Improving electrical energy and producing thermal energy in solar photovoltaic system by integrating phase change materials

Solar energy is a natural source that provides clean and renewable energy, which supplies two types of energy: thermal energy and photovoltaic energy. Whereas, the most effective way to exploit this energy is photovoltaic cells. However, for all the incident solar radiation, the solar panels can absorb a limited quantity of energy. While, the rest of radiation energy gets lost as heat, that increases the temperature of the photovoltaic cells, this is the reason why the productivity of electricity is decreased. Therefore, to exceed this issue and benefit from the two sources of sun radiation, a hybrid thermo-electrical system is proposed. The system is a solar panel surrounded by the phase change material that can absorb the temperature to increase the efficiency of solar system and use this energy to produce a hot water.


1. Introduction
Solar energy is the most widespread and distributed energy in the world, which is presently one of the greatest renewable and green energy sources, has been extensively focused and investigated for producing heat and electricity. Using solar energy in our standard of living can be decreasing energy costs and protect the environment. Therefore, the photovoltaic (PV) cells exploit the photoelectric effect to produce direct current by absorbing of solar radiation. This effect allows cells to convert light energy from photons into electricity through a semiconductor material carrying electric charges. However, only a limited sun radiation on a (PV) can be transformed into electrical energy, the rest of this radiation is transformed into a heat, which is not only a waste of energy but also affect the PV cell converting solar energy into electricity and decrease their productivity [1]. For that reason, we proposed hybrid system based on the Phase change materials this system can provides an electrical energy from the photovoltaic cells with more productivity and thermal energy come from the heat of photovoltaic panel and directly from sun radiation, which produce hot water. Phase change materials (PCMs) are materials that have a large capacity to store energy in the form of latent heat, the PCM changes phase (it liquefies) by absorbing heat from the ambient atmosphere and when the temperature drops, it becomes solid by restoring the stored heat. In this study, a numerical investigation pretend the increase the efficiently of solar cells by using the PCM systems be enabling electrical energy production when solar panels have a constant temperature. The system can also use extra indirect temperature from panels and extra direct temperature from air and the sun to produce and store thermal energy via the PCM integrated into the solar collectors (1).
The proposed solar energy system ( Figure 3) has the following components: -The solar collector is the main component, and it produces electrical energy on the basis of the PV effect. The collector has dual functions; the first is the conversion of energy from the sun into electrical energy, and the second is the conversion of energy from the PV collector into thermal energy in the PCM for storage and later use to produce hot water.
-The PCM tank (1,2) is a critical element of thermal energy production, and it is designed to store heat and release it when needed or in the absence of the sun. In addition, using PCMs in the systems allows the use of reduced tanks volume without reducing the energy stored. Therefore, energy capacity is based on thermal stratification process that the warm water will always settle on top of cold water.
This system aims to support and complement the previous systems proposed, and therefore develop a perfect and homogeneous system for the heating and storage of thermal energy.

Physical model
The heat transfer (3) in the PCM layer is achieved by phase change. In fact, there are diverse methods for heating as radiation and conduction. It assumed that the PCM volume variation during the change phase was negligible. In addition, the PCM is isotropic, homogeneous and the thermo-physical properties are assumed constants (4). In this way, we consider the system described in Figure 2. The PCM unit has two types of plate the horizontal plate is rectangular shape (5) that is filled by paraffin with length L = 1,5 m and width l=0,1m, and vertical plate is rectangular shape (5) that is filled by paraffin with length L = 1 m and width l=0,1m ( Figure 4).
The objective is to have a tool for predicting the performance of a hybrid system based on solar panel integrate the PCM. We consider a photovoltaic cell as a rectangle the PCMs placed in a rectangular attachment of which all the faces of the rectangle are assumed to be adiabatic except that in contact with the photovoltaic cell the variation of the volume during the change of phase of PCM is considered negligible.The proposed system as shown in figure 2:  The boundary conditions and the gravity direction used for numerical simulation are depicted. At the top and the bottom surfaces, an adiabatic wall is defined. To avoid heat exchange between PCM and air, the interface domain is considered as a wall (6). The adiabatic boundary condition stopped heat dissipation that is coming from the fluid heat transfer during the charging process. It is assumed that the top surface of the PCM plate receives hot temperature when the bottom one receives cold temperature.
The boundary conditions are indicated in Figure 4 (7).

Governing equations
The natural convection effect in liquid phase helps to provide the governing equations for transient analysis of the PCM melting process include the mass conservation equation, the momentum conservation equation and the energy conservation equation (8).
Under assumption of laminar and incompressible flow, appealing the Boussinesq approximation in modelling the buoyancy force(9), the governing equations can be written as follows (10).
Mass conservation equation: (u,v) velocity components in x and ydirections respectively.
Momentum conservation equation: Projection on x: With: P the pressure, ρ the density, β coefficient of thermal expansion, μ dynamic viscosity, g acceleration of gravity, FL liquid fraction, Am constant of the pasty zone and real number of low value (10 -4 ) to avoid division by zero, T: temperature.
Projection on y : With H: specific enthalpy and : thermal conductivity.
The constant of the pasty zone (Am) measures the velocity loss in the zone where phase change takes place. Once the transition is higher, the zone speed being solidified tends to zero fast. In contrast, if the Am is lower the solidification rate is increased and the liquid fraction value is reduced (order of 10 -2 ). The proposed model uses the "enthalpic" method to take into account the phase change; it defines the total enthalpy H as the sum of sensible enthalpy S and latent heat L: = + = ∫ 1 + P < (5) With < : Latent heat absorbed locally.

