Numerical Study of PCM Melting in Evacuated Solar Collector Storage System

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1 Numerical Study of PCM Melting in Evacuated Collector Storage System MOHD KHAIRUL ANUAR SHARIF, SOHIF MAT, MOHD AFZANIZAM MOHD ROSLI, KAMARUZZAMAN SOPIAN, MOHD YUSOF SULAIMAN, A. A. Al-abidi. Energy Research Institute, Universiti Kebangsaan Malaysia Bangi, Selangor. MALAYSIA. Abstract: - domestic hot water is a cost effective and efficient way to pre-heat domestic water for use in buildings. Latent heat thermal energy storage promises high performance and reliability of high storage density at nearly constant temperature energy with advantage for removing the discrepancy between energy supply and demand as well as for improving the efficiency of solar energy systems. This study numerically investigates the melting process of phase change material (PCM) in evacuated solar collector incorporating an absorber RT82 phase change material storage system (ESPCMS). The numerical simulation is conducted to verify the effect of PCM melting process at different solar radiation intensity at 400W/m 2, 600W/m 2 and 800W/m 2 at no-flow in u- tube heat transfer fluid. A 3D numerical model is developed using the Fluent 14.0 software program. The pure conduction and natural convection are considered in the literature. The simulated results show that the increasing of solar intensity has significant effects on the time of completion of the melting of the PCM in the ESPCMS. Key-Words: - Melting, Evacuated solar collector PCM storage system, Phase change material, Fluent. 1 Introduction The increase in the fuel prices and the huge raise in greenhouse gas emission are the main driving force behind the efforts for utilizing of different sources of renewable energy. Since the solar energy considers as one of the periodic energy sources, which led to low efficiency of most solar energy systems, it is becoming increasingly difficult to ignore the mismatch between the energy supply and energy demand for the thermal solar application. Most the thermal solar energy requires thermal energy storage to eliminate the mismatch between energy supply and demand and to improve their efficiency. The storage of energy in suitable forms, which can conventionally be converted into the required form, is a present day challenge to the technologists. Energy storage not only reduces the mismatch between supply and demand but also improves the performance and reliability of energy systems and plays an important role in conserving the energy [1]. Latent heat energy storage system (LHTES) promises high performance and reliability with the advantages of high storage density and nearly constant thermal energy [2]. PCMs are limited in their usage as thermal energy storage because of their low thermal conductivity, a disadvantage that lengthens the time necessary to complete the melting and solidification processes. Several researchers investigate LHTES that can operate under both consecutive and simultaneous charging and discharging modes. There has been research published on LHTES including studies of various PCM thermal properties, LHTES geometry, heat transfer mechanisms, and heat transfer enhancement design. There are also many studies conducted for PCM heat transfer and thermal properties and organic PCMs have been shown to be less corrosive, less sub-cooling effect, and less deviation of thermal properties during melting and freezing cycles then in organic. Many storage system geometries have been considered as well, and vertical cylindrical containers and multi-tube arrays have been shown to be the best geometry for enhancing heat transfer due to the large heat transfer surface area and the presence of natural convection. The numerical study PCM melting in a high efficiency evacuated solar collector incorporating an integrated absorber phase change material has been conducted. The simulation also used to determine the optimum operating parameters (inlet water temperature, flow rate and solar radiance intensity) to enhance the performance ISBN:

2 of the LHTES.PCM has been shown to be safe, relatively inexpensive, and has a melting temperature in range 50 c - 85 C suitable for use with SDHW [3]. PCM used in the simulation as its properties, high capacity energy storage and temperature range inside solar evacuated tube is high suitable for RT82 melting temperature. In this study type of vacuum tube collector, where the absorber strip is located in an evacuated and pressure proof glass tube. In this paper the melting fraction of PCM and heat transfer to HTF inside the evacuated tube is studied numerically. 2 PHYSICAL AND NUMERICAL MODELS Figure 1. shows the physical configuration of the ESPCMS, which has PCM container copper tube radius of 38.1 mm with thickness of 1.2 mm. In middle u- tube radius ru 6.1mm and distance between centre to centre are 20mm, with 2 mm thickness as shown in Figure 2. Copper pipes were used to ensure high thermal conductivity. The inner U-tubes were used for the HTF (water), whereas the outer tube as a container was used for the PCM that is based on a commercially available material, Rubitherm GmbH RT82. The PCM container will be placed in the middle of evacuated tube where water as heat transfer fluid (HTF) flows through in the middle of PCM container directly in a U-Tube Copper piping as shown in Figure 1. The thermophysical properties are listed in Table 1: Figure 1: Cross Section of Evacuated Tube with PCM Container Storage System T1 (a) 20mm Ø6.1mm (0.25 inches) Ø38.1mm (1.5 inches) (b) FIGURE 2 : Schematic diagram of PCM Container (a) and (b) ISBN:

3 Table 1. Thermo-physical of the PCM Property Unit Value Density of PCM, solid,ρs kg/m Density of PCM, liquid, ρl kg/m Specific heat of PCM, liquid, Cp,l, Cp,s J/kg K 2000 Latent heat of fusion, L J/ kg Melting temperature, Tm K Thermal conductivity, k W/m K 0.2 Thermal expansion coefficient 1/K Dynamic Viscosity, Kg/m.s Governing equation The mathematical equations of the melting process of the PCM inside the container of the ESPCMS indicate a laminar, unsteady, and incompressible flow do not write the same just paraphrase. The thermal resistances of the container and u-tubes, as well as the viscous dissipation, are negligible. The effect of natural convection during melting is considerable, whereas the thermophysical properties of the HTF and PCMs are independent of temperature. The viscous incompressible flow and the temperature distribution are solved using the Navier- Stokes and thermal energy equations, respectively. The continuity, momentum, and thermal energy equations can be written as follows [4]. Continuity: p is the pressure, g is the gravity acceleration, k is the thermal conductivity, and h is the sensible enthalpy. The sensible enthalpy can be expressed as: h = h ref + cp T (4) T ref T H, can be defined as: H = h + H (5) Where h ref is the reference enthalpy at the reference temperature T ref, cp is the specific heat, H is the latent heat content that may change between zero (solid) and L (liquid), the latent heat of the PCM, and β is the liquid fraction which happens during the phase change between the solid and liquid state when the temperature is T l > T > T s, so it can be written as: t (ρ) + i (ρu i ) = 0 (1) Momentum: β = H L (6) t (ρu i ) + i ρu i u j = µ jj u i i p + ρg i + S i (2) Energy Equation: t (ρh) + i (ρ H) + i (ρu i h) = i (k i T) (3) Where ρ is the density of PCM (RT82),µ i is the fluid velocity, µ is the dynamic viscosity, 0 if T < T s β = 1 if T < T l (7) (T T s )/(T l T S ) if T l > T > T s The source term S i in momentum equation, Eq. (2), is defined as: Numerical modeling S i = C(1 β) 2 u i β 8 (8) ISBN:

4 A commercial computational program (Fluent 14.0), solidification and melting models, were used to simulate the melting process of the PCM. A ESPCMS dimensions (r, θ) with tilt angle 7 from horizontal were drawn and meshed in a geometric model Ansys Fluent 14.0 and define the boundary layers and zone types.. A half-section computational grid was used to reduce the time required for simulation. The PRESTO scheme was used for the pressure correction equation, and a SIMPLE algorithm was used for the pressure velocity coupling. The under relaxation factors for the pressure, velocity, energy, and liquid fraction were 0.3, 0.7, 1, and 0.9, respectively. Different grid meshing and time steps for the models were carefully tested during the primary calculations. The independency of the time steps from the melt fraction was examined for the simulations at 0.5 and 0.2s (chosen). In addition as shown in Figure 3, two grid sizes of 172,449 (chosen) and cells were investigated to validate the independency of the grid size from the numerical solution. The grid size of cells with 0.2 s time step was chosen for the calculation because it is sufficient to achieve the predetermined convergence of energy equation (10-5 ). 3 Result and discussion Fig. 4 shows the effect of the solar radiation on the melting fraction of the PCM inside the ESPCMS under non flowrate of HTF, thickness, and boundary conditions. As can be seen in Fig.4, the melting rate time of PCM increased with higher solar radiation. In addition, the time required of the RT82 melting fraction is shortening. This effect increased with more solar radiation, indicating that the dilution time is shorter. Figure 3: Numerical study grid meshing in Ansys Fluent 14.0 The simulation results plotted in Fig. 5 show temperatures profiles against time similar to melting fraction where it increase with faster as the solar radiation increase. The simulated maximum temperature of HTF was cut-off when the simulation melting fractions become 1. Figure 6 a,b,c and d shows focusing on solar radiation 600W/m 2 simulation result, complete melting was achieved after 75 minute. Approximately 90% of the PCM melted after 60min. With putting tilt angle 7 from horizontal its shows that convection and conduction mechanisms controlled the heat transfer in the PCM and occurred immediately after 30 minutes. 4 Conclusion Three heating capacity were investigated by PCM melting in the ESPCMS. The effect of solar radiation on the melting process was studied; as the solar radiation increase, the average temperature of the PCM increased and a shorter time was required for PCM melting. With tilt angle 7 from horizontal it was obvious, convection and conduction were observed in the melting process. More studies will be conducted in the future on the effect of the solar radiation as well flowrate of HTF will be simulated and experimental validation will be conducted. Melting Fraction Flow Time W/m2 600W/m2 Figure 4 : Melting Fraction with Effect of ISBN:

HTF Temperature 500 480 460 440 420 400 380 360 340 320 300 Flow Time

5 HTF Temperature Flow Time W/m2 600W/m2 800W/m2 Figure 5 : HTF with Effect of a. 600W/m 2 Melting Fraction Time : 0 Minutes b. 600W/m 2 Melting Fraction Time : 30 Minutes ISBN:

6 c. 600W/m 2 Melting Fraction Time : 60 Minutes d. 600W/m 2 Melting Fraction Time : 67 Minutes Figure 6 : a,b,c and d Melting Fraction with Time Flow. References: [1] Garg HP, Mullick Bhargava AK, Thermal Energy Storage, Reidel Publishing Co [2] Atul Sharma, V.V. Tyagi, Review on thermal energy storage with phase change materials and applications, Renewable and Sustainable Energy Reviews 13 (2009),pp [3] S.D. Sharma, Latent heat storage materials and systems: A review, International Journal of Green Energy, 2: pp.1-56, 2005 [4] Abduljalil A. Al-Abidi, Sohif Mat, CFD applications for latent heat thermal energy storage: a review, Renewable and Sustainable Energy Reviews 20 (2013), pp ISBN:

7 Nomenclature C Mushy zone constant kg/m 3.s Greek Letters C p Specific heat of PCM kj/kg ρ Fluid density, kg/m 3 g Gravity acceleration m/s 2 Β Liquid fraction h sensible enthalpy, J/kg µ Dynamic viscocity kg/m.s H Enthalpy, J/kg L Latent heat fusion, J/kg k thermal conductivity (w/m.k) L Length (m) T Temperature ( k) u Velocity m/s Subscripts Ini HTF M Ref s l Initial Heat transfer fluid Melting Reference Solidus of the phase change materials Liquidus of the phase change materials ISBN:

Study on the flow and thermal characteristics of a heat storage system

THE ASIAN SYMPOSIUM ON COMPUTATIONAL HEAT TRANSFER AND FLUID FLOW - 2011, 22 26 SEPTEMBER 2011, KYOTO, JAPAN Study on the flow and thermal characteristics of a heat storage system Chung-Jen Tseng, Tzu-Yu