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A combination of fins-nanoparticle for enhancing the discharging of phase-change material used for liquid desiccant air conditioning unite
Journal of Energy Storage 24 (2019) 100784
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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
A combination of fins-nanoparticle for enhancing the discharging of phasechange material used for liquid desiccant air conditioning unite
T
⁎
Ammar M. Abdulateefa,b, , Jasim Abdulateefa,b, Abduljalil A. Al-Abidia,c, Kamaruzzaman Sopiana, Sohif Mata, Mustafa S. Mahdib a
Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Department of Mechanical Engineering, University of Diyala, 32001 Diyala, Iraq c Department of HVAC Engineering, Sana’a Community College, P.O. Box 5695 Sana’a, Yemen b
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase-change material Triplex-tube heat exchanger Fins-nanoparticle Discharging time
A combination of fins-nanoparticle is essential for enhancing the Thermal Energy Storage (TES) that reduces the mismatch between energy supply and energy demand and this employs for Liquid Desiccant Air Conditioning Unite. Major problem is that most Phase-Change Materials (PCMs) have low thermal conductivity (k≤ 0.2 W/m K), resulting in an incomplete charging and discharging processes. Triplex-Tube Heat Exchanger (TTHX) was numerically and experimentally designed, adopted and tested with Alumina nanoparticle (Al2O3) and Paraffin (RT82) that has a discharging temperature of 65 °C . The both-sides freezing was used as a major method and the experimental findings displayed the influence of mass flow rates on the PCM discharging basing on the change in these rates of 16.2, 29.4 and 37.5 kg/min, respectively. The solidification rate was minimized as the angle direction increased from θ = 90° to θ = 270°. Other important findings were that with fins-nanoparticle, an enhancement for the cooling rate of the PCM, compared with these without nanoparticle. Furthermore, the PCM model was solved by the enthalpy-porosity and the finite-volume methods with the Software Ansys Fluent. The solidification time was reduced for TTHX with longitudinal fins and TTHX with triangular fins to 33% and 34% under the effect of 10% nanoparticle, compared with pure Paraffin, respectively. The total energy released for the PCM and nano-PCM was considered. Close agreement obtained between numerical and experimental findings.
1. Introduction Enhancement in Thermal Energy Storage (TES) reduces the mismatch between energy supply and energy demand. This will ensure reliability and increase the efficiency of many energy technologies, such as solar thermal systems. The main challenges that were faced by researchers are the environmental impact, limited energy supply and fossil fuel sources. The issues of the intermittent availability, the relatively high cost of the installation and the maintenance made the move to more sustainable and environmentally friendly sources, such as solar and wind energies to be less competitive in energy markets. The TES has the ability to be an alternative solution to correct the intermittency, the use of facilitating more efficient and promoting broader usage of the aforementioned sources. A low thermal conductivity (k≤0.2 W/m K) is one of the most frequently stated problems with Phase-Change Materials (PCMs) that are used in TES applications. That may cause an incomplete charging and
⁎
discharging processes. The performance of these systems is limited by various temperatures within the PCM, resulting material failure and system overheating. PCM has a major area of interest within the field of engineering applications, such as building heating, water heating, solar energy systems, electronic cooling, drying technology, refrigeration and cold storage, air conditioning and waste heat recovery [1]. A considerable amount of literature has been published on different kinds of heat exchangers, such as concentric cylinder, shell and tube and Triplex-Tube Heat Exchanger (TTHX), which definitely focused on the behavior of PCM used. Abdulateef et al. [2] have reviewed geometric and design parameters of the various fins employed for enhancing PCM-TES systems. Hosseini et al. [3] conducted a combined experimental and numerical study to investigate the influences of increasing Heat-Transfer Flow (HTF) inlet temperature on the charging and discharging of Paraffin (RT50) inside a shell and tube unite. Seddegh et al. [4] investigated and compared the thermal behavior of PCM in a vertical and horizontal shell and tube-Latent Heat Thermal Energy
Corresponding author at: Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia. E-mail address: [email protected] (A.M. Abdulateef).
https://doi.org/10.1016/j.est.2019.100784 Received 28 February 2019; Received in revised form 5 May 2019; Accepted 20 May 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 24 (2019) 100784
A.M. Abdulateef, et al.
Nomenclature Al2O3 B C CFD CNTs Cp d Fe3O4 g GNS h H HTF k L LHTES p PEG PCM r RD S t T TTHX TES
Tm u v x, y
Aluminum oxide Boltzmann constant (J/K) Mushy zone constant (kg/m3 s) Computational fluid dynamics Carbon nano tubes Specific heat (kJ/kg K) Diameter of nanoparticle (nm) Ferrousferric oxide Gravity acceleration (m/s2) Graphene nano sheet Sensible enthalpy (J/kg) Enthalpy (J) Heat-transfer fluid Thermal conductivity (W/m K) Latent heat of fusion (J/kg) Latent heat thermal energy storage Pressure (Pa) Polyethylene glycol Phase-change material Tube radius (mm) Raw diatomite Momentum source term (Pa/m) Time (min) Temperature (°C or K) Triplex-tube heat exchanger Thermal energy storage
Melting temperature (°C or K) Velocity component (m/s) Velocity component (m/s) x, y-component in a Cartesian coordinate system
Greek letters
β ε ϕ γ μ θ ρ ζ
Thermal expansion coefficient (1/K) Constant Volumetric fraction of nanoparticle Liquid fraction Dynamic viscosity (kg/m s) Angle (o) Fluid density (kg/m3) Correction factor
Subscribes i Ini l m np npcm o pcm ref s
Storage (LHTES) system with no fins using a combined conduction and convection heat-transfer model. A horizontal shell and tube unite has a better performance in heat-transfer, especially during part-load energy charging. Tay et al. [5] simulated PCM-LHTES units, including plain tube, pinned tube and circular finned tube. The circular finned tube design was found to yield better average effectiveness and shorter phase-change duration in the solidification process. Xiaohu et al. [6] numerically studied a shell and tube unit with annular fins. An optimal fin parameter, such as fin number (N = 31), thickness (t/l = 0.0248) and interval (l/L = 0.0313) has been employed for maximizing performance. Rahimi et al. [7] evaluated an experimental test to investigate the charging and discharging processes in a plate finned tube heat exchanger. Sciacovelli et al. [8] simulated the use of two kinds of tree shaped fins, namely: a single bifurcation and a double bifurcation configuration, to optimize and accomplish the maximum work for LHTES system. Consequently, there is a large volume of published studies describing the significant role of the designed fin shapes for enhancing the heat-transfer rate in PCM-LHTES systems. On the other hand, the solid-liquid interface moves away from the heat-transfer surface, and the heat flux reduced with the unloading of the LHTES. This was due to augmenting thermal resistance of the growing layer of the molten and solidified medium of PCM. This influence can also be reduced by dispersing high thermal conductivity particles, such as Aluminum, Copper, Silver and Graphite. Arasu and Mujumdar [9] numerically reported the Paraffin Wax melting by dispersing different volumetric concentrations of Alumina (Al2O3) inside a square enclosure heated from one side. Mahdi and Nsofor [10] have presented a numerical study to illustrate the effect of utilizing fins, nanoparticle and a combination of nanoparticle-fins in the PCM-TTHX. The results indicated that PCM melting is improved by these techniques. Darzi et al. [11] simulated horizontal annulus configurations consist of two circular cylinders in addition to one elliptical cylinder and one finned cylinder, which are separately in a circular cylinder. Enhancing the melting and solidification rates can be effectively seen because of increasing the volume fractions of Copper nanoparticle concentration. Nurten et al.
Inner Initial time Liquid Middle Nanoparticle Nano-PCM Outer Phase-change material Reference Solid
[12] enhanced the thermal conductivity of PCM by preparation of Paraffin Nano Magnetite (Fe3O4) composites. The results clearly indicate that the dispersion of Fe3O4 nanoparticles is an efficient and cost effective method. Wentao et al. [13] reported novel magnetic, sunlightdriven energy conversion and storage nanocomposites based on Fe3O4 functionalized Graphene Nano Sheet (Fe3O4-GNS) embedded form stable Polymer-PCMs. Yuang et al. [14] employed Polyethylene Glycol (PEG) composite PCM for enhancing solar thermal energy conversion and storage capacity, supported by Silver nanoparticle functionalized Graphene Nano Sheet. Ahmet et al. [15] also investigated PEG600 that incorporated with Raw Diatomite (RD)/Carbon Nano Tubes (CNTs) precomposites to abolish its leakage problem during phase-change, to amplify LHTES capacity depending on the increased impregnation ratio and to reduce the heat during the charging and discharging times. On the other hand, the latest progress for enhancing TES that has used metal foam was by Xiaohu et al. [16] who simulated a shell and tube unite and employed an open-cell metal foam with a porosity of 0.94 and pore density of 15 pore/inch for PCMs domains. The influences of the metal foam location and porosity on the heat storage were analyzed. Xiaohu et al. [17] also displayed the melting behaviors of pure PCM and PCM embedded in open-cell metal foam. The experiments are carried out at 0°, 30°, 60° and 90° inclination angles, respectively. The melting time is reduced by 12.28%, 22.81% and 34.21% at 0°, 30° and 60°, respectively compared with the case at 90°. Another published about estimating the effective thermal conductivity of PCMs having two distinctive phases (solid, liquid or liquid-solid mixed phases) was presented by [18]. Generally, there seems to be some evidences to indicate that this paper covers nanoparticle, metal foam and so on, which have been played an important role for enhancing heat-transfer rates and thermal conductivity of PCM during the charging and discharging processes, respectively. The specific objective of this research was to evaluate the performance of the TTHX unite using a combination of fins-nanoparticle, included: triangular fins-nanoparticle and longitudinal finsnanoparticle during the PCM discharging and employed the energy 2
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3. Numerical approach
released for Liquid Desiccant Air Conditioning Unite.
