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Evaluation of water and paraffin PCM as storage media for use in thermal energy storage applications: A numerical approach
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Evaluation of Water and Paraffin PCM as Storage Media for use in Thermal Energy Storage Applications: A Numerical Approach Christos Pagkalos , George Dogkas , Maria K. Koukou , John Konstantaras , Kostas Lymperis , Michail Gr. Vrachopoulos PII: DOI: Reference:
S2666-2027(19)30006-0 https://doi.org/10.1016/j.ijft.2019.100006 IJTF 100006
To appear in:
International Journal of Thermofluids
Received date: Revised date: Accepted date:
3 October 2019 22 November 2019 22 November 2019
Please cite this article as: Christos Pagkalos , George Dogkas , Maria K. Koukou , John Konstantaras , Kostas Lymperis , Michail Gr. Vrachopoulos , Evaluation of Water and Paraffin PCM as Storage Media for use in Thermal Energy Storage Applications: A Numerical Approach, International Journal of Thermofluids (2019), doi: https://doi.org/10.1016/j.ijft.2019.100006
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Evaluation of Water and Paraffin PCM as Storage Media for use in Thermal Energy Storage Applications: A Numerical Approach Christos Pagkalos, George Dogkas, Maria K. Koukou*, John Konstantaras, Kostas Lymperis and Michail Gr. Vrachopoulos National and Kapodistrian University of Athens, General Department, Energy and Environmental Research Laboratory, 34400, Psachna Campus, Evia, Greece *Corresponding Author: email: [email protected]; [email protected]
Abstract Two different heat storage media, namely water and paraffin phase change material, are studied in order to evaluate and compare them for use in thermal energy storage systems. Using commercial computational fluid dynamics software, the charging process of the two materials is simulated and the results focus on the energy stored, the storage material temperature and heat transfer fluid outlet temperature. The geometric characteristics of the computational domain are selected in accordance with commercially used heat exchangers for thermal storage applications. Two different tube lengths and two different feed rates are used for the comparison of the charging process, while the heat transfer fluid temperature and the initial temperature of the components of the system are the same for all the simulations. Results show that for the same volume of storage mediums, the phase change material can store more energy than water, for the same temperature of the heat transfer fluid, as expected. As the feed rate is doubled, the same energy is stored in the media, but in approximately half the time. The simulation yielded an increase of the storage capacity of the examined system by approximately 4.1 times when phase change material is used instead of water and a significantly lower storage medium temperature change during the process. It is also shown that the stored energy is about 6.6 times greater for the 6.6 m long tube than the 1 m long tube and is proportional to the heat storage medium mass. However, the duration of the process is not proportional to the heat storage medium mass and when using water as heat storage medium, it almost doubles for a 6.6 times longer tube, while when using phase change material the duration increases approximately only 1.5 times.
Keywords: CFD, PCM, water, storage, latent heat, sensible heat
1
Nomenclature CFD HSM HTF LHS PCM SHS
Computational Fluid Dynamics Heat Storage Medium Heat Transfer Fluid Latent Heat Storage Phase Change Material Sensible Heat Storage
1 Introduction Today, the environmental sustainability challenge is one of the main concerns for the modern world. The depletion of fossil fuels and the environmental pollution are the driving forces for the technologies of the future. Among the ways to achieve environmental sustainability is the use and optimization of renewable energy. Energy storage is a crucial part of energy saving and power supply, as energy demand and energy availability often do not coincide in time. There are three main ways for storing thermal energy, sensible, latent and thermo-chemical energy storage [1,2]. In SHS, thermal energy is stored by raising the temperature of a material, typically solid or liquid. The LHS, can be achieved using PCMs, i.e. materials characterized by high latent heat of fusion, which through melting or solidification are able to store or provide heat respectively [3,4]. Finally, thermochemical storage is accomplished through a reversible chemical reaction and it has the advantage of nearly no losses during heat exchange and a high energy density. The LHS system is a superior way of storing thermal energy because of its high storage density and isothermal nature of the storage process. A comparison between latent and sensible heat storage shows that storage densities typically 5 to 10 times higher can be realized using latent heat storage units [5-7]. Researchers have reported comprehensive lists of possible candidates for latent heat storage covering a wide range of temperatures [8,9]. Many reviews between different phase change materials for different applications have been published [4-6] and the most recent advances on material aspects of PCMs can be found in [9]. In PCM systems for thermal storage, PCMs are mainly used as waste heat storage tanks for industrial processes, buildings applications, agriculture applications or even fuel cells [9]. PCMs are used according to their working temperature ranges (-20 °C to +200 °C or above). There are four different temperature ranges according to the application: low temperature range (-20 °C to +5 °C) where the PCMs are typically used for domestic and commercial refrigeration; low to medium temperature range (+5 °C to +40°C) where the PCMs are typically applied for heating and cooling applications in buildings; medium temperature range for solar based heating, hot water and electronic applications (+40°C to +80°C); and high temperature range (+80 °C to +200°C or above) for absorption cooling, waste heat recovery and electricity generation [7]. Comparison of the storage capacity between SHS and LHS systems has been conducted mainly on solar applications for domestic hot water production. Huang et al. examined with mathematical models the efficiency of a solar thermal energy storage system for domestic hot water production [10]. The system incorporated both a water tank and a PCM tank. The storage capacity of the system increases with the addition of the PCM tank. The models results yielded an increase of 30 % of the solar fraction when both tanks were used compared to the single water tank. However due to the low heat transfer rate of the PCM tank compared to the water tank, the presence of the latter was found to be necessary for completing a full solar charge of the system during daylight. The stored energy of the single water tank was either higher or lower than that of the combination of both water and PCM tank, depending on the area of the solar collectors and the diameter of the heat exchanger tubes. Castell et al. investigated experimentally two tanks of the same size [11]. One tank was filled only with water and the second tank was also filled with water but at the top, PCM was placed. The tank with the added PCM exhibited increased stored energy compared to the pure water tank. Prieto et al. have applied several models in order to study the performance of a heating microcogeneration system coupled with a thermal energy storage system [12]. Three thermal energy storage 2
systems were investigated: one with palmitic acid PCM, one with RT60 paraffin PCM and one with water. The system with palmitic acid exhibited longer charging and discharging duration than that with RT60 and RT60 in turn exhibited longer duration than the sensible heat system with water. In addition, higher heat transfer rate and accumulated energy were observed at the palmitic acid system than in the water system. Nkwetta et al. examined a domestic hot water tank with the use of TRNSYS software and compared a tank with water only and tanks with water and some PCMs [13]. The combined water and PCM tanks resulted to an improvement of the stored energy in comparison to the pure water tank. In addition, the bigger the amount of PCM the higher the stored energy according to their model. Canbazoglou et al. experimentally compared a conventional open-loop solar water heating system for domestic hot water production with a similar system which combined PCM [14]. The duration of solar charging, the mass of the produced hot water and the stored energy of the combined system were theoretically calculated to be 2.6–3.5 times higher than those of the conventional system when specific PCM types were used. Moreover, the combined system exhibited more stable temperature and it weighted less than the system using with only water in the tank. In this work, the performance of two different HSMs is evaluated, through CFD analysis. The computational domain considered, is a finned tube, immersed in the storage medium (water or PCM) through which hot water flows, in order to charge the system. The fins are used to make the area of the heat transfer larger and the dimensions take into account commercially available heat exchanger patterns [15,16]. The fins are considered to be from aluminum and the tube from copper. The domain represents a single tube inside a heat exchanger and the dimensions correspond to the Luvata pattern 1022 [16] one of the commercially used heat exchangers for heating /cooling or domestic hot water applications. The volume of the storage medium depends on the length and the radius of the tube as well as the number of fins.
