Monocrystalline Silicon Photovoltaic Cells

Lamination of Monocrystalline Silicon Photovoltaic Cells with Polymer Composite Material

Abstract

Alternative energy sources, particularly photovoltaic panels, are gaining increasing importance in our lives as energy demand grows daily. In this study, the lamination of photovoltaic cells—which generate electricity directly from solar energy—using polymer composite structure to obtain a semi-flexible configuration was investigated. The production method of the composite material for photovoltaic cell lamination was determined, and the matrix and fiber suitable for the intended application were researched. During material selection and lamination technique determination, maximum efficiency obtainable from photovoltaic cells was targeted.

1. Introduction

Solar energy is increasingly rising in the ranking of energy sources used worldwide. Energy derived from the sun is obtained through photovoltaic cells by converting solar radiation into heat and electrical energy. This energy production method is a renewable energy source and therefore offers significantly greater environmental protection advantages compared to other energy sources, alongside wind energy, hydroelectric energy, geothermal energy, biomass energy and similar energy sources. The increased use of solar energy for electricity generation reduces the use of energy production based on fossil fuel consumption. Its application field is quite broad. Photovoltaic cells are used to obtain electricity from solar energy. According to data from Turkey Electricity Transmission Corporation for 2013, annual electricity consumption in our country was determined as 225 billion 337 million [kWh], yet only 2.6% of this comes from renewable energy sources. Turkey's annual sunshine duration is 2,623 hours; considering this data, it is evident that a large portion of the annual electricity requirement could be met from solar energy. The lamination of commonly used photovoltaic cells is performed with materials that are inflexible and have low strength-to-weight ratios, or the structure becomes quite heavy after lamination. This situation restricts the application field of photovoltaic cells. In this study, the use of polymer composite structure for photovoltaic cell lamination was investigated. The objective of using polymer composite structure is to render photovoltaic cells semi-flexible, expanding their field of application, and enabling their use on curved surfaces such as in solar vehicles without compromising aerodynamic form. In selecting the matrix and fiber of the composite material, the efficiency of the photovoltaic cell, the refractive index of the material, mechanical properties and thermal resistance data were considered. Furthermore, the production method of the composite material was determined as vacuum-assisted resin transfer molding [1] according to its application field.

2. Photovoltaic Cell Selection

In the study, silicon photovoltaic cells were selected considering energy efficiency, price, lifetime and performance variables, as well as being readily accessible and efficient. Research has shown that monocrystalline silicon photovoltaic cells maintain their efficiency for longer periods and have greater lifetimes compared to polycrystalline cells. Based on data and advantages obtained from the Sunpower C60 Solar Cell [2] product catalog, it was determined that the C60 Monocrystalline Silicon Solar Cell is suitable for this study.

2.2.1. Advantages of C60 Monocrystalline Silicon Solar Cells Maximum Light Absorption:

Because the contacts of C60 Solar cells are located on the rear surface, there are no losses from fine lines on the front surface. As a result, approximately 10% more sunlight is utilized compared to conventional cells.

Additionally, they have better performance at elevated temperatures compared to conventional cells, less light-induced degradation, can generate energy across a wider light spectrum, and are longer-lasting cells. Table 1. Typical Electrical Characteristics of Selected Cells Under Standard Test Conditions

The selected C60 Solar Cell is a G-type cell. The cell's maximum power value is 3.34 [Wp], efficiency value is 21.8%, and voltage at maximum power operating point is 0.574 [V] (Table 1). These values are suitable for the photovoltaic cell's application field. The cell dimensions are given in Figure 1.

Figure 1. Dimensions in mm of the selected cell

Short-circuit current (Isc) is the current flowing through the photovoltaic cell when the voltage value in the cell is zero. Short-circuit current is produced by light-generated carrier electrons and is the maximum current value obtainable from photovoltaic cells. Examining Table 1, the short-circuit current of the selected photovoltaic cell is 6.24 [A]. Open-circuit voltage (Voc) is the voltage value obtained without connecting a load to the photovoltaic cell (when current value is zero). The open-circuit voltage of the selected photovoltaic cell is 0.682 [V].

3. Polymer Composite Structure Design

Composite material is defined as material created by combining two or more materials at the macroscopic level and possessing new mechanical properties. Composite materials can be classified according to matrix type. In this study, the use of polymer matrix composite material was considered appropriate. In polymer matrix composites, various resins are used as the matrix and various fibers as reinforcement material. To impart superior properties to the structure, different particles, coatings or regional variations can be applied.

3.1. Matrix Selection

The matrix is the primary component of the two important components of composite material, forms the structure's shape and contributes to the technical properties of the composite material. It transfers forces to the fibers and ensures their uniform distribution, increases the toughness of the composite material and prevents crack formation. In this study, polymer matrix materials were preferred (Table 2). Since it will be used in photovoltaic cell lamination, the matrix was primarily determined according to its refractive index value. The investigated matrix materials are: PMMA (Polymethyl methacrylate) [6], EPX 200 Resin [3], Polipol 354 [8], Crystal Clear 200 Resin [9], and Araldite LY 1564 SP.

