75 PERFORMANCE ANALYSIS OF LATENT THERMAL ENERGY SYSTEMS DURING
MELTING AND SOLIDIFICATION WITH PCM Shekh Irfan Kadari, Vishwajeet Kureel
Abstract - Latent thermal energy storage could be a way for storing energy for later use in different application. This can be utilized in many applications like insulated buildings, natural phenomenon and house shuttles. The major problem in such systems is comparatively low charging and discharging power i.e. the time to complete melting and solidification either in low thermal physical philosophy or in giant attributable. By using phase change materials like RT-35 many ways to raise the performance of the system are examined. Present work is dealing with the evolution of latent thermal energy storage in exceedingly coaxal element through each melting and solidification using PCMs RT35. It is a comparative study of simplified 2-D model of horizontally and vertically oriented shell and tube device for RT-35 PCMs. Ansys Fluent 18.1 is used as a Platform for CFD Programme through 4 different models. The results show that the vertical orientation performs higher in all cases, melting and solidification for RT-35.
1 INTRODUCTION
Thermal energy storage (TES) is a type of energy storage system in which energy is stored in the form of heat, in which the carrier material energies when there is an increment in temperature whereas de- energizes if temperature decreases. By using this property it enables to use various materials with different thermal properties and get different results, which can help in different application of either heating or cooling. TES is very useful to maintain energy demand and supply on daily, weekly or even seasonal basis. TES is mainly used to overcome the mismatch between energy generation and use. This mismatch can be in time, temperature, power or location. It also helps to reduce maximum demand, load, carbon emission and costs, so it is useful to increase overall efficiency of energy systems. Such types of systems are most commonly used in solar thermal energy systems. Due to various advantages, it has many other applications such as for storing heat in building and houses, utilize waste heat and district heating systems and to couple heat pumps and combined heat and power (CHP) generators in district heating networks.
With increasing use of renewable energy sources during last few decades’
importance of research and new innovations in energy storage systems is also increasing. Renewable energy sources like wind, solar and tides do not generate energy with a constant rate as it can be utilized. So traditional fossil
fuels switch to systems with high impact of renewable energies, and it introduces gap between supply and demand.
Thermal energy storage is having less cost than electricity storage and it has high potential of intermittent energy sources such as wind solar or hydro into HVAC sector. TES offers many advantages to heating and cooling networks (DHC), including maximum loads, improved efficiency and integrating other heat sources as industrial application of waste heat or seawater.
1.1 Selection Criteria for PCMS
The phase change material should possess the following thermodynamic properties
Melting temperature in the desired operating temperature range
High latent heat of fusion per unit volume
High specific heat, high density, and high thermal conductivity
Small volume changes on phase transformation and small vapor pressure at operating temperatures to reduce the containment problem
Congruent melting
Kinetic properties
High nucleation rate to avoid supercooling of the liquid phase
High rate of crystal growth, so that the system can meet demands of heat recovery from the storage system
Chemical properties
Chemical stability
76
Complete reversible freeze/melt cycle
No degradation after a large number of freeze/melt cycle
Non-corrosiveness, non-toxic, non- flammable and non-explosive materials
Economic properties
Low cost
Availability
2 LITERATE REVIEW
Kannan, J. Prakash, D. Roan et.al.
2021 developed An off-grid star combisystem with integral heat of transformation storage unit is intended for active house and water heating throughout day and passive house heating throughout night.
The star combi-system performance shows that the typical energy saving quantitative relation and energy saving obtained for house heating were ~ ninety three and ~ a pair of.8 kWh, severally. it had been discovered that the air temperature within the state change material integrated house unit (area coverage
~60%; melting point: twenty three
°C) stay ~ four to six °C cooler than corresponding while not state change material integrated house unit. The semi-permanent performance analysis of the solar combi-system shows that the off- grid system will with efficiency maintain the house temperature in a very comfortable zone throughout winter and summer season together with water heating for residential application. it's evident that the off-grid state change materials integrated star comb-is ystem will result in vital energy saving in residential buildings each in colder and hotter climatic conditions.
