in order to absorb the heat from the PCMs. In this method, which was named as super-fast discharge (SFD-IBF), the solidification rate enhanced up to two orders of magnitude compared to the conventional solidification method, which is quite noticeable compared to previous studies.

As mentioned above, most previous studies examined the effect of adding nanoparticles and the use of high thermal conductivity metal foam and fin on reducing the melting and freezing times of PCMs. Additionally, some research has focused on the effect of the intermediate fluid without phase change in direct contact with the PCM on the melting and freezing speed of the PCM, in which the intermediate fluid absorbs and releases thermal energy in the form of sensible heat. Moreover, in reference [31], in which the IBF method was introduced, only the influence of this technique on the solidification process was investigated, and its impact on the melting process was not studied. . It should be noted that the reversibility of the storage system is feasible only if it goes through a complete phase changing cycle [32]. In other words, a full cycle of charge and discharge processes for the phase change material is essential in order to analyze the reversibility and effectiveness of the IBF method. In the present study, the effect of an IBF (acetone) on both melting and freezing speeds of a PCM (paraffin) is investigated for the first time. Also, for the first time, the effect of reservoir pressure as an important parameter in the boiling rate of acetone on the melting and freezing speed of paraffin has been evaluated. In this study, the intermediate boiling fluid (acetone), in direct contact with paraffin, changes phase from liquid to vapor and vice versa as it absorbs and releases heat, during solidification and melting processes of paraffin respectively. There is no direct contact between the condenser surface and liquid paraffin which prevents the formation of a solid layer of paraffin as a thermal resistance on the condenser surface and results in a much faster solidification process. Finally, the effect of different amounts of acetone on melting and freezing times of will be discussed. This research improves our understanding of the complex interaction between the intermediate boiling fluid and the PCM during the melting and freezing processes.

Section snippets

Experimental setup

As presented in Fig. 1-a, the cylindrical body of the setup is Pyrex glass with an internal diameter, outer diameter, and height of 80, 100, and 1500 mm, respectively. Two steel flanges with a thickness of 100 mm are located at both ends to seal and stabilize the setup. These two flanges are connected via four screws. A plate heater which is connected to a DC power supply is used for heating purposes. Moreover, the condenser is structured from a copper coil with four loops with the internal and

Results

In this study, in order to show the effectiveness of the IBF method on increasing the freezing and melting rates of paraffin as a PCM, the results for acetone as the IBF and air are compared with each other. The outcomes of the experiments are as follows:

Conclusion

In the present study, an innovative and promising technique yet steeped with great areas for further research was introduced for enhancing the melting and solidification time of phase change materials. In this method, paraffin was used as PCM, and acetone was employed as the intermediate boiling fluid (IBF). The following results can be drawn from the tests:

  • To choose the intermediate fluid and PCM, the intermediate fluid's boiling temperature must be lower than the solidification temperature of

Authorship statements

Conception and design of study: Hossein Hosseininaveh (First Author), Omid Mohammadi (Second Author)

Acquisition of data: Hossein Hosseininaveh (First Author), Omid Mohammadi (Second Author), Shahin Faghiri (Third Author)

Analysis and interpretation of data: Hossein Hosseininaveh (First Author), Omid Mohammadi (Second Author), Mohammad Behshad Shafii (Forth Author)

Declaration of Competing Interest

There is no conflict of interest for the paper entitled as “A comprehensive study on the complete charging-discharging cycle of a phase change material using intermediate boiling fluid to control energy flow”.

Acknowledgments

We would like to express our gratitude to the Deputy ofResearch and Technology of Sharif University of Technology andSharif Energy Research Institute (SERI) for providing a comfortableworking environment to carry out experiments.

References (33)

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