2 Sensible and latent heat storage mechanisms 2.1 Sensible heat storage
SHS systems utilize the heat capacity during the process of charging or discharging. The temperature of the storage material rises when energy is absorbed and drops when energy is dissipated (Fig. 1). In SHS systems, the amount of energy stored depends on the specific heat capacity of the medium, the temperature difference from start till the end of the process and the mass of the medium. Heat from a HTF is transferred through the tube and fins to the storage medium increasing its temperature. The amount of heat stored in or released form a material can be described by Eq. (1) which when solved provides Eq (2). ∫
(1) (2)
where Q [J] is the amount of thermal energy stored (or released), Ti [K] is the initial temperature of medium, Tf [ K] is the final temperature of the medium, m [kg] is the mass of the material used to store thermal energy and Cp [J/kg∙K] is the specific heat capacity of the material used to store thermal energy. SHS often use solids like stone or brick, or liquids like water, as storage materials. The higher the specific heat and density of the material, the more energy will be stored in a given volume of the material. However, there are several other parameters which also affect the performance of the system, the temperature of operation, the thermal conductivity, the thermal diffusivity, the compatibility between the storage material and the container, the stability of the material at the highest temperature of the cyclic operation and of course the cost of the system. One of the most attractive features of sensible heat storage systems is that charging and discharging operations can be expected to be completely reversible for an unlimited number of cycles [1,2].
3
Figure 1. Temperature and stored heat proportionality in the case of sensible heat process.
2.2 Latent heat storage
LHS is based on the phase transition of a material. PCMs have considerably higher thermal energy storage density compared to SHS materials and are able to absorb or release large quantities of energy, i.e. latent heat, at constant temperature undergoing a change of phase. The materials may also store energy due to sensible heat due to temperature changes at the beginning and the end of a process like in Fig. 2. However, such energy storage is small compared to latent heat of phase transformation. LHS is based on heat absorption or release when a storage material undergoes a phase change from solid to liquid or liquid to gas and vice versa. The storage capacity of the LHS system with a PCM medium is calculated by Eq. (3) which is integrated to Eq. (4): ∫
∫
(3) (
)
(4)
where Tm [K] is the melting temperature, cp,s [J/kg∙K] is the specific heat capacity of solid PCM, cp,l [J/kg∙K] is the specific heat capacity of liquid PCM, β is the dimensionless fraction of PCM melted and Δhm [J/kg] is the heat of fusion per mass unit. The first term in Eq. (4) represents the heat transfer between the storage material and the HTF before the PCM melts. The second term represents the energy transfer during phase change and the last term represents the sensible energy transferred between the PCM and the HTF after the PCM has melted. When the PCM changes phase, a large amount of energy, i.e. the latent heat, can be stored or released at an almost constant temperature [9]. Thus, a small difference in temperature can be used for storing and releasing the stored energy. The system with PCM depends on the phase change of the material for harvesting and releasing the energy. Heat storage through phase change has the advantage of compactness since the latent heat values of most materials are large. It has the added advantage of heat supply at constant temperature. The various phase changes that can be found are melting, evaporation, lattice change and crystal bound water content. Phase change of a certain material can be in the following form: solid– solid, solid– liquid, solid– gas, liquid– gas and vice versa [3,4,5]. In solid–solid transitions, heat is stored as the material is transformed from one crystalline to another. These transitions generally have smaller latent heat and smaller volume changes than solid–liquid transitions. Solid–solid PCMs offer the advantages of less stringent container requirements and greater design flexibility. Solid–liquid transformations have comparatively smaller latent heat than liquid–gas. However, these transformations involve only a small change in volume (about 10 % or less). Therefore, solid–liquid transitions have proved to be economically attractive for use in thermal energy storage systems.
4
However, not all PCMs can be used for thermal storage. An ideal PCM candidate should fulfil a number of criteria such as: high heat of fusion and thermal conductivity, high specific heat capacity, small volume change, non-corrosiveness, non-toxicity and exhibit little or no decomposition or supercooling [9]. There is a large number of organic and inorganic PCM that meet the required thermodynamic criteria for operation in the desired temperature range of 0-140 oC, but many of them cannot be used because of problems of chemical stability, toxicity, corrosion, volume change, availability at reasonable price, etc. There is a vast number of materials with published properties available for latent heat storage [1,15,25].
Figure 2. Temperature and stored heat relationship in the case of combined sensible and latent heat process
3. CFD model setup The CFD simulated domain was defined based on a real, small scale experimental rig which utilized a finned-tube heat exchanger immersed in PCM or water [17]. In Fig. 3 the heat exchanger is immersed in paraffin PCM. The rig was built to study the heat transfer phenomena taking place is such systems assisting the development of real scale devices for thermal energy storage applications [17,18,19]. The total length of the heat exchangers tube was 6.6 m. The problem of paraffin phase change conjugated to conduction in the tube wall and to the forced convection of the HTF is unsteady. The simplifying assumptions, which the mathematical model relies on, for the description of the physics of the problem are: the PCM is homogeneous and isotropic; natural convection of the PCM is neglected; azimuthal temperature is everywhere negligible (2D solution) [20]. The enthalpy-porosity technique [21,22,23] is used, for modeling the solidification/melting process.
Figure 3. Photograph of the experimental thermal energy storage small rig.
5
For the simulations, the commercial CFD solver ANSYS Fluent was used. The parameters that were investigated were: the energy stored inside the media, the outlet HTF temperature and the temperature of the storage medium. A heat exchanger working with water as the HTF is used to transfer energy from the hot water to the PCM. The most widely used HTF is pure water. Many different fluids have been applied over the years. Propylene glycol-water mix, ethylene glycol water mix, synthetic oil and silicone oil are some examples [24]. The geometry selected is that of a commercial heat exchanger, in order to examine the behavior of the storage media, in use with commercial heat exchangers. A 2D axisymmetric computational domain was created with the characteristics shown in Table 1 and it is depicted in Fig. 4. The simulations were conducted for either 1 m or 6.6 m tube length and for either a laminar flow at 30 l/hr or a turbulent at 60 l/hr. For all cases, PCM and water were tested as HSMs. One-meter long tube cases offer short simulation time and they are adequate for the full development of the flow which takes about 0.15 m. The long tube on the other hand, has a length (6.6 m) equal to the circuit length of the experimental rig illustrated in Fig. 3 for future comparison of CFD and experimental results. The flow rates were the same as those applied in a preceding work [17] and they allow the comparison of process duration between a laminar and a turbulent case. Table 1. Properties of the 2D axisymmetric computational domain
Circuits configuration Inner tube diameter (mm) Outer tube diameter (mm) Tube material Fins material Radius of the fins (mm) Fins Pitch (mm) Fins thickness (mm) Heat storage medium volume (m3) 1 m long tube Heat storage medium volume (m3) 6.6 m long tube
Single tube 1 or 6.6 m long 8.6 10 Copper Aluminium 13.8 5.0 0.3 0.0008804 0.0058107
Figure 4. Part of the axisymmetric computational domain.
For both tube lengths (1 and 6.6 m) and both flow rates (30 and 60 l/hr) the computational mesh form was the same and it was created using ANSYS software (Fig. 5). The details of the mesh are shown in Figure 5. The majority of the cells has rectangular shape and the mesh is inflated near the 6
water-tube interface. The mesh selected has 81200 elements for the 1 m tube and 487200 elements for the 6.6 m tube.
