THERMAL CHARACTERISTICS OF PHASE CHANGE MATERIAL USED AS THERMAL STORAGE SYSTEM BY USING SOLAR ENERGY

In this paper, the melting processes of phase change material in a shell and tube heat exchanger by using solar thermal energy have been investigated numerically and experimentally. All experimental were outdoor tested at AL-Mussaib city-Babylon-Iraq (Lat 32.5 o North, and long 44.3 o East) with N-S collector direction at tilt angle of 32.5 o with the horizontal. The phase change material used in this work is black color Iraqi origin pure Paraffin with amount of 12 kg. In the experimental setup evacuted tube solar collector is employed for melting phase change material in shell regime. Different volume flow rates for the water flow inside the inner tube of heat exchanger namely (200, 300, and 500 LPH) for Reynolds number namely (15000, 23000, 38000) respectively were used for each season from August 2016 to January 2017. The numerical investigation involves a three dimension numerical solution of model by a commercial package ANSYS FLUENT 15.0. The boundary conditions of the model that solved by the numerical solution have been taken from the experimental tests. The experimental results indicated that the inner tube inlet and ambient temperatures has a significant effects on the melting process compared with the volume flow rates. Studying phase change material temperature distribution, it is exposed that a melting temperature of the phase change material in summer season needed time of (3-4) hours only, while it needed more time; (14-16) hours in winter season. Increasing solar radiation and ambient temperature reduces the melting time of phase change material. Increasing water temperature difference of inner tube increased the heat gained for phase change material. The results obtained from numerical solution presented the static temperature contours and showed that the temperature distribution of phase change material give good validations with experimental results with percentage deviation of 2.7%. The present experimental results have been compared with the previous studies and give a good agreement with increasing for present work of 25.9 %.


INTRODUCTION
Thermal energy storage (TES) is presented as the temporary holding of thermal energy in the form of cold or hot substances for later use Energy request change on seasonal, weekly and daily, bases. These request can be matched with the help systems that a labor synergistically,, and the agree with the storage of energy by heating, cooling ,melting, vaporizing or solidifying a material and the thermal energy be obtainable when the process is reversed. A considerable technology in systems included renewable energies as well as other energy resources as it can make their a process more efficient, particularly by bridging the interval between intervals when energy is harvested and intervals when it is necessary. That is beneficial for balancing between the equipping and request of energy (Fernandez, 2012 of the infiltration method on the rate of paraffin saturation was found, yielding better results with the vacuum system (Sinaringati et al., 2016), investigated experimentally the utilization of the paraffin and beeswax materials as a heat energy sources for infant incubator and made comparison between them. The results showed that the phase change material can keep heat energy in the infant incubator room at temperature above 32°C for more than 8 hours. The beeswax performed better heat energy storage than paraffin and best performance when used beeswax as the PCM for infant incubator or any other practical application.
The aim of this work is investigation the effect of using solar energy on the thermal characteristics of Phase change materials (PCM) which used for providing storage thermal energy under outdoor test for Iraq climate conditions at AL-Mussaib city-Babylon-Iraq ( Lat 32.5 º North, and long 44.3 º East).

Experimental rig
The Experimental rig includes vacuum tubes solar collector (VTSC) using with shell and tube heat exchanger is designed and fabricated in order to study the effect of different ambient conditions and collector water flow conditions on the thermal performance of PCM. The experimental rig and a schematic diagram of the comprehensive experimental system are shown connecting plastic pipes of (3/4 inch) to connect the parts of the setup system, glass wool insulations has a thickness of 15 mm to insulate all system pipes, and global valves for controlling the amount of water flow and Senior safety valves system used for preventing exposed to high-pressure upon the arrival of water at high temperatures.

Measurements and Instrumentation
The measurement devices that used in this work includes: Ten standard thermocouples type (K) with 0.4 mm diameter and 1 m long its locations distributed in the test rig are shown in Fig. 2 are used in this work which connected to Digital data logger type of 12 channels temperature recorder with an accuracy of (±4%). Nuritech flowmeter with range of (1.8-19 lpm) to measure water volumetric flow rate, Kipp and Zonen Pyranometer having a measuring range of up to 4000 W/m 2 to measure solar intensity which installed on the collector at tilt angle of 32.5 º with the horizontal, and Lutran multifunctional anemometer device to measure the wind speed and with measurement range of (0.4-30) m/s.

Phase Change Materials
There is one type of PCM used in this work, the type is black Iraqi origin pure Paraffin with amount of 12 kg which shown in the Fig. 3.The physical properties of PCM are shown in Table   1.