Zhi Li ,Yiji Lu, Rui Huang et al 2021 considered Thermal energy storage (TES) technology to Balance the demand and supply and overcoming the intermittency and fluctuation nature of real- world heat sources, making a more flexible, highly efficient and reliable thermal energy systems. This review aims to recognize potential methods to
design and optimize LTES heat exchangers for heat recovery and storage, bridging the knowledge gap between the present studies and future technological developments.
The key focuses of this work can be described as follows:
1. Insight into moderate-high temperature phase change materials and thermal conductivity improvement methods.
2. Various configurations of latent thermal energy storage heat exchangers and relevant heat
transfer enhancement
techniques
3. Applications of latent thermal energy storage heat exchangers with different thermal sources, including solar energy, industrial waste heat and engine waste heat, are discussed in detail.
Ahmed H.N. Al-Mudhafar et al 2020 to make-out improvement in thermal performance of PCM thermal energy storage system a modified webbed tube heat exchanger was introduced and numerical investigation is made.
The performance of this heat exchanger is compared with webbed tube heat exchanger and the triple tube heat exchanger. 2- D numerical models were developed which enabled us to simulate conduction and natural convection heat transfer mechanisms. And the whole process of solidification was monitored. It concluded that the PCM solidification process accelerates by 41% when applying the modified webbed tube heat exchanger compared to the webbed tube heat exchanger.
Hussam Jouhara et al 2020 presented different methods of thermal energy storage including sensible heat storage, latent heat storage and thermo chemical energy storage, focusing mainly on phase change materials (PCMs) as a form of suitable solution for energy utilization to fill the gap between demand and supply to improve the energy efficiency of a system. . The
77 article presents a classification of
PCMs according to their chemical nature as organic, inorganic and eutectic and by the phase transition with their advantages and disadvantages. In addition, different methods of improving the effectiveness of the PCM materials such as employing cascaded latent heat thermal energy storage system, encapsulation of PCMs and shape- stabilization are presented in the paper. Furthermore, the use of PCM materials in buildings, power generation, food industry and automotive applications are presented and the modeling tools for analyzing the functionality of PCMs materials are compared and classified.
3 PROBLEM
There is need of a numerical analysis of latent thermal energy storage in co-axial transient heat problem. In this case both conduction and convection are incurred and dominant at different stages of the process. Due to its nature, analytical solutions are impossible in real practice.
This work need to examine the nature of heat transfer in thermal energy storage, with the help of simulations in the CFD on Ansys 18.1. The geometries considered are one thin two- dimensional tubes orientated horizontally and one vertically orientated rectangle. This is possible due to the mirror effect of the vertical case, which requires simulation of only one side of the tube. The simulations will be of for cases, melting and solidification.
3.1 Objective
In a time of environmental issues and climate change, the search for more efficient energy systems is a constant process. More efficient usage of LTES could be one way to more efficiently utilise already harvested energy. In this work, LTES in a coaxial component is simulated and analysed in four different cases. The purpose is to see if there is a difference in performance between horizontal and vertical orientation of the heat exchanger, during charging and discharging.
4 METHODOLOGY
It is impossible to do a transient heat transfer problems in multiple dimensions, along with conduction and convection both analytically. It is essential to know all the mechanisms involved to setup the simulations correctly, with appropriate boundary conditions and meshing. It is also important to be able to interpret and justify the obtained results. this chapter presents the governing equations and boundary condition will be considered.
Also the methods used when simulaton and the different cases considered, including PCM will be presented.
4.1 Governing Equations
When there is an analysis of the phase- changing process, here the enthalpy porosity method is used, which is described by Esapour et. al. and the various assumptions and equations are derived. The assumptions are ; flow is considered laminar and the viscous dissipation term is considered negligible.