Results and discussion
The proposed solar power system aims to improve electrical energy production using a phase change material (PCM) that gets rid of the increase in undesirable temperatures and uses thermal energy to heat water. For this reason, a numerical study based on MATLAB and ANSYS software is used to get the results as a function of enthalpy, temperature, time, stocked energy and melting evolution. To validate the model, a comparison of the predicted temperature in the numerical simulation with the experimental results is performed. The system geometry, material parameters and boundary conditions are as close as possible [9].

Enthalpy Vs temperature in PCM (fusion vs solidification)
The obtained results show that PCMs absorb large amounts of heat, followed by a temperature increase until they reach the phase change temperature (melting temperature). However, PCMs keep absorbing heat until all the material is converted to the liquid phase. As a result, this simulation can describe the PCM behaviour (11).
Consequently, the enthalpy changes as a function of temperature, and the system absorbs or releases energy until the steady state is achieved, as shown in Figure 5 Figure 5 shows the enthalpy evolution H as a function of temperature (12). Therefore, the curve is decomposed into three parts: initially, the PCM stocks energy using sensible heat. Then comes the PCM phase change, where the enthalpy saves the same value when changing its phase. In this case, the PCM stocks a large amount of heat using latent heat, and then the PCM is completely liquified.
When the PCM layer is integrated into the module, the PCM absorbs thermal energy from the PV system, and its temperature is decreased compared to the system without the PCM, as shown in Figure 6:  Figure 6, It can be understood that the expected temperatures on the front surface of PV/PCM system are of a consistent form. After some time the temperature of the MCP increasing that caused the productivity of the hot water, although the temperature of panel PV still constant the front surface does not change significantly, however as less heat is lost from the system, the time necessary for the phase change material melting is reduced. The temperatures increase quickly for the first 3 min, the temperatures on the front surface of PV/PCM system then still basically constant through the PCM transition and then increased quickly at phase change achievement. After complete melting, the liquid PCM is higher than that for the solid PCM the amount of temperature rise is slowly than that during the solid stage. The temperatures aluminium plate is cooled by the water and on the front surface of the PV/PCM system.

Simulation using finite element method
In order to improve the production and storage of thermal energy, a 2D numerical simulation using ANSYS FLUENT software (13)(14) based on finite volume approach is proposed. In addition, the paraffin PCM is used for the melting process that had a very high latent heat and can be found everywhere.
Assuming that x = 0 at the outer wall of the plate, the plate is initially at Ti= 57 °C, the fluid temperature of the heat transfer T 8 = 69 °C. The phase change temperature of the paraffin Tf = 64 °C .
The geometrical model is uniformly meshed using symmetric square cells as depicted in Figure 10.
A very fine mesh was applied to the 2D rectangular PCM which predicting correctly its features. The selected elements are 1056 and the nodes are 1139.

Fig. 7 : Geometry and mesh
In order to improve the computational efficiency, a three-dimensional model with double precision and standard turbulence (ke) is used to calculate the underrelaxation factors. The control parameters are also used by default. Furthermore, the SIMPLEC method is adopted in pressure-velocity coupling mode, the mode of retention is the PRESTO method, and the rest are all defaults (9).
At the start, the PCM is in the solid state when the collector and the tank absorb the heat. As a result, the PCM begins the fusion process, and the heat transmits through the PCM by conduction due to the temperature dissimilarity between the system temperature and the PCM. Then, the PCM absorbs heat. As 30 minutes passes, the absorbed heat is then used for the phase change from a solid state to a liquid state. After the beginning of the fusion phase, the natural convection and heat conduction are contemporary in the system. The increase in the melting process increases the natural convection. Then, the half of the PCM is melted after approximately 90 minutes.
The temperature of the system is in steady-state condition when all of the PCMs are liquified and heated to the inlet temperature. Upon comparison, the simulation results are in good agreement with the experiment results, when considering total melting time and detailed fractional melting (15).

Conclusion
In this study, the hybrid thermophotovoltaic system is based on PCMs that can produce electricity, which are surrounded by PCMs that can absorb heat in order to increase the efficiency of the solar power system and use the absorbed thermal energy to produce hot water. For this reason, a numerical study based on MATLAB and ANSYS software is employed to attain the results as functions of enthalpy temperature, time, stocked energy and melting evolution. To validate the model, a comparison of the predicted temperature in the numerical simulation with the experimental results is performed. The system geometry, material parameters and boundary conditions are as close as possible.
The proposed system is able to store energy in the collector and in the tank, which is why the system without PCM realises the energy faster than the proposed system. Therefore, the overall operation of the heating system can accelerate the heating; if the tank is exhausted, the collector quickly heats the water using the stocked energy in the PCM and energy from the sun. The numerical analysis and the results of the experiment show that the integration of the PCMs makes it possible to reduce the temperature of the photovoltaic module by 30°C; this decrease improves the efficiency of the PV module. This difference in energy temperature heats the water for approximately four hours each time. The fundamental objective of this work has therefore been achieved. However, there are some problems related to the problem of PCM volume change during integration with the photovoltaic module; if a stabilised form of the PCM is used, the problem will be solved.