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Abstract

The current study aims to improve the energy storage capability of the thermal energy storage system by utilizing multiple phase-change materials. In this regard, a new thermal energy storage system has been studied when multiple phase-change material with different melting temperatures are employed. In this study, three-set, each having 70 encapsulated phase-change materials, are arranged horizontally in the order of decreasing melting temperature for the charging cycle and increasing melting temperature in the case of discharging cycle. Water is employed as heat transfer fluid, and the SAE 304 spherical capsule with a diameter of 60 cm is used to encapsulate the phase-change material. The arrangement showed an excellent thermal response for input parameters such as inlet temperature, flow rate. Further, it was observed that as the heat transfer fluid flow rate increases from 1 litres per minute (LPM) to 5 LPM, the melting and solidification time decreases from 10% to 30% and 14% to 28%, respectively. It was also noticed that as the flow rate increases, exergy and overall efficiency decreases.

Keywords

Charging and discharging energy efficiency phase-change material latent heat thermal energy storage thermal energy storage

Introduction

Fossil fuel depletion, increasing energy demand, and rapid consumption of nonrenewable sources in various industries are the few concerns that force humans to find alternative renewable energy sources. Energy storage has emerged as a potential method up to some extent. Thermal energy storage (TES) is one of the most effective energy storage techniques to bridge the gap between demand and supply and give a possible solution for energy wastages and surplus energy.¹ TES systems are broadly categorized as sensible, latent, and thermochemical storage. Latent heat thermal energy storage systems (LHTESS) are more efficient than the other TES techniques.² LHTESS mainly consists of the charging and discharging process. In this process, energy is stored during the phase change of the charging process (solid to liquid) and retrieved during the phase change of the discharging process (liquid to solid). However, due to its compactness, economical, and capacity to hold more energy, LHTESS using phase-change material (PCM) has become one of the most popular techniques for energy storage.³ It is also the first choice of many researchers.

With a massive increase in the world's population and also due to huge development in industrial, marine, aeronautical, and aerospace fields, energy which mainly consists of fossil fuel, coal, natural gas, etc., is decreasing in overall quantity because these energy resources are the most important factor in the development of those fields.⁴ Researchers and scientists are concerned about the widening gap between supply and demand for energy and the enormous amount of fossil fuel consumption.⁵ Even though renewable energy can solve these problems, the best solution is to improve thermal energy management practices and store excess energy.⁶ However, extra energy can be stored using a PCM-based TES system, which can help to reduce costs by bridging the supply–demand gap.⁷ Alam et al.⁸ used a TES system in which sodium nitrate with a melting point of 306°C was used as a PCM, and the air was used as a heat transfer fluid (HTF). Fleischer⁹ conducted an extensive study on the use of PCM in building materials, especially PCM in the roofing of the buildings. Zalba et al.¹⁰ carried out a review on TES systems, particularly the use of PCM with special focus on type of material, its heat transfer rate, and application. The authors reviewed about 150 PCMs and reported that PCM can be used in different applications such as ice storage, building materials.

Study on the most recent developments in PCM-based LHTESS particularly, temperature range between 0 and 60°C was carried out by Agyenim et al..¹¹ Tatsidjodoung et al.¹² published a comparative study of conventional TESS with the recent PCM-based LHTESS. Other PCM-based TES with little temperature difference have low and high direct storage density. Application of PCMs in hot water generation and building heating was thoroughly investigated by Gasia et al..¹³ They circulated liquid PCM throughout the melting process to enhance heat transmission. It was found that encapsulating PCM with molten salt as HTF improves the working of the packed bed storage system. The enthalpy approach was used to look at the phase transition process within the phase-changing material capsule.¹⁴ Dogkas et al.¹⁵ compared PCM-filled TES tanks to tanks filled with water. The result showed that the PCM-filled tank had a 35.5% higher energy storage capacity than the water-filled tank. Yasiri and Szabó¹⁶ and Avignon and Kummert¹⁷ studied the behavior of real-scale PCM storage and documented their findings. One of the study's primary discoveries was that subcooling and hysteresis effects do not behave as expected.

Liu and Bruno¹⁸ used ice-filled rectangular PCM slabs. Based on a one-dimensional approach and taking temperature variations along the flow direction of the HTF into account. The model developed was found to be very close to reality. Further, researchers also developed new innovative techniques such as composite PCM fins, photovoltaic (PV), and solar water heating to enhance the energy storage and other thermophysical properties of the PCM prepared a hybrid PCM by doping magnesium particles in ternary carbonate salt and found that the thermal conductivity of the ternary salt was increased by 46% for 2% wt. of magnesium particles. Abdulateef et al.¹⁹ found that the use of fins in the PCM enhances the thermal conductivity of the PCM. Browne et al.²⁰ reviewed about the use of PCM in heat management of PV and building integrated PV and also described technical and economic aspects. They discovered that PCM helps to improve the performance of PV in the short-term run.