These assumptions make it possible to write the continuity-equation as in
...4.1 5 GENERAL RESULTS
The results from the simulations of the four cases, shows that the vertical orientation enables both faster melting and solidification. In this section, contours of liquid fraction, contours of temperature and temperature plots are presented.
5.1 Liquid Fraction
In figure 5.1, 5.2, 5.3 and 5.4, contours of the liquid fraction during the processes are shown.
Figure 5.1: Liquid fraction contours of case 2: Solidification in horizontal
component.
78 Figure 5.2: Liquid fraction contours of
case 3: Melting in vertical component.
Figure 5.3: Liquid fraction contours of case 2: Solidification in horizontal
component
Figure 5.4: Liquid fraction contours of case 4: Solidification in vertical
component.
5.2 Temperature
In figure 5.5, 5.7, 5.9 and 5.11, temperature contours of temperature change in the different cases are presented. Graphs of the average temperature, i.e. the temperature gradient, during the process are also presented in figure 5.6, 5.8, 5.10 and 5.12.
Figure 5.5: Temperature contours of case 1: Melting in horizontal
component.
Figure 5.6: Temperature plot of case 1:
Melting in horizontal component, shows the average temperature in the PCM in relation to time.
Figure 5.7: Temperature contours of case 2: Solidification in horizontal
component.
Figure 5.8 Temperature plot of case 2:
Solidification in horizontal component, shows the average temperature in the
PCM in relation to time.
Figure 5.9: Temperature contours of case 3: Melting in vertical component.
Figure 5.10: Temperature plot of case 3: Melting in vertical component, shows the average temperature in the
PCM in relation to time.
79 Figure 5.11: Temperature contours of
case 4: Solidification in vertical component.
Figure 5.12: Temperature plot of case 4: Solidification in vertical component,
shows the average temperature in the PCM in relation to time.
5.3 Performance Comparison
The purpose of the simulations is to see which of the orientations is best. In figure 5.13 and figure 5.14, a comparison between the two different orientations is made, both during melting (figure 5.5) and solidification (figure 5.6). As depicted in the figures, the vertical orientation is better in both cases, i.e. the time to complete melting and solidification is the shortest. In the case of melting, the vertical orientation is approximately 200% faster than the horizontal orientation. In the case of solidification, vertical orientation is approximately 90%
faster than horizontal orientation.
Figure 5.13: Comparison of liquid fraction during melting between horizontal and vertical orientation in
relation to time.
Figure 5.14: Comparison of liquid fraction during solidification between
horizontal and vertical orientation in relation to time.
5.4 Energy Comparison
The reason of using LTES in the first place is to store energy for later use.
Thus, it is important to know how much energy is stored in the PCM. In figure 5.15, 5.16, 5.17 and 5.18 the accumulated energy received by or released from the PCM are shown. As can be seen, the energy transferred in the different cases is approximately the same.
This is because the transferred energy is calculated with respect to a full size tube, with a mass of 0,6 kg. The simulations are done until the PCM has fully changed state, not until the temperature is the same. Thus, the energy content would, in a full- scale simulation, be the same in both orientations, albeit the transfer rate would not be the same.
Figure 5.15: Plot of accumulated energy in relation to time in case 1:
Melting in horizontal component.
80 Figure 5.16: Plot of accumulated
energy in relation to time in case 2:
Solidification in horizontal component.
Figure 5.17: Plot of accumulated energy in relation to time in case 3:
Melting in vertical component.
Figure 5.18: Plot of accumulated energy in relation to time in case 4:
Solidification in vertical component.