Figure 5. Computational grid used in the simulations.
Regarding boundary conditions constant velocity applied at the inlet of the tube and for the outlet, constant pressure was defined. The outer walls of the entire domain are considered adiabatic. The solidification /melting model is enabled and for the mushy zone parameter the default value was selected. Finally, according to the flow rate value, the laminar model was applied for 30 l/hr and the k-ε turbulent for 60 l/hr. The material properties used in the simulations are shown in the Table 2. Unlike SHS systems which operate in a broad range of temperatures, LHS systems operate in a much narrower temperature range. It is within this narrow range that the change of phase of a material has to be completed. Therefore, the melting point of the PCM has to be matched to the operating temperature of the system. The PCM selected for the simulations is A44 PCM from PCM PRODUCTS Ltd [25] which has a nominal melting point of 44 oC which matches the operating temperature range of the system. For the PCM two different values for density exist one for solid phase and one for the liquid. In our simulation only the liquid density is used in order to make a simpler, less computational expensive model. Moreover, the defined inlet temperature for the HTF is 52 oC and the initial temperature of the fins, tube, storage medium and stagnant water in the tube is 39 o C. Table 2. Material properties used in the simulations. Material Thermal conductivity [W/m∙K] Density [kg/m3] Specific Heat [J/kg∙K] Viscosity [kg/m∙s] Melting heat of fusion [kJ/kg] Solidus temperature [oC] Liquidus temperature [oC] s –solid; l – liquid
Water 0.6 998.2 4182 0.001003 -
4 Results and discussion 4.1 Results from the 1 m tube 7
PCM A44 [25] 0.24 912 (s), 775 (l) 2400 (s), 1800 (l) 0.007 (l) 250 42.85 45.85
Copper 387.6 8978 381 -
Aluminium 218 2719 871 -
For the 1 m long geometry the mass of PCM is 0.68 kg and the mass of HSM water is 0.89 kg, so there is more mass available for the HSM water for energy storage. However, the latent heat of fusion of the PCM has also a strong impact on the store energy ability of the system. In Figures 6-9 PCM and HSM water are compared for the two different HTF flow rate values. The process ends when the HTF outlet temperature becomes equal to the inlet temperature. In Fig. 6 the HSM average temperature is plotted against time, yielding a sharp increase of the water temperature which finally becomes equal to the 52 oC of HTF inlet temperature. On the other hand when PCM is used, the temperature of the medium increases sharply before and after the phase change region like in Fig. 2. As the process of transferring heat from the HTF to the system begins, part of the PCM absorbs heat and changes phase while the remaining is still in solid phase. As the process progresses, the PCM that already has melted absorbs energy by means of sensible heat, increasing its temperature, while the remaining starts melting. So, the energy storing process combines both sensible and latent heat. It can be seen from Fig. 6, the part of the combined stored energy is far greater than the sensible only part, which is at the start and the end of the process. Between the phase change region the rate of temperature increase is low and it is defined by the change of PCM properties with the temperature and the liquid fraction every time. Because the HSM exhibits different conditions along the length of the tube, the average HSM temperature is an indicative statistical value of the condition of the HSM. Furthermore, for both materials, the duration of the process is longer for the 30 l/hr flow rate compared to 60 l/hr. Results of the stored energy value and the duration of the process for all cases of the 1 m long tube are presented in Table 3. There is a significant difference in the stored energy between water and PCM. Similar increase in the storage capacity of thermal energy storage tanks with the addition of PCM into the water volume, has been observed both theoretically [12,13] and experimentally [11,14]. Table 3. Stored energy and process duration for PCM and HSM water for 1 m long tube at both flow rates.
Stored energy [kWh] – 30 l/hr Stored energy [kWh] – 60 l/hr Duration [hr] – 30 l/hr Duration [hr] – 60 l/hr
HSM Water
PCM A44
0.014 0.015 0.55 0.29
0.059 0.061 2.17 1.11
The stored energy in the LHS system increases rapidly for all 4 cases, but for the SHS this increase ends soon. In contrast, stored energy continues to increase when PCM is used and only near the end of the process the increase rate diminishes. Considering the flow rate effect on the stored energy increase rate, it is obvious from Fig. 7 that the higher the flow rate, the faster the energy is stored. Considering the HSM temperature as a function of the stored energy shown in Fig. 8, as expected the function is linear for the sensible storage medium. Any small differences between the two flow rates for HSM water are attributed to laminar and turbulent model small differences in convergence criteria and to the heat capacity of the tube and the fins which create a minor thermal inertia effect. For the PCM, the two flow rate values have also some small differences which are explained by the same reasons referred at the discussion of Fig. 6. The HTF outlet temperature as a function of the stored energy is depicted in Fig. 9. At all 4 cases when the process ends, the HTF water exits the tube at the same temperature as it enters because there is no more temperature gradient between the water inside the tube and the HSM. When water is used as HSM, the HTF outlet temperature is the same for both flow rates. When PCM is used as HSM, the HTF does not have the time to fully exchange heat with the PCM and for the fast flow rate of 60 l/hr the temperature of the outlet water is lower than for the slower flow rate. This is true at the first part of the process. At the remaining part of the process, the outlet temperature of the fast flow becomes higher than the slow flow. Moreover, independently of the flow rate, with the use of PCM as HSM, the outlet HTF temperature is more constant compared with the use of HSM water as the system is charged with energy. This stable outlet temperature is beneficial for the sizing of the system equipment in actual installations [18,19,26]
8
0.07
324
0.06
Stored energy [kWh]
Temperature [K]
326
322 320 318 316
PCM 1m 30 l/h Water 1m 30 l/h PCM 1m 60 l/h Water 1m 60 l/h
314 312 310 0
1000
2000
3000 4000 Time [s]
5000
6000
0.04 0.03 PCM 1m 30 l/h Water 1m 30 l/h PCM 1m 60 l/h Water 1m 60 l/h
0.02 0.01 0
7000
Figure 6. Storage medium average temperature vs time of the 1 m long tube for 30 and 60 l/hr flow rate. 326
0
1000
2000
3000 4000 Time [s]
5000
6000
7000
Figure 7. Stored energy vs time of the 1 m long tube for 30 and 60 l/hr flow rate. 325
324 Temperature [K]
322 Temperature [K]
0.05
320 318 316
PCM 1m 30 l/h Water 1m 30 l/h PCM 1m 60 l/h Water 1m 60 l/h
314 312 310 0
0.02
0.04 0.06 Stored energy [kWh]
323 PCM 1m 30 l/h Water 1m 30 l/h PCM 1m 60 l/h Water 1m 60 l/h
322 321
0.08
Figure 8. Storage medium average temperature vs stored energy of the 1 m long tube for 30 and 60 l/h flow rate.
324
0
0.02
0.04 0.06 Stored energy [kWh]
0.08
Figure 9. Outlet water temperature vs stored energy of the 1 m long tube for 30 and 60 l/h flow rate.
4.2 Results from the 6.6 m tube
For the 6.6 m long geometry the mass of PCM is 4.5 kg and the mass of HSM water is 5.8 kg. In Fig. 10-13 the results of the CFD simulation are presented for the case of the 6.6 long tube. As expected, the temporal evolution of the HSM temperature and the stored energy is similar with the 1 m long tube geometry. Again, the outlet temperature is more stable during the process when PCM is used instead of water as it was also experimentally shown by [14]. Furthermore, the relationship of the HSM and the outlet temperature with the stored energy is also similar with the 1 m long tube geometry. The duration of the process and the stored energy are larger in the 6.6 m long tube simulation as expected and they are listed in Table 4.