3.1. Physical models 2. Experiments and procedures In this study, the TTHX model used for our application consists of (1) TTHX with internal longitudinal fins and (2) TTHX with internal triangular fins as illustrated in Fig. 4. The difference between TTHX with longitudinal fins and TTHX with triangular fins was the base (thickness) of triangular fin, which was 25 mm. The both-sides freezing is the major method used to supply the cooling rate from both inner and outer tubes. The experimental data illustrated that the minimum temperature required to operate the PCM-TTHX unite was approximately 65 °C .
2.1. Experimental apparatus The operation range of most engineering applications must match the PCM melting temperature employed in PCM-TES systems. Akgun et al. [19] examined commercial- grade PCMs that displayed stable properties after 1000 to 2000 cycles. Table 1 and Fig. 1a, illustrate the selected Paraffin (RT82) (RUBITHERM GmbH-Germany) with thermophysical properties used for Liquid Desiccant Air Conditioning Unite. The minimum discharging temperature requires for this unite is in the range of 65 ℃ −70 ℃ [28]. The nanoparticle that selected to improve the thermal conductivity of Paraffin is Al2O3 as shown in Fig. 1b. The melting point of Al2O3 (2345 ℃) is much higher than the Paraffin (RT82) as illustrated in Table 1. Thus, the Al2O3 remained all the time in the solid state at the average charging and discharging temperatures of 90 °C and 65 °C, respectively. The LHTES system consists of many parts, see Fig. 2. The basic unite is a TTHX as shown in Fig. 3, and consists of inner, middle and outer tubes with 76.2 mm, 381 mm and 500 mm diameters with 3 mm thickness and 3000 mm length, respectively. Eight internal longitudinal fins were equally positioned and welded around the circumference of the inner tube at 45°. Each longitudinal fin has 121 mm pitch, 2 mm thickness and 2800 mm length. The inner and middle tubes and fins are made of copper, while the outer tube is made of steel. The inner and outer tubes were used for HTF, while the middle tube was filled in by 100 kg of the Paraffin (RT82). Furthermore, the Alumina nanoparticles have regularly been dispersed and distributed by 10 kg into three slots of the TTHX as displayed in Fig. 2. There are three main concentrations of Al2O3 used (0%, 5% and 10% by weight) to improve the thermal conductivity of the PCM. As mentioned in Table 1, selecting the nanoparticle to avoid accumulating that maybe occurred in experimental processes. Some nanoparticles, such as Copper maybe accumulated sometimes on the bottom due to its highest density. A 70-mm thick Glass Wool insulation (k = 0.04 W/m K) was wrapped around the TTHX and pipes to reduce heat lost and to make the surface adiabatic. Two Thermocouples (T-type) on the top and bottom of the Storage Tank were fitted in the axial direction and were used to measure the temperatures of the PCM solidification, 500 mm and 1100 mm away from the HTF tube entrance, respectively. Thermocouples (J-type) and Flow Meter are currently the most popular tools for measuring the HTF inlet and outlet temperatures and HTF rate, respectively. A Data Logger and a Personal Computer achieved the data recording for the discharging processes (PCM temperature and HTF rate).
3.2. Governing equations There are some assumptions can be considered:
• The melting behaves as an Incompressible Newtonian Fluid. • The convective motion is laminar and two-dimensional. • No slip conditions were applicable for the velocity components at the boundaries. • The thermo-physical properties of the Paraffin (RT82) are constant for both solid and liquid phases.
During the PCM melting, the influence of the natural convection is specifically pointed by invoking the Boussinesq approximation, which is valid for the density variations of the buoyancy force; otherwise, this influence is ignored. The density variation is defined by:
ρ = ρl /(β (T − Tl ) + 1)
(1)
where ρl represents the PCM density at the melting temperature of Tl , and β is the thermal expansion coefficient. The temperature distribution and viscous Incompressible Flow are solved by the Navier-Stokes and thermal energy equations, respectively. The continuity, momentum and thermal energy equations are expressed by [20]: The continuity equation:
∂ (ρv ) ∂ (ρu) ∂ρ =0 + + ∂y ∂x ∂t
(2)
The momentum equation:
∂p ∂ (ρvu) ∂ (ρuu) ∂ (ρu) ∂ ∂u ∂ ∂u + ρ g x + Sx = (μ )+ (μ )− + + ∂x ∂y ∂x ∂x ∂y ∂y ∂x ∂t (3a)
∂p ∂ (ρvv ) ∂ (ρuv ) ∂ (ρv ) ∂ ∂v ∂ ∂v + ρ g y + Sy = (μ ) + (μ ) − + + ∂y ∂y ∂x ∂x ∂y ∂y ∂x ∂t (3b)
2.2. Discharging process
The energy equation:
∂ (ρvh) ∂ (ρuh) ∂ (ρh) ∂ ⎛ ∂T ⎞ ∂ ⎛ ∂T ⎞ + = + + k ⎜k ⎟ ∂y ⎝ ∂y ⎠ ∂y ∂x ⎝ ∂x ⎠ ∂x ∂t
After the PCM is entirely charged, the discharging process starts. The water was circulated by the Pumps from the Water Storage Tank (500 L) to the TTHX. The valves were closed to prevent the water from going to Evacuated Tubes Solar Collector as shown in Fig. 2. The cooling rate is transferred to the inner and outer tubes of the TTHX and then by the conduction heat-transfer to the PCM or nano-PCM in the middle tube with both-sides freezing method. The initial temperature of the Paraffin was set at 90 °C. The inlet HTF was at average discharging temperature of 65 °C and continued to the TTHX during a closed cycle. However, the Alumina nanoparticle remained in a solid state at HTF temperature during this process. The HTF temperature cannot also remain constant and continues to circulate until the PCM was completely discharged.
⎜
⎟
(4)
Table 1 Thermo-physical properties of Paraffin (RT82) and Alumina (Al2O3). Properties
Paraffin (RT82)
Al2O3
Density, solid, ρs (kg/m ) Density, liquid, ρl (kg/m3) Specific heat, C pl , C ps (J/kg K)
950 770 2000
3600 – 765
Latent heat of fusion, L (J/kg) Dynamic viscosity, μ (kg/m s) Melting temperature, Tm (°C ) Thermal conductivity, k (W/m K) Thermal expansion coefficient (1/K)
176000 0.03499 78.15 -82.15 0.2 0.001
– – 2345 36 –
3
3
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Fig. 1. The granules survey of (a) Paraffin (RT82) and (b) Alumina (Al2O3). Fig. 2. Schematic of the experimental apparatus of the LHTES system. It includes: 1. Evacuated tubes solar collector, 2. Flow meter, 3. TTHX, 4. T-type thermocouple, 5.J-J-type thermocouple, 6. Internal longitudinal fins, 7. diaphragm expansion tank, 8. Pump, 9. Data logger, 10. Computer, 11. Water storage tank, 12. Electrical heater, 13. Pipes, 14. Valve twoways and 15. Valve three-ways.
where ρ represents the density of the PCM, u and v are the fluid velocities, μ is the dynamic viscosity, p is the pressure, g is the gravitational acceleration, k is the thermal conductivity, h is the sensible enthalpy and T represents the temperature. The sensible enthalpy equation can be expressed by:
h = href +
∫T
T
ref
Cp ΔT
(5)
where href represents the reference enthalpy at the reference temperature Tref ; Cp is the specific heat. The total enthalpy H equation can be defined by: Fig. 3. Triplex-Tube Heat Exchanger (TTHX).