Although the literature survey gives an idea about various methods to enhance the TES capacity of the TES system. The enhancement is still within the limits and there is scope for further improvement in the storage capacity. The technique must be developed to enhance the thermal performance of the energy storage system and show good thermal conductivity and better control over the charging and discharging process. The present study focuses on using multiple PCMs to enhance the energy storage capacity of TES systems. The system's performance is tested using three PCMs, each with a different melting temperature, arranged in an increasing and decreasing order of melting temperatures. Water is used as the HTF. The output of the experimental investigations is discussed in the following sections.

The novelty of the current works lies in the orientation of the cylinder and the PCM arrangement inside the cylinder. The horizontal direction of the cylinder and encapsulated PCM arrangement inside the cylinder gives better control over the charging and discharging process and heat transfer process in comparison to the vertical orientation of the cylinder. The charging rate can be enhanced by choosing a different set of PCM. For example, if three set of PCMs with a melting temperature of (30, 40, and 50°C) is used during the charging process, the charging rate can be enhanced by just replacing PCMs with a melting temperature of (20, 30, and 40°C). However, encapsulation of the PCM minimizes the loss during the charging–discharging process and reduces the adverse effect on the environment.

Experimental setup

In the current experimental work, a new packed bed consisting of multiple PCM is developed to investigate the charging and discharging process of an LHTESS, as shown in Figure 1. It mainly consists of an HTF tank with municipal water supply connections, electric heaters with digital temperature controllers to maintain uniform temperature during the charging and discharge process, centrifugal pump, and rotameter to regulate and direct the HTF from the tank to the heat exchanger. Height of the storage tank is taken as 104 cm, and the diameter is taken as 40 mm. The complete set up is well insulated with 10 mm thick insulating material. Proper PVC fittings are done to complete the setups and prevent leakages. The packed bed consists of 210 encapsulated PCM arranged in three-sets, with each set of 70 from top to bottom, having a specific melting temperature. The average size of the capsule is 60 mm, and the average porosity of the bed and bed to the particle is 0.40 and 15, respectively. Thermocouple (J type) is used to record the temperature of the energy storage tank. Five thermocouples are placed in every layer of the PCM. Water is used as an HTF digitally controlled heater, and the centrifugal pump is used to maintain the temperature and flow of the HTF.

Figure 1.

Model of the experimental setup.

Experimental procedure

The experimental procedure begins with heating the HTF to the required temperature with the support of digitally controlled electric heaters. The hot water is allowed to flow horizontally with the support of a centrifugal pump to maintain the steady temperature heaters during the complete charging process. The thermostat and rotameter are employed to monitor the inlet temperature and velocity of the HTF. As the charging proceeds, the melting of the PCM capsules begins. The practice is continued until the charging of all the capsules is completed, once the charging is done, the heating is stopped all the temperatures are recorded. Water is allowed to flow for the discharging process. It extracts heat from the PCM, and the PCM capsules start solidifying. The process is continued until the output and input temperature of the HTF is the same. The testing is performed for the three different flow rates of water, that is, 1, 3, and 5 litres per minute (LPM).

Encapsulation of PCM

SAE 304 stainless steel is used for the encapsulation of the PCM. Easy availability, low cost, and high thermal conductivity are some of the desirable properties of the SAE 304, which are best suited for the current study. The high thermal conductivity increases the performance of the system. The 210 spherical shape capsules with 3 layers of 70 capsules are filled with different PCMs with distinct temperatures used for this study. First, the PCMs are melted separately and then poured into the sphere manually. After that, the spheres are tightened with the help of washers and bolts to avoid leakages. Every 100 g of PCM is poured into each sphere. A total of 70*3 kg of PCM is run inside the spherical capsule. After the filling is completed, each capsule is manually inspected to ensure leakage-free Encapsulation. PCM layers are arranged in the order of decreasing melting temperature.

PCM arrangement in the cylinder

The arrangement of PCM is the most critical aspect of the experiment. The 105 cm length of the cylinder is divided into three zones filled with three different PCMs having three different temperatures. Each zone has 70 kg of PCM. A total of 210 kg of PCM is loaded into the cylinder. The length of each zone is given as L1 = L2 = L3 = L/3. The schematic diagram of the PCM arrangements is shown in Figure 2.

Figure 2.

Schematic diagram of the phase-change material (PCM) arrangement.