9
326
0.5 Stored energy [kWh]
Temperature [K]
324 322 320 318 316
PCM 6.6 m 30 l/h Water 6.6m 30 l/h PCM 6.6 m 60 l/h
314 312 0
5000
10000
0.3 0.2
PCM 6.6 m 30 l/h Water 6.6m 30 l/h
0.1
Water 6.6m 60 l/h
310
0.4
PCM 6.6 m 60 l/h Water 6.6m 60 l/h
0 0
15000
5000
Time [s]
324
324
322
322
320 318 316
PCM 6.6 m 30 l/h
314
Water 6.6 m 30 l/h
312
PCM 6.6 m 60 l/h
0.1
0.2 0.3 Stored energy [kWh]
0.4
320 318 316
PCM 6.6 m 30 l/h Water 6.6m 30 l/h PCM 6.6 m 60 l/h Water 6.6m 60 l/h
314 312
Water 6.6 m 60 l/h 0
15000
Figure 11. Stored energy vs time of the 6.6 m long tube for 30 and 60 l/hr flow rate. 326
Temperature [K]
Temperature [K]
Figure 10. Storage medium average temperature vs time of the 6.6 m long tube for 30 and 60 l/hr flow rate. 326
310
10000 Time [s]
310 0
0.5
Figure 12. Storage medium average temperature vs stored energy of the 6.6 m long tube for 30 and 60 l/h flow rate.
0.1
0.2
0.3
0.4
0.5
Stored energy [KWh] Figure 13. Outlet water temperature vs stored energy of the 6.6 m long tube for 30 and 60 l/h flow rate.
Table 4. Stored energy and process duration for PCM and HSM water for 6.6 m long tube at both flow rates.
Stored energy [kWh] – 30 l/hr Stored energy [kWh] – 60 l/hr Duration [hr] – 30 l/hr Duration [hr] – 60 l/hr
HSM Water 0.09 0.11 1.11 0.55
PCM A44 0.39 0.41 3.33 1.69
4.3 Comparison between the two tube lengths
The energy that is stored inside the LHS system is expected to be directly proportional to the mass of the storage medium. As it can be seen in Table 5, the stored energy is about 6.6 times greater for the 6.6 m long tube than the 1 m long. Similar increase of the stored energy with the PCM has been derived by the model of [13]. However, the duration of the process is not proportional to the HSM mass. For HSM water, it almost doubles when the length becomes 6.6 larger, while for PCM it increases only approximately 1.5 times respectively. Therefore, if a fast process is required, it is favorable to use a longer tube configuration in order to utilize its greater ability to store energy.
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Table 5. Comparison between the 1 m and 6.6 m long tube geometry in terms of stored energy and process duration for PCM and HSM water at both flow rates.
Stored energy ratio 6.6 to 1 m 30 l/hr Stored energy ratio 6.6 to 1 m 60 l/hr Duration ratio 6.6 to 1 m 30 l/hr Duration ratio 6.6 to 1 m 60 l/hr
HSM Water 6.59
PCM A44 6.60
6.66
6.67
2.00
1.54
1.90
1.52
5 Conclusions In this work a CFD comparison between a sensible heat storage medium (water) and a latent heat storage medium (organic paraffin PCM A44) has been conducted. As it was expected using PCM as a storage medium a greater amount of stored energy for the same volume of storage medium is accomplished, but longer time period is needed compared to the use of water as storage medium for storing the energy. It was found that PCM can store 4.1 times more energy than water for the same volume of storage tank. The duration of the charging process with PCM was found to be approximately 3.9 or 3.0 times longer than with water for the same volume of storage tank, depending on the tube length. In terms of temperature stability, the CFD simulation yielded that the PCM system stores most of its energy while the average PCM temperature changes no more than 5 oC. In contrast, water has to raise its temperature by about 54 oC in order to store the same amount of energy as PCM A44. Comparing a laminar flow at 30 l/hr and a turbulent flow at 60 l/hr, the heat transfer rate results to be higher at the faster flow rate. However, the amount of energy that is stored does not change with the flow rate and this is true for both heat storage media. The process-average heat transfer rate ratio of the turbulent to the laminar is higher for PCM than for water. Additionally, this ratio increases with the increase of the tube length. In particular, for the PCM the process-average heat transfer rate is 53 and 72 % higher for the turbulent flow compared to the laminar for 1 m and 6.6 m long tube respectively. For the water, it is 14 and 31 % higher for the turbulent flow respectively. Finally, comparing a heat exchanger with short tube (1 m) and a longer one (6.6 m), it is concluded that the stored energy is directly proportional to the mass of the medium and thus the length of the tube. On the other hand, with the utilization of the longer tube, the duration of the process increases, but less than 6.6 times. For water the increase is about 2 times and for PCM about 1.5 time.
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[5] M.M. Farid, A.M. Khudhair, A.K. Siddique, A. Sari., A review on phase change energy storage: materials and applications, Energy Conversion and Management, 45, 1597-1615 (2004). [6] A. Khyad, H. Samrani, M. N. Bargach, State of the art review of thermal energy storage systems using PCM operating with small temperature differences: Focus on Paraffin, J. Mater. Environ. Sci. 7 (4) 1184-1192 (2016). [7]. Kun Du John Calautit Zhonghua Wang Yupeng Wu Hao Liu, A review of the applications of phase change materials in cooling, heating and power generation in different temperature ranges, Applied Energy, 220, 242-273 (2018). [8] M. Mofijur, T.M. Mahlia, A.S. Silitonga, H.C. Ong, M. Silakhori, M.H. Hasan, N. Putra and S.M. Ashrafur Rahman, Phase Change Materials (PCM) for Solar Energy Usages and Storage: An Overview, Energies, 12, 3167, 1-20 (2019). [9] C. Elias, V.N. Stathopoulos, A comprehensive review of recent advances in materials aspects of phase change materials in thermal energy storage, Energy Procedia, 161, 385-394 (2019). [10] H. Huang, Y. Xiao, J. Lin, T. Zhou and Y. Liu, "Improvement of the Efficiency of Solar Thermal Energy Storage Systems by Cascading a PCM Unit with a Water Tank," Journal of Cleaner Production, pp. 1-14, 2019. [11] A. Castell, C. Sole, M. Medrano, M. Nogues and L. Cabeza, "Comparison of Stratification in a Water Tank and a PCM-Water Tank," Journal of Solar Energy Engineering, vol. 131, pp. 1-5, 2009. [12] M. Prieto, B. Gonzalez and E. Granado, "Thermal Performance of a Heating System Working with a PCM Plate Heat Exchanger and Comparison with a Water Tank," Energy and Buildings, vol. 122, pp. 89-97, 2016. [13] D. N. Nkwetta, P.-E. Vouillamoz, F. Haghighat, M. El-Mankibi, A. Moreau and A. Daoud, "impact of Phase Change Materials Types and Positioning on Hot Water Tank Thermal Performance: Using Measured Water Demand Profile," Applied Thermla Engineering, vol. 67, pp. 460-468, 2014. [14] S. Cambazoglu, A. Sahinaslan, A. Ekmekyapar, G. Aksoy and F. Akarsu, "Enhancement of Solar Thermal Energy Storage Performance Using Sodium Thiosulfate Pentahydrate of a Conventional Solar Water-Heating System," Energy and Buildings, vol. 37, pp. 235-242, 2005. [15] C. Pagkalos, M.Gr. Vrachopoulos, J. Konstantaras, K. Lymperis, Comparing water and paraffin PCM as storage mediums for thermal energy storage applications, in Proceedings of International Conference on Advances in Energy Systems and Environmental Engineering (ASEE19), Wroclaw, Poland, 09-12.06.2019. Available in: E3S Web of Conferences 116, 00057 (2019) Open Access. [16] Luvata coils catalogue [17] M.K. Koukou, M.Gr. Vrachopoulos, N. S. Tachos, G. Dogkas, K. Lymperis, V. Stathopoulos, Experimental and computational investigation of a latent heat energy storage system with a staggered heat exchanger for various phase change materials”, Thermal Science and Engineering Progress 7, 87–98, (2018) Open Access. [18] M.K. Koukou, M.Gr. Vrachopoulos, G. Dogkas, C. Pagkalos, K. Lymperis, L. Coelho, and A. Rebola, Testing the performance of a prototype thermal energy storage tank working with organic phase change material for space heating application conditions, in Proceedings of International Conference on Advances in Energy Systems and Environmental Engineering (ASEE19), Wroclaw, Poland, 09-12.06.2019. Available in: E3S Web of Conferences 116, 00038 (2019) Open Access. [19] G. Dogkas, J. Konstantaras, M.K. Koukou, V. N. Stathopoulos, L. Coelho, and A. Rebola, Evaluating a Prototype Compact Thermal Energy Storage Tank Using Paraffin-based Phase Change Material for Domestic Hot Water Production, in Proceedings of International Conference on Advances in Energy Systems and Environmental Engineering (ASEE19), Wroclaw, Poland, 09-12.06.2019. Available in: E3S Web of Conferences 116, 00016 (2019) Open Access.