H = h + ΔH
(6)
where ΔH represents the latent heat content of the PCM; which changes 4
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the tube wall represented the HTF temperature [22,23] and was approximately 90 °C for melting and 65 °C for solidification. The boundary conditions of the TTHX can be written by: Both-sides heating or freezing method:
at r = ri → T = THTF
(10)
at r = rm → T = THTF
(11)
Initial temperature of all models:
at t = 0
→
(12)
T = Tini
In case of the nano-PCM, the same conservation equations, boundary and initial conditions are applied. The difference is that the melting temperature of the Al2O3 is much higher than that of the Paraffin (RT82) as shown in Table 1. Therefore, the Alumina nanoparticle remained in the solid state at the HTF temperature (90 °C and 65 °C ) during the melting and solidification processes. 3.4. Thermo-physical properties Depending on a commercially available material (Rubitherm GmbH-Germany (RT82)) supplied by company and nanoparticle. Table 1 illustrates the thermo-physical properties of the Paraffin (RT82) and Al2O3. In this research, the properties of the nano-PCM can be determined by [24]: The density equation:
ρnpcm = ϕρnp + (1 − ϕ) ρpcm
(13)
The specific heat capacity equation:
Cp, npcm =
ϕ (ρCp)np + (1 − ϕ)(ρCp)pcm ρnpcm
(14)
The latent heat equation: Fig. 4. Physical configurations of the TTHX model with (a) Internal longitudinal fins and (b) Internal triangular fins.
Lnpcm =
between zero (solid) and L (liquid); and γ is the liquid fraction, which is generated during the phase-change between the solid and liquid states when the temperature is Tl > T > Ts . This can be written as:
γ = ΔH / L
(7)
γ=0 ifT < Ts γ=1 if T > Tl T − Ts γ= if Ts < T < Tl Tl − Ts
(8)
Sx = C (1 −
Sy = C (1 − γ )2 where C (1 −
u γ3 + ε
(15)
μnpcm = 0.983e (12.959ϕ) μpcm
(16)
The effective thermal conductivity of the nano-PCM includes the effects of the particle size (dnp ), particle volume fraction (ϕ) and temperature dependence, as well as the properties of the Paraffin (RT82). The particle subject to Brownian motion is given by Vajjha et al. [25].
knpcm =
knp + 2kpcm − 2(kpcm − knp ) ϕ knp + 2kpcm + 2(kpcm − knp ) ϕ + 5 × 10 4γk ξ ϕρpcm Cp,pcm
kpcm
BT f (T ,ϕ) ρnp dnp
(17)
(9a) where B is the Boltzmann constant (1.381 ×
v γ3 + ε
u γ )2 3 γ +ε
ρnpcm
The dynamics viscosity of nano-PCM is given by Vajjha et al. [25]:
The source term S in the momentum equations and Eqs. (3a and 3b) can be defined in x and y directions, respectively as:
γ )2
(1−ϕ)(ρL)pcm
γk =
(9b) and C (1 −
v γ )2 3 γ +ε
are the "porosity function"
J/K) and
8.4407(100ϕ)−1.07304
f (T ,ϕ) = (2.8217 × 10−2 ϕ+ 3.917 × 10−3)
defined by Brent et al. [21]. C is a constant, which describes how sharply the velocity is reduced to zero when the material solidifies. This constant varies between 104 and 107 (105 is considered for this study). ε is a small number (0.001) to prevent division by zero.
10−23
− 3.91123 × 10−3)
(18)
T + (−3.0669 × 10−2 ϕ Tref (19)
where Tref =273 K represents the reference temperature. There is a correction factor ξ in the Brownian motion term because there should be no Brownian motion in the solid phase [9]. Its value is the same as for the liquid fraction γ in Eq. (8). In this study, we applied the effects of the particle size (dnp =20 nm), particle volume fraction (ϕ= 10%) and reference temperature (Tref =273 K) in Eq. (17).
3.3. Boundary and initial conditions At the initial time, the Paraffin (RT82) was in the solid state, and the temperature was 27 °C for melting process. The constant temperature of 5
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3.5. Computational methodology The most popular methods for solving the PCM model were the enthalpy-porosity and the finite-volume [26]. That was with the Software Ansys Fluent 16 and the procedure for that can be summarized as shown below:
• Drawing and meshing the model in two-dimensions (r , θ) and de• • • • •
fining the boundary layer conditions and zone types by Software Gambit 2.4.6. To input the data and find the solutions for problems, the PCM model was exported to the Fluent 16 Software. Applying the equations for the continuity, momentum and energy in addition to solving these equations with the boundary and initial conditions by finite-volume method basing on the transient time and pressure-based solver with absolute velocity. Discretizing the equations of momentum and energy by the secondorder upwind and discretizing the pressure by the PRESTO method. Employing the SIMPLE scheme in pressure-velocity coupling and applying the under-relaxation factors (0.3, 0.2, 0.7, 1 and 0.9) for the pressure, velocity, momentum, energy and liquid fraction, respectively. However, applying the convergence for the energy equation (10−6) and the velocity equations (10−3). The final set selected the data (0.1 s) as the standard value, and (0.5 and 1 s) were for the some measurements. These were for the timestep sensitivity.
Fig. 6. Validation of the experimental and numerical data of the pure PCM and nano-PCM in a TTHX with internal longitudinal fins.
℃. Furthermore, the HTF discharging temperature for both-sides freezing method is 65 ℃ with an experimental mass flow rate of 37.5 kg/min. 4. Results and discussions Much of the heat exchanger research has focused on designing and employing the fins to improve the heat-transfer. That was because of its simplicity, easy fabrication and low construction cost [2]. Recent developments in the field of TES have led to a renewed interest in dispersing nanoparticle to enhance the thermal conductivity of PCM.
Fig. 5 shows the results obtained from this analysis, the grid sizes were 66,536 and 56,200 for the internal triangular fins and internal longitudinal fins, respectively. Further details in Computational Fluid Dynamics (CFD) related with this study can be found in Ref. [27].
4.1. Experimental results 4.1.1. Average charging temperature Figs. 7 shows the charging temperature of the PCM and nano-PCM versus time for both-sides heating method with a flow rate of 37.5 kg/ min. Data for these experiments were collected with the average charging temperature of 90 ℃. The reason for taking this temperature with time is the intermitted nature of the solar irradiance. It should not be less than the Paraffin (RT82) melting temperature (78.15 ℃− 82.15 ℃) to achieve the process.
3.6. Numerical model validation Fig. 6 shows the validation of the experimental and numerical average temperature of the pure PCM and nano-PCM in a TTHX with internal longitudinal fins. The early stage of the solidification process represents the sensible cooling of the PCM as a liquid. With time, the experimental findings showed close agreement with the numerical results based on the four Thermocouples of T-type inserted into the PCM and nano-PCM, while the numerical findings depended on the full section of the model. The percentage errors between the numerical and experimental results did not exceed 3% for two cases. The data were recorded when the average temperature of the Paraffin (RT82) was 90
4.1.2. Nanoparticle dispersed technique Fig. 8 illustrates the experimental data for the effect of the nanoparticle concentrations on the PCM solidification. Generally, Increasing
Fig. 5. Distribution of the grids-size number in the middle tube of the TTHX with (a) Triangular fins and (b) Longitudinal fins. 6
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solidification rate reduced as the angle direction increased from θ = 90° to θ = 270°. This was due to the cooling rate at the upper tube was higher than the lower tube. Thus, the solid layer can fully be seen on the top of tube, as fast as on the bottom of the same tube. The most surprising aspect of the data is in Fig. 11b that also illustrates the average temperature variations of the PCM along the angular direction after depressing 10% of nanoparticle. The main reason for improving discharge rate, compare with the pure PCM was that depressing the nanoparticle that improved the thermal conductivity of the Paraffin (RT82) (0.2 W/m K) to 25%. 4.1.6. Thermodynamic analysis of nano-PCM discharging process Fig. 12 indicates the inlet and outlet temperatures of the HTF, the average temperature of the nano-PCM via the time for the TTHX with internal longitudinal fins. For the PCM solidification process, the discharging temperature was at 65 ℃ with a flow rate of 37.5 kg/min. The 10% of Al2O3 depressed in the PCM-TTHX unit to enhance the thermal conductivity and cooling rate. With nanoparticle, there was no change to the deference between the inlet temperature and outlet temperature. As the phase-change begun, the difference between the inlet temperature, outlet temperature and PCM temperature was gradually reduced. This was for a long time until the PCM was entirely discharged.
Fig. 7. The charging temperature versus time for both-sides heating method with a flow rate of 37.5 kg/min.
4.2. Uncertainty analysis The uncertainty of a measured quantity is an error of measurement analysis for illustrating the level of confidence of the experimental results. There are two components can be split for the uncertainty of PCM thermal properties: random and systematic components. For example, the onset point, peak point and latent heat of fusion, which occurred during the charging and discharging methods. Random components can be defined as the standard error, which can statistically analyze by (1) determining the mean and (2) calculating the standard deviation. This is for a finite set of measurements. In these experiments, some measurements were ignored from the set depended on the Chauvenet’s criterion to reject certain measurement, according to Holman [29]. Ultimately, it can be depending on the modified measurement set, a new mean and standard deviation can effectively be determined. Table 2 shows the experimental data on the new mean and the standard uncertainties for the Paraffin (RT82) thermal measurement with confidence interval 90% of its thermal properties.