Material selection

Paraffin C₁₆–C₁₈, Paraffin, Polyglycol E600, and Paraffin C₁₃–C₂₄ are used as PCM-1, PCM-2, and PCM-3 for the multiple PCM arrangements. As shown in Table 1, all the other materials such as PCM tank, HTF tank, and shell are made up of high thermal conductivity materials to enhance the system's thermal performance.

Table 1.

Properties of phase change materials (PCMs).^21,22

S. No.	PCM	Melting temperature (^°C)	Latent heat of fusion (kJ/kJ)	Thermal conductivity (W/m.K)	Density (kg/m³)
1.	Paraffin C16–C18	20	152	NA	NA
2.	Polyglycol E600	22	127.5	0.189	1126 (solid)	1132 (liquid)
3.	Paraffin C13–C24	24	185	0.21	0.76 (solid)	0.9 (liquid)

Energy and exergy mathematical formulation

Mathematical calculations during melting and solidification are given as follows:

E_{in} = \dot{m} C_{H T F} \int_{0}^{t} [T_{in} - T_{o u t}] d t

(1)

E_{out} = \dot{m} C_{H T F} \int_{0}^{t} [T_{out} - T_{in}] d t

(2)

E_in and E_out represent the PCM's absorbed heat and the water's absorbed heat from the surrounding environment. The mass flow rate is m and the fluid's specific heat is C_HTF.

The storage tank's thermal efficiency can be calculated as

η = \frac{E_{out}}{E_{in}}

(3)

The following equations can be used to calculate the amount of exergy:

X_{in} = \dot{m} C_{H T F} \int_{0}^{t} [(T_{in} - T_{out}) - T_{0} \ln (\frac{T_{in}}{T_{out}})]

(4)

X_{out} = \dot{m} C_{H T F} \int_{0}^{t} [(T_{out} - T_{in}) - T_{0} \ln (\frac{T_{out}}{T_{in}})]

(5)

X_{stored} = \dot{m} C_{H T F} \int_{0}^{t} [(T_{in} - T_{out}) (1 - \frac{T_{0}}{T_{melt}})] d t

(6)

T_in and T_out, T_melt, and T_o are the input, output, melting, and ambient room temperature. Calculation of the TES unit's charging, discharging, and overall efficiencies are given by the equations below:

ε_{charging} = \frac{X_{stored}}{X_{in}}

(7)

ε_{discharging} = \frac{X_{out}}{X_{stored}}

(8)

The total uncertainty (δR) of the experimental results depend upon number of independent variables. If X₁, X₂, …, X_n are the error in measurement of each factor, then total uncertainty (δR) is given by,

δ R = \sqrt{\sum_{n = 1}^{N} {(\frac{δ R}{δ X_{n}} δ X_{n})}^{2}}

(9)

In the present study, the uncertainty analysis was performed for the discharging, charging, volumetric thermal storage capacity of the volume flow rate, and temperature difference. The volume measurement was accurate to within 1%. As a result, E_in and E_out had maximum uncertainties of 8.25% and 3.48%, respectively, whereas

ε_{charging}

and

ε_{discharging}

had maximum uncertainties of 20.1% and 20.12%, respectively.

Results and discussion

Charging process

The purpose of the charging process is to store latent heat in the encapsulated PCM. In this process, water is first heated with the help of electric heaters until the required temperature is achieved, and then it is allowed to pass through the PCM bed using a centrifugal pump. Hot HTF passes through the PCM bed. PCMs absorb the latent portion of the heat and start melting by changing their phase from solid to liquid. The PCM bed is charged with different flow rates, and the charging is continued until the PCM bed temperature reaches 75°C. The ambient temperature during the process was 32–34°C. The thermocouple was placed inside the PCM bed to record the inside temperature of the encapsulated PCM, and data was stored in the computer coupled to it. As the flow rate of the HTF enhances, the charging of the encapsulated PCM becomes faster, and it takes less time for the charging process, as shown in Figures 3, 4, and 5, respectively.

Figure 3.

Input–output and ambient temperature for 1 LPM flow.

Figure 4.

Input–output and ambient temperature for 3 LPM flow.

Figure 5.

Input–output and ambient temperature for 5 LPM flow.

Discharging process

The purpose of discharging process is to extract the heat from the PCM bed. The process began by passing the cold water through the PCM bed from top to bottom. Encapsulated PCM releases the latent portion of the heat and starts solidifying here. Also, the process is accelerated as the flow rate of the HTF increases. At the beginning of the process, the input temperature of PCM is taken as 34°C, and the ambient temperature is also 34°C. Still, as the discharging proceeds, there is an increase in the HTF temperatures. The process is continued till the temperature of the HTF reached to 34°C. The variation of temperature during discharging for different conditions are shown in Figures 6, 7, and 8, respectively.

Figure 6.

Input–output and ambient temperature for 1 LPM flow during discharging.