[20] FLUENT 15.0 Theory Guide [21] V.R. Voller, Modeling Solidification Processes, Technical report. Mathematical Modeling of Metals Processing Operations Conference, Palm Desert, CA American Metallurgical Society (1987). 12
[22] V.R. Voller, A.D. Brent, C. Prakash, The modeling of heat, mass and solute transport in solidification systems, Int. J. Heat Mass Transf., 32, 9, 1719-1719 (1989). [23] V.R. Voller, C. Prakash, A Fixed-Grid Numerical Modeling Methodology for Convection-Diffusion Mushy Region Phase-Change Problems, Int. J. Heat Mass Transfer., 30, 1709-1720 (1987). [24] B. Ramlow, B. Nusz, Solar water heating: a comprehensive guide to solar water and space heating systems. Books for wiser living, 2006, Gabriola Island, B.C.: New Society Publishers. XV [25] http://www.pcmproducts.net/," PCM Products Ltd., [Online]. Available: http://www.pcmproducts.net/. [Accessed September 2019]. [26] Tesse2b project deliverable 8.5: Training material, [Online]. http://www.tesse2b.eu/Content/images/tesse2b/Training_Material.pdf. [Accessed July 2019].
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Available:
Evaluation of Water and Paraffin PCM as Storage Media for use in Thermal Energy Storage Applications: A Numerical Approach Christos Pagkalos , George Dogkas , Maria K. Koukou , John Konstantaras , Kostas Lymperis , Michail Gr. Vrachopoulos PII: DOI: Reference:
S2666-2027(19)30006-0 https://doi.org/10.1016/j.ijft.2019.100006 IJTF 100006
To appear in:
International Journal of Thermofluids
Received date: Revised date: Accepted date:
3 October 2019 22 November 2019 22 November 2019
Please cite this article as: Christos Pagkalos , George Dogkas , Maria K. Koukou , John Konstantaras , Kostas Lymperis , Michail Gr. Vrachopoulos , Evaluation of Water and Paraffin PCM as Storage Media for use in Thermal Energy Storage Applications: A Numerical Approach, International Journal of Thermofluids (2019), doi: https://doi.org/10.1016/j.ijft.2019.100006
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Evaluation of Water and Paraffin PCM as Storage Media for use in Thermal Energy Storage Applications: A Numerical Approach Christos Pagkalos, George Dogkas, Maria K. Koukou*, John Konstantaras, Kostas Lymperis and Michail Gr. Vrachopoulos National and Kapodistrian University of Athens, General Department, Energy and Environmental Research Laboratory, 34400, Psachna Campus, Evia, Greece *Corresponding Author: email: [email protected]; [email protected]
Abstract Two different heat storage media, namely water and paraffin phase change material, are studied in order to evaluate and compare them for use in thermal energy storage systems. Using commercial computational fluid dynamics software, the charging process of the two materials is simulated and the results focus on the energy stored, the storage material temperature and heat transfer fluid outlet temperature. The geometric characteristics of the computational domain are selected in accordance with commercially used heat exchangers for thermal storage applications. Two different tube lengths and two different feed rates are used for the comparison of the charging process, while the heat transfer fluid temperature and the initial temperature of the components of the system are the same for all the simulations. Results show that for the same volume of storage mediums, the phase change material can store more energy than water, for the same temperature of the heat transfer fluid, as expected. As the feed rate is doubled, the same energy is stored in the media, but in approximately half the time. The simulation yielded an increase of the storage capacity of the examined system by approximately 4.1 times when phase change material is used instead of water and a significantly lower storage medium temperature change during the process. It is also shown that the stored energy is about 6.6 times greater for the 6.6 m long tube than the 1 m long tube and is proportional to the heat storage medium mass. However, the duration of the process is not proportional to the heat storage medium mass and when using water as heat storage medium, it almost doubles for a 6.6 times longer tube, while when using phase change material the duration increases approximately only 1.5 times.