3.1.1. Matrix Materials

Polymethyl methacrylate (PMMA) is a hard and transparent material. Compared to other thermoplastic materials, it is more resistant to weather conditions. It can be used as a glass fiber reinforced thermoplastic matrix. In the study, considering the refractive index values according to the wavelength of incident light, it was anticipated that it could be a suitable matrix. Polypol 354 polyester, which is a fully transparent casting-type polyester, has the fundamental property of producing very light-colored transparent products. EPX 200 resin is a special thick-consistency resin. It has high strength and chemical resistance. It is resistant up to 120°C temperature. Therefore, it can be easily used in environments exposed to high temperatures. Cure time is 2 [hours], full hardening time is 24 [hours]. Its density value is 1.15 [g/cm³]. It has a transparent appearance.

* This value was taken based on the general properties of the resins.

Table 2. Properties of selected matrix materials

Crystal Clear 200 resin can be used for processes requiring transparency. Low viscosity, making mixing and pouring easy. At room temperature, it hardens with a very low rate of shrinkage. When hardened, it is UV resistant and not brittle. F-1564 Epoxy can be used in low-viscosity laminates with high flexibility. It is nearly transparent. F-3487 can be used as hardener.

3.2. Reinforcement (Fiber) Material Selection

Reinforcement materials placed within the matrix that have higher strength than the matrix are called fiber material. The mechanical properties of fiber material are quite influential on the mechanical properties of the produced composite material. In this study, glass fiber was selected as the fiber material considering its refractive index and strength properties.

3.2.1. Glass Fiber

One of the most commonly used fiber types in fiber-reinforced composite production. Glass fiber is produced by passing molten glass through a specially designed furnace with small holes at its base under pressure. It has high tensile strength (Table 3). Its thermal resistance is low. It is resistant to chemical materials. It does not have moisture absorption properties. It does not conduct electricity. After curing with suitable resin, the refractive index for photovoltaic cell lamination becomes close to the refractive index of water.

Table 3. Glass Fiber Properties

3.2.1. Chopped (Seamless) Glass Fiber (BMC1-06)

Obtained by assembling bundles that have been chopped to a specific length with a binder. These are fibers chopped to specific lengths and packaged for use in ready-to-mold compounds and thermoplastic compounds. Its strength is lower than other glass fiber. However, it is more suitable for our working conditions.

3.2.2. Woven Glass Fiber

Woven glass fibers are structures used to improve the strength values of composite materials to be produced. They are used in multi-layer production to increase bending and impact resistance. They provide good casting, wetting and cost-effectiveness. By arranging the weave at specific angles, different strength and application ranges are created, but they are not as good as chopped (seamless) glass fiber regarding transparency. For this reason, their use on the top surface of the photovoltaic cell (sun-facing) in lamination is not very suitable. In lamination, their use on the rear surface of the photovoltaic cell (non-sun-facing) was intended to obtain better strength values.

4. Polymer Composite Production Method

The selected production method differs depending on the fiber material, matrix material (resin, epoxy), part shape, desired mechanical properties and production speed. Certain considerations must be observed in the production of composite materials. These considerations are: the fiber material showing equal spacing and homogeneous distribution, fibers being sensitive to mechanical contact and thus being thoroughly wetted by the matrix, and the creation of a strong interface between fiber and matrix. In this study, vacuum-assisted resin infusion molding [1][7] was selected as the production method suitable for photovoltaic cell lamination. Figure 2 shows the layer sequence in the application of the method.

Figure 2. Vacuum-Assisted Resin Infusion Molding [1] Method

4.1. Vacuum-Assisted Resin Infusion Molding Method

In this method operating on the principle of resin advancement in a vacuum environment, the objective is production of the finished product without manual handling. This innovative method, often used for producing composite elements with complex structures, requires that the resin impregnated in the materials has appropriate viscosity. In cases involving narrow gap dimensions and long flow paths, the resin must be impregnated into the fibers in as short a time as possible.

4.1.1. Production Stages of Composite Material Using Vacuum-Assisted Resin Infusion Molding Method

• The mold surface must not contain foreign matter such as oil or dust. Oil and dust are removed and cleaned from the surface, • Release film is applied to the cleaned mold surface, • Fibers are cut to fit the mold. They are laid out sequentially and according to intended use, • A layer providing surface roughness is laid out throughout the mold, • Vacuum and resin lines are set up around the mold. The mold is sealed with a vacuum bag and the tightness of the created system is checked, • Air in the system is evacuated by vacuum pump, • According to the determined amount, resin is prepared with the necessary hardener. The resin placed in a reservoir is connected to the line and transferred to the system, • The process continues until the resin contacts every point of the mold. Then waiting continues during the cure time, • When the cure process is complete, the vacuum bag and lines are removed and the part is separated from the mold.