5.5 Discussion
In this section, the different cases, their results and the effective heat transfer mechanisms will be discussed. As can be seen in figure 5.1, depicting case 1, the heat from the tube is transferred to the solid state PCM in the surrounding shell via conduction. The PCM closest to the tube then starts melting. As the melting continues and thus the liquid fraction increases, the buoyancy effect becomes more prominent. This leads to circulation of melted material. The heat transfer process is at this stage dominated by natural convection. This leads to an even
faster melting process due to the vortex formations, as can be seen in figure 5.6 approximately at time step 4,000. When all of the PCM above the tube has melted, the melting process slows down, also shown in figure 5.6, approximately at time step 9,000. This is because all of the remaining PCM is below the the tube, which makes the natural convection process less prominent. The melting process continues, though with less speed due to the conduction now dominating the process, until all of the PCM has melted. In case 2, depicted in figure 5.2, the PCM closest the tube starts to cool. When the PCM closest to the tube has solidified, though uneven due to convection, the heat transfer rate slows down. This slows the solidification process down, as can be seen in figure 5.8, where there is a significant change in the curve around time step 4,000. This occurs due to the fact that the transfer process is now dominated by conduction in the solid material. The solidification process then continues in this fashion until all of the PCM has solidified. In the vertical melting case, depicted in figure 5.3, the process is much alike case 1.
Heat is transferred from the tube to the solid PCM in the shell in form of conduction, which starts melting the PCM closest to the tube. Pretty quick, the buoyancy effect becomes prominent.
Liquid PCM with lower density starts rising to the top of the geometry, thus forming vortexes in the PCM. This increases the rate of melting, at this stage dominated by natural convection.
Because all of the PCM and the tube heating it, are on the same level, the convection continues with almost the same speed until all of the PCM has melted, unlike case 1. There is no significant decrease of the melting speed once the buoyancy effect becomes prominent, which can be seen in figure 5.10, which has approximately the same look through all of the process. The last case, case 4, vertical solidification, is much alike case 2. The PCM cools and the material closest to the tube solidifies.
The conduction in the solidified material slows the rate of solidification down, thus slowing the rate of the whole process down, which can be seen in figure 5.12, where a significant change of the
81 curvature can be seen at time step 2,000.
The process is at this stage dominated by conduction, and continues until all of the material has solidified. The heat transfer processes of melting and solidification yielded in the numerical analysis, are the processes expected from the studies in the introduction chapter, which suggests that the simulations are somewhat accurate. Though without actual real- worldexperiments, the predictions are less valuable and reliable.
6 CONCLUSION
Two different orientations of a shell and tube heat exchanger were compared and their performance studied during melting and solidification. The results from the numerical Analysis show that:
The melting time for the horizontal orientation is approximately 33,000 seconds and the melting time for the vertical orientation is about 11,000 seconds.
The solidification time for the horizontal orientation is approximately 55,000 seconds and the solidification time for the vertical orientation is about 29,000 seconds.
A vertical orientation of the heat exchanger induce a melting time about 200% faster than the horizontal orientation.
A vertical orientation of the heat exchanger induce a solidification time about 90% faster than the horizontal orientation.
The main reason for the difference of performance in time is the effect of gravity upon the heat exchanger, which induce a larger portion of natural convection.
LTES:s should be designed to maximise natural convection as the main heat transfer mechanism.
The models considered in this thesis are both unrealistic to use in practice, due to the long charging and discharging time. More efficient geometries should be explored.
7 FUTURE SCOPE
One of the drawbacks with the analysis here conducted is the simple geometry.
The simplified two- dimensional model is just that, a model, and a more thoroughly analysis, simulation and comparison of a
full-scale shell and tube heat exchanger in horizontal and vertical orientation is desired. There is a variety of other suggestions in the current research on how to improve charging and discharging performance of LTES, of many mentioned in the literature study. One on the most promising and common is a multi- ube heat exchanger. Such an exchanger in a vertical orientation should be examined.
It is not only the orientation and tube setup that should be considered, but as mentioned before, also which PCM is used, and research on new PCM:s.
Finally, to validate the results, a full scale live model of the setup here considered should be analyzed.
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