Keywords: CFD, PCM, water, storage, latent heat, sensible heat
1
Nomenclature CFD HSM HTF LHS PCM SHS
Computational Fluid Dynamics Heat Storage Medium Heat Transfer Fluid Latent Heat Storage Phase Change Material Sensible Heat Storage
1 Introduction Today, the environmental sustainability challenge is one of the main concerns for the modern world. The depletion of fossil fuels and the environmental pollution are the driving forces for the technologies of the future. Among the ways to achieve environmental sustainability is the use and optimization of renewable energy. Energy storage is a crucial part of energy saving and power supply, as energy demand and energy availability often do not coincide in time. There are three main ways for storing thermal energy, sensible, latent and thermo-chemical energy storage [1,2]. In SHS, thermal energy is stored by raising the temperature of a material, typically solid or liquid. The LHS, can be achieved using PCMs, i.e. materials characterized by high latent heat of fusion, which through melting or solidification are able to store or provide heat respectively [3,4]. Finally, thermochemical storage is accomplished through a reversible chemical reaction and it has the advantage of nearly no losses during heat exchange and a high energy density. The LHS system is a superior way of storing thermal energy because of its high storage density and isothermal nature of the storage process. A comparison between latent and sensible heat storage shows that storage densities typically 5 to 10 times higher can be realized using latent heat storage units [5-7]. Researchers have reported comprehensive lists of possible candidates for latent heat storage covering a wide range of temperatures [8,9]. Many reviews between different phase change materials for different applications have been published [4-6] and the most recent advances on material aspects of PCMs can be found in [9]. In PCM systems for thermal storage, PCMs are mainly used as waste heat storage tanks for industrial processes, buildings applications, agriculture applications or even fuel cells [9]. PCMs are used according to their working temperature ranges (-20 °C to +200 °C or above). There are four different temperature ranges according to the application: low temperature range (-20 °C to +5 °C) where the PCMs are typically used for domestic and commercial refrigeration; low to medium temperature range (+5 °C to +40°C) where the PCMs are typically applied for heating and cooling applications in buildings; medium temperature range for solar based heating, hot water and electronic applications (+40°C to +80°C); and high temperature range (+80 °C to +200°C or above) for absorption cooling, waste heat recovery and electricity generation [7]. Comparison of the storage capacity between SHS and LHS systems has been conducted mainly on solar applications for domestic hot water production. Huang et al. examined with mathematical models the efficiency of a solar thermal energy storage system for domestic hot water production [10]. The system incorporated both a water tank and a PCM tank. The storage capacity of the system increases with the addition of the PCM tank. The models results yielded an increase of 30 % of the solar fraction when both tanks were used compared to the single water tank. However due to the low heat transfer rate of the PCM tank compared to the water tank, the presence of the latter was found to be necessary for completing a full solar charge of the system during daylight. The stored energy of the single water tank was either higher or lower than that of the combination of both water and PCM tank, depending on the area of the solar collectors and the diameter of the heat exchanger tubes. Castell et al. investigated experimentally two tanks of the same size [11]. One tank was filled only with water and the second tank was also filled with water but at the top, PCM was placed. The tank with the added PCM exhibited increased stored energy compared to the pure water tank. Prieto et al. have applied several models in order to study the performance of a heating microcogeneration system coupled with a thermal energy storage system [12]. Three thermal energy storage 2
systems were investigated: one with palmitic acid PCM, one with RT60 paraffin PCM and one with water. The system with palmitic acid exhibited longer charging and discharging duration than that with RT60 and RT60 in turn exhibited longer duration than the sensible heat system with water. In addition, higher heat transfer rate and accumulated energy were observed at the palmitic acid system than in the water system. Nkwetta et al. examined a domestic hot water tank with the use of TRNSYS software and compared a tank with water only and tanks with water and some PCMs [13]. The combined water and PCM tanks resulted to an improvement of the stored energy in comparison to the pure water tank. In addition, the bigger the amount of PCM the higher the stored energy according to their model. Canbazoglou et al. experimentally compared a conventional open-loop solar water heating system for domestic hot water production with a similar system which combined PCM [14]. The duration of solar charging, the mass of the produced hot water and the stored energy of the combined system were theoretically calculated to be 2.6–3.5 times higher than those of the conventional system when specific PCM types were used. Moreover, the combined system exhibited more stable temperature and it weighted less than the system using with only water in the tank. In this work, the performance of two different HSMs is evaluated, through CFD analysis. The computational domain considered, is a finned tube, immersed in the storage medium (water or PCM) through which hot water flows, in order to charge the system. The fins are used to make the area of the heat transfer larger and the dimensions take into account commercially available heat exchanger patterns [15,16]. The fins are considered to be from aluminum and the tube from copper. The domain represents a single tube inside a heat exchanger and the dimensions correspond to the Luvata pattern 1022 [16] one of the commercially used heat exchangers for heating /cooling or domestic hot water applications. The volume of the storage medium depends on the length and the radius of the tube as well as the number of fins.
2 Sensible and latent heat storage mechanisms 2.1 Sensible heat storage
SHS systems utilize the heat capacity during the process of charging or discharging. The temperature of the storage material rises when energy is absorbed and drops when energy is dissipated (Fig. 1). In SHS systems, the amount of energy stored depends on the specific heat capacity of the medium, the temperature difference from start till the end of the process and the mass of the medium. Heat from a HTF is transferred through the tube and fins to the storage medium increasing its temperature. The amount of heat stored in or released form a material can be described by Eq. (1) which when solved provides Eq (2). ∫
(1) (2)
where Q [J] is the amount of thermal energy stored (or released), Ti [K] is the initial temperature of medium, Tf [ K] is the final temperature of the medium, m [kg] is the mass of the material used to store thermal energy and Cp [J/kg∙K] is the specific heat capacity of the material used to store thermal energy. SHS often use solids like stone or brick, or liquids like water, as storage materials. The higher the specific heat and density of the material, the more energy will be stored in a given volume of the material. However, there are several other parameters which also affect the performance of the system, the temperature of operation, the thermal conductivity, the thermal diffusivity, the compatibility between the storage material and the container, the stability of the material at the highest temperature of the cyclic operation and of course the cost of the system. One of the most attractive features of sensible heat storage systems is that charging and discharging operations can be expected to be completely reversible for an unlimited number of cycles [1,2].
3
Figure 1. Temperature and stored heat proportionality in the case of sensible heat process.
2.2 Latent heat storage
LHS is based on the phase transition of a material. PCMs have considerably higher thermal energy storage density compared to SHS materials and are able to absorb or release large quantities of energy, i.e. latent heat, at constant temperature undergoing a change of phase. The materials may also store energy due to sensible heat due to temperature changes at the beginning and the end of a process like in Fig. 2. However, such energy storage is small compared to latent heat of phase transformation. LHS is based on heat absorption or release when a storage material undergoes a phase change from solid to liquid or liquid to gas and vice versa. The storage capacity of the LHS system with a PCM medium is calculated by Eq. (3) which is integrated to Eq. (4): ∫
∫
(3) (
)
(4)
where Tm [K] is the melting temperature, cp,s [J/kg∙K] is the specific heat capacity of solid PCM, cp,l [J/kg∙K] is the specific heat capacity of liquid PCM, β is the dimensionless fraction of PCM melted and Δhm [J/kg] is the heat of fusion per mass unit. The first term in Eq. (4) represents the heat transfer between the storage material and the HTF before the PCM melts. The second term represents the energy transfer during phase change and the last term represents the sensible energy transferred between the PCM and the HTF after the PCM has melted. When the PCM changes phase, a large amount of energy, i.e. the latent heat, can be stored or released at an almost constant temperature [9]. Thus, a small difference in temperature can be used for storing and releasing the stored energy. The system with PCM depends on the phase change of the material for harvesting and releasing the energy. Heat storage through phase change has the advantage of compactness since the latent heat values of most materials are large. It has the added advantage of heat supply at constant temperature. The various phase changes that can be found are melting, evaporation, lattice change and crystal bound water content. Phase change of a certain material can be in the following form: solid– solid, solid– liquid, solid– gas, liquid– gas and vice versa [3,4,5]. In solid–solid transitions, heat is stored as the material is transformed from one crystalline to another. These transitions generally have smaller latent heat and smaller volume changes than solid–liquid transitions. Solid–solid PCMs offer the advantages of less stringent container requirements and greater design flexibility. Solid–liquid transformations have comparatively smaller latent heat than liquid–gas. However, these transformations involve only a small change in volume (about 10 % or less). Therefore, solid–liquid transitions have proved to be economically attractive for use in thermal energy storage systems.
4
However, not all PCMs can be used for thermal storage. An ideal PCM candidate should fulfil a number of criteria such as: high heat of fusion and thermal conductivity, high specific heat capacity, small volume change, non-corrosiveness, non-toxicity and exhibit little or no decomposition or supercooling [9]. There is a large number of organic and inorganic PCM that meet the required thermodynamic criteria for operation in the desired temperature range of 0-140 oC, but many of them cannot be used because of problems of chemical stability, toxicity, corrosion, volume change, availability at reasonable price, etc. There is a vast number of materials with published properties available for latent heat storage [1,15,25].