Fig. 8. Effect of the nanoparticle concentrations on the PCM discharging process.
the Al2O3 concentration reduces the PCM discharging time. Thus, the Paraffin (RT82) with 10% Al2O3 was the best choice where the discharging rate enhancement was pronounced. 4.1.3. Discharging method mechanism As mentioned earlier, the discharging process starts after the PCM is entirely charged. Fig. 9 shows the charging and discharging temperatures of the PCM and nano-PCM using both-sides heating and freezing methods, respectively with a flow rate of 37.5 kg/min. For both-sides freezing method, the cold water was supplied from both inner and outer tubes of the TTHX, in which an average discharging temperature supplied to the Paraffin (RT82) by conduction at 65 ℃. The temperature readings for PCM depended on Thermocouples (T-type) location as shown in Fig. 2. Another important finding was that after using 10% of nanoparticle, the enhancement in discharging rate was better than the pure PCM. This was due to increasing the cooling rate between the surface of the fins, middle tube and the PCM liquid. With time, the cooling rate is improved because of augmenting the thermal resistance owing to increase the solid layer front of the PCM in the TTHX.
4.3. Numerical results 4.3.1. Fins technique for enhancing the PCM solidification After the Paraffin (RT82) is entirely melted, the solidification process starts between the cold wall of the middle tube, the cold wall of fins and the PCM liquid. A thin layer of the Paraffin solid is gradually formed at the early stage to surround the fin and the tube surfaces. The
4.1.4. The effect of the HTF mass flow rate The influence of mass flow rate on the PCM solidification for bothsides freezing method was carried out depending on the change in mass flow rate of 16.2, 29.4 and 37.5 kg/min as shown in Figs. 10a and b for PCM and nano-PCM. Increasing the mass flow rate increases the cooling rate that causes decreasing the discharging time. This was due to increase the thermal resistance owing to increase the solid layer front of the Paraffin (RT82), especially after depressing nanoparticle, compare with the pure PCM. The PCM was fully solidified in shorter time at mass flow rate of 37.5 kg/min. Fig. 9. Charging and discharging temperatures of the PCM and nano-PCM using both-sides heating and freezing methods, respectively with a flow rate of 37.5 kg/min.
4.1.5. Angular temperature variations Fig. 11a displays the average temperature variations along the angular direction for both-sides freezing method at Ti = 65 ℃. The 7
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Fig. 12. The inlet and outlet temperatures of the HTF, the average temperature of the PCM versus the time for the TTHX with longitudinal fins. Table 2 The new mean and the standard uncertainties of the PCM thermal properties. Solidification process Onset point (℃ )
Peak point (℃ )
Heat of fusion (kJ/kg)
81.86 ± 0.02828
78.158 ± 0.090358
207.807 ± 1.359165
part of Paraffin retained in a liquid state because of the limitation of the natural convection effectiveness. There were small convection cells created and extended after 10 min inside the middle tube and specifically on fins walls. With increasing the time, large convection cells were formed at 60 min. The solid fraction of the Paraffin is increased and extended to reach on the bottom part of the middle tube at 240 min. There are some factors were dominated during this process, including the effects of natural convection driven by buoyancy and the quick of solid layer of the PCM, which is squeezed because of its high density. The solid fraction was effectively increased in all directions of the tube with time. As a result, the liquid fraction contours of the PCM solidification in the TTHX are displayed in Fig.13. The Paraffin (RT82) was fully solidified at 780 min and 668 min for longitudinal and triangular fins, respectively.
Fig. 10. a The effects of mass flow rate for both-sides freezing method on the solidifying PCM. b The effects of mass flow rate for both-sides freezing on the solidifying PCM under effect of the nanoparticle.
4.3.2. Nanoparticle dispersed technique Eqs. (17–21) calculated some of the thermo-physical properties of the nano-PCM under the effect of different volumetric concentrations of nanoparticle. The thermal conductivity and dynamic viscosity of the nano-PCM were higher than those of the pure PCM as shown in Table 3. It can also be seen that the specific heat and latent heat of the nanoPCM were lower than those of the PCM without nanoparticle. The dynamic viscosity of the nano-PCM is enhanced, and this may be affected on the natural convection effectiveness that contributed the melting and solidification. This variation in the thermal conductivity and dynamic viscosity accepted well with the findings reported in Ref. [9]. 4.3.3. Fins-nanoparticle for enhancing the PCM solidification As mentioned earlier for the behavior of pure PCM solidification, the behavior of the nanoPCM was similar, but the reduction in solidification time was improved. The cooling rate between the fins wall, the middle tube wall, the nanoparticle surface and the Paraffin liquid is directly started after the PCM is entirely melted. At the early stage, a very thin layer of PCM solid is created to surround the fins surface and the cold wall of the middle tube under the effect of nanoparticle. The rest of the PCM is retained liquid without any phase-transition because the effects of the natural convection at the beginning were limited. The Paraffin solid layer also gradually grows up at various times because of increasing the natural convection effectiveness. Over time, the
Fig. 11. a The average temperature variation along the angular direction for both-sides freezing method. b The average temperature variation along the angular direction for both-sides freezing method under effect of the nanoparticle.
8
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Fig. 13. Liquid fraction contours of the PCM in the TTHX obtained using longitudinal and triangular fins.
consumes a short time to produce the biggest demand thermal energy released, compared with the same PCM volume. At the same time, the liquid fraction values for nano-PCM were also lower than those of pure PCM as shown in Table 4. The total energy more stores inside the PCM, but consumes a longer time to produce the biggest demand thermal energy released for Liquid Desiccant Air Conditioning Unite.
Table 3 Variation of the thermal conductivity and dynamic viscosity of the nano-PCM. Volumetric concentration ϕ (%)
Thermal conductivity k (W/m K)
Dynamic viscosity μ (kg/m s)
Simple PCM Nano-PCM with 1% Al2O3 Nano-PCM with 4% Al2O3 Nano-PCM with 7% Al2O3 Nano-PCM with 10% Al2O3
0.2 0.206 0.225 0.245 0.265
0.03499 0.0121161 0.0485 0.084812 0.121161
4.5. Comparison of PCM solidification time Fig. 16 compares the numerical data on the PCM solidification time for the TTHX with longitudinal fins and TTHX with triangular fins, with and without nanoparticle. As a general result, the thermal conductivity of the Paraffin (RT82) (0.2 W/m K) enhanced to 25% by dispersing 10% of Al2O3. The most interesting findings were that the solidification time was minimized with internal longitudinal fins-nanoparticle and internal triangular fins-nanoparticle to 33% and 34%, respectively, compared with the pure Paraffin. The main reason for that is, to enhance the cooling rate between the fins wall, the middle tube wall and the Paraffin into TTHX by combination of fins-nanoparticle.
convection cells extended to reach on the bottom part of the middle tube. Thus, the solid fraction of the PCM was strongly extended in all directions with time. The Paraffin solid was squeezed due to its high density, and the natural convection effect was driven by buoyancy. As a result, the liquid fraction contours of the PCM solidification under the effect of 10% nanoparticle can be seen in Fig.14. The times required for achieving the full PCM solidification in the TTHX with longitudinal fins and the TTHX with triangular fins were 520 min and 441 min, respectively.
5. Conclusions 4.4. Total energy released The purpose of this work was to assess the performance of the TTHX unite using a combination of fins-nanoparticle during PCM discharging and employed the released energy for Liquid Desiccant Air Conditioning Unite. The both-sides freezing was used as main method to experimentally investigate the influence of mass flow rate (16.2, 29.4
Fig. 15 illustrates the total energy released for the PCM and nanoPCM in TTHX with internal triangular fins as a sample simulated using the Software Ansys Fluent 16 at the different times of the freezing process. The nano-PCM has a lower energy released capacity, but 9
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Fig. 14. Liquid fraction contours of the PCM in the TTHX obtained using longitudinal and triangular fins-nanoparticle.
Fig. 16. Comparison of PCM solidification time for TTHX with and without nanoparticle.
Fig. 15. The total energy released for the PCM and nano-PCM in TTHX-internal triangular fins.
and 37.5 kg/min) on the PCM solidification. This study has also shown that the discharging rate minimized as the angle direction increased from θ = 90° to θ = 270°. Numerically, the Paraffin (RT82) was fully solidified at 780 min and 668 min for TTHX with longitudinal fins and TTHX with triangular fins, respectively. However, the total energy more stores inside PCM, but consumes a longer time to produce the biggest demand thermal energy released. The most obvious findings to emerge from this research are with nanoparticle, an important enhancement for the PCM discharging (solidification), compared with these without nanoparticle. Close agreement can be seen between numerical and experimental data. Ultimately, the combination of fins-nanoparticle has a
Table 4 The Liquid fraction and total energy released of the PCM and nano-PCM in TTHX with internal triangular fins. Time (min)
30 60 100 160
Liquid fraction
Total energy released (kJ)
PCM
nano-PCM
PCM
nano-PCM
0.798801 0.676442 0.558266 0.443534
0.752145 0.608758 0.477617 0.358431
240.9688 215.6957 192.0092 168.8889
163.4294 145.8297 129.4984 113.767
10
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number of important implications for future practice in TES systems. [14]
Acknowledgements The authors gratefully appreciate the financial supports provided by Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia (UKM), Malaysia.
[15]
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Contents lists available at ScienceDirect
Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
A combination of fins-nanoparticle for enhancing the discharging of phasechange material used for liquid desiccant air conditioning unite
T
⁎
Ammar M. Abdulateefa,b, , Jasim Abdulateefa,b, Abduljalil A. Al-Abidia,c, Kamaruzzaman Sopiana, Sohif Mata, Mustafa S. Mahdib a
Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Department of Mechanical Engineering, University of Diyala, 32001 Diyala, Iraq c Department of HVAC Engineering, Sana’a Community College, P.O. Box 5695 Sana’a, Yemen b
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase-change material Triplex-tube heat exchanger Fins-nanoparticle Discharging time
A combination of fins-nanoparticle is essential for enhancing the Thermal Energy Storage (TES) that reduces the mismatch between energy supply and energy demand and this employs for Liquid Desiccant Air Conditioning Unite. Major problem is that most Phase-Change Materials (PCMs) have low thermal conductivity (k≤ 0.2 W/m K), resulting in an incomplete charging and discharging processes. Triplex-Tube Heat Exchanger (TTHX) was numerically and experimentally designed, adopted and tested with Alumina nanoparticle (Al2O3) and Paraffin (RT82) that has a discharging temperature of 65 °C . The both-sides freezing was used as a major method and the experimental findings displayed the influence of mass flow rates on the PCM discharging basing on the change in these rates of 16.2, 29.4 and 37.5 kg/min, respectively. The solidification rate was minimized as the angle direction increased from θ = 90° to θ = 270°. Other important findings were that with fins-nanoparticle, an enhancement for the cooling rate of the PCM, compared with these without nanoparticle. Furthermore, the PCM model was solved by the enthalpy-porosity and the finite-volume methods with the Software Ansys Fluent. The solidification time was reduced for TTHX with longitudinal fins and TTHX with triangular fins to 33% and 34% under the effect of 10% nanoparticle, compared with pure Paraffin, respectively. The total energy released for the PCM and nano-PCM was considered. Close agreement obtained between numerical and experimental findings.
1. Introduction Enhancement in Thermal Energy Storage (TES) reduces the mismatch between energy supply and energy demand. This will ensure reliability and increase the efficiency of many energy technologies, such as solar thermal systems. The main challenges that were faced by researchers are the environmental impact, limited energy supply and fossil fuel sources. The issues of the intermittent availability, the relatively high cost of the installation and the maintenance made the move to more sustainable and environmentally friendly sources, such as solar and wind energies to be less competitive in energy markets. The TES has the ability to be an alternative solution to correct the intermittency, the use of facilitating more efficient and promoting broader usage of the aforementioned sources. A low thermal conductivity (k≤0.2 W/m K) is one of the most frequently stated problems with Phase-Change Materials (PCMs) that are used in TES applications. That may cause an incomplete charging and
⁎
discharging processes. The performance of these systems is limited by various temperatures within the PCM, resulting material failure and system overheating. PCM has a major area of interest within the field of engineering applications, such as building heating, water heating, solar energy systems, electronic cooling, drying technology, refrigeration and cold storage, air conditioning and waste heat recovery [1]. A considerable amount of literature has been published on different kinds of heat exchangers, such as concentric cylinder, shell and tube and Triplex-Tube Heat Exchanger (TTHX), which definitely focused on the behavior of PCM used. Abdulateef et al. [2] have reviewed geometric and design parameters of the various fins employed for enhancing PCM-TES systems. Hosseini et al. [3] conducted a combined experimental and numerical study to investigate the influences of increasing Heat-Transfer Flow (HTF) inlet temperature on the charging and discharging of Paraffin (RT50) inside a shell and tube unite. Seddegh et al. [4] investigated and compared the thermal behavior of PCM in a vertical and horizontal shell and tube-Latent Heat Thermal Energy
Corresponding author at: Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia. E-mail address: [email protected] (A.M. Abdulateef).
https://doi.org/10.1016/j.est.2019.100784 Received 28 February 2019; Received in revised form 5 May 2019; Accepted 20 May 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature Al2O3 B C CFD CNTs Cp d Fe3O4 g GNS h H HTF k L LHTES p PEG PCM r RD S t T TTHX TES
Tm u v x, y
Aluminum oxide Boltzmann constant (J/K) Mushy zone constant (kg/m3 s) Computational fluid dynamics Carbon nano tubes Specific heat (kJ/kg K) Diameter of nanoparticle (nm) Ferrousferric oxide Gravity acceleration (m/s2) Graphene nano sheet Sensible enthalpy (J/kg) Enthalpy (J) Heat-transfer fluid Thermal conductivity (W/m K) Latent heat of fusion (J/kg) Latent heat thermal energy storage Pressure (Pa) Polyethylene glycol Phase-change material Tube radius (mm) Raw diatomite Momentum source term (Pa/m) Time (min) Temperature (°C or K) Triplex-tube heat exchanger Thermal energy storage
Melting temperature (°C or K) Velocity component (m/s) Velocity component (m/s) x, y-component in a Cartesian coordinate system
Greek letters
β ε ϕ γ μ θ ρ ζ
Thermal expansion coefficient (1/K) Constant Volumetric fraction of nanoparticle Liquid fraction Dynamic viscosity (kg/m s) Angle (o) Fluid density (kg/m3) Correction factor
Subscribes i Ini l m np npcm o pcm ref s
Storage (LHTES) system with no fins using a combined conduction and convection heat-transfer model. A horizontal shell and tube unite has a better performance in heat-transfer, especially during part-load energy charging. Tay et al. [5] simulated PCM-LHTES units, including plain tube, pinned tube and circular finned tube. The circular finned tube design was found to yield better average effectiveness and shorter phase-change duration in the solidification process. Xiaohu et al. [6] numerically studied a shell and tube unit with annular fins. An optimal fin parameter, such as fin number (N = 31), thickness (t/l = 0.0248) and interval (l/L = 0.0313) has been employed for maximizing performance. Rahimi et al. [7] evaluated an experimental test to investigate the charging and discharging processes in a plate finned tube heat exchanger. Sciacovelli et al. [8] simulated the use of two kinds of tree shaped fins, namely: a single bifurcation and a double bifurcation configuration, to optimize and accomplish the maximum work for LHTES system. Consequently, there is a large volume of published studies describing the significant role of the designed fin shapes for enhancing the heat-transfer rate in PCM-LHTES systems. On the other hand, the solid-liquid interface moves away from the heat-transfer surface, and the heat flux reduced with the unloading of the LHTES. This was due to augmenting thermal resistance of the growing layer of the molten and solidified medium of PCM. This influence can also be reduced by dispersing high thermal conductivity particles, such as Aluminum, Copper, Silver and Graphite. Arasu and Mujumdar [9] numerically reported the Paraffin Wax melting by dispersing different volumetric concentrations of Alumina (Al2O3) inside a square enclosure heated from one side. Mahdi and Nsofor [10] have presented a numerical study to illustrate the effect of utilizing fins, nanoparticle and a combination of nanoparticle-fins in the PCM-TTHX. The results indicated that PCM melting is improved by these techniques. Darzi et al. [11] simulated horizontal annulus configurations consist of two circular cylinders in addition to one elliptical cylinder and one finned cylinder, which are separately in a circular cylinder. Enhancing the melting and solidification rates can be effectively seen because of increasing the volume fractions of Copper nanoparticle concentration. Nurten et al.
Inner Initial time Liquid Middle Nanoparticle Nano-PCM Outer Phase-change material Reference Solid
[12] enhanced the thermal conductivity of PCM by preparation of Paraffin Nano Magnetite (Fe3O4) composites. The results clearly indicate that the dispersion of Fe3O4 nanoparticles is an efficient and cost effective method. Wentao et al. [13] reported novel magnetic, sunlightdriven energy conversion and storage nanocomposites based on Fe3O4 functionalized Graphene Nano Sheet (Fe3O4-GNS) embedded form stable Polymer-PCMs. Yuang et al. [14] employed Polyethylene Glycol (PEG) composite PCM for enhancing solar thermal energy conversion and storage capacity, supported by Silver nanoparticle functionalized Graphene Nano Sheet. Ahmet et al. [15] also investigated PEG600 that incorporated with Raw Diatomite (RD)/Carbon Nano Tubes (CNTs) precomposites to abolish its leakage problem during phase-change, to amplify LHTES capacity depending on the increased impregnation ratio and to reduce the heat during the charging and discharging times. On the other hand, the latest progress for enhancing TES that has used metal foam was by Xiaohu et al. [16] who simulated a shell and tube unite and employed an open-cell metal foam with a porosity of 0.94 and pore density of 15 pore/inch for PCMs domains. The influences of the metal foam location and porosity on the heat storage were analyzed. Xiaohu et al. [17] also displayed the melting behaviors of pure PCM and PCM embedded in open-cell metal foam. The experiments are carried out at 0°, 30°, 60° and 90° inclination angles, respectively. The melting time is reduced by 12.28%, 22.81% and 34.21% at 0°, 30° and 60°, respectively compared with the case at 90°. Another published about estimating the effective thermal conductivity of PCMs having two distinctive phases (solid, liquid or liquid-solid mixed phases) was presented by [18]. Generally, there seems to be some evidences to indicate that this paper covers nanoparticle, metal foam and so on, which have been played an important role for enhancing heat-transfer rates and thermal conductivity of PCM during the charging and discharging processes, respectively. The specific objective of this research was to evaluate the performance of the TTHX unite using a combination of fins-nanoparticle, included: triangular fins-nanoparticle and longitudinal finsnanoparticle during the PCM discharging and employed the energy 2
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3. Numerical approach
released for Liquid Desiccant Air Conditioning Unite.
3.1. Physical models 2. Experiments and procedures In this study, the TTHX model used for our application consists of (1) TTHX with internal longitudinal fins and (2) TTHX with internal triangular fins as illustrated in Fig. 4. The difference between TTHX with longitudinal fins and TTHX with triangular fins was the base (thickness) of triangular fin, which was 25 mm. The both-sides freezing is the major method used to supply the cooling rate from both inner and outer tubes. The experimental data illustrated that the minimum temperature required to operate the PCM-TTHX unite was approximately 65 °C .
2.1. Experimental apparatus The operation range of most engineering applications must match the PCM melting temperature employed in PCM-TES systems. Akgun et al. [19] examined commercial- grade PCMs that displayed stable properties after 1000 to 2000 cycles. Table 1 and Fig. 1a, illustrate the selected Paraffin (RT82) (RUBITHERM GmbH-Germany) with thermophysical properties used for Liquid Desiccant Air Conditioning Unite. The minimum discharging temperature requires for this unite is in the range of 65 ℃ −70 ℃ [28]. The nanoparticle that selected to improve the thermal conductivity of Paraffin is Al2O3 as shown in Fig. 1b. The melting point of Al2O3 (2345 ℃) is much higher than the Paraffin (RT82) as illustrated in Table 1. Thus, the Al2O3 remained all the time in the solid state at the average charging and discharging temperatures of 90 °C and 65 °C, respectively. The LHTES system consists of many parts, see Fig. 2. The basic unite is a TTHX as shown in Fig. 3, and consists of inner, middle and outer tubes with 76.2 mm, 381 mm and 500 mm diameters with 3 mm thickness and 3000 mm length, respectively. Eight internal longitudinal fins were equally positioned and welded around the circumference of the inner tube at 45°. Each longitudinal fin has 121 mm pitch, 2 mm thickness and 2800 mm length. The inner and middle tubes and fins are made of copper, while the outer tube is made of steel. The inner and outer tubes were used for HTF, while the middle tube was filled in by 100 kg of the Paraffin (RT82). Furthermore, the Alumina nanoparticles have regularly been dispersed and distributed by 10 kg into three slots of the TTHX as displayed in Fig. 2. There are three main concentrations of Al2O3 used (0%, 5% and 10% by weight) to improve the thermal conductivity of the PCM. As mentioned in Table 1, selecting the nanoparticle to avoid accumulating that maybe occurred in experimental processes. Some nanoparticles, such as Copper maybe accumulated sometimes on the bottom due to its highest density. A 70-mm thick Glass Wool insulation (k = 0.04 W/m K) was wrapped around the TTHX and pipes to reduce heat lost and to make the surface adiabatic. Two Thermocouples (T-type) on the top and bottom of the Storage Tank were fitted in the axial direction and were used to measure the temperatures of the PCM solidification, 500 mm and 1100 mm away from the HTF tube entrance, respectively. Thermocouples (J-type) and Flow Meter are currently the most popular tools for measuring the HTF inlet and outlet temperatures and HTF rate, respectively. A Data Logger and a Personal Computer achieved the data recording for the discharging processes (PCM temperature and HTF rate).
3.2. Governing equations There are some assumptions can be considered:
• The melting behaves as an Incompressible Newtonian Fluid. • The convective motion is laminar and two-dimensional. • No slip conditions were applicable for the velocity components at the boundaries. • The thermo-physical properties of the Paraffin (RT82) are constant for both solid and liquid phases.
During the PCM melting, the influence of the natural convection is specifically pointed by invoking the Boussinesq approximation, which is valid for the density variations of the buoyancy force; otherwise, this influence is ignored. The density variation is defined by:
ρ = ρl /(β (T − Tl ) + 1)
(1)
where ρl represents the PCM density at the melting temperature of Tl , and β is the thermal expansion coefficient. The temperature distribution and viscous Incompressible Flow are solved by the Navier-Stokes and thermal energy equations, respectively. The continuity, momentum and thermal energy equations are expressed by [20]: The continuity equation:
∂ (ρv ) ∂ (ρu) ∂ρ =0 + + ∂y ∂x ∂t
(2)
The momentum equation:
∂p ∂ (ρvu) ∂ (ρuu) ∂ (ρu) ∂ ∂u ∂ ∂u + ρ g x + Sx = (μ )+ (μ )− + + ∂x ∂y ∂x ∂x ∂y ∂y ∂x ∂t (3a)
∂p ∂ (ρvv ) ∂ (ρuv ) ∂ (ρv ) ∂ ∂v ∂ ∂v + ρ g y + Sy = (μ ) + (μ ) − + + ∂y ∂y ∂x ∂x ∂y ∂y ∂x ∂t (3b)
2.2. Discharging process
The energy equation:
∂ (ρvh) ∂ (ρuh) ∂ (ρh) ∂ ⎛ ∂T ⎞ ∂ ⎛ ∂T ⎞ + = + + k ⎜k ⎟ ∂y ⎝ ∂y ⎠ ∂y ∂x ⎝ ∂x ⎠ ∂x ∂t
After the PCM is entirely charged, the discharging process starts. The water was circulated by the Pumps from the Water Storage Tank (500 L) to the TTHX. The valves were closed to prevent the water from going to Evacuated Tubes Solar Collector as shown in Fig. 2. The cooling rate is transferred to the inner and outer tubes of the TTHX and then by the conduction heat-transfer to the PCM or nano-PCM in the middle tube with both-sides freezing method. The initial temperature of the Paraffin was set at 90 °C. The inlet HTF was at average discharging temperature of 65 °C and continued to the TTHX during a closed cycle. However, the Alumina nanoparticle remained in a solid state at HTF temperature during this process. The HTF temperature cannot also remain constant and continues to circulate until the PCM was completely discharged.
⎜
⎟
(4)
Table 1 Thermo-physical properties of Paraffin (RT82) and Alumina (Al2O3). Properties
Paraffin (RT82)
Al2O3
Density, solid, ρs (kg/m ) Density, liquid, ρl (kg/m3) Specific heat, C pl , C ps (J/kg K)
950 770 2000
3600 – 765
Latent heat of fusion, L (J/kg) Dynamic viscosity, μ (kg/m s) Melting temperature, Tm (°C ) Thermal conductivity, k (W/m K) Thermal expansion coefficient (1/K)
176000 0.03499 78.15 -82.15 0.2 0.001
– – 2345 36 –
3
3
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Fig. 1. The granules survey of (a) Paraffin (RT82) and (b) Alumina (Al2O3). Fig. 2. Schematic of the experimental apparatus of the LHTES system. It includes: 1. Evacuated tubes solar collector, 2. Flow meter, 3. TTHX, 4. T-type thermocouple, 5.J-J-type thermocouple, 6. Internal longitudinal fins, 7. diaphragm expansion tank, 8. Pump, 9. Data logger, 10. Computer, 11. Water storage tank, 12. Electrical heater, 13. Pipes, 14. Valve twoways and 15. Valve three-ways.
where ρ represents the density of the PCM, u and v are the fluid velocities, μ is the dynamic viscosity, p is the pressure, g is the gravitational acceleration, k is the thermal conductivity, h is the sensible enthalpy and T represents the temperature. The sensible enthalpy equation can be expressed by:
h = href +
∫T
T
ref
Cp ΔT
(5)
where href represents the reference enthalpy at the reference temperature Tref ; Cp is the specific heat. The total enthalpy H equation can be defined by: Fig. 3. Triplex-Tube Heat Exchanger (TTHX).
H = h + ΔH
(6)
where ΔH represents the latent heat content of the PCM; which changes 4
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the tube wall represented the HTF temperature [22,23] and was approximately 90 °C for melting and 65 °C for solidification. The boundary conditions of the TTHX can be written by: Both-sides heating or freezing method:
at r = ri → T = THTF
(10)
at r = rm → T = THTF
(11)
Initial temperature of all models:
at t = 0
→
(12)
T = Tini
In case of the nano-PCM, the same conservation equations, boundary and initial conditions are applied. The difference is that the melting temperature of the Al2O3 is much higher than that of the Paraffin (RT82) as shown in Table 1. Therefore, the Alumina nanoparticle remained in the solid state at the HTF temperature (90 °C and 65 °C ) during the melting and solidification processes. 3.4. Thermo-physical properties Depending on a commercially available material (Rubitherm GmbH-Germany (RT82)) supplied by company and nanoparticle. Table 1 illustrates the thermo-physical properties of the Paraffin (RT82) and Al2O3. In this research, the properties of the nano-PCM can be determined by [24]: The density equation:
ρnpcm = ϕρnp + (1 − ϕ) ρpcm
(13)
The specific heat capacity equation:
Cp, npcm =
ϕ (ρCp)np + (1 − ϕ)(ρCp)pcm ρnpcm
(14)
The latent heat equation: Fig. 4. Physical configurations of the TTHX model with (a) Internal longitudinal fins and (b) Internal triangular fins.
Lnpcm =
between zero (solid) and L (liquid); and γ is the liquid fraction, which is generated during the phase-change between the solid and liquid states when the temperature is Tl > T > Ts . This can be written as:
γ = ΔH / L
(7)
γ=0 ifT < Ts γ=1 if T > Tl T − Ts γ= if Ts < T < Tl Tl − Ts
(8)
Sx = C (1 −
Sy = C (1 − γ )2 where C (1 −
u γ3 + ε
(15)
μnpcm = 0.983e (12.959ϕ) μpcm
(16)
The effective thermal conductivity of the nano-PCM includes the effects of the particle size (dnp ), particle volume fraction (ϕ) and temperature dependence, as well as the properties of the Paraffin (RT82). The particle subject to Brownian motion is given by Vajjha et al. [25].
knpcm =
knp + 2kpcm − 2(kpcm − knp ) ϕ knp + 2kpcm + 2(kpcm − knp ) ϕ + 5 × 10 4γk ξ ϕρpcm Cp,pcm
kpcm
BT f (T ,ϕ) ρnp dnp
(17)
(9a) where B is the Boltzmann constant (1.381 ×
v γ3 + ε
u γ )2 3 γ +ε
ρnpcm
The dynamics viscosity of nano-PCM is given by Vajjha et al. [25]:
The source term S in the momentum equations and Eqs. (3a and 3b) can be defined in x and y directions, respectively as:
γ )2
(1−ϕ)(ρL)pcm
γk =
(9b) and C (1 −
v γ )2 3 γ +ε
are the "porosity function"
J/K) and
8.4407(100ϕ)−1.07304
f (T ,ϕ) = (2.8217 × 10−2 ϕ+ 3.917 × 10−3)
defined by Brent et al. [21]. C is a constant, which describes how sharply the velocity is reduced to zero when the material solidifies. This constant varies between 104 and 107 (105 is considered for this study). ε is a small number (0.001) to prevent division by zero.
10−23
− 3.91123 × 10−3)
(18)
T + (−3.0669 × 10−2 ϕ Tref (19)
where Tref =273 K represents the reference temperature. There is a correction factor ξ in the Brownian motion term because there should be no Brownian motion in the solid phase [9]. Its value is the same as for the liquid fraction γ in Eq. (8). In this study, we applied the effects of the particle size (dnp =20 nm), particle volume fraction (ϕ= 10%) and reference temperature (Tref =273 K) in Eq. (17).
3.3. Boundary and initial conditions At the initial time, the Paraffin (RT82) was in the solid state, and the temperature was 27 °C for melting process. The constant temperature of 5
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3.5. Computational methodology The most popular methods for solving the PCM model were the enthalpy-porosity and the finite-volume [26]. That was with the Software Ansys Fluent 16 and the procedure for that can be summarized as shown below:
• Drawing and meshing the model in two-dimensions (r , θ) and de• • • • •
fining the boundary layer conditions and zone types by Software Gambit 2.4.6. To input the data and find the solutions for problems, the PCM model was exported to the Fluent 16 Software. Applying the equations for the continuity, momentum and energy in addition to solving these equations with the boundary and initial conditions by finite-volume method basing on the transient time and pressure-based solver with absolute velocity. Discretizing the equations of momentum and energy by the secondorder upwind and discretizing the pressure by the PRESTO method. Employing the SIMPLE scheme in pressure-velocity coupling and applying the under-relaxation factors (0.3, 0.2, 0.7, 1 and 0.9) for the pressure, velocity, momentum, energy and liquid fraction, respectively. However, applying the convergence for the energy equation (10−6) and the velocity equations (10−3). The final set selected the data (0.1 s) as the standard value, and (0.5 and 1 s) were for the some measurements. These were for the timestep sensitivity.
Fig. 6. Validation of the experimental and numerical data of the pure PCM and nano-PCM in a TTHX with internal longitudinal fins.
℃. Furthermore, the HTF discharging temperature for both-sides freezing method is 65 ℃ with an experimental mass flow rate of 37.5 kg/min. 4. Results and discussions Much of the heat exchanger research has focused on designing and employing the fins to improve the heat-transfer. That was because of its simplicity, easy fabrication and low construction cost [2]. Recent developments in the field of TES have led to a renewed interest in dispersing nanoparticle to enhance the thermal conductivity of PCM.
Fig. 5 shows the results obtained from this analysis, the grid sizes were 66,536 and 56,200 for the internal triangular fins and internal longitudinal fins, respectively. Further details in Computational Fluid Dynamics (CFD) related with this study can be found in Ref. [27].
4.1. Experimental results 4.1.1. Average charging temperature Figs. 7 shows the charging temperature of the PCM and nano-PCM versus time for both-sides heating method with a flow rate of 37.5 kg/ min. Data for these experiments were collected with the average charging temperature of 90 ℃. The reason for taking this temperature with time is the intermitted nature of the solar irradiance. It should not be less than the Paraffin (RT82) melting temperature (78.15 ℃− 82.15 ℃) to achieve the process.
3.6. Numerical model validation Fig. 6 shows the validation of the experimental and numerical average temperature of the pure PCM and nano-PCM in a TTHX with internal longitudinal fins. The early stage of the solidification process represents the sensible cooling of the PCM as a liquid. With time, the experimental findings showed close agreement with the numerical results based on the four Thermocouples of T-type inserted into the PCM and nano-PCM, while the numerical findings depended on the full section of the model. The percentage errors between the numerical and experimental results did not exceed 3% for two cases. The data were recorded when the average temperature of the Paraffin (RT82) was 90
4.1.2. Nanoparticle dispersed technique Fig. 8 illustrates the experimental data for the effect of the nanoparticle concentrations on the PCM solidification. Generally, Increasing
Fig. 5. Distribution of the grids-size number in the middle tube of the TTHX with (a) Triangular fins and (b) Longitudinal fins. 6
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solidification rate reduced as the angle direction increased from θ = 90° to θ = 270°. This was due to the cooling rate at the upper tube was higher than the lower tube. Thus, the solid layer can fully be seen on the top of tube, as fast as on the bottom of the same tube. The most surprising aspect of the data is in Fig. 11b that also illustrates the average temperature variations of the PCM along the angular direction after depressing 10% of nanoparticle. The main reason for improving discharge rate, compare with the pure PCM was that depressing the nanoparticle that improved the thermal conductivity of the Paraffin (RT82) (0.2 W/m K) to 25%. 4.1.6. Thermodynamic analysis of nano-PCM discharging process Fig. 12 indicates the inlet and outlet temperatures of the HTF, the average temperature of the nano-PCM via the time for the TTHX with internal longitudinal fins. For the PCM solidification process, the discharging temperature was at 65 ℃ with a flow rate of 37.5 kg/min. The 10% of Al2O3 depressed in the PCM-TTHX unit to enhance the thermal conductivity and cooling rate. With nanoparticle, there was no change to the deference between the inlet temperature and outlet temperature. As the phase-change begun, the difference between the inlet temperature, outlet temperature and PCM temperature was gradually reduced. This was for a long time until the PCM was entirely discharged.
Fig. 7. The charging temperature versus time for both-sides heating method with a flow rate of 37.5 kg/min.
4.2. Uncertainty analysis The uncertainty of a measured quantity is an error of measurement analysis for illustrating the level of confidence of the experimental results. There are two components can be split for the uncertainty of PCM thermal properties: random and systematic components. For example, the onset point, peak point and latent heat of fusion, which occurred during the charging and discharging methods. Random components can be defined as the standard error, which can statistically analyze by (1) determining the mean and (2) calculating the standard deviation. This is for a finite set of measurements. In these experiments, some measurements were ignored from the set depended on the Chauvenet’s criterion to reject certain measurement, according to Holman [29]. Ultimately, it can be depending on the modified measurement set, a new mean and standard deviation can effectively be determined. Table 2 shows the experimental data on the new mean and the standard uncertainties for the Paraffin (RT82) thermal measurement with confidence interval 90% of its thermal properties.