Figure 7.

Input–output and ambient temperature for 3 LPM flow during discharging.

Figure 8.

Input–output and ambient temperature for 5 LPM flow during discharging.

Effect of the mass flow rate on multiple PCM

The TES system used in the current study consists of three consecutive layers of encapsulated PCM with different melting temperatures arranged horizontally to decrease the melting temperature for the charging cycle and increase the melting temperature for the discharging cycle. There different layers are used to maintain the temperature rise among the HTF. Figure 9 and Supplementary Figures 10 and 11 indicate the temperature variation in the packed bed consisting of PCM capsules. As the flow rate increases, the charging and discharging time for the thermal energy system decreases. It was also noticed that the temperature variation between the HTF and PCM plays a critical role in heat extraction and heat transfer process. As the HTF reaches PCM-1, it starts melting in the case of multiple PCM. It reaches its melting temperature, and the charging stops since the temperature of PCM-2 is less than PCM-1. Charging of PCM2 starts until it reaches its melting temperature, also similar process is followed for PCM3. It is concluded that encapsulated multiple PCM arrangements facilitate the charging and discharging process and can be observed in Supplementary Figure 12. The comparative melting time at various HTF temperatures is shown in Supplementary Figure 12.

Figure 9.

Multiple phase-change material (PCM) capsule temperature at 1 LPM during charging.

Energy and exergy analysis

Equations (1) and (2) calculate the energy transferred to and extracted from PCM by the HTF during the charging and discharging cycle. The values of important parameters and the overall efficiency of the TES system are calculated using equation (3) and are shown in Supplementary Table 2. It can be noted that charging efficiency increases as the mass flow rate increases. However, the discharging and overall efficiency decreases, as shown in Supplementary Figure 13. Equations (4), (5), and (6) give the exergy input–output and are stored during the charging and discharging process. Equations (7) and (8) provide the exergy with the efficiency during the charging and discharging process. All the data is tabulated in Supplementary Table 3. Results show that mass flow rate has an adverse effect on the exergy efficiency of the TES system as the mass flow rate increases, exergy efficiency decreases, as shown in Supplementary Figure 14.

Single PCM unit versus multiple PCM units

It was observed that the multiple PCM units gave better performance than the single PCM. This happens only because the multiple PCM units are capable of maintaining the temperature change between the HTF and PCM. This results in the higher heat transfer and extraction by the HTF enhancing the exergy efficiency of the multiple PCM when compared to the single PCM. The higher exergy efficiency indicates the efficiency of the system is improved as shown in Supplementary Figure 15.

Conclusion

The current investigation is based on the study of latent heat storage systems when three different PCMs with different melting temperatures were used. During the investigation, different process parameters were considered to analyze the impact of mass flow rate on the melting and solidification process. The important findings of this study are summarized below.

Energy efficiency of LHTESS increases from 54.7% to 74.3% as the HTF flow rate is increased from 1 to 5 LPM.

HTF flow rate has a significant impact on the charging–discharging rate of PCM. As HTF flow rate increased from 1 LPM to 5 LPM, the melting and solidification time decreased from 10% to 30% and 14% to 28%, respectively.

Exergy efficiency decreased from 21.4% to 15.3% as the mass flow rate increased from 1 LPM to 5 LPM.

The results obtained in this study indicate that the energy storage capacity of the LHTES system can be enhanced using multi PCM rather than using single PCM. It can be summarized that the use of multiple PCM enhances the storage capability of the TES system.

Supplemental Material

sj-docx-1-pie-10.1177_09544089221110983 - Supplemental material for Experimental analysis of latent heat thermal energy storage system using encapsulated multiple phase-change materials

Supplemental material, sj-docx-1-pie-10.1177_09544089221110983 for Experimental analysis of latent heat thermal energy storage system using encapsulated multiple phase-change materials by Santosh Kumar Singh, Sujit Kumar Verma, Rahul Kumar, Abhishek Sharma, Ramanpreet Singh and Nishant Tiwari in Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering

Supplemental Material

sj-docx-2-pie-10.1177_09544089221110983 - Supplemental material for Experimental analysis of latent heat thermal energy storage system using encapsulated multiple phase-change materials

Supplemental material, sj-docx-2-pie-10.1177_09544089221110983 for Experimental analysis of latent heat thermal energy storage system using encapsulated multiple phase-change materials by Santosh Kumar Singh, Sujit Kumar Verma, Rahul Kumar, Abhishek Sharma, Ramanpreet Singh and Nishant Tiwari in Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Sujit Kumar Verma

Abhishek Sharma

Ramanpreet Singh

Nishant Tiwari

Supplemental material

Supplemental material for this article is available online.

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Supplementary Material

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