Figure 2. Temperature and stored heat relationship in the case of combined sensible and latent heat process
3. CFD model setup The CFD simulated domain was defined based on a real, small scale experimental rig which utilized a finned-tube heat exchanger immersed in PCM or water [17]. In Fig. 3 the heat exchanger is immersed in paraffin PCM. The rig was built to study the heat transfer phenomena taking place is such systems assisting the development of real scale devices for thermal energy storage applications [17,18,19]. The total length of the heat exchangers tube was 6.6 m. The problem of paraffin phase change conjugated to conduction in the tube wall and to the forced convection of the HTF is unsteady. The simplifying assumptions, which the mathematical model relies on, for the description of the physics of the problem are: the PCM is homogeneous and isotropic; natural convection of the PCM is neglected; azimuthal temperature is everywhere negligible (2D solution) [20]. The enthalpy-porosity technique [21,22,23] is used, for modeling the solidification/melting process.
Figure 3. Photograph of the experimental thermal energy storage small rig.
5
For the simulations, the commercial CFD solver ANSYS Fluent was used. The parameters that were investigated were: the energy stored inside the media, the outlet HTF temperature and the temperature of the storage medium. A heat exchanger working with water as the HTF is used to transfer energy from the hot water to the PCM. The most widely used HTF is pure water. Many different fluids have been applied over the years. Propylene glycol-water mix, ethylene glycol water mix, synthetic oil and silicone oil are some examples [24]. The geometry selected is that of a commercial heat exchanger, in order to examine the behavior of the storage media, in use with commercial heat exchangers. A 2D axisymmetric computational domain was created with the characteristics shown in Table 1 and it is depicted in Fig. 4. The simulations were conducted for either 1 m or 6.6 m tube length and for either a laminar flow at 30 l/hr or a turbulent at 60 l/hr. For all cases, PCM and water were tested as HSMs. One-meter long tube cases offer short simulation time and they are adequate for the full development of the flow which takes about 0.15 m. The long tube on the other hand, has a length (6.6 m) equal to the circuit length of the experimental rig illustrated in Fig. 3 for future comparison of CFD and experimental results. The flow rates were the same as those applied in a preceding work [17] and they allow the comparison of process duration between a laminar and a turbulent case. Table 1. Properties of the 2D axisymmetric computational domain
Circuits configuration Inner tube diameter (mm) Outer tube diameter (mm) Tube material Fins material Radius of the fins (mm) Fins Pitch (mm) Fins thickness (mm) Heat storage medium volume (m3) 1 m long tube Heat storage medium volume (m3) 6.6 m long tube
Single tube 1 or 6.6 m long 8.6 10 Copper Aluminium 13.8 5.0 0.3 0.0008804 0.0058107
Figure 4. Part of the axisymmetric computational domain.
For both tube lengths (1 and 6.6 m) and both flow rates (30 and 60 l/hr) the computational mesh form was the same and it was created using ANSYS software (Fig. 5). The details of the mesh are shown in Figure 5. The majority of the cells has rectangular shape and the mesh is inflated near the 6
water-tube interface. The mesh selected has 81200 elements for the 1 m tube and 487200 elements for the 6.6 m tube.
Figure 5. Computational grid used in the simulations.
Regarding boundary conditions constant velocity applied at the inlet of the tube and for the outlet, constant pressure was defined. The outer walls of the entire domain are considered adiabatic. The solidification /melting model is enabled and for the mushy zone parameter the default value was selected. Finally, according to the flow rate value, the laminar model was applied for 30 l/hr and the k-ε turbulent for 60 l/hr. The material properties used in the simulations are shown in the Table 2. Unlike SHS systems which operate in a broad range of temperatures, LHS systems operate in a much narrower temperature range. It is within this narrow range that the change of phase of a material has to be completed. Therefore, the melting point of the PCM has to be matched to the operating temperature of the system. The PCM selected for the simulations is A44 PCM from PCM PRODUCTS Ltd [25] which has a nominal melting point of 44 oC which matches the operating temperature range of the system. For the PCM two different values for density exist one for solid phase and one for the liquid. In our simulation only the liquid density is used in order to make a simpler, less computational expensive model. Moreover, the defined inlet temperature for the HTF is 52 oC and the initial temperature of the fins, tube, storage medium and stagnant water in the tube is 39 o C. Table 2. Material properties used in the simulations. Material Thermal conductivity [W/m∙K] Density [kg/m3] Specific Heat [J/kg∙K] Viscosity [kg/m∙s] Melting heat of fusion [kJ/kg] Solidus temperature [oC] Liquidus temperature [oC] s –solid; l – liquid
Water 0.6 998.2 4182 0.001003 -
4 Results and discussion 4.1 Results from the 1 m tube 7
PCM A44 [25] 0.24 912 (s), 775 (l) 2400 (s), 1800 (l) 0.007 (l) 250 42.85 45.85
Copper 387.6 8978 381 -
Aluminium 218 2719 871 -
For the 1 m long geometry the mass of PCM is 0.68 kg and the mass of HSM water is 0.89 kg, so there is more mass available for the HSM water for energy storage. However, the latent heat of fusion of the PCM has also a strong impact on the store energy ability of the system. In Figures 6-9 PCM and HSM water are compared for the two different HTF flow rate values. The process ends when the HTF outlet temperature becomes equal to the inlet temperature. In Fig. 6 the HSM average temperature is plotted against time, yielding a sharp increase of the water temperature which finally becomes equal to the 52 oC of HTF inlet temperature. On the other hand when PCM is used, the temperature of the medium increases sharply before and after the phase change region like in Fig. 2. As the process of transferring heat from the HTF to the system begins, part of the PCM absorbs heat and changes phase while the remaining is still in solid phase. As the process progresses, the PCM that already has melted absorbs energy by means of sensible heat, increasing its temperature, while the remaining starts melting. So, the energy storing process combines both sensible and latent heat. It can be seen from Fig. 6, the part of the combined stored energy is far greater than the sensible only part, which is at the start and the end of the process. Between the phase change region the rate of temperature increase is low and it is defined by the change of PCM properties with the temperature and the liquid fraction every time. Because the HSM exhibits different conditions along the length of the tube, the average HSM temperature is an indicative statistical value of the condition of the HSM. Furthermore, for both materials, the duration of the process is longer for the 30 l/hr flow rate compared to 60 l/hr. Results of the stored energy value and the duration of the process for all cases of the 1 m long tube are presented in Table 3. There is a significant difference in the stored energy between water and PCM. Similar increase in the storage capacity of thermal energy storage tanks with the addition of PCM into the water volume, has been observed both theoretically [12,13] and experimentally [11,14]. Table 3. Stored energy and process duration for PCM and HSM water for 1 m long tube at both flow rates.