Fig. 8. Effect of the nanoparticle concentrations on the PCM discharging process.
the Al2O3 concentration reduces the PCM discharging time. Thus, the Paraffin (RT82) with 10% Al2O3 was the best choice where the discharging rate enhancement was pronounced. 4.1.3. Discharging method mechanism As mentioned earlier, the discharging process starts after the PCM is entirely charged. Fig. 9 shows the charging and discharging temperatures of the PCM and nano-PCM using both-sides heating and freezing methods, respectively with a flow rate of 37.5 kg/min. For both-sides freezing method, the cold water was supplied from both inner and outer tubes of the TTHX, in which an average discharging temperature supplied to the Paraffin (RT82) by conduction at 65 ℃. The temperature readings for PCM depended on Thermocouples (T-type) location as shown in Fig. 2. Another important finding was that after using 10% of nanoparticle, the enhancement in discharging rate was better than the pure PCM. This was due to increasing the cooling rate between the surface of the fins, middle tube and the PCM liquid. With time, the cooling rate is improved because of augmenting the thermal resistance owing to increase the solid layer front of the PCM in the TTHX.
4.3. Numerical results 4.3.1. Fins technique for enhancing the PCM solidification After the Paraffin (RT82) is entirely melted, the solidification process starts between the cold wall of the middle tube, the cold wall of fins and the PCM liquid. A thin layer of the Paraffin solid is gradually formed at the early stage to surround the fin and the tube surfaces. The
4.1.4. The effect of the HTF mass flow rate The influence of mass flow rate on the PCM solidification for bothsides freezing method was carried out depending on the change in mass flow rate of 16.2, 29.4 and 37.5 kg/min as shown in Figs. 10a and b for PCM and nano-PCM. Increasing the mass flow rate increases the cooling rate that causes decreasing the discharging time. This was due to increase the thermal resistance owing to increase the solid layer front of the Paraffin (RT82), especially after depressing nanoparticle, compare with the pure PCM. The PCM was fully solidified in shorter time at mass flow rate of 37.5 kg/min. Fig. 9. Charging and discharging temperatures of the PCM and nano-PCM using both-sides heating and freezing methods, respectively with a flow rate of 37.5 kg/min.
4.1.5. Angular temperature variations Fig. 11a displays the average temperature variations along the angular direction for both-sides freezing method at Ti = 65 ℃. The 7
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Fig. 12. The inlet and outlet temperatures of the HTF, the average temperature of the PCM versus the time for the TTHX with longitudinal fins. Table 2 The new mean and the standard uncertainties of the PCM thermal properties. Solidification process Onset point (℃ )
Peak point (℃ )
Heat of fusion (kJ/kg)
81.86 ± 0.02828
78.158 ± 0.090358
207.807 ± 1.359165
part of Paraffin retained in a liquid state because of the limitation of the natural convection effectiveness. There were small convection cells created and extended after 10 min inside the middle tube and specifically on fins walls. With increasing the time, large convection cells were formed at 60 min. The solid fraction of the Paraffin is increased and extended to reach on the bottom part of the middle tube at 240 min. There are some factors were dominated during this process, including the effects of natural convection driven by buoyancy and the quick of solid layer of the PCM, which is squeezed because of its high density. The solid fraction was effectively increased in all directions of the tube with time. As a result, the liquid fraction contours of the PCM solidification in the TTHX are displayed in Fig.13. The Paraffin (RT82) was fully solidified at 780 min and 668 min for longitudinal and triangular fins, respectively.
Fig. 10. a The effects of mass flow rate for both-sides freezing method on the solidifying PCM. b The effects of mass flow rate for both-sides freezing on the solidifying PCM under effect of the nanoparticle.
4.3.2. Nanoparticle dispersed technique Eqs. (17–21) calculated some of the thermo-physical properties of the nano-PCM under the effect of different volumetric concentrations of nanoparticle. The thermal conductivity and dynamic viscosity of the nano-PCM were higher than those of the pure PCM as shown in Table 3. It can also be seen that the specific heat and latent heat of the nanoPCM were lower than those of the PCM without nanoparticle. The dynamic viscosity of the nano-PCM is enhanced, and this may be affected on the natural convection effectiveness that contributed the melting and solidification. This variation in the thermal conductivity and dynamic viscosity accepted well with the findings reported in Ref. [9]. 4.3.3. Fins-nanoparticle for enhancing the PCM solidification As mentioned earlier for the behavior of pure PCM solidification, the behavior of the nanoPCM was similar, but the reduction in solidification time was improved. The cooling rate between the fins wall, the middle tube wall, the nanoparticle surface and the Paraffin liquid is directly started after the PCM is entirely melted. At the early stage, a very thin layer of PCM solid is created to surround the fins surface and the cold wall of the middle tube under the effect of nanoparticle. The rest of the PCM is retained liquid without any phase-transition because the effects of the natural convection at the beginning were limited. The Paraffin solid layer also gradually grows up at various times because of increasing the natural convection effectiveness. Over time, the
Fig. 11. a The average temperature variation along the angular direction for both-sides freezing method. b The average temperature variation along the angular direction for both-sides freezing method under effect of the nanoparticle.
8
Journal of Energy Storage 24 (2019) 100784
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Fig. 13. Liquid fraction contours of the PCM in the TTHX obtained using longitudinal and triangular fins.
consumes a short time to produce the biggest demand thermal energy released, compared with the same PCM volume. At the same time, the liquid fraction values for nano-PCM were also lower than those of pure PCM as shown in Table 4. The total energy more stores inside the PCM, but consumes a longer time to produce the biggest demand thermal energy released for Liquid Desiccant Air Conditioning Unite.
Table 3 Variation of the thermal conductivity and dynamic viscosity of the nano-PCM. Volumetric concentration ϕ (%)
Thermal conductivity k (W/m K)
Dynamic viscosity μ (kg/m s)
Simple PCM Nano-PCM with 1% Al2O3 Nano-PCM with 4% Al2O3 Nano-PCM with 7% Al2O3 Nano-PCM with 10% Al2O3
0.2 0.206 0.225 0.245 0.265
0.03499 0.0121161 0.0485 0.084812 0.121161
4.5. Comparison of PCM solidification time Fig. 16 compares the numerical data on the PCM solidification time for the TTHX with longitudinal fins and TTHX with triangular fins, with and without nanoparticle. As a general result, the thermal conductivity of the Paraffin (RT82) (0.2 W/m K) enhanced to 25% by dispersing 10% of Al2O3. The most interesting findings were that the solidification time was minimized with internal longitudinal fins-nanoparticle and internal triangular fins-nanoparticle to 33% and 34%, respectively, compared with the pure Paraffin. The main reason for that is, to enhance the cooling rate between the fins wall, the middle tube wall and the Paraffin into TTHX by combination of fins-nanoparticle.
convection cells extended to reach on the bottom part of the middle tube. Thus, the solid fraction of the PCM was strongly extended in all directions with time. The Paraffin solid was squeezed due to its high density, and the natural convection effect was driven by buoyancy. As a result, the liquid fraction contours of the PCM solidification under the effect of 10% nanoparticle can be seen in Fig.14. The times required for achieving the full PCM solidification in the TTHX with longitudinal fins and the TTHX with triangular fins were 520 min and 441 min, respectively.
5. Conclusions 4.4. Total energy released The purpose of this work was to assess the performance of the TTHX unite using a combination of fins-nanoparticle during PCM discharging and employed the released energy for Liquid Desiccant Air Conditioning Unite. The both-sides freezing was used as main method to experimentally investigate the influence of mass flow rate (16.2, 29.4
Fig. 15 illustrates the total energy released for the PCM and nanoPCM in TTHX with internal triangular fins as a sample simulated using the Software Ansys Fluent 16 at the different times of the freezing process. The nano-PCM has a lower energy released capacity, but 9
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Fig. 14. Liquid fraction contours of the PCM in the TTHX obtained using longitudinal and triangular fins-nanoparticle.
Fig. 16. Comparison of PCM solidification time for TTHX with and without nanoparticle.
Fig. 15. The total energy released for the PCM and nano-PCM in TTHX-internal triangular fins.
and 37.5 kg/min) on the PCM solidification. This study has also shown that the discharging rate minimized as the angle direction increased from θ = 90° to θ = 270°. Numerically, the Paraffin (RT82) was fully solidified at 780 min and 668 min for TTHX with longitudinal fins and TTHX with triangular fins, respectively. However, the total energy more stores inside PCM, but consumes a longer time to produce the biggest demand thermal energy released. The most obvious findings to emerge from this research are with nanoparticle, an important enhancement for the PCM discharging (solidification), compared with these without nanoparticle. Close agreement can be seen between numerical and experimental data. Ultimately, the combination of fins-nanoparticle has a
Table 4 The Liquid fraction and total energy released of the PCM and nano-PCM in TTHX with internal triangular fins. Time (min)
30 60 100 160
Liquid fraction
Total energy released (kJ)
PCM
nano-PCM
PCM
nano-PCM
0.798801 0.676442 0.558266 0.443534
0.752145 0.608758 0.477617 0.358431
240.9688 215.6957 192.0092 168.8889
163.4294 145.8297 129.4984 113.767
10
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number of important implications for future practice in TES systems. [14]
Acknowledgements The authors gratefully appreciate the financial supports provided by Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia (UKM), Malaysia.
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