5. Production Trial Runs

The cells to be used in our main production target are C60-type photovoltaic cells with monocrystalline rear surfaces and rear contacts. However, before proceeding to main production, to practice, gain experience, identify where errors might occur and select correct materials, trial production was conducted with cells having fine lines on the front surface and front contacts.

Figure 3. Operations in trial runs

5.1. Soldering of Photovoltaic Cells 5.1.1. Soldering of Front-Surface-Connected Photovoltaic Cells

• First, the photovoltaic cell was carefully placed on the work table where the soldering operation would be performed, on a flat surface, • In the second step, with the aid of a solder iron, solder was applied to the soldering points on the cell and to the flat connecting wire, • After solder and paste application, three separate connecting wires were individually soldered flat on the contact points of the sun-receiving surface (top surface) of the photovoltaic cell, • The wires soldered on the top surface were soldered with another wire extending beyond the cell to reach all wires, • The same operation was performed on the contact points on the rear surface of the photovoltaic cell, and flat connecting wires on the rear surface were soldered with another wire at the edge opposite to where the wires were joined on the top surface, • In this manner, two front-surface-contacted photovoltaic cells were prepared for lamination.

5.2. Photovoltaic Panel Lamination Process

• Resin sealant layer was cut according to working conditions and laid out on the heating table, • All edges of the sealant layer were adhered to the production workbench with sealant tape (Figure 3(a)),

Figure 4. Trial production using vacuum-assisted resin transfer method

• Since lamination of two separate cells was performed, single-layer chopped glass fiber was placed on one side of the working area and double-layer chopped glass fiber on the other side. Photovoltaic cells were placed on top of these glass fibers with the sun-receiving surface facing downward. 45° woven glass fiber was placed on top. Release fabric was laid out to cover all fabric surfaces, • To impregnate glass fibers with resin more uniformly, resin distribution film was laid on top of the fabrics. Resin feed tubes and vacuum tubes were positioned in the system (Figure 3(d)), • The working area was sealed with a vacuum bag. Holes were opened for resin feed inlets. System tightness was established and checked, • With proper system tightness, the resin and hardener preparation stage was begun. The resin used in this production is Araldite LY 1564 SP. The hardener used is F-3487. After mixing resin and hardener in a 1/3 ratio for a few minutes, infusion was performed from the resin feed tube (Figure 4), • Finally, the production workbench was set to 80°C and curing was performed for 480 minutes.

5.2.1. Laminated Photovoltaic Cells

The front-surface-contacted photovoltaic cells produced as the first stage of the research can be examined in Figure 5. Test data for these cells are presented in the relevant section.

Figure 5. Produced front-surface-contacted photovoltaic cells

Following trial production, measurement of the electrical performance of the laminated cells was undertaken.

Figure 6. Test setup and schematic representation

6. Tests Performed and Results

The cells laminated with the selected polymer composite structure were tested under sunlight in an outdoor environment for testing. The irradiance value was measured with a portable irradiance meter, and cell temperature value was measured with an infrared portable thermometer during testing. The purpose of the test is to determine the efficiency loss of the cell after lamination. Cell efficiency standard is determined under laboratory conditions according to AM 1.5 standard, under 1,000 [W/m²] irradiance at 25°C. In other words, it is determined by the ratio of energy obtained from the cell relative to energy provided at this irradiance value. In this study, differences between non-laminated and produced cells were observed in an outdoor environment.

Table 4. Measurements during testing

When comparing the current-voltage values of the laminated photovoltaic cell and the non-laminated photovoltaic cell, the loss resulting from lamination was found to be around twenty percent. The efficiency loss as a result of lamination, based on measured values from efficiency calculations, indicates a lamination that could be suitable for use. However, different matrices and fibers should be tested, and the technique and use of layers should be improved. Additionally, in the use of polymer composite structure in photovoltaic cell lamination, to prevent the structure from yellowing over time and reducing cell efficiency, the selected matrix materials should be given attention to this property, and if necessary, appropriate chemical treatment should be applied after production.

Acknowledgments

The authors express their thanks to Research Assistant Akar Doğan and Sonay Doğan for their dedicated assistance during this research conducted within the framework of an undergraduate graduation project in the Department of Mechanical Engineering at Dokuz Eylül University, for material support, and to the Mechanical Laboratory and Solaris Solar Car Team. Deniz Aksoy Dokuz Eylül University Faculty of Engineering Department of Mechanical Engineering       Ali Kaan Kara Dokuz Eylül University Faculty of Engineering Department of Mechanical Engineering     Emin Kaan Türkay Dokuz Eylül University Faculty of Engineering Department of Mechanical Engineering       Hasan Yıldırım Dokuz Eylül University Faculty of Engineering Department of Mechanical Engineering       Dr. Aytaç Gören Dokuz Eylül University (DEÜ) Department of Mechanical Engineering (MM) Automatic Control and Robotics Laboratories (ACRL) Université de Picardie Jules Verne (UPJV) Laboratory of Innovative Technologies (LTI)    

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