Procedure of Experimental Test
The main experimental procedure in the present work is included using the solar collector type of evacuated tube for melting PCM. All experimental works were outdoor tested at AL-Mussaib city-Babylon-Iraq (Lat 32.5 º North, and long 44.3 º East) with N-S collector direction at tilt angle of 32.5 º with the horizontal due to a high incident solar radiation absorbed at this angle (Ashrae, 1999). Fig. 1 shows the experimental setup of utilization the paraffins as heat energy storage in shell regime of the heat exchanger. The outer shell wall was made from aluminum as a circular cylinder shape with dimensions of 400 mm on height, 200 mm on diameter and a rectangular tube was made from stainless steel with cros section dimensions of (60 x 80) mm and 1 mm thickness wall with 400 mm hight. Previously physical properties of paraffins were measured using differential scanning calorimetry (DSC) (Abhat, (1983), Lane, (1983). 12 kg of melted paraffin was inserted in the shell regime while water flow through the tube which circulated from and to solar collector. The steps adopted for each experimental test include; Kufa Journal of Engineering, Vol. 9, No. 1, 2018 9 connect all thermocouples to data loggers, then adjustment date and time recorder of data loggers. Data from all thermocouples were recorded every 30 minutes then checked, fill the storage tank with source water through the intake valve, close the intake valve after the water exit from ventilation pipe, solar radiation data was recorded every 30 minute by LOGBOX data logger. The experimental tests were carried out for melting PCM, experimental data was collected from 1 st of August 2017 to 31st of January 2017 as shown in Table 2, each test starting at 8:00 am and terminated at 17:00 pm. Three different water circulating volume flow rates were employed through the system for every experimental stage, namely (200, 300,500) Liter per hour (LPH). The experimental tests were implemented for different weather conditions which included partly cloudy and clear sky.

THERMAL PERFORMANCE ANALYSIS
The value of instantaneous thermal efficiency ηth for the water solar collector has been calculated from the energy balance of the receiver. The useful energy as heat gain (Qu) transported to the receiver defined as (Ma et al., 2011): Where: Ti and To represent the inlet fluid and exit fluid temperatures, respectively.
The instantaneous thermal efficiency of the water solar collector defined as the ratio of heat gain (Qu) supplied to aperture area (Aa), and the solar radiation (I) which is incident on the area of aperture (Aa) (Ma et al., 2011).
In sensible heat storage (SHS), thermal energy is stored by raising the temperature of a solid or liquid. SHS system utilizes the heat capacity and the change in temperature of the material during the process of charging and discharging. The amount of heat stored depends on the specific heat of the medium, the temperature change and the amount of storage material, sensible heat storage system with a PCM medium is given by the following equations (Lane, 1983).
In latent heat storage system charging and discharging phenomenon occur when the storage material undergoes phase change either from solid to liquid, liquid to gaseous or solid to solid.
The storage capacity of latent heat storage system with a PCM medium is given by the following equations (Lane, 1983).

NUMERICAL SOLUTION
The geometry of the shell and tube heat exchanger is created in ANSYS-FLUENT-15. A cylindrical polar coordinate system (r, θ, z) is used with a mesh size of 8903 nodes and 12792 elements as shown in the Fig. 4. The continuity equation, momentum equation, and energy equation are (Bird et al., (1987), Bird et al., (2002)): Continuity equation Equations from (9) to (13) are solved for steady state cases by using of ANSYS-FLUENT-15.
The specifications and initial and boundary conditions of the models that solved in this work are shown in Table 3.     of the contact area of the PCM near T3 is more than T6 and unsymmetrical in its locations from the bulk temperature of water flow inside rectangular inner tube.   shows the ambient temperature with daily time for three days on August 2016, which represented a test days for 200, 300, and 500 lph.      Temp.

Numerical Results
The numerical results are obtained by employing ANSYS Fluent 15.0 software that demonstrated in section (4) for examine the effect of geometry and conditions of flow on thermal characteristics of PCM, the geometries and condition of the model for the present work are given in Table 2. One model of shell and tube is tested numerically for steady state, turbulent flow.
Static temperature contour of the PCM for the Reynlds Number of 15000 is given in Fig. 27, the effect of cross heat transfer in the shell region is visible from the temperature contour. It can be shown that the temperature at inner tube is higher than PCM temperature in the shell side and there is no symmetric distribution of PCM temperature due to rectangular shape of inner tube. The results of the analysis of the numerical simulation are used to estimate the PCM temperature distribution. Fig. 28 gives a comparison of the temperature distributions obtained from the experiment and those obtained by using the numerical simulation in the different angular directions. The same inlet boundary conditions are employed for numerical model and experimental tests. The comparison gave good validation with small mean difference; the percentage deviation for temperatures is 2% for (T3, T4, and T5) direction and 2.7% for (T6, T7, and T8) direction. The comparison shows a good agreement between the computational and experimental values which indicated that the present numerical models can be used to compute the performance of system.

CONCLUSIONS
The melting processes in a shell and tube heat exchanger by using solar thermal energy have been investigated numerically and experimentally. The PCM that employed in this work is black color Iraqi origin pure Paraffin (PCM) with amount of 12 kg. The main experimental procedure in the present work is included using the solar collecor type of evacuted tube for melting PCM in shell regime. Different volume flow rates for the water inside the inner tube of heat exchanger for Reynolds number namely (15000, 23000, 38000) were studied. The influence of the inner tube inlet and ambient temperatures were investigated .The results indicated that the inner tube inlet and ambient temperatures has a significant effects on the melting process compared with the volume flow rates. Studying PCM temperature distribution, it is exposed that a melting temperature of the PCM in summer season needed time of (3-4) hours only, while it needed more time (14-16) hours in winter season. Increasing solar radiation and ambient temperature reduces the melting time for both PCM types. Increasing water temperature difference of inner tube increased the heat gained for PCM.