Stored energy [kWh] – 30 l/hr Stored energy [kWh] – 60 l/hr Duration [hr] – 30 l/hr Duration [hr] – 60 l/hr
HSM Water
PCM A44
0.014 0.015 0.55 0.29
0.059 0.061 2.17 1.11
The stored energy in the LHS system increases rapidly for all 4 cases, but for the SHS this increase ends soon. In contrast, stored energy continues to increase when PCM is used and only near the end of the process the increase rate diminishes. Considering the flow rate effect on the stored energy increase rate, it is obvious from Fig. 7 that the higher the flow rate, the faster the energy is stored. Considering the HSM temperature as a function of the stored energy shown in Fig. 8, as expected the function is linear for the sensible storage medium. Any small differences between the two flow rates for HSM water are attributed to laminar and turbulent model small differences in convergence criteria and to the heat capacity of the tube and the fins which create a minor thermal inertia effect. For the PCM, the two flow rate values have also some small differences which are explained by the same reasons referred at the discussion of Fig. 6. The HTF outlet temperature as a function of the stored energy is depicted in Fig. 9. At all 4 cases when the process ends, the HTF water exits the tube at the same temperature as it enters because there is no more temperature gradient between the water inside the tube and the HSM. When water is used as HSM, the HTF outlet temperature is the same for both flow rates. When PCM is used as HSM, the HTF does not have the time to fully exchange heat with the PCM and for the fast flow rate of 60 l/hr the temperature of the outlet water is lower than for the slower flow rate. This is true at the first part of the process. At the remaining part of the process, the outlet temperature of the fast flow becomes higher than the slow flow. Moreover, independently of the flow rate, with the use of PCM as HSM, the outlet HTF temperature is more constant compared with the use of HSM water as the system is charged with energy. This stable outlet temperature is beneficial for the sizing of the system equipment in actual installations [18,19,26]
8
0.07
324
0.06
Stored energy [kWh]
Temperature [K]
326
322 320 318 316
PCM 1m 30 l/h Water 1m 30 l/h PCM 1m 60 l/h Water 1m 60 l/h
314 312 310 0
1000
2000
3000 4000 Time [s]
5000
6000
0.04 0.03 PCM 1m 30 l/h Water 1m 30 l/h PCM 1m 60 l/h Water 1m 60 l/h
0.02 0.01 0
7000
Figure 6. Storage medium average temperature vs time of the 1 m long tube for 30 and 60 l/hr flow rate. 326
0
1000
2000
3000 4000 Time [s]
5000
6000
7000
Figure 7. Stored energy vs time of the 1 m long tube for 30 and 60 l/hr flow rate. 325
324 Temperature [K]
322 Temperature [K]
0.05
320 318 316
PCM 1m 30 l/h Water 1m 30 l/h PCM 1m 60 l/h Water 1m 60 l/h
314 312 310 0
0.02
0.04 0.06 Stored energy [kWh]
323 PCM 1m 30 l/h Water 1m 30 l/h PCM 1m 60 l/h Water 1m 60 l/h
322 321
0.08
Figure 8. Storage medium average temperature vs stored energy of the 1 m long tube for 30 and 60 l/h flow rate.
324
0
0.02
0.04 0.06 Stored energy [kWh]
0.08
Figure 9. Outlet water temperature vs stored energy of the 1 m long tube for 30 and 60 l/h flow rate.
4.2 Results from the 6.6 m tube
For the 6.6 m long geometry the mass of PCM is 4.5 kg and the mass of HSM water is 5.8 kg. In Fig. 10-13 the results of the CFD simulation are presented for the case of the 6.6 long tube. As expected, the temporal evolution of the HSM temperature and the stored energy is similar with the 1 m long tube geometry. Again, the outlet temperature is more stable during the process when PCM is used instead of water as it was also experimentally shown by [14]. Furthermore, the relationship of the HSM and the outlet temperature with the stored energy is also similar with the 1 m long tube geometry. The duration of the process and the stored energy are larger in the 6.6 m long tube simulation as expected and they are listed in Table 4.
9
326
0.5 Stored energy [kWh]
Temperature [K]
324 322 320 318 316
PCM 6.6 m 30 l/h Water 6.6m 30 l/h PCM 6.6 m 60 l/h
314 312 0
5000
10000
0.3 0.2
PCM 6.6 m 30 l/h Water 6.6m 30 l/h
0.1
Water 6.6m 60 l/h
310
0.4
PCM 6.6 m 60 l/h Water 6.6m 60 l/h
0 0
15000
5000
Time [s]
324
324
322
322
320 318 316
PCM 6.6 m 30 l/h
314
Water 6.6 m 30 l/h
312
PCM 6.6 m 60 l/h
0.1
0.2 0.3 Stored energy [kWh]
0.4
320 318 316
PCM 6.6 m 30 l/h Water 6.6m 30 l/h PCM 6.6 m 60 l/h Water 6.6m 60 l/h
314 312
Water 6.6 m 60 l/h 0
15000
Figure 11. Stored energy vs time of the 6.6 m long tube for 30 and 60 l/hr flow rate. 326
Temperature [K]
Temperature [K]
Figure 10. Storage medium average temperature vs time of the 6.6 m long tube for 30 and 60 l/hr flow rate. 326
310
10000 Time [s]
310 0
0.5
Figure 12. Storage medium average temperature vs stored energy of the 6.6 m long tube for 30 and 60 l/h flow rate.
0.1
0.2
0.3
0.4
0.5
Stored energy [KWh] Figure 13. Outlet water temperature vs stored energy of the 6.6 m long tube for 30 and 60 l/h flow rate.
Table 4. Stored energy and process duration for PCM and HSM water for 6.6 m long tube at both flow rates.
Stored energy [kWh] – 30 l/hr Stored energy [kWh] – 60 l/hr Duration [hr] – 30 l/hr Duration [hr] – 60 l/hr
HSM Water 0.09 0.11 1.11 0.55
PCM A44 0.39 0.41 3.33 1.69
4.3 Comparison between the two tube lengths
The energy that is stored inside the LHS system is expected to be directly proportional to the mass of the storage medium. As it can be seen in Table 5, the stored energy is about 6.6 times greater for the 6.6 m long tube than the 1 m long. Similar increase of the stored energy with the PCM has been derived by the model of [13]. However, the duration of the process is not proportional to the HSM mass. For HSM water, it almost doubles when the length becomes 6.6 larger, while for PCM it increases only approximately 1.5 times respectively. Therefore, if a fast process is required, it is favorable to use a longer tube configuration in order to utilize its greater ability to store energy.
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Table 5. Comparison between the 1 m and 6.6 m long tube geometry in terms of stored energy and process duration for PCM and HSM water at both flow rates.
Stored energy ratio 6.6 to 1 m 30 l/hr Stored energy ratio 6.6 to 1 m 60 l/hr Duration ratio 6.6 to 1 m 30 l/hr Duration ratio 6.6 to 1 m 60 l/hr
HSM Water 6.59
PCM A44 6.60
6.66
6.67
2.00
1.54
1.90
1.52
5 Conclusions In this work a CFD comparison between a sensible heat storage medium (water) and a latent heat storage medium (organic paraffin PCM A44) has been conducted. As it was expected using PCM as a storage medium a greater amount of stored energy for the same volume of storage medium is accomplished, but longer time period is needed compared to the use of water as storage medium for storing the energy. It was found that PCM can store 4.1 times more energy than water for the same volume of storage tank. The duration of the charging process with PCM was found to be approximately 3.9 or 3.0 times longer than with water for the same volume of storage tank, depending on the tube length. In terms of temperature stability, the CFD simulation yielded that the PCM system stores most of its energy while the average PCM temperature changes no more than 5 oC. In contrast, water has to raise its temperature by about 54 oC in order to store the same amount of energy as PCM A44. Comparing a laminar flow at 30 l/hr and a turbulent flow at 60 l/hr, the heat transfer rate results to be higher at the faster flow rate. However, the amount of energy that is stored does not change with the flow rate and this is true for both heat storage media. The process-average heat transfer rate ratio of the turbulent to the laminar is higher for PCM than for water. Additionally, this ratio increases with the increase of the tube length. In particular, for the PCM the process-average heat transfer rate is 53 and 72 % higher for the turbulent flow compared to the laminar for 1 m and 6.6 m long tube respectively. For the water, it is 14 and 31 % higher for the turbulent flow respectively. Finally, comparing a heat exchanger with short tube (1 m) and a longer one (6.6 m), it is concluded that the stored energy is directly proportional to the mass of the medium and thus the length of the tube. On the other hand, with the utilization of the longer tube, the duration of the process increases, but less than 6.6 times. For water the increase is about 2 times and for PCM about 1.